Design method suitable for high-capacity chained energy storage type static synchronous compensator

Abstract

The invention discloses a design method suitable for a high-capacity chain type energy storage static synchronous compensator. Firstly, a chain type energy storage unit is connected to the direct current side of the chain type static synchronous compensator, high-voltage direct hanging is achieved in a chain type energy storage mode, meanwhile, the energy storage capacity is increased, and the effect that active power and reactive power can be stably compensated at the same time is achieved. Then, in a control part of the system, applying a proportional resonance regulator under an alpha-beta static coordinate system to realize instantaneous power zero static error control; sOC balance control between phase clusters is achieved by injecting zero-sequence voltage, and SOC balance of units in the phase clusters is achieved by superposing corresponding alternating-current voltage on each cascade unit. Finally, simulation verification is carried out on the established model, it can be seen that the device can carry out active power and reactive power exchange on the electric power system, and meanwhile the device can have a good supporting effect on the voltage of the electric power system. The high-capacity chained energy storage type static synchronous compensator designed by the method can stably operate in a power system and has good dynamic performance.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a design method suitable for a large-capacity chain energy storage type static synchronous compensator. Background Art

[0002] Static synchronous compensator STATCOM (static synchronous compensator) is a parallel type reactive power compensation device that can be used in both transmission and distribution networks. It stabilizes the voltage at the connection point, enhances the system voltage stability, improves the system load capacity, or provides support for the voltage of key nodes of the system to achieve the purpose of enhancing the system stability limit by absorbing or emitting continuously adjustable reactive power. STATCOM with chain topology can be designed with a higher control bandwidth due to its high equivalent switching frequency so that it has harmonic compensation capability. Traditional STATCOM only has reactive power exchange capability, but not active power exchange capability. In order to achieve better results, STATCOM is sometimes required to have a certain ability to exchange active power with the power grid to achieve the purpose of suppressing the power pulsation of new energy generation, shaving peaks and filling valleys, compensating for the resistive voltage drop of the circuit, damping the oscillation of the system, and extending the support time of the power grid under fault conditions.

[0003] As the most promising power generation methods for large-scale development and application, wind power generation, photovoltaic power generation and other renewable energy sources are becoming more mature and widely used. Renewable energy has the characteristics of intermittency, volatility and randomness, which means that its large-scale grid connection will seriously affect the stability, power quality and economy of the power grid. Energy storage technology, as one of the effective measures to address the above problems, can smooth active power fluctuations and regulate reactive power. It can also be applied to the fields of peak shaving, frequency regulation and black start of the power grid. With the rapid development of battery technology, battery energy storage system (BESS) has significant advantages in capacity, power energy density, life, operating efficiency and maintenance. It is more suitable for high-power and large-capacity energy storage and has gradually become a research hotspot and development direction.

[0004] Each phase of the chain-type BESS is composed of multiple H-bridge (HB) energy storage units cascaded. This circuit topology can use power switching devices with lower withstand voltage to achieve energy conversion at higher voltage levels without the need for a step-up transformer. It has obvious advantages in volume, weight, floor space and cost, and is simple to control, easy to implement modularization and has good scalability. Therefore, this topology is widely used in various medium- and high-voltage large-capacity occasions. In recent years, there has been relatively little research on the control strategy of the power conditioning system (PCS) of the chain-type BESS. BESS based on other energy storage forms and topologies are applied to wind farm grid connection, power system stability control, etc., but no in-depth research has been conducted on the PCS control strategy. The DC / DC converter is applied to the delta-type chain-type BESS, and its control strategy is systematically studied. However, the addition of the DC / DC converter reduces the system efficiency and complicates the control strategy. Active power control and state of charge (SOC) balancing control strategies are proposed for the chain-type BESS, but reactive power control and four-quadrant operation are not considered.

