TP-6953 7/19136 Section 7 Troubleshooting
Protective Relay Intended Function Potential Causes Troubleshooting
Over Frequency
(on the Decision-
Makerr 3500
controller:
AC Frequency:
High Warning)
To protect the customer
loads from poor quality
power.
System Control mode. System Control mode will force all generators in the system to control real power to a target set by the
speed bias. If the actual power is lower than the target, the speed will increase. If the actual power is
higher than the target, the speed will decrease. Make sure that no controllers are receiving a system
load control input. Make sure that no controllers are configured for System Control mode in software.
System Sync mode. System Sync mode will force all generators in the system to match frequency to a bias level s et by the
speed bias (which could be from 95% to 105% of rated speed). Make sure that no controllers are
receiving a system sync control input. Make sure that no controllers are configured for System Control
Mode in software.
Baseload. Baseload mode on a controller may decrease the f requency of the paralleling bus if attempting to
unload real power when the other generators are not able to support the load. Check for other nodes
configured in Baseload mode. Check if this node is configured for Baseload.
Excessive restriction on return
line.
On certain units, excessive restriction on the return line will cause seals to fail in the injector pump
which may supply unwanted fuel to the engine causing it to ’run away’ (accelerate without being
commanded to do so by the ECM or governor). Verify backpressure at the return line. Ensure that no
valves exist in the return line that can be accidentally closed (without knowledge of the implications).
Frequency Trim disabled. The Frequency Trim corrects the generator output frequency to a nominal level under normal
operation. If it is disabled, the generator frequency could fall anywhere between 95% and 105% of
nominal. Make sure that Trims are enabled on at least 1 of the paralleling generators.
Incorrect Protective Relay setting. Ensure that protective relay settings are determined by a comprehensive coordination study using
recent and accurate system data.
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