WO2004027959A1

WO2004027959A1 - System and method for stabilising a power system

Info

Publication number: WO2004027959A1
Application number: PCT/AU2003/001253
Authority: WO
Inventors: Andrew Mark Tuckey; Juergen Zimmermann
Original assignee: Powercorp Pty Ltd
Priority date: 2002-09-23
Filing date: 2003-09-23
Publication date: 2004-04-01
Also published as: AR041364A1; CA2499918A1; AU2002951587A0

Abstract

A power system stabiliser (19) for stabilising a power generation system (9) having a grid (11) supplying a load (17) where power fluctuations may arise as a consequence of variations in the power generation system (9) or the load (17). The power system stabiliser (19) includes sensors for sensing a property of the power generation system (9), being a grid frequency and/or a grid voltage of the power generation system (9). Power system interface means in the form of a grid interface (21) is also included for electrically connecting with the power generation system (9). The grid interface (21) allows flow of electrical energy between the power system stabiliser (19) and the power generation system (9). Load interface means in the form of a load interface (25) is provided for electrically connecting with a stabilising load (20).The power system stabiliser (19) also comprises a link (29) for electrically connecting the grid interface (21) and the load interface (25).

Description

"System and Method for Stabilising a Power System"

Field of the Invention

This invention relates to a system and method for stabilising a power system, where predictable and/or unpredictable power fluctuations arise in the power system, caused by the loading of, or the energy generated by, the power system.

The invention has particular, although not exclusive, utility in remote locations using renewable energy sources such as wind or sun for generating the power supplied by such systems and which feed into a utility grid that may have varying load demands placed thereon. The invention also has utility in power systems that may have no renewable energy sources to supplement them and rely solely on conventional power generation sources from fossil fuels such as gas and/or diesel driven generation sets.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Background Art

The following discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge of the person skilled in the art as at the priority date of the application.

The quality of the power delivered, as well as the efficiency of such delivery, is an ongoing consideration in the design and construction of power generator plants or systems and power systems to be connected to utility grid systems or individual or groups of customers. This is true regardless of whether the power system utilises conventional energy sources (such as gas, diesel or a mains connected utility) or renewable energy sources (such as solar, wind, bio-mass, micro-hydro energy, tidal energy, wave energy or geo-thermal energy), although the problem is more pronounced in the case of power systems utilising renewable energy sources.

To illustrate, renewable energy power generation systems ("RPGSs"), such as those that utilise wind and/or solar energy, generate power having inherent fluctuations resulting from the prevailing environmental condition, i.e. a sudden gust of wind or cloud cover obscuring the sun.

This problem can be overcome by combining the RPGSs with conventional power generation systems ("CPGSs") with sufficient spinning reserve to cover these fluctuations, but this means that at least one CPGS has to be on-line all of the time and the CPGSs are not necessarily loaded to their optimum efficient operating point. It may also mean that more CPGSs are on-line than are required with the extra CPGS or CPGSs providing the spinning reserve.

In such situations, since some CPGSs have a minimum loading requirement, it may mean that the amount of energy that could be generated by the RPGSs is not fully utilised.

Furthermore, to accommodate sudden decreases in the load on CPGSs, either by sudden increases in the available renewable energy supply or sudden decreases in the total system load, the CPGSs must be loaded such that they can decrease their output power quickly. This extra constraint provides additional restriction on the operating region of the CPGSs and has led to the development of systems designed for stabilising power generated by RPGSs or combined RPGSs/CPGSs rather than supplementing it at times of need.

Fig 1A illustrates the basics of a power stabilising system. The power stabilising system comprises a main grid line 37' to which various power sources 39a', 39b' are connected for supplying power to the grid. Various loads (not shown) are connected in and out over time, and a power system stabiliser 41' is connected to smooth out fluctuations from the power sources 39a', 39b' or for peak lopping of the consumer load. The utility 39a' is indicative of RPGSs, while 39b' is indicative of CPGSs.

The power system stabiliser 41 ' commonly consists of a battery/inverter system and this system is described in more detail below.

As shown in Fig. 1 B, a battery/inverter system 43' is connected to the utility grid and load 45' in conjunction with a synchronous compensator 47' (also known as a synchronous condenser). Essentially, a synchronous compensator comprises a synchronous alternator connected to a three-phase power system to provide voltage support.

The battery/inverter system 43' provides the frequency control, and the synchronous compensator 47' supplies fault current and provides voltage control. Thus power generation is provided with continuous stabilising of power fluctuations as well as backfeeding of power into the utility grid.

In this arrangement, the fluctuations from the RPGS are smoothed by controlling the power going into the battery/inverter system 43'; absorbing energy when the RPGS is generating more power, and supplying energy when the RPGS is generating less power. Furthermore, due to the large amount of energy stored within such a system, this system can operate on a second-by-second basis, on a sub-second basis and even on a minute-by-minute basis.

The problem with this arrangement, however, is that the battery/inverter system 43' is only designed for a limited number of charge/discharge cycles before it loses its ability to hold charge, recharging is generally slower than discharging, and recharging times can be significant. This is unacceptable in a RPGS where it is vital to be able to recharge the battery as quickly as it has been discharged in order to have its full capacity available again in the shortest possible time. This can be compensated for by oversizing the batteries to cope with fast recharging, but this leads to higher capital and maintenance costs and does not solve the problem of the battery having a finite number of charge/discharge cycles. Furthermore, operating the synchronous compensator 47' in addition to the battery/inverter system 43' adds additional standby losses to the system as well as capital cost.