[0005] The present invention aims to connect the chain energy storage unit to the DC side of the chain static synchronous compensator, and realize high voltage direct hanging by chain energy storage, while increasing the capacity of energy storage, so as to achieve the effect of stable compensation of active and reactive power at the same time. In the control part of the system, the proportional resonant regulator is applied in the α-β static coordinate system to realize instantaneous power zero-static error control; the SOC balance control between phase clusters is realized by injecting zero-sequence voltage, and the SOC balance of units within the phase cluster is realized by superimposing the corresponding AC voltage on each cascade unit. Summary of the invention

[0006] The technical problem to be solved by the present invention is to design a method suitable for a large-capacity chain energy storage type static synchronous compensator to solve the problem that the existing traditional static synchronous compensator can only compensate reactive power but cannot exchange active power.

[0007] The present invention is achieved by adopting the following technical solutions:

[0008] A design method for a large-capacity chain energy storage static synchronous compensator. First, the main circuit topology of the chain BESS-STATCOM is established, and the high-voltage direct hanging method is adopted to connect to the power grid. Then, a power control strategy that can control the active power and reactive power output of the device is designed, that is, an instantaneous power control strategy. The state of charge balance control strategy, inter-phase cluster balance control and intra-phase cluster unit balance control are designed. Among them, the inter-phase cluster balance control adopts the method of injecting zero-sequence voltage to make the phase cluster with a lower state of charge less discharge and more charge, and the phase cluster with a higher state of charge more discharge and less charge, so as to achieve SOC balance between phase clusters; the intra-phase cluster unit balance control achieves the balance of SOC of each energy storage unit in the same phase by superimposing an AC voltage signal on the modulation signal of each energy storage unit. Finally, the voltage signal is input to the PWM regulator, and the control signal is output to control the IGBT of the device to realize the functions of active exchange and reactive compensation.

[0009] As a preferred embodiment of the present invention, the scheme comprises the following steps:

[0010] Step A1: Establish the main circuit topology of the H-bridge cascaded chain STATCOM, and connect a battery model in parallel on its DC side for energy storage.

[0011] Step A2: Design an instantaneous power control strategy for active and reactive power output and a balanced control strategy between phase clusters and a balanced control strategy for unit within a phase cluster for balanced state of charge.

[0012] Step A3: Input the voltage signal to the PWM regulator, and output a control signal to control the IGBT of the device to achieve the functions of active power exchange, reactive power compensation and voltage support.

[0013] The main circuit topology structure of the chained BESS-STATCOM is established in step A1, and the specific method is as follows:

[0014] The chain BESS-STATCOM consists of a PCS based on H-bridge cascade and a DC-side battery unit. Each phase bridge arm of the chain BESS-STATCOM is called a phase cluster, which is composed of n energy storage units based on H-bridge circuits in series; the DC side of each energy storage unit is composed of a battery unit composed of multiple battery modules in series and a capacitor in parallel. The three-phase converter is connected to the medium and high voltage power grid in a star connection through a connecting reactor.

[0015] In the main circuit established, the grid voltage and the three-phase are assumed to be symmetrical, that is,

[0016]

[0017] Ignoring line impedance, the PCS voltage equation is

[0018]

[0019] Where: u ao 、u bo and u co Output three-phase voltage for PCS; i a 、i b and i c is the three-phase current flowing through the PCS; L is the connected reactor. The equation (2) is transformed to the α-β stationary coordinate system by Clark transformation:

[0020]

[0021] Where: u sα and u sβ is the component of the grid voltage in the α-β stationary coordinate system; u α and u β is the component of the PCS output three-phase voltage in the α-β stationary coordinate system; i α and i β are the components of the current in the α-β stationary coordinate system. Equation (3) is the mathematical model of PCS in the α-β stationary coordinate system.

[0022] Step A2 designs the instantaneous power control strategy of active and reactive power output and the inter-phase cluster balanced control and intra-phase cluster unit balanced control strategies for charge state balance. The specific strategies are:

[0023] The PCS control strategy is mainly divided into instantaneous power control and SOC balance control. The unipolar carrier phase-shifted sine pulse width modulation technology (CPS-SPWM) adopted this time can achieve a higher equivalent switching frequency at a lower switching frequency, and has significant advantages such as switch load balance and superior harmonic characteristics.

