10 03 Excitation System.pdf

1
Vlady Pollero - Electrical Rotating Machines Department (GEN)
STATIC EXCITATION SYSTEM FOR SYNCHRONOUS GENERATOR
COMPANY
CONFIDENTIAL
COURSE ON EXCITATION SYSTEM – AL SHABAB and WEST DAMIETTA, NOVEMBER 2016

2
COMPANY
CONFIDENTIAL
• To provide the direct current (DC) to the rotor of the synchronous
generator.
• To regulate the stator voltage, the reactive power or the power factor of
the generator with a fast dynamic response
• To protect the generator and the excitation system by tripping the unit
in case of dangerous situation
• To limit in real time the working point of the generator and keep the
operation inside a safe area; to optimize the dynamic response of the
generator
• To allow an easy operator interface
• To provide suitable tools for diagnostics and special tests
Application field of this kind of excitation system:
• synchronous generators for gas turbines
• synchronous generators for steam turbines
• synchronous generators for hydraulic turbines
• synchronous condensers (reactive power compensators)
Excitation system purpose

3
Main Components
COMPANY
CONFIDENTIAL
Static Excitation System
The system is composed by two main components:
• An excitation transformer with the primary winding connected to the power supply source (Generator Terminals)
and the secondary winding that lowers the voltage to a level suitable for the thyristor power converter
• An electrical cubicle with a thyristor power converter and a control system

4
+
STATIC
EXCITATION
CUBICLE
MAIN
GENERATOR
EXCITATION
TRANSFORMER
POWER SUPPLY
SLIP
RINGS
BRUSHES
Brushless or Static.
COMPANY
CONFIDENTIAL
Excitation System Types
+
--
STATIC
EXCITATION
CUBICLE
MAIN
GENERATOR
BRUSHLESS
AUXILIARY
GENERATOR
ROTATING DIODES
RECTIFIER BRIDGE
POWER SUPPLY
EXCITATION
TRANSFORMER
The excitation system of Giza
North Project is of STATIC type:
the excitation current provided
by the static thyristor converter
is directly applied to the rotor of
the generator by means of
brushes and slip rings.
Only for comparison, the
BRUSHLESS excitation systems
has an additional auxiliary
generator and a diode rectifier
installed on the same shaft of the
main generator.

5
Advantages and Disadvantages
COMPANY
CONFIDENTIAL
Excitation System Types
RELIABILITY OF THE STATIC COMPONENTS
FAST FIELD DE-EXCITATION
COMPLETE REDUNDANCY (BRUSHLESS IS NOT DOUBLED)
DYNAMIC RESPONSE
SIMPLIFIED MAINTENANCE
SHORT TIME TO RESTORE EVENTUAL FAULTS
GLOBAL EFFICENCY
SHORTER DIMENSIONS OF ROTATING PARTS
BRUSHES WEAR, CAUSING POWDER AND REQUIRING MAINTENANCE
ADVANTAGES of Static Exciter compared to Brushless
DISADVANTAGES of Static Exciter compared to Brushless

6
Overall Dimensions, front, side and top view
COMPANY
CONFIDENTIAL
Cubicle Outline

7
Front View
COMPANY
CONFIDENTIAL
Cubicle Outline
PC-Panel (HMI),
Main Operator
Interface
Regulators
Power
Converters

8
Front View (Door Opened Views)
COMPANY
CONFIDENTIAL
Cubicle Outline
PLC
Regulators
Power
Converters

9
Power Section
• The Power Section of the Excitation Cubicle is composed by a Totally
Controlled Three-Phase Graetz Bridge.
• Each Branch of the Bridge is provided with one Thyristor.
• The Bridge is protected by 3 fuses on the supply line, or, more often, by a
fuse for each thyristor.
• Each thyristor is paralleled to a ‘ Snubber ’, which is an RC circuit for
protecting the thyristor against transient spikes due to the commutations
COMPANY
CONFIDENTIAL
Static Excitation Cubicle

