CN115864374A - Transient stability improving method for energy storage MMC-synchronous machine parallel power supply system - Google Patents
Transient stability improving method for energy storage MMC-synchronous machine parallel power supply system Download PDFInfo
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Abstract
The invention discloses a transient stability improving method of an energy storage MMC-synchronous machine parallel power supply system. The method can enable the energy storage MMC to work in an area beneficial to transient stability of the parallel system, and improve the transient stability of the system; the MMC does not change the original inner ring controller structure and outer ring controller structure of the MMC, and the control strategy during the steady-state operation is not influenced. After the method is used, compared with the traditional MMC fault ride-through strategy, the power angle swing of the synchronous machine is reduced during the fault period of the energy storage MMC-synchronous machine parallel power supply system, the critical removal time is increased, the transient power angle instability risk is reduced, and the stability and the reliability of the power supply system are improved.
Description
Technical Field
The invention belongs to the technical field of power transmission and distribution of a power system, and particularly relates to a transient stability improving method of an energy storage MMC-synchronous machine parallel power supply system.
Background
With the rapid development of power electronic devices, a voltage source converter based flexible direct current (VSC-HVDC) technology is also widely used. Compared with a traditional direct current system based on a semi-controlled device, the VSC-HVDC has the advantages of being flexible in control, free of phase change voltage provided by a power grid, capable of independently controlling active power and reactive power, capable of providing synchronous alternating current power supply support for a passive network and the like, has the advantages of supplying power to the passive network, independently controlling the active power and the reactive power, capable of flexibly achieving trend reversal and the like, is widely applied to scenes of new energy grid connection, interconnection among alternating current large power grids, offshore wind power access, direct current power distribution networks and the like, and is huge in development prospect. The modular multilevel converter MMC has the advantages that harmonic components are few, the power device series connection technology is not needed, and the like, and becomes a preferred voltage source converter in large-scale new energy base grid connection. Meanwhile, MMC-HVDC is used as an important asynchronous machine power supply, and can replace a synchronous machine power supply to supply power to a system in a future power system.
When the MMC-HVDC is used as an asynchronous machine power supply, two typical control strategies of a network following type and a network construction type are mainly adopted. The network following MMC usually adopts current vector control, an outer loop controller realizes decoupling control of active/passive quantity, an active control loop usually fixes active power, a reactive loop can adopt a fixed reactive power/alternating voltage control strategy, and a Phase Locked Loop (PLL) is adopted to track voltage of a grid-connected point so as to realize synchronization with an active power grid. When the MMC is provided with the energy storage device, the MMC can adopt network following type control and network construction type control, and can have active and reactive throughput capacity within the capacity range of the energy storage device, so that the energy storage MMC has stronger flexibility in application scenes of grid-connected operation, power supply to a passive network and the like.
With the increase of the demand of electric energy and the increase of environmental protection pressure, the demand of clean energy is continuously increased, and the leading position of the power supply of the traditional synchronous machine is broken in the future; with the gradual replacement of the synchronous machine power supply by the asynchronous machine power supply, the energy storage MMC with the four-quadrant operation capability has wider application prospect. Among the existing MMC fault ride-through strategies, the two most commonly used strategies are: (1) MMC injects Reactive current to the system in the fault period [ S.De Rijcke, H.Ergun and D.Van Hertem, et al, "group image of Voltage Control and Reactive Power Support by Wind Turbines required With Direct-Drive Synchronous Machines," IEEE trans.Sustain energy, vol.3, no.4, pp.890-898, oct.2012]; (2) during the Fault period, the injection active and reactive current components [ MA S, GENG H and LIU L, et al, grid-Synchronization Stability Improvement of Large Scale Wind Farm During Grid Fault [ J ]. IEEE trans. Power Syst.2018,33 (1): 216-226] of the MMC are adjusted according to the Grid-connected impedance angle of the MMC.
The two fault ride-through strategies are mainly used for improving the grid-connected transient stability of the MMC, however, in the energy storage MMC-synchronous machine parallel power supply system, the active throughput capacity of the energy storage MMC can not be exerted, and the transient stability of the system is not favorably improved. Therefore, another study on a transient stability promotion strategy of the parallel power supply system of the energy storage MMC-synchronous machine is needed to realize the stable and reliable operation of the parallel power supply system of the energy storage MMC-synchronous machine in the future.
