SE514068C2 - Rotary power system stabilizer - Google Patents

Abstract

In the invention, electrical machines (2) with variable rotational speed are utilised as stabilisers in an electric power network. The motion of the rotor (10) corresponds to a certain kinetic energy, and this kinetic energy is used as energy storage for the stabiliser. The electrical machine (2) and its rotational speed are regulated by controlling the current that is sent through the rotor (10). The voltage source and the control means (18) for this are supplied by power from a source, which is independent of the voltage of the electric power network. The voltage source comprises preferably a brushless regulating machine (20) arranged at the same shaft (22) as the electrical main machine (2). By further directly monitoring temperatures in stator (14) and rotor (16) windings, the electrical main machine (2) may temporarily be utilised for large power transmissions, which by far exceed the nominal power.

Description

Next to! »Intill m 10 15 20 30 514 068 g '|| i» |' 2 The power grid should also be operated within certain given voltage margins. Several devices such as transformers and motors are designed for operation within given voltage ranges. The voltage in an AC power network is regulated by controlling the reactive power balance in the network. In the event of a lack of reactive power or in the event of an unfavorable distribution between where reactive power is produced and consumed, the network cannot at certain times be able to maintain the voltage and a so-called voltage collapse is obtained, after which the network breaks down.

In addition to energy conversion, synchronous machines are also used as synchronous compensators without being connected to any drive or mechanical load and therefore do not perform any energy conversion. These machines rotate synchronously with the power grid and can deliver / consume reactive power. They also help to stabilize the power grid in the event of malfunctions by regulating the magnetization of the machine and by increasing the synchronous compensator's short-circuit power of the power grid. The ability of a synchronous machine to stabilize the power grid by modulating active power is limited by the fact that the active power that can be regulated must be provided from the magnetic energy in the machine's air gap or from rotating mass through a transient speed variation when the machine's angle changes.

ASH stands for Asynchronous Hydro (also known as ASG - Asynchronous Generator) and is an asynchronous machine where the rotor winding is fed via slip rings from a converter that is connected to the same power network as the machine's stator poles. These machines have been used primarily in connection with pumped storage power plants. The advantages are fl yours, including that the power plant's efficiency is improved and that you can make a so-called load sequence in pump operation, ie. that the power output from the power grid can be regulated in pump operation. Such machines are in operation at your locations in Japan and are described by Takao Kuwabars et. al i ”Design and dynamic response characteristics of 400 MW adjustable speed pumped storage unit for Ohkawachi power station” in IEEE Transactions on Energy Conversion, vol. 10 15 20 30 514 068 | f ||. | ' 3 11, no. 2, June 1996, p. 376 - 384. An ASH can also be operated as a synchronous machine by supplying direct current to the rotor. However, one should be aware that this can cause thermal overload on a phase if this stationary conducts full excitation current.

An ASH can be used to improve the stability of the network by being able to quickly regulate the active power delivered to the network from the machine in the event of malfunctions and faults in the network. Unlike a synchronous machine, you can regulate active and reactive power independently of each other. The improvement of the power grid's stability enables higher utilization of the power grid by allowing larger amounts of power to be transmitted. This is described by Jan O. Gjerde et. al. i "Integration of Adjustable Speed Hydro Machines in Established Networks", Proc. of Hydropower into the Next Century - III (Hydropower '99), Gmunden, Austria, 1999, p. 559-567. Another advantage of ASH machines is that the working power of the converter is small in relation to the total working power of the machine, typically 15-30% in plants that have been put into operation.

A problem with today's ASH machines is that the frequency converter that is connected to the power grid will quickly be disconnected in the event of a malfunction.

The machine therefore becomes sensitive to faults in the power grid. This is especially true when the part of the converter that is connected to the power grid needs reactive power to commutate and in particular when the converter is a cyclo-converter, where the mains voltage should also have a good symmetry between the phases. Two U.S. Patents 4,812,773 and 4,870,339 disclose various methods for maintaining a controllable alternating voltage in the stator of the machine in the event of failure of the connected power grid. This is absolutely necessary to maintain the magnetization of the machine so that it can be phased in again when the voltage in the power grid returns.

FACTS - an acronym for “Flexible AC Transmission System” is a collective term for several different components that include power electronics and are used to regulate power output and 1 ll. l lill) U) 10 15 20 3G 514 068 |||; - | 4 the voltage distribution in a power grid. The definition set by the IEEE (Institute of Electrical and Electronics Engineers) states that FACTS is: “Alternating current transmission systems incorporating power electronic-based and other static controllers to enhance controllability and increase power transfer capability. In simple terms, FACTS components can be described as equipment, which in most cases includes power electronic valve elements (thyristors and transistors), in order to be able to quickly and accurately vary the voltage, current, impedance and / or phase angle in or between the nodes (nodes) to which the component is connected. Since the introduction of these components creates opportunities to control the power fl fate, they can be used to increase the power system's transmission capacity between different areas or points.

In addition, the introduction of the components can reduce the transmission losses in the transmission system by better distributing the power output between different transmission paths.

FACTS components can be connected to the power grid by shunt connection, series connection or a combined shunt and series connection.

Shunt-coupled units are mainly used for input / output of reactive power and power electronic components are then used for a quick control of reactive power input / output. An example is SVC - “Static Var Compensator” which consists of parallel connected reactors (coils) and capacitors and where the power electronics are used to control the reactive power production / consumption to the unit. This is usually done by controlling the current through the reactors. If full controllability is desired, ie in 100% of the power to the SVC unit, the power electronic components should be dimensioned for the unit's total reactive power. However, an SVC generates harmonics and filtering is therefore necessary. Furthermore, the ability of an SVC to inject reactive current into the power grid will be reduced with voltage, ie. when 10 15 20 30 3 514 068 11 '5 the need is often greatest. However, a "Static Synchronous Compensator" (STATCOM) does not have this problem because shunt-connected capacitors and reactors are not included in the reactive power production / consumption. The reactive power is generated directly by power electronic valve elements and the unit actually functions as a synchronous compensator without rotating mass and with controllable and limited short-circuit power. However, all reactive power should pass through the inverter, so this must be dimensioned for full power.

Series-connected elements in transmission contexts consist mainly of capacitors and are used mainly to compensate for the reactance of a power line, and thus to reduce the "electrical length" of the line.

A series-connected capacitor injects a voltage that is phase-shifted 90 electrical degrees against the current in the power line. The amplitude of the injected voltage can be controlled, such as in a thyristor controlled series capacitor (TCSC).

A UPFC - "Universal Power Flow Controller" - consists of two inverters, one in series and the other connected in shunt with the transmission system.

This means that a UPFC can both regulate a reactive power that can be injected in shunt with the electric power line as well as a controllable voltage that can be injected in series, which means that the voltage can be regulated both to amplitude and phase. A UPFC can thus simultaneously, and independently, control both the active and reactive power lines in a transmission line, thereby combining the possibility of power control and voltage regulation. UPFC is a flexible tool for controlling power flows, at the price of a complicated circuit solution with expensive converters.

FACTS have great dynamics, which makes it possible to use them to improve the dynamic properties of the power grid. They can also compensate for asymmetric operation, e.g. when the voltage between the different phases differs.

