2.18 Overexcitation (Volt/Hertz) Protection (ANSI 24)
115
7UM61 Manual
C53000-G1176-C127-3
Figure 2-35 Logic Diagram of the Overexcitation Protection
2.18.2 Setting Notes
General Overexcitation protection is only effective and available if address 143 OVEREXC.
PROT. is set to Enabled during configuration. If the function is not required
Disabled is set. Address 4301 OVEREXC. PROT. serves to switch the function ON
or OFF or to block only the trip command (Block relay).
Overexcitation protection measures the voltage/frequency quotient which is propor-
tional to the induction B. The protection must intervene when the limit value for the pro-
tected object (e.g. unit transformer) is exceeded. The transformer is for example en-
dangered if the power station block is switched off at full-load operation and the
voltage regulator does not respond fast enough or not at all to avoid related voltage
increase.
Similarly a decrease in frequency (speed), e.g. in island systems, can lead to an inad-
missible increase in induction.
In this way the U/f protection monitors the correct functioning both of the voltage reg-
ulator and of the speed regulation, in all operating states.
Independent
Stages
The limit-value setting at address 4302 U/f > is based on the induction limit value
relation to the nominal induction (B/B
N
) as specified by the manufacturer of the object
to be protected.
A pickup message is transmitted as soon as the induction limit value U/f set at address
4302 is exceeded. A warning message is transmitted after expiry of the corresponding
4303 T U/f > time delay.
The 4304 U/f >>, 4305 T U/f >> trip stage characteristic serves to rapidly switch
off particularly strong overexcitations.
The time set for this purpose is an additional time delay which does not include the
operating time (measuring time, drop-out time).
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