[0024] The instantaneous power control of the device's output active and reactive power is completed in the α-β stationary coordinate system. According to the instantaneous power theory, the instantaneous active power p and instantaneous reactive power q in the α-β stationary coordinate system can be obtained:

[0025]

[0026] Formula (4) shows that by controlling i α and i β It can effectively control p and q. The command current can be obtained from formula (4): and

[0027]

[0028] Where p * ,q *They are the command signals for instantaneous active power and instantaneous reactive power respectively.

[0029] From formula (3), we can know that the PCS output voltage command signal in the α-β stationary coordinate system is and for

[0030]

[0031] In the formula, both the command current and the actual current are power frequency periodic signals. The conventional proportional integral (PI) regulator cannot eliminate the steady-state error, so the PR regulator is used to achieve error-free tracking of the command current signal. In formula (6), K P , K R are the proportion and resonance coefficient of the proportional resonant regulator respectively.

[0032] When the reference signal is a periodic signal with a certain frequency, the resonant regulator can keep the frequency and shape of the controlled quantity unchanged, and only integrate its amplitude, so as to achieve the error-free tracking of the output of the controlled object to its reference signal. The transfer function of the PR regulator is

[0033]

[0034] Where: ω0 is the reference signal frequency, and the amplitude gain of the PR regulator is infinite at ω0. The regulator parameters can be designed using the method of dominant pole configuration of the open-loop system, and the Bode diagram is used to verify the performance of the designed system. Similar to the traditional PI regulator, the proportional coefficient K P Determines the bandwidth, stability and dynamic response of the controller; resonance coefficient K R It determines the bandwidth near the resonant frequency. In order to effectively deal with the possible fluctuations in the grid frequency, the value is relatively large in practical applications. At the same time, the PR regulator has a simple discrete iterative control algorithm and is easy to implement digitally.

[0035] Since the amplitude gain of the PR regulator is infinite at ω0, the feedforward voltage has no effect on the current closed-loop control; the coefficient of the second term is 1, thus achieving error-free tracking of the command current. Therefore, equation (6) can be rewritten as

[0036]

[0037] Then transform it to the abc stationary coordinate system to obtain the PCS output three-phase voltage command signal and

[0038] During operation, the SOC state of the chain energy storage unit battery also needs to be balanced, which involves the following control strategy. The DC side voltage of the H-bridge energy storage unit of the chain BESS is provided by a battery with self-stabilizing characteristics, so there is no need to balance the DC side voltage, only the SOC of the battery needs to be balanced. The SOC of the battery is

[0039]

[0040] SOC balancing includes inter-phase cluster balancing control and intra-phase cluster unit balancing control strategies. First, define the following variables: n is the number of energy storage units in series in phase a, S aj is the SOC of the jth battery cell of phase a cluster, j = 1, 2, ..., n, then the average SOC of phase a cluster is

[0041] S a =(S a1 +S a2 +…+S an ) / n (10)

[0042] The average three-phase SOC is

[0043] S=(S a +S b +S c ) / 3 (11)

[0044] The inter-phase cluster balancing control achieves inter-phase cluster SOC balancing by injecting zero-sequence voltage into the PCS output three-phase voltage command signal without changing the three-phase line voltage output by the PCS. The difference between the three-phase SOC average value and the SOC average value of each phase is

[0045]

[0046] Transform to α-β stationary coordinate system

[0047]

[0048] make

[0049] Where: ΔS represents the imbalance of SOC between phase clusters; represents the distribution of SOC imbalance among phase clusters. Then the reference zero-sequence voltage is

[0050]

[0051] Where: K0 is the proportionality coefficient, and its value is related to the specific battery selection; θ i is the phase angle of phase a current. The three-phase active power generated by the zero-sequence voltage is

[0052]

[0053] In the formula: U0, I a ,I b and I c are the phasor representations of the zero-sequence voltage and three-phase current, respectively; U0 and I are the amplitudes of the zero-sequence voltage and three-phase current, respectively. From formula (17), it can be seen that the sum of the three-phase active power generated by the zero-sequence voltage is 0, indicating that the injected zero-sequence voltage will not affect the total power and three-phase current of the PCS.