A further example of a power stabilising system is shown at Fig. 1C, where the battery/inverter is replaced with a flywheel system 49'. The flywheel system 49' comprises a flywheel 51' connected to the rotor of a motor/generator 53', which in turn is connected to a bi-directional converter 55' and then to a bi-directional inverter 57' to provide for frequency regulation with changes in flywheel speed. Thus, power generation may be provided with continuous stabilising of power fluctuations and backfeeding of power into the utility grid. However, in such arrangements, the synchronous compensator 47' is still required in order to provide fault current and, in some systems, voltage control.

As with the battery/inverter system, the fluctuations from the RPGS in this arrangement are smoothed by controlling the power going into the flywheel system 49'; absorbing energy when the RPGS is generating much power, and supplying energy when the RPGS is generating less power. However, unlike the battery/inverter system, this system can run only on a second-by-second basis, or on a sub-second basis, but not on a minute-by-minute basis.

The problem with this arrangement is that, in order to provide sufficient fault current to the RPGS, a synchronous compensator 47' still has to be used, adding to the additional standby losses and the capital cost of the system. Additionally, the inverter 57' needs to have a rating equal to the rated power output of the flywheel/inverter system.

Yet another method of effecting power system stabilisation for renewable energy sources is to use dynamic dumping resistors. Dynamic dumping resistors work on the basis that there is always excess renewable energy available and by dumping the excess energy dynamically, the frequency can be controlled. To elaborate, the fluctuations from a renewable energy source are smoothed by controlling the power used by the dump load; absorbing more energy when the renewable energy source generator is generating more power, and absorbing less energy when the renewable energy source generator is generating less power.

Dynamic dumping resistor systems can run on a second-by-second basis, on a sub-second basis, or on a minute-by-minute basis.

However, dynamic dumping resistor systems can never supply energy since they store no energy. The energy such systems "dump" is considered as waste energy and is not the primary product of the system. Sometimes the "dumped" energy is put to a useful purpose, such as space heating or water desalination.

Furthermore, dynamic load dumping systems only work when there is a constant (not average) oversupply from the RPGS. For example, in the case of wind turbines, the wind speed has to be constantly high or a large number of high capacity wind turbines need to be installed. This limits this type of controlling method to locations with very good wind resources or where extremely high energy costs exist such that installation of sufficient wind turbines to achieve the requisite capacity is justified. In any event, it is often necessary to operate the system in conjunction with a synchronous compensator, thus adding to the capital costs of the plant.

Disclosure of the Invention

It is therefore an object of the present invention to provide an improved power stabilising system that eliminates, or alleviates, some or all of the problems mentioned above.

In accordance with one aspect of the present invention, there is provided a power system stabiliser for stabilising a power system supplying a load where power fluctuations may arise as a consequence of variations in the power system or the load, comprising:

sensing means for sensing a property of the power system; power system interface means for electrically connecting with the power system to allow flow of electrical energy between the power system and the power system stabiliser; and

control means for controlling the flow of electrical energy between the power system and the power system stabiliser;

wherein the control means is responsive to the sensing means to control the flow of electrical energy between the power system and the power system stabiliser to maintain the property of the power system at a predetermined value to stabilise the power system.

Preferably, the power system stabiliser further comprises load interface means for electrically connecting with a stabilising load to allow flow of electrical energy between the power system stabiliser and the stabilising load, the load interface means being electrically connected with the power system interface means to allow flow of electrical energy therebetween, and the control means controlling the flow of electrical energy between the power system and the power system interface means and the load interface means, wherein the control means is responsive to the sensing means to control the flow of electrical energy between the power system and the power system interface means and the load interface means to maintain the property of the power system at the predetermined value to stabilise the power system.

Preferably, the property of the power system sensed by the sensing means is a grid frequency of the power system.

Preferably, when the property is the grid frequency of the power system, the flow of electrical energy is controlled by the control means so that input real power to the power system stabiliser is generated to maintain the grid frequency at the predetermined value.

Preferably, the property of the power system sensed by the sensing means is a grid voltage of the power system. Preferably, when the property is the grid voltage of the power system, the flow of electrical energy is controlled by the control means so that input reactive power to the power system stabiliser is generated to maintain the grid voltage at the predetermined value.

Preferably, the sensing means is integrated with the control means.

Preferably, the control means further controls the flow of electrical energy between the power system and the power system interface means and between the load interface means and the stabilising load.

Preferably, the control means further controls the reactive power between the power system and the power system interface means.

Preferably, the control means comprises first control means to control flow of electrical energy between the power system and the power system interface means, and second control means to control flow of electrical energy between the load interface means and the stabilising load.

Preferably, the first control means includes a switching power supply providing a direct current power supply to the second control means; the sensing means senses a voltage of the direct current power supply; and the second control means controls flow of electrical energy in response to the voltage sensed by the sensing means to control the voltage level of the direct current power supply.

Preferably, the first control means receives three phase power from the power system and the first control means controls flow of electrical energy between the power system and the power system interface means in response to the sensing means.

In accordance with a further aspect of the present invention, there is provided a method for stabilising a power system supplying a load where power fluctuations may arise as a consequence of variations in the power system or the load, the method comprising: sensing a property of the power system; and

controlling the flow of electrical energy from the power system to maintain the property of the power system at a predetermined value to stabilise the power system.

Preferably, the method comprises sensing a grid frequency of the power system.

Preferably, the method comprises controlling the flow of electrical energy to generate an input real power to the power system stabiliser to maintain the grid frequency at the predetermined value.

Preferably, the method comprises sensing a grid voltage of the power system.

Preferably, the method comprises controlling the flow of electrical energy to generate an input reactive power to the power system stabiliser to maintain the grid voltage at the predetermined value.

