CN104242333A - Self-excited starting method for modular multilevel inverter flexible direct current transmission system - Google Patents

Abstract

The invention discloses a self-excited starting method for a flexible direct current transmission system of a modular multilevel converter, comprising the following steps: 1. closing a direct current isolating switch; 2. closing a circuit breaker on the side of the alternating current system of a second converter station; 3. . Close the current-limiting resistor bypass switches of the first converter station and the second converter station; 4. Set the initial reference value of the DC voltage controller of the first converter station to the current DC voltage value, and the reactive power reference value to be 0, and unlock the first converter station; 5. Set the DC voltage reference value of the converter at the first converter station; 6. When the AC side current and DC voltage of the first converter station become stable, unlock the second converter station. 7. Set the constant DC voltage control reference value of the first converter station as the rated value; 8. Set the constant active power of the second converter station, the constant reactive power of the first converter station and the second converter station The power control reference value is the value required by the system. It has the advantages of reducing the inrush current generated by the AC and DC systems during the charging process.

Description

Self-excited starting method of modular multilevel converter flexible direct current transmission system

technical field

The invention relates to a flexible direct current transmission technology of a power system, in particular to a self-excited starting method of a modular multilevel converter flexible direct current transmission system.

Background technique

Modular Multilevel Converter (MMC) is a new type of voltage source inverter. MMC adopts a strict modular design. Its core unit—Sub Module (SM), It is a half-bridge structure composed of two IGBTs with anti-parallel diodes and a DC capacitor. The topology is shown in Figure 1. The converter is composed of three phase units (Phase Unite), and each phase unit contains up and down symmetrical The converter leg (Converter Leg), and each converter leg is composed of N (N is the total number of modules of the bridge arm) sub-modules and a bridge arm reactor connected in series.

Before the converter station is put into operation, it is necessary to precharge the capacitors of its sub-modules and ensure that its DC side voltage reaches the rated value. When the converter is normally unlocked, it is necessary to ensure that the difference in the equivalent DC voltage generated by the superposition of the conduction sub-modules in the phase units of the converter stations on both sides is small, so as to avoid causing a large DC impact current. On the other hand, when the inverter voltage at the AC outlet of the converter station is greatly different from the AC voltage at the system valve side when unlocking, it will also cause an excessive current impact on the AC system, and in severe cases, the switching device may be damaged. Therefore, it is necessary to adopt appropriate strategies to prevent the generation of inrush current.

In order to complete the normal start-up of the flexible DC transmission system of the modular multilevel converter and ensure the stable and fast charging of the capacitors of each sub-module of the converter, the self-excited charging method is generally preferred. Xu Zheng, Tu Qingrui and others' "Three-phase Modular Multilevel Converter Starting Method Without Auxiliary DC Power Supply" (Application Publication No.: CN101795057A) proposed a three-phase A self-excited charging method for a phase-modular multilevel converter (method 1), which uses the AC system line voltage as a charging power source, and controls the charging and discharging states of each bridge arm sub-module by detecting the flow direction of the bridge arm current. The specific process is: lead the AC voltage to the inverter through the current-limiting resistor, block the trigger pulse of all sub-module IGBTs, detect the current of each bridge arm, and turn on the IGBT on the sub-module to be charged when the current is charging the bridge arm , make it input, and charge the sub-module; when the current discharges the bridge arm, turn off the upper and lower IGBTs of the sub-module to make it latched, and the capacitor of the sub-module is bypassed. Repeat this until the capacitor voltage of the sub-module reaches the rated voltage, turn on the lower IGBT of the sub-module to cut it off, and then charge the next sub-module, repeat the above steps until the capacitor voltage of all the sub-modules of the bridge arm reaches the rated voltage. Near the value, the converter completes charging.

The Chinese patent with the publication number CN102170140A discloses "a start-up method for a modular multi-level converter flexible direct current transmission system", which proposes a no-load start-up method for MMC for flexible direct current transmission (method 2). The capacitor is uncontrolled rectified and charged through the current-limiting resistor. After the capacitor voltage is stable, the current-limiting resistor is bypassed, the converter station is unlocked, and the system is directly put into closed-loop control of the capacitor voltage. The specific process is divided into three stages: In the first stage, first block the trigger pulses of all modules, open the bypass switch of the current limiting resistor, and perform uncontrolled rectification and charging on the capacitors of the MMC sub-modules of the two converter stations. After the capacitor voltage is stable, enter In the second stage, unlock the converter station MMC2, and make it enter the constant AC voltage closed-loop control. After the capacitor voltage of the converter station MMC2 sub-module is stable, enter the third stage, close the current limiting resistor bypass switch, and unlock the converter station MMC1 , and make it enter the constant DC voltage closed-loop control. After the capacitor voltage of the sub-modules of the two converter stations is stable, the charging ends.

