WO2022041364A1

WO2022041364A1 - Smooth switching method for control policy of voltage-source-type converter

Info

Publication number: WO2022041364A1
Application number: PCT/CN2020/116988
Authority: WO
Inventors: 李周; 李亚洲
Original assignee: 东南大学溧阳研究院; 东南大学
Priority date: 2020-08-26
Filing date: 2020-09-23
Publication date: 2022-03-03
Also published as: CN112086988B; CN112086988A

Abstract

Disclosed is a smooth switching method for a control policy of a voltage-source-type converter. The method comprises: collecting an actual electrical quantity value; confirming the current control policy after a control policy switching signal is received; and a control system performing initial value setting on a proportional integral regulator in a control loop, locking a set value, and modifying a control goal and switching the control policy. By means of the present invention, when a control goal of a voltage-source-converter is changed, a control policy of the voltage-source-type converter can be switched in a rapid and smooth manner, thereby reducing the transient impact, and improving the stability of a power system. By means of the present invention, overcurrent and overvoltage can be prevented in a transient state, a control goal of a converter station can be achieved in a steady state, and rapid regulation can be ensured.

Description

A smooth switching method of voltage source converter control strategy

technical field

The invention belongs to the technical field of power electronics, and relates to a control strategy smooth switching method of a voltage source converter.

Background technique

As an important technology in smart grid, high voltage direct current transmission technology based on voltage source converter (VSC-HVDC) has shown its specific advantages over traditional HVDC. VSC-HVDC can supply power to passive networks, realize large-scale renewable energy grid integration, build multi-terminal DC grids, independently and quickly control active and reactive power, and achieve reverse power supply without voltage polarity reversal. With the increasing demand for new energy sources such as photovoltaics and wind power and passive network power supply, voltage source converters have gradually attracted attention as an important way for passive network power supply and new energy grid connection.

In recent years, in order to achieve high flexibility and controllability of flexible DC grids, the control objectives of voltage source converters have become more complex, and the control strategies have become more diverse. In order to reduce the transient impact of the converter station, and at the same time ensure a relatively fast adjustment speed, in order to reduce the transient impact of the converter station while realizing the tasks such as the start-up process of the converter station, the response when the converter station fails, and the function switching of the converter station, the realization of the power system For the purpose of stability, the traditional method adopts the method of blocking the converter to reduce the transient impact. This method needs to clear the control system integrator and restart the control strategy when unlocking. The whole process increases the adjustment time and cannot meet the high flexibility and controllability requirements of flexible DC transmission.

SUMMARY OF THE INVENTION

Aiming at the deficiencies of the prior art, the present invention provides a method for smoothly switching the control strategy of the voltage source converter, which can ensure that the current control strategy is smoothly switched to the target control when the control target of the voltage source converter changes. The strategy makes it unnecessary to block the converter when the control strategy is switched, and the whole process is fast and smooth, which can reduce the transient impact and improve the stability of the power system.

In order to solve the above-mentioned technical problems, the present invention provides the following technical solutions:

A method for smooth switching of a control strategy of a voltage source converter, comprising the following steps:

(1) The control system collects the actual electrical value;

(2) The control system receives the control strategy switching signal;

(3) The control system confirms the current control strategy;

(4) The control system adjusts the initial value of the proportional-integral regulator in the control loop, locks the setting value, modifies the control target and switches the control strategy at the same time.

Further, the actual electrical quantities include the following values: three-phase AC voltage, AC current, active power and reactive power data collected from the common coupling point of the voltage source converter, and phase data collected by using a phase-locked loop. , the DC voltage data collected from the DC grid connection point.

Further, the control strategy includes a power and reactive power decoupling type control strategy and an active and reactive power non-decoupling type control strategy.

