WO2012152975A1

WO2012152975A1 - System and method for increasing the current injected into the electrical grid by electrical generators during power dips generated in the grid

Info

Publication number: WO2012152975A1
Application number: PCT/ES2012/070332
Authority: WO
Inventors: Dionisio Ramirez Prieto; Sergio Martinez Gonzalez; Carlos Antonio Platero Gaona; Rosa María DE CASTRO FERNANDEZ; Alfonso Cuesta Garcia; Guillermo Guillen Guedez; Ana Diez Castro
Original assignee: Universidad Politécnica de Madrid; Alstom Technology Ltd.
Priority date: 2011-05-12
Filing date: 2012-05-10
Publication date: 2012-11-15
Also published as: ES2405505B2; ES2405505A1

Abstract

The invention relates to a system for increasing the current injected into the electrical grid by electrical generators during power dips generated in the grid, comprising: a power transformer (2) of the electrical generator (1) with taps in the windings thereof; an on-load tap changer (3) with electronic switches for performing tap changes in the power transformer (2); a measuring system (5) for obtaining the measurement of the phase voltages on one of the two sides of the power transformer (2); and a control system (4) for receiving the measurements taken by the measurement system (5), and analysing said measurements in order to detect power dips in the electrical grid, to determine the tap of the power transformer (2) to be switched in order to increase the current injected into the electrical grid, and to order the actuation of the switches of the tap changer (3) in order to perform the previously determined tap change.

Description

System and procedure to increase the current injected into the electrical network by electric generators during voltage dips produced in the network

Sector of the technique

The invention is framed in the technical sector of the generation and transport of electrical energy. More specifically, in the production of electrical energy with devices that have low percentage short-circuit power, that is, low contribution to the short-circuit current. Typically, although not exclusively, in the generation of electrical energy from renewable or dispersed sources with generators that include stages of energy conversion through power electronics.

State of the art

An electric power system is the set of electrotechnical installations for the generation, transport and use of electrical energy in a geographical area. Thus, the mission of such a system is to supply electric power to consumers who demand it within a certain geographical area. Additionally, this supply must be carried out in compliance with adequate safety, quality and environmental conditions, at the lowest possible cost.

Electric generators are one of the fundamental elements in an electric power system. Traditionally, these generators are, for the most part, large synchronous rotary electric machines. Thus, the operation of electric power systems has been very defined by the operating characteristics of this type of networked machines. Particularly, in what relates to the present invention, the philosophy of protection of electrical power systems and associated facilities and equipment, are based on how these generators behave in the face of the different failures or disturbances to which the power may be subjected. electric power system.

In recent years, mainly due to the increase in the use of renewable sources of energy for the generation of electricity, this situation is changing. Other types of electric generators are being installed whose operating characteristics are different from those of synchronous generators. While the degree of penetration of these new generators in the systems of Electric power was not significant, the operating characteristics of the systems were not substantially altered. However, the increase in its level of penetration has been revealing alterations in the functioning of the systems that have forced the operators of electric power systems to impose ever stricter requirements on these new generators. These requirements are of various kinds, but it can be said that, in general, they are aimed at the new generators incorporating complementary systems so that the characteristics of joint operation are similar to those of synchronous generators. Thus, for example, the capacity to withstand voltage sags is already a requirement that is included in many network codes and, as a consequence, generators manufacturers already include it among the features of their equipment.

Currently, in electric power systems with a high level of penetration of unconventional electric generators, a problem related to their lower capacity to contribute to the short-circuit current in the event of a defect begins to be revealed, compared with that of synchronous generators. . This capacity is especially low in generators, or groupings of generators, in which all the electricity produced is poured into the electricity grid through an electronic converter. For example, in photovoltaic solar plants or in wind generators with full power electronic converter. It should be noted that a superficial analysis of the situation could conclude that it is beneficial that, in the event of a fault in the electrical network, the short-circuit current supplied by these generators is percentage lower than that provided by conventional generators. This is partially true, for example, in regard to the dimensioning of the switching device of the electric power system. However, it is important to note that a good part of the protection philosophy of said system is based on the fact that insulation defects are detected due to the high currents associated with them, at least in conventional systems. The decrease in the contribution of the new generators to the short-circuit current, therefore, poses a problem for the protection of the electric power system.