The disadvantage of the above method 1 is that this self-excited charging using the AC system requires a specially designed control program for inverter charging, which increases the complexity of the controller design, and this method is only for the charging method of the inverter , the DC line and the charging process of other equipment in the system have not been involved. The disadvantage of the above method 2 is that when unlocking the converter station MMC2, the capacitor voltage of each bridge arm sub-module is 1/2N times the peak voltage of the AC valve side. It is possible that the minimum operating voltage has not yet been reached, resulting in the failure of the sub-module to achieve controlled operation; when the converter station MMC2 is unlocked, the AC current limiting resistor is still connected in series to the line, which can reduce the overcurrent of the converter station MMC1, but the instant converter station MMC2 is unlocked. The DC voltage drops to half of that before unlocking, and the MMC2 converter of the converter station will have a short-term inrush current; and this method is suitable for the case where one end of the converter station must be started passively with no load, and the normal start of the converter station , the converter stations on both sides can be connected to the AC local power supply at the same time to start.

Contents of the invention

The purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art, and provide a self-excited starting method for a flexible direct current transmission system of a modular multilevel converter. Before that, make the sub-module capacitance and DC line charging reach the rated value, and suppress the inrush current problem generated during the charging process by formulating a suitable control strategy.

The purpose of the present invention is achieved through the following technical solutions: a self-excited starting method for a modular multilevel converter flexible direct current transmission system, based on a modular multilevel converter flexible direct current transmission system, the modular multilevel converter The three phases on the AC side of the converter station at both ends of the level converter flexible direct current transmission system are connected in series with a current-limiting resistor of a certain resistance, and a bypass switch is connected in parallel at both ends of each current-limiting resistor; the modular multi-level The converter is composed of three-phase six bridge arms, each bridge arm is composed of a reactor and several sub-modules with the same structure in series, each sub-module is a half-bridge structure, and the sub-modules include two IGBTs and corresponding Freewheeling diodes and DC side capacitors; when starting, the AC sides of the converter stations at both ends are connected to the local AC power grid through the connection transformer. The starting method includes the following steps:

(1) Set the first converter station as constant DC voltage control and constant reactive power control; set the second converter station as constant active power control and constant reactive power control;

(2) Disconnect the circuit breakers on the AC system side of the first converter station and the second converter station, disconnect the bypass switches of the current limiting resistors on both sides of the system, close the DC isolation switch, and block the trigger pulses of all sub-module IGBTs of the two stations ;

(3) Close the circuit breaker on the AC system side of the first converter station;

(4) Monitor the capacitor voltage of the sub-module of the first converter station, and close the AC system side circuit breaker of the second converter station when the capacitor voltage of the sub-module of the first converter station tends to be stable;

(5) Monitor the phase current of the AC side of the second converter station and the capacitor voltage of the sub-module, and when the current and voltage tend to be stable, close the current-limiting resistor bypass switches of the first converter station and the second converter station;

(6) Set the initial reference value of the DC voltage controller of the first converter station to the DC voltage value at the current moment, the reference value of the reactive power to zero, and unlock the converter of the first converter station;

(7) Monitor the AC side current of the first converter station and the capacitor voltage of the sub-modules of the two stations. When the current and voltage tend to be stable, set the DC voltage command value of the converter of the first converter station to 2U _sm (twice the AC phase voltage peak value on the valve side of the system), and add slope control;

(8) Monitor the AC side current of the first converter station, the capacitor voltage and DC voltage of the sub-modules of the two stations, and when the current and voltage tend to be stable, unlock the second converter station and set the active power and reactive power controllers initially The reference value is given as zero;

(9) Monitor the AC side current of the second converter station and the capacitor voltage of the sub-modules of the two stations. When the current and voltage tend to be stable, set the reference value of the fixed DC voltage controller of the first converter station as the rated value;

(10) Monitor the capacitor voltage and DC voltage of the sub-modules of the two stations. After the control requirements are met, set the control command values of the constant active power and constant reactive power of the second converter station to the system requirement values, and add slope control;

The improvement of the starting method provided by the present invention is that step (4) tends to be stable and means that U _c2 is approximately 1/2N times of the peak value of the system valve side line voltage, and step (5) tends to be stable means that U _c2 is approximately 1/N times of the peak value of the side line voltage of the system valve, at this time, the sub-module controller of the second converter station has reached the minimum working voltage.

