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STATIC EXCITATION EQUIPMENT Page 1 of 7

1. SYSTEM DESCRIPTION

Brief Description of the Excitation System (Ref sheet YU101)

The static excitation equipment regulates the voltage and/or the flow of reactive
power during parallel operation) from the synchronous machine (generator) by direct
control of the rotor current (field current) using (static) thyristor converters.

The entire unit can be broken down into the following major groups:

™ The Excitation Transformer.. T1

™ The Control Unit .. .. REG
™ Thyristor Converter .. .. TY
™ The Field Flashing .. .. FF
™ The Field Breaker .. .. FB

In “shunt” excitation, the excitation power is drawn from the generator stator. The
field current for the rotor flows across the excitation transformer -T1, AC Breaker
Q11, the converter TY, and the field circuit-breaker -Q1. The excitation transformer -
T1 reduces the generator voltage to the input voltage required for the converter,
provides the galvanic isolation between the generator terminals and the field winding,
and acts at the same time as the commutating reactance for the converter. The
converter TY converts the 3-phase AC current into the regulated DC current If.

In excitations with shunt-connected supply, there is not enough remanent voltage in

the rotating generator to build up the generator voltage autonomously via the
converter. To accomplish this, special field flashing equipment is needed. When the
field flashing equipment is being supplied with power from a DC power source (power
station battery), a resistor is used to limit the field flashing current. When it is being
supplied from an AC power grid, a transformer serves as the adapter needed.
Excitation of the generator is started by closing the field circuit-breaker -Q1 and the
field flashing breaker -Q2. This supplies current to the field, which excites the
generator up to 10 ... 30% U. The generator then supplies voltage to the converter via
the excitation transformer. Starting from approx. 10% of the generator voltage the
firing electronics and the converter are able to continue the voltage build-up, so that
the field flashing circuit is relieved of current. Once the voltage exceeds approx. 70%
of U the field flashing breaker is finally opened, having no current. The diode bridge at
the input to the field flashing breaker prevents a backflow of current to the field
flashing source.
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The converter TY has been broken down for this purpose into three autonomous
converter blocks TY1 to TY3 . Each converter block is capable of meeting the system
requirement.

Each block has its own Final Pulse Stage, redundant cooling fans and monitoring of
the elements.

Redundancy in the regulator section is ensured by means of TWO fully separate

channels with independent measuring inputs and extensive monitoring
(“SUPERVISION”)

Channel 1 is built as AUTO Channel and Channel 2 as MANUAL Channel. AUTO

channel works as voltage regulator, which has a PID control algorithm. AUTO channel
also contain various limiters and corrective control circuits to ensure the use and
stable operation of the synchronous machine up to its operating limits. Each of these
channels possess a Gate Control Unit with a subsequent Intermediate Pulse Stage to
generate the firing pulses for the thyristor converter. The Intermediate Pulse Stage of
the Channel which is active, transmits the firing pulses galvanically separated to the
common pulse bus at the input to the Final Pulse Stage. Various monitoring functions
of the Auto channel and pulse monitoring on the common pulse bus initiate an
automatic switch-over to hot stand by channel in case of a malfunction.

MANUAL Channel works as a simple field-current regulator with a Pl control

algorithm. It serves as a back-up channel in case of a malfunction on the AUTO
channel, and is also used for commissioning and for special machine tests. This
channel contains no special limiters to ensure that the synchronous machines will
always remain within its operating limits. For this reason, operating using the field
current regulator requires expert adjustments of the machine’s operating point and
continuous monitoring of the machine by the operating staff. Whenever the
generator’s operating limits are exceeded, the safety devices respond immediately by
shutting down the excitation and the generator.

