Optimizing Security and Sensitivity for Line Differential Protection Applications

摘要

Numerical phase segregated line differential relays employing digital communications over dedicated or multiplexed communication networks have been employed for various types of line topologies for numerous reasons: 1. Electrically short lines 2. High and variable load transport with directional changes ("wild" power systems) 3. Taps on electrically short lines 4. Lines subject to power swings 5. Multiple and variable mutual coupling situations in power corridors 6. Unavailability of VTs for distance measurement, or difficulty obtaining polarizing quantities due to system configuration Key to the security and sensitivity of these relay systems is how they handle the effects of the following situations for in zone and out of zone faults: 1. Measurement error due to CT loading 2. Measurement error due to CT saturation for heavy currents 3. Different DC decay times due to different CT set secondary circuit time constants 4. Application of differently rated CT sets with different saturation thresholds and secondary CT circuit time constants 5. Timing errors if applied in multiplexed systems with split path possibilities (switched networks) 6. External events such as out of zone transformer switching with accompanying inrush and effects on different CT sets 7. Clearing of external faults where the CT sets have different rates of DC offset (and therefore decay) due to different CT secondary circuit X/R component 8. Sensitivity of switch onto fault detection We will explore the use of an adaptive strategy to automatically select the optimal element for security and sensitivity when challenged by the above situations. We will also explore the use of dynamic restraining techniques to accommodate rapidly changing conditions of CT loading, CT saturation and communication network timing errors.