NL8004597A - METHOD AND APPARATUS FOR THE OPTIMAL USE OF VARIABLE NON-MANAGABLE SOURCES OF ENERGY. - Google Patents

Abstract

A method and an installation are described for the optimum use of at least one variable power source which is difficult to control. In known power systems with a power source which is difficult to control, it is normal to couple to it one or more synchronous A.C. generators which can be connected with a rectifier and an inverter to an autonomous A.C. network. In this arrangement, the A.C. network is connected to a second synchronous A.C. generator which is mechanically connected, such that it can be disconnected therefrom, to a controllable auxiliary machine, such as e.g. a diesel generating set. To keep the frequency of the network constant if too much power is offered or, above all, if the power offered drops, such an amount of power is supplied from the autonomous A.C. network (5) to an additional usable load (2) that the frequency of the network (5) remains constant. The excess power to be diverted can be used, for example, for the preparation of hot water or for the electrolysis of water, combined with the use of fuel cells, or for battery charging or for pumping water to increased heights. <IMAGE>

Description

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In the name of: Stichting Energieonderzoek Centrum Nederland in The Hague.

Title: Method and structure for optimal use of variable non-controllable energy sources

The invention relates to a method for optimally utilizing one or more variable uncontrollable energy sources, by using an energy system comprising one or more of these variable energy sources (1), one or more electric alternators (5) coupled thereto. 3), a mutator inverter (8), an alternating current network (5) connected to this mutator with one or more electric alternating current generators (6), which are coupled to one or more controllable energy sources (7).

The above-mentioned method generally does not present any problems, if a large alternating current network is present, which can absorb the energy generated by the variable energy sources.

After all, the adaptation to the fluctuating electrical power and the compensation of any required reactive power are left to the conventional power stations in the grid.

13 However, the situation is completely different if such a large network is not present and more particularly if one wants to operate a small autonomous network almost entirely by means of wind energy. In favorable wind conditions, people would like to supply a grid almost entirely by means of wind energy. In connection with the price of fossil fuels and more particularly of diesel oil, it is desired to use as little of this as possible for a long time and then also use the power supplied by the wind or another variable energy source at the most optimal way to use. The problem of high fuel prices and the absence 25; Being of a large coupled AC electric grid mainly occurs in insulated places, for which it is not economical to lay long connecting cables and to build coupling stations for an electrical grid.

The object of the invention is to satisfy this long-felt technical need in 3D. The solution found proves to be technically very reliable. O 80 04 59 7

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- 2 - ** ν to be visible, which is surprising to the expert in view of the low grid stability of small autonomous electrical grids.

According to the invention, these uncontrollable variable energy sources (1) are made to supply energy in an energy system in which, moreover: a) all electric generators (3, 6) are synchronous; b) the alternating current network (5) is autonomous; c) the commutation energy is supplied by the (synchronous) electric generators 10 (6) associated with the controllable energy sources (7); d) the controllable energy sources (7) are coupled to the generators (3, 6) and are thereby completely disconnectable; e) the energy system has an additional useful electrical load (2).

The invention also relates to an energy system in which the aforementioned method is applied and in which, in particular, the commutation energy is supplied by the synchronous generators coupled to the variable controllable energy sources (1), because a high efficiency converter (grid-commutated ) (8) is linked to the alternating network (5).

The advantages of the invention are: 1. A great saving of fuel, for example diesel oil.

2. Preventing no-load from a diesel engine or other liquid fuel engine, which can cause pollution.

25 3. A high efficiency of the variable, as it were, energy obtained for nothing, through the use of a grid-commutated inverter while retaining the good voltage and frequency of the autonomous grid. Here, use is made of the fact that a synchronous capacitor is formed by the synchronous machine (6) and the converter of the commutator (8).

It is noted that an autonomous network (5) is understood to mean a network that derives all or most of its power from the variable energy sources (1).