The object of the present invention is to provide a solution to this problem, by means of a new way of using a transformer with sockets and with a changer thereof in charge. Apparently similar equipment has been known and used for many decades, as can be seen, for example, in patent document US1641271, of 1927. However, its use in electric power systems is oriented to the change of outlets in a quasi-permanent regime , never during test holes and always on balanced systems and for voltage control, in which the speed of response of the tap control is not important.

This conventional use of a transformer with tap-changeover has resulted in the functional specialization of the equipment to solve such problem, as can be seen, for example, in patent documents US201 10005910, US201 10000769 and US20100263996. Thus, the current equipment is optimized for a quasi-permanent operation, with few maneuvers per hour, and its operation is aimed at controlling the voltage. In summary, there is a certain technical prejudice regarding the use of these devices as slow voltage regulators.

Description of the invention

The present invention proposes a special use of a transformer with change of taps on load to act on the intensity, instead of the voltage, during the transient regime associated with a short circuit, and the specification of its characteristics to fulfill such function. On the other hand, unlike the conventional use of transformers with on-load tap-changers, where their function is the control of a quantity (voltage), the function of the present invention is to multiply, instead of controlling, another quantity ( the current).

On the other hand, to achieve this multiplication of the current, it is not enough to make a change of taps of the transformer that allows the change of intensity ratio described above. In addition, it must be combined with a procedure to operate the generator (including the converter) as a source of intensity, instead of as a source of voltage, to ensure that, once the intakes are changed, the nominal value of the current of the electronic converter. In addition to this main function, the method should include monitoring and controlling the voltage on the converter side, since the system described in the present invention can only be used when there is a decrease in line voltage (typically due to a short circuit) since, when changing the transformation ratio to multiply the current in one of the windings of the transformer, a multiplication of the voltage in the opposite winding connected to the electronic converter also takes place, being able to block it and even destroy it. Keep in mind that the value of the voltage on the DC bus of An inverter is directly related to the voltage of the network to which it is connected, due to the antiparallel diodes connected to its static switches. Consequently, when changing the sockets the voltage increases in the windings of the transformer connected to the inverter, so that the rectification made by the diodes will charge the capacitors of the DC bus of the inverter, increasing its voltage, being able to destroy them and the switches static of the inverter. For this reason, the control system must make a measurement of the mains voltage and make the change in the transformation ratio only in cases where it detects a voltage drop.

When the control system detects a single-phase or two-phase gap, the regulation of the electronic converter reacts by trying to cancel the reverse sequence and restore the direct sequence, injecting the appropriate currents through the lines within the limits of the converter and switching the transformer connections of the same way as in the case of three-phase gaps.

Brief description of the figures

For the best understanding of what is described in this specification, figures are included in which some possible embodiments of the invention are represented exclusively by way of illustrative examples and without limitation.

Figure 1 shows a connection diagram of a possible embodiment of the system of the invention, in the case of a three-phase four-wire system with a star-star transformer with several sockets.

Figure 2 shows a connection diagram for the case of a three-phase three-wire system with a star-star transformer with two sockets on the network side.

Figure 3 shows the detail of the maneuver of one of the phases of the static tap-changer after detection of the voltage gap associated with the short circuit at some point of the electric power system.

Figure 4 shows, in an analogous way, the detail of the maneuver of one of the phases of the static tap-changer after detecting the recovery of the voltage once the short-circuit has been eliminated. Figure 5 shows a laboratory test in which the voltage level of the network has been recorded in a steady state and during a gap, and in addition the three sinusoidal currents corresponding to each phase of the three-phase network have been registered, measurements on the transformer's network side.

Description of a preferred embodiment

The invention relates to a system and method that increases the current that a generator system contributes to the electrical network during voltage dips, but without increasing the current provided by the electric generator.

Figure 1 shows a connection diagram of a possible embodiment of the system of the invention, in the case of a three-phase four-wire system with transformer (2) star-star with several sockets. The generator (1) or set of generators (including electronic converters if applicable) are shown, whose short circuit current is intended to increase with the present invention. Only its connection terminals with the outside are shown. The first three, starting at the top, are the phase terminals (R, S, T), while the fourth is the neutral terminal (N). Also shown is the electric power system (6) to which the energy produced by the generator (1) is poured. The system of the invention is represented by the remaining four blocks: the power transformer (2), the on-load tap-changer (3), the control system (4) and the measuring system (5).

