Tianshu Bi

WAMS Implementation in China and the Challenges for Bulk Power System Protection

Synchronized phasor measurement technology has been gaining increasingly interests in China due to its wide area time synchronization capability and fast transmission speed. Over 10 years of implementation of wide area measurement system (WAMS) in Chinese power grid have shown its great value in power system dynamic monitoring and potential applications in system modeling and validation, feedback control and system wide protection. This paper reviews the overall implementation of WAMS in China and analyzes the corresponding characteristics of WAMS. Moreover the challenges for bulk power system protection with synchronized phasor measurement technology are presented also.

Transmission line parameters identification based on moving-window TLS and PMU data

The accuracy of transmission line parameters is crucial to power system normal operation. The development of PMU and WAMS supply a new way to obtain the real-time value of line parameters. In this paper, it is proposed that the moving-window total least squares algorithm for transmission line parameters identification directly based on PMU data. The method treats parameters constant during a time window and establish TLS model. Meanwhile, Kernel density estimation and point estimation are used to discuss the credibility of too many identification results. Comparison simulation with single sample computation demonstrates the presented algorithm can alsomaintain accuracy and stability under noisy condition. In addition, practical PMU data identification of one 500 kV transmission line verifies moving-window TLS effective.

Flow transferring identification algorithm with consideration of transient period

Cascading trips caused by flow transferring are crucial for lots of well-known blackouts. For a power system with heavy load flow, once there is a fault, the system might suffer flow transferring after the fault clearing. However, for relays, they cannot know the sensing overcurrent is caused by internal faults or flow transferring. The relays will trip the overcurrent after the predefined time delay, which might result in flow transferring again and worsen the system state. Therefore, if relays pickup due to flow transferring, the back-up relay should be blocked until the thermal limit of the line reaches. In this paper, Flow Transferring Relativity Factor (FTRF) is suggested to estimate the flow distribution after a line is tripped. Moreover, based on the relationship of node injection and branch currents, the impact of transient period on the flow distribution is analyzed. Finally, a flow transferring identification algorithm is presented to distinguish flow transferring from internal fault caused by fault clearing. The simulation results of 10-generator 39-bus New England system show that the proposed flow transferring identification algorithm works well.

Rotor position measurement method for generator power angles

In traditional engineering applications, the internal voltage and power angle of generators are often measured by the electrical calculation method. The internal voltage and power angle of generators are calculated by direct-axis reactance Xd, quadrature-axis reactance Xq, terminal voltage and current which measured real-time. This calculation method may result in deviation because the value of Xd and Xq may be changed when the generator running in different status. This paper proposes a kind of rotor position measurement method. The internal voltage and power angle of generators are gotten by using generator phase keying signal and terminal voltages. This method is independent of generator equivalent calculative models and the value of synchronous reactance. It has high accuracy and suitable for real-time power angle measurement when power grid is in a disturbing or transient processing.

Wams Based Flow Transferring Indentification Algorithm and its implementation

When relays trip a faulted line in power system, the flow on the faulted line will be transferred to the other lines. Those lines might be overloaded and then cascading tripped by the backup relays, which worsen the system state and even cause blackout. Generally, these kind of cascading trips are the origins of a lot of well-known blackouts. In this paper, by analyzing the defects of traditional backup protections, a new wide area backup protection scheme based on WAMS is presented to identify flow transferring. A new concept of flow transferring relativity factor (FTRF) is proposed to estimate the flow distribution after flow transferring occurs; and the method of calculating FTRF matrix online is also deduced. The simulation of 10-unit and 39-bus New England network shows that the proposed wide area backup protection scheme can identify flow transferring successfully and hence be superior to the traditional backup protection.

Configuration and setting of protection for large-scale wind farm

Wind turbine generator is characterized with certain unique features during their normal and faulty operating conditions. Conventional protection for a wind farm faces challenges regarding the coordination, selectivity and sensitivity of relays. In this paper, the coordination between the wind turbine generators protections and the wind farm operation rules required by the national standard in China is discussed. Principles to choose an appropriate fuse for step up transformer are put forward. Configuration, setting and performance of two kinds of collector lines over-current protection are analyzed and compared, and the better one is suggested. Besides, protections for main transformer and buses are also suggested. The aimed study of wind farm protection is carried out on a practical wind farm in a certain province in China as an example. Setting results show that the over-current protection of collector line satisfies the requirements of selectivity and sensitivity.

Relay replacement strategy based on the least unit life cycle cost with minimum maintenance model

Protection device is repairable system, and its routine maintenance is Minimum Maintenance. With the number of maintenance increasing, both the reliability of relay and the economy of maintenance are decreasing. Therefore, it should be replaced at a proper time. The optimal replacement strategy was studied based on relay reliability and maintenance economy according to field data. It first constructed a minimum maintenance model. Then, based on the constructed model, it formulated the replacement strategy as an optimal problem with the objective of least unit life cycle cost. Finally, it obtained the optimal replacement cycle for a field-operating relay with the proposed strategy. In addition, in order to simulate the actual situation with more accuracy, it considered the double protection in the proposed strategy.

A phase selector for transmission line protective relaying using the multi-parameter comparison criterion

A novel phase selector is proposed based on the multi-parameter comparison of fault components. Each of three phases is used as the reference phase in turn to calculate the positive sequence component and the sum of the zero and negative sequence components. The phase difference between the positive sequence component and the sum of the zero and negative sequence components is calculated under three different reference phases. Therefore, a multi-parameter criterion is proposed based on the three cosine values to accomplish the phase selection. The proposed method has an obvious boundary for faulty phase discrimination. Furthermore, the proposed method has good performance of sensitivity and reliability, and its effectiveness is verified by the simulations.

Impact of fault clearing time on shaft torque amplification in series compensated system

Series capacitor compensation in AC transmission systems may cause subsynchronous resonance (SSR) that can lead to turbine-generator shaft fatigue. One of the SSR phenomenons is referred to as shaft torque amplification, which might cause serious damage to the shaft. A methodology considering shaft torque as the torsional system response of applied force is given in this paper. The maximum shaft torque is finally deduced as an analytic function of fault clearing time using a simplified two-DOF model. The conclusion could be obtained that fault clearing at odd numbers of half cycles of the dominant torsional mode will generally provide maximum torques. Simulation results demonstrate the validity of the conclusion, which helps in choosing a proper fault clearing time that could avoid shaft damage and decrease reduction of shaft life efficiently.

Researches on the time-varying failure rate characteristic for the HVDC protection devices in China Southern Power Grid

In China Southern Power Grid (CSG), the SIMADYN D (SD) of SIEMENS plays an important role for the HVDC protection system. Based on the field failure data of SD, this paper studies the time-varying failure rate (TFR) characteristic for the protection devices of HVDC in the CSG. According to the concept of TFR, failure mode is divided into two types, one is the aging failure and the other is the random failure. According to the above failure classification, the failure characteristic of different modules is also discussed.