An overview of the various FACTS components and their use and 10 15 20 3G i 514 068 1 || 6 technical positions are given in the article “FACTS - powerful systems for exable power transmission” by R. Grünbaum et. al. in ABB Review, no. 5, 1999, p. 4-17.

For all FACTS components, it generally applies that they traditionally do not contain any significant energy storage. The energy stored in capacitors and reactors is severely limited in relation to the unit's operating power and these units can therefore not supply or subtract active power from the power grid. However, by connecting an energy source to the DC side of a STATCOM, for example, this can be realized. In existing experimental facilities, either batteries (Battery Energy Storage - BES) or superconductive coils (Superconductive Magnetic Energy Storage - SMES) are used as energy storage. In an article by Y. Mitani et. al. "Application of Superconductive Magnet Energy Storage to Improve Power System Dynamic Performance", IEEE Transactions on Power Systems, vol. 3, no. 4, nov. 1988, p. 1418-1425, it is shown how such a unit can be used to improve the stability of the power grid. A disadvantage of both SMES and BES is that the converter must be designed for full power and that energy storage is expensive and that some types of batteries contain heavy metals and other environmentally harmful materials.

The placement of different types of compensators in the power grid is very important for them to have the desired effect. To stabilize electromechanical oscillations between generators or groups of generators, for example, shunt-coupled reactive compensators should be placed at equal electrical distances from the two generators (or groups of generators) in order to have a good effect. However, in order to improve the reactive balance of the power grid, these units should be located near or in large load areas. Shunt-connected devices that can regulate active power, such as an ASH, should be placed close to other power producers to have a stabilizing effect. 10 15 20 25 30 'i 514 068 7 For all power electronic converters, it generally applies that their overload capacity is limited by short "ternary" time constants. A converter, for example, has a very small ability to withstand overload. A converter should therefore be dimensioned for maximum load in current and voltage.

A power grid that holds a large proportion of non-thermal power, e.g. hydro power grids or wind power grids, may experience large seasonal or daily variations in the power grid's power fl fate. There is thus a need to be able to use stabilizers which can compensate for both reactive and / or active power ”so that they can be used under different operating conditions in the power grid.

A special embodiment of rotary electric machines is shown in WO 97/45919, where the high-voltage stator winding is based on cable technology. No transformer for connection to a high-voltage network is then required.

Machines made with this type of stator winding are characterized by the fact that it has a very low current density in the stator conductors and by the fact that the cooling takes place substantially at ground potential in the stator plate package. The machines have, by suitable design of the area and / or protection of the rotor windings, a good ability to temporarily, for tens of minutes and even individual hours, generate very large reactive power. An embodiment of protection is described in WO 98/34312, where the temperature of the rotor is imaged in a relay protection stationary located in the plant.

A common problem with prior art stabilizers is that it is difficult to provide stabilizers with sufficient energy storage resources. The costs and complexity of large-capacity stabilizers are very high, and people deliberately choose to use only smaller stabilizers and instead switch off parts of the electricity grid if the fault situations become sufficiently serious. Another common problem with prior art stabilizers is that they cannot be used in a flexible way to compensate for both reactive and active power. It is thus a general object of the present invention to provide stabilizers for electric power systems which provide greater energy storage capacities. A further general object of the present invention is to provide stabilizers which are smaller, more flexible and simpler than today's plants.

The above objects are achieved by devices and methods according to the appended claims. In general terms, the invention relates to a rotary power system stabilizer, comprising a main machine in the form of an asynchronous machine with a wound rotor. The rotor speed can be varied to change the amount of stored energy. The power system stabilizer is characterized in that the stator of the main machine is connected to a power grid, that the rotor winding of the main machine is connected to a converter and that the converter derives its active power from a voltage source which is preferably independent of the power grid. The voltage source is preferably a rotating electrical control machine having a common axis with the asynchronous machine.

The converter is arranged for control and regulation of the main machine's active and reactive power production / consumption. The converter regulates whereby the magnetization of the machine and the current in the rotor winding power conversion is regulated. The converter is preferably located co-rotating with the common axis of the main machine and the control machine.

The shaft of the power system stabilizer can be coupled to a flywheel to strengthen the energy storage capacity of the rotor. The power system stabilizer must primarily be able to dampen fluctuations in the power grid during and after operational disturbances, ie improve the network's transient stability, and therefore work actively only for short periods.

The power system stabilizer according to the present invention can be overloaded for such a shorter time, which makes it possible to keep its nominal power, and thus cost, low. By directly monitoring temperatures in the stator and / or rotor windings, the overload utilization of the main electrical machine can take place temporarily during large power transmissions, which far exceed the rated power, without risking any damage to the machine.

The power system stabilizer can be used in several ways. It can be used as a shunt-coupled element in the power grid, where it stabilizes the power grid during malfunctions and in the event of a fault condition in the power grid by supplying / consuming active and reactive power. The active power is absorbed / emitted by changing the speed of the machine's rotor, where mechanical energy is stored due to the moment of inertia of the rotor. During normal operation, the rotary power system stabilizer can compensate for reactive power and then works mainly as a rotating synchronous compensator. The power system stabilizer can also be used as a power converter, where the machine, in addition to the functions above, converts mechanical energy into electrical energy, or electrical energy into mechanical energy, by connecting the shaft of the power stabilizer to a turbine or mechanical load.

The power system stabilizer is normally operated close to its synchronous speed. The active electrical power supplied from the inverter to / from the rotor winding therefore constitutes only a small proportion of the active electrical power supplied by the power system stabilizer to / from the power grid. The rotating asynchronous machine therefore constitutes an active power amplifier for the converter which is connected to the rotor of the machine.

The invention also relates to a method for stabilizing a power system, comprising the main steps of transmitting electrical power between a power line and a rotating main machine and of regulating this electrical power by changing the speed of the main machine. An apparatus and method according to the present invention has a number of advantages. The rotary stabilizer, unlike static compensators, has a large energy storage in the rotating shaft, which can be used for transient input and output of active power for damping power oscillations in the electricity grid. In the event of transient overload of the machine, its properties can be significantly increased. The energy storage can be strengthened in a cost-effective way by using flywheels. Unlike traditional rotary compensators, the rotary stabilizer can compensate for asymmetric operation (minus sequence system). Used as a shunt-coupled reactive compensator, the machine can for limited periods of e.g. 5 - 60 minutes is significantly overloaded with reactive power in that the temperature margins of the insulation can be used in a controlled manner by measuring the temperature in the stator and / or rotor winding. A machine with high-voltage cable winding also has a large thermal time constant, which means that the period when the machine is not in thermal equilibrium increases. During this period, the load can be further increased. Overloadability means that the nominal power of the machine can be reduced. The rotary stabilizer has an internal emf which in principle can be controlled even in the event of a fault in the power grid as long as the control machine can deliver power to the converter. The machine can thus have a controlled input of active and / or reactive power during transient processes in the event of a fault in the power grid. The machine therefore has a controllable short-circuit effect. The rotor winding can be designed with frequency-dependent resistance so that the machine's ability to withstand large network disturbances increases.

Unlike static compensators, the machine can, for example, be used as a combined power converter and compensator in a power station to dampen oscillations both in power grids and e.g. waterways.