The new energy system can be understood as follows: the variable energy sources (1) always provide a maximum of energy in view of the circumstances present, which with a high efficiency through 80 04 5 9 7 * 4 - 3 -% t of the commutator (8) is supplied to the autonomous network (5). If the grid (5) is supplied with too much energy from the variable energy sources (1), this would result in an increase in the grid frequency without adequate regulation. This undesired frequency-5 increase is now prevented by dissipating the excess energy by means of an adjustable useful electrical load (2). Obviously in this case the variable controllable energy source (7) is completely disconnected from the synchronous generators (6), which in this case also supply the commutation energy, which in this case turns out to be very low; loss in commutation (8) less than 10%.

In many cases one or more wind turbines will be used as the variable energy source. A property of many wind turbines is that, if the blade position is adjusted to maximum energy output as much as possible, they are not self-starting. In an appendix added to the description entitled: "Turbine properties" with accompanying figure 3, this is further elucidated.

- A preferred embodiment of the new energy system uses as the energy source one or more wind turbines (1), which are then previously provided with electric synchronous generators (3) coupled thereto by means of a three-phase connection between the controllable energy sources. (7) start coupled synchronous generators (6) asynchronously. The synchronous generators (3) are then provided with, for example, an extra cage.

This additional provision ensures that wind turbine blades are always in the optimum position for energy recovery.

In this case, the wind turbines are restarted after standstill due to wind or storm protection by means of the electric generators (6), which in turn are connected to the controllable energy sources (7). The controllable energy source (7) starts 30 · .. 'in this case the wind turbines (1).

The additional payload (2) is in a preferred embodiment on a small scale in the autonomous electrical network (5).

Another place for the extra useful electrical load (2) is placement in the commutator (8).

355 Yet another conceivable place for the additional payload (2) 80 04 59 7

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- 4 - \ is placement between the commutator (8) and the generators connected to the non-controllable energy sources (1). This has the advantage that the energy losses in the commutator are minimal (3).

As mentioned before, it is possible to place the extra useful load (2) in the net (5). The additional energy obtained can be used in this case and the other cases for the production of hot water or for the electrolysis of water, whereby hydrogen and oxygen are formed. It is of course possible to use the additional load (2) necessary for the control of the system for other useful applications. One can think of charging batteries, which is generally less interesting, because it requires extensive protection equipment.

One can also think of the production of, for example, chlorine bleach lye.

15 The additional payload (2) in all these cases is adapted to the needs of the total energy system by means of an adjustable rectifier (10).

Although other inverters with lower efficiencies are also applicable, a grid-commutated inverter (8) will preferably be used for supplying the energy from the variable energy source (1) of the autonomous grid (5).

In this case, the grid-commutated inverter (8) is a six-thyristor line commutated inverter (11), as shown in Figure 2.

Since the wind speed generally fluctuates very sharply, it is possible that with insufficient dimensioning of the system, especially when the wind is used as the sole energy source, the mains frequency sometimes varies too much. This can be prevented by using machines 3Q with a high moment of inertia as generators (6) connected to the controllable energy sources (7). All this is further elucidated in appendix 2 added at the end of the description entitled: "System management".

Preferably, as controllable variable energy sources, one or more diesel machines (7) will be used because of the good control characteristics and reliability of these machines.

Switching these machines on and off (7), if there is too little or 80 04 59 7 ✓ 4 - 5 - too much energy supply, is absolutely no problem for the skilled person.

The energy system according to the invention can best be realized for an autonomous network (5), which mainly runs on wind energy, by simultaneously applying three control systems as follows: a) A speed control (12 ) of the wind turbines (1), which is connected to the electric synchronous generators (3) coupled to these wind turbines and based on the power Pw delivered by the wind to these 1Ό turbines (1) and the angular speed of the rotor of these turbines (13).

b) A frequency control (14) of the electric autonomous network (5), which is connected to an adjustable rectifier (10), via which the extra useful electric load (2) can draw the energy from the autonomous network (5) and based on the power Pw »delivered by the wind to the turbines, the electrical load P ^ of the autonomous network (5) and the mains frequency (15) of this autonomous network.

c) A voltage control (16) of the autonomous electric network (5), which is connected to the synchronous generators (16) in the autonomous network (5). and based on the mains voltage (17).

Preferably, in the previously given control system, the mains voltage is kept constant by field energization of the synchronous generators (6), as required by the measured mains voltage.

