Protection History: Generation Protection

Author: Walter Schossig, Germany

Overvoltage-, Self- & Overexcitation Protection

Overvoltage Protection       

To avoid dangerous increases of the voltage in the windings of the machine and connected assets the rise-in-voltage protection was developed.

The main reasons recognized:

  • Increase with the voltage drop during sudden load-rejection as in case of trip during a short circuit. Soft generators (huge scattering to limit short-circuit currents) got a huge voltage drop (0.4...0.5 of the nominal voltage). Due to that and if the generator was not equipped with a voltage controller, an increase of voltage could occur up to 140...150% of the nominal voltage after 1...2 s (depending on the inertia)
  • Additional temporary overvoltage caused by increase of rotation speed and normal operation of the controller of the drive machine. Ignoring the impact of the magnetic time constant, the stator voltage increases quadratic with the rotation speed. Since the magnetic time constant in reality is much bigger than the time constant of the increase of rotation speed, in case of a fast increase of speed the increase of voltage can be estimated as linear. The biggest temporary difference of rotation speed occurring for instance at steam engines can be 20...30% of the nominal rotation speed. A normal controlled switching off of such a machine could cause a voltage increase of 1.7...1.8 of the nominal value. Since turbines in hydro power stations could reach an increase of rotation speed up to the doubled nominal voltage the danger of voltage increase due to failed turbine controllers is very high
  • Increase of voltage due to the impact of the capacitance of long lines on the excitation of the machine. This especially happens in case of a small apparent power of the machine compared with the capacitive reactive power of the line and especially if the active load of the machine is small. This could happen if for instance in the grid load has been switched off and the machine stays connected with the transmission line

The voltage controllers in normal case could not catch the increase of voltage in such cases without special fast-response regulators. The rise-in voltage protection is not needed for hydro power turbines. For thermal stations it was not needed in any case, especially if voltage controllers do exist. If the over-current protection operates during switching off and de-excitation, the over-voltage protection was still required since the load- rejection could also happen in case of switching of a circuit breaker.

Very seldom the rise-on voltage protection operates on a decrease of a field. It was more common, that the relay operates with switching off and de-excitation since the rise of voltage was a disturbance causing an interruption of the operation in any case.

Figure 2 shows an Oerlikon relay for maximum voltage with stator- and rotor-protection.

Siemens presented the rise-in voltage relay RV5 in 1936 (Figure 1). It was usable for increases of frequency of up to 200%. It was connected with a supporting reactor, this means a transformer for the turn-to-turn-fault protection, to the generator  (Figure 3). The reactor has a second secondary winding for measurement, control and synchronization purposes which saved the measurement current transformers.

BBC produced a rise-in voltage-relay SG1 in 1950. It was developed for hydro power stations and equipped with a voltage display. During a small increase of voltage as (1.2 Un) it operated with a time delay; during an increase of  1.4 Un it was not delayed (Figure 4, the Figure on page 70).

The instantaneous over- or under- voltage relay RGB was produced by BBC in 1945. There have been several variants to visualize the status if the voltage was permanently a multiple of the startup voltage:

  • Maximal voltage-relay RBGmax
  • Minimal voltage-relay RBGmin
  • Special model with increased permanent voltage RBGw
  • Maximal voltage-relay as special model with increased falling edge voltage RBGMmax (Figure 5)
  • Minimal voltage-relay as special model with increased operate voltage RBGMmin

Single-step rate-of-change voltage relays V4s 118 H 125 by Siemens have been used in the 1950s  (Figure 6). To cover short-term overvoltages a timing element was added to the measuring element.

AEG delivered the overvoltage relay RUZf. To protect generators they have been modified - in normal case with a resistance. The relay started up at 20% over-voltage. If the voltage does not drop off to the nominal value within 2 seconds an auxiliary relay tripped the circuit breaker und the de-excitation breaker. For machines  in hydro power plants, as in the Austrian KW Opponitz (Wienstrom), the relay RUZfmod had 2 systems (Figure 7).

Westinghouse's solution for an overvoltage protection as a part of a protection scheme was proposed in 1967 (left hand side of Figure 10).

Increase of short circuit currents in the high-voltage grids with solid grounding caused high ground-fault currents. A limitation was possible by isolating the different neutrals of the transformers. This was very effective for unit transformers. Opening a circuit breaker that is connecting the unit with a grounded grid, e.g. in case of load shedding, could cause dangerous overvoltages.

Nuclear power plants Beznau I and II (NOK, Switzerland) have been put into operation in 1969 and 1971. In this station a star-point switch was used as current arrester  (Figure 12 and Figure 16). The power output of both plants together was 700 MW. The generators are operated in unit connection. Four three-phase transformers with 220 MVA; 15.5/250 kV each supply the 220 kV grid. The neutrals on the high-voltage side are protected by surge arresters and grounded with current arresters. The current arresters are tripped together with the circuit breaker. The operating time of 19 ms allows an earthing before the contacts of the circuit breaker are opened.

The core of the current arrester is a breaker gap (air pressure 15 bar) with a moving and a fix contact. The fix one is connected to the bushing, the flexible is a switching tube with a plunger.

This plunger is locked by the air pressure but can be unlocked in case of a trip in a time of less than 1 ms. The housing of the surge arrester is isolated and grounded via an earthed current transformer. This ensures a switching of the arrester only in case of neutral current of less than 200A (limited breaking capacity). Control, measurement and protection devices are situated as usual in a cubicle.

ZPA produced a voltage relay VT12 in 1976 (Figure 9). It could be used as over-voltage; under-voltage and under-voltage time relay. The setting range was 1... 2 Un or 0.45... 0.85 Un, 0.4..12s and ratio 0.87 – 0.95.

Rate-of-change voltage protection is two-stage in normal case. The operating time of the first stage was chosen in such manner, that in case of increase of voltage due to decrease of current the voltage controller could operate at first. The second stage was used for the supervision of the operation of the voltage controller to avoid a voltage higher than the maximum allowed voltage of the generator. Depending from the scheme of the plant (Figure 11) it might get dangerous for the generator, the unit transformer and the station service. Since the unit transformers very often have been equipped with tap changers (e.g. ± 11 %) there was no possibility to realize the overvoltage protection for generator and unit transformer with a common relay.

A single-stage relay was sufficient for the generator. The transformer was equipped with two- stage relay. This was especially necessary for unit transformers (connected to the 220 and 380 kV directly). It was allowed to operate these transformers in the 380 kV grid with 420 kV (insulation voltage). For 5 minutes even 540 kV have been accepted, and for 5 seconds 580 kV (to handle the increase of voltage in case of load-shedding at a turbine-generator unit.)

A small automation company (BRA Saalfeld) in Eastern Germany developed an overvoltage protection after the development of the operators of a power station in 1973. The name of the device was SSR175 and could be used in 380- and 220-kV-transformer units.

The small company - BRA Saalfeld developed according to requirements of the power station operator VNE, the rise-in voltage protection SSR175 in 1973. It was used for 220 kV and 380 kV unit transformers (Figure 18).

The startup voltage was between 1.0 and 1.3 . The reset ratio at 1.3 Un was 0.98.

The static relay 7RE21-Z1 was produced by Siemens in 1984. It was a two-stage overvoltage relay  (Figure 13 and Figure 17)

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