The figure is included for the purpose of illustrating the invention with an example, but without limitation. Thus, for example, although the figure represents a three-wire four-wire system with a star-star connection, the invention is applicable to single-phase or three-phase systems, with three or four wires, of any primary-secondary combination (triangle, star or zig- zag).

To increase the short-circuit current of generators or groupings of electric generators, the system includes:

- A power transformer (2) with several sockets in one or several of its windings, distributed to obtain one or several ratios of intensity transformation, including values ostensibly different from the nominal.

- A tap-changer (3) for this transformer that allows the direct change to the desired tap and / or the change through the intermediate taps, with a suitable action time to the rapidity with which it is intended to increase the current. Typically, this actuation time must be of the order of a wave period of the industrial frequency of the electrical power system to which it is connected or less, although, depending on the protection philosophy of the network, it may be greater. In Figure 1 you can see a functional diagram of the tap changer (3). The switches shown are not limiting of any type of technology, mechanical or solid state, that meets the specifications of time of action.

- A measurement system (5) of intensities and / or voltages. The most basic form of implementation of the system of the invention requires only the measurement of the voltage of all the phases (for example: three, in the three-phase case, one, in the single phase, etc.), on either side of the power transformer. For example, in Figure 1 you can see a measurement system of the three phase voltages on the network side of the power transformer, at the outlet of the tap changer (3). If you want to have more control over the system of the invention, or even integrate its control with that of the generator (including the electronic converter), the line currents can be added to the measurement system.

- A control system (4) that, depending on the previous measures, allows governing the change of taps (3) of the transformer (2). In the example of Figure 1 is represented a control system (4) that receives, as inputs, the measurements of the three phase voltages and acts, as outputs, on the independent opening of the nine switches that make up the tap changer (3).

Figure 2 shows a connection diagram for the case of a three-phase three-wire system with transformer (2) star-star with two sockets on the network side. In this case, the generator (1) includes a generating electric machine (7) and an electronic converter (8). Also shown is the electrical system (6) to which the energy produced by the generator is poured. The system of the invention is represented by the three remaining blocks: the power transformer (2), the static tap changer (3) and the control system (4). The control system (4) receives the measurement signals from the converter (9) and the electrical network (10). In turn, the control system outputs the trigger signals of the static switches of the electronic converter (11) formed by pulse trains generated with the spatial vector modulator (SVM), which activate and deactivate the six IGBTs. that make up the inverter, and the static tap changer (12).

The transformer sockets (2) can be in any one of the three-phase windings (primary or secondary) to adjust the transformation ratio of tensions and currents. The sockets can be selected by activating or deactivating electronic switches built with antiparallel thyristors, instantaneous conduction and forced or natural blocking.

The control system (4) is based on a microprocessor that implements the regulation of the power delivered to the network by the electric generator (1) through an electronic converter (8) as well as the control of the outlets (3) of the transformer .

The control system (4) measures the value of the voltage in the electrical network and, when it detects that a voltage gap has occurred, decides which three-phase socket it must switch taking into account that the voltage rise that will be experienced in terminals of the electronic converter (8) due to the change in the transformation ratio must be admissible for it. Admissible is that network voltage value less than or equal to the maximum value of alternating voltage that the inverter in turn can generate. In this way, it is guaranteed that the inverter is able to flow power to the network.

The selection criterion of the socket is based on injecting the maximum three-phase current into the line without blocking or destroying the increase in terminal voltage of the electronic converter (8).

The control system (4) carries out the measurement of two of the three voltages of the network for the detection of the voltage gap, calculating with them the module of the spatial phasor network voltage.

In the preferred embodiment, the control system performs the detection of single-phase or two-phase unbalanced gaps by the direct and inverse sequence separation algorithm called "Delayed Signal Cancellation" (DSC). Under normal conditions, the voltage decomposition of the network only presents a direct sequence, whereas if an unbalanced fault occurs, a reverse sequence component also appears.