25; The invention is explained below without intention of limitation in two figure descriptions 1 and 2 and two appendices, "Turbine properties" with associated figure 3 and "System control".

In Figure 1, 1 represents a two-bladed wind turbine which may, for example, have a horizontal axis. This wind turbine 1 is coupled by means of a gearbox 18 to a synchronous electric generator 3. The generated electrical energy is supplied as direct voltage via a converter 8 to an autonomous network 5. The autonomous network 5 can, if the wind does not supply enough energy or , if this is necessary in view of the stability of the mains voltage and mains frequency 80 04 59 7 X.

It is also apparent that electrical energy is also received from a synchronous electric generator 6, which can be driven by means of a coupling 9 by a diesel machine 7. The diesel machine 7 can hereby be completely disconnected from the associated electric generator 6. When the diesel machine is completely disconnected, the synchronous generator continues to run and thereby supplies the mutation energy for the converter 8, if the system is also provided with an adjustable electrical load 2. In figure 1, the adjustable electric load 2 is supplied with its energy by means of an adjustable rectifier 10, which in this case is placed in the autonomous network 5. The commutator (8) consists of a system of a number of horn rectifiers 20 and converters 4. In figure 1 it is indicated by means of three small oblique stripes, that the generators supply three-phase current and also where this three-phase current is respectively delivered to the commutator 8 , the autonomous network 5 and the adjustable rectifier 10, which is placed in front of the adjustable electric load 2. The commutator (8) contains a self-induction schematically indicated by 19. In the diagram shown in figure 1 the electrical load (2) serves to produce hot water, however it is possible to use the useful additional load 2 for other purposes.

Figure 2 shows a control scheme in detail; three control systems are simultaneously applied, namely: a) A control system at the speed of the wind turbine 12, which is set in such a way that the blades of the wind turbine 1 are always positioned in such a way that the maximum amount of energy is taken from the wind . This is achieved by simultaneously arranging on the electric generator (3) a control (12) in which the angular speed (13) of the rotor of the wind turbine (1) and the power Pw supplied by the wind are supplied as variables.

This is further explained in the appendix "Turbine properties".

b) A frequency control 14 of the autonomous het 5, which in this case is connected to the adjustable rectifier (10), which supplies the energy to the adjustable useful electric load (2).

The mains frequency 80 04 59 7 <- * - 7 - frequency (15) and are supplied as variables to this control (14). the power P ^ delivered by the network and the power Pw delivered by the wind, respectively

In this regulation, first of all, the mains frequency and furthermore a difference between the power Pw supplied by the wind and the power P vermogen taken from the mains.

If the wind does not supply enough power, the power consumed by the additional payload (2) will in the first place be reduced to a minimum value and the diesel machine (7) from figure 1 will be switched on in the event of a further reduction.

ZQ c) A voltage control (16) of the electric autonomous wet (5), in which the measured mains voltage (17) is supplied as a parameter and which is connected to the windings of the electric generator (6). The control works by varying the field excitation of the generator (6).

15 The mutator. 8 from FIG. 1 is now shown in a more elaborate form 11. The rectifying portion 20 from FIG. 1 is now shown by 21, while the converting portion is shown by 22. In 11, a 6 thyristor line commutated inverter is used in this manner. It is hereby possible to limit the reactive power of the conversion portion 22 by means of extinction angle limitation.

The use of a grid commutated inverter 8 or 11, as mentioned earlier, is an important part of the new system.

This DC switching makes the system very flexible. The rotor speed of the wind turbine is hereby chosen independently of the grid frequency, so that it always delivers a maximum amount of energy. Multiple wind turbines or other variable energy sources can be connected in parallel in the same energy system, with each wind turbine being individually controllable if desired.

30 80 0 4 5 9 7 - 8 - ·. ·

Appendix 1, Turbine Properties.

It is known that in general the wind speed varies very quickly, with fluctuations in the very short term as well as in the longer term. The short-term fluctuations can be taken into account by means of the very fast control properties of the system. The longer-term fluctuations, on the other hand, can be better absorbed by means of the useful extra tax (2). With further development of fuel cells it is possible to bring this additional load (2) back into the network at a later time in times of little supply of wind energy or peak load in the autonomous network (5). It is therefore very important that a system can optimally absorb the offered wind energy.