The starting method provided by the present invention is improved in that, when the converter is unlocked in the step (6), the DC voltage controller command value is the actual value of the DC voltage at the moment of unlocking; when unlocking, it is adopted to appropriately increase the total number of input modules The method of (n+num) causes a small current impact when unlocking, and then gradually reduces to n, so that the converter station can operate normally and stably.

As in the starting method provided _by the present invention, the improvement is that, after the first converter station is unlocked, the DC voltage is raised in two steps. In the second converter station, a small DC and AC current impact can be generated when unlocking, and then the DC voltage is raised to the rated value after stabilization, so that the converter station can operate normally and steadily.

The working principle of the present invention: during the starting process of the self-excited starting method of the present invention, the converter stations at both ends are connected to the local AC power grid through the connecting transformer. The method first charges the converter without current control, and then unlocks the converter Station, enter the current and voltage closed-loop control to charge the sub-module capacitor voltage and DC line to the rated value, and effectively reduce the impact current generated by the AC and DC systems during the charging process by formulating a suitable unlocking control strategy for the converter station.

Compared with the prior art, the present invention has the following advantages and effects:

1. The present invention effectively suppresses the inrush current generated by the system when unlocking the first converter station.

2. The present invention solves the problem that the working power supply of the sub-module controller cannot be put into normal operation when the second converter station is unlocked.

3. The present invention effectively suppresses the inrush current generated by the system when unlocking the second converter station.

4. The present invention solves the problem of DC line charging.

Description of drawings

Fig. 1a is a schematic diagram of the main circuit topology of a three-phase modular multilevel converter.

Fig. 1b is a schematic structural diagram of a half-bridge sub-module of a three-phase modular multilevel converter.

Fig. 2 is a structural schematic diagram of a modular multi-level flexible direct current transmission system at both ends.

Fig. 3 is an operation flow chart of the starting method provided by the present invention.

Detailed ways

The present invention will be further described in detail below in conjunction with the embodiments and the accompanying drawings, but the embodiments of the present invention are not limited thereto.

Example

As shown in Fig. 1a, it is the basic topological structure diagram of the three-phase modular multilevel converter of the present invention, which is composed of six commutation bridge arms in three phases, and each bridge arm is composed of a reactor L and N sub-modules are connected in series. All sub-modules have the same internal structure, adopting a half-bridge structure, which is composed of IGBTs (VT1, VT2) with self-shutoff capability, freewheeling diodes VD1, VD2 connected in antiparallel with IGBTs, and a DC capacitor C. The reference direction of the specified bridge arm current i _xyz is shown in Figure 1b, where x=p and n correspond to the upper and lower bridge arms respectively, y=a, b and c correspond to the three phases a, b and c of the AC system respectively, z=1 and 2 correspond to the first converter station and the second converter station respectively. U _cj is the capacitor voltage of the sub-module, where j=1 and 2 correspond to the first converter station and the second converter station respectively. When the bridge arm current i _xyz >0, the sub-module capacitor has the current condition for charging.

As shown in Figure 2, it is a two-terminal modular multi-level flexible DC power transmission system, the converter station MMC1 (namely: the first converter station) and the converter station MMC2 (namely: the second converter station) adopt Modular multi-level converters, the three phases on the AC side of the two stations are each connected in series with a current-limiting resistor R of a certain resistance, a bypass switch S _R is connected in parallel at both ends of each current-limiting resistor, and the DC at both ends of the converter station A DC isolation switch S _k is connected in series in the positive and negative lines on the positive and negative sides.

