Kuang Li

Strategies and Operating Point Optimization of STATCOM Control for Voltage Unbalance Mitigation in Three-Phase Three-Wire Systems

Reactive power control through the static compensator (STATCOM) has gained wide attentions due to its outstanding performance. But for a STATCOM with traditional control strategies, unbalanced utility voltages will greatly affect the performance of the STATCOM and, in severe cases, may even cause the shutdown of the STATCOM for overcurrent protection. This paper proposes novel control strategies to ensure normal operation of STATCOM in three-phase three-wire systems when severe system voltage unbalance occurs and, furthermore, to mitigate voltage unbalance at the point of common coupling (PCC) whether it is caused by the utility or load. The control laws, operation principles, compensation characteristics, and operating point optimization of the proposed control strategies are analyzed and compared in detail. The power flow in the STATCOM and corresponding dc-side voltage-control schemes for these control strategies are also introduced. It is shown that the proposed voltage-controlled current source (VCCS) strategy and modified voltage-controlled voltage source strategy are valid for voltage unbalance compensation, especially the VCCS strategy for low rating under the same performance. Finally, the simulation and experimental investigations were carried out with these three parameter-optimized control strategies, respectively, for light and heavy load conditions. The results verified that the current passing through the STATCOM is under control and the voltage at the PCC is more balanced than before compensation. The compensation performance of the STATCOM is thus proven satisfactory

Novel load ripple voltage-controlled parallel DC active power filters for high performance magnet power supplies

The cooling storage ring of the heavy ion research facility in Lanzhou (HIRFL-CSR) is one of the five mega-science projects undertaken by Chinese Academy of Science within its ninth and tenth five-year plan period. The HIRFL-CSR facility can providing heavy ion beams in wide energy range, manifold and with high quality. This paper presents a hybrid filter structure consisting of a passive filter and a parallel dc active power filter (DCAPF) for a sample magnet power supply in HIRFL-CSR. The configuration of the power system with the DCAPF is introduced. Then a novel feedback control scheme for the DCAPF is proposed with detecting the output ripple voltage across the magnet load. For detailed discussion, a simple model of the whole system has been derived and the stability characteristic is analyzed. Through simulation and experiment investigation, the DCAPF is successfully applied in a 120-kW magnet power supply in HIRFL-CSR finally. The ripple factor and tracking accuracy of the output current through the load achieve 8.87 ppm and 100 ppm, respectively. Therefore, the rigor requirements of low output current ripple and fast ramp rate are both satisfied with the DCAPF proposed.

Comparison of four control methods to active power filters applied in accelerator power supplies

This paper proposes a hybrid filter structure consisting of a passive power filter and a DC active power filter (DCAPF) applied in high current, high precision and low ripple accelerator power supplies. The configuration of the power system with the DCAPF is firstly introduced. Then four control schemes to obtain the reference signal for the DCAPF control circuit is presented. For each method, the basic operation principle of eliminating output voltage and current ripples is analyzed in detail. Furthermore, the compensation characteristics of each method is discussed and compared. To verify the proposed circuit configuration and the control methods, the testing results of the practical field operation of the DCAPF with a 120kW accelerator power supply is introduced. By adopting the control method of detecting the load ripple voltage, the output current ripple factor and tracking accuracy of the accelerator power supply achieve 8.87/spl times/10/sup -6/ (peak-to-peak) and 1/spl times/10/sup -4/ respectively, which is very satisfactory for this kind of power supplies. The dynamic response of the accelerator power supply is also greatly improved through the application of the DCAPF.

A novel control approach to the DC active power filter used in a low ripple and large stable/pulse power supply

In this paper, a hybrid filter structure, which consisting of a passive and a shunt-connected DC active power filter, is proposed for a special high precision, low ripple and large power DC stable/pulse power supply. The main circuit configuration of DC active power filter and its basic concept for suppression harmonics are analyzed in detail. A novel control strategy of DC active power filter with detecting the output ripple voltage, i.e. the reference signal of the DC active power filter is obtained by detecting the output ripple voltage across the DC load, is presented. The advantage of the control approach is that, it is very easy to meet the requirements of low current ripple and fast response in DC stable/pulse power supply, whereas the precision of the circuit transducer and components is not need special request. To validate the proposed concept, a prototype of 12-pulse rectifier system and DC active power filter are manufactured and corresponding experiment investigation is done. Tested by a 120 kW practical industrial application stable/pulse power supply with the DC active power filter and adopting the control approach proposed, its output current ripple factor achieves 8.87/spl times/10/sup -6/ (peak to peak), in addition to others advantages include fast dynamic response, high efficiency and low cost.