Brief Description of the Drawings

As previously described, the following drawings refer to various prior art arrangements, wherein:

Figure 1 A is a schematic block diagram of a typical utility grid power system using renewable energy and conventional power generation sources with a power system stabiliser;

Figure 1 B is a schematic block diagram of a power system stabiliser using conventional battery energy storage; and

Figure 1C is a schematic block diagram of a power system stabiliser using conventional flywheel energy storage. With respect to the following description of preferred embodiments of the invention, the description is made with reference to the following drawings, wherein:

Figure 2 is a schematic block diagram showing a power system stabiliser according to a first embodiment of the invention connected into a typical grid power system and a first load;

Figure 3A is a graph of AC Real Power against Grid Frequency exemplifying the use of the power system stabiliser with a nominal 100% load at nominal grid frequency using soft under frequency protection with 100% pseudo spinning reserve;

Figure 3B is a further graph of AC Real Power against Grid Frequency exemplifying the use of the power system stabiliser with a nominal 50% load at nominal grid frequency using full active frequency control with 50% pseudo spinning reserve;

Figure 4 is a schematic block diagram showing the power system stabiliser according to the first embodiment of the invention connected to a second load;

Figure 5 is a schematic block diagram showing the power system stabiliser according to the first embodiment of the invention connected to a third load;

Figure 6 is a schematic block diagram showing a power system stabiliser according to a second embodiment of the invention connected to a first load;

Figure 7 is a schematic block diagram showing the power system stabiliser according to the second embodiment of the invention connected to a second load;

Figure 8 is a schematic block diagram showing the power system stabiliser according to the second embodiment of the invention connected to a third load;

Figure 9 is a schematic block diagram showing a power system stabiliser according to a third embodiment of the invention connected to a load; Figure 10 is a schematic block diagram showing a power system stabiliser according to a fourth embodiment of the invention connected to a load;

Figure 10A is a graph of AC Real Power against Grid Frequency exemplifying the use of the power system stabiliser with a nominal 0% load at nominal grid frequency using full active frequency control with 100% spinning reserve, according to the fourth embodiment;

Figure 11 is a schematic block diagram showing a power system stabiliser according to the first embodiment of the invention connected to two loads.

Best Mode(s) for Carrying Out the Invention

Several preferred embodiments of the invention will now be described by way of example only.

In a first preferred embodiment of the present invention there is provided a power system stabiliser 19 for stabilising a power generation system 9 having a grid 11 supplying a load 17. Power fluctuations may arise as a consequence of variations in the power generation system 9 or the load 17. The power system stabiliser 19 is shown schematically in Figure 2.

The power generation system 9 is controlled by a power system controller (not shown).

The power generation system 9 consists of a renewable energy generator 13 utilising a renewable energy source and a conventional energy generator 15 utilising a conventional energy source. The renewable energy source utilised may include, but is not limited to, wind, solar, biomass, micro-hydro, tidal, wave, or geo-thermal energy sources. The conventional energy source may include, but is not limited to, gas, diesel, or a mains connected utility energy source. In this arrangement, the grid 11 is a three-phase ac wired power system that draws power from the power generation system 9. The grid 11 supplies power to numerous loads 17 variably connected to the grid 11.

The power system stabiliser 19 generally comprises sensors (not shown) for sensing a property of the power generation system 9, power system interface means in the form of an AC grid interface 21 for electrically connecting with the power generation system 9 to allow for the flow of electrical energy between the power system stabiliser 19 and the power generation system 9, and load interface means in the form of an AC load interface 25 for electrically connecting with a stabilising load 20.

In alternative exemplary embodiments, the power system interface means is integrated with the load interface means.

In alternative exemplary embodiments, multiple stabilising loads may be connected to the power system stabiliser 19. Figure 11 shows such an arrangement where the power system stabiliser is electrically connected with two stabilising loads 20, 20'.

Returning to Figure 2, the power system stabiliser 19 also comprises a link 29 for electrically connecting the grid interface 21 and the load interface 25 to allow for the flow of electrical energy therebetween.

Control means for controlling the flow of electrical energy between the grid interface 21 and the load interface 25 is also provided. The control means is responsive to the sensors to control the flow of electrical energy between the grid interface 21 and the load interface 25 to maintain the property of the power generation system 9 at a predetermined value to stabilise the power generation system 9.

In the several preferred embodiments of the invention described herein, the sensors are integrated with the control means. This is not an essential requirement, however, and in other embodiments of the invention the sensors may not be integrated with the control means.

The control means comprises first control means in the form of a grid interface control system 23 to control the flow of electrical energy between the power generation system 9 and the grid interface 21 , and second control means in the form of a load interface control system 27 to control the flow of electrical energy between the load interface 25 and the stabilising load 20.

The grid 11 supplies input real power and input reactive power to the grid interface 21. The input real power, minus losses, flows along the link 29 to the load interface 25 at a dc-link voltage.

The grid interface 21 and the grid interface control system 23 enable dynamic control of the input real power and input reactive power supplied to the power system stabiliser 19.

The grid interface 21 has high-speed switching transistors (not shown), input filter inductors (not shown) and capacitors (not shown). The combination of transistors, filter inductors and capacitors enables a three-phase ac connection with the grid 11. The high-speed switching transistors may be MOSFETs, IGBTs, GTOs, IGCTs, Thyristors or similar devices known to those skilled in the art.

By using high-speed switching transistors, such as those mentioned above, in the grid interface 21 the current going into the grid interface 21 is controlled. Alternatively, the operation of the high-speed switching transistors is controlled such that the back electromotive force ("EMF") or pseudo-back EMF of the grid interface 21 is controlled. In this manner both the input real power and the input reactive power supplied to the power system stabiliser 19 can be controlled.