BRIEF DESCRIPTION OF THE DRAWINGS The invention and further objects and advantages thereby achieved are best understood by reference to the following description and the accompanying drawings, in which: Fig. 1a is a schematic illustration of a power system stabilizer according to the present invention. , comprising a transformer, but without connection to drive machine or load; Fig. 1b is a schematic illustration of a power system stabilizer according to the present invention, comprising high voltage cable windings, but without connection to drive machine or load; Fig. 2a is a schematic illustration of a power system stabilizer according to the present invention, comprising a transformer and connected to a drive machine or load; Fig. 2b is a schematic illustration of a power system stabilizer according to the present invention, comprising high voltage cable windings and connected to drive or load; Fig. 3 is a schematic illustration of a first embodiment of a stabilizer according to the present invention, comprising a brushless main machine; Fig. 4 is a schematic illustration of a second embodiment of a stabilizer according to the present invention, comprising a main machine with brushes; Fig. Sa is an equivalence diagram per phase for the main machine in ñg. 3; Fig. Sb is a reformulated equivalence scheme corresponding to that in fi g. 5a; Fig. 6 is a view diagram of an operating situation in which the stabilizer according to the present invention operates stationary as a synchronous compensator; Fig. 7 shows the position of the stator fl destiny referred to the winding axes of the stator and rotor windings; Fig. 8 is a block diagram for controlling the stabilizer in a stator fl fate-oriented system with the converter connected to the rotor of the main machine; Fig. 9a is a diagram showing power oscillations between a power system stabilizer according to the present invention and a Cl power net; Fig. 9b is a diagram showing speed variations occurring in a power system stabilizer according to the present invention during the power oscillations in fi g. 9a; Fig. 10 is a diagram showing non-periodic power oscillations between a power system stabilizer according to the present invention and an electric power grid; Fig. 11 schematically illustrates a power grid with a production area and a load area; Fig. 12 schematically illustrates a power grid with two production areas and a load area, where a power system stabilizer is connected to one of the production areas; Fig. 13 schematically illustrates a power grid with two production areas and a loader area, where a power system stabilizer is connected to the load area; Fig. 14 shows how the control system for the voltage and power regulators shown in fi g. 8 is extended to be able to perform active and reactive power modulation; and Fig. 15 is a flow chart illustrating a stabilization method according to the present invention.

DETAILED DESCRIPTION As stated above, a preferred embodiment of the invention consists in summary of a power system stabilizer built up of a rotating electric machine (main machine) where the stator winding is shunt-connected to a power grid. The machine's rotor winding is designed as a as phase AC winding and this is fed from a converter.

The converter is further connected to a rotating machine (control machine) mounted on the same shaft as the main machine.

Initially, a brief description is given of how the stabilizer is connected to a power grid and what effects can be controlled to stabilize the power grid. Next, the power-carrying parts of the rotary stabilizer are first described, i.e. stator and rotor for main machine and control machine, and the converter, which is connected between the control machine and the rotor winding of the main machine. After that, the control and monitoring of the stabilizer is described and how this is done with sensors and digital control units placed in the rotor and / or stator of the machine. Finally, it is specified how the stabilizer can be operated and used in a power grid.

For AC machines, it generally applies that they are made for energy conversion, ie. that they work either as motors or generators. The shaft of the machine is then connected to a mechanical load or a drive machine.

According to a first embodiment, the power system stabilizer is the main machine, as shown in ñg. 1a, in principle an asynchronous machine 2, where the stator winding is a 3-phase alternating current winding which is connected to a power grid 1 via a transformer 3 and a coupling means 4. I fi g. 1a, the main engine is not connected to any prime mover or mechanical load and its possible power exchange with the power grid is then: Q: The stabilizer can stationary supply / consume reactive power to / from the power grid. This value can be quickly changed from a stationary value Q1 to a new stationary value Q2. The maximum power is given by stationary thermal limits in the main machine.

AQ: This symbolizes the ability of the stabilizer, for a limited period of time, in the event of a malfunction or fault to deliver / absorb a reactive power that may be greater than what is stationary thermally permissible. This is explained in more detail below.

AP: This symbolizes the ability of the machine, for a limited period of time, to emit / receive active electrical power that deviates from the shaft power by allowing the machine's speed to change. Energy can then be obtained from rotating mass by reducing the speed or stored in rotating mass by increasing the speed. This is described in more detail below.

In the embodiment in fi g. 1b, the main machine of the power system stabilizer is an asynchronous machine 2 ', where the stator winding is a high-voltage cable winding.

The winding is then connected directly via the coupling means 4 to the power grid without an intermediate transformer. I ñg. lb, the main machine is also not connected to any drive machine ("prime mover") or mechanical load and its possible power exchange with the power grid is then analogous to what is stated in connection with fi g. la.

The stabilizer can, as in ñg. 2a, connected to a loader / drive machine 5.

The drive machine supplies mechanical energy to the shaft ("prime mover"). Instead, a mechanical load draws mechanical energy from the stabilizer shaft.

The drive machine can be, for example, a turbine (water, meadow or gas turbine), an internal combustion engine (piston engine or Stirling engine) or an electric motor. The mechanical load can be a pump, an electric generator or a brake. When such a machine is connected to the shaft of the stabilizer, it can, in addition to the previously mentioned quantities, stationary supply or receive active power from the power grid, ie. work as a generator or motor.

I ñg. 2b illustrates an asynchronous machine with high-voltage cable winding, connected to a load / drive machine 5. This configuration can supply or receive active power from the power grid in the same way as above.

Fig. 3 shows an embodiment of a power system stabilizer according to the present invention. A main machine 2 is here an asynchronous machine with wound rotor 10. The stator 12 of the main machine has a 3-phase winding 14 connected via a transformer 3 to a power grid. The rotor winding 16 can in principle have a constant greater than or equal to two. The synchronous speed set up by the S-phase winding 14 in the stator 12 is determined by the frequency in the power grid and by the number of poles with which the winding 14 is made. The speed of the rotor 10 can be changed in relation to this by providing an alternating current in the rotor winding 16 of the main machine 2. This current is supplied by a converter 18. The frequency of this current is determined by the difference between the synchronous speed, the rotor speed and the machine pole. A control machine 20 is mounted on the same common shaft 22. In this example, the control machine 20 is a synchronous machine where the anchor winding 24 is located in the rotor 26. The co-rotating converter 18 can therefore transmit power between the control machine rotor 26 and the main machine rotor 10. When the main machine 2 rotates synchronously speed, all power is transferred between the rotor 10 of the machine and the stator 12 by rotation induction. Therefore, no active electrical power is applied to the rotor winding 16 at this time (disregarding losses). When the machine 2 rotates asynchronously, a certain proportion of the power in the stator 12 will be transmitted from the rotor 10 transformatively.

This electrical energy should therefore be supplied to the rotor winding 16 from the converter 18. The converter 18 thus provides for the magnetization of the main machine 2 and for supplying / receiving control power (active electric power) to / from the rotor winding 16 from / to the main machine 2.

The task of the control machine 20 is to function as a voltage source for the converter 18 so that it can magnetize the main machine 2 and to transmit the control power to mechanical power on the common shaft 22 by alternately operating as a motor or generator. In a preferred embodiment, the control machine 20 operates in operation as a synchronous machine.

