CN107134800B - Bipolar VSC passive control method and device for a direct current transmission system - Google Patents

Abstract

The invention relates to a bipolar VSC passive control method and device of a direct current transmission system. After one pole VSC on the inverter side is controlled to be unlocked, it is controlled by a constant AC voltage, and then the other pole VSC on the inverter side is controlled. Phase-lock the output phase of one of the VSCs above, and control the other VSC to unlock after phase-locking. The invention can realize the stable unlocking of the two-pole VSCs, keep the voltage phases of the two-pole VSCs synchronously after unlocking, and supply stable power to the same passive network at the same time.

Description

Bipolar VSC passive control method and device for a direct current transmission system

technical field

The invention belongs to the technical field of direct current transmission, and in particular relates to a bipolar VSC passive control method and device for a direct current transmission system.

Background technique

Line Commutated Converter Based High Voltage Direct Current (LCC-HVDC) has been widely used in large-capacity long-distance power transmission and back-to-back interconnection of asynchronous grids. Failure, inability to supply power to passive systems, and the need to consume a large amount of reactive power during operation; while the voltage source converter based high voltage direct current transmission system (Voltage Source Converter Based High Voltage Direct Current, VSC-HVDC) has the advantages of independent control of active power and reactive power, no commutation failure, and power supply to passive systems, but it has higher construction costs and smaller transmission capacity than LCC. Among the many VSC-HVDC transmission topologies, the Modular Multilevel Converter Based High Voltage Direct Current (MMC-HVDC) has all the advantages of VSC-HVDC, but due to its high cost and inability to Effectively deal with shortcomings such as DC faults, making it unsuitable for long-distance, high-power transmission occasions.

In order to combine the advantages of LCC-HVDC and VSC-HVDC, the two DC transmission systems, a hybrid DC transmission system came into being. The rectification side of the hybrid DC transmission system uses LCC, and the inverter side uses VSC, which fully utilizes the advantages of large capacity and long distance of the LCC DC transmission system, and VSC does not have commutation failure and can supply power to the passive AC system. Prospects.

Generally, in a hybrid DC power transmission system, the VSC terminal at the inverter side can work in two operating modes: active mode and passive mode. In the active mode, the VSC terminal module performs active charging through the AC grid connected to it; in the passive mode, the VSC terminal module performs passive charging through the DC line connected to it. After the charging is completed, the VSC is unlocked according to the corresponding control mode in the two modes to realize the power transmission of the DC transmission system. In the prior art, a paper titled "LCC-MMC Hybrid HVDC Transmission System" was published in the journal "Acta Electrotechnical Society" in Volume 28, Issue 10 in October 2013, which discloses a double-ended unipolar DC transmission system. The control method of the power transmission system uses the LCC to provide a stable DC voltage to complete the capacitor charging of each sub-module of the MMC and the unlocking of the IGBT.

In the DC system, when the bipolar VSC on the inverter side is connected to the same passive network, only the DC voltage provided by the rectifier side is used to charge and unlock the bipolar VSC, and there will be a voltage phase deviation after the bipolar VSC is unlocked. When the voltage phase deviation of the VSC is large, it will cause a certain impact on the passive network, and when the impact is severe, the DC transmission system will not work stably. In addition, when the DC voltage provided by the rectification side is used to charge and unlock the two-pole VSC, there is also a problem of power coordination control of the two-pole VSC.

Contents of the invention

The object of the present invention is to provide a bipolar VSC passive control method and device for a direct current transmission system, which is used to solve the problem of unlocking control of a direct current system with a bipolar VSC connected to the same passive network.

In order to solve the above technical problems, the present invention proposes a bipolar VSC passive control method of a direct current transmission system, including the following method schemes:

Method scheme one includes the following steps:

1) The first pole VSC and the second pole VSC on the inverter side are passively charged by the DC voltage provided by the rectification side;

2) After the first pole VSC on the control inverter side is unlocked, a constant AC voltage control is applied to the first pole VSC to obtain a three-phase modulation reference voltage of the first pole VSC, thereby establishing an AC voltage on the grid side; The second pole VSC phase-locks the phase of the grid-side AC voltage established by the first pole VSC, and controls the second pole VSC to unlock after phase locking.

Method scheme two, on the basis of method scheme one, the constant AC voltage control is as follows: correspondingly process the difference between the grid-side AC voltage reference value and the d-axis component of the grid-side voltage to obtain the d-axis reference value of the first pole modulation voltage value;

Corresponding processing is performed on the difference between 0 and the q-axis component of the grid-side voltage to obtain the q-axis reference value of the first pole modulation voltage, and the d-axis reference value of the first pole modulation voltage and the q-axis reference value of the first pole modulation voltage are calculated Parker transforms inversely to obtain the three-phase modulation reference voltage value of the first pole VSC.