In one exemplary embodiment of this, the control of power is effected by sensing the grid voltage by the sensors and controlling the high-speed switching transistors such that the grid-side current of the ac interface is sinusoidal and has some phase relationship with the grid voltage sensed. This allows the magnitude of the input real power and input reactive power to be varied by varying the magnitude of the grid-side current and phase angle of the grid side current.

In alternative exemplary embodiments, the grid interface 21 and the grid interface control system 23 are set up so as to act as a grid connected inverter and emulate a synchronous generator output with dynamic control of the sinusoidal or pseudo- sinusoidal internally generated back EMF or pseudo-back EMF and the power angle. For example, the grid connected inverter is effected by controlling the high-speed switching transistors such that a sinusoidal or other waveform back EMF or pseudo-back EMF is generated inside the grid interface 21 with controllable amplitude, frequency and phase. This back EMF interacts with the grid voltage and grid characteristics such that real and reactive power flows from the grid 11 into the grid interface 21.

The grid interface 21 further comprises filters to reduce/eliminate radio interference and switching frequency harmonics injected into the power generation system 9 and grid 11.

The grid interface 21 and the grid interface control system 23 are configured to operate in one of the following ways:

(a) Fully-Independent (Stand Alone): - as an independent power system component, with no control from other components in the power generation system 9, such as the power system controller;

(b) Semi-Independent: - as a component that communicates with an upper-level control system (not shown) of the power system controller of the power generation system 9; or

(c) Non-independent: - as a component integrated in a power generation subsystem (not shown) of the power generation system

9, namely as part of an inner kernel of the power system controller, for example, operating with tight integration of the conventional energy generator 15. Each configuration will now be described in further detail where like numerals reference like parts.

Fully-Independent Configuration

In this configuration, the sensors for sensing a property of the power generation system 9 are integrated in the grid interface control system 23.

The properties of the power generation system 9 sensed by the sensors are a frequency of the grid 11 and a voltage of the grid 11. The flow of electrical energy is controlled by the control means so that input real power to the power system stabiliser 19 is generated to maintain the grid frequency at a predetermined grid frequency, and input reactive power to the power system stabiliser 19 is generated to maintain the grid voltage at a predetermined grid voltage.

This operation will now be described in further detail.

The control of input real power drawn from the grid 11 on the ac side of the power system stabiliser 19 is governed by the frequency of the grid 11. Control of input reactive power drawn from the grid 11 on the ac side of the power system stabiliser 19 is governed by the voltage of the grid 11.

In controlling the input real power drawn from the grid 11 on the ac side of the power system stabiliser 19, the grid interface control system 23 (via the sensors) determines the frequency of the grid 11 and sets the real power according to the determined frequency.

To determine the frequency of the grid 11 , the grid interface control system 23 measures 3-phase voltages of the grid 11 to generate a set of measured values. The set of measured values from the 3-phase system is then transformed to equate to a set of measured values for a 2-phase system. From this set of measured values for a 2-phase system a rotating voltage vector is obtained. The grid interface control system 23 then measures the radian frequency of the rotating voltage vector to determine the frequency of the grid 11. It should be noted that this type of measuring method is not prone to measurement noise, as is the case with zero-voltage-crossing measuring methods. However, zero-voltage-crossing methods are used in alternative embodiments.

Furthermore, this type of frequency measuring method can determine changes in the grid frequency very dynamically, and certainly in less than one voltage cycle.

Once the frequency of the grid 11 has been determined, the grid interface control system 23 uses the frequency of the grid 11 as a variable in an algorithm processed by the control means to determine the amount of real power to be drawn from the grid. In the embodiment described, the algorithm is programmed into the grid interface control system 23 to determine when the grid frequency falls below a predetermined level and then commences reducing the magnitude of the input real power in a linear arrangement commensurate with the difference between the grid frequency and the predetermined level. To elaborate, Figure 3A shows a graph of an AC Real Power level against a Grid Frequency value exemplifying the use of the algorithm.

Normally, when a load 17 is added to a power generation system 9 greater than the spinning reserve on that power generation system, the frequency of the grid 11 will decrease. If the frequency of the grid 11 decreases too much the power generation system 9 will cease to operate and the result will be a blackout. By using the abovementioned algorithm with the correlation of AC Real Power to Grid Frequency shown in Figure 3A, the power system stabiliser 19 will reduce the input real power drawn from the grid 11 and thus reduce the total load on the grid 11. The input real power to the power system stabiliser 19 will be 100% of the value of the stabilising load 20 until the grid frequency decreases to 49.5hz. At this grid frequency the load reduction will start and the input real power drawn from the grid will reduce linearly to 0% at 49.0Hz. If, with this reduction in total load on the grid 11, there is now sufficient spinning reserve on the grid to supply power to the added load 17, that is, the spinning reserve is greater than zero including the addition of the load 17, the grid frequency will reduce no further, and no blackout will occur. For example, the supply of input real power to the power stabilising system 19 may be 100kW for a 50Hz power system. If the frequency of the grid 11 falls below 49.5Hz, the supply of input real power may be set to linearly decrease such that the supply of input real power will be 100kW at 49.5Hz and OkW at 49.0Hz. In this manner, an under frequency problem can be limited by reducing the load the grid sees at the input to the power system stabiliser 19. This is referred to as soft-under-frequency protection.