Disclosure of Invention
In view of the above, the invention provides a transient stability improving method for an energy storage MMC-synchronous machine parallel power supply system, which can optimize a current injection strategy of the energy storage MMC during a fault, improve the transient stability of the system, and has good robustness; compared with the traditional MMC fault ride-through strategy, the control method has the advantages of simple implementation, strong applicability and higher practical value in engineering design.
A transient stability improving method for an energy storage MMC-synchronous machine parallel power supply system comprises the following steps:
(1) For an energy storage MMC-synchronous machine parallel power supply system, when an alternating current side short circuit fault occurs, acquiring the rotor angular frequency of the synchronous machine and transmitting the rotor angular frequency to the energy storage MMC;
(2) The energy storage MMC calculates the angle difference between the power angle of the synchronous machine and the voltage phase of the grid-connected point according to the angular frequency of the rotor;
(3) Determining an active component instruction value and a reactive component instruction value of the injection current of the energy storage MMC during the fault period according to the angle difference;
(4) The energy storage MMC is switched from a normal working mode to a current saturation mode, and the injected current of the energy storage MMC during the fault period tracks the instruction value, so that the transient stability of the system is improved.
Further, in the step (1), a Wide Area Measurement System (WAMS) is used to acquire the rotor angular frequency of the synchronous machine.
Further, in the step (2), an angle difference between a synchronous machine power angle and a grid-connected point voltage phase is calculated through the following formula;
wherein: theta gs Is the angle difference between the synchronous machine power angle and the grid-connected point voltage phase during the fault period, t f For duration of fault, θ gs0 Is the angle difference between the synchronous machine power angle and the grid-connected point voltage phase under the steady state, omega 0 For rating the angular frequency, omega, of the system g For the rotor angular frequency, omega, of synchronous machines s For the grid-connected point voltage angular frequency, t represents time.
Further, the grid-connected point voltage angular frequency ω s The detection is carried out through a phase-locked loop of the energy storage MMC.
Further, in the step (3), an active component command value and a reactive component command value of the energy storage MMC injected current during the fault are determined through the following expressions;
wherein: i is d * And I q * Respectively an active component instruction value and a reactive component instruction value, I, of the energy storage MMC injection current during the fault max In order to limit the current of the energy storage MMC,
an angle reference is injected for the current.
Further, the current injection angle reference value
Wherein theta is gs Is the angle difference between the synchronous machine power angle and the grid-connected point voltage phase during the fault.
Further, when the energy storage MMC detects that the voltage amplitude of the grid-connected point falls below the voltage threshold in the step (4), the normal operation mode is switched to the current saturation mode.
Further, in the step (4), the injection current of the energy storage MMC is made to track the instruction value during the fault period, that is, the instruction value is used as a reference value of the inner ring current controller of the energy storage MMC, and when the voltage of the grid-connected point is recovered to be higher than the voltage threshold value, the energy storage MMC is switched back to the normal working mode, so that fault ride-through is completed.
Compared with the prior art, the invention has the following beneficial technical effects:
1. the invention provides a feasible control strategy for the energy storage MMC-synchronous machine parallel power supply system, can adjust the injection current of the MMC by using the rotating speed information of the synchronous machine, ensures that the power supply system is positioned in a region with stronger transient stability, and plays a certain guiding role in the design of future engineering.
2. The design of the controller of the invention only adjusts the current set value during the MMC fault period through the information collected from the WAMS system, the original inner and outer ring controller structures of the MMC do not need to be changed, and the control strategy during the steady state operation is not affected, thus the invention can be suitable for the network-forming type MMC and the traditional network-following type MMC, the implementation is simple, and the invention has good economic benefit.
3. Under the condition of considering communication delay, the transient stability of the system is hardly influenced after the control strategy is adopted, so that the method has strong robustness and extremely high engineering value.
4. When the power angle of the synchronous machine changes in the fault period, the current instruction value of the MMC can be dynamically adjusted, and the MMC can be guaranteed to operate in the transient stability optimal area all the time. Compared with the conventional MMC fault ride-through strategy, when the MMC adopts the control strategy of the invention, the power angle swing-out degree is reduced during the fault period of the synchronous machine, the Critical Clearing Time (CCT) is increased, and the transient stability of the system is improved, so the invention has strong applicability under various working conditions and has great practical engineering significance.
Drawings
Fig. 1 is a schematic view of a topology structure of a parallel power supply system of an energy storage MMC-synchronous machine in an embodiment of the present invention.