Method scheme three, on the basis of method scheme two, the d-axis reference value of the first pole modulation voltage is calculated by the following formula:

In the formula, U _{d1_ref} is the d-axis reference value of the first pole modulation voltage, K _p is the proportional coefficient, is the per-unit value of the grid-side AC voltage reference value U _{ac_ref} , U _{ac_d} is the d-axis component of the grid-side voltage, and T _i is the integral time constant;

The q-axis reference value U _{q1_ref} of the first pole modulation voltage is calculated by the following formula:

In the formula, U _{q1_ref} is the q-axis reference value of the first pole modulation voltage, K _p is the proportional coefficient, U _{ac_q} is the q-axis component of the grid side voltage, and T _i is the integral time constant.

Method scheme four, on the basis of method scheme one, after the second pole VSC is unlocked, a dual-loop control is adopted in which the outer loop is a power loop and the inner loop is a current loop.

Method scheme five, on the basis of method scheme four, the dual-loop control in which the outer loop is a power loop and the inner loop is a current loop is specifically: perform PI control on the difference between the active power reference value and the active power measured value, and obtain the inner loop Current d-axis reference value, PI control is performed on the difference between the reactive power reference value and the measured value of reactive power to obtain the inner loop current q-axis reference value, the inner loop current d-axis reference value and the inner loop current q-axis The reference value passes through the inner loop current controller to obtain the d-axis reference value of the second pole modulation voltage and the q-axis reference value of the second pole modulation voltage respectively, and performs inverse Parker transformation to obtain the three-phase modulation voltage reference value of the second pole VSC.

In order to solve the above problems, the present invention also proposes a bipolar VSC passive control device for a direct current transmission system, including the following device solutions:

Device solution one, including a charging unit and an unlocking control unit, wherein:

The charging unit is used to passively charge the DC voltage provided by the first pole VSC and the second pole VSC on the inverter side through the rectification side;

The unlocking control unit is used to control the first pole VSC on the inverter side to be unlocked, and to control the first pole VSC with a constant AC voltage to obtain a three-phase modulation reference voltage of the first pole VSC, thereby establishing a grid-side AC voltage; The second pole VSC on the inverter side phase-locks the phase of the grid-side AC voltage established by the first pole VSC, and controls the second pole VSC to unlock after phase locking.

The device scheme 2, on the basis of the device scheme 1, also includes corresponding processing for the difference between the grid-side AC voltage reference value and the d-axis component of the grid-side voltage to obtain the d-axis reference value of the first pole modulation voltage; Perform corresponding processing on the difference with the q-axis component of the grid-side voltage to obtain the q-axis reference value of the first pole modulation voltage, and perform Parker inversion on the d-axis reference value of the first pole modulation voltage and the q-axis reference value of the first pole modulation voltage Transform to obtain the unit of the three-phase modulation reference voltage value of the first pole VSC.

The device scheme three, on the basis of the device scheme two, also includes a calculation unit: the d-axis reference value for the first pole modulation voltage is calculated by the following formula:

In the formula, U _{d1_ref} is the d-axis reference value of the first pole modulation voltage, K _p is the proportional coefficient, is the per-unit value of the grid-side AC voltage reference value U _{ac_ref} , U _{ac_d} is the d-axis component of the grid-side voltage, and T _i is the integral time constant;

The q-axis reference value U _{q1_ref} of the first pole modulation voltage is calculated by the following formula:

In the formula, U _{q1_ref} is the q-axis reference value of the first pole modulation voltage, K _p is the proportional coefficient, U _{ac_q} is the q-axis component of the grid side voltage, and T _i is the integral time constant.

Device solution 4, on the basis of device solution 1, further includes a unit for dual-loop control in which the outer loop is a power loop and the inner loop is a current loop after unlocking the second pole VSC.

Device plan five, on the basis of device plan four, also includes PI control for the difference between the active power reference value and the measured value of active power to obtain the d-axis reference value of the inner loop current, and the reactive power reference value and reactive power The difference between the measured power values is controlled by PI to obtain the q-axis reference value of the inner loop current, and the d-axis reference value of the inner loop current and the q-axis reference value of the inner loop current pass through the inner loop current controller to obtain the second pole modulation respectively The d-axis reference value of the voltage and the q-axis reference value of the second pole modulation voltage are subjected to inverse Parker transformation to obtain the unit of the three-phase modulation voltage reference value of the second pole VSC.

