Reverse Power Protection; Functional Description - Siemens SIPROTEC 4 7UT6 Series Manual

Functions

2.12 Reverse Power Protection

2.12 Reverse Power Protection
Reverse power protection is used to protect a turbo-generator unit on failure of energy to the prime mover
when the synchronous generator runs as a motor and drives the turbine taking motoring energy from the
network. This condition endangers the turbine blades the and must be interrupted within a short time by trip-
ping the network circuit-breaker. For the generator, there is the additional risk that, in case of a malfunc-
tioning residual steam pass (defective stop valves) after the switching off of the circuit breakers, the turbine-
generator-unit is speeded up, thus reaching an overspeed. For this reason, the decoupling should only be
performed after the detection of active power input into the machine. The reverse power protection can be
used as a criteria for the decoupling in the system.
The reverse power protection can only be used for a 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, allows for a logical calculation of the active power. This is therefore only
possible for 7UT613 and 7UT633.

Functional Description

2.12.1
Reverse Power Determination
The reverse power protection in 7UT613 / 7UT633 calculates the active power from the symmetrical compo-
nents of the fundamental waves of voltages and currents.
There are two measurement methods:
•
The "precise" measuring procedure is especially suited for reverse power protection on generators, as in
this case a very low active power is calculated from a very high apparent power (for small cos φ). The
positive sequence systems from voltages and currents are used to obtain a very high accuracy of the last
16 cycles. The evaluation of the positive phase-sequence systems makes the reverse power determina-
tion independent of current and voltage asymmetries and corresponds to actual loading of the drive end.
By taking the error angles of the voltage and current transformers into account, the active power compo-
nent is exactly calculated even with very high apparent powers and low cos φ. The angle correction is
performed by a correction angle φ
priately determined by the commissioning of the protective device in the system (see Subsection
"Mounting and Commissioning", "Checking the Voltage Connections and Polarity Check").
•
The "fast" measurement also uses the positive-sequence components of currents and voltages that are
calculated 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 of real power.
Pickup Seal-In Time
To ensure that frequently occurring short pickups can cause tripping, a selectable prolongation of these pickup
signals is provided. Should new fault detection signals appear within this seal-in time the pickup is maintained,
so that a delayed tripping can take place.
Delay and Logic
Two delay times are available for the delay of the trip command.
When used as a reverse power protection for generators, bridging a perhaps short power input during
synchronisation or during power swings caused by system faults, the trip command is delayed by a selectable
time T-SV-OPEN. In case of a closed turbine emergency tripping a short delay T-SV-CLOSED is, however,
sufficient. The state of the emergency tripping valve must then be given to the device via a binary input "RLS
Fast". The delay time T-SV-OPEN is still effective as back-up stage.
In other applications only the delay T-SV-OPEN is generally needed, as they act independently to the
mentioned binary input. Of course you can also use the two-stage protection as needed, in order to -
dependent on an external criteria - achieve two different trip delays.
224
(see Section 2.1.4.2 General Power System
corr
Data), which is appro-
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016