Fig. 2 is a control schematic block diagram of the energy storage MMC fault ride-through strategy of the present invention.
Fig. 3 is a waveform diagram of an output current of an energy storage MMC in a transient state process when a three-phase metallic grounding short-circuit fault occurs in a middle point of a line 2 and the fault duration is 250ms under the control strategy of the present invention.
Fig. 4 is a schematic diagram showing a comparison of power-angle waveforms of a synchronous machine in a transient process when a three-phase metallic grounding short-circuit fault occurs in a middle point of a line 2 and the fault duration time is 400ms under the control strategy of the present invention and the conventional control strategy respectively.
Fig. 5 is a schematic diagram showing a comparison of PCC voltage amplitude waveforms of an MMC during a transient state when a three-phase metallic grounding short-circuit fault occurs at a midpoint of a line 2 and the fault duration is 400ms, respectively, under the control strategy of the present invention and the conventional control strategy.
Detailed Description
In order to more specifically describe the present invention, the following detailed description is provided for the technical solution of the present invention with reference to the accompanying drawings and the specific embodiments.
The invention discloses a transient stability improving method of an energy storage MMC-synchronous machine parallel power supply system, which comprises the following steps:
(1) For the energy storage MMC-synchronous machine parallel power supply system, a Wide Area Measurement System (WAMS) is adopted during a fault period to obtain the rotor angular frequency of the synchronous machine and transmit the rotor angular frequency to the MMC, so that the dynamic regulation of the active component current and the reactive component current of the MMC is realized.
When setting for not adopting transient stability to promote the strategy, adopt reactive current priority control strategy, specifically during energy storage MMC trouble: when short-circuit fault occurs on the AC side, the MMC detects the voltage amplitude U of the grid-connected point s Drop to voltage threshold U th Then, the normal operation mode is switched to the current saturation mode, and the d-axis and q-axis current set values I are obtained d * 、I q * Satisfies the following conditions:
in the above formula, I max Is the current limit value of the VSC. When a transient stability lifting strategy is adopted, when the energy storage MMC enters a current saturation mode during the fault period, the reference value of the current injection angle is
d. q-axis current set value I d * 、I q * Satisfies the following conditions: />
(2) For the energy storage MMC-synchronous machine parallel power supply system, firstly, the angle difference between the power angle of the synchronous machine and the voltage phase of the grid-connected point of the MMC is calculated.
According to the voltage drop degree of the MMC grid-connected point under the fault condition, whether the MMC enters a current saturation mode is judged, if the MMC enters the current saturation mode, a rotating speed signal omega of a synchronous machine is transmitted to a communication device during the fault period g To the control center of the WAMS and from the control center to the MMC via the communication means. PCC Voltage phase θ of MMC during Fault s Angle delta with synchronous machine g Difference value θ of gs Satisfies the following conditions:
in the above formula, ω 0 For system nominal angular frequency, omega s PCC voltage angular frequency theta detected for phase locked loop of MMC gs0 The difference between the PCC voltage phase of the MMC and the power angle of the synchronous machine under the steady state is shown, delta represents deviation, d represents differentiation, and t is time. Therefore, when the failure duration t f Knowing, theta can be obtained gs It satisfies the condition:
(3) And (3) for the energy storage MMC-synchronous machine parallel power supply system, determining the instruction values of active and reactive components of the injection current during the fault period of the MMC according to the angle difference obtained by calculation in the step (2).
Obtaining the difference theta between the PCC voltage phase of the MMC and the power angle of the synchronous machine gs And then, when the transient stability of the system is strongest, the electromagnetic power output during the fault period of the synchronous machine is maximized, and the optimal current injection angle of the MMC is obtained at the moment
Satisfies the following conditions:
therefore, let the current injection angle reference value be
Thereby calculating d and q axis current set value I of MMC d * 、I q * ：
(4) And (4) for the energy storage MMC-synchronous machine parallel power supply system, enabling the injected current during the fault period of the MMC to track the instruction value of the MMC according to the result of the step (3), and finishing the transient stability promotion strategy of the energy storage MMC-synchronous machine parallel power supply system.
When the MMC enters a current saturation mode due to voltage drop during fault, calculating the I obtained in the step (3) d * 、I q * The result is used as a reference value of the inner ring current controller of the energy storage MMC, and the design method of the inner ring current controller is basically the same as that of the inner ring current controller of the traditional MMC; when the fault is cleared, the PCC voltage of the MMC is restored to the voltage threshold U th And at the moment, the MMC is switched back to the normal working mode to complete fault ride-through.