The beneficial effects of the present invention are: after one pole VSC on the inverter side is controlled to be unlocked, it is controlled by a constant AC voltage, and then the other pole VSC on the inverter side is controlled to phase-lock the output phase of one pole VSC , control the other pole VSC to unlock after phase locking. The invention can realize the stable unlocking of the two-pole VSCs, keep the voltage phases of the two-pole VSCs synchronously after unlocking, and supply stable power to the same passive network at the same time.

Description of drawings

Figure 1 is a topology diagram of a bipolar passive hybrid DC transmission system;

Figure 2 is a topological structure diagram of one pole VSC in a bipolar VSC;

Fig. 3 is a schematic block diagram of a pole 1 controller of a bipolar VSC;

Figure 4 is a block diagram of a pole 2 controller for a bipolar VSC.

Detailed ways

The specific embodiments of the present invention will be further described below in conjunction with the accompanying drawings.

An embodiment of a bipolar VSC passive control method for a direct current transmission system of the present invention:

In the bipolar passive hybrid DC power transmission system shown in Figure 1, the rectifier side uses LCC to connect to the AC power grid, and the inverter side uses VSC to connect to the passive network. There is a smoothing reactor in series on the side, and a diode in series on the DC side of the VSC end. The VSC end adopts an MMC structure. The converter is composed of three-phase 6 bridge arms, and each bridge arm is composed of bridge arm inductance and the same number of sub-modules. As shown in Figure 2, the sub-module adopts a half-bridge structure, including 2 IGBTs, 2 anti-parallel diodes and 1 energy storage capacitor.

For the hybrid DC system in Figure 1, control the unlocking of the bipolar LCC terminal on the rectifier side, and use the stable DC voltage provided by the bipolar LCC to provide pole 1 and pole 2 on the inverter side (the first unlocked pole of the bipolar VSC VSC is called pole 1, and the last unlocked pole VSC is called pole 2) The capacitor in the sub-module is charged. After the charging is completed, unlock VSC terminal 1. After pole 1 is unlocked, a constant AC voltage control is used to establish the VSC terminal AC. side three-phase voltage. Since VSC terminal 1 and pole 2 are connected in the same passive network, after VSC terminal 1 is unlocked at this time, there is already an AC voltage on its AC side, and the control pole 2 phase-locks the voltage phase output by pole 1, and the The voltage phase of pole 2 is synchronized with the voltage phase of pole 1, and then the pole 2 is unlocked by means of constant active power control and constant reactive power control. After the VSC terminal 2 is unlocked, the power distribution of the two poles can be completed through the active power and reactive power output by the control pole 2, and the bipolar coordinated control of active power and reactive power can be realized.

When the VSC terminal is passively charged through the DC side, the unlocking of the VSC terminal 1 is controlled by a constant AC voltage. The control process is:

As shown in Figure 3, the difference between the per unit value of the grid-side AC voltage reference value U _{ac_ref} and the d-axis component U _{ac_d} of the grid-side voltage is controlled to obtain the d-axis reference value U _{d1_ref} of the first pole modulation voltage. Control the difference of the side voltage q-axis component U _{ac_q} to obtain the q-axis reference value U _{q1_ref} of the first pole modulation voltage, and then perform inverse Parker transformation to obtain the three-phase modulation voltage reference values U _{a1_ref} , U _{b1_ref} , U _{c1_ref} of pole 1. Among them, U _{ac_ref} increases from 0 with a certain slope, so that the established grid-side AC voltage rises steadily.

When the VSC terminal 1 is unlocked, since the two poles of the VSC terminal are connected in the same passive network, there is already an AC voltage on the AC side of the VSC terminal 2 at this time, and the voltage phase of the control terminal 2 is locked to the phase of the terminal 1. Active power control and constant reactive power control methods unlock pole 2, and the control process of pole 2 is:

As shown in Figure 4, PI control is performed on the difference between the active power reference value P _{ac_ref} and the measured active power value P _{ac_meas} to obtain the inner loop current d-axis reference value I _{d_ref} , and the reactive power reference value Q _{ac_ref} and the measured reactive power value The difference of the value Q _{ac_meas} is controlled by PI to obtain the reference value of the inner loop current q-axis I _{q_ref} , and the second-pole modulation voltage d-axis reference value U _{d2_ref} and the second-pole modulation voltage q-axis reference value U are respectively obtained through the inner loop current controller _{q2_ref} , the three-phase modulation voltage reference values U _{a2_ref} , U _{b2_ref} , and U _{c2_ref} of pole 2 are obtained by inverse Parker transformation. Among them, P _{ac_ref} and Q _{ac_ref} increase or decrease from 0 with a certain rate of increase or decrease, so that the output active power and reactive power change smoothly.

