Research On Key Techniques Of Cascaded Distribution Static Synchronous Compensator

With the revival of the traditional industry and the development of information technology and power electronic technique, the power quality problem of the distribution grid caused by reactive power, harmonics and the unbalanced load has become more and more serious. At the same time, the power consumers have had higher requirement for the power quality. The DSTATCOM can effectively address the power problems mentioned above with excellent operation performance, including highly accurate static compensation, fast dynamic response, wide operating range, and low output harmonic. DSTATCOM with cascaded topology is the core equipment used for the power quality control in distribution grid for the characteristics of modularity, high efficiency, redundancy operation and small volume. And the power links are connected in series to improve AC output voltage to coordinate with grid voltage rank and the application demand. It has a great research value. This paper does a deep research on several key techniques in the cascaded DSTATCOM with three-phase star configuration.Firstly, this paper explains the fundamental operating principle of cascaded DSTATCOM with three-phase star configuration, including the main circuit topology, modulation method and fixed-point frequency phase locked loop (PLL) technique. Mathematical models of single link, single-phase cluster and three-phase converter chain have been established using the switch cycle average method. To design the control system, simplifying the cycle average model gets the input and output characteristic model under both abc static coordinate and dq rotating coordinate. When the three phase load is unbalanced, symmetrical component method has been used to get the equivalent model of each sequency. Based on the instantaneous reactive power theory and test method for dq transformation, the referential current test method with complete compensation and selective compensation under power factor compensation mode, and the referential current test method under unbalanced compensation mode both have been introduced.Secondly, this paper introduces the state variable decoupling and voltage feedforward control strategy for the cascaded DSTATCOM with mathematical model under the dq rotating coordinate. The current control technique under different compensation mode has also been studied. Under the power factor compensation mode, after analyzing the design technique of traditional digital PI controller, an integrated controlling method combining PI and repetitive control has been introduced to improve the harmonic wave compensation and depression ability. Considering the unsatisfactory dynamic properties of the integrated controller, an optimal repetitive controller based on error regulator and advanced internal model is presented to solve the contradiction between the static tracing accuracy and the dynamic responding speed and the parameter robustness has also been improved correspondingly. Under the PCC voltage control mode, by analyzing the mathematical relation, add a voltage control loop before the q axis current inner loop to keep the PCC voltage stable. Under the unbalanced compensation mode, an individual sequence control strategy aiming at controlling different sequences successively of three-phase four-line equipment is presented and the balanced compensation of grid current is realized under unbalanced condition of load and PCC voltage.Then, this paper introduces the cacaded DSTATCOM’s hierarchical control strategy for the dc-side voltage’s self-balance. The control method of the dc-side voltage has been clearly divided into three levels which are general voltage regulation control, voltage balance control between clusters and single phase balance control in a cluster. Soft-start and the general voltage control is achieved by using methods of pre-charge current limitation, pre-modulation signal and the voltage reference gradually-increasing. The balance control method for unipolar frequency doubling CPS-PWM modulation is presented based on voltage vector superposition principle to realize the balance control for phase to phase voltage and single phase voltage. The design of traditional hierarchical controller is complicated which cost many storage resources and it is hard to realize the in-operation fault redundance. To solve the problems above, a new method based on NLM modulation is introduced aiming at balancing the single phase voltage and realizing the in-operation fault redundance. This new method can both simplify the controller design and easily realize in-operation fault redundance when single link is failure.Finally, based on PEBB concept, a type of high reliable HPEBB module used for the lOMvar/lOkV cascaded DSTATCOM is introduced into the main circuit and the controller to improve the compensation performance and reliability of the device. This HPEBB achieves high voltage and large capacity in the main circuit and the controller, as well as high modularization, controllability, scalability, and reliability. The section that introduces the main circuit focuses on the electrical, structural, and thermal dissipation designs according to the module’s performance index. The section that discusses the controller emphasizes the principle of the complex programmable logic device (CPLD) based universal HPEBB controller. The hardware principle of the controller and the realization of controlling core have been introduced. This section also analyzes typical reliability and anti-interference issues produced by the transient process of the mixed-signal circuit based on CPLD filtering. As a result, the reliability of HPEBB module has been improved. The validity and reliability of the module design are proved by the HPEBB module test-bed and the industrial prototype practical application.