Page 250 - Uninterruptible Power Supplies

Some System Failures: The Light of Experience!
248 Chapter Ten

An Unstable Power Supply
This incident occurred at a remote location supplied from an overhead
line connected to a diesel engine generating station. The equipment of
interest was a rotary-type of UPS in which the synchronous machine
was supplied with power from a line commutated inverter. On two occa-
sions a fault on the overhead line was followed by a commutation fail-
ure within the equipment. It was important to find an explanation for
the commutation failures and a reconstruction of the sequence of
events follows.
The overhead line failures would have been phase-to-phase or phase-
to-earth faults probably caused by bird strikes. The supply voltage
would have been severely depressed and at that time several dynamic
features would have come into play:

■ Energy would have been drawn from the UPS flywheel for a few
tenths of a second until the battery contactor had closed, the rotor
speed is therefore reduced leading to a low output frequency.
■ When the battery contactor closed, the inverter would immediately
go into current limit in order to accelerate the rotor and correct the
low frequency.
■ The generating station voltage regulator would have increased exci-
tation in an attempt to restore the voltage.

As a result of these dynamic features, when the fault was finally
cleared the supply voltage from the overhead line suddenly increased
to 118 percent of nominal and this voltage was applied to the line com-
mutated inverter already in current limit. Commutation was not pos-
sible and the failure led to a short circuit of the dc supply and a
complete failure of the UPS output.
The equipment included a rectifier contactor, and to prevent a
recurrence the control circuit was changed to ensure that on a loss of
supply the contactor opened and remained open for 2 s. The equip-
ment was quite capable of accepting the 118 percent voltage surge
under normal conditions and the short delay ensured that on restora-
tion of supply the inverter would have come out of its current-limited
mode of operation.
This incident demonstrates the complexity of the occasional failures
experienced on systems. It is not always easy to reconstruct the events
leading to a catastrophic failure. After this incident the fault conditions
were reproduced within the manufacturer’s works to demonstrate that
the reconstruction was credible.

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