The control machine 20 may have a different pole number than the main machine 2 so that the frequency therein can be increased. The control machine 20 has DC-supplied field windings 34 in the stator 28. These are supplied in normal operation via an AC-DC converter 42 connected to the same three-phase lines as the stator windings 14 of the main machine, via a transformer 44. In the event of failure of the connected power grid or other types of malfunctions, the field winding to the control machine is provided from a battery backup 65 or by providing the control machine 20 with permanent magnets. In the first case, the inverter will be a UPS (Uninterruptable Power Supply). 10 15 20 514 068 16 I fi g. 5a shows the equivalence diagram for the main machine per phase. The resistance in the stator and rotor windings is disregarded here. XM is the excitation reactance, while XLs denotes the leakage reactance in the stator winding and XLR is the leakage reactance in the rotor winding. IM denotes the excitation current, the ice current through the stator windings and the IR current through the rotor windings. s denotes the lag of the rotor in relation to the synchronous speed of the stator. The stator has a generic reference, while the rotor has a motor reference.

The power system stabilizer normally operates at a speed that is close to the synchronous. From the equivalence scheme it is then seen that the voltage in the rotor winding is low, since the lag s is very small. If the stabilizer is to be operated within a speed range, such as: 10% of the synchronous, the proportion of active electrical power to be applied to the rotor winding 16 will not exceed 10% of that of the main machine. This means that the power to the inverter is small in relation to the total power of the main machine (and thus the stabilizer). In addition, the inverter should be able to magnetize the main engine of the stabilizer, which means that the power of the inverter must be slightly greater. The power of the control machine is therefore determined by the maximum control power and by the reactive power requirements of the inverter.

Fig. 4 shows an alternative embodiment according to the present invention.

It is broadly similar to the previously described embodiments and common parts will not be described again. The anchor winding 24 'of the control machine 20' is located in the stator 28 in this case. The inverter 18 'is stationary and connected to the rotor winding 16 of the main machine 2 via brushes 30 and slip rings 32. The field winding 36 of the control machine 20' is consequently arranged at the rotor 26. the DC converter via brushes 38 and slip rings 40. Fig. Sb shows a reformulated equivalence scheme for the asynchronous machine with wound rotor where XM is the excitation reactance, while XLs denotes the leakage reactance in the stator winding. The sum of these constitutes the reactance of the stator winding and is denoted XA. The IR value and phase of the rotor current are related to the stator side of the machine.

Fig. 6 shows a pointer diagram for an operating situation where the stabilizer acts stationary as a synchronous compensator. No mechanical drive or load is connected to the shaft. The limit of the maximum emitted stationary reactive power is then normally given by the thermal stress in the rotor. It is therefore normally the need for magnetomotor power (MMK) in the rotor that determines the main dimensions of the machine. When the stabilizer acts as a compensator for reactive power, the rotor current can go out fi g. 5b is derived from: Lwigia fl yfi) XM (JS XM From this it is seen that the magnetization requirement (rotor current IR) becomes lower the higher the magnetization reactance XM is. conventional synchronous machine, it is seen that the MMK requirement will be lower than what is normally allowed in a conventional synchronous machine, this is partly due to the requirements for stability in the event of disturbances in the power grid.Because, via the converter 18 connected to the rotor winding 16, the rotor current can be controlled in amplitude and phase, the rotor and stator fate in this machine will not be asynchronous, therefore synchronism is not lost as in a conventional synchronous machine, and the stability requirements can be moderated, thus reducing the MMK requirement in the rotor compared to a conventional synchronous compensator. This means that to dimension the stabilizer for stationary reactive power compensations compared to a conventional synchronous machine, one can manufacture the machine with smaller dimensions. This is due to the fact that it is the rotor that is dimensioning for the machine and / or that the losses in the rotor can be reduced by reducing the current density in the rotor winding.

Faults in the power grid, such as a short circuit or earth fault, will normally lead to a disturbance in the operation of the power grid. When the fault still remains in the mains, the currents will increase and the voltages will drop in the mains. The biggest change, of course, is near the wrong place. While the fault is present, synchronous machines in the power grid will have their torque balance changed by changing the electrical load torque while the mechanical one is approximately unchanged. The transient processes in a power grid are very complicated, so that it may be desirable, for example, that while the fault remains, provide the grid with reactive power, in order to maintain the best possible torque balance. At other times this is not desirable because the fault currents will then be too large. On the contrary, one may then have a desire to limit both active and reactive power (current) which is input in the event of a fault. Since the converter 18 in Figure 3 is fed from the control machine 20, the measurement to the converter 18 will not be affected by the error. The converter 18 can therefore itself, in the event of a fault in the power network, supply the rotor winding 16 with the desired current and voltage. In this way, the input reactive power from the stabilizer can be controlled during fault situations. In traditional ASH, as described earlier, the inverter will switch off in the event of an excessive reduction of the voltage in the power grid and / or in the event of excessive asymmetry. In the stabilizer according to the invention, on the other hand, there is a controllable short-circuit effect during the entire fault process. Furthermore, in a traditional synchronous compensator, it will normally only be possible to control the short-circuit current after the subtranscent process has ended.

Even after this, the dynamics of the control in a synchronous machine are limited due to the large time constant of the field winding. The relationship between the field winding voltage and the internal voltage of the machine can be considered as a first order system with a characteristic time constant. This time constant is the transient reactance of the machine at open stator poles and is mainly determined by the field winding. This is referred to in the literature as Tho. This has a size of 1-8 seconds for large synchronous machines.

Since the main machine in the rotary stabilizer is an asynchronous machine, the rotor winding, unlike a synchronous machine, is designed as an alternating current winding. This has a lower time constant and in addition to changing the amplitude of the current, it can also change its phase.

To further increase the robustness of the stabilizer in the event of a fault and to increase its tightening capacity, the rotor winding 16 can be designed so that its resistance increases with the frequency of the current. This is done by using the leakage i in the rotor to give a current displacement, ie. that the current in the rotor winding 16 is not distributed homogeneously over its cross section, but is compressed against the part of the winding 16 which is closest to the air gap in the machine. This effect increases with frequency.

Usually the line where there is an error will be disconnected. The rotary stabilizer will no longer be connected to the power grid if it is connected to the power line that is disconnected. The power grid control system will usually try to make a quick reconnection, after the line has first been disconnected. It is then important that the stabilizer can behave analogously to a traditional synchronous generator, so that it can be phased in again on the network. The stabilizer according to the invention has its own separate power network in the rotor, consisting of the rotor winding 16 in the main machine 2, the converter 18 and the control machine 20. Since the control machine 20 does not lose its magnetization, kinetic energy in the rotating parts can be used to magnetize the system. to the rotor of the main machine itself if the external power grid is disconnected.

The magnetization of the control machine does not require very much power and it can, as previously mentioned, also be secured by supplying it with the battery backup 65, or by making the control machine 20 with permanent magnets. Even if the external power grid should fall off for a certain time (in the worst case a few minutes), the voltage in the main engine 2 of the stabilizer can be regulated to the correct frequency and phase so that automatic connection can be made. Once it has been detected that disconnection has taken place, the stabilizer control system (which is described in more detail below) can be allowed to generate a reference value for the stator flow, for example that it rotates at the same speed as before the fault.

From fi g. 6 it is seen that when the stabilizer acts as a compensator close to its nominal power, an increase in active power AP or in reactive power AQ will easily lead to an overload of the machine. Nominal effects are normally set today according to the manufacturer's recommendations, possibly with a certain margin. The manufacturer's recommendations also usually apply to stationary situations, where ambient temperatures etc. are assumed to be higher than normal.

