3.5.5 Dynamic Frequency Hz/S
Some frequency relays require a dual ramp to perform this test. Three sets of data need to be
established to perform the test. These sets of data are the pre-fault, fault, and post fault
conditions. The dynamic frequency test is executed from the Manual Frequency screen. This
test requires a single voltage generator. The change in frequency happens when the frequency
passes through the positive going zero-crossing. The rate of change is defined as the delta
(Hz/S). The frequency is changed every cycle at the positive going zero crossing. The test limits
the delta setting to a maximum value of 10Hz/sec with anything greater being treated as a Step
test. The frequency increment is calculated for each cycle prior to starting the dynamic ramp. If
we use a delta of 1 Hz/sec and want to ramp from 60 to 50 hertz, then the time would be ten
seconds to go from 60 hertz to 50 hertz in increments of 0.01667 hertz steps per cycle.
The first set defines the pre-fault values, which are necessary to energize the relay before the
fault condition. The following diagram shows the frequency decreasing. However, the frequency
ramp could be increasing as well. The pre-fault and fault values determines the direction of the
ramp. If the fault value is less than pre-fault value then the ramp is decreasing. If the fault value
is greater than the pre-fault value then the ramp is incremented up from the pre-fault value.
There isn’t any change to the generator outputs during pre-fault.
60.00
Hz
T ime
Fault Freq
Set point
Prefault T ime
Delta 1
Delta 2
Duration
Prefault
Post Fault
Dynamic Frequency
Duration is the time at the fault frequency. The dynamic fault requires the V/I Generators to ramp
to the actual fault value. Delta 1 is the fault ramp and the change in frequency divided by time
(Hz/S). The set point indicates the frequency when the relay first detects the fault. Once the set
point is reached the relay starts its timer. If the fault is not cleared within the timeout, the relay
will trip. The fault frequency is below the set point to ensure the relay trips for a decreasing ramp.
The fault value is greater than the set point for the increasing ramp. Once the ramp has reached
the set point, the timer is enabled. The fault condition is maintained for the time indicated in the
duration.
The post fault requires the V/I Generators to ramp with the generators in the fault condition. This
post fault ramp will ramp back to the pre-fault condition. The relay is also in a fault condition and
could trip. The post fault will be able to detect the relay trip and record the trip time. The post
fault timer will start at the beginning of the ramp and will not wait for the set point. Delta 2 is the
post fault ramp and is the change in frequency divided by time (Hz/S).
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