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harmonics will be rejected by the internal DFFT-digital
filter. Whereas the RMS

RMS

RMS-metering instrument measures

the RMS-

RMS-

RMS-value of the input signals.

6.5.2 Checking the operating and

resetting values of the over/
undervoltage functions

Note:

When the measuring voltage is connected or
disconnected, vector surge tripping or df/dt tripping can
occur. In order to ensure a trouble-free test procedure,
the vector surge function or df/dt function of the relay
have to be blocked before tests are started.
Apply three voltages with the rated value and gradually
increase (decrease) the voltages until the relay starts, i.e.
at the moment when the LED U> (or U<) lights up or the
voltage alarm output relay (contact terminals D4/E4) is
activated. Read the operating voltage indicated by the
voltmeter. The deviation must not exceed 1% of the set
operating value.
Furthermore, gradually decrease (increase) the voltages
until the relay resets, i.e. the voltage alarm output relay
is disengaged. Check that the dropout to pickup ratio for
voltage is greater than 0.97 (for overvoltage function) or
smaller than 1.03 (for undervoltage).

6.5.3 Checking the relay operating time

of the over/undervoltage functions

To check the relay’s operating time, a timer must be
connected to the trip output relay (contact terminals D1/
E1). The timer should be started simultaneously with the
voltage change from sound condition to a faulty condition
and stopped by the trip relay contact. The operating time
measured by timer should have a deviation about 3% of
the set value or < 20 ms.

6.5.4 Checking the operating and reset-

ting values of the over/underfre-
quency functions

Note:

Due to frequency changes, vector surge tripping or df/dt
tripping can occur during frequency tests. In order to
ensure a trouble-free test procedure, the vector surge
function or df/dt function of the relay have to be blocked
before tests are started.
During frequency tests, each of the frequency elements
should be tested separately. This makes it necessary that
the other frequency elements of the relay have to be
blocked by setting the frequency pickup values f

- f

“EXIT”. For testing the pickup and dropout to pickup
values, the test frequency has to be increased (decreased)
until the relay is engergized. This is indicated by lighting
up of LEDs f

- f

When comparing the values displayed with those of the
frequency meter, the deviation must not exceed 0.01 Hz.
The dropout to pickup values are ascertained by
increasing (decreasing) the test frequency slowly until the
output relay releases.
The dropout to pickup value for overfrequency must be >
0.99, and for underfrequency < 1.01.

6.5.5 Checking the relay operating time

of the over/underfrequency
functions

The operating time of the over/underfrequency functions
can be tested in the similar manner as in chapter 6.5.3
for over/undervoltage functions.

6.5.6 Checking the vector surge function

With the help of an advanced relay test equipment a
phase shift (vector surge) on the voltage signal can be
obtained to test the vector surge function of

MRN3

relay.

If there is no such testing facility available, a very simple
simulation circuit may be used to test the vector surge
function of the relay with a sufficient accuracy. Figure 6.2
shows the possibility to simulate a phase shift by means of
a RC circuit. Closing or opening the switch S1 causes the
phase angle of the input voltage to change depending on
the adjustable resistor R.
The phase angle obtained may be calculated with the
following formula and is almost independent on the test
voltages.
In case of a 3-phase vector surge, the angle

ΔΘ

can be

calculated with the following formula if the parameters R

R and C are known:

ΔΘ =

arctg

- arctg

Example:

= 1 Ohm,

R = 363 Ohm, C = 3

then:

ΔΘ