In addition, manufacturers usually set the recommendations themselves with certain margins. In most electrical machines and transformers today, there are normally certain thermal margins that can be used to overload an electrical machine, at least temporarily. To prevent the main machine from being damaged when this is done, the temperature in the rotor and / or stator winding can be monitored during operation with temperature sensors. I fi g. 3, the main machine is thus provided with a temperature sensor 60 on the rotor winding 16 and a temperature sensor 64 on the stator winding.

The machine i fi g. 4 is similarly provided with temperature sensors 60, 64. This then makes it possible to work outside the nominal capacity of the machine for a limited period, as stated in fi g. 6.

Compared with existing shunt-coupled compensators according to the prior art, this means that the stabilizer's ability to deliver / consume reactive power in a cost-effective way can be increased far beyond the stabilizer's nominal power for a shorter period. In an SVC, this is not possible, without the entire plant having to be dimensioned up to its peak power. Overloadability is not possible there. For a STATCOM system, the entire reactive power must be delivered through a 10 15 20 FC (71 Lv.

F) 514 068 21 power electronic converter. This, on the other hand, has a very small overloadability. This means that the plant must be dimensioned for its peak power. In the power system stabilizer according to the invention, on the other hand, only the frequency converter should be dimensioned to be able to cope with the increased rotor power. Since the power on this converter is low in relation to the total power of the stabilizer, the overhead costs for this will be correspondingly lower.

The stabilizer can also deliver / consume active power to / from the power grid.

This can be done by changing the speed of the stabilizer and / or by having a drive / mechanical load connected to the stabilizer. First, an embodiment is described in which the machine has no drive machine or mechanical load coupled to its shaft, which corresponds to the systems in ñg. la and ñg. lb.

The energy stored in the rotating parts of the stabilizer is largely determined by the moment of inertia and the speed. For electric machines it is often expressed in the time constant H for the inertia of the machine, which is given by: J (and SN H: NI-I where J indicates the moment of inertia of the machine, wo nominal mechanical speed (rad / s) and SN power of the machine. H therefore indicates the amount stored energy in rotating parts at nominal speed in relation to the power of the machine, for larger electrical machines this is typically in the range of 2-8 seconds.

Fig. 9a shows a time course of emitted active power P from the power system stabilizer. The effect varies between Po and -Po, but the mean value of the effect is zero. When the power oscillates after a sine function as in ñg. 9a, the converted energy A1 in a half period will be) PT, where P is the maximum active power and T is the period of oscillation. During the next half period, the same energy A2 will then be converted again, but now in the opposite direction. 10 15 20 30 514 068 22 The speed in such a process will then have a process which is given in ñg. 9b.

A typical period time for power oscillations based on pole wheel oscillations, which is due to velocity oscillations between the speeds of synchronous machines, in synchronous machines is 0.5 -2 seconds. For a sine-varying power variation as in ñg. 9a, the need for an energy storage then becomes between 0.32 P and 1.3 P. In the article "Application for Superconducting Magnet Energy Storage to Improve Power System Dynamic Performance", by Y. Mitani et. al i IEEE Trans. on Power Systems, vol. 3, no. 4, November 1998, p. 1418-1425 and in the article “Active-Power Stabilizers for Multimachine Power Systems: Challenges and Prospects” by I. Kamwa et. al. and IEEE Trans. on Power Systems, vol. 13, no. 4, November 1998, the amount of energy that should be able to be stored in relation to the effect of the stabilizer is described in more detail. It is then shown that even with an energy storage for E measured in watts, you can get a sufficient attenuation of oscillations in the power grid. Now assume that the speed variation is 110% from the synchronous speed. This means that about 20% of the rotating energy can be utilized for stabilization, which means that the inertia constant H should be in the order of 1.6 - 6.5 seconds for this example. If the stabilizer does not have sufficient natural moment of inertia, this can be increased in a cost-effective way by using flywheels on the stabilizer. This is shown schematically in ñg. 3 with the reference numeral 72. This will have the greatest benefit for fast-rotating machines, ie where the speed is high because more energy is then stored in a flywheel with the same physical dimensions. As can be seen from the article "Large-Scale Active-Load Modulation for Angle Stability Improvement" by Kamwa et. al., IEEE Transactions on Power Systems, vol. 14, no. 2, May 1999, p. 582-590, you typically only need to modulate 5% of the active power. This means that an output of 1000 MW in a power line can be stabilized by modulating an output of 50 MW. The rotary stabilizer thus provides a cost-effective energy storage with a capacity for E of the same order of magnitude as P and when one also takes into account that the stabilizer according to the invention can be significantly overloaded, one realizes how powerful the stabilizer is in relation to to its rated electrical power. It is only the converter that is connected to the rotor windings of the main machine that should be dimensioned in power, but this converter constitutes only a small proportion of the stabilizer's total power.

For occasions where the frequency of commuting is so low that the need for modulated energy is large in relation to the power, mechanical loads can be used. I fi g. 4 a brake 70 is arranged at the rotor shaft 22. This brake 70 can be used to get rid of too high rotational energies. This further increases the visibility of the stabilizer, by allowing the average value of power taken from the power grid to differ from zero and, if necessary, disposing of residual energy via the brake 70. The brake 70 can, if necessary, be performed with cooling devices which make it stationary for a period of minuter your minutes use the brake 70.

Fig. 10 shows another time course for delivered active power P from the power system stabilizer. The effect varies non-periodically, but the effect is controlled at each time according to the needs. The speed of the power system stabilizer will also vary non-periodically due to the power fl fates.

The integral below the power curve corresponds in the same way as above to the energy delivered to the electric power system. As long as this integral remains smaller than the allowable speed variation in the stabilizer, no energy needs to be supplied or removed from the stabilizer. However, if the power oscillations are too large, a drive or load must be coupled to the stabilizer shaft to moderate its speed. the main parts of I fi g. 3 also illustrates the monitoring systems. A static control unit 48 is arranged to supply the control and alternating current direct current converter 42 with control signals via a control connection 52. The alternating current direct current converter 42 supplies the field winding 34 of the control machine with suitable excitation current. Correspondingly, a rotating control unit 46, which is arranged co-rotating with the common shaft 10 and thus with the rotor 10 of the main machine, is arranged to supply the converter 18 with suitable control signals via a control connection 66.

The converter 18 thus supplies the rotor winding 16 of the main machine with current of suitable phase, amplitude and frequency. How this control takes place is described in more detail below. The co-rotating control unit 46 further comprises a communication means 56 for wireless communication with a communication means 54 in the static control unit 48. The control units 46 and 48 can thus exchange information. Both control units 46, 48 comprise process units 47, 49 for processing signals and data.

The co-rotating control unit 46 is connected to the temperature sensor 60 and is thereby provided with temperature data about the rotor winding. Furthermore, there is a sensor 58, for measuring current / voltage in the rotor winding of the main machine, and a sensor 62, for measuring current / voltage in the winding of the control machine, connected to the control unit 46, which monitors the current and / or voltage to the rotor windings 16 resp. the current and / or voltage between the control machine 20 and the converter.

