Page 89 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 87

There must be provision for occasionally connecting to “dead bus-
bars” when, obviously, synchronizing is impossible and the control sys-
tem must take care of this condition. It arises when a single set or the
first of multiple sets is started and connected to “dead busbars.”
For manual synchronizing the synchroscope is arranged to be easily
visible from each generator panel, and each panel includes raise/lower
controls for speed and excitation so that the operator can make fine
adjustments while watching the synchroscope. It is likely that the
electromechanical synchroscope will be replaced by fully electronic ver-
sions, but clear visibility is essential. It is usual to include a check syn-
chronizer in the control system; the check synchronizer prevents circuit
breaker closure unless the voltage, frequency, and phase relationship
are within limits.
For automatic synchronizing the control system takes care of the
adjustments to voltage, frequency, and phase relationship, and closes
the circuit breaker when conditions are within limits.

Test Load Facility
Consideration should be given to including in the switchboard a
switched outlet to which a test load can be connected when required.
The circuit should be fused or otherwise protected and fully rated for the
output of one set. For a multiple-set installation this will usually be ade-
quate, but for a large important installations the rating could be
increased. This facility is useful after major work has been undertaken
on a set and avoids the need for temporary connections within the
switchboard in order to connect a test load. The connection of a perma-
nently installed test load to this circuit is discussed in Chap. 3.

Standard Reference Conditions
The standard conditions for switchgear are specified in ISO 8528 as:
Altitude. Not exceeding 2000 m above sea level. Note: For high-volt-
age switchgear BS EN 60694 sets the maximum at 1000 m above sea
level.
Air temperature. Maximum not exceeding 40°C and 24-h average
not exceeding 35°C.
Relative humidity. Not exceeding 50 percent at 40°C. Note: For
high-voltage switchgear more complex rules are given in BS EN
60694.

It should be noted that, at altitudes above the reference level, the
reduced density of the air leads to a reduction of the dielectric strength
and of the cooling effect, and special considerations are demanded.

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