The energy storage MMC-synchronous machine parallel power supply system adopted in the embodiment is as shown in fig. 1, and a grid connection point of the synchronous machine and the energy storage MMC passes through a line Z 1 Connected and the grid-connected point of the energy storage MMC and the alternating current grid are connected through a line Z 2 The specific parameters of the main loop of the system are shown in Table 1, X in FIG. 1 S And X G And respectively representing the leakage reactance of the connecting transformer of the synchronous machine and the energy storage MMC.
TABLE 1
Under the steady state operation state, the energy storage MMC adopts the network construction type control and utilizes a phase-locked loop to detect the voltage frequency of a grid-connected point. The MMC operates as an inverter station, the output active power is 200MW, and the reactive power is 0; the output complex power of the synchronous machine is (200 + j39.2) MVA, where j is an imaginary unit. In a steady state, the PCC voltage phase theta of the MMC can be obtained according to load flow calculation s ＝4.58 ° Angle delta with synchronous machine g ＝14.51 ° The phases are based on the phase of the ac grid voltage. Therefore, θ in steady state gs0 ＝14.51-4.58＝9.93 ° (ii) a PCC voltage phase theta of MMC when fault happens s Angle delta with synchronous machine g All change, so θ gs Is calculated by the information obtained by the WAMS system, and the optimal current injection angle of the MMC is adjusted according to the methods of the steps (2) and (3)
And finally obtaining the active and reactive current instruction values of the MMC. The control strategy of the energy storage MMC is shown in FIG. 2, wherein a mode 0 in FIG. 2 represents a steady-state operation mode, and a mode 1 represents a current saturation mode; after the energy storage MMC detects the voltage drop during the fault, the mode 0 is switched to the mode 1, wherein i dref 、i qref Current instruction values of d and q axes of an inner ring of the MMC under a steady state or a fault are respectively; after the fault is recovered, the energy storage MMC is switched from the mode 1 to the mode 0, normal operation is recovered, and s is a Laplace operator.
When the control strategy of the invention is adopted, when a three-phase metallic grounding short-circuit fault occurs at the midpoint of the line 2 shown in fig. 1 at the 10 th s, and the fault duration is 250ms, the output current waveform of the energy storage MMC in the transient process is shown in fig. 3.
When the control method and the conventional control strategy of the present invention are respectively used, when a three-phase metallic grounding short-circuit fault occurs at the midpoint of the line 2 shown in fig. 1 at 9s, and the fault duration is 400ms, the power angle waveform of the synchronous machine in the transient process is shown in fig. 4, wherein the conventional control strategy is adopted during the fault period of the MMCReactive current priority injection control strategy and maintenance
And is not changed.
When the control method and the traditional control strategy of the invention are respectively used, when a three-phase metallic grounding short-circuit fault occurs at the midpoint of the line 2 shown in fig. 1 at the 9 th time, and the fault duration is 400ms, the voltage waveform of the grid-connected point of the MMC in the transient process is shown in fig. 5.
Table 2 shows simulation results of the CCT of the system when the control method and the conventional control strategy of the present invention are used, respectively, where the CCT of the system is defined as the clearing time of the fault when the system is subjected to critical power angle instability. The second column of table 2 shows the CCT simulation results of the system under the conventional reactive power priority control strategy; the third column shows the CCT simulation result of the system under the control method of the invention; the fourth column is the CCT simulation result of the system under the condition of considering time delay and adopting the control method of the invention.
TABLE 2
For the above example, it can be seen from fig. 3 that the dq axis injection current of the MMC during the fault can successfully follow the command value, which means that the current inner loop will not be unstable and the dq axis injection current will not remain constant, since the control strategy of the present invention can dynamically adjust the phase of the injection current of the MMC according to the power angle of the synchronous machine and the PCC voltage phase difference value
Therefore, after the control strategy is adopted, the injection current of the MMC can be always positioned in a region beneficial to the transient stability of the system.
The simulation results of fig. 4 show that, for the same fault duration, when the MMC adopts the conventional control strategy, the power angle δ of the synchronous machine after the fault is cleared is δ g The unstable equilibrium point is crossed, and the power angle instability of the system occurs in the first swing period. However, when the present invention is adoptedIn control strategy, the power angle delta of the synchronous machine after fault clearing g The system can return to a stable equilibrium point, and finally the system can recover stable operation after a transient process.
