CN114156904B - Indirect detection method of capacitance voltage of chained STATCOM submodules - Google Patents

Abstract

A chain STATCOM sub-module capacitor voltage indirect detection method, belonging to the field of power system reactive power compensation technology. The purpose of this invention is to use the AC valve side voltage data, combined with the corresponding sub-module input information provided by the latest level approach modulation NLM, to implement a chain-type STATCOM sub-module capacitance voltage indirect detection method for indirect detection of the capacitance voltage of each sub-module in the chain. The steps of the invention are: precharging the DC side capacitance of the sub-module in the chain of the unconnected STATCOM, using the AC valve side voltage transformer to realize the synchronization of the STATCOM to the grid, and initiating the indirect detection method of the sub-module DC voltage. The invention greatly reduces the manufacturing and maintenance costs of STATCOM equipment, solves the technical problem of difficulty in connecting STATCOM to the grid due to the lack of DC capacitive voltage sensor hardware support in the indirect detection method, not only saves the hardware cost of STATCOM, but also has engineering practicality value.

Description

Indirect detection method of capacitance voltage of chained STATCOM submodules

Technical field

The invention belongs to the technical field of reactive power compensation of power systems.

Background technique

Chain STATCOM is a kind of dynamic reactive power compensation equipment, which is mostly used in medium and high voltage distribution network systems. STATCOM can be equivalent to an AC voltage source with the same frequency as the power grid whose amplitude and phase can be controlled. Its topology is shown in Figure 1. The parameter U _c-ij is the capacitance voltage of the i-th phase j-th H-bridge submodule of STATCOM, L is the grid-connected inductance, v _{gA, B, and C} are the three-phase voltage of the grid, v _{oA, B, and C} are the AC valve sides of STATCOM Output voltage, i _{cA, B, C} is the STATCOM compensation current. As long as the phase and amplitude of the STATCOM AC valve side output voltage v _{oA, B, C} relative to v _{gA, B, C} are changed, the current flowing through the STATCOM can be controlled. Phase and amplitude. Implemented reactive power compensation for the system

The control strategy of STATCOM using NLM modulation technology is shown in Figure 2. In the figure is the cluster voltage, which is the sum of the capacitance voltages of N sub-modules in the m-phase chain, U _DCref is the cluster voltage command, I _qref is the reactive current command, I _pref is the active current command, and ω ₀ t is the real-time phase of the grid voltage. The voltage external loop control controls the flow of active power by comparing the relationship between the cluster voltage command and the cluster voltage to achieve cluster voltage stability; the in-chain voltage equalization control balances the distribution of active power between H-bridge sub-modules by adjusting the output voltage of the H-bridge sub-modules. Distribution to achieve balanced distribution of DC side capacitor voltages of sub-modules within the chain; current internal loop control is used to track and compensate for reactive current in real time. In order to avoid capacitor overvoltage and reduce the harmonic content of STATCOM compensation current, sub-module capacitor voltage control is essential.

Sub-module capacitor voltage control is inseparable from the acquisition of capacitor voltage. The traditional way of acquiring capacitor voltage is to use hardware sensor detection. However, as the voltage level of the power grid increases, the number of chain H-bridge sub-modules will increase proportionally. The technical solution that relies on DC voltage sensing for sub-module capacitance voltage detection will cause a significant increase in production costs. At the same time, due to multi-channel detection The difficulty of DC voltage balance control of sub-modules in the chain caused by signal transmission delay and unreliability also increases.

Contents of the invention

The purpose of the present invention is to use the AC valve side voltage data, combined with the corresponding sub-module input information provided by the nearest level approximation modulation NLM, to implement a chain-type STATCOM sub-module capacitance voltage indirect detection method for indirect detection of the capacitance voltage of each sub-module in the chain.

The steps of the present invention are:

S1. Precharge the DC side capacitor of the sub-module in the chain that is not connected to the grid STATCOM.

