TB XRW1-4 04.01 E
11
4.3.1
Selection of star or delta connection
All connections of the input voltage transformers are led
to screw terminals. The nominal voltage of the device is
equal to the nominal voltage of the input transformers.
Dependent on the application the input transformers
can be connected in either delta or star. The connec-
tion for the phase-to-phase voltage is the delta connec-
tion. In star connection the measuring voltage is re-
duced by 1/ 3 . During parameter setting the connec-
tion configuration either Y or
∆
has to be adjusted.
Fig. 4.1: Input v.t.s in delta connection (
∆
)
Fig. 4.2: Input v.t.s in star connection (Y)
4.4
Principle of frequency supervision
The frequency element of
XRW1-4
protects electrical
generators, consumers or electrical operating equip-
ment in general against over- or underfrequency.
The relay has independent three frequency elements
f
1
- f
3
with a free choice of parameters, with separate
adjustable pickup values and delay times.
The measuring principle of the frequency supervision is
based in general on the time measurement of complete
cycles, whereby a new measurement is started at each
voltage zero passage. The influence of harmonics on
the measuring result is thus minimized.
J
KJ
6
6
Fig. 4.3: Determination of cycle duration by means of zero
passages.
In order to avoid false tripping during occurence of in-
terference voltages and phase shifts the relay works
with an adjustable measuring repetition (see chapter
6.2.3)
Frequency tripping is sometimes not desired by low
measured voltages which for instance occur during al-
ternator acceleration. All frequency supervision func-
tions can be blocked with the aid of an adjustable
voltage threshold U
B
in case the measured voltage
value is below this value.
4.5
Measuring of frequency gradient
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 intrasys-
tem occurs for the following reasons:
•
It must be prevented that the electrical generators
are damaged when mains voltage recovering
asynchrone, e.g. after a short interruption.
•
In order to prevent uncontrollable islands, users
generating their own power must decouple from the
remaining faulty grid.
