Siemens 7SA511 manual Overview of Hardware and Protection Functions

7SA511 Line Protection Relay Reference A

A. Method of Operation

A.1 Overview of Hardware and Protection Functions

The 7SA511 relay is a microprocessor-based, high speed, numerical distance relay designed to provide fast, reliable, and selective clearance of all kinds of ground and phase faults on overhead lines and cables being fed from one point or multiple points. It can also be used as local backup and/or bus protection through reverse interlocking or to provide backup protection for generators, and it can be installed on grounded or ungrounded systems. Figure A-1 shows the basic structure of the unit.

The relay provides full digital processing of all functions, from data acquisition of measured values to the trip commands for the circuit breaker. Protection functions provide by the 7SA511 relay are:

Distance protection (21/21G) with directional discrimination Power swing protection (78)

Emergency overcurrent protection (50/51/50G/51G) Pilot protection (85)

High-resistance ground fault protection (50G/51G) with directional discrimination in grounded systems

￿Automatic reclose (79)

Ground fault detection with directional discrimination in ungrounded (isolated or compensated) systems

The analog input module (see Figure A-1) contains input amplifiers, sample and hold elements for each input, multiplexers, and analog/digital converters. Analog current input signals enter into the relay isolating transformers and then pass through anti-aliasing filters to the analog input module. The anti-aliasing filters’ bandwidth and processing speed are optimized to match the protection algorithms. The input signals are sampled 20 times per power cycle (one sample every 0.83 milliseconds at 60 Hz) and converted to digital signals.

The 7SA511 relay has seven voltage transformers and four current transformers. The first three voltage transformers are wired in a wye to measure the phase-to-ground voltages. The second three voltage transformers are wired in a delta to measure the phase-to-phase voltages. The three delta terminals are internally connected in parallel with the three wye terminals. This connection scheme results in a total of 4 inputs - one for each of the three phases and the neutral. The remaining voltage transformer input VE is used for measurement of the displacement voltage VE from the broken-delta windings of the VTs. Three of the five current inputs are for input of the phase currents of the protected line. The remaining current input is used for one of the following:

Residual current of the protected line

￿Ground current of a parallel line for parallel line compensation

Neutral current of a grounded infeed transformer for directional ground fault protection

July 27, 1995 A-5