Further, when the target control strategy to be switched in the step (2) is an active and reactive power decoupling control strategy, the initial value setting of the proportional-integral regulator in the step (4) includes the following steps:

1) Set the electrical quantity reference value of the control strategy to be achieved as the current actual electrical quantity value, and set the initial value of the outer loop integrator to the actual electrical quantity value;

2) Set the initial value of the inner loop proportional integrator that generates the modulating wave voltage in the control loop. The setting formula is as follows:

Among them, U _c and U _S are the rms voltage at the outlet of the AC side of the converter station and the rms value of the AC voltage at the common connection point, v _dreset and v _qreset are the integrator setting values in d-axis control and the integral in q-axis control are the active and reactive powers injected at the common connection point, _i _sd and _i _sq are the dq-axis components of the AC side current of the converter station, X _c is the commutation reactance of the converter station, R _c is the equivalent resistance of the converter station.

Further, the actual electrical quantity value includes at least: active power, reactive power and DC voltage reference value before control switching.

Further, the active and reactive power decoupling control strategies include: a constant active and reactive power control strategy, a constant DC voltage and reactive power control strategy, a constant active power AC voltage control strategy, a constant DC voltage and AC voltage control strategy, and a DC voltage slope control strategy. .

Further, when the target control strategy to be switched in the step (2) is an active and reactive non-decoupling control strategy, the initial value setting of the proportional-integral regulator in the step (4) includes the following steps:

1) Set the electrical quantity reference value of the control strategy to be achieved as the current actual electrical quantity value, and set the initial value of the proportional integrator in the control loop as the actual electrical quantity value;

2) In the control loop, the phase of the AC side voltage is tracked, the phase is maintained during switching, and the initial phase angle of the amplitude-phase control after switching is set.

Further, the actual electrical value includes: the AC voltage amplitude before switching.

Further, the active and reactive power non-decoupling control strategy includes an amplitude-phase control strategy.

With respect to the prior art, the beneficial effects of the present invention are as follows:

1. The design of the scheme of the present invention is ingenious and reasonable. This scheme realizes that when the control strategy of the voltage source converter is switched, it is not necessary to block the converter. By setting the initial value of the proportional integrator in the control system and the reference value of the control strategy, the The fast and smooth switching process reduces the transient impact and ensures the smooth operation of the system.

2. The method of the present invention can prevent overcurrent and overvoltage in a transient state, and can achieve the control objective of the converter station in a steady state, while ensuring the rapidity of regulation.

3. The method of the present invention improves the stability of the voltage source type converter, improves the stability of the power system connected with the voltage source type converter, and can be widely used in the field of power electronics, such as the start-up process of the converter station, Response in the event of a converter station failure, and functional switching of the converter station, etc.

Description of drawings

1 is a schematic diagram of a method for smooth switching of a control strategy of a voltage source converter provided by the present invention;

Fig. 2 is the control strategy frame diagram used by VSC (voltage source converter) in the present invention, wherein (a) is the active and reactive power decoupling type control frame of VSC, (b) is the amplitude and phase control frame of VSC;

Fig. 3 is to verify the double-ended VSC system simulation model diagram of the present invention;

Fig. 4 is the dynamic simulation result diagram of the present invention switched to amplitude-phase control in the double-terminal VSC system model, wherein (a) is the PCC line voltage (effective value) and A-phase current (real-time value) of VSC1, (b) is PCC active power and reactive power of VSC1, (c) is the DC voltage and DC current of VSC1.

Description of reference numbers:

U _dc - DC voltage measurement value at PCC point; U _dcref - DC voltage reference value at PCC point;

u _ac — AC voltage measurement value at PCC point; u _acref — AC voltage reference value at PCC point;

P _s —active power measurement value at PCC point; P _sref —active power reference value at PCC point;

Q _s —measured value of reactive power at PCC point; Q _sref —reference value of reactive power at PCC point.

detailed description

In order to deepen the understanding and understanding of the present invention, the present invention will be further described and introduced below with reference to the accompanying drawings.

Example 1:

Referring to FIG. 1 and FIG. 2 , the voltage source converter control strategy referred to in the present invention includes power and reactive power decoupling type control and active and reactive power non-decoupling type control. Active and reactive power decoupling control includes but is not limited to constant active and reactive power control, constant DC voltage and reactive power control, constant active power AC voltage control, constant DC voltage AC voltage control, DC voltage slope control, etc. Active and reactive power non-decoupling control includes but is not limited to amplitude and phase control. The method for smooth switching of control strategies provided by the present invention is applicable to switching of these types of control strategies.