Both the channels are equipped with tracking equipment so that the inactive channel
always generates the same control variable as the active channel during steady-state
operation. This ensures smooth switch-over from one channel to other . .
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Dynamic braking is employed to bring the hydro generators rapidly to standstill

condition. This is an additional braking force which is applied in conjunction with
normal mechanical brakes to cut down the stopping time. Dynamic braking is applied
at a predetermined speed.
After unloading, machine is taken out of grid by opening Generator Circuit Breaker
and is de-excited. Once the machine speed comes down to a predetermined speed,
the shorting isolator IS closes to short circuit the stator terminals. AC Breaker Q11 is
opened and Q12 & Q13 are closed. Regulation switches over to Manual mode. A
preset Excitation is applied to the generator field through thyristor bridge of Static
Excitation Equipment by rectifying the secondary voltage of transformer T11 to
generate armature current not exceeding the rated value. Mechanical energy available
in the rotating system gets converted into electrical and is dissipated in the form of
heat thus bringing the machine quickly to a speed at which mechanical brakes can be
applied. Excitation is switched off and Field breaker Q1, AC breakers Q12 & Q13 are
tripped following which the dynamic braking isolator is also opened. An over current
relay is provided to protect auxiliary supply circuit of the dynamic braking equipment
against the over current.

2. Power Supply System

Principle of primary power supply
In the shunt excitation system here, the excitation transformer also provides the power
supply for the electronic equipment and, for the converter fans. The fans receive their
supply when the Field breaker is closed.

A station battery supply is absolutely necessary for the control of the field circuit
breaker. It is the power source for the electronic devices till the generator is able to
supply voltage.

Auxiliary power to the field flashing equipment must be present in order to build up
the generator excitation. The power supply for standstill heating and the cubicle
lighting is of secondary importance for operation of the plant. The synchronous
voltages Usyn are each supplied to three channels separately across transformers.
The Gate Control Units need these voltages to enable them to issue the pulses at a
given firing angle relative to the input voltage of the converter.

For testing, the synchronous voltages and the AC power supply to the electronic
equipment can be switched over to an auxiliary power supply while the generator is at
a standstill using TEST-SERVICE switch . The electronic equipment and the ignition
circuits can then be tested while at standstill.
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3. Digital Voltage Regulator, DVR

3.1 Principle of Operation of the Regulator AVR
To regulate the voltage and the reactive power of a synchronous machine, the field
voltage must be adjusted quickly to the changes in the operating conditions (with a
response time that does not exceed a few ms).

The DVR digital voltage regulator calculates the control variable from the measured
and reference data in very short time intervals. This results outwardly in a quasi-
continuous behavior with a negligible delay time (as in an analog regulator).

The calculations are made in the binary number system. Analog measurement
signals, such as those for generator voltage and generator current, are converted into
binary signals in analog/digital converters. The set-points and limit values have
already been defined in digital (binary) form.

3.2 Operation of the AUTO Channel (Ref sheet YU105)

The functions of the automatic voltage regulator AVR are:
™ to regulate the generator voltage,
™ to regulate the effect of the reactive and/or active current on the voltage (droops),
™ to limit Volt/Hz,
™ to limit max. field current,
™ to limit inductive stator current,
™ to limit capacitive stator current,
™ to limit the load angle and
™ to stabilize the power system
™ to regulate MVAR/PF
™ dead line chatging.

4. Converter
4.1 Converter Power Section
The thyristor converter consists of three independent parallel rectifier blocks TY1 ,
TY2, and TY3 which are always in service. Even if two blocks fail the remaining
block take over automatically the full design current of the excitation circuit.
4.2 Converter Cooling
A cooling system is needed to dissipate heat losses in the converter blocks and
electronics. Each converter block has therefore been equipped with 2 fans (100%
redundancy) supplied with power from the station aux. supply or from the converter’s
primary voltage through a transformer in case of failure of aux. supply. The fans are
protected with motor protection circuit breakers.
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An air flow monitoring unit is provided for monitoring the air flow through the thyristor
bridge. If a circuit breaker failure is detected, or if the air flow monitor drops off at one
of the thyristor bridges, the bridge involved is immediately set out of operation by
blocking its firing pulses.

4.3 Thyristor Converter Monitoring

A thyristor bridge in which defects occur that could threaten the safety of operation or
cause secondary damage is switched off automatically, i.e., its firing pulses are
blocked.

