Functions
234
7UM62 Manual
C53000-G1176-C149-3
2.36
DC Voltage/DC Current Protection (ANSI 59NDC/51NDC)
General
To detect DC voltages, DC currents and small AC quantities, the 7UM62 is equipped
with a measuring transducer input (TD1) that can be used either for voltages (
±
10 V)
or currents (
±
20 mA). Higher DC voltages are connected via an external voltage
divider. The DC voltage/DC current protection can be used, for example, for the
monitoring of the excitation voltage of synchronous machines (see Figure 2-113), or
for the detection of earth faults in the DC section of the start-up converter of a gas
turbine set (see Figure 2-114).
2.36.1 Functional Description
A measuring transducer performs the analog/digital conversion of the measured
quantity. The measuring transducer provides for galvanic isolation, a digital filter
integrates the measurement voltage over two cycles and suppresses high ripple
content or non-periodic peaks. A mean value of 32 samples is generated. Since the
absolute values are sampled, the result is always positive. Thus, the polarity of the
voltage is of no concern. When no suitable measured AC quantities are present
("operating condition 0 "), the DC voltage protection is still operative. The mean value
is then calculated over 4 x 32 measured value samples.
If, in special cases, an AC voltage should be measured via this analog input, the
RMS
value should be set on the protection. The factor 1.11 between r.m.s. and mean value
is recognized within the protection function.
Optionally, this function can also be used for the monitoring of small currents, provided
that the TD input has been configured as current input and the settings of the
associated jumpers on the C–I/O–6 have been changed. If the jumper settings do not
match the configuration parameters, an error message is output.
The protection can be set to operate for overvoltage or undervoltage. Pickup can be
blocked via a binary input, and the output signal can be time delayed.
Typical applications are shown in the Figures 2-113 and 2-114.
Excitation Voltage
Monitoring
Figure 2-113 illustrates the excitation voltage monitoring. The excitation voltage is
stepped down to a processable level by a voltage divider, and fed to the measuring
transducer.
Figure 2-113 DC Voltage Protection Used for Excitation Voltage Monitoring
Excita-
tion
system
10:1
20:1
U
=
0 – 8,5 V
Measuring
transducer
Input
filter
7UM62
G
Ro
tor
3PP1326–0BZ–V718 160
∼
∼
TD1