When the control system detects a single-phase or two-phase gap, the regulation of the electronic converter reacts by trying to cancel the reverse sequence and restore the direct sequence, injecting the appropriate currents through the lines within the limits of the converter and switching the transformer connections of the same way as in the case of three-phase gaps. The switching of the three-phase socket, whose purpose is to increase the current in the line, can be done eliminating turns in the three-phase winding connected to the network or adding turns in the three-phase winding connected to the electronic converter.

To make a switchover with less transient current in the windings of the transformer, the control system measures the current in the three lines and performs the switching of the taps when the current in each line goes through zero.

Once the voltage gap is detected, the control system (4) disconnects a part of the three-phase winding connected to the network. For this, when the current in each of the phases passes through zero, short-circuits the corresponding winding by closing the switch in parallel with it and opens the switch in series, according to the sequence of maneuver shown in Figure 3 , in three stages from left to right corresponding to three consecutive instants, where only one of the phases of the tap changer (3) has been represented. In the first stage (left), the upper static switch is shown in driving and the lower one in blocking. In the second stage (center), the upper static switch is shown in block and the lower one in conduction, but in the zero crossing of the current. And in the third stage (right), the upper static switch is shown in block and the lower one in conduction with a non-zero value of the current.

Once the control system (4) detects that the voltage gap in the network has disappeared, once the short circuit is eliminated, it reconnects each one of the windings successively as each of the line currents passes through zero, according to the sequence of maneuver shown in Figure 4, in three stages from left to right corresponding to three consecutive instants, where only one of the phases of the tap changer has been represented. In the first stage (left), the upper static switch in blocking and the lower in driving are shown. In the second stage (center), the upper static switch is shown in conduction, but in the zero crossing of the current, and the lower one in blocking. And in the third stage (right), the upper static switch is shown in conduction, with a non-zero value of the current, and the lower one in blocking.

In the preferred embodiment, the regulation of the electronic converter for connection to the grid is carried out with current control to ensure that the tap change does not affect the current obtained from the electric generator when the voltage change at the terminals of the power supply increases with the tap change. said investor. Figure 5 shows a laboratory test in which the voltage level of the network has been recorded in a steady state and during a gap. Likewise, the three sinusoidal currents corresponding to each phase of the three-phase network, measured on the transformer's network side, have been recorded. It is checked that, during the voltage drop, the current in the line increases considerably due to the switching of the sockets.

Another object of the present invention is a method, implemented in the control system, for increasing the short-circuit current of the generator or set of generators. The procedure is cyclical and begins with the calculation of the effective value of the voltages of the phases, from their measurements, and the comparison with their nominal value. If the difference is within the normal operating range of the generator or the voltage regulator (if any), the system of the invention should not act. If the difference is greater, the transformer tap is taken to bring its generator-side voltage to the level closest to its nominal value, which implies implicitly increasing the intensity on the network side to the highest value compatible with a voltage admissible by the generator, in addition to operating the generator (including the electronic converter) as a source of intensity. Once the tap is calculated, the control system orders the action of the tap changer switches to carry out the calculated change, and a new process cycle is started.

Claims

one . - System for increasing the current injected into the electrical network by electric generators during voltage dips produced in the network, characterized in that it comprises:

- a power transformer (2) of the electric generator (1) with a plurality of sockets in at least one of its windings;

- a tap-changer (3) with a plurality of electronic switches in charge of making tap changes in the power transformer (2) according to certain control commands;

- a measurement system (5) in charge of obtaining the measurement of the phase voltages on one of the two sides of the power transformer (2);

- a control system (4) configured for:

* receive the measurements made by the measurement system (5);

* Analyze these measures for, when a voltage gap occurs in the electrical network:

- detecting said voltage gap;

- determining, according to a certain selection criterion, the power transformer (2) to be switched to increase the current injected into the electric network;

- order the action of the tap changer switches (3) to carry out the previously determined tap change.

2. - System according to claim 1, characterized in that the measurement system (5) is additionally configured to obtain the measurement of the current of the different phases, and where the control system (4) is configured to order the actuation of the tap changer switches (3) when the current in each of the phases is close to zero.