The static control unit 48 is similarly connected to the temperature sensor 64 for measuring the temperature of the stator winding. Furthermore, a sensor 50 is connected to the control unit 48 for measuring the current and / or the voltage to / from the stator 12 of the main machine. Knowing the properties of the transformer 3, this sensor 50 indirectly senses the properties of the power grid. Alternatively, a sensor 51 can be arranged on the power grid side of the transformer 3, i.e. in the electric power plant in which the stabilizer is located, for direct measurement somewhere in the electric power plant. However, such a solution normally requires p.g.a. the higher voltages more expensive technical solutions. The sensor 50 or the sensor 51 can thus detect disturbances in the power grid, e.g. regarding the rms value of the voltage, phase and / or amplitude and / or the rms value of the current, phase and / or amplitude, and their frequency. In this way, data can be obtained concerning both electrical and thermal parameters, and this data is communicated between the control units 46, 48, preferably via a radio connection. The co-rotating control unit 46 can therefore be provided with current information as a basis for controlling the rotor current through the converter 18 in a suitable manner.

I ñg. 4 shows the corresponding control units 46, 48. The rotating control unit 46 is here primarily responsible for collecting data from the temperature sensor 60 and the sensor 58. This data is processed in the process unit 47 and transmitted wirelessly to the static control unit 48. The static control unit 48 is here responsible for the control. of both the AC-DC converter 42 and the converter 18 '.

The stabilizer according to fi g. 3 thus has an integrated control and regulation system which includes rotating 47 and stationary 49 digital processors with brushless digital communication 54, 56 and sensors 50, 51, 58, 60, 62, 64 for measuring and monitoring current quantities. The processors 47, 49 normally operate in a master / slave relationship with the rotating processor 47 as a slave. Essentially, the stationary processor 49 controls the power conversion, measures and monitors quantities associated with the stator 12 of the electrical machine, and communicates with other external control systems. The main task of the rotating processor 47 is to control the converter 18 rotating with the axis of the electric machine and to measure and monitor quantities associated with the rotor 10 of the machine.

The rotary processor 47 is programmed so that in the event of repeated and severe disturbances in the wireless digital communication 54, 56, it can autonomously control and regulate the stabilizer for a period of time.

The main machine can be regulated according to the same principles that are normally used for ASH machines. The control method can be based on a so-called ot-ß transformation, where the machine's dynamic system of equations and physical currents and voltages in the rotor and stator are transformed into a system consisting of fictitious rotor and stator windings which are oriented on a specific in relation to the fate of the stator fl. It can then be shown that one component of the rotor current controls torque and thus active power P, while the other component controls the pole voltage of the stator and thus reactive power Q. The regulator structure for this is shown in fi g. 8, where 91 is a voltage regulator or regulator for reactive power and 92 is a regulator for active power. The current regulator is denoted by 90. The voltages uar, ubf, um are connected to a modulator in the converter. A more detailed description is given in "Consequences of introducing Adjustable Speed Hydro (ASH) in Established Power Networks", Proc. or PSCC'99, vol. 1, p. 150-156, Norwegian Univ. For Technology and Science, Trondheim, 1999 by Jan O Gjerde et. al. When the stabilizer is connected to a turbine and works as a generator, the speed is controlled by the turbine which has its own control system. Examples of this are shown in the publication “Design of dynamic response Characteristics of 400 MW adjustable speed pumped storage unit for Ohkawachi Power Station”, IEEE Trans. on Energy Conversion, vol. ll, no. 2, June 1996, p. 376-384, by T.

Kuwabara et.al.

Calculation of on and off times for the inverter connected to the rotor can be done as shown in Figure 7. Stator position (WS) in relation to the stator windings os can be determined in several ways, for example by mounting sensors in the machine air gap or by integrating measured stator voltage . By integrating the measured speed, the position o of the rotor in relation to the stator is obtained. Thus, of can be calculated and the ot and ß component of the rotor current can be transformed into real phase currents in the rotor. The speed of the machine can, for example, be measured / calculated by integrating the voltage from the control machine.

Measuring voltage values in high voltage equipment is costly because you need measuring equipment that is connected to high voltage conductors and that therefore needs to be insulated against these voltages. For electrical machines 10 15 20 514 oesi 27 with high voltage stator winding, it is expensive to equip the machine with its own voltage transformers for measuring pole voltage, while measuring current is cheap because it is not necessary to connect galvanically. This normally requires a voltage measurement on the machine's poles in order to be able to phase it into the power grid when it is to function as a generator.

There will always be voltage measurement on the busbar as this is necessary for phasing in other lines and machines, etc. The stabilizer according to the invention can be phased in against an external power network without its own measurement of the voltage at the machine's poles. The measurement of the voltage to which the machine is to be phased in is transferred to e.g. the desktop computer. This computer then calculates the position of the stator fl for the voltage to be in phase with the measured voltage. Then, by controlling the amplitude and frequency and phase of the rotor current, you can generate a voltage out of the machine that is in phase with the measured one. This represents a clear improvement in relation to the state of the art because you do not need your own voltage transformers for phasing. Since the dynamics of the machine is significantly faster than that of the drive machine, it does not have to run stably at a speed that coincides with the synchronous one, since the difference between the synchronous speed and the actual speed of the stabilizer can be compensated for by controlling the frequency of rotor currents. This is not possible for a synchronous machine.

By having a rotating digital control unit and a communication line between the rotor and stationary parts of the stabilizer, you can use advanced monitoring systems in the rotor, even if these require extensive signal processing. Partial discharges in the insulation can, when the discharge level becomes too great, destroy the insulation, which can lead to a breakdown.

In the publication "Continuous On-line Partial Discharge Monitoring of Power Generators", 1996 IEEE Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, p. 496-499 by A. Kheirmand et. al., describes an INTECH system which during operation must be able to monitor the level of partial discharges in the stator winding of a rotating AC machine. A similar system, when having a rotating digital control unit, can also be used to monitor the insulation in the rotor.

The rotary stabilizer according to the invention is flexible in its use.

To exemplify this, a more detailed account will be given of how it is used in a power grid. I ñg. 11 shows a power network consisting essentially of a production area 80 and a load area 82. In order to be able to effectively stabilize power oscillations between these areas, a tensioning means 84A, 84B can be connected to the power network. Active power modulation will be most effective when the actuator is located near a producer 88, such as the actuator 84A, or a consumer 86 of active power, such as the actuator 84B. The stabilizer can then be placed either in the production area 80 or in the load area 82. If the stabilizer 84A is located in the production area 80, it can also be used as a power converter by connecting it to a turbine. For example, if the stabilizer 84B is located in the load area 82, it may also act as a synchronous compensator. Its ability to overload means that in the event of a voltage collapse, it can be significantly overloaded during a critical period of typically 1 - 30 minutes.

Power fluctuations can occur in electric power grids as a result of poorly attenuated low-frequency (eg 0.1 to 2 Hz) oscillations between the rotors on generators. There are two main groups of oscillatory problems, characterized by the fact that the oscillations take place either between different areas (inter-area) or more locally (intra-area). The introduction of power system stabilizers (PSS) on generators can actively contribute to dampening power oscillations. A large part of the damping moment, regarding power oscillations between areas, is added via a modulation of the loads of the electricity power grid.

