METHOD OF CONTROLLING A RENEWABLE POWER GENERATION PLANT

Abstract

Provided is a renewable power generation plant, a computer program product and method of controlling a renewable power generation plant including a power converter for connecting the renewable power generation plant to a power transmission network; a circuit breaker arrangement between the power converter and the power transmission network, including a circuit breaker for each phase of the renewable power generation plant; and a converter controller configured to generate control signals for the power converter and control signals for the circuit breaker arrangement; which method includes the steps of detecting the occurrence of a phase-to-ground fault event in one of the phases of the power transmission network; issuing control signals to the circuit breaker arrangement to keep the circuit breakers closed during the phase-to-ground event; and issuing control signals to the power converter to ride through the phase-to-ground fault event.

Claims

1. A method of controlling a renewable power generation plant comprising: a power converter for connecting the renewable power generation plant to a power transmission network; a circuit breaker arrangement between the power converter and the power transmission network, having a circuit breaker for each phase of the renewable power generation plant; and a converter controller configured to generate control signals for the power converter and control signals for the circuit breaker arrangement; which method comprises the steps of detecting the occurrence of a phase-to-ground fault event in one of the phases of the power transmission network; issuing control signals to the circuit breaker arrangement to keep the circuit breakers closed during the phase-to-ground event; and issuing control signals to the power converter to ride through the phase-to-ground fault event.

2. The method according to claim 1, wherein the occurrence of a phase-to-ground fault event is deduced from analysis of grid-side voltage and current values.

3. The method according to claim 1, wherein the control signals issued to the power converter are generated to inject negative sequence current to counteract a grid-side transient overvoltage resulting from the phase-to-ground fault event.

4. The method according to claim 1, wherein the converter controller is configured to issue gate switching signals to deliver active power into the power transmission network during a phase-to-ground fault event.

5. The method according to claim 1, wherein the converter controller is configured to issue gate switching signals to deliver reactive power into the power transmission network during a phase-to-ground fault event.

6. The method according to claim 1 wherein delivery of reactive power is given priority over delivery of active power during a phase-to-ground fault event.

7. The method according to claim 1, wherein detection of a phase-to-ground fault event is assigned a lower priority than the detection of an over-voltage fault.

8. The method according to claim 1, wherein detection of a phase-to-ground fault event is assigned a lower priority than the detection of a low-voltage fault.

9. A renewable power generation plant comprising: a power converter for connecting the renewable power generation plant to a power transmission network; a circuit breaker arrangement between the power converter and the power transmission network, comprising a circuit breaker for each phase of the renewable power generation plant; and a converter controller configured to generate control signals for the power converter and control signals for the circuit breaker arrangement; the converter controller further comprises a fault detection module configured to detect the occurrence of a phase-to-ground fault event in a single phase of the power transmission network, and wherein the converter controller is configured to issue control signals to the circuit breaker arrangement to keep the circuit breakers closed during the phase-to-ground event, and to issue control signals to the power converter to ride through the phase-to-ground fault event.

10. The renewable power generation plant according to claim 9, wherein the fault detection module is configured to distinguish a phase-to-ground fault event from a high-voltage fault event and/or to distinguish a phase-to-ground event from a low-voltage fault event.

11. The renewable power generation plant according to claim 9, realized as a wind turbine generator.

12. The renewable power generation plant according to claim 9, wherein the power converter comprises a machine-side converter and a grid-side converter, each with a power semiconductor switch arrangement for each phase, and wherein the converter controller is configured to issue phase-width modulated gate switching signals for the machine-side converter and to issue phase-width modulated gate switching signals for the grid-side converter.

13. The renewable power generation plant according to claim 9, realized as photovoltaic generator.

14. The renewable power generation plant according to claim 9, wherein the power converter comprises a grid-side converter with a power semiconductor switch arrangement for each phase, and wherein the converter controller is configured to issue phase-width modulated gate switching signals for the grid-side converter.

15. A computer program product, comprising a computer readable hardware storage device having computer readable program code stored therein, said program code executable by a processor of a computer system to implement a method comprising a computer program that is directly loadable into a memory of a controller of a renewable power generation plant and which comprises program elements for performing steps of the method according to claim 1 when the computer program is executed by the controller.

Description

BRIEF DESCRIPTION

[0033] Some of the embodiments will be described in detail, with references to the following Figures, wherein like designations denote like members, wherein:

[0034] FIG. 1 shows a simplified block diagram of a power transmission network fed by two renewable power generation plants;

[0035] FIG. 2 shows embodiments of a renewable power generation plant according to embodiments of the invention;

[0036] FIG. 3 shows embodiments of a renewable power generation plant according to embodiments of the invention;

[0037] FIG. 4 shows embodiments of a renewable power generation plant according to embodiments of the invention;

[0038] FIG. 5 is a flow chart to illustrate steps of the inventive method;

[0039] FIG. 6 illustrates aspects of the inventive method;

[0040] FIG. 7 illustrates aspects of the inventive method; and

[0041] FIG. 8 illustrates aspects of the inventive method.