In an alternative embodiment, the results of another linear algorithm using the correlation of AC Real Power to Grid Frequency shown graphically at Figure 3B is processed by the grid interface control system 23. The graph in Figure 3B illustrates the use of the power system stabiliser 19 as a full active frequency controller actively controlling the grid frequency. Such a power system stabiliser 19 dampens power system frequency transients and gives protection for under frequencies due to insufficient spinning reserve in the power system 9, and protection for over frequencies due to excessive generation, which may be caused by a sudden reduction in load 17. When controlled in this way, the power system stabiliser 19 gives 50% psuedo-spinning reserve capability, meaning that the amount of spinning reserve necessary on the power system may be reduced by 50% of the value of the stabilising load 20.

The grid interface control system 23 determines a voltage of the grid 11. Once the voltage of the grid 11 has been determined, the grid interface control system 23 uses the voltage of the grid 11 as a variable in the algorithm programmed into the grid interface control system 23 to determine the supply of input reactive power.

Semi-Independent Configuration

In this configuration, the upper-level control system of the power system 9 allocates predetermined set-point values for the supply of input real power and input reactive power to the power system stabiliser 19.

The grid interface control system 23 then controls the operation of the high-speed switching transistors in the grid interface 27 in such a way that the actual real power and the actual reactive power drawn from the grid 11 is the same as the desired input real power and the desired input reactive power drawn from the grid 11 respectively.

Additionally, the grid interface control system 23 measures the frequency of the grid 11 and the voltage of the grid 11 in the same manner as described for a Fully Independent Configuration. The frequency of the grid 11 is used as a variable in a first algorithm programmed into the grid interface control system 23 to calculate the desired supply of actual input real power to the power system stabiliser 19 and the voltage of the grid 11 is used as a variable in a second algorithm programmed into the grid interface control system 23 to calculate the desired supply of the actual input reactive power to the power system stabiliser 19. This configuration uses the predetermined set-point value of input real power unless the grid frequency drops below a predetermined value, in which case it uses the value calculated by the first algorithm. By doing this the predetermined set-point is predominantly used, but the pseudo-spinning reserve capability and soft under frequency protection are preserved.

Furthermore, this configuration uses the predetermined set-point value of input reactive power unless the grid voltage deviates outside some predetermined limits, in which case it uses the value calculated by the second algorithm. By doing this the predetermined input reactive power set-point is predominantly used, but the voltage control capability is preserved.

For example, the desired supply of input real power to the power stabilising system 19 may be set to 50kW by the upper level control system for a 50Hz power system. If the frequency of the grid 11 falls below 49.5Hz, the supply of input real power may be set to linearly decrease such that the supply of input real power will be 50kW at 49.5Hz and OkW at 49.0Hz. In this manner, an under frequency problem can be limited by reducing the total load on the grid 11. This is referred to as soft-under-frequency protection. Non-lndependent Configuration

In this configuration, the grid interface 21 and the grid interface control system 23 comprise part of an inner kernel of a power system generation control means. For example, the grid interface 21 and the grid interface control system 23 may be integrated with the conventional energy generator 15 so that the control of supply of the real power and reactive power attained by the grid interface control system 23 is operable against the conventional energy generator 15.

The frequency control algorithms and voltage control algorithms of the grid interface control system 23 may be adaptive. For example, the grid interface control system 23 may measure how many conventional energy generators 15 are present on the grid 11 by measuring the inertia of the grid 11 and change the supply of real input power and/or reactive input power accordingly.

With reference to Figure 4 (illustrating an ac motor load as the stabilising load 20a) and Figure 5 (illustrating other types of ac loads as the stabilising load 20b), the load interface 25 and the load interface control system 27 act to transform the dc-link voltage into an output voltage of the load interface 25 suitable for powering the ac load. Further, in such a situation, the load interface 25 comprises an inverter which changes the dc-link voltage into a 3-phase ac voltage for powering the stabilising load 20.

The amount of power flowing to the stabilising load 20 is dependent on the dc-link voltage. In this configuration, the control of supply of the input real and input reactive power provided by the grid interface control system 23 is independent of the load interface control system 27 controlling the supply of power to the stabilising load 20.

For very fast dynamic response, and to avoid dc-link voltage overshoots, the power system stabiliser 19 comprises a feed-forward control line 37 between the grid interface control system 23 and the load interface control system 27.

This feed-forward control line 37 from the grid interface control system 23 transmits the input real power being drawn from the grid to the load interface control system 27. This enables the load interface control system 27 to control the load interface 25 so that it draws an equivalent amount of power from the dc link 29. This gives superior transient dynamics and reduces the amount of dc link variation when compared to the method where the load interface control system 27 waits until the dc link voltage has risen before it increases the power drawn by the load interface 25.

In accordance with a second preferred embodiment of the present invention as illustrated in Figure 6, where like numerals reference like parts, the stabilising load 20 is a dc load 31. Accordingly, load interface 25 and load interface control system 27 act to transform the dc-link voltage into an output voltage suitable for powering the dc load 31.

The output voltage from the load interface 25 can vary to effect a change in power used by the dc load 31.

In the second embodiment, the sensors sense the dc-link voltage of the direct current power supply to the load interface 25 and load interface control system 27, and the load interface control system 27 controls the flow of electrical energy in response to the dc-link voltage sensed by the sensors to control the dc-link voltage.

When the dc load 31 is resistive, the load interface control system 25 (via the sensors) measures the dc-link voltage and varies the duty cycle, and hence the output voltage, of the load interface 25 such that the input real power into the link 29 from the grid interface 21 (minus losses) flows to the dc load 31. To implement this in the embodiment being described, the load interface 25 comprises a chopper type dc/dc converter controlled by hysteresis band current control provided by the load interface control system 27.