As shown in Figure 3, it is the operation flow of the self-excited starting process of the flexible direct current transmission system of the present invention, which specifically includes the following steps:

(1) Set the converter station MMC1 to constant DC voltage control and constant reactive power control, and the command value of reactive power is zero. Set the converter station MMC2 to constant active power control and constant reactive power control, and the initial active power and reactive power command values are zero;

(2) Disconnect the circuit breakers S ₁ and S ₂ of the AC system side of the converter station MMC1 and MMC2 of the converter station, disconnect the bypass switches S _R1 and S _R2 of the current limiting resistors, and close the DC isolation switches S _k1 and S of the two stations _k2 , block the trigger pulses of all sub-modules VT1 and VT2 of the two stations, and prepare for the starting charging of the converter;

(3) Close the circuit breaker S ₁ on the AC system side of the converter station MMC1, the AC system valve side line voltage, the converter bridge arm of the converter station MMC1, the DC line and the bridge arm phase unit of the converter station MMC2 form a charging circuit. When the current i _xyz of each bridge arm of the converter station MMC1 >0, the current charges the capacitor C through the anti-parallel diode VD1, and at the same time, only N submodules of a single bridge arm are charged at the same time; when the current i _xyz of the bridge arm of the converter station MMC1 < At 0, the current flows out through the anti-parallel diode VD2, the capacitor is bypassed, and its voltage remains unchanged. The charging current i _xyz of the upper and lower bridge arms of the converter station MMC2 is equal in size and direction, and when i _xyz >0, a total of 2N sub-modules of the phase unit of the bridge arm are charged; when i _xyz <0, the capacitor voltage of the phase unit sub-module constant;

(4) Monitor the capacitor voltage of the sub-modules of each station. When the capacitor voltage of the sub-modules of the converter station MMC1 tends to be stable, close the AC circuit breaker S ₂ on the side of the converter station MMC2, and do not control the sub-modules of the converter station MMC2. rectification charge. When the phase current tends to be stable and U _c1 and U _c2 are approximately 1/N times the peak value of the valve side line voltage, close the bypass switches S _R1 and S _R2 and exit the current limiting resistor;

(5) When the capacitor voltage of the sub-module of the converter station MMC2 tends to be stable, unlock the converter station MMC1, and set the initial reference value of the DC voltage control as the DC voltage value at the current moment. '=n+num (if the level number of the output voltage of the converter is n+1, the number of sub-modules put into each phase during normal operation is n, generally n≤N) the sub-modules are put into, so that the n' sub-modules The upper IGBT (VT1) of the converter station is turned on, and the lower IGBT (VT2) of the remaining 2N-n' sub-modules are turned on, and then gradually decrease from n'→n. At this time, the inverter output phase voltage amplitude of the MMC1 AC outlet of the converter station increases Larger and closer to the phase voltage value of the valve side of the system, effectively reducing the AC current impact when unlocking. At the same time, in order to keep the capacitor voltage balance of each bridge arm sub-module, in the unit control period, the upper IGBT (VT1) of the sub-module with low voltage is selected to be turned on, and the lower IGBT (VT2) of the sub-module is selected to be turned off; the upper IGBT of the sub-module with high voltage is selected (VT1) is turned off, and the lower IGBT (VT2) is turned on;

(6) Monitor the phase current of the AC side of the converter station MMC1 and the capacitor voltage of each sub-module of the two stations. After stabilization, set the reference value of the DC voltage to 2U _sm (U _sm is the peak value of the phase voltage on the valve side of the system), and follow a certain slope Rising slowly, the charging current is limited, during which the sub-module capacitor voltages of the converter station MMC1 and converter station MMC2 remain relatively consistent and rise steadily;

(7) Monitor the capacitor voltage and DC voltage of the sub-modules of the two stations. When the DC voltage reaches 2U _sm , unlock the converter station MMC2 (see step (5) for specific operations, where n'=n), active power and reactive power The command value is zero;

(8) After the AC current on the MMC2 side of the converter station and the capacitor voltage of the sub-module are stabilized, the command value of the DC voltage controller of the given converter station MMC1 is the rated value;

(9) After the DC voltage rises to the rated value according to the slope, the power command values of the two stations are given, and the constant active power and constant reactive power modes are started during normal operation. At this point, the start-up process of the entire flexible DC transmission system is over.

According to the designed start-up operation process, the obtained sub-module capacitor voltage waveforms of converter station MMC1 and converter station MMC2, where U _c1 and U _c2 are the sub-module capacitor voltages of converter station MMC1 and converter station MMC2 respectively . According to the designed operation process, the sub-module capacitor voltage is relatively stable at each stage, and finally reaches the rated value.