DETAILED DESCRIPTION

[0042] FIG. 1 shows a very simplified block diagram of a power transmission network 3 fed by two renewable power generation plants 1. Of course, the power transmission network 3 can be fed by any number of power generation plants including such renewable power generation plants 1. For example, each power generation plant 1 may be a wind turbine of a wind park, and all wind turbines of a wind park feed into the power transmission network 3 through a bus B.

[0043] The power transmission network 3 is a three-phase network, for example a 20 kV network, and power may be assumed to be transported in overhead power cables suspended from masts, with a separate cable for each phase 3A, 3B, 3C. Occasionally, a single phase-to-ground fault F may occur, for example if a branch of a tree makes contact with a power cable, in this case the power cable carrying phase 3C. This fault F will short the affected phase to ground.

[0044] To respond to faults, the power transmission network 3 has a fault response system, shown here to comprise a control relay 30 and a circuit breaker unit 31 between the power transmission network 3 and each bus B. The control relay 30 is configured to monitor phase measurements 32 (voltage and current) and to issue control signals 33 to the circuit breaker unit 31 to open the circuit breaker(s) of one or more phases 3A, 3B, 3C, depending on the nature of the fault. For example, in the case of a single-pole fault F on phase 3C, the control relay 30 will issue control signals 33 to the circuit breaker unit 31 to open the circuit breaker of phase 3C. The control relay 30 continues to monitor the phase voltages and currents in order to determine when the affected phase 3C has been restored. Once that phase 3C is deemed to be healthy again, the control relay 30 issues control signals 33 to the circuit breaker unit 31 to re-close the circuit breaker of phase 3C. The sequence of fault occurrence, pole-breaker opening, dead-time interval and pole-breaker re-closing is referred to as a “fault event”. A single-pole fault followed by opening of the circuit breaker and subsequent re-closure of the circuit breaker is referred to as single-pole auto-reclose (SPAR) when performed by the control relay 30 of the power transmission network 3. Manual intervention is not required.

[0045] A renewable power plant such as a wind turbine or photovoltaic plant must comply with the applicable grid code. A grid code is a set of requirements that defines how an energy generation facility must be connected to the transmission network 3 to ensure safe, secure and proper operation. Up until now, response to a single phase-to-ground fault F has not been incorporated as part of any grid code for a renewable power plant. This is because, historically, this type of fault does not present a problem to a conventional power generation plant such as a nuclear power plant, a fossil-fuel power station etc., or any large synchronous generator that can ride through a fault and subsequent pole reclose events. A renewable power plant such as wind turbine can handle unbalanced operation for only a brief time, for example a few cycles (about 100 ms), so that the established response to a SPAR event has simply been to shut down and re-start when the affected phase has been restored. However, it takes considerable time to restart a renewable power generation plant, and the accumulated downtime from several such SPAR events can significantly reduce the annual output power (AEP).

[0046] FIG. 2 shows a first embodiment of a renewable power generation plant 1 according to the invention. Here, the renewable power generation plant 1 comprises a wind turbine generator WTG of the full-converter type. The wind turbine generator WTG generates three-phase power that is up-converted in a pulse-width modulated (PWM) power converter 10.

[0047] The renewable power generation plant 1 is configured to feed power into a high-voltage transmission network 3. The power converter 10 converts the low-voltage (e.g. 690 V) three-phase power generated by the wind turbine 1 into a higher voltage level. Generally, the power generation and transmission system use two step-up transformers, for example a power generation unit transformer to step-up from a low voltage (e.g. 690 V) to a mid-voltage (e.g. 20 kV) and a substation transformer to step-up from the mid-voltage to the grid voltage (e.g. 220 kV).

[0048] The renewable power generation plant 1 further comprises a converter controller 11 that monitors machine-side voltage/current measurements 111 as well as network-side voltage/current measurements 115. The network-side measurements 115 will be affected by any disturbance in the power transmission network 3.

[0049] The power converter 10 comprises a machine-side converter 102 and a grid-side converter 104. The converter controller 11 receives the machine-side measurements 111 and the network-side measurements 115. On the basis of this information, the converter controller 11 is configured to issue appropriate switching pulses 112 to the machine-side converter 102 and switching pulses 114 to the grid-side converter 104. The switching pulses 112, 114 may be assumed to comprise a set of PWM gate signals to the power electronics switches of the power converter. During normal operation, the machine-side voltages are up-converted for the grid-side phases 1A, 1B, 1C that can be fed via the bus into the transmission network 3.

[0050] To respond to a high-voltage fault or a low-voltage fault in the grid, the converter controller 11 comprises a fault detection module 12 and a fault ride-through module 13.