If the dc-link voltage rises above a first predetermined threshold value, for example 800V, then the chopper switch turns on, and power flows to the dc load 31 , thereby reducing the dc-link voltage. Conversely, if the dc-link voltage drops below a second predetermined threshold value, for example 700V, then the chopper switch turns off and power stops flowing to the dc load 31 and thereby causing the dc-link voltage to rise. In this way, the duty cycle of the chopper is controlled to keep the dc-link voltage within the limits set by the first and second predetermined threshold values and the power flowing from the grid interface 21 to the dc load 31.

Furthermore, in this embodiment, the grid interface control system 23 operates independently of the load interface control system 27.

The load interface 25 could also have filters (not shown) to reduce/eliminate radio interference and switching frequency harmonics.

The dc load is not limited to being resistive, however, and other types of dc loads may be used. In this regard, Figure 7 shows a dc motor 33a as the dc load, while Figure 8 shows other dc loads 33b which could include, but are not limited to, electrolysis apparatus etc.

Some loads can not take all the power required all the time. For example, when the load is a motor, it takes a finite amount of time for the motor to accelerate to operating speed, and while running at a lower speed the amount of power that can be fed to the motor may be limited. Thus, to allow the power system stabiliser 19 to operate with very fast dynamics it may be necessary to add a dump load to the link 29 so that the supply of input real power can be independently controlled even if the load can not take all the power that is supplied. In this case the dump load dissipates the transient energy.

Accordingly, such an arrangement is the subject of a third preferred embodiment of the invention, as shown in Figure 9, where like numerals reference like parts. This embodiment, as illustrated, includes an ac motor load 20. However, it should be appreciated that any type of ac load can be applied if an ac load interface 25 is used, and any type of dc load can be applied if a dc load interface is used. The variation between this embodiment and other embodiments is the inclusion of a dump load 34 as described in the previous paragraph to ac and dc systems. The dump load 34 is connected into the dc-link 29 between the grid interface 21 and the load interface 25, via a dc-dc dump load interface 36 and its associated dump load control 38.

In a fourth embodiment, again where like numerals reference like parts and as illustrated in Figure 10 of the drawings, the power system stabiliser 19 is able to operate in a bi-directional manner by adding a power source 40 to link 29.

Figure 10 shows one instance of this where the power source 40 comprises the output of a diesel generator 41 connected to link 29 via an autotransformer 43 and a rectifier 45. In this instance, power flowing from the grid 11 into the power system stabiliser 19 and to the load 17 would be the same as described previously. However, the power system stabiliser 19 could supply power from the diesel generator 41 to the grid 11 via link 29.

Furthermore, in this embodiment, the power system stabiliser 19 is used to hold down the frequency of the grid 11 by increasing the load on the power generation system 9. Additionally, the power system stabiliser 19 is used to hold up the frequency of the power generation system 9 by supplying power to the power generation system 9.

Figure 10A shows a graph of AC Real Power levels against a Grid Frequency value exemplifying the use of the power system stabiliser 19 of the fourth embodiment as a full active frequency controller. This type of control also dampens power system frequency transients, gives protection for under frequencies due to insufficient spinning reserve in the power generation system 9, and protection for over frequencies due to excessive generation which may be caused by a sudden reduction in load 17 on the power generation system 9. Such a power system stabiliser 19 gives 100% real spinning reserve capability meaning that the power source 40 provides 100% of a power system stabiliser 19 rating in real spinning reserve. Additionally, it provides 100% extra loading capability meaning that it can absorb 100% of the power system stabiliser 19 rating should there be an over supply of generating capacity. ln summary, the power system stabiliser 19 as described in the aforementioned embodiments, can:-

(a) provide pseudo-spinning reserve in a power system;

(b) provide frequency control in a power system;

(c) provide voltage control in a power system;

(d) provide reactive power support; and

(e) reduce harmonics in the power system.

The power system stabiliser 19 achieves these effects by the following means,

(a) Pseudo-spinning reserve in a power system.

When the power system stabiliser is operating above zero real power the input real power drawn from the power system can be reduced very quickly (dynamically) and controllably at any point in time. This reduction in the required input real power is commonly known as "load-shedding," however the power system stabiliser 19 described herein can do this in a continuous (non discrete steps) and dynamically controllable manner. This may be called "dynamic load changing" to distinguish it from load-shedding.

This ability to dynamically change the input real power to the power system stabiliser means that the amount of spinning reserve required for the power system can be reduced. For example, if a power system would normally require 100kW of spinning reserve to accommodate a sudden increase in load of 100kW, a power system stabiliser system could be used to reduce the required spinning reserve. If a power system stabiliser was able to reduce its input real power by 50kW then only 50kW of real spinning reserve would be required. If the power system stabiliser could reduce its power real input by 100kW, then zero real spinning reserve would be required. This shows that the power system stabiliser acts as pseudo- spinning reserve.

One way for the power system stabiliser 19 to achieve dynamic load changing is by measuring power system frequency and adjusting the input real power according to an algorithm programmed into the power system stabiliser 19 as previously described. In this manner, the power system stabiliser 19 has power system frequency measuring techniques built into it.

Another way for the power system stabiliser to achieve dynamic load changing is by being able to set the instantaneous input real power value according to an input from some other control system.

(b) Frequency control in a power system.

The power system stabiliser can achieve frequency control by changing its input real power according to the measured frequency of the power system. This way it increases its input real power (absorbs excess power) when there is an over supply of power to the power system, thereby holding the power system frequency down; and decreases its input real power when there is an under supply of power to the power system, thereby holding the power system frequency up. Some simple or complicated control algorithm can be used to achieve this.

(c) Voltage control in a power system.

