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Short Line Distance Protection Solution Based on Current Restoration Disclosure Number: IPCOM000217307D
Publication Date: 2012-May-07
Document File: 5 page(s) / 146K

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Short Line Distance Protection Solution Based on Current Restoration

Jianping Wang, Jiuping Pan


Majority of large city power grids (typically in the voltage range from 60kV to 130kV) are characterized by high load densities, meshed network topologies comprising of short distance lines, strict requirements for reliability and excessive reliance on power import from outside sources. One major concern of such city power grid is the increased short circuit levels that require upgrade of circuit breakers with higher interrupting capability and also current transformers with higher transient performance features.

The measure of a current transformer (CT) performance is the ability to reproduce accurately the primary current on the secondary side of the CT in terms of its magnitude and shape. Fault currents, which may be much greater than rated currents and which may be non- symmetrical (such as DC components in the fault currents), often cause distortion of the secondary current due to CT saturation effects. CT saturation leads to inaccurate current measurement and therefore may cause malfunction of protective relays and control devices that use currents as input signals. For example, a distorted current waveform may result in delayed operation of inverse-time over-current relays due to underestimation of RMS value of current waveforms. Distance relays may experience both over-reach and under-reach problems in fault impedance calculations due to inaccurate current measurement.

Challenge for short HV line protection

One challenge in large city power grid protection system is the requirement of fast and reliable short line protection schemes. Shorter lines can be protected by protections relying on different communication links, such as pilot wire and leased lines, etc. These relay systems, however, are very costly to install, operate and maintain. It is thus desirable to implement distance protection schemes for short distance lines. Currently, the application of distance protection is limited to certain line length, for instance longer than 10-15 km. This limitation is mainly due to measurement errors introduced by saturated current transformers. Many studies have shown that CT saturation causes distance relays to underestimate correct distance, which causes over- reach problems of distance protection. It is feasible to use coordinated distance zone 2 protection schemes known as permissive overreach transfer tripping scheme which permit transfer tripping via zone 2 distance relays, while reducing the zone 1 setting reaches. But it is much desirable to extend the coverage of zone 1 distance protection schemes immure to saturation of current transformers. The most challenge is to obtain accurate fault current on the faulted short line even when the line side CT severely saturated.

Several methods have been discussed in the literature on handing CT saturation problems. These methods can be generally divided into two major categ...