Attenuation of (inter-area) oscillations takes place by increasing the attenuation of the oscillating modes of interest, which ideally takes place by applying a braking torque proportional to the speed deviation in the machine. Two practical alternatives to stabilize via the excitation control of electrical machines are to modulate the electrical or mechanical moment. In practice, improved attenuation can be obtained by allowing a power system stabilizer, PSS, to provide an additional signal to the machine's excitation system. The inputs to a PSS can be, for example, the speed deviation of the rotor shaft, the frequency at the generator terminal or the integral of the electrical power.

Placement of devices for Stabilization of the power grid is a complicated subject that requires a comprehensive analysis at every opportunity. However, based on simplified models, one can understand how one should try to use different devices. In the article "Utilizing HVDC to Damp Power Oscillations" by T.

Smed and G. Andersson, IEEE-Transactions on Power Delivery, vol. 8, no. 2, April 1993, p. 620-627 gives some instructions. Devices that can mainly modulate active power should be placed close to consumers or producers of active power, while devices that can mainly modulate reactive power should be placed in an electrical midpoint between producers / consumers of active power. According to the article, the mains frequency constitutes a suitable input signal for the control of the active power modulation, while the derivative of the amplitude of the voltage constitutes a suitable input signal for the reactive power modulation.

The instantaneous value for active electrical power from the stabilizer's main machine can be calculated, for example, by multiplying the measured instantaneous values of current and voltage belonging to the same phase. The reactive power from the stabilizer's main machine can be calculated by measuring the efficiency values for current and voltage as well as the phase angle between current and voltage belonging to the same phase.

Fig. 14 shows how the control system in fi g. 8 may be amended for this purpose.

The signals AU and Aw are processed in their own filters / signal processors denoted by their transfer functions Gp and Gu. These then generate a signal AQ representing a transient change of the stationary reference of the stabilizer voltage / reactive power, and a signal AP representing a transient change of the reference of the active power of the machine.

I fi g. 12, there is a power grid with two production areas 80A and 8OB and a load area 82. Suppose now that these production areas 80A, 8OB differ in diurnal and / or seasonal variation. If the production area 80A is based on hydropower with large reservoirs, this will be dominant in the winter. The power will then mainly fl surface from the production area 80A to the load area 82. The production area 8OB can be based on, for example, hydropower with river power plants. Its production is then greatest at large watercourses in watercourses, e.g. in the spring. It will then mainly have an effect fl fate from the area 8OB to the load area 82.

By placing a stabilizer 84 in the area 8OB, this can be used itself if one has seasonal variations in the power output as described. When the production area 80A dominates, the stabilizer 84 will be able to deliver reactive power to the power grid to achieve an optimal voltage distribution in the power grid. In case of failure, the stabilizer can supply a regulated temporary reactive power AQ to stabilize active power oscillations between the production area 80A and the load area 82. When the production area 8OB is dominant, the active power can be modulated as in Figure 11 to stabilize oscillations between the production area 8OB and the load area 82.

Fig. 13 describes a power grid with two dominant production areas 8OC, 80D which supply power to a larger load area 82. Here the stabilizer 86 can be used stationary as a synchronous compensator and deliver reactive power in the load area 82, delivering a temporary reactive power AQ for the two production areas 8OC, 80D, and deliver a temporary active power AP to stabilize oscillations between the load area 82 and one of the production areas 8OC or 80D. An example of this is given in the article "Large-Scale Active-Load Modulation for Angle Stability Improvement" by I. Kamwa et. al., IEEE Transactions on Power Systems, vol. 14, no. 2, May 1999, p. 582-590.

Fig. 15 shows a flow chart summarizing in a simple manner the control method according to the invention. The process starts in step 100. In step 102, an effect is transmitted either from the main engine of the stabilizer to the power grid or from the power grid to the main engine of the stabilizer. In step 104, this power transfer is regulated. The control is done by changing the speed of the main machine. This is accomplished by transmitting a current of suitable frequency, phase and amplitude through the rotor windings of the main machine.

The process ends in step 106.

Those skilled in the art will recognize that various modifications and changes may be made to the present invention without departing from the scope of the invention as defined by the appended claims. It is e.g. possible to use a different type of voltage source instead of a co-rotating control machine. In order for the stabilizer to be able to maintain its good properties in the event of a fault in the power grid, this voltage source should be independent of the power grid. The voltage source can e.g. consists of some energy storage or separate machine. The separate machine can be of any type capable of providing the necessary currents and voltages to control the rotor of the main machine. It may well have a shaft that is separated from the main machine, although some features are then impossible.

Claims (62)