Then, by changing the P _{ac_ref} and Q _{ac_ref} of the VSC terminal 2 and the corresponding reference value of the lifting rate to complete the power distribution of the two poles, and realize the bipolar coordinated control of active power and reactive power. During operation, if pole 2 fails, pole 2 will stop operating, and pole 1 will continue to operate; if pole 1 fails, pole 1 will stop operating, and at the same time, the control strategy of pole 2 will switch to constant AC voltage control and continue to operate.

In the present invention, the DC voltage is established after unlocking the LCC terminal on the rectification side, passively charges the sub-module at the VSC terminal on the inverter side through a DC line, and then unlocks the VSC terminal 1, thereby establishing a three-phase voltage on the AC side of the VSC terminal, and charging the VSC terminal at the VSC terminal The AC side of pole 2 generates an AC voltage. Finally, after pole 2 is phase-locked to the voltage phase of pole 1, pole 2 is unlocked by means of constant active power and constant reactive power control. After the VSC terminal 2 is unlocked, the power distribution of the two poles can be completed through the active power and reactive power output by the control pole 2, and the bipolar coordinated control of active power and reactive power can be realized. The invention has simple logic, is easy to implement in engineering, can realize stable unlocking of two-pole VSCs, and can supply stable power to the same passive network at the same time.

The present invention is used to solve the unlocking control problem of connecting a DC system with a bipolar VSC to the same passive network. Therefore, the control method of the present invention is not only applicable to the bipolar passive hybrid DC transmission system shown in Figure 1, namely The converter on the rectification side is not limited to LCC, and can also be a bipolar VSC or other types of converters, as long as the converter on the rectification side can provide a stable DC voltage for the inverter side.

The method of unlocking the bipolar LCC in this embodiment can be set according to the needs, either unlocking the bipolar LCC at the same time to charge the capacitors of VSC terminal 1 and pole 2, and then unlocking pole 1 and pole 2 after charging is completed; or using Unlock the LCC of pole 1 to terminal first, and then unlock the LCC of pole 2 to terminal, and then unlock pole 1 and pole 2; it can also be unlocked separately at the two stages of rectifier side and inverter side, such as unlocking pole 1 to terminal After LCC, unlock pole 1 on the VSC terminal, then unlock the LCC on the opposite end of pole 2, and finally unlock pole 2 on the VSC terminal.

An embodiment of a bipolar VSC passive control device for a direct current transmission system of the present invention:

It includes a charging unit and an unlocking control unit, wherein the charging unit is used to passively charge the first pole VSC and the second pole VSC on the inverter side through the DC voltage provided by the rectification side; the unlocking control unit is used to control the inverter side After the first pole VSC is unlocked, control the first pole VSC with a constant AC voltage to obtain the three-phase modulation reference voltage of the first pole VSC; control the output of the second pole VSC on the inverter side to the first pole VSC The phases of the three-phase AC voltage are phase-locked, and the second pole VSC is controlled to be unlocked after phase-locking.

The bipolar VSC passive control device of the DC transmission system referred to in the above embodiments is actually a computer solution based on the method flow of the present invention, that is, a software framework, which can be applied to the converter station. The above-mentioned device That is, the processing process corresponding to the method flow. Since the introduction of the above method is clear and complete enough, and the device claimed in this embodiment is actually a software framework, no detailed description is given here.

Claims (8)

1. A method of passive control of a bipolar VSC of a direct current transmission system, comprising the steps of:

1) the first pole VSC and the second pole VSC on the inverting side are passively charged through the direct-current voltage provided by the rectifying side;

2) after a first pole VSC on the control inversion side is unlocked, constant alternating voltage control is carried out on the first pole VSC to obtain three-phase modulation reference voltage of the first pole VSC, and therefore network side alternating voltage is established; controlling a second-pole VSC on an inversion side to carry out phase locking on the phase of the network-side alternating-current voltage established by the first-pole VSC, and controlling the second-pole VSC to unlock after the phase locking;

the constant alternating voltage control comprises the following steps: correspondingly processing the difference between the grid-side alternating voltage reference value and the grid-side voltage d-axis component to obtain a first polar modulation voltage d-axis reference value;

and correspondingly processing the difference between 0 and the q-axis component of the network side voltage to obtain a q-axis reference value of the first pole modulation voltage, and performing park inverse transformation on the d-axis reference value of the first pole modulation voltage and the q-axis reference value of the first pole modulation voltage to obtain a three-phase modulation reference voltage value of the first pole VSC.

