Siemens SIPROTEC 4 7UT6 Series Manual Download

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Forward Power Supervision

The forward power supervision monitors wether the active power undershoots one set value or overshoots a
separate second value. Each of these functions can initiate different control functions.
When, for example, with generators operating in parallel, the active power output of one machine becomes so
small that other generators could take over this power, then it is often appropriate to shut down the lightly
loaded machine. The criteria here is that the forward power supplied into the network falls below a certain
value.
In some applications it may be useful to give a control command when the issuing real power exceeds a
specific value. If only one of two parallel connected transformers is active, the second one can be activated, as
soon as the transferred power exceeds a preset ratio.
When a fault in a utility network is not cleared within critical time, the utility network should be split or for
example, an industrial network with internal supply decoupled from it. As criteria for decoupling, in addition
to power flow direction, are undervoltage, overcurrent and/or frequency. As a result, the 7UT6x can also be
used for network decoupling.
The forward power supervision can only be used for three-phase protective objects. This understands that the
device is connected to a voltage transformer set and that this voltage, together with an assigned corre-
sponding current transformer, allow for a logical calculation of the active power. This is therefore only
possible for 7UT613 and 7UT633.
When the circuit breakers are deactivated, the P<-Stufe stage should be blocked via external signals.

Functional Description

Determining Real Power

The forward active power supervision in 7UT613 / 7UT633 calculates the active power from the symmetrical
components of the fundamental waves of voltages and currents.
There are two measurement methods:

•

The “exact” measurement method averages the active power via the last 16 cycles of the measured quan-
tities. The evaluation of the positive phase-sequence systems makes the active power definition inde-
pendent of current and voltage asymmetries. If an exact derivation of real power at high apparent
powers (low cos φ) is desired, it will be necessary to consider the angle error of voltage and current trans-
formers. The angle correction is performed by a correction angle φ

corr

(see Section

2.1.4 Power System

Data 1

•

The “fast” measurement method calculates the positive-sequence components of currents and voltage
over a cycle. A short tripping time is hereby achieved. It is therefore well suited in system applications
where short tripping times are more desired than high accuracy, e.g. used for purposes of network
decoupling.

Time Delay, Logic

The P<-stage as well as the P>-stage each have a time delay. The respective command is issued after end of
the resulting delay and each can trigger a control activity.
Every stage can be blocked separately via binary inputs; a another binary input blocks the entire forward
power supervision. The P<-stage is blocked internally when the broken wire or voltage failure is recognised or
voltage transformer protection breaker failure (via the respective binary input) is indicated (see also Section
“Technical Data”).

Siemens SIPROTEC 4 7UT6 Series, Manual

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