[0051] A circuit breaker unit 16 is arranged between the power converter 10 and the power transmission network 3, and comprises a circuit breaker 16A, 16B, 16C for each grid-side phase 1A, 1B, 1C of the renewable power generation plant 1.

[0052] The following discussion relates to the detection of a SPAR event in the power transmission network 3, i.e. a single-pole phase-to-ground fault as described in FIG. 1, followed by a subsequent auto reclose of the circuit breaker of the affected phase, whereby SPAR is effected by the relevant control relay 30 and circuit breaker unit 31. From the occurrence of the fault F to the restoration of the affected phase in the power transmission network 3, voltage imbalances are seen in the “unaffected” grid-side phases of the renewable power generation plant 1.

[0053] In this exemplary embodiment of the invention, the converter controller 11 also comprises a SPAR ride-through module 14. Using the network-side measurements 115, the converter controller 11 can detect the occurrence of a SPAR event in the power transmission network 3. The SPAR ride-through module 14 of the converter controller 11 responds by issuing control signals 116 to the circuit breaker unit 16 to keep closed the circuit breaker 16A, 16B, 16C for the affected grid-side phase, i.e. phase 1C using the example phase-to-ground fault in phase 3C of the transmission network 3 described in FIG. 1 above. The result of keeping phase 1C connected to the affected grid-side phase 3C means that the voltage imbalances (TOVs) following the SPAR event would also be “seen” by the power converter 10.

[0054] To deal with this, the SPAR ride-through module 14 in the converter controller 11 of the inventive renewable power generation plant 1 is configured to adjust the switching pulses 112, 114 of the power converter 10 to minimize the voltage imbalances, i.e. to ride-through the SPAR event.

[0055] In this way, the renewable power generation plant 1 remains connected to the transmission network 3 throughout the entire SPAR event. Because the phases 1A, 1B remain connected to the corresponding unaffected grid phases 3A, 3B, there is no need to carry out any re-synchronization sequence after the faulty phase is restored, and the affected phase 1C can immediately resume feeding into grid-side phase 3C.

[0056] The fault detection module 12, the fault ride-through module 13 and the SPAR ride-through module 14 can be implemented as software modules running on a processor of a plant controller, for example a wind turbine controller if the renewable power generation plant 1 is a wind turbine.

[0057] FIG. 3 shows a second embodiment of a renewable power generation plant 1 according to the invention. Here, the renewable power generation plant 1 comprises a wind turbine generator WTG of the doubly-fed type. The principle of operation of the converter controller 11 is the same as in FIG. 2 above, and the converter controller 11 will issue appropriate switching control signals to the power electronics switches of the machine-side converter 102 and the grid-side converter 104.

[0058] FIG. 4 shows a third embodiment of a renewable power generation plant 1 according to the invention. Here, the renewable power generation plant 1 comprises a photovoltaic plant PV that generates a DC voltage. In this case, the power converter comprises only a grid-side converter 104, and the converter controller 11 will issue appropriate switching control signals to the power electronics switches of the grid-side converter 104.

[0059] FIG. 5 shows a simplified flow-chart to illustrate steps of the inventive method.

[0060] In a first step 51, the magnitudes and phase angles of the three-phase voltages are measured. In a subsequent step 52, the magnitudes and phase angles of the instantaneous currents are measured. With this information, the positive sequence, negative sequence and zero sequence values are computed in stage 53. Higher-priority faults are dealt with in various stages collectively referred to as block 54 here. Response to a fault may be referred to as fault ride-through (FRT). Common fault response routines are low-voltage ride-through (LVRT), over-voltage ride-through (OVRT), etc. After the higher-priority faults have been ruled out, the positive sequence, negative sequence and zero sequence values and analyzed in block 55 to detect the occurrence of a SPAR event.

[0061] The occurrence of a SPAR event can be identified by detecting characteristics in the voltages and currents following such a fault event. The detection logic may be based on knowledge of fault characteristics, for example it may be assumed that no other type of voltage fault would result in a per-unit negative sequence voltage above 0.5. Analysis of the relevant parameters can take into consideration the voltage and current unbalances that follow a SPAR event as well as the contribution of the positive and negative sequence voltages. This will allow the controller to differentiate a SPAR event from other types of grid transients. If SPAR event detection returns “no”, the control sequence can terminate. If it returns “yes”, SPAR fault-ride-through is initiated in block 56. As described above, SPAR response can comprise the steps of keeping all pole-breakers closed and adjusting the gate-switching signals of the power converter; reducing reactive power to a low level; “freezing” the PLL; deactivating a frequency alarm; and performing over-voltage parameter tune-up. In the case of a wind turbine, the reactive power is reduced to a low level, and efforts are made to maintain the voltage at the wind turbine terminals to within a specific range, for example ±10% of nominal voltage. In normal conditions, a power generation unit will operate within certain frequency limits and will be disconnected from the power network system when the frequency is outside of these limits. Since transient frequencies outside of these limits may appear during the SPAR event, the frequency alarm is deactivated immediately when a SPAR event is detected in order to avoid tripping of the converter. Similarly, a SPAR event includes a step of opening the faulty phase, which usually results in transient over-voltages. When a SPAR event is detected by the inventive method, any relevant over-voltage parameter is briefly adjusted to avoid any tripping of the converter.