[0054] The unit balancing control in the phase cluster achieves the SOC balancing of each unit in the phase cluster by superimposing an AC voltage signal on the modulation signal of each energy storage unit. Taking phase a as an example, the difference between the SOC of each unit in the phase and the average SOC of phase a is

[0055] ΔS aj =S a -S aj (17)

[0056] Then the voltage signal superimposed by each unit is

[0057] u 1aj =K1ΔS aj cosθ i (18)

[0058] Where: K1 is the proportionality coefficient, and its value depends on the specific battery selection. The sum of the voltages superimposed on phase a is

[0059]

[0060] This formula shows that the SOC balance of units within a phase cluster will not affect the instantaneous power control and the balanced control between phase clusters. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 The present invention provides a flow chart of a design method suitable for a large-capacity chain energy storage type static synchronous compensator.

[0062] Figure 2 This is the main circuit block diagram of BESS-STATCOM.

[0063] Figure 3 This is the block diagram of the PCS control strategy.

[0064] Figure 4 This is the block diagram of the instantaneous power control module of PCS.

[0065] Figure 5 This is the block diagram of the inter-phase cluster balancing control strategy.

[0066] Figure 6 This is the block diagram of the unit balancing control strategy within the phase cluster.

[0067] Figure 7 Simulation results for a given reactive power output for the device.

[0068] Figure 8 Simulation results for a given active power output for the device.

[0069] Fig. 9 Output the simulation results for the given active and reactive power for the device.

[0070] Fig.10 This is the simulation result of the bus voltage after the device stabilizes the parallel capacitor.

[0071] Fig.11 The simulation results of the bus voltage after the parallel inductor is stabilized.

[0072] Fig.12 This is a comparison of the bus voltage before and after the device is connected when a three-phase ground short circuit occurs on the bus. DETAILED DESCRIPTION

[0073] The present invention provides a design method for a large-capacity chain energy storage type static synchronous compensator. In order to make the purpose, technical solution and effect of the present invention clearer, the specific implementation scheme of the present invention is described in detail below with reference to the accompanying drawings and examples. The specific examples described in the present invention are only used to explain the present invention and are not used to limit the present invention.

[0074] 1. Introduction to the specific embodiments of the invention

[0075] Figure 1 This is a flow chart of a design method for a large-capacity chain energy storage static synchronous compensator provided by the present invention. Figure 1 , the design method described in the present invention includes:

[0076] A1: Establish a 35KV AC bus power system model, establish the main circuit model of the H-bridge cascaded chain STATCOM, and connect a battery model in parallel on its DC side for energy storage and access the system.

[0077] A2: Establish the instantaneous power control system model of active and reactive power output and the inter-phase cluster balanced control and intra-phase cluster unit balanced control system models for charge state balance.

[0078] A3: Input the voltage signal to the PWM regulator, and output the control signal to control the IGBT of the device to achieve the functions of active power exchange and reactive power compensation. Further, the following specific steps are included:

[0079] A1: Establish a 35KV AC bus power system model, establish the main circuit model of the H-bridge cascaded chain STATCOM, and connect a battery model in parallel on its DC side for energy storage and access the system.

[0080] The chain BESS-STATCOM main circuit structure adopted by the present invention is a three-phase bridge arm, each phase bridge arm is composed of 6 H-bridge-based energy storage units connected in series, and the DC side of each energy storage unit is composed of a battery unit composed of multiple battery modules connected in series and a capacitor in parallel, such as Figure 2 shown.

[0081] A21: Construct PCS control strategy control block diagram and simulation model.

