CHAPTER 6: SETPOINTS
S3 PROTECTION
350 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
6–109
Volts per Hertz (24)
Volts-per-hertz (V/Hz) protection can be applied to transformer feeders to detect
overexcitation that can be dangerous for the transformer. The flux in the transformer core
is directly proportional to the voltage and inversely proportional to the frequency. When V/
Hz ratios are exceeded, saturation of the magnetic core of the transformer occurs. This
causes saturation of the normal iron path designed to carry that flux, which instead
follows leakage paths, causing damage as well as iron burning from the interlamination
iron core.
This can happen in the following cases:
•
In an islanded power system operating condition, where the sudden loss of load
causes a voltage rise. Unsuccessful reduction of the voltage in such disturbances.
•
When a generator voltage runaway condition causes the Generator Step-up Unit and
the auxiliary transformer to encounter high voltages.
•
At low frequencies where overfluxing typically occurs, full protection is especially
important. Consider a generator coasting down with full excitation applied. If the
exciter does not have a V/Hz limiter, the voltage regulator attempts to maintain rated
voltage as speed decreases. To avoid transformer (and generator) damage, the V/Hz
protection must remove the transformer from service, and also trip the generator
exciter breaker.
The volts-per-hertz (V/Hz) value is derived from the maximum of the three-phase voltage
inputs. If there is no voltage on the relay terminals, the V/Hz value is automatically set to 0.
The measured voltage from the V/Hz ratio is expressed with reference to the nominal
voltage entered under Voltage Sensing setup. The measured frequency is expressed with
respect to the nominal frequency entered under Power System setup. These setpoints can
be found at the following paths:
Voltage:
PATH
:
SETPOINTS > S2 SYSTEM SETUP > VOLTAGE SENSING > VT SECONDARY
Frequency:
PATH
:
SETPOINTS > S2 SYSTEM SETUP > POWER SYSTEM > NOMINAL FREQUENCY
For example, if the Phase VT Secondary and Nominal Frequency are set to 120 V and 60
Hz, respectively, these values define the base unit as 1 x (V/Hz). The volts-per-hertz ratio
after division of these nominal settings is 120/60 = 2. If the Pickup setpoint from the V/Hz
element is set to 1.05 x (V/Hz), this means that in order for the element to pick up, the
actual volts-per-hertz ratio after division should be 2 *1.05 = 2.1. The ratio of 2.1 can be
achieved if, for example, the measured voltage is 126V and frequency is 60 Hz, or the
voltage is constant at 120 V and the frequency is 57.14 Hz. To check back the pickup
setting, we use the base (V/Hz) unit = 120/60 = 2, such that the pickup setting value is 2.1/2
= 1.05 x (V/Hz).
The element has a linear reset characteristic, and the reset time can be programmed to
match the cooling characteristics of the protected equipment. The element will fully reset
from the trip threshold once the set reset time has elapsed. The V/Hz element may be used
as an instantaneous element with no intentional time delay, or as a Definite or Inverse
timed element. The characteristics of the inverse curves are shown below.
PATH
:
SETPOINTS > S3 PROTECTION > SETPOINT GROUP 1(2) > VOLTS PER HERTZ
FUNCTION
Range: Disabled, Trip, Alarm, Latched Alarm
Default: Disabled
For details see