10
Logic Diagram
COMPANY
CONFIDENTIAL
G
Regulator 1
AVR1
Regulator 2
AVR2
Crow-Bar
Discharge
Resistance
EXCITATION
TRANSFORMER
Set-point
EXCITATION
CUBICLE
ALTERNATOR
Commands
and signals
from remote
Rectifier 1
Rectifier 2
+
_
Signals
to remote

11
COMPANY
CONFIDENTIAL
Interface with Power Plant
STATIC
EXCITER
CONTROL
SYSTEM (STCS)
GENERATOR
(STATOR PTS AND
CTS, ROTOR)
FIELD (GENERATOR
BREAKER)
PROTECTION
SYSTEM
EXCITATION
TRANSFORMER

12
COMPANY
CONFIDENTIAL
Equivalence between Permanent Magnet and Electrical Magnet
Excitation System Generalities
Generator with Flux given by
ROTATING PERMANENT MAGNET
D.C. CURRENT INJECTED INTO
ITS FIELD WINDINGS
N
S
N
S
+
_
Iexc
VR
VS
VT
R S T
R S T
Same resulting three-phase voltage system
Supposing constant and equal the rotor revolution speed in both following cases,
the difference between a generator with its internal flux made by a Rotating
Permanent Magnet and a generator with its internal flux made by d.c. current
injected into its Rotor Windings is mainly the following.

13
COMPANY
CONFIDENTIAL
Permanent Magnet
ITS FIELD WINDINGS
N
S
N
S
+
_
Iexc
VR
VS
VT
R S T
R S T
In the first case (Permanent Magnet), the Three-Phase Voltage System
generated on the Stator Terminals is unchangeable and its amplitude value is a
direct function of the revolution speed only. This is due to the unchangeable
“Power” of its fixed magnet.

14
COMPANY
CONFIDENTIAL
Electrical Magnet
ITS FIELD WINDINGS
N
S
N
S
+
_
Iexc
VR
VS
VT
R S T
R S T
In the second case (Electrical Magnet), on the contrary, changing the d.c. Field
current, the Three-Phase Voltage System on Stator Terminals is changing too
and this gives the enormous consequent advantage to have a Control on the
Stator Parameters, based on specific Logic.

15
COMPANY
CONFIDENTIAL
Generator connected to the Grid
Generalities
• When Generator is synchronized, Grid creates on Stator Winding a strong rotating
Electromagnetic Field, which interacts with the Rotor.
• This Magnetic Connection is strictly dependent by the Air Gap Magnetic Flux, and
is stronger with higher Excitation Current.
• Energy Exchange between Generator and Grid is controlled by Regulation of that
Flux.
• Consequently, a Rotating Permanent Magnet could not control that exchange,
while a Rotating “Magnet” produced by Field Excitation (DC Current) can properly
regulate that Exchange.

16
COMPANY
CONFIDENTIAL
Stator Voltage Generation
Generalities
The DC current injected into the Rotor Winding creates the
Electromagnetic Flux, which follows the magnetic path through the
iron of the Stator and Rotor cores and links to the 3 Stator Windings.
In case of variation of the linked Flux a voltage is generated on the
Stator Winding; the variation is given by the rotation.
The Voltage induced in the Stator windings depends on:
• Rotation Speed, that in normal operation is quite constant, about
3000 rpm;
• Rotor Current, that can be regulated by the Excitation System.
Generator / Motor Operation
• A machine is a Generator if the Rotor Magnet, powered by the
mechanical couple of the Turbine, rotates in advance of the
Stator Electromagnetic Field and drags that.
• A Machine is a Motor if the Rotor Magnet lags behind of the
Stator Electromagnetic Field and is dragged by that.
• The Angle Displacement between axes of Rotor and Stator
magnets is called “ load angle ”.
• During starting phase the machine is used as a motor, while
during operation at rated speed (at load or no load) that is always
used as a generator.
U1
U2
V1
V2
W1
W2
ω
N
S
+
_
Iexc
VR
VS
VT
R S T