The general formula for the power delivered by the shaft of a wind turbine is: 15

Pw - Cp i TT R2 p v3 (a) 20 where v: the wind speed R: rotor diameter p: airtightness

Cp: power coefficient that depends on the quotient λ, the rotor peripheral speed and the wind speed

The coefficient is therefore a function of the quantity λ, where: λ; 30 7 where ω represents the angular velocity.

In figure 3 a curve of the coefficient Cp as a function of λ is given for a two-bladed wind turbine with horizontal axis. From 35 80 04 59 7 - 9 - v: t this figure shows that with an optimal value of λ the quantity Cp has a value of 0.46. Slightly different curves may occur with other turbines, but the general picture is the same.

Figure 3 shows that the power that can be obtained from wind is small at low values of λ. This means that the wind provides too little torque at low rotor speeds to compensate for losses in the entire energy system. This makes it impossible to start the wind turbine by using only the wind.

The losses in the energy system are discussed in the following 10 Appendix "System Management".

In that case, the rotor speed is adjusted to the wind speed in order to achieve the optimum value of λ as much as possible. The synchronous generator (3) will therefore in many cases not be used as a synchronous generator. Since the wind turbine is not self-starting, a device as shown in claim 3 can be used if at least one does not have access to energy sources other than a diesel machine.

From formula (a) it appears that the amount of energy Pw delivered is proportional to the third power of the wind speed and therefore fluctuates even more strongly in view of the large fluctuations thereof.

These particularly large fluctuations of Pw can be compensated, inter alia, by applying a synchronous generator (6) in the autonomous network (5) with a great moment of inertia J, which will be discussed in the following Appendix "System control".

25 80 04 59 7 - 10-

Appendix 2, System Management

The power balance of the system shown in Figures 1 and 2 when using the diesel engine (7) is given by: 5 ^ diesel + + + J Ir <b> where: IQ ^ diesel ^ et power 08 delivered by the diesel engine ( 7)

Pw the power supplied by the wind to the autonomous network (5) P 'the power required by the autonomous network (5)

Af

PV power loss in the synchronous generator (6)

Pg the power going into the useful extra load is called water production, electrolysis of water and the like J moment of inertia of the synchronous generator (6) ω * angular velocity of the synchronous generator (6) in rad / s (ω1 is another angular velocity then w which relates to the wind turbine (1)).

20 If P> P + P rw rv r% 25, the diesel machine (7) can in principle be switched off. In this case, ^ diesel * De win <* is currently the only energy source for the autonomous network (5). However, the rotary synchronous generator (6) remains an essential part of the entire energy system. The generator (6) now functions as a voltage source of the three-phase network (5) and takes care of the commutation in the converter (22). The reactive power required by the grid load P ^ and the converter (22) is provided by the synchronous generator (6). The synchronous generator (6) functions as a synchronous capacitor and the mains voltage can be maintained at a constant value by field energies of the synchronous generator.

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All wind energy excess which is not otherwise required for the grid load Pj or for the loss Py in the synchronous generator (6) is fed as much as possible to the useful additional load Pe * It is noted that when starting a wind turbine (1), 5 has stood still, must take this PV into account. It is also not possible for this reason to start the wind turbine only by means of wind energy.

However, if there is a large supply of wind energy, formula (b) shows that, with the help of the additional payload (2) or Pe 10, it is possible to control the power flow to the network (5). The speed of rotation of the synchronous generator (6) which determines the frequency of the autonomous network can also be influenced. In view of the particularly rapid fluctuations in the supply of wind energy, it is important to smooth out these fluctuations as well as fluctuations due to different mains loads P ^ by means of a large moment of inertia J of 15. the synchronous generator (6).

The rotating mass of the generator (6) in this case is able to store or deliver a certain amount of energy. As soon as the amount of stored energy threatens to become too large or too small, more or less energy will be supplied to the extra useful load (2) or Pe. It is possible to switch on the diesel machine (7), if at a certain moment too little energy is supplied for the system, due to the wind to maintain the energy requirement of the system.

