gxue Lin

Study of HVDC Light for its enhancement of AC/DC interconnected transmission systems

This paper illustrates a study on HVDC light for its enhancement of AC/DC hybrid transmission system and multi-infeed HVDC system. In the sense of the realizing independent controlling of active and reactive power, HVDC light is more effective compared to the conventional HVDC system. An AC/DC parallel transmission system and a three-infeed HVDC system are modeled using PSCAD/EMTDC. Simulations corresponding to different situations are performed. From the simulation results, HVDC light is proven to be able to improve the rotor angle stability and dynamic performance of AC/DC interconnected transmission system, and support the AC voltage during the system fault to enlarge the margin of the voltage stability which is the basic difference from conventional HVDC. HVDC light can also prevent commutation failure in multi-infeed HVDC system and help the recovery of HVDC system too.

Studies of Commutation Failures in HVDC System Based on Hypersim

The study of commutation failures requires more detailed and accurate simulation of HVDC systems. Hypresim is an integrated digital simulator, which could provide reliable, extensible and high-speed electromagnetic transient simulation for power systems. Its special characteristics are introduced in this paper. A HVDC system is established based on Hypersim. Modeling details of the ac system, dc lines, dc converters and their control systems are presented. Commutation failures are triggered by external disturbances on the ac bus and internal faults on the thyristor valve; the dynamic behavior of the HVDC system is demonstrated; detailed analysis is also given. A protection method against commutation failures is proposed. Simulation result indicates that the probability of failure could be reduced.

High speed identification and protection for commutation failure in HVDC systems based on wavelet entropy

The objective of this paper is to propose a new method of identification and protection for commutation failure in HVDC system. The technique of wavelet entropy is introduced by combining wavelet transform with entropy principles. It is used to decompose the transient signal of HVDC system into different resolution levels and extract the feature of the signal. According to the calculation of wavelet entropy, the criterion is defined, which is helpful to make a definite discrimination of commutation failure from dc line short circuit fault and normal operation. By moving sampling windows, the analysis and detection of fault can be realized online. The wavelet protection controller based on wavelet entropy is designed in MATLAB/Simulink. When the controller detects the commutation failure, it will improve the desired value of ignition delay angle and extend the margin of commutation. The control strategy can effectively prevent and mitigate commutation failures caused by different faults. The simulation results show that the controller is with well performance.

Analysis of Reactive Power Exchanging between Converter Station from YunGuang ±800Kv UHVDC Projection

The paper analysis the yunguang UHVDC system requires exchanging reactive power with AC busbar while transmitting active power. In order to satisfy the operation requirement of converter station under different AC and DC operation modes, the paper centers on switching AC filter and reactive power values under different active power transmission, operate modes and DC voltages. The conclusion has been applied in yunguang UHVDC and is very useful for the UHVDC project.

Influence of load model on UHVDC Emergency DC Power Support strategy

The mechanism of UHVDC Emergency DC Power Support (EDCPS) to enhance transient stability of multi- machine system is analyzed briefly based on EEAC (Extended Equal Area Criteria), and the influence of load model characteristic including ZIP static load model and IM (Induction Motor) model on UHVDC EDCPS effect after heavy disturbance is further studied, and the corresponding UHVDC EDCPS Strategies should be modified with different load models to keep system stable. Studies reveal that the influence of sending-grid ZIP characteristic on post-fault EDCPS effect is completely opposite to that of receiving-grid. Under heavy disturbance IM load absorbs more active power than that of ZIP load and the influence of sending-grid IM load on post-fault EDCPS angel stability is completely opposite to that of receiving-grid. As for the influence on EDCPS effect between synthesis load and ZIP load, total load of both sending-grid and receiving-grid should be weighed first. If total load of sending-grid is smaller than that of receiving-grid, synthesis load model with IM will deteriorate post-fault EDCPS effect, and DP should be increaseed or T should be advanced appropriately.

Diagnosis of commutation failure in HVDC systems based on wavelet transforms

Fast diagnosis of commutation failure in HVDC systems can improve the level of operation in the power systems. It is also the base to realize the prevention of commutation failure. By application of wavelet transform, the rapidity and correctness of faults diagnosing are enhanced. AC voltage and DC voltage in inverter side are taken as the signals for wavelet analysis. Wavelet coefficient d6 changed suddenly when fault occurred. This paper studied a diagnosing method based on the variance of wavelet coefficient d6. With moving sample windows, the sampling signals in the windows are analyzed with wavelet and the variance of wavelet coefficient d6 in real time. Therefore, the fault diagnoses are realized in real-time. The functions are realized in MATLAB/Simulink. The results of simulations show that the method can output the alarm signals after about half cycle when fault occurred. It can also tell out the AC fault without CF and DC line fault with CF fault. Above all, the diagnosing method using wavelet analysis ameliorates the rapidity and correctness of faults diagnosing.

Computing resources assignment in RTDS simulators with subgraph isomorphism based on genetic algorithm

To construct the largest power simulation model with real time digital simulators (RTDS), assignment of the computing resource is an important issue to make the model feasible. For automatic computing resource assignments, the interesting idea of subgraph isomorphism is proposed. Computing units (racks) are connected with communication wires, whereas the power grid is connected with electrical lines. The two systems can be taken as two graphs with physical correspondence. To assign the computing resources is to find a relationship of subgraph isomorphism between two graphs. Genetic algorithm is applied to solve the problem. According to the definition of subgraph isomorphism, the object function is given as the number of ones in the expended incidence matrix defined here. By selection, crossover and mutation in the GA, the problem can be solved and the assignment can also be achieved directly. There are many feasible assignments received.

Study on the control strategies and dynamic performance of DC distribution network

Some changes are required to improve the characteristics of power system to implement the developing smart grid technology. For distribution system, DC mode is a very good complementation of AC techniques. The control strategies and dynamic performance of DC distribution system are studied to investigate the feasibility of the DC distribution mode. A novel prototype of distribution system is provided. Three different control strategies are applied in different converters for the demand of corresponding functions. The simulation results show the advantages of DC distribution system: the converters can provide accurate active power transaction and balance reactive power in the stable situation, while provide the ability to mitigate the voltage sags during the faults in AC distribution system and improve the power quality in transient situation.

Studies on Voltage Sag in Distribution Network Containing Distributed Generations

The operational state of distribution network will be changed when a large number of distributed generations (DGs) is installed in, which may cause power quality problems. In this paper, a voltage sag analysis model of distribution network containing DGs is built and simulated based on EMTDC/PSCAD. The voltage sag characteristics are extracted precisely from the simulation results by the Moving Window Method. Simulation results show that interconnecting inverter-based DG to distribution network can mitigate the voltage sag to some extent. The control strategy, installed position and output power and of DGs also exert certain influences on voltage sag. All these conclusions are available for reference on mitigating voltage sag with DGs installed in the distribution network.

Study on the Reliability of Integration between Smart Micro-Grid and Distribution Network

The general definition and operational characteristics of distributed generations (DGs) and micro-grid are summarized in this paper. Three technical requirements are proposed to achieve the most reliable interconnection between micro-grid and the external grid. A typical analysis model of micro-grid containing wind power generation and photovoltaic generation is built and simulated based on EMTDC/PSCAD. Through simulation the general requirements of reliability between the micro-grid and the external grid is tested and verified. Other particular requirements that contribute in the reliable interconnection between micro-grid and the external grid in terms of power electronic device, power quality, relay protection and control strategy, etc, are also obtained in this paper. The conclusions gave in this paper are available for the reference on achieving reliable interconnection between the micro-grid and external grid.