Distance Analysis for Power Systems

Author:
Mohamed Ibrahim, Fellow IEEE, USA

Digital fault recording equipment is basically applied on the bulk power system for post mortem analysis following the occurrence of system disturbances. In addition, DFR analysis can confirm the operation of the relay elements which activates the tripping of the associated circuit breakers to clear the fault. Accurate RMS measurements as well as a host of software packages can be executed to verify system model and assess the impact of disturbances on power system equipment. DFR analysis is presently being investigated for a new concept referred to as event based maintenance (EBM) for application to periodic testing of numerical relaying. Self-monitoring features of numerical relaying coupled with DFR analysis can be used to steer the periodic minimum testing requirements for numerical relaying. This concept if successful will replace the present injection testing methods and will result in cost savings and methods improvements.

Benefits of Disturbance Analysis Function for Power Systems
Analysis of all disturbances on the system is an important function, however, it is realized that this function may demand additional manpower that may not be available within the present utility environment. Indeed, it should be realized that the knowledge gained from analyzing mundane operations may prove to be very valuable. The following will provide a summary of some of the benefits that are gained from analyzing system disturbances:

  • Provide vital parameters to Energy Control Centers like; fault type and fault location, needed to restore the system
  • Monitor the performance of the relaying system and associated inputs, outputs, communication system and power system equipment (i.e. circuit breakers, transformers)
  • Obtain RMS ground current calculations to confirm power system short circuit model
  • Develop statistics regarding fault summary
  • Optimize the performance of the relaying system by optimizing the design process through analysis feed-back process
  • Review of mundane operations that result in successful fault clearing can reveal valuable power system phenomena, relay hidden failures and correction of system design and modeling errors
  • Support the research effort to justify the use of event based maintenance concept as a substitute for injection testing methods for the numerical relaying systems

Phenomena Related to System Faults and the Clearing Process of Faults from the Power System
Power system disturbances are mainly classified as shunt faults. Disturbance analysis provides classification of the fault as phase or ground shunt faults and description of their causes. Fault type and fault location are two essential parameters that needed to be given to Energy Control Centers to be able to restore the system as soon as possible. Arc-over at voltage peak represents a slow fault creation mechanism when insulation fails, while lighting hits are random and may produce fault incident points at zero voltage. Faults occurring on the power systems can be therefore either symmetrical or asymmetrical depending on the fault incident angle. In addition, DFR analysis will provide description about the nature of fault currents and their different modes of clearings from the power system. Fault modes of clearing is determined as either high speed or sequential or delayed local backup (breaker failure) or remote delayed backup. Power system phenomena related to the fault clearing process can also be determined from disturbance analysis.

The following case studies will illustrate some of the power system shunt fault clearing modes:

Case Study 1: Sub-cycle relay operating time
Figure 1 shows the 765 kV system simplified one line diagram and the monitored DFR currents and voltages used to analyze the A-C-g fault. Figure 2 reveals a Sub-cycle relay operating time during an EHV A-C-g double phase-to-ground which has resulted in a total clearing time of 2 cycles for phase C and 2.5 cycles for phase A of the fault. Total fault clearing time consists of relay operating time and breaker interrupting time. The EHV circuit breaker has a two-cycle specified nominal interrupting time. The sub-cycle operating time to clear this A-C-g fault is done by the electromechanical induction cup relay. Electromechanical induction cup or cylinder relays can rotate in less than 8 ms in the presence of enough fault currents. The performance of these relays in the presence of enough fault currents is hard to match with the present state-of-art numerical relaying technology. Based on nominal interrupting time of two cycles, it can be postulated that a total clearing time of two cycles is accomplished by a relay operating time of less than one cycle (sub-cycle).

Case Study 2: Sequential clearing of a phase C-to-ground fault
Sequential clearing occurs for systems with weak contribution from one (local) end and strong current contribution from the other (remote) end. Ground fault levels are impacted by the positive and zero sequence sources. When the strong end clears first, the ground fault will be fed as stub from the weak end forcing the ground current to increase slightly and reach the relay pick up setting to operate and clear the fault sequentially.

Sequential clearing should be avoided for certain power system configurations to avoid prolonging of faults on the system. This can be accomplished by applying sensitive current differential pilot relay schemes or distance based pilot scheme with weak-in-feed logic.

Figure 3 shows the 345 kV system simplified one line diagram and the monitored DFR currents and voltages used to analyze the C-g fault. Figure 4 reveals line L1 DFR record where substation Y clears the fault in 7.5 cycles by Z1 ground distance element. Current contribution for substation X goes up allowing substation X to clear the C-g fault sequentially by the pilot ground distance element after an additional 5.5 cycles with a total clearing time of 13 cycles.

 

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