Under the premise of ensuring that the grid-connected knife switch is disconnected, the 4 IGBTs in all H-bridge sub-modules are turned off, and the sub-module capacitor precharging program is started. After completing the functional self-test of the DC voltage source, first select the STATCOM The mth phase is used as the charging phase. The DC side capacitors of each sub-module in this phase chain are used to charge in sequence. When the charging current is less than 0.01A, the next uncharged sub-module capacitor is charged until the A, B, C three-phase chain All sub-module DC side capacitors have been charged;

S2. Use AC valve side voltage transformer to realize STATCOM simultaneous grid connection

Compare the grid voltage phase information obtained using the PLL phase-locked loop with the STATCOM AC valve side voltage information detected by the voltage transformers PT _A , PT _B , and PT _C installed on the STATCOM AC valve side to confirm that the STATCOM output AC voltage is consistent with After the grid-side voltage phase amplitude is consistent, the STATCOM can be connected to the grid simultaneously by closing the grid-connection switch;

The specific steps are as follows:

(a) After confirming that the DC side capacitors of all sub-modules in the three-phase chain are charged, first read the grid voltage information from the grid-side voltage transformers PT _gA , PT _gB , and PT _gC , and use the PLL phase-locked loop to obtain the three-phase voltage of the grid phase information;

(b) Set the reactive current reference value I _qref = 0, and the active current reference value I _pref according to U _{DC ref} and The difference is calculated by the PI controller, combined with ω ₀ t and based on the NLM algorithm to output the control signal. The control sub-module performs correct actions. When the AC valve side voltage is consistent with the grid voltage, the grid-connected knife switch is commanded to close to achieve STATCOM synchronization. grid connection;

S3. Indirect detection method of DC voltage of promoter module

Use the STATCOM AC valve side output voltage to calculate the DC side capacitor voltage of a specific sub-module. The specific execution process is as follows: First, based on the relationship between the STATCOM output reference voltage u _ref and the DC side capacitor reference voltage U _dcref of a single sub-module, find out the required input The number of H-bridge sub-modules k, where The active power flow direction is determined based on the positive and negative relationship between the STATCOM output voltage and current. When the active power flows to the STATCOM, k H-bridge sub-modules with lower capacitance voltages are selected from the N sub-modules in the chain through sorting of sub-module capacitance voltages; when the active power flows to the STATCOM, When STATCOM flows out, select k H-bridge sub-modules with higher capacitance voltage and put them in; combined with the AC valve side voltage measured by PT _ABC , calculate the sub-module capacitance voltage:

Suppose the number of sub-modules in each phase chain is N. If only one H-bridge sub-module is input in a phase chain, the capacitor voltage of this sub-module is equal to the absolute value of the AC valve side output voltage of the phase; if there are N H-bridges in the chain When all sub-modules are put in, the sub-module capacitor voltage is the average of the AC valve side voltage. If there are j modules in a phase chain, then there exists/> input situation of different combinations, where 1＜j＜N, according to the H-bridge sub-module number of each phase input, write the /> at time t _x according to formula (1) different voltage relationships, because/> Thus, the capacitor voltage of each H-bridge sub-module is obtained;

u _c-m1 +u _c-m2 …+u _c-mj ＝|v _om (t _x )| (1)

In formula (1), v _om (t _x ) is the output voltage of the mth phase t _x on the AC valve side of STATCOM obtained through the AC valve side PT _ABC .

The invention greatly reduces the manufacturing and maintenance costs of STATCOM equipment, and solves the technical problem of difficulty in connecting STATCOM to the grid due to the lack of DC capacitance voltage sensor hardware support in the indirect detection method. It not only saves the hardware cost of STATCOM, but also has engineering practicality. value.

Description of the drawings

Figure 1 is a STATCOM topology diagram;

Figure 2 is a control strategy diagram;

Figure 3 is a sub-module precharging flow chart;

Figure 4 is the NLM algorithm flow chart;

Figure 5 is a timing diagram showing the relationship between the capacitor voltage of the NLM submodule and the AC valve side output voltage;

Figure 6 is the capacitor voltage diagram of the three sub-modules in the A-phase chain;

Figure 7 is the voltage and current relationship diagram on the A-phase grid side;

Figure 8 is a comparison chart between direct and indirect detection effects of phase A capacitor voltage.