Specifically, the method for smooth switching of the control strategy of the voltage source converter provided by the present invention includes the following steps:

(1) The control system collects the actual electrical values, specifically: collect the three-phase AC voltage, AC current, active power and reactive power data from the AC common coupling point of the voltage source converter, and use the phase-locked loop to collect the phase Data, collect DC voltage data from the DC grid connection point. The control system is the existing control system of the voltage source converter, and the control system collects the actual electrical value at all times.

(2) The control system receives the control strategy switching signal; this step can ensure that the current control strategy switching process of the system is the switching process of the system operation, such as switching from the amplitude and phase control to the constant active and reactive power control strategy, which can prevent erroneous control Switching of strategies.

(3) The control system confirms the current control strategy;

(4) The control system sets the initial value of the proportional-integral regulator in the control loop, then locks the set value, modifies the control objective, and switches the control strategy.

The following example describes the initial value tuning of the proportional-integral regulator in detail:

Figure 2(a) is a block diagram of active and reactive power decoupling control. Different control objectives can be achieved by setting control parameters, including constant active and reactive power control, constant DC voltage reactive power control and DC voltage slope control and a series of An improved control strategy based on this extension. In the present invention, when the target control strategy after switching is active and reactive power decoupling control, in order to ensure smoothness and rapidity before and after switching control, it is necessary to maintain constant quantities during switching control, including control target reference value, outer loop and inner ring

The integral value of the proportional integral link. The initial value setting method of the proportional integral regulator is as follows:

1) Set the electrical quantity reference value of the control strategy to be achieved as the current actual electrical quantity value, such as the active power, reactive power and DC voltage reference value before control switching, etc. The above control strategy has an outer loop proportional integrator, The initial value of the outer loop integrator needs to be set correspondingly to the actual electrical value; the actual electrical value is collected in real time.

Among them, U _c and U _S are the rms voltage at the outlet of the AC side of the converter station and the rms value of the AC voltage at the common connection point, v _dreset and v _qreset are the integrator setting values in d-axis control and the integral in q-axis control device setting value. _Ps and _Qs are the active and reactive power injected at the point of common connection (PCC point). i _sd and i _sq are the dq-axis components of the AC side current of the converter station, X _c is the commutation reactance of the converter station, and R _c is the equivalent resistance of the converter station.

Referring to Fig. 2(b), it is a block diagram of a typical amplitude-phase control. The amplitude-phase control adopts indirect current control, and the active and reactive powers are coupled with each other. In the present invention, when the target control strategy after switching is amplitude and phase control, in order to ensure smoothness and rapidity before and after switching control, the amount that needs to remain unchanged during switching control is mainly AC voltage amplitude and phase. The initial value setting method of the proportional-integral regulator is as follows:

1) Set the electrical quantity reference value of the control strategy to be achieved as the current actual electrical quantity value. The initial value of the proportional integrator in the control loop is set to the actual electrical value, that is, the AC voltage amplitude before switching, and the actual electrical value is collected in real time.

2) In the control loop, the phase tracking of the AC side voltage is carried out, the phase is maintained during switching, and the initial phase angle of the amplitude-phase control after switching is set to the phase angle before switching, and the set phase angle and The above-mentioned AC voltage amplitude can output the corresponding modulated wave.

Application example:

The present invention will be further described in detail below in conjunction with specific embodiments, which are to illustrate the present invention rather than limit the present invention to the following examples.

Referring to Figure 3, it is a double-terminal seven-level voltage source converter system, the AC bus voltage is 500kV, the MMC AC voltage is 255kV, the DC voltage is ±250kV, the sub-module capacitance is 1000uF, and the equivalent generator output is 700MW active power, The equivalent motor consumes 100MW of active power, and the DC grid undertakes 600MW of output. At 6s, a power switching control signal is generated, and the constant active and reactive power control is switched to the amplitude and phase control, and the smooth switching method provided by the present invention is adopted. According to the model, PSCAD simulation is carried out to obtain the dynamic characteristics of the voltage source converter when the control strategy is switched.

