Instantaneous Overcurrent Protection Research Articles

A Rotor Winding Internal Short-Circuit Fault Protection Method for Variable-Speed Pumped Storage Units

In electrical machinery, the rotor windings’ internal short-circuit faults are addressed by the instantaneous over-current protection of the power electronic excitation device, which has low sensitivity and has difficulty meeting the safety requirements. In this paper, a rotor windings’ internal short-circuit fault protection method is proposed based on the harmonic characteristics of the circulating current between stator branches. The magnetomotive force distribution of the short-circuit coils in the rotor windings is theoretically deduced, and the characteristic frequencies of the circulating current between stator branches are analyzed. On this basis, the protection criterion of the rotor windings’ internal short-circuit fault is constructed by using the harmonic component of the circulating current. Then, an analytic model of the variable-speed pumped storage unit is established based on the multi-loop method, and the finite element method is used to verify the correctness of the proposed modeling method. An actual large variable-speed pumped storage unit is taken as an example, and the possible faults under different slip ratios are simulated. In the simulation results, the stator branch circulation has the obvious characteristic frequency harmonic components, which is consistent with the theoretical analysis. It verifies the effectiveness of the proposed protection method. Finally, it is analyzed and verified that the proposed protection has a strong maloperation prevention ability under other kinds of faults.

Methodology of Adaptive Instantaneous Overcurrent Protection Setting

This paper proposed a methodology of adaptive instantaneous overcurrent protection (AIOCP) setting that ensures that the protection coverage remains unchanged regardless of the operating condition of the electrical network. The methodology calculates the protection setting parameters based on the real-time estimation of the Thevenin equivalent circuit (TEC). The estimation algorithm uses the positive-sequence voltage and current values and a system of non-linear equations, which is solved iteratively by the Gauss–Newton method. The proposed methodology calculates the IOCP settings in real time by using the real-time estimation of the TEC; therefore, any change in the electric network conditions is represented in the TEC, and the calculated setting keeps the desired protection coverage constant. Simulation results demonstrate that the proposed AIOCP can keep constant the protection coverage overcoming the classical problems of classical IOCP: sub-allocation and over-coverage.

Analysis on 2nd Main Transformer Trip Accident of a Certain Substation of the West Inner Mongolia Power Grid

This paper mainly analyzes the causes of a 10KV line and 2nd transformer circuit breaker tripping, also analyzes the action mechanism of the relay protection. Because of the permanent phase fault at the head of the line, instantaneous over-current protection(SegmentⅠ) as its main protection of the phase fault made the 921 breaker triping, the action was selective.And the permanence of this fault led to the failure of the reclosing; Reversed the TA(current transformer) for measuring with the TA for differential protection in high voltage side of the transformer, this fault can also make the TA of the differential protection circuit quickly saturated when the external short-circuit current flowing through the TA. When the above saturation happens, a big differential current generating from differential circuit will lead to sampling error. This is the direct cause of the mal-operation of the differential protection which resulting in the 2nd transformer tripping. In response to this accident, put forward specific prevention measures.

Design and implementation of a PWM solid-state synchronous condenser

A new PWM solid-state synchronous condenser (SSSC) is designed and implemented in this paper. The power circuit of this SSSC is simple as only four power semiconductor switches are used. The SSSC allows leading or lagging reactive power compensation, and continuous control of reactive power. It has a rapid response, which allows an almost instantaneous reactive current control, with low harmonic distortion in the line current. Because it uses a current control, instantaneous overcurrent protection is provided naturally, thereby increasing the reliability. The capability of the proposed synchronous condenser is estimated. To ensure a clean line current waveform is obtained, the current-tracking mechanism is analysed with a graphical aid and a critical condition is presented. The performance of the SSSC is demonstrated with some simulation and experimental results.