17
COMPANY
CONFIDENTIAL
Coupling between Rotor and Electrical Grid
Generalities
• In order to deliver big amounts of Energy to the Grid, Turbine gives High Torque
Couples to the Generator Shaft.
• But those High Couples, applied to Rotor Body/Magnet, succeed to drag the Stator
Magnet only if Magnetic Flux inside Air Gap is very strong (Rotor-Stator link, in
this case, must be very “solid” to do it).
• Strong Flux means high intensities of Excitation Current on the Rotor Winding (this
is Over-Excitation Status).
• On the contrary, when the Excitation Current is reduced, the possibility to transmit
Energy to the Grid is correspondingly reduced because Flux is reduced.
• In that conditions it is hazardous to have Strong Couples on the shaft because it
becomes easier to tear the magnetic link between rotor and stator (Under-
Excitation Status).

18
COMPANY
CONFIDENTIAL
General Scheme
Power Converter
• DC Output Current of the Thyristor Rectifier, that goes through the Rotor Winding, can flow only in one direction
EXCITATION
TRANSFORMER
GEN
ROTOR
WINDING
BRUSH
BRUSH
FUSE
SECOND
REDUNDANT
RECTIFIER
• DC Average Output Current of the converter, that supplies the generator field, can be only POSITIVE
• DC Average Output Voltage from the converter can be POSITIVE, zero or NEGATIVE

19
COMPANY
CONFIDENTIAL
Thyristor
Power Converter
+
_
Current
Va
Vc
Vg
anode
cathode gate
• Thyristor is a Controlled Diode having 3 poles: Anode, Cathode and Gate.
• Its Conduction is conditioned by 2 status: potential of Anode Va higher than
potential of Cathode Vc, and pulse Vg on the Gate. In this condition Current
can flow through the Thyristor.
• It can be assimilated at a Breaker, that closes when only the pulse on the
Gate is given while Va > Vc.
• Output Voltage of Graetz Bridge can be changed simply controlling the
pulses on the Gates.

20
COMPANY
CONFIDENTIAL
Thyristor
Power Converter
Necessary condition for Diode Conduction :
Va – Vc > 0,7 V
Necessary conditions for Thyristor Conduction:
Va – Vc > 0,7 V
&
Pulse VG on the gate
+
_
Current
Va
Vc
anode
cathode
+
_
Current
Va
Vc
Vg
anode
cathode gate

21
COMPANY
CONFIDENTIAL
Operation Principle
Power Converter
• The Output Average Voltage Vexc
(left side) is, in any instant, the
highest among the actual
differences among the 3 phase
voltages VR , VS , VT at that instant
(phase-to-phase values).The result
is a regular sequence of the 6
typical ondulating peaks during
each 20 ms (cycle 50Hz). Its
average value is directly
proportional to VR , VS , VT feeding
values only.
Time
Vexc
VR – VT
VR VS VT
V
S
VR
V
R
VT VS
VRS VRT VST VSR VTR VTS
1 period of the fundamental
20 ms (50Hz) – 16.6 ms (60Hz)
V
R
V
S
V
T
V
R
V
S
V
T
V
R
V
S
V
T
R+
R -
S+
S -
T+
T -
Phase voltages
Vexc
R
S
T
+
--
i
i
iexc
VR
VS
VT
Vexc
DIODES BRIDGE (only for example)

