Circuit Breaker Derating due to High DC Time Constants

Overview

Due to expansion plans and new generation connections, EirGrid and System Operator for Northern Ireland (SONI) are expecting increased fault levels with high DC time constants in some areas of the all-island transmission grid. PSC provided guidance on industry best practice and a recommended method to de-rate the circuit breaker AC component accordingly.

Challenge

EirGrid and SONI wanted to investigate methods for assessing the asymmetrical breaking duty in HV circuit breakers exposed to DC time constants higher than the IEC 62271-100 standard test parameters and to develop an accurate and practical methodology to de-rate the AC component, if appropriate, in a cost-effective manner. This methodology should reflect international best practice and must not compromise safety as far as reasonably practicable.

Solution

PSC initially reviewed and discussed the published literature relating to international standards, technical recommendation and international best practice with regards to de rating of circuit breakers. PSC then developed a methodology to calculate de-rating factors using an accurate and simplified method for calculating major loop area. The basis of the methodology is the comparison of the energy under that area for the DC time constant at which the circuit breaker (CB) was tested and the DC time constant of the system short-circuit current the CB is required to interrupt. A comparison of de-rating factors calculated by the different published methods including for constant arc voltage and constant arc resistance was presented to compare the strengths and weaknesses of each method. PSC recommended the CB de-rating method to be adopted as the preferred option by EirGrid and SONI and performed calculations for various 3-phase short-circuit fault scenarios.

Results

PSC presented an assessment of a range of accurate and practical methods for investigating asymmetrical breaking duty and de-rating circuit breakers which proposed a cost-effective methodology. A recommendation was also given to EirGrid and SONI to adopt a special case dc time-constant and accept a specific limit on the major current loop length for new switchgear to be installed in areas of the all-island transmission system with high DC time constants.