Himanshu J. Bahirat

Application of Super Conducting fault current limiter in Indian grid

With the fast load growth, the grid is pushed to operate near to the limits. The fault current levels at critical substations can overshoot the limits on various equipments like busbars, circuit breakers and transformers. The various conventional remedial measures to reduce fault currents are reported in literature. However, there are time critical situations which can be better tackled by recent devices like fault current limiters (FCLs). Super Conducting FCL (SCFCL) can limit the fault current below the prospective peak values. The paper presents a simple steady state model for SCFCL. An analytical approach to determination of SCFCL impedance is presented. An utility test system is selected to demonstrate the effectiveness of the SCFCL. The location and sizing issues of the SCFCL to minimize cost and obtain desired fault current reduction are also addressed.

Short circuit currents of DFIG based wind turbines

The share of wind energy is growing day by day and asynchronous machines like doubly fed induction generators are finding their place in the grid. These asynchronous generators under transient conditions like fault behave in a different manner than synchronous ones. In order to achieve fault ride through capabilities for such generators special protection scheme is needed. One of the solution technique is to use crowbar resistances on rotor winding to protect the rotor side converter. During a fault, the current profile of DFIG is different compared to a conventional generator. This paper focuses on the short circuit current profiles of DFIG for wind turbines. An analytical expression is described for an asymmetrical fault current which is slip and initial condition dependent. Crowbar protection scheme and its effect on short circuit characteristics has been discussed. A brief analysis on the effects of these fault currents on generator circuit breaker is considered. Simulation results compared with theoretical analysis is presented for verification.

Controlled switching of power circuit breakers

This paper presents an overview of importance and significance of controlled switching in complex power networks. The main objective of controlled switching is to avoid transients in the system. In a power system network transients are the redistribution of stored energy among the system capacitance and inductance whenever there is a change of state. So controlled switching is the method to optimize the instant of switching the circuit breaker resulting in minimal energy redistribution.

Superconducting fault current limiters for multi-terminal DC grid applications

During the last decade there has been an increasing interest in the High Voltage DC grids amongst researchers and policy makers. Protection has been identified as a critical component in the realization of these multi-terminal direct current (MTDC) systems. The fault currents in the DC systems are limited by the circuit resistance and inductance and thus are likely to reach very high values. This may also lead to voltage collapse. In order to limit the fault current magnitudes and to prevent voltage collapse use of superconducting fault current limiters (SCFCL) in MTDC system is proposed in this paper. The effectiveness of SCFCL in limiting the fault current along with sizing of SCFCL resistor are explored in this paper. The paper also discusses the effect of initial discharge and critical current setting of SCFCL.

Impact on superconducting fault current limiters on circuit breaker capability

There has been significant capacity addition to the power system leading to increased fault current at substations buses. This call for costly equipment upgrades and replacements. In order to defer investments utilities are exploring use of superconducting fault current limiters (SCFCL) and current limiting reactors (CLR). The SCFCL can be built with either resistive or inductive shunts. Also, it might be possible to include a recovery switch in series with the superconducting element. The choice of shunt is likely to have significant impact on the current limiting capability of the SCFCL. Also, for correct application design the circuit breaker transient recovery voltage (TRV) should be within its capability limits. It is expected that the TRV will be impacted by the choice of shunt and use of the recovery switch. The paper analyzes the effect of type of shunt on efficacy of limiting fault current and impact on TRV using EMTP-ATP. It is observed that the use of resistive shunt may be better for limiting first peak of the current and limiting TRV values. The inductive shunt reduces the steady state fault current effectively. The use of recovery device has adverse impact on the TRV characteristics of the system.

Thévenin Equivalent of Voltage-Source Converters for DC Fault Studies

This paper presents a simple modulation index-dependent Thévenin equivalent circuit model of the voltage-source converters (VSCs) and the associated anti-parallel diode rectifier. The model derivation uses orthogonal transformation and a transformer analogy to transfer impedances to the dc side of the converter. The paper proposes a switched equivalent representation of a three-phase diode rectifier formed due to the antiparallel diodes when VSC switching is inhibited. The proposed model is implemented in Alternate Transients Program-Electromagnetic Transients Program and simulation results are compared with the switched model. The proposed model is shown to under predict the fault currents for pole-ground (PG) faults and to provide a nearly exact match for pole-pole faults. The rectifier equivalent provides increased accuracy in the transient region with the inclusion of the source inductance. A multiterminal dc (MTDC) system model is used to study effects of aggregation which gives increased accuracy of the equivalent circuit. It is shown that the MTDC system simulation with the proposed Thévenin equivalent is about 3 times faster than full-switched circuit simulation for a time step of 5 μs. The MTDC system simulation with the Thévenin equivalent circuit is still accurate and 126 times faster when the time step is increased to 750 μs.