Detailed ways

The purpose of the present invention is to provide an indirect detection method of STATCOM submodule DC voltage suitable for nearest level approach modulation, using the input submodule command information obtained by the nearest level approach modulation (NLM) algorithm, combined with the AC valve side voltage sensor ( The three-phase voltage information of STATCOM output collected by PT _{A, B, C} ) is used to obtain the real-time voltage of the sub-module capacitance in each phase chain through the constructed inversion detection algorithm. At the same time, a proposed sub-module capacitor precharging based on DC power supply is provided. This method solves the technical problem of difficulty in connecting STATCOM to the grid due to the lack of hardware support for DC capacitive voltage sensors in this indirect detection method. It not only saves the hardware cost of STATCOM, but also has engineering practical value.

The specific steps of the present invention are:

(1) Precharge the DC side capacitor of the sub-module in the chain that is not connected to the grid STATCOM

Under the premise of ensuring that the grid-connected knife switch is disconnected, the four IGBTs in all H-bridge sub-modules are turned off. Start the sub-module capacitor precharging program. The precharging process is shown in Figure 3. After completing the functional self-test of the DC voltage source, first select the mth phase of STATCOM as the charging phase, and use the DC side capacitors of each sub-module in this phase chain to charge in sequence. When the charging current is less than 0.01A, the next phase is charged. The uncharged sub-module capacitors are charged until the DC side capacitors of all sub-modules in the A, B, and C three-phase chain are fully charged.

(2) Utilize AC valve side voltage transformer to realize STATCOM simultaneous grid connection

Compare the grid voltage phase information obtained using the PLL phase-locked loop with the STATCOM AC valve side voltage information detected by the voltage transformers PT _A , PT _B , and PT _C installed on the STATCOM AC valve side to confirm that the STATCOM output AC voltage is consistent with After the grid-side voltage phase amplitude is consistent, the STATCOM can be connected to the grid simultaneously by closing the grid-connection switch;

The specific steps are as follows:

(a) After confirming that the DC side capacitors of all sub-modules in the three-phase chain are charged, first read the grid voltage information from the grid-side voltage transformers PT _gA , PT _gB , and PT _gC , and use the PLL phase-locked loop to obtain the three-phase voltage of the grid phase information;

(b) According to the control structure in Figure 2, set the reactive current reference value I _qref = 0, and the active current reference value I _pref according to U _{DC ref} and The difference is calculated by the PI controller, combined with ω ₀ t and based on the NLM algorithm to output the control signal. The control sub-module performs correct actions. When the AC valve side voltage is consistent with the grid voltage, the grid-connected knife switch is commanded to close to achieve STATCOM synchronization. Connected to the grid.

(3) Indirect detection method of DC voltage of promoter module

Based on the voltage information provided by the valve side PT _c , combined with the specific switching number of the submodule in each phase chain calculated by the NLM algorithm, the STATCOM AC valve side output voltage is used to calculate the DC side capacitor voltage of the specific submodule. The specific execution algorithm is as follows.

The flow of the NLM algorithm is shown in Figure 4. First, based on the relationship between the STATCOM output reference voltage u _ref and the DC side capacitance reference voltage U _dcref of a single sub-module, the number of H-bridge sub-modules that need to be invested is calculated. The active power flow direction is determined based on the positive and negative relationship between the STATCOM output voltage and current. When the active power flows to the STATCOM, k H-bridge sub-modules with lower capacitance voltages are selected from the N sub-modules in the chain through sorting of sub-module capacitance voltages; when the active power flows to the STATCOM, When STATCOM flows out, k H-bridge sub-modules with higher capacitor voltages are selected and put into operation. During the NLM calculation process, the H-bridge submodule number of each phase input is output, and combined with the AC valve side voltage measured by PT _ABC , the submodule capacitance voltage is obtained. The specific calculation algorithm is as follows.