The power system stabiliser can achieve voltage control of the power system by changing its input reactive power according to the measured power system voltage. This way it absorbs excess reactive power when there is an over supply of reactive power to the power system, thereby holding the power system voltage down; and decreases its input reactive power when there is an under supply of reactive power to the power system, thereby holding up the power system voltage. Some simple or complicated control algorithm can be used to achieve this. The input reactive power can also be negative or positive, and can exist with or without a real load, i.e. the real and reactive input power can be controlled independently.

(d) Reactive power support.

The grid interface of the power system stabiliser can supply or consume reactive power, thus supporting the reactive power of the power system. This reactive power can flow with or without a real load, i.e. the real and reactive input power can be controlled independently.

(e) Harmonics in the power system.

Power Quality and the ability to actively cancel out harmonics are becoming an increasingly important issue in power systems. The power system stabiliser 19 is able to provide this function of cancelling out external harmonics that are present in the power system.

It should be appreciated that the scope of the present invention is not limited to the particular embodiments described herein. Accordingly, variations to certain components of the system as dictated by conventional engineering practice or the practical application of the invention to a particular site and which form part of the common general knowledge of the field of the invention, but which do not depart from the general spirit and principles of the present invention, are envisaged to fall within the scope of the invention and not detract from it. In particular, while the present invention is most suitable for use with renewable energy power generation systems because the level of penetration of the renewable energy can be increased, and thus assist in earlier repaying the additional capital costs associated with such generation systems, it should not be considered as limited to use with such systems. For example, the present invention can be used with conventional power generation systems to provide greater fuel efficiency by carrying the spinning reserve in the power system stabiliser 19.

Claims

The Claims Defining the Invention are as Follows

1. A power system stabiliser for stabilising a power system supplying a load where power fluctuations may arise as a consequence of variations in the power system or the load, comprising:

sensing means for sensing a property of the power system;

power system interface means for electrically connecting with the power system to allow flow of electrical energy between the power system and the power system stabiliser; and

wherein the control means is responsive to the sensing means to control the flow of electrical energy between the power system and the power system stabiliser to maintain said property of the power system at a predetermined value to stabilise the power system.

2. A power system stabiliser as claimed in claim 1 , wherein the power system stabiliser further comprises load interface means for electrically connecting with a stabilising load to allow flow of electrical energy between the power system stabiliser and the stabilising load, the load interface means being electrically connected with the power system interface means to allow flow of electrical energy therebetween, and the control means controlling the flow of electrical energy between the power system and the power system interface means and the load interface means, wherein the control means is responsive to the sensing means to control the flow of electrical energy between the power system and the power system interface means and the load interface means to maintain said property of the power system at the predetermined value to stabilise the power system.

3. A power system stabiliser as claimed in claim 1 or 2, wherein the control means processes an algorithm programmed into the control means to control the flow of electrical energy.

4. A power system stabiliser as claimed in any one of the preceding claims, wherein said property of the power system sensed by the sensing means is a grid frequency of the power system.

5. A power system stabiliser as claimed in claim 4, wherein the flow of electrical energy is controlled by the control means so that input real power to the power system stabiliser is generated to maintain the grid frequency at the predetermined value.

6. A power system stabiliser as claimed in claim 4 or 5, wherein the flow of electrical energy is dynamically controlled by the control means so that input real power to the power system stabiliser is generated to maintain the grid frequency at the predetermined value.

7. A power system stabiliser as claimed in any one of claims 4 to 6, wherein the sensing means measures 3-phase voltages of a grid of the power system to generate a set of measured 3-phase voltage values; transforms the set of measured 3-phase voltage values to a set of 2-phase voltage values; obtains a rotating voltage vector from the set of 2-phase voltage values; and measures the radian frequency of the rotating voltage vector to sense the grid frequency.

8. A power system stabiliser as claimed in any one of claims 4 to 6, wherein the sensing means senses the grid frequency by a zero voltage crossing method.

9. A power system stabiliser as claimed in any one of claims 4 to 8, wherein the flow of electrical energy is controlled by the control means processing an algorithm programmed into the control means to determine when the grid frequency falls below the predetermined value and then reduce the magnitude of the input real power in a linear arrangement commensurate with the difference between the sensed grid frequency and the predetermined value to maintain the grid frequency at the predetermined value.

10. A power system stabiliser as claimed in any one of claims claim 1 to 3, wherein said property of the power system sensed by the sensing means is a grid voltage of the power system.

11. A power system stabiliser as claimed in claim 10, wherein the flow of electrical energy is controlled by the control means so that input reactive power to the power system stabiliser is generated to maintain the grid voltage at the predetermined value.

12. A power system stabiliser as claimed in claim 10 or 11 , wherein the flow of electrical energy is dynamically controlled by the control means so that input reactive power to the power system stabiliser is generated to maintain the grid voltage at the predetermined value.

13. A power system stabiliser as claimed in any one of claims 10 to 12, wherein the flow of electrical energy is controlled by the control means processing an algorithm programmed into the control means to determine the input reactive power to be generated to maintain the grid voltage at the predetermined value.

14. A power system stabiliser as claimed in any one of the preceding claims, wherein the sensing means is integrated with the control means.

15. A power system stabiliser as claimed in any one of claims 2 to 14, wherein the control means further controls the flow of electrical energy between the power system and the power system interface means and between the load interface means and the stabilising load.

16. A power system stabiliser as claimed in any one of claims 2 to 15, wherein the control means further controls the reactive power between the power system and the power system interface means.

17. A power system stabiliser as claimed in any one of claims 2 to 16, wherein the control means comprises first control means to control flow of electrical energy between the power system and the power system interface means, and second control means to control flow of electrical energy between the load interface means and the stabilising load.

