THE PROBLEMS TESTING DIGITAL
RATE-OF-CHANGE IN FREQUENCY RELAYS
By V. Gurevich, Ph.D.
The frequency of alternating current
in electric power networks is the paramount parameter of the operating mode
of a network. Even small frequency deviations can bring about serious malfunctions in the networks and demands for
urgent corrective action. In many cases, a
parameter for an emergency mode in a
network is not just the absolute value of
frequency, rather it includes the frequency’s change in time.
Such a parameter as Rate-of-Change
of Frequency (ROCOF) is today the principal parameter that is monitored by
numerous specialized digital relays (the
ANSI code for relays of this type is
81RL), which are available on the market
(for example, UFD34, MRF2, G59,
PPR10, LMR-122D, FCN950, KCG593,
MFR 3, MFR 11, LS 4, VAMP 210,
SPCF 1D15, 256-ROCL and many other
types).
There are two cases in which
ROCOF relay protection is used in the
core:
1. For automatic load shedding, that
is, for switching-off part of the loading
upon detection of sweeping changes of
frequency. It is necessary to note that in
the emergency mode in a high-voltage
power network, changes of frequency
can be distinctive in different sections of
a network depending on the power of the
separate substations which are available
in this network. In addition to sweeping
decreases of frequency in a branch network, there are overflows of power
between the energy sources powering
this network, accompanied by frequency
fluctuations in the network as a whole.
Thus, the absolute value of the downgraded frequency is not constant and
therefore cannot serve as the criterion for
pickup adjustment of the frequency relay,
switching-off a part of the loading. A
much more reliable criterion for load
shedding is the ROCOF function which
is used as an additional criterion upon
detection of the decrease of an absolute
value of frequency below the set level.
2. The case of an instantaneous
Fig. 1. Principle for measurement frequency of the distorted periodical signal.
interdiction of repeated connection of the
generator to a power network if it has
been already disconnected, even for a
short-term (isolated from a network). In
the latter case, this protection is referred
to as “loss of mains”, or “loss of grid”, or
“islanding protection”. A high-voltage
circuit breaker tripping and the separation of a section of a network with the
generator (that is, the formation of an isolated “island”) from the main network
(that is “loss of mains”) leads to an
infringement in the balance of power in
the isolated section of the network creating oscillations, accompanied by frequency fluctuations. Very quickly, however, the frequency can return to normal
under the action of an automatic voltage
excitation controller in the generator or if
the load of the generator is insignificant.
However, the situation remains potentially dangerous, as the frequency of the
generator can change at any moment during the change of its load; thus the automatic reclosing of the circuit breaker will
lead once more to the emergency mode
of operation. For this reason, ordinary
frequency relays are not applied in this
situation. The ROCOF relays are capable
of detecting the frequency fluctuations of
fractions of a second at once after the circuit breaker trip, stopping the automatic
reclosing.
The set point ROCOF for the protective relays is calculated according to concrete parameters of a network, the generator, loads and can be essentially different for various networks. For example, in
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