Reverse-Power Protection

In the first years a reverse power has been indicated by an annunciation only. H&B produced a reverse-current and direction-of-current indicator in 1894 (Fig. 1).  The rotating red disk in front of white plate showed the irregularity.

Directional relays have been used to distinguish between short-circuits at busbar or in feeders and failures in the generator. They could detect if the current flows from the generator into the grid or in reverse direction. These relays used current transformers in the generator circuit breakers; this location was the border where the overcurrent should trip without time delay.

A combined overcurrent and reverse-power relays for generators was shown by AEG in 1903 (Fig. 5). An aluminum-disk was driven by a magnetic three-leg core. The outer legs have been excited by the voltage, the middle one by current. At normal direction of current even in case of a huge overcurrent the relay is delayed, in case of reverse current it operates more or less immediately. In 1920 generators were equipped with at least two-phase or better three-phase reverse-current tripping device with sensitive setup: the relays should only trip in case of internal faults. Backup protection was realized by high-current tripping devices with a long delay. In case of tripping of the generator circuit-breaker the generator had to be de-excited. This avoids fire in the winding of the generators.

Only one power relay was used at this time because the engineers thought that only in case of a failure a power in that direction could occur. Such a "reverse power" is possible in case of power swing, bad synchronization or during a short circuit with up to 15% of the nominal power of the generator. Such a reverse relays should not endanger normal operation. The setting should be above the value mentioned or with a time longer than the power swing (1.5 s). Of course in that case the efficiency of the protection was quite poor.

The clearing time was long (for a generator protection) and the relay operates only in case of a terminal short-circuit because the voltage collapses and an active power of more than 10% could not be measured anymore. This "dead zone" could be avoided particularly with a directional relay used in a 30°- or 50°-scheme. Now the relay starts up even in case of inductive reactive currents during unequal excitation.

On Ascension Day 1924 a disaster occurred in a steam station in Erfurt (Germany) during the taking of a generator out of service. The bolt of the trip valve was full of salt and could not interrupt completely the steam supply. Now the rotor was fastened up and the new installed generator was destroyed completely.

In the 1920s AEG developed the RR2 power relays. See Fig. 2. They consist of two induction driving elements with a common Ferraris-disk. Springs hold them in the middle position. The driving elements work in Aaron-circuit. An arm moved according to the amount and direction of power. It was more or less a wattmeter with a contact. The switching capacity was poor and an auxiliary relays was necessary.

Reverse power protection was later used for protection of steam turbines.

The turbine operates as a synchronous motor and it could be damaged.

Circuit and view of a 2-pole reverse-power protection CG90c (BBC) is shown in Fig. 3 and Fig. 10.

To avoid a tripping of the protection in case of turbine blade salt deposits, the tripping signal is active only if the valve 2 is closed (Fig.4).

ZPA produced the reverse-power relay GSCT12 (Fig. 9) in the early 1970s. For measuring a ferrodynamic relay SW in Figure 8 was used. An advantage of this device was the sensitivity for harmonics because it trips on the mean value of the products of voltage and current. A torque was produced only in case of equal fundamental or harmonic.

These relays could be used for earth-fault detection too. The successor was the static relay GSCT12X in 1981.

BBC produced a static PPX110/111 (Fig. 6) in the 1970s. This relay was used for supervision and tripping of generators, but it also could detect if a generator still receives energy in case of a leak valve. Another usage was for huge changes of load which could cause an out-of-step of the generator. All these conditions could be supervised and evaluated with a counter.

A definite time reverse power relay, type WCG, produced by GEC in 1988 is shown in Fig. 11.