Zhichang Yuan

DC voltage balancing technique using multi-pulse optimal PWM for cascade H-bridge inverters based STATCOM

A novel multi-pulse optimal PWIM technique for STATCOM based on cascade H-bridge inverters is proposed. The harmonics are suppressed using optimization techniques. Furthermore the fundamental output voltages of all the inverter cells are controlled to be equal to make all the inverter cells equally share the exchanged power with power system, which is to maintain the dc voltages as balance as possible. At the same time the switching-pattern swapping technique is used, and the dc voltages can be fairly balanced. The validity of the proposed method is verified using the experimental results of a laboratory prototype of 30 kVAr STATCOM based on a 17-level cascaded H-bridges inverter.

Battery energy storage system load shifting control based on real time load forecast and dynamic programming

Battery energy storage system (BESS) is one of the key technologies for smart grid and load shifting is one of the fundamental functions of BESS. BESS load shifting performance is determined by the availability of accurate load curves and optimization approaches. In this paper, a real-time control strategy based on load forecast and dynamic programming methods is presented. The predicted load curve is updated on-line through regress forecasting. The proposed optimization model is solved by using dynamic programming technique. The objective is peak shaving and prolonging the battery lifetime, and the constraints considered include battery state-of-charge (SOC), cycling times per day, converter capacity and step power. The above control strategy was successfully applied to the 5MW*4hour lithium-Ion BESS demonstration project in Biling substation, China Southern Power Grid. The simulation results based on the actual load data of Biling substation demonstrate the effectiveness.

Nanao multi-terminal VSC-HVDC project for integrating large-scale wind generation

Due to the renewable generation target and the continuously increasing electrical demand, a large number of wind farms are being planned in China. Meanwhile, the corresponding transmission infrastructures have to be developed to facilitate the increased renewable penetration. HVAC, conventional HVDC and voltage source converter (VSC) based HVDC are possible options for building new lines. Compared to the other approaches, VSC-HVDC offers better transmission efficiency, flexible and independent power control, reactive power compensation, and unlimited terminal configurations. Therefore, a multi-terminal VSC-HVDC system is being constructed to extract wind generation from Island of Nanao to the mainland of China. This paper presents a comprehensive architecture of the transmission network, as well as the corresponding control system. As a key factor of running a multi-terminal HVDC system, the system coordinated control has been investigated to cooperate with the system start-up and shutdown processes. In addition, the control strategy of online re-commissioning of a station after maintenance has been designed for this project, using disconnectors instead of DC circuit breakers.

Nonlinear Controller for Cascade H-Bridge Inverter-Based STATCOM

Cascade H-bridge multilevel inverter technique brings a lot of advantages to static synchronous compensator over conventional multi-pulse STATCOM. Based on the theory of the exact linearization via feedback, a MIMO nonlinear controller is proposed. The averaged model of STATCOM based on cascade multilevel inverter is established and the nonlinear control law is analyzed. Simulation results show, under this controller, STATCOM can achieve fast dynamic control of reactive power, with dc link voltage controlled at almost a constant value. Experimental results of a prototype STATCOM based on 17-level cascade H-bridges inverter are also shown to verify the control law

Optimal Battery Energy Storage System Charge Scheduling for Peak Shaving Application Considering Battery Lifetime

Peak shaving is one of the most important applications of battery energy storage system. In order to prolong battery life or to study the relationship between the battery lifetimes, the charge-discharge cycles and the depth of discharge, constraints concerning charge-discharge cycles and depth of discharge should be added to the optimization model. Therefore, algorithms based on continuous models fail to solve this kind of problem. This paper presents a dynamic programming algorithm containing multi-modules of states. The remaining capacity is discretized and modularized. Tests are carried out using a set of predicted load data. The results show the effectiveness of the method.