2. A method of passive control of a bipolar VSC of a direct current transmission system according to claim 1, characterised in that the first pole modulation voltage d-axis reference value is calculated by:

in the formula of U_{d1_ref}Is a d-axis reference value of the first-pole modulation voltage, K_pIs a coefficient of proportionality that is,is a network side AC voltage reference value U_{ac_ref}Per unit value of U_{ac_d}Is the d-axis component of the net-side voltage, T_iIs an integration time constant;

the first pole modulation voltage q-axis reference value U_{q1_ref}Calculated by the following formula:

in the formula of U_{q1_ref}For the first pole modulating the q-axis reference value, K_pIs a proportionality coefficient, U_{ac_q}Is the q-axis component of the net side voltage, T_iIs the integration time constant.

3. A method of passive control of a bipolar VSC of a dc transmission system according to claim 1, characterised in that the unlocking of the second VSC takes a double loop control with an outer loop being a power loop and an inner loop being a current loop.

4. A method of bipolar VSC passive control of a dc transmission system according to claim 3, wherein the dual loop control with the outer loop being a power loop and the inner loop being a current loop is specifically: and PI control is carried out on the difference between the active power reference value and the active power measured value to obtain an inner ring current d-axis reference value, PI control is carried out on the difference between the reactive power reference value and the reactive power measured value to obtain an inner ring current q-axis reference value, the inner ring current d-axis reference value and the inner ring current q-axis reference value pass through an inner ring current controller to respectively obtain a second polar modulation voltage d-axis reference value and a second polar modulation voltage q-axis reference value, and inverse park transformation is carried out to obtain a three-phase modulation voltage reference value of the second polar modulation voltage.

5. A bipolar VSC passive control device for a direct current transmission system, comprising:

a charging unit: the passive charging device is used for passively charging a first pole VSC and a second pole VSC on the inversion side through direct-current voltage provided by the rectification side;

an unlocking control unit: after the first pole VSC used for controlling the inversion side is unlocked, the first pole VSC is controlled by constant alternating voltage to obtain three-phase modulation reference voltage of the first pole VSC, and therefore network side alternating voltage is established; controlling a second-pole VSC on an inversion side to carry out phase locking on the phase of the network-side alternating-current voltage established by the first-pole VSC, and controlling the second-pole VSC to unlock after the phase locking;

the device also comprises a step of carrying out corresponding processing on the difference between the grid-side alternating voltage reference value and the grid-side voltage d-axis component to obtain a first polar modulation voltage d-axis reference value;

and correspondingly processing the difference between 0 and the q-axis component of the network side voltage to obtain a q-axis reference value of the first pole modulation voltage, and performing park inverse transformation on the d-axis reference value of the first pole modulation voltage and the q-axis reference value of the first pole modulation voltage to obtain a three-phase modulation reference voltage value unit of the first pole VSC.

6. A bipolar VSC passive control device of a direct current transmission system according to claim 5, characterized by further comprising a calculation unit: the d-axis reference value for the first pole modulation voltage is calculated by:

in the formula of U_{d1_ref}Is a d-axis reference value of the first-pole modulation voltage, K_pIs a coefficient of proportionality that is,is a network side AC voltage reference value U_{ac_ref}Per unit value of U_{ac_d}Is the d-axis component of the net-side voltage, T_iIs an integration time constant;

the first pole modulation voltage q-axis reference value U_{q1_ref}Calculated by the following formula:

in the formula of U_{q1_ref}For the first pole modulating the q-axis reference value, K_pIs a proportionality coefficient, U_{ac_q}Is the q-axis component of the net side voltage, T_iIs the integration time constant.

7. A bipolar VSC passive control arrangement of a direct current transmission system according to claim 5, characterized by means for dual loop control with an outer loop being a power loop and an inner loop being a current loop after unlocking the second pole VSC.

8. A bipolar VSC passive control device of a DC power transmission system according to claim 7, characterized by further comprising means for PI controlling the difference between the active power reference value and the actual measured value of active power to obtain an inner loop current d-axis reference value, PI controlling the difference between the reference value of reactive power and the actual measured value of reactive power to obtain an inner loop current q-axis reference value, passing the inner loop current d-axis reference value and the inner loop current q-axis reference value through an inner loop current controller to obtain a second modulated voltage d-axis reference value and a second modulated voltage q-axis reference value, respectively, and performing inverse park transformation to obtain a three-phase modulated voltage reference value of the second VSC.