22
COMPANY
CONFIDENTIAL
Operation Principle
Power Converter
• The Output Average Voltage of the Thyristor Rectifier
is not directly proportional to the Supply Voltage, as it
would be for a Diode Bridge, but it is function of the
controlling signal on the Gate of Thyristors.
• Every 3,3 ms (one sixth of the 20 ms period), 2
Thyristors are switched on: one of the Upper Row an
one of the Lower Row (but never on the same
column). In this way the line-to-line Voltage between
two Input Phases is directly applied to the rotor of the
generator.
• The DC Average Output voltage from the Rectifier can
be POSITIVE, ZERO or NEGATIVE. The negative
value can not be supplied continuously, but only as
long as the Rotor Reactance is charged with some
current: this is always true during the short electrical
transients, when a negative voltage is requested to
get a fast dynamic response.
• Theoretically Switching Angle could vary between
a=0° (same as a diode bridge) and a=180°.
• But Thyristors are not ideal components, so they need
a short time for the commutation: this limits the angle
range between 5 °(Positive Ceiling) and
150°(Negative Ceiling).
R
S
T
+
_
Iexc
VR
VS
VT
Rotor
R+ S+ T+
R - S - T -
R+ means thyristor of
the phase R, side
positive (+)
Average value of the rotor voltage
- VCeiling (α=150°)
0
Static Exciter Continuous Rated Voltage
Generator Rated Rotor Voltage
Transient
operation
+ VCeiling (α=5°)
Transient
operation
Continuous
operation

23
COMPANY
CONFIDENTIAL
Voltage Waveform
Power Converter
“Natural” instant
of firing of the
thyristor R+
VT VR VS VT VR VS VT
Phase voltages
Delay angle α
0°
60° 60° 60° 60° 60°
R+ R-
T- S+ T+ S-
Average
positive
Time
Vexc Rotor voltage
for α = 30°
α
α
α
α
α
α
Average = 0
Average
negative
Time
Time
Vexc
Vexc
Rotor voltage
for α = 90°
Rotor voltage
for α = 140°
• By changing the Delay of the Thyristor Firing Time (angle a) referred
to the “natural” firing instant of the diode bridge, it is possible to
reduce the output voltage, even to zero or to a negative voltage.
Working conditions with α > 90°,
and consequently with average voltage
negative, can be for transient times only

24
COMPANY
CONFIDENTIAL
Excitation Voltage
Power Converter
7.8 7.805 7.81 7.815 7.82
-100
0
100
200
300
400
500
600
700
800
900
1000
Low Voltage High Voltage Negative Voltage
• Static Exciter controls Generator by regulating the value of Field
Voltage (mean value of the Saw Teeth).
• For few seconds (< 10 s) Excitation Voltage can be forced to
double of Rated Value, in order to obtain fast corrective action
during transients (Ceiling Voltage).
3.8 3.805 3.81 3.815 3.82
0
100
200
300
400
500
600
700
800
900
1000
3 3.005 3.01 3.015 3.02
-600
-400
-200
0
200
400
600

25
COMPANY
CONFIDENTIAL
Excitation Voltage
Power Converter
• The Thyristors Bridge can deliver negative output voltages, even if the
semiconductors are controlled diodes (unidirectional).
• This is possible because Bridge feeds a big inductive load (Rotor Winding of
Generator), and any voltage variation on that produces current changes very slow,
compared with voltage changes (inside an inductance any current variation is
braked).
• Consequently, with Thyristors Pulses delayed at α > 90°, Voltage transmitted to
the Field is negative but Thyristors remain in conduction until current is positive
(current is decreasing because of the voltage inversion).
• If Current reached zero, Thyristors and Bridge would be switched-off.