Development of a scaled STATCOM prototype based on 21-level cascade H-bridge inverter

This paper is intended to present a laboratory scaled STATCOM prototype using 21-level cascade H-bridge inverter, which is developed to verify the control schemes that will be adopted in a plusmn50 MVar STATCOM project. A novel multi-pulse optimal PWM technique for STATCOM based on cascade H-bridge inverters is proposed. The harmonics are suppressed using optimization techniques. Furthermore the fundamental output voltages of all the inverter cells are controlled to be equal to make them equally share the exchanged power with power system, which is to maintain the DC voltages as balance as possible. Combined with the switching-pattern swapping technique, the DC voltages can be fairly balanced. The redundant operation is achieved through storing switching angles for various cases of N. A nonlinear control scheme based on the theory of exact linearization via feedback is proposed to improve the dynamic performance of the STATCOM. The validity of the proposed method is verified using the experimental results of the laboratory scaled prototype

A Novel Strategy on Smooth Connection of an Offline MMC Station Into MTDC Systems

Themodular multilevel converter-based multiterminal HVDC (MMC-MTDC) technology has been applied to the Nanao project, which was commissioned successfully in China, and has shown many advantages. However, how to connect an offline MMC station into an operating MTDC system smoothly without expensive dc circuit breakers presents a huge challenge. Making full use of the structural characteristics of MMC, the proposed strategy implements the smooth connection by sequential operations of the dc disconnectors (DSs) at the positive and negative polarities, without the need for changing system parameters or hardware. The charging circuits of MMC are studied fully after every operation of DSs, and a four-order state equation is built to explore the potential maximum surge current after closing the DSs. Case study and simulation are performed based on the PSCAD/EMTDC model of the Nanao three-terminal MMC-HVDC system to validate the effectiveness of the proposed strategy. Besides easy operation and high reliability, the strategy neither influences the operating system nor increases any cost.

Control strategy of system coordination in Nanao multi-terminal VSC-HVDC project for wind integration

The ±160 kV three-terminal voltage source converter (VSC) based HVDC system under construction in Island of Nanao, China, is expected to be the first multi-terminal VSC-HVDC industrial project. The system has been designed to extract wind energy from the wind intensive areas on the island to the bulk power system at the mainland of China. At present, the system is being tested online, and planned to be committed by the end of 2013. Due to the outstanding controllability of VSC-HVDC system, the control strategy becomes critical for coordination between each converter station, as well as the interconnected DC and AC system. This paper presents the detailed control schemes for each individual active and reactive power parameter according to different operation modes, including paralleled operation of AC and DC circuits, DC operation only, and working as a reactive power compensator. In this system, the power flow management ensures efficient power transmission in normal operation, and fast response in emergency condition, which improves the ability of fault ride-through. Also, the converter station is capable to provide a stable AC system for connecting wind farms.

Optimal control strategy for cascaded STATCOM under unsymmetrical power system conditions

This paper presents a optimal control method for cascaded STATCOM under unsymmetrical conditions by power faults. Cascaded H-bridge multilevel converter is considered suitable for power system application. Since unsymmetrical faults have much higher probability than symmetrical faults in power system, it is importance for STATCOM to operate continuously under unsymmetrical conditions. While isolated capacitors in each H-bridge makes it more difficult to balancing 3-phase dc voltage. The mathematical model of cascaded STATCOM and its operation principle under unsymmetrical power conditions are analyzed. A optimal control method based on energy balance of 3-phase dc capacitors, including selection method of 3-phase current reference to ensure dc voltage stability and continuous operation. The validity is examined by digital simulation under ±50Mvar cascaded STATCOM connected to 110kV power grid.

Research on HVDC Model in Transient Voltage Stability Analysis of AC/DC Transmission Systems

Fast Simplified models of HVDC are widely used in large-scale AC/DC power system transient voltage stability analysis. An important concern of different models is that to what extent the analysis results are impacted by using simplified models. Under voltage reactive power characteristic of HVDC is analyzed based on CIGRE HVDC test benchmark. It is observed that reactive power absorbed by HVDC is proportional to 1.6-th power of AC bus voltage, while the reactive power supplied by fixed capacitors of converter station is proportional to square of AC bus voltage. The above feature makes HVDC harmful to transient voltage stability. Among different simplified models of HVDC, constant power and constant impedance model are quite inconsistent with actual HVDC model, while Quasi-Steady-State (QSS) model is fairly consistent with actual HVDC model. So QSS model is suitable for large-scale AC/DC power system transient voltage stability analysis. Main conclusions are validated through simulations on Chinese Southern Power Grid.