[0082] PCS control strategy block diagram Figure 3 , mainly divided into instantaneous power control and SOC balance control. Figure 4 It is the instantaneous power control module of PCS. The specific process is: the three-phase voltage is transformed into the α-β stationary coordinate system by a Clark transformation, and then the instantaneous active power p and instantaneous reactive power q are controlled to obtain the command current. The result of subtracting the command current from the actual current value after Clark transformation is input into the PR regulator to achieve error-free tracking of the command current signal. P , K R They are the proportional and resonant coefficients of the proportional resonant regulator respectively. Proportional coefficient K P Determines the bandwidth, stability and dynamic response of the controller; resonance coefficient K R Determines the bandwidth near the resonant frequency. In order to effectively deal with the possible fluctuations in the grid frequency, the value is relatively large in practical applications. At the same time, the PR regulator has a simple discrete iterative control algorithm and is easy to implement digitally. The PR regulator generates a PCS output voltage command signal U in the α-β stationary coordinate system. αref and U βref , and then transformed to the abc stationary coordinate system, the PCS output three-phase voltage command signal U aoref , U boref and U c oref.

[0083] SOC balancing includes inter-phase cluster balancing control and intra-phase cluster unit balancing control strategy. See the block diagram of inter-phase cluster balancing control strategy for details. Figure 5 The inter-phase cluster balancing control is to balance the SOC between each phase cluster. The cluster balancing control injects zero-sequence voltage into the PCS output three-phase voltage command signal without changing the three-phase line voltage output by the PCS, thereby achieving inter-phase cluster SOC balance.

[0084] Subtract the SOC of the abc three-phase cluster from the average SOC of the three phases to obtain ΔS a , ΔS b and ΔS c, and then transformed to the α-β stationary coordinate system through Clark transformation to obtain the distribution of imbalance ΔS and SOC imbalance between phase clusters Then the reference zero-sequence voltage U can be obtained 0ref .

[0085] The block diagram of the unit balance control strategy within the phase cluster is shown in Figure 6 The unit balancing control in the phase cluster achieves the SOC balancing of each unit in the phase cluster by superimposing an AC voltage signal on the modulation signal of each energy storage unit. First, the difference between the SOC of each unit in the phase and the average SOC of the phase is calculated, and then the voltage signal superimposed by each unit is calculated to obtain the balancing voltage u 1aj 、u 1bj and u 1cj .

[0086] Carrier Phase Shifted Sinuous Pulse Width Modulation (CPS-SPWM) is suitable for high-power power electronic devices. It is a relatively mature and widely used modulation strategy for cascaded H-bridge inverters. Its basic principle is to compare the triangular carrier and the sinusoidal modulation wave to generate a PWM drive signal.

[0087] 2. Technical feasibility verification of the present invention

[0088] In the 35KV AC system built as described above, the BESS-STATCOM device is connected, and the simulation verification results of fixed active power, fixed reactive power output and simultaneous active and reactive power output are as follows: Figure 7 , Figure 8 and Fig. 9 As shown in the figure; after the capacitor and inductor are connected in parallel on the bus, the device performs reactive power compensation on the bus to stabilize the AC voltage. Fig.10 and Fig.11 As shown; the simulation verification of constant AC voltage is carried out after the busbar has a three-phase short circuit, and the results are as follows Fig.12 As shown, the effectiveness of the present invention can be verified.

[0089] First, in the 35KV system, under normal operation, after the device is connected to the AC system and stabilized, a Q is given at 0.7s. ref =50MVar, and after 1.2s the command signal is changed to Q ref =-50MVar, such as Figure 7 As shown in the figure, it can be seen that the device can stably output the given reactive power; under normal operating conditions, after the device is connected to the AC system and stabilized, a P is given at 0.7s. ref =10MW, and after 1.2s the command signal changes to Pref =-10MW, such as Figure 8 As shown, it can be seen that the device can stably output the given active power. Under normal operating conditions, after the device is connected to the AC system and stabilized, a P is given at 0.7s. ref =10MW, Q ref =50MVar, and after 1.2s the command signal changes to P ref =-10MW, Q ref =-50MVar, such as Fig. 9 As shown, it can be seen that the device can stably output the given active power.