Referring to Figure 4, after the 6s control strategy is switched, the original constant active and reactive power control is switched to the amplitude-phase control, the VSC DC side voltage oscillates slightly from the original 502kV, the oscillation amplitude is about ±3kV, and the DC current is hardly affected. . The rms value of the PCC voltage on the AC side of the VSC produces a small oscillation of ±2kV from 501kV, the AC current produces a small oscillation of ±0.15kA from 1kA, and the active power produces an oscillation of ±40MW from 600MW. The reactive power support changes from 0 to -10MVar.

In the above dynamic process, due to the small reactive power support deviation in the amplitude and phase control after switching, a small oscillation occurs. The whole oscillation process lasts for about 1 s, and the oscillation amplitude of each electrical quantity is small, which shows that the present invention has good rapidity. The characteristics of smoothness and smoothness improve the stability of the voltage source converter and the power system connected to the voltage source converter.

It should be noted that the above-mentioned embodiments are only preferred embodiments of the present invention, and are not used to limit the protection scope of the present invention. Equivalent replacements or substitutions made on the basis of the above solutions all belong to the protection scope of the present invention. .

Claims

A method for smooth switching of a control strategy of a voltage source converter, characterized in that it includes the following steps:

(1) The control system collects the actual electrical value;

(2) The control system receives the control strategy switching signal;

(3) The control system confirms the current control strategy;

(4) The control system adjusts the initial value of the proportional-integral regulator in the control loop, locks the setting value, modifies the control target and switches the control strategy at the same time.
The method for smooth switching of a control strategy of a voltage source converter according to claim 1, wherein the actual electrical value includes the following values: a three-phase AC voltage collected from a common coupling point of the voltage source converter , AC current, active power and reactive power data, the phase data collected by the phase-locked loop, and the DC voltage data collected from the DC grid connection point.
The method for smooth switching of a control strategy of a voltage source converter according to claim 1, wherein the control strategy includes a control strategy of decoupling of power and reactive power and a control strategy of non-decoupling of active and reactive power.
The method for smoothly switching control strategies of a voltage source converter according to claim 1 or 3, wherein when the target control strategy to be switched in the step (2) is an active and reactive power decoupling control strategy, The initial value setting of the proportional-integral regulator in the step (4) includes the following steps:

1) Set the electrical quantity reference value of the control strategy to be achieved as the current actual electrical quantity value, and set the initial value of the outer loop integrator to the actual electrical quantity value;

2) Set the initial value of the inner loop proportional integrator that generates the modulating wave voltage in the control loop. The setting formula is as follows:

Among them, U c and U S are the rms voltage at the outlet of the AC side of the converter station and the rms value of the AC voltage at the common connection point, v dreset and v qreset are the integrator setting values in d-axis control and the integral in q-axis control are the active and reactive powers injected at the common connection point, i sd and i sq are the dq-axis components of the AC side current of the converter station, X c is the commutation reactance of the converter station, R c is the equivalent resistance of the converter station.
The method for smooth switching of a control strategy of a voltage source converter according to claim 4, wherein the actual electrical values at least include: active power, reactive power and DC voltage reference values before control switching.
The method for smoothly switching control strategies of a voltage source converter according to claim 3, wherein the active and reactive power decoupling control strategies include: a constant active and reactive power control strategy, a constant DC voltage and reactive power control strategy, Constant active AC voltage control strategy, constant DC voltage AC voltage control strategy, DC voltage slope control strategy.
The method for smooth switching of a control strategy of a voltage source converter according to claim 1 or 3, characterized in that, when the target control strategy to be switched in the step (2) is a control strategy of active and reactive power without decoupling , the initial value setting of the proportional-integral regulator in the step (4) includes the following steps:

1) Set the electrical quantity reference value of the control strategy to be achieved as the current actual electrical quantity value, and set the initial value of the proportional integrator in the control loop as the actual electrical quantity value;

2) In the control loop, the phase of the AC side voltage is tracked, the phase is maintained during switching, and the initial phase angle of the amplitude-phase control after switching is set.
The method for smooth switching of a control strategy of a voltage source converter according to claim 7, wherein the actual electrical value comprises: the AC voltage amplitude before switching.
The method for smooth switching of a control strategy of a voltage source converter according to claim 3, wherein the active and reactive non-decoupling control strategy includes an amplitude-phase control strategy.