18. A power system stabiliser as claimed in claim 17, wherein the first control means controls a high speed switching transistor to control flow of current between the power system and the power system interface means.

19. A power system stabiliser as claimed in claim 17, wherein the first control means controls a high speed switching transistor to control a back electromotive force of the power system interface means.

20. A power system stabiliser as claimed in claim 17, wherein the first control means controls a high speed switching transistor to control a pseudo back electromotive force of the power system interface means.

21. A power system stabiliser as claimed in claim 17, wherein the first control means comprises a switching power supply providing a direct current power supply to the second control means; the sensing means senses a voltage of the direct current power supply; and the second control means controls flow of electrical energy in response to the sensed voltage of the direct current power supply to control the voltage level of the direct current power supply.

22. A power system stabiliser as claimed in claim 17 or 21 , wherein the first control means receives three phase power from the power system and the first control means controls flow of electrical energy between the power system and the power system interface means in response to the sensing means.

23. A power system stabiliser as claimed in any one of the preceding claims, wherein the power system is controlled by a power system controller.

24. A power system stabiliser as claimed in claim 23, wherein the control means is not controlled by the power system controller.

25. A power system stabiliser as claimed in claim 23, wherein the control means is controlled by the power system controller.

26. A power system stabiliser as claimed in claim 25, wherein the power system controller sets the predetermined value of said property of the power system.

27. A power system stabiliser as claimed in claim 23, wherein the control means is integrated with the power system controller.

28. A power system stabiliser as claimed in any one of the preceding claims, wherein the power system stabiliser further comprises a power source for generating electrical energy, the power source being electrically connected with the power system interface means to allow flow of electrical energy therebetween.

29. A method for stabilising a power system supplying a load where power fluctuations may arise as a consequence of variations in the power system or the load, comprising:

sensing a property of the power system; and

controlling the flow of electrical energy from the power system to maintain said property of the power system at a predetermined value to stabilise the power system.

30. A method for stabilising a power system as claimed in claim 29, wherein the method further comprises controlling the flow of electrical energy by processing an algorithm.

31. A method for stabilising a power system as claimed in claim 29 or 30, wherein the method further comprises sensing a grid frequency of the power system.

32 A method for stabilising a power system as claimed in claim 31 , wherein the method further comprises controlling the flow of electrical energy to generate input real power to the power system stabiliser to maintain the grid frequency at the predetermined value.

33. A method for stabilising a power system as claimed in claim 31 , wherein the method further comprises dynamically controlling the flow of electrical energy to generate input real power to the power system stabiliser to maintain the grid frequency at the predetermined value.

34. A method for stabilising a power system as claimed in any one of claims 31 to 33, wherein the method further comprises measuring 3-phase voltages of a grid of the power system to generate a set of measured 3-phase voltage values; transforming the set of measured 3-phase voltage values to a set of

2-phase voltage values; obtaining a rotating voltage vector from the set of 2- phase voltage values; and measuring the radian frequency of the rotating voltage vector to sense the grid frequency.

35. A method for stabilising a power system as claimed in any one of claims 31 to 33, wherein the method further comprises sensing the grid frequency by a zero voltage crossing method.

36. A method for stabilising a power system as claimed in any one of claims 31 to 35, wherein the method further comprises controlling the flow of electrical energy by processing an algorithm determining when the grid frequency falls below the predetermined value and then reducing the magnitude of the input real power in a linear arrangement commensurate with the difference between the sensed grid frequency and the predetermined value to maintain the grid frequency at the predetermined value.

37. A method for stabilising a power system as claimed in claim 29 or 30, wherein the method further comprises sensing a grid voltage of the power system.

38. A method for stabilising a power system as claimed in claim 37, wherein the method further comprises controlling the flow of electrical energy to generate input reactive power to maintain the grid voltage at the predetermined value.

39. A method for stabilising a power system as claimed in claim 37, wherein the method further comprises dynamically controlling the flow of electrical energy to generate input reactive power to maintain the grid voltage at the predetermined value.

40. A method for stabilising a power system as claimed in any one of claims 37 to 39, wherein the method further comprises controlling the flow of electrical energy by processing an algorithm determining the input reactive power to be generated to maintain the grid voltage at the predetermined value.

41 A method for stabilising a power system as claimed in any one of claims 29 to 40, wherein the method further comprises controlling the flow of electrical energy between the power system and the power system interface means, and controlling the flow of electrical energy between the load interface means and the stabilising load.

42. A method for stabilising a power system as claimed in claim 41 , wherein the method further comprises controlling flow of current between the power system and the power system interface means.

43. A method for stabilising a power system as claimed in claim 41 , wherein the method further comprises controlling back electromotive force of the power system interface means.

44. A method for stabilising a power system as claimed in claim 41 , wherein the method further comprises controlling pseudo back electromotive force of the power system interface means.

45. A method for stabilising a power system as claimed in any one of claims 29 to 44, wherein the method further comprises supplying electrical energy to the power system.

46. A method for stabilising a power system as claimed in any one of claims 29 to 40, wherein the method further comprises controlling the reactive power between the power system and the power system interface means.

47. A power system stabiliser as claimed in any one of claims 2 to 28, wherein the load interface means is integrated with the power system interface means.

48. A power system stabiliser as claimed in any one of claims 2 to 28, wherein the power system interface means is integrated with the load interface means.

49. A power system stabiliser substantially as hereinbefore described with reference to Figures 2 to 5; Figures 6 to 8; Figure 9; Figures 10 and 10A; or Figure 11 of the accompanying drawings.