10 15 20 7.5 30 514 068 32 PATENT REQUIREMENTS Power system stabilizer comprising a rotary electric main machine (2, 2 ') with power line connections, a converter (18, 18') and a voltage source, characterized in that windings (14) in a stator (12) in the electric main machine (2, 2 ') is connected to the power grid connections; a rotor (10) in the main electric machine (2, 2 ') comprises alternating current windings (16); one terminal of the inverter (18, 18 ') is connected to the AC windings (16) of the rotor; the second terminal of the converter (18, 18 ') is connected to the voltage source; whereby electrical power is exchanged via the power line connections by changing the speed of the rotor (10). Power system stabilizer according to claim 1, characterized in that the voltage source is a voltage source independent of the power lines; Power system stabilizer according to Claim 1 or 2, characterized in that the voltage source is a control machine (20, 20 '). Power system stabilizer according to Claim 1, 2 or 3, characterized in that the control machine (20, 20 ') and the main machine (2, 2') have a common axis (22). Power system stabilizer according to Claim 4, characterized in that the inverter (18 ') is arranged at the static parts of the main machine and is connected to the rotor line connections (16) of the main machine via brushes (30) and slip rings (32). 10 15 20 3G 514 068 33 Power system stabilizer according to claim 5, characterized by a control system, which comprises a first control unit (48) for controlling the static converter (18 '). Power system stabilizer according to claim 6, characterized in that the control system comprises a second control unit (46) which is arranged co-rotating with the common rotor (22). Power system stabilizer according to Claim 6 or 7, characterized in that the first control unit (48) is arranged for controlling the voltage source. Power system stabilizer according to claim 4, characterized in that the control machine (20) and the main machine (2) are brushless and that the converter (18) is arranged co-rotating at the axis (22) of the rotor. Power system stabilizer according to claim 9, characterized by a control system, which comprises a first control unit (46). for controlling the converter (18), which first control unit (46) is arranged co-rotating at the axis (22) of the rotor. Power system stabilizer according to claim 10, characterized in that the control system comprises a second control unit (48) which is arranged for controlling the voltage source. Power system stabilizer according to claim 8 or 11, characterized in that the control system (46, 48) comprises an electric power grid sensor (50; 51) for sensing an electrical disturbance in the electric power grid. Power system stabilizer according to claim 12, characterized in that the electrical disturbance is a disturbance of at least one quantity selected from the group: the amplitude of the voltage; 10 15 20 25 3 C- 514 068 34 the effective value of the voltage; the phase of voltage; the frequency of the voltage; current amplitude; the efficiency of the current; phase of the current; and the frequency of the current. Power system stabilizer according to claim 12 or 13, characterized in that the control system comprises a first temperature sensor (64) for sensing the stator temperature. Power system stabilizer according to claim 14, characterized in that the control system comprises a second temperature sensor (60) for sensing the rotor temperature, which second temperature sensor (60) is connected to the co-rotating control unit (46). Power system stabilizer according to claim 14 or 15, characterized in that the control system comprises communication means (54, 56) for wireless communication between the control units (46, 48). Power system stabilizer according to one of Claims 12 to 16, characterized by a transformer (3) arranged between the stator winding (14) and the power line connections. Power system stabilizer according to Claim 17, characterized in that the control system's electrical mains sensor (50) is arranged for sensing voltage and / or current in the connection between the transformer (3) and the stator winding (14). 10 15 20 30 1514 oss 35 Power system stabilizer according to one of Claims 1 to 18, characterized by a flywheel arranged on the shaft (22) of the main electric machine. Power system stabilizer according to one of Claims 1 to 19, characterized by a driving means arranged for applying a force to the shaft (22) of the main electric machine. Power system stabilizer according to claim 20, characterized in that the driving means is a turbine. Power system stabilizer according to claim 20, characterized in that the driving means is an internal combustion engine. Power system stabilizer according to one of Claims 1 to 22, characterized by a loading means arranged for absorbing the driving force of the shaft (22) of the main electric machine. Power system stabilizer according to claim 23, characterized in that the loading means is a brake (70). Power system stabilizer according to claim 23, characterized in that the load means is an electric generator. Power system stabilizer according to one of Claims 1 to 25, characterized in that the rotor winding (16) is arranged to have a current constriction which depends on the frequency of the rotor current. Power network comprising power lines and a shunt stabilizer (84, 84A, 84B), said shunt stabilizer comprising a rotary electric main engine (2, 2 ') connected to the power lines, a converter (18, 18') and a voltage source, characterized in that Windings (14) in a stator (12) in the main electrical machine (2, 2 ') are connected to the power lines; a rotor (10) in the main electric machine (2, 2 ') comprises alternating current windings (16); one terminal of the inverter (18, 18 ') is connected to the AC windings (16) of the rotor; the second terminal of the converter (18, 18 ') is connected to the voltage source; electrical power being exchanged between the power lines and the shunt stabilizer (84, 84A, 84B) by changing the speed of the rotor (10). Power grid according to Claim 27, characterized in that the voltage source is a source of voltage independent of the power lines; Power network according to Claim 27 or 28, characterized in that the voltage source is a control machine (20, 20 '). Power grid according to Claim 27, 28 or 29, characterized in that the control machine (20, 20 ') and the main machine (2, 2') have a common axis (22). Power grid according to Claim 30, characterized in that the current generator (18 ') is arranged at the static parts of the main machine and is connected to the rotor windings (16) of the main machine via brushes (30) and slip rings (32). Power grid according to claim 31, characterized by a control system, which comprises a first control unit (48) for controlling the static converter (18 '). Power grid according to claim 32, characterized in that the control system comprises a second control unit (46) which is arranged co-rotating with the common rotor (22). 10 15 20 3G 514 oss 37 Power network according to Claim 32 or 33, characterized in that the first control unit (48) is arranged for controlling the voltage source. Power grid according to Claim 30, characterized in that the control machine (20) and the main machine (2, 2 ') are brushless and in that the converter (18) is arranged co-rotating at the axis (22) of the rotor. Power grid according to claim 35, characterized by a control system, which comprises a first control unit (46) for controlling the converter (18), which first control unit (46) is arranged co-rotating at the axis (22) of the rotor. Power network according to claim 36, characterized in that the control system comprises a second control unit (48) which is arranged for controlling the voltage source. Power grid according to Claim 34 or 37, characterized in that the control system (46, 48) for controlling the voltage source and the converter comprises an electric power grid sensor (50; 51) for sensing an electrical disturbance in the electric power grid. Power grid according to claim 38, characterized in that the electric disturbance is a disturbance of at least one quantity selected from the group: the amplitude of the voltage; the effective value of the voltage; the phase of voltage; voltage frequency; current amplitude; the efficiency value of the current; phase of the current; and the frequency of the current. 10 15 20 3 C- 514 068 38 Power grid according to claim 39, characterized in that the control system comprises a first temperature sensor (64) for sensing the stator temperature. Power grid according to claim 40, characterized in that the control system comprises a second temperature sensor (60) for sensing the rotor temperature, which second temperature sensor (60) is connected to the co-rotating control unit (46). Power network according to claim 40 or 41, characterized in that the control system comprises communication means (54, 56) for wireless communication between the control units (46, 48). Power network according to one of Claims 38 to 42, characterized by a transformer (3) arranged between the stator winding (14) and the power lines. Power network according to Claim 43, characterized in that the control system's electrical power network sensor (50) is arranged for sensing the voltage in the connection between the transformer (3) and the stator winding (14). Power grid according to one of Claims 27 to 44, characterized by a flywheel arranged on the shaft (22) of the main electric machine. Power grid according to one of Claims 27 to 45, characterized by a driving means arranged for applying a force to the shaft (22) of the main electric machine. Power grid according to claim 46, characterized in that the driving means is a turbine. 10 15 20 30 514 068 39 Power grid according to claim 46, characterized in that the driving means is an internal combustion engine. Power grid according to one of Claims 27 to 48, characterized by a load means arranged for absorbing the driving force of the shaft of the main electric machine. Power grid according to claim 49, characterized in that the loading means is a brake (70). Power grid according to claim 49, characterized in that the drive means is an electric generator. Power network according to one of Claims 27 to 51, characterized in that the rotor winding (16) is arranged to have a current constriction which is due to the TOÉOTSÉTÖIDITICHS ffCkVCnS. 53. A method for stabilizing the voltage in a power system, comprising the step of: transmitting electrical power between a power line and a rotating electric main machine (2, 2 '), characterized by the step: regulating the output of the electric main machine (2, 2') applied electrical power by changing the speed of the main electric machine. A method according to claim 53, characterized in that the regulating step comprises the steps of: providing a rotor current through rotor windings (16) on the main electric machine (2, 2 '); controlling the amplitude, phase and frequency of the rotor voltage to obtain the desired amplitude, phase and frequency of the voltage across stator windings (14) on the main electrical machine (2, 2 '). 10 15 20 3 C '514 068 40 Method according to claim 54, characterized in that the control power for the rotor winding of the main machine is provided by a control machine (20, 20). Method according to Claim 55, characterized in that a shaft of the control machine (20, 20 ') is driven mechanically by a shaft (22) of the main electric machine (2, 2'). A method according to any one of claims 53 to 56, characterized by the step of: sensing the voltage / current of the power line to detect disturbances in its amplitude, rms value, phase or frequency; wherein the regulation takes place based on at least one of the detected disturbances. A method according to claim 57, characterized by the step of: sensing the temperature of the stator windings (14) of the main machine; whereby the regulation takes place based also on the stator temperature. A method according to claim 57 or 58, characterized by the step of: sensing the temperature of the rotor windings (16) of the main machine; whereby the regulation takes place based also on the rotor temperature. Method according to Claim 59, characterized in that the control step produces electrical effects in excess of rated power, valid for continuous operation, for the main electrical machine (2, 2 ') for a limited time. Method according to one of Claims 53 to 60, characterized by the step of: transmitting control information between the stationary and rotating parts of the main machine. 514 068 41 A method according to any one of claims 53 to 61, characterized by the step of: transforming the voltage across the stator windings (14) of the main machine into a suitable mains voltage.