Analysis and Design of Fully Digital and Direct Current Controlled Voltage Source Converters Connected to the Grid

Direct current control has been used in many voltage source power converters connected to utility grid, such as Static Var Generators (SVGs) and parallel Active Power Filters (APFs). In their stationary coordinates control systems, there are usually two closed control loops. One is the current loop and the other is the voltage loop. In practical applications, because the voltage source converters are connected to the grid, the output current will be affected and very often a command-tracking error exists in the current control loop. In this paper, a detailed analysis and design considerations are carried out to investigate this command-tracking error caused by the source voltage. Based on the obtained current loop transfer function and block diagram of the system, analytical expressions of the current control error are derived. The variables that affect the current control errors are then identified. The relationship between the circuit/control parameters and the current control errors are extensively explored. The command-tracking error affects the peak value of the modulation wave. When we design a fully digital control system, the peak value of the modulation wave is smaller than the peak value of the carrier wave. Then, according to choosing the values of the variables that affect the command-tracking error, the methods to design a fully digital direct current controlled voltage source converters connected to the grid are presented. The computer simulation investigations and hardware experimental results verify the analysis and design considerations.

Static Var generator control strategy for unbalanced systems in medium voltage applications

This paper proposes a novel static Var generator (SVG), which employs series-connected IGBTs in each leg of the bridge for medium voltage blocking capability. The SVG is used to compensate reactive power, negative sequence current, current harmonics and unbalanced source voltages. The configuration of the AC power system with the SVG is firstly introduced. Then the operation principles of the SVG to compensate the current or voltage distortions and unbalances are described. Two types of control strategies of the whole system, i.e., controlling the SVG operating as a voltage controlled voltage source (VCVS) and controlling the SVG as a current source (VCCS), are analyzed and discussed in detail. Especially, the approaches to generating the reference signal of for the SVG control circuit is presented according to the different compensation objectives mentioned above. All these control strategies and algorithms are realized in digital circuits based on a DSP controller. The simulation investigations and experiments on a low voltage prototype were carried out. The results showed that the behavior of the SVG is satisfactory both in steady state and in transient process. The proposed control strategies and algorithms are verified.

A Transformerless 2-Level Inverter Based Static Var Generator with Multiple Functions in Medium Voltage Application

This paper proposes a transformerless 2-level inverter based static var generator (SVG), which employs 20 series-connected IGBTs in each leg of the bridge for medium voltage blocking capability. The multifunctional SVG is used to compensate reactive power, negative sequence current, current harmonics and unbalanced utility voltages. Especially, a voltage controlled current source (VCCS) strategy is introduced and analyzed to mitigate grid voltage unbalance. Furthermore, the approaches to generating the reference signal for the SVG control circuit is presented according to different compensation objectives. Successful field installation and test results under 6kV voltage level showed satisfactory performance of the proposed SVG

Control and optimization of VCVS static var generators for voltage unbalance mitigation

Reactive power control through static var generator (SVG) has been gaining wide attentions due to its outstanding performance. But for a SVG with traditional control strategies, unbalanced utility voltages have greatly affect the performance of the SVG and in severe case may even cause shut-down of the SVG for overcurrent protection. This paper proposes a voltage controlled voltage source (VCVS) strategy to assure normal operation of SVG with series-connected IGBTs when severe system voltage unbalance happens, and furthermore to mitigate voltage unbalance at the point of common coupling (PCC) whether it is caused by utility or load. The control law, operation principle, compensation characteristics and parameter optimization of the proposed control strategy are analyzed in detail. Based on the VCVS control, a modified VCVS strategy is proposed and the difference between them is presented. Finally, the simulation and experimental investigations were carried out respectively for resistive and rectifier load conditions. The results verified that the current passes through the SVG is under control and the voltage at PCC is much more balanced than that before compensation. The compensation performance of the SVG is thus proved satisfactory

An Analysis on the Influence of Input Filters to SVG systems and Corresponding Design Considerations

Previous publications regarding the design and specification of input filters for Static Var Generators (SVG) usually deals with the input filter only, and seldom pay attention to the influence of input filters to SVG systems performance. In this paper a detailed analysis on the input filter influence on SVG systems and corresponding design consideration are presented. Three types of input filters, i.e., L filter, LC filter, and LCL filter, are examined separately. Through frequency domain methods, such as transfer functions and Bode plots, the influence of each type of input filter to the SVG system stability and filtering characteristics is extensively investigated. As direct current- control taken as a major control strategy for SVG, all the different situations when adopting different current detection points are covered in the analysis. A comparison between LC filter and LCL filter is also presented with optimized filter parameters. Based on the analysis, the design considerations for different type of input filter under different current diction scheme are discussed, which lead to filter parameters different to traditional design. All the computer simulation and hardware experimental results verified the validity of the above analysis and design.