ON INDEPENDENT BACKUP POWER SUPPLY SOURCES

N. A. Seulin, A. G. Milyushkin

(Presented by the scientific seminar of the Department of Electric Power Stations and Electrical Systems and Networks)

Consumers of a backed-up load node impose different requirements on the reliability of their power supply and on the quality of electric power. Backing up the power supply from independent sources makes it possible, more fully than is possible when duplicating elements of the power-supply scheme, to satisfy the specified requirements of individual consumers and to achieve the optimal degree of backup for the load node.

The supply of individual electrical receivers or of a group of receivers, or even of an entire district, may be provided—depending on the specific conditions—from an independent backup supply source. The choice of the type of independent backup source depends on many factors, the principal ones being:

a) the permissible duration of interruption of the power supply, during which damage is absent or is insignificant;

b) the consumers’ power;

c) the permissible deviation of voltage and frequency.

If the permissible duration of interruption of the power supply exceeds 2 min (the starting time of a backup unit from a cold reserve), then fully automated power stations with gas-turbine, diesel, or gasoline engines may be used as backup sources, depending on the consumers’ power [1].

When the permissible interruption time is more than 10–20 sec, fully automated diesel generators or units with gasoline engines may also serve as backup sources. Diesel generators have found wide application abroad for backing up the power supply of the most critical consumers in multi-story buildings, large airports, hospitals, etc. (emergency lighting, some elevators, water-supply and sewerage pumps).

When the permissible duration of interruption of the power supply is up to 0.2–0.3 sec, backup power supply can be provided by units of rapid or instantaneous readiness. There are several schemes for such units. In all schemes, however, when the voltage in the network disappears or drops sharply, the backup of the power supply is carried out by means of the inertia of a rotating flywheel mounted on the same shaft as a synchronous machine. Through an electromagnetic clutch the unit can be coupled to a diesel engine, which, after acceleration, takes over the load. The flywheel is designed in such a way that

so that, when power from the grid is interrupted before the diesel starts, the frequency does not fall below the maximum permissible value.

To reduce the weight of the flywheel and the frequency deviation when the flywheel speed changes, it is recommended to install an adjustable electromagnetic clutch between the generator and the flywheel; this makes it possible to maintain a constant generator speed when the flywheel speed varies over fairly wide limits (down to one half). For the same purpose, it is possible to install on the shaft of the instant-readiness unit a hydraulic motor supplied from a tank containing water under pressure [2].

For accident-free and economical operation of highly critical consumers (control computers, control and monitoring circuits for certain continuous chemical processes, communications facilities, radio, television, tunnel emergency lighting, operating rooms, etc.), uninterrupted power supply is required under very stringent requirements for the quality of electric power (frequency, voltage).

In such cases it is advisable to place loads that are critical and sensitive to power quality on a special power-supply system, completely isolating them from any disturbances in external networks. Independent power-supply units with rotating machines do not meet all the requirements imposed by critical consumers on backup power-supply installations.

Installations consisting of a storage battery, a charging rectifier, and a thyristor inverter with electronic control have considerably better technical characteristics. Static converters are simple, noiseless in operation, fast-acting, have very high frequency stability (independent of the load), increased reliability, ease of maintenance, and do not interfere with radio reception.

In normal operation the battery is charged through the rectifier, and if the voltage from the grid disappears, the battery supplies the load through the inverter.

To reduce the cost of the battery, its capacity is calculated for a limited period (0.5–1 hour), using a combined emergency power-supply system in which, after the diesel generator has been started, the entire load is transferred to it and the battery is switched to charging.

When there are large loads that do not permit an interruption, their supply can be provided from a generator which at first is rotated by a DC motor supplied by the battery; then the generator shaft is connected by an electromagnetic clutch to the diesel engine.

Backup supply for highly critical consumers for which an interruption in electric power supply is altogether impermissible, and for which a short-term change in frequency even by 1% or a voltage drop to 8% may lead to irreparable consequences (for example, the computer complex of stations for tracking space objects), is provided by local diesel stations with not fewer than three units, two of which operate at 50% load, while the third—standby—unit is switched on automatically when one of the operating units is disconnected.

REFERENCES

1. N. A. Seylin, Economic efficiency of local backup supply for remote consumers. Elektrichestvo, 1966, No. 5.
2. Fritz Rathman. Uninterrupted emergency electric-power supply from units with a flywheel. Electro-Technik, 1969, No. 24.