[0090] Then, during the operation of the 35KV system, in the normal operating state, after the device is connected to the AC system and stabilized, a 100uF capacitor is connected to the AC bus at 0.7s, causing voltage fluctuations on the AC bus. At this time, the device is adjusted to work in the constant voltage mode and the capacitor is removed at 1.2s. Fig.10 As shown in the figure, we can see the supporting effect of the device on the AC bus voltage. Under normal operating conditions, after the device is connected to the AC system and stabilized, a 0.07H inductor is added to the AC bus at 0.7s, causing the voltage fluctuation on the AC bus. At this time, the device is adjusted to work in the constant voltage mode, and the inductor is removed at 1.2s, as shown in the figure. Fig.11 As shown, we can see the supporting effect of the device on the AC bus voltage.

[0091] Finally, a three-phase ground short circuit fault is set on the 35KV bus at 0.5s, with a ground resistance of 5Ω and a duration of 0.2s. The bus voltage before and after the device is connected is compared. Fig.12 As shown in the figure, the bus voltage is significantly more stable when a fault occurs after the device is connected than when it is not connected, and can be roughly maintained near the rated voltage of 35KV, indicating that the device has a good supporting effect on the bus voltage under fault conditions.

[0092] The above simulation results verify the effectiveness and feasibility of the parameter design method of the present invention.

[0093] In summary, the design of the large-capacity chain energy storage type static synchronous compensator described in the present invention can exchange active power and reactive power for the power system, and at the same time can provide good support for the voltage of the power system.

[0094] Finally, it should be noted that the above examples of the present invention are merely examples for illustrating the present invention, and are not intended to limit the embodiments of the present invention. Although the applicant has described the present invention in detail with reference to the preferred embodiments, for those of ordinary skill in the art, other different forms of changes and modifications can be made based on the above description. It is impossible to list all the embodiments here. Any obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A design method for a large-capacity chain energy storage static synchronous compensator, characterized in that: The following steps are involved: S1: Establish a main circuit model of an H-bridge cascaded chain static synchronous compensator, and connect a battery model in parallel on its DC side for energy storage; S2: Establish a control module of the energy storage static synchronous compensator, including an instantaneous power control link for controlling the output of active power and reactive power and a balancing control link for balancing the battery state of charge (SOC), and output the control signal to the CPS-SPWM modulation module; S3: The signal modulated by CPS-SPWM is input to the inverter of the H-bridge circuit, which controls the on and off of the IGBT to output AC current, and inputs the active and reactive power into the system in parallel with the energy storage static synchronous compensator to realize the functions of compensation and voltage support; S4: The chain-type main circuit structure using H-bridge cascade can be connected to the AC bus using a high-voltage direct connection method in systems of 35KV and below, without the need to add a DC-DC conversion module to increase the output voltage of a single battery. At the same time, the voltage and capacity of a single battery can be appropriately reduced, which has better feasibility; the battery can charge the DC capacitor, and in the event of a fault, the battery can support the voltage drop of the DC capacitor, thereby enabling the device to operate stably.

2. A design method for a large-capacity chain energy storage static synchronous compensator according to claim 1, characterized in that: In the control link, the specific strategies for controlling the active and reactive power output by the control device and the charge and discharge control between the energy storage batteries in the device are as follows: S21: In the power control link in step 2, instantaneous power control is adopted for the control of active power and reactive power. The specific method is: firstly, the measured bus voltage is converted to the α-β stationary coordinate system, and then according to the instantaneous power theory, the reference value of the current can be obtained from the given values ​​of active power and reactive power, and then the voltage reference value is obtained as the output signal; S22: For the charge and discharge control of the energy storage battery in the device, the control link of balancing the battery state of charge (SOC) is divided into two parts, namely: inter-phase cluster balancing control and intra-phase cluster unit balancing control. The inter-phase cluster balancing control adopts the method of injecting zero-sequence voltage to make the phase cluster with lower state of charge discharge less and charge more, and the phase cluster with higher state of charge discharge more and charge less, so as to achieve SOC balance between phase clusters; The unit balancing control within the phase cluster achieves the balancing of the SOC of each energy storage unit in the same phase by superimposing an AC voltage signal on the modulation signal of each energy storage unit.

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