The realization of the control process of step (5) can utilize the control program of the valve level control layer during the normal operation of the converter, without additional design; in order to suppress the inrush current when the converter station MMC1 is unlocked in step (5), the present invention controls the DC voltage The control strategy and modulation algorithm are appropriately improved, that is, the actual value of the DC voltage is used as the initial reference value of the DC voltage, so that it will not produce a large inrush current due to the rapid change of the DC voltage reference value; on the other hand, the moment of unlocking due to Insufficient capacitor voltage of the sub-module of the converter leads to a large difference between the AC voltage output by the inverter at the AC outlet of the converter and the AC voltage on the valve side, causing excessive AC impact. The number of modules is invested to ensure that the output voltage amplitude of the converter outlet is relatively large as much as possible, and the impact current when the converter is unlocked is reduced.

The above-mentioned embodiment is a preferred embodiment of the present invention, but the embodiment of the present invention is not limited by the above-mentioned embodiment, and any other changes, modifications, substitutions, combinations, Simplifications should be equivalent replacement methods, and all are included in the protection scope of the present invention.

Claims (4)

1. A self-excited starting method for a modular multilevel converter flexible direct current transmission system, based on a modular multilevel converter flexible direct current transmission system, the modular multilevel converter flexible direct current transmission system The three phases of the AC side of the converter station at both ends are each connected in series with a current-limiting resistor of a certain resistance, and a bypass switch is connected in parallel at both ends of each current-limiting resistor; the modular multi-level converter consists of three-phase six bridges Each bridge arm is composed of a reactor and several sub-modules with the same structure in series. Each sub-module is a half-bridge structure. The sub-module includes two IGBTs and corresponding freewheeling diodes and DC side capacitors; start , the AC sides of the converter stations at both ends are connected to the local AC power grid through the connecting transformer. It is characterized in that the self-excited starting method includes the following steps: (1) Set the first converter station to constant DC voltage control and constant reactive power control, set the second converter station to constant active power control and constant reactive power control, close the DC isolation switch, and disconnect the two Bypass switch for side AC circuit breaker and current limiting resistor; (2) Close the circuit breaker on the AC system side of the first converter station, and close the circuit breaker on the AC system side of the second converter station when the capacitor voltage of the sub-module of the first converter station tends to be stable; (3) When the capacitor voltage of the sub-module of the second converter station tends to be stable, close the current-limiting resistor bypass switches of the first converter station and the second converter station; (4) Set the initial reference value of the DC voltage controller of the first converter station to the DC voltage value at the current moment, the reference value of the reactive power to zero, and unlock the first converter station; (5) When the AC side current of the first converter station tends to be stable, set the DC voltage reference value of the first converter station to 2U _sm , that is, twice the peak value of the AC phase voltage at the valve side of the system, and add the slope control; (6) When the AC side current and DC voltage of the first converter station tend to be stable, the second converter station is unlocked, and the initial reference values of its constant active power and constant reactive power controllers are zero; (7) When the AC side current of the second converter station tends to be stable, set the constant DC voltage control reference value of the first converter station as a rated value; (8) When the capacitor voltage and DC voltage of the two sub-modules reach the control requirements, that is, the rated value, set the constant active power of the second converter station, the constant reactive power of the first converter station and the second converter station The control reference value is the value required by the system, and the slope control is added. 2. self-excited starting method as claimed in claim 1, is characterized in that, in step (3), described becoming stable means that U _c1 and U _c2 are approximately 1/N times of system valve side line voltage peak value, At this time, the voltages obtained from the working power sources of the sub-module controllers of the converter stations at both ends are above the minimum working voltage, and both the first converter station and the second converter station are unlocked. 3. The self-excited starting method as claimed in claim 1, characterized in that, when unlocking the converter in the step (4), an appropriate increase in the total number of modules n+num is adopted at the initial stage of unlocking, wherein, num The value range is: 0<num<N-n, so that a small AC current impact occurs when unlocking, and then gradually decreases to n, so that the sub-modules of the first converter station enter the rated input state. 4. The self-excited starting method according to claim 1, characterized in that in step (4), the DC voltage is raised after the first converter station is unlocked, and the method of raising the DC voltage includes the following steps: Step 41. Raise the DC voltage to 2U _sm , that is, twice the peak value of the AC phase voltage on the valve side of the system; Step 42. When unlocking the second converter station, first make the DC and AC current impacts lower than the rated value during unlocking, and then raise the DC voltage to the rated value after the second converter station operates stably, so that the second converter station normal steady state operation.