3. System according to claim 2, characterized in that, in order to carry out the maneuvering sequence for the tap change of each phase, the control system (4) is configured so that, in the zero crossing of the phase current, the Switch in parallel with the winding and open the switch in series.

4. - System according to any of the preceding claims, characterized in that the control system (4) is additionally configured to detect, depending on the measurements made by the measurement system (5), the completion of the voltage gap and order the action of the tap-changer switches (3) to make the changeover to the nominal operating outlet.

5. - System according to claim 4 when dependent on the 2, characterized in that to perform the maneuvering sequence for the change to the nominal tap in each phase the control system (4) is configured to, in the zero step of the phase current, open the switch in parallel with the winding and close the switch in series.

6. - System according to any of the preceding claims, characterized in that the analysis of the measures to determine the voltage gaps comprises calculating the effective value of the phase voltages and compare them with their nominal value, so that if the difference is higher at a certain margin, the presence of a voltage gap is determined.

7. System according to any of the preceding claims, characterized in that the measurement system (5) is configured to obtain the measurement of the ac voltage of the network on the side of the electronic converter (8) of the generator (1), and wherein the control system (4) is configured to select that tap that generates the maximum current injection in the network and such that the terminal voltage of the electronic converter (8) is equal to or less than its rated voltage.

8. System according to any of the preceding claims, characterized in that the control system (4) is configured to detect single-phase or two-phase unbalanced gaps by the DSC direct and inverse sequence separation algorithm.

9. System according to any of the preceding claims, characterized in that the switching of the tap to increase the current injected into the electrical network is done by one of the following ways:

- eliminating turns in the winding of the transformer (2) connected to the network;

- adding turns in the winding of the transformer (2) connected to the generator (1).

10. - System according to any of the preceding claims, characterized in that the control system (4) is configured to perform the switching to the outlet determined in one of the following ways:

- directly;

- sequentially, through intermediate taps.

eleven . - Procedure to increase the current injected into the electrical network by electric generators during voltage dips produced in the network, where the power transformer (2) of the electric generator (1) has a plurality of sockets in at least one of its windings , characterized in that the method comprises:

- obtain the measurement of the phase voltages on one of the two sides of the power transformer (2);

- Analyze these measures so that, when a voltage gap occurs in the electricity network:

* detect said voltage gap; * determine, according to a certain selection criteria, the power transformer (2) to be switched to increase the current injected into the electric network;

* Carry out the previously determined tap change on the power transformer (2).

12. - Method according to claim 1, characterized in that it additionally comprises:

- obtain the measurement of the current of the different phases;

- carry out the tap change when the current in each of the phases is close to zero.

13. Method according to claim 12, characterized in that the switching sequence for the tap change of each phase comprises, in the zero crossing of the phase current, closing the switch in parallel with the winding and opening the switch in series.

14. - Method according to any of claims 1 to 13, characterized in that it additionally comprises detecting the completion of the voltage gap and making the change to the nominal operating outlet.

15. - Method according to claim 14 when it depends on the 12, characterized in that the sequence of maneuver for the change to the nominal tap in each phase comprises, in the zero crossing of the phase current, opening the switch in parallel with the winding and close the switch in series.

16. - Method according to any of claims 1 to 15, characterized in that the analysis of the measures to determine the voltage gaps comprises calculating the effective value of the phase voltages and compare them with their value nominal, so that if the difference is greater than a certain margin, the presence of a voltage gap is determined.

17. - Method according to any of claims 1 to 16, characterized in that it additionally comprises:

- obtain the measurement of the tension of c.a. of the network on the side of the electronic converter (8) of the generator (1), and

- select the tap that generates the maximum current injection in the network and such that the terminal voltage of the electronic converter (8) is equal to or less than its rated voltage.

18. - Method according to any of claims 1 to 17, characterized in that it comprises detecting single-phase or two-phase unbalanced gaps by means of the DSC direct and inverse sequence separation algorithm.

19. - Method according to any of claims 1 to 18, characterized in that the switching of the tap to increase the current injected into the electrical network is done by one of the following ways:

20. - Method according to any of claims 1 to 19, characterized in that the switching to the determined outlet is carried out in one of the following ways:

- directly;

- sequentially, through intermediate taps.