Suppose the number of sub-modules in each phase chain is N. If only one H-bridge sub-module is input in a phase chain, the capacitor voltage of this sub-module is equal to the absolute value of the AC valve side output voltage of the phase; if there are N H-bridges in the chain When all sub-modules are put in, the sub-module capacitor voltage is the average of the AC valve side voltage. If there are j modules in a phase chain (1<j<N), then there is/> According to the input status of different combinations, according to the H-bridge sub-module number of each phase input, write the /> at time t _x according to formula (1) equations related to different voltages. Because/> Therefore, the capacitor voltage of each H-bridge sub-module can be calculated

u _c-m1 +u _c-m2 …+u _c-mj ＝|v _om (t _x )| (1)

In formula (1), v _om (t _x ) is the output voltage of the mth phase t _x on the AC valve side of STATCOM obtained through the AC valve side PT _ABC .

Taking N=3 as an example, the timing relationship between the submodule capacitor voltage and the AC valve side output voltage is shown in Figure 5. During the T ₁ period, only one sub-module is turned on. At this time, the absolute value of the output voltage on the AC valve side is equal to the capacitor voltage of the H-bridge sub-module. During the T ₂ period, when two H-bridge sub-modules are turned on, three different voltage relationships can be obtained:

Among them, v _om (t ₁ ), v _om (t ₂ ), and v _om (t ₃ ) are the output voltages of the i-th phase t ₁ , t ₂ , and t ₃ of the STATCOM AC valve side obtained through PT _c respectively. Solve the equations (1) Obtain the capacitance voltage of 3 sub-modules. During period T ₃ , all three H-bridge sub-modules in the chain are put into operation. At this time, the capacitor voltage is the average of the AC valve side voltage.

Extended to the number of modules being N, when j modules are invested, there is Different investment situations can be listed/> equation, because/> Therefore, no matter how many H-bridge sub-modules are put in, the capacitor voltage of each H-bridge sub-module can be calculated. The technical advantage of the present invention is that the sub-module DC voltage indirect detection method involved in the present invention can omit all H-bridge sub-module DC voltage sensors in the STATCOM chain, and only needs to rely on the voltage data provided by the AC valve side voltage transformer, combined with the latest power By using the corresponding sub-module input information provided by the flat approximation modulation NLM and using the sub-module DC voltage indirect detection method, real-time detection of the capacitance voltage of each sub-module in the chain can be achieved, thus significantly reducing the manufacturing and maintenance costs of STATCOM equipment.

Example verification:

In order to verify the feasibility of the sub-module capacitance voltage indirect detection method, a chain STATCOM reactive power compensation simulation system based on NLM modulation technology was built in the Simulink environment. STATCOM supplements the inductive reactive power consumed by the load in the network. The switching device uses insulated gate bipolar transistor (IGBT). The rated capacity of STATCOM is 1kVar, the rated voltage of the grid is 110V, the number of chain sub-modules is 3, and the sampling interval is 1.2 e-3s, connect the reactor 3mH, and the DC side capacitance of the sub-module is 2mF.

STATCOM obtains the sub-module capacitance voltage through the indirect detection method involved in the present invention to perform reactive power compensation control. At 0.4s, STATCOM starts to perform reactive power compensation. At 0.605s, the load reactive power suddenly increases to 1.5 times the original reactive power value.

The sub-module capacitor voltage equalization effect is shown in Figure 6. The capacitor voltage of each sub-module reaches a stable equilibrium after STATCOM is put in for 0.1s. The reactive power compensation effect is shown in Figure 7: After STATCOM performs reactive power compensation, the angle between the grid voltage and current The current lags the voltage by 40.7° and is quickly adjusted to the same phase as the voltage and current; after a sudden increase in load reactive power, the grid voltage and current phase angle returns to the same phase relationship within 0.02s. STATCOM has a fast response speed. In this process, the capacitance voltage of a single sub-module is obtained through direct detection and indirect detection. The comparison results are shown in Figure 8. The comparison shows that the indirect capacitance voltage detection method proposed by the present invention has high measurement accuracy.