The simulation result of fig. 5 shows that, when the MMC adopts the conventional control strategy for the same fault duration, the power angle of the system is unstable, the voltage generates non-periodic oscillation, and the stable operation state cannot be recovered. When the control strategy is adopted, the system can keep the power angle stable, and the system voltage can finally be oscillated and subsided through a transient process. The simulation results of fig. 4 and fig. 5 together illustrate that the control strategy of the present invention can effectively improve the transient stability of the energy storage MMC-synchronous machine parallel power supply system.
The CCT simulation results in table 2 show that, when the control strategy of the present invention is adopted, the CCT is increased by 39ms compared with the conventional control strategy, which shows that the control strategy of the present invention is more advantageous in improving the transient stability of the system compared with the conventional control strategy, and the CCT result is hardly affected when the delay of the 60ms communication system is considered, which shows that the control strategy of the present invention still has good robustness when the delay is considered.
The foregoing description of the embodiments is provided to enable one of ordinary skill in the art to make and use the invention, and it is to be understood that other modifications of the embodiments, and the generic principles defined herein may be applied to other embodiments without the use of inventive faculty, as will be readily apparent to those skilled in the art. Therefore, the present invention is not limited to the above embodiments, and those skilled in the art should make improvements and modifications to the present invention based on the disclosure of the present invention within the protection scope of the present invention.
Claims (8)
1. A transient stability improving method for a parallel power supply system of an energy storage MMC-synchronous machine comprises the following steps:
(1) For an energy storage MMC-synchronous machine parallel power supply system, when an alternating current side short circuit fault occurs, acquiring the rotor angular frequency of the synchronous machine and transmitting the rotor angular frequency to the energy storage MMC;
(2) The energy storage MMC calculates the angle difference between the power angle of the synchronous machine and the voltage phase of the grid-connected point according to the angular frequency of the rotor;
(3) Determining an active component instruction value and a reactive component instruction value of the energy storage MMC injected current during the fault period according to the angle difference;
(4) The energy storage MMC is switched from a normal working mode to a current saturation mode, and the injected current of the energy storage MMC during the fault period tracks the instruction value, so that the transient stability of the system is improved.
2. The transient stability improvement method of claim 1, wherein: and (2) acquiring the rotor angular frequency of the synchronous machine by using a Wide Area Measurement System (WAMS) in the step (1).
3. The transient stability improvement method of claim 1, wherein: in the step (2), the angle difference between the synchronous machine power angle and the voltage phase of the grid-connected point is calculated by the following formula;
wherein: theta gs Is the angle difference between the synchronous machine power angle and the grid-connected point voltage phase during the fault period, t f For duration of fault, θ gs0 Is the angle difference between the synchronous machine power angle and the grid-connected point voltage phase under the steady state, omega 0 For rating the angular frequency, omega, of the system g For the rotor angular frequency, omega, of synchronous machines s For the grid-connected point voltage angular frequency, t represents time.
4. The transient stability improvement method of claim 3, wherein: the grid-connected point voltage angular frequency omega s The method is obtained through phase-locked loop detection of the energy storage MMC.
5. The transient stability improvement method of claim 1, wherein: determining an active component instruction value and a reactive component instruction value of the energy storage MMC injected current during the fault period through the following expressions in the step (3);
wherein: i is d * And I q * Respectively an active component instruction value and a reactive component instruction value, I, of the energy storage MMC injection current during the fault max In order to limit the current of the energy storage MMC,
an angle reference is injected for the current.
6. The transient stability improvement method of claim 5, wherein: the current injection angle reference value
Wherein theta is gs Is the angle difference between the synchronous machine power angle and the grid-connected point voltage phase during the fault.
7. The transient stability improvement method of claim 1, wherein: and (4) when the energy storage MMC detects that the voltage amplitude of the grid-connected point falls below a voltage threshold value, switching the normal working mode to a current saturation mode.
8. The transient stability improvement method of claim 1, wherein: in the step (4), the injection current of the energy storage MMC is enabled to track the instruction value during the fault period, that is, the instruction value is used as the reference value of the inner ring current controller of the energy storage MMC, and when the voltage of the grid-connected point is recovered to be higher than the voltage threshold value, the energy storage MMC is switched back to the normal working mode, so that fault ride-through is completed.
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