This happens whenever:

♦ The Final Pulse Stage fails, which is detected by internal monitors (supply voltage,
sustained pulse, short-circuit on the output end).
♦ The power supply to both the fans fails.
♦ The fan air flow, as monitored by the Air flow monitor, fails or is insufficient,
♦ Isolator on AC/DC side is open.
♦ Heat sink temperature exceeds the set value.
Further, an alarm is initiated whenever conduction of thyristor fails.

5 Field Current Circuit

5.1 Field Circuit-Breaker
refer to block diagram P/YU101)
The circuit-breaker in the field circuit is used to isolate the field circuit from the
converter. It is capable of switching off the synchronous machine from full load under
the maximum conditions of a 3-phase short-circuit.

In addition to its main contacts, the field circuit-breaker also has a de-excitation
contact with which the field energy stored in the field can be dissipated across the de-
excitation resistor. The de-excitation contact closes shortly before the main contacts
open so as to ensure proper commutation of the field current from the main contacts
to the de-excitation contact when the breaker is switched off.

The field circuit-breaker is switched on by electromagnetic force and is kept switched

on by a mechanical latch. When the latch is released by a trip coil, the circuit-breaker
opens. The circuit-breaker also has auxiliary contacts that report its status.
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5.2 Field Flashing

In shunt supplied excitation circuits (excitation transformer connected to the generator
terminals), the generator does not have enough remanence voltage for a generator
voltage build-up via the converter. In this case, a field flashing circuit is provided. It
consists of the field flashing contactor , the diode bridge and a transformer or blocking
diodes and a voltage dropping resistor used to adapt the auxiliary input voltage to
the voltage needed for field flashing, when power is supplied from the auxiliaries
network. Because the field flashing contactor is not able to switch off the energy
stored in the field, the control ensures that the contactor can only reopen if the field
circuit-breaker has already been opened (generating the TRIP order) or, in a
normal field flashing sequence, when the converter has taken over the field current.

Field flashing occurs in the following stages:

♦ The excitation is switched on, closing the field flashing contactor ( Field Circuit
Breaker is already closed ).
♦ The start-up excitation current flows through the rotor, driving the generator
voltage up to approx. 15% U
♦ After about 10% U, the firing pulses to the converter are released and it begins to
excite the generator to its rated voltage.
♦ After about 30% U, the field flashing contactor opens (with no current, since the
converter is now supplying the current).

The diode bridge at the input to the field flashing contactor prevents a feed-back from
the converter to the source of field flashing while the contactor is still closed.

5.3 De-excitation
When malfunctions occur, the stored field energy must be dissipated as quickly and
safely as possible to protect the generator. This is done by the converter, the field
circuit-breaker, and the discharge resistor (voltage dependent).

De-excitation (with opening of the field circuit-breaker) takes place in the following
stages:
♦ The converter, drives to its inverter limit position (negative ceiling voltage),
recovers a portion of the field energy into the network. A trip command is given to
the field circuit-breaker.
♦ The de-excitation contact closes, diverting the field voltage to the discharge
resistor.
♦ Then, immediately, the main contacts open, building voltage. The field voltage
commutates to the de-excitation resistor.
♦ The current diminishes.
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Due to the reversal of the field voltage by the converter, the field current commutates
from the main contacts of the field circuit-breaker to the de-excitation resistor in a very
early phase. This reversal of the field voltage prevents burn-off on the main contacts
and provides effective protection for the field circuit-breaker.

Depending on the operating policy, an operational shut-down of the excitation can

also be effected with the field circuit-breaker closed. This method is useful mainly
when the excitation is switched on and off frequently. In this case, the converter is
merely driven into the inverter limit position so that the field energy is recovered into
the network. The converter then blocks since it is supplying positive current only.

6. Excitation Transformer
The excitation transformer matches the generator voltage to the field voltage (required
ceiling voltage). It also serves as a commutation reactance for the thyristor converter
and as a potential isolator between the network and the excitation circuit. In addition,
the transformer functions as a current limiter in that it makes it possible to keep any
short-circuits in the excitation circuit under better control.

The excitation transformer is equipped with temperature monitoring devices

(thermistors) which set off an alarm when the temperature exceeds a first max.limit,
and then trips the excitation if the temperature continues rising to a second (higher)
limit.