The above simulation analysis proves that: the indirect detection method involved in the present invention can quickly and accurately detect the DC side capacitor voltage of each H-bridge sub-module, and use it for chain STATCOM control to obtain better dynamic reactive power compensation effect.

Claims (1)

1. A chain type STATCOM submodule capacitor voltage indirect detection method is characterized by comprising the following steps of: the method comprises the following steps:

s1, pre-charging direct-current side capacitors of in-chain submodules of non-grid-connected STATCOM

On the premise of ensuring the disconnection of a grid-connected disconnecting link, 4 IGBTs in all H bridge submodules are turned off, a submodule capacitor pre-charging program is started, after the function self-checking of a direct-current voltage source is completed, firstly, the m phase of a STATCOM is selected as a charging phase, all submodule direct-current side capacitors in a phase chain are utilized to be charged in sequence, and when charging current is smaller than 0.01A, the next uncharged submodule capacitor is charged until all submodule direct-current side capacitors in a A, B, C three-phase chain are charged;

s2, achieving STATCOM synchronous grid connection by utilizing alternating-current valve side voltage transformer

To obtain the power grid voltage phase information by using the PLL and to install the voltage transformer PT at the STATCOM AC valve side _A 、PT _B 、PT _C Comparing the detected STATCOM alternating-current valve side voltage information, and after confirming that the phase amplitude of the STATCOM output alternating-current voltage is consistent with that of the network side voltage, closing a grid-connected disconnecting link to realize synchronous grid connection of the STATCOM;

the specific method comprises the following steps:

(a) After confirming that all submodules in a three-phase chain are charged with direct-current side capacitors, a network side voltage transformer PT is firstly used for charging _gA 、PT _gB 、PT _gC Reading power grid voltage information, and acquiring phase information of three-phase voltage of a power grid by using a PLL (phase locked loop);

(b) Setting reactive current reference value I _qref =0, active current reference value I _pref According toU _{DC ref} And (3) withDifferential effort through PI controlCalculated by the device, combined->The control signal is output based on an NLM algorithm, the submodule is controlled to perform correct action, and when the voltage of the alternating-current valve side is consistent with the voltage of the power grid, the grid-connected disconnecting link is commanded to be switched on, so that synchronous grid connection of the STATCOM is realized;

s3, direct-current voltage indirect detection method for starting submodule

The DC side capacitor voltage of a specific submodule is calculated by using the STATCOM AC valve side output voltage, and the specific implementation process is as follows:

first, output the reference voltage according to STATCOMReference voltage +.>The relation between the H bridge sub-modules is obtained to calculate the number of H bridge sub-modules needed to be inputkWherein->The method comprises the steps of carrying out a first treatment on the surface of the Determining an active current direction according to the positive-negative relation between the output voltage and the current of the STATCOM, and selecting N submodules in a chain through submodule capacitor voltage sequencing when the active current flows to the STATCOMkThe input of the H bridge submodule with lower capacitor voltage is performed; when the active power flows out of the STATCOM, selectkThe H bridge submodule with higher capacitor voltage is put into operation; binding PT _ABC The measured ac valve side voltage is used to calculate the capacitance voltage of the sub-module:

let the number of sub-modules in each phase chain beNIf only one H-bridge submodule is put into a phase chain, the capacitance voltage of the submodule is equal to the absolute value of the output voltage of the alternating current valve side; if in-chainNAll H bridge submodules are put into operation, and at the moment, the capacitance voltage of the submodules is the average value of the alternating-current valve side voltageIf there is one phase chain injWhen the module is put in, there is +.>The input conditions of different combinations, 1<j<NAccording to the H bridge submodule number input by each phase, writing according to a formula (1)t _x Time->A different voltage relationship because of->Thereby deriving a capacitance voltage for each H-bridge sub-module;

（1）

in the formula (1)Through the alternating current valve side PT _ABC The mth phase of the STATCOM alternating current valve sidet _x Output voltage at time.