REG216, CompactREC216 Numerical Generator ProtectionNumerical Control Unit

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DESCRIPTION OF FUNCTION AND APPLICATION

Summary

Both analogue and binary I/P signals pass through a signal con ditioning stage before being processed by the main processor: Analogue signals go through the chain comprising the input transformer, shunt, low-pass filter (anti-alias filter), amplifier, sampling (sample and hold), multiplexer and A/D converter. As digital signals they are then separated by digital filters into real and apparent components before entering the main processor. The binary signals are isolated by opto-couplers in the input cir cuits and are evaluated by the main processor. Only now are the protection algorithms and the logic functions processed in the main processor.

A delay function and a logic function are also included. The de lay function enables a time to be defined between pick-up (I/P of the function) and its operation. A setting is also provided for the reset time. The logic function provides an AND, OR or RS gate for combining different O/P signals (of functions and binary I/P’s).

The distance protection function is equipped with overcurrent or underimpedance starters. In ungrounded systems, all the usual cyclic and acyclic phase preference schemes are available.

The residual (neutral) current and/or residual (zero-sequence) voltage are monitored to detect earth faults.

The first distance zone, the overreaching zone and the reverse zone measure simultaneously. All of the distance zones have wide setting ranges and can be set completely independently of each other, also with respect to whether they measure in the forwards or reverse direction. There are three directional dis tance zones and a fourth, which can be either directional or non directional as demanded by the application. The overreaching and reverse zones are for use in transfer tripping schemes with signal transmission between the units at the two ends of a line. The operating characteristics are polygons with the reactance borders slightly inclined to give an ideal tripping area. For close three-phase faults with very low voltages, the use of a reference voltage comprising a healthy voltage and the voltage of a mem ory feature ensures a reliable directional decision.

Compensation of the mutual zero-sequence impedance of paral lel circuits can be achieved by appropriate selection of the zero sequence impedance factor (k0) or the residual current of the parallel circuit using k0m.

A v.t. supervision feature (fuse failure) is already incorporated. Its measurement can be based on either the zero-sequence component (U0 . not I0) and/or the negative-sequence compo nent (U2 . not I2). The latter is of special advantage in un grounded systems or systems with poor grounding.

An independent back-up overcurrent function becomes a short zone protection, as soon as the line isolator is opened. Tripping of the overcurrent back-up protection is uninfluenced by any signal, which may be blocking the distance protection (e.g. v.t. supervision or power swing blocking).

The power swing blocking function monitors the variation of the quantity U . cos . This method of detecting power swings is en tirely independent of the characteristic and location of the dis tance protection. Power swings with frequencies between 0.2 and 8 Hz are detected.

The sensitive E/F protection for ungrounded systems and sys tems with Petersen coils measures both in the forwards and re verse directions. A characteristic angle of 90 (U0 . I0 . sin ) is chosen for ungrounded systems and one of 0 (U0 . I0 . cos ) for systems with Petersen coils.

An logic, which can be freely programmed with the aid of FUPLA (function block programming language), provides convenient fa cility for achieving special circuits needed for specific applica tions.

The auto-reclosure function enables up to four three-phase re closure cycles to be carried out, each with independently set dead time for fast or slow auto-reclosure.

Where necessary, a large variety of supplementary protection and logic functions is contained in the RE. 216 and RE. 316*4 function software libraries.

The distance protection logic gives the user access for blocking or enabling purposes to a wide range of functions, including for example the kind of transfer tripping scheme, switch-onto-fault logic, zone extension logic, v.t. supervision logic and whether the protection should trip just the phase concerned or all three phases for an E/F.

The memory of the event recorder function has sufficient capac ity for up to 256 binary signals and their relative time tags.

The memory of the disturbance recorder registers 12 analogue and 16 binary signals. The number of events it can actually re cord depends on the total duration of an event as determined by the amount of pre-event data (event history) and the duration of the event itself.

Where necessary, a large variety of supplementary protection and logic functions is available in the MODURES 216 function software library, which can provide other protection functions such as restricted earth fault protection or standstill protection.

Restricted E/F protection for a transformer

Basic requirements A restricted E/F scheme must be able to detect E/F’s in the protection zone remain stable during both phase and earth through-faults. The scheme is designed to remain stable in the case of a solidly grounded star-point for an external E/F current in the case of an impedance grounded star-point for the highest external phase and earth fault current. When designing a scheme, it is assumed that one c.t. is fully saturated and none of the others are.

Design

The E/F current is determined by a) the generator and step-up transformer reactances when the HV circuit-breaker is open (see Fig. 4.1) b) in addition to a) by the HV power system when the HV cir cuit-breaker is closed (see Fig. 4.2). As a result of the current distribution for a through-fault, the star point c.t. conducts the highest current in the case of a solidly grounded transformer as shown in Fig. 4.2. Apart from the burden, the high fault level results in a high c.t. flux and the probability of it saturating is then also high. The influence of through faults on the circulating current circuit is limited, especially if the connections between the c.t. cores can be kept short. It is for this reason, that phase faults are ne glected when designing a scheme for a solidly grounded system. Phase faults have to be taken into consideration, however, where a system is impedance grounded. The value of the stabilising resistor is chosen such that the volt age drop caused by the highest external E/F and possibly phase fault current across the secondary winding and leads of the saturated c.t. cannot reach the pick-up setting of the protection (see Fig. 4.3).

The knee-point voltage of the c.t’s is specified such that the c.t’s can supply sufficient current during an internal fault to enable the protection to trip. The knee-point voltage Uk of the c.t’s must therefore be appreciably higher than the voltage drop Ua

Symbols used:

IE I2 I2N I1N IN R2 RL Ua , Ui Uk Û I RS IF Equations: primary star-point current (AC component) for a through-fault secondary current of the non-saturated c.t’s c.t. secondary rated current c.t. primary rated current protection rated current secondary resistance of the saturated c.t. at 75°C lead resistance according to Fig. 4.3 voltage drop across the circulating current circuit for external, respectively internal faults. knee-point voltages of the c.t’s peak value of voltage current setting stabilising resistor highest primary fault current (AC component) for an internal E/F.