English
Static synchronous compensator (STATCOM) is the core of flexible AC transmission system. This article will introduce the control mode of STATCOM.
The following is a list of contents
Physical quantity control mode
control strategy
According to the control physical quantity, it can be divided into direct current control and indirect current control. Direct current control technology is to use tracking PWM control technology to feedback control the instantaneous value of current waveform. It directly commands the occurrence of current. It has simple structure, fast current regulation response and good robustness to disturbance. However, it is only suitable for small and medium capacity occasions, and has great limitations for large capacity occasions. Indirect current control is to indirectly control the AC side current through the extremely thin amplitude and phase of AC voltage of STATCOM inverter. It is simple and easy to realize, but the dynamic performance is poor. It is suitable for high-capacity STATCOM. In order to reduce harmonics, multiplex, multilevel or PWM technology can be used to improve the waveform in indirect current control.
The multiplicity and chain structure of the main circuit design of STATCOM device are common technologies to improve the capacity. Multiple structure is to use several single-phase or three-phase inverters to generate square wave voltages with phase difference of several degrees, and use transformers to connect square wave voltages with different phases in series, which can effectively improve capacity and voltage and reduce harmonics, but it will also bring many problems, such as high price, increased device loss and floor area, Moreover, the ferromagnetic nonlinearity of the transformer also brings difficulties to the design. The DC capacitors of each inverter bridge of chain STATCOM are independent of each other, and there is the problem of capacitor voltage imbalance. The main causes of capacitor voltage imbalance are the difference of hybrid loss, parallel loss and input pulse delay. The capacitor voltage balance can be realized by adjusting the phase difference between the inverter bridge and the system and the modulation ratio of each inverter bridge.
In terms of control strategy, it can be divided into open-loop control, closed-loop control and hybrid control of the two. Generally speaking, it is voltage loop and current loop in terms of control. It is mentioned in domestic literature that the control of STATCOM is a voltage and current double loop control based on SVPWM. The reactive current in the load current is detected by phase-locked loop (PLL) and low-pass filter (LPF), and the decoupling control of STATCOM reactive current and active current in DQ plane is realized through dq transformation. At the same time, the output of the DC voltage outer loop controller is coupled to the active current control loop to realize the DC voltage stabilizing control.
From the perspective of control technology, there are PI control method, PI inverse control method, robust adaptive control, recursive neural network adaptation, sliding mode variable structure and fuzzy control method. Among them, the robust adaptive control method makes it difficult to select the fuzzy control coefficient; Neural network adaptive method does not depend on the establishment of system model, but its real-time performance is not good; Sliding mode variable structure linearization is difficult. In practical application, the traditional PI control is still in the majority. Another literature proposes a passivity control (PBC) method, establishes the Euler Lagrange system model of STATCOM, and introduces the variable scale method of nonlinear programming for optimization.
In order to achieve better compensation effect, the traditional reactive power compensation device can be operated and controlled jointly with STATCOM, so as to avoid the problems of switching devices with the increase of compensation current, the increase of switching loss and the decrease of efficiency in order to obtain the ideal output current waveform. Hybrid static synchronous var compensator (hSTATCOM) is based on Deadbeat control, that is, the pulse width of the next switching cycle is calculated according to its state equation and the expected value of output reactive current. Using the method of combining active and passive compensation, TSC is used as the main compensation means in the passive part, which does not produce harmonics and has low loss; Active compensation is used to realize the continuous regulation of compensation current, and reactive power can be continuously regulated in both directions. A new hybrid device composed of SVC and STATCOM and a joint operation scheme based on fuzzy prediction are proposed in domestic literature, that is, using small capacity STATCOM to suppress flicker and large capacity SVC to compensate reactive power, which avoids the complexity of algorithm when STATCOM adopts asymmetric control. The joint control operation mode has simple algorithm and clear control purpose, but its structure may be complex, so it will be developed in specific fields.
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