EMR-5000
IM02602012E
81R Rate of Change of Frequency (df/dt)
Electrical generators running in parallel with the mains (e. g.: industrial internal power supply plants) should be
separated from the mains when failure in the intra-system occurs for the following reasons:
•
Damage to electrical generators must be prevented when mains voltage is recovering asynchronously (e. g.: after a
short interruption).
•
The industrial internal power supply must be maintained.
A reliable criterion of detecting mains failure is the measurement of the rate of change of frequency 81R (df/dt). The
precondition for this is a load flow via the mains coupling point. At mains failure, the load flow change spontaneously
leads to an increasing or decreasing frequency. At active power deficit of the internal power station, a linear drop of the
frequency occurs and a linear increase occurs at power excess. Typical frequency gradients during application of
"mains decoupling" are in the range of 0.5 Hz/s up to over 2 Hz/s.
The protective device detects the instantaneous frequency gradient 81R (df/dt) of each mains voltage period. Through
multiple evaluations of the frequency gradient in sequence, the continuity of the directional change (sign of the
frequency gradient) is determined. Because of this special measuring procedure, a high safety in tripping and thus a
high stability against transient processes (e. g.: switching procedure) are achieved.
The frequency gradient (rate of change of frequency [df/dt]) may have a negative or positive sign, depending on
frequency increase (positive sign) or decrease (negative sign).
In the frequency parameter sets, the User can define the kind of df/dt mode:
•
Positive df/dt = the frequency element detects an increase in frequency;
•
Negative df/dt = the frequency element detects a decrease in frequency; and
•
Absolute df/dt (positive and negative) = the frequency element detects both, increase and decrease in frequency.
This protection element provides a tripping threshold and a tripping delay. If the frequency gradient df/dt exceeds or
falls below the set tripping threshold, an alarm will be issued instantaneously. If the frequency gradient remains still
above/below the set tripping threshold until the tripping delay has elapsed, a tripping command will be issued.
Working Principle
(Please refer to block diagram on next page.)
The frequency element supervises the three phase voltages »
VA«, »VB« and »VC«. If any of the evaluated three
phase voltages is below 15% Vn, the frequency calculation is blocked. According to the frequency supervision mode
set in the Device Planning (81R), the phase voltages are compared to the set frequency gradient (df/dt) threshold.
If in
any of the phases, the frequency gradient exceeds or falls below the set pickup threshold (acc. to the set df/dt mode)
and if there are no blocking commands for the frequency element, an alarm is issued instantaneously and the tripping
delay timer is started. When the frequency gradient still exceeds or is below the set pickup threshold after the tripping
delay timer has elapsed, a tripping command will be issued.
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