255 80 04 53 7

Claims (14)

Method for optimally utilizing one or more variable non-controllable energy sources, by using an energy system comprising one or more of these variable energy sources (1), one or more electric alternators (3) coupled thereto, a mutator (inverter) (8), an alternating current network (5) connected to this mutator with one or more alternating electric generators (6), which are coupled to one or more controllable energy sources (7), characterized in that these are not controllable energy sources (1) supplies energy in a 10 energy system in which: a) all electric generators (3, 6) are synchronous; b) the alternating current network (5) is autonomous; c) the commutation energy is supplied by the (synchronous) electric generators 15 (6) associated with the controllable energy sources (7); d) the controllable energy sources (7) are coupled to the generators (3, 6) and are thereby completely disconnectable; e) the energy system is equipped with an additional useful electrical load (2). 20 2 «Energy system for applying the method as claimed in claim 1, comprising one or more variable non-controllable energy sources (1), one or more associated alternating current electric generators (3), a mutator (inverter) (8), one of these mutator (8) connected electric alternating current network (5) with one or more electric alternating current generators (6) coupled to one or more controllable energy sources (7), characterized in that: a) all electric generators (3, 6) be in sync; b) the alternating current network (5) is autonomous; 3Q (c) a high efficiency converter (grid-commutated) (8) is coupled to the AC grid (5); d) the controllable energy sources (7) are provided with couplings (9), which can completely disconnect them during operation of the system from the associated generators (6); E) the energy system is equipped with a so-called extra useful load (2). 80 04 59 7 '' - 13 - ^ = Γ Energy system according to claim 2, characterized in that the variable energy source is one or more wind turbines (1), which are provided with electric synchronous generators (3) coupled thereto, with means for connecting them by means of a three-phase connection start asynchronously between the synchronous generators (6) coupled to the controllable energy sources (7). Energy system according to claim 2 or 3, characterized in that the additional useful electrical load (2) is located in the network (5). Energy system according to claim 2 or 3, characterized in that the additional payload (2) is located in the commutator (8). Energy system according to claim 2 or 3, characterized in that the additional payload (2) is placed between the commutator (8) and the synchronous generators (3) connected to the uncontrollable variable energy sources (1). Energy system according to claim 4, characterized in that the additional payload (2) is a hot water production unit or hydrogen and oxygen production unit, which can receive electrical energy by means of an adjustable rectifier (10). Energy system according to Claims 2 to 7, characterized in that a mains-commutated inverter (8) is used for converting the alternating voltage obtained from the synchronous generators (3) connected to the variable energy sources (1). . Energy system according to claim 8, characterized in that this mains commutated inverter (8) is a six-thyristor line commutated in verter (11). Energy system according to any of claims 2 to 9, characterized in that the synchronous generators (6), which can be connected to the controllable energy sources (7), have a high moment of inertia. Energy system according to any one of claims 2 to 10, characterized in that diesel machines are used as variable controllable energy sources (7). Energy system according to any one of claims 2 to 4 and 7 to 11, wherein one or more wind turbines supply energy to an electric mains, characterized in that three control systems can be used simultaneously, namely: 80 04 59 7 S> Si 4 a) a control of the speed (12) of the wind turbines (1), which is connected to the electric synchronous generators (3) coupled to these wind turbines and based on the wind turbines (1) output power Pw and the angular velocity 5 of the rotor of these turbines (13); b) a frequency control (14) of the electric autonomous network (5), which is lit on an adjustable rectifier (10) through which it can draw the additional useful electric load (2) from the energy of the autonomous network (5) and based on the power P ^ delivered by the wind to the turbines, the electrical load P of the autonomous network (5) and the mains frequency (5) of this autonomous network; c) a voltage control (16) of the autonomous electric network (5), which is connected to the synchronous generators (6) in the autonomous network (5) and based on the mains voltage (17). Energy system according to claim 12, characterized in that the mains voltage can be kept constant by field energization of the synchronous generators (6). 14. The invention as shown in the description with drawings. 2Q 80 04 59 7