[0062] Following a single-pole fault, sub-cycle TOVs will be present. Once a sub-cycle TOV has passed, the converter controller assesses its capability of delivering negative sequence currents with the aim of remaining connected to the grid. Then, based on the overall current capacity, the converter controller will determine the available positive sequence active current. In this way, the renewable power generation facility can continue delivering at least some quantity of active power during the SPAR event. The effect of a single phase-to-ground fault in the grid on revenue and AEP of the renewable power generation plant will therefore be kept to a favorable minimum. The inventive method therefore has the potential for increasing the performance of a renewable power generation plant, by maintaining operation and avoiding loss of revenue during SPAR events.

[0063] The sequence of events can be preceded by a step to enable/disable SPAR, for example to ensure that the SPAR response described here is only implemented in a power distribution network that includes a fault response system that identifies and responds to a phase-to-ground fault by carrying out a SPAR routine. FIGS. 6-8 illustrate aspects of the inventive method. FIG. 6 shows exemplary timing of a p.u. value of voltage/current for a phase of the transmission network prior to, during and after a SPAR event F.sub.event. A phase-to-ground fault occurs in one phase of the transmission network at time t.sub.F and persists until time t.sub.R as indicated in FIG. 7. The duration of the phase-to-ground fault may be in the order of a few milliseconds, several seconds, or longer.

[0064] As a result of the fault, anomalies appear in the grid-side voltages/currents of the renewable power generation plant 1 as described above. The nature of the fault event is detected quickly (within a few cycles) by the fault detection module 13 and the converter controller 11 responds as described above, and normal operation is restored within a favorably short time. In normal operation (in the time prior to time t.sub.F, and in the time following time t.sub.R) the phasors of each set (positive sequence, negative sequence, zero sequence) are all the same size, and the positive sequence and negative sequence phasors are at 1200 to each other.

[0065] FIG. 7 shows an exemplary voltage sequence and current sequence following a SPAR event, with the fault at 3.5 s and grid-side pole re-closure shortly afterward. The diagram shows positive sequence voltage U.sub.pos, negative sequence voltage U.sub.neg, positive sequence current I.sub.pos and negative sequence current I.sub.neg. In the inventive method, negative sequence and positive sequence voltage and current are monitored to identify a SPAR event, i.e. to differentiate between a SPAR event and any other type of fault. In this example, the SPAR logic commences at 3.5 s with the wind turbine still connected to the power network system.

[0066] The behavior of the system following the fault may be summarized as follows: positive sequence voltage increases; negative sequence voltage increases; a highly unbalanced voltage condition follows; a high negative sequence current is injected. Here, the negative sequence voltage in excess of 0.5 p.u. allows identification of a SPAR event, since no other voltage fault event would result in such a negative voltage magnitude. To ride through the SPAR event, the voltage limit alarm is increased for a short period of time. A high negative sequence current I.sub.neg is injected to assist in fault ride-through. The controller is configured to interpret the high negative sequence current I.sub.neg as a characteristic of the SPAR event. In stable operation of a network, negative current has a per-unit value of 0.0. A “high negative sequence current” may be understood to be a p.u. negative sequence current in excess of 0.4.

[0067] FIG. 8 shows two exemplary negative sequence current plots to illustrate the difference between negative sequence current response in a single-phase event without SPAR, and in a SPAR event. Following a single-phase event without SPAR, the negative current I.sub.sp_neg is relatively low. In SPAR fault-ride through using the inventive method, the per-unit negative sequence current I.sub.neg is considerable, in this example it exceeds the single-phase event negative current I.sub.sp_neg by a factor of ten and reaches a p.u. value of 1.0 as shown in FIG. 7. The effect of the injected negative sequence current I.sub.neg is to counteract a transient overvoltage. In this example, the fault controller 11 detects passing of the SPAR event and initiates completion of the SPAR ride-through sequence so that the converter controller issues control signals to resume normal operation of the power converter.

[0068] Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. For example, during a SPAR event ride through, the converter can stop switching the power devices of the converter, so that neither active power nor reactive power is injected into the grid. However, the converter is on a stand-by state ready to resume operation as soon as the SPAR event passes. The method described herein is applicable to any renewable power plant that is connected to the grid via a converter, such as wind farms and photovoltaic parks. For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.