26
COMPANY
CONFIDENTIAL
Converter Cooling related to Size
Module with 2 Thyristors, heat sinks and accessories
Low current size (up to about 1300 A): air cooling with
natural circulation in opened cycle
Middle current size (between 1300 A and about 4000 A):
air cooling with forced circulation in opened cycle
High current size (over about 4000 A): water cooling,
with forced circulation in closed cycle
AL SHABAB
AND
WEST DAMIETTA
COOLING TYPE

27
COMPANY
CONFIDENTIAL
Thyristors Module
Power Converter
Each rectifier is composed by 3 modules. Each module includes 2
Thyristors and the related accessories (heat sinks, fuses, insulating
transformer for the gate pulses, snubbers resistances and capacitors).
In case of Thyristor replacement:
• Remove the fuses and the gate connections;
• Unbolt the module from the copper bars;
• Release the nuts in the bell insulators, that keeps compressed
the whole sandwich of Thyristors and heat sinks;
• Clean the old grease and put some new silicone grease on the
Thyristor surfaces.
Example of Thyristor; main data:
• Maximum Junction Temperature = 125 °C
• Manufacturer: POSEICO
• Model: AT655S28
• PRV (Peak Reverse Voltage) = 2200 V
Cooling:
• The Thyristor commutations (that occur each 3,3 ms) cause a
considerable production of losses and heat.
• Bridges are cooled with forced air circulation in open cycle.
• Air enters the cabinet from the bottom openings, flows through
the thyristor heat exchangers, and exits from the upper part of
the cubicle.
• A temperature switch is placed on each thyristor module.

28
COMPANY
CONFIDENTIAL
2 Thyristors Module (1/3 of the Single Rectifier)
Power Converter

29
COMPANY
CONFIDENTIAL
2 Thyristors Module with Heat Sinks and Accessories
Power Converter
AIR COOLING WITH NATURAL CIRCULATION IN OPEN CYCLE

30
COMPANY
CONFIDENTIAL
Power Converter
AIR COOLING WITH FORCED CIRCULATION IN OPEN CYCLE

31
COMPANY
CONFIDENTIAL
Power Converter
WATER COOLING WITH FORCED CIRCULATION IN CLOSED CYCLE

32
COMPANY
CONFIDENTIAL
Thyristor Rectifier
Power Converter
THYRISTOR RECTIFIER (COMPOSED BY 3 MODULES)

33
COMPANY
CONFIDENTIAL
Excitation System
• The Power Section of the Excitation Cubicle is
composed by a totally controlled Three-Phase
Thyristor Bridge (known as ‘Graetz’ configuration).
• The Bridge is protected by one fuse in series of
each thyristor
• Each Thyristor is paralleled to a ‘snubber’, which
is an RC circuit for protecting the Thyristor against
transient spikes due to the commutations
• Gates of Thyristors are connected to both
Regulators through separate insulating
transformers
• Thyristor Rectifier is dimensioned for the
maximum continuous current that can be
requested by Generator Rotor in the heaviest
operating condition from the rotor point of view,
that is with the worst combination of voltage and
frequency variation: with high voltage and low
frequency the rotor current requested is higher
• A second thyristor rectifier, identical to the first, is
installed to get a 100% redundancy
Power Converter Generalities

34
COMPANY
CONFIDENTIAL
Redundancy
Excitation System
• 2 identical regulators are provided, for a 100 % ‘hot’ redundancy (the stand-by regulator is active, the program is
cycling, but no commands are given to the thyristors) . In case of failure, there is an automatic bumpless
commutation to the stand by regulator.
• 2 identical rectifiers are provided, for a 100 % ‘cold’ redundancy (the stand-by bridge doesn’t carry any current).
In case of failure, there is an automatic bumpless commutation to the stand by rectifier.
• 2 identical internal feeders for the supply of the control are provided, for a 100 % redundancy.
• the feedback of the stator voltage is 100% redundant: in case of lack of a feedback voltage, there is an automatic
switch to the stand-by regulator.
REGULATOR 1
(AUTOMATIC AND
MANUAL)
AVR 1
REGULATOR 2
(AUTOMATIC AND
MANUAL)
AVR 2
PLC
MONITORING LOGICS
CONVERTER 1
CONVERTER 2
INTERNAL FEEDER 1
INTERNAL FEEDER 2
FAN 2/2
FAN 2/2
FAN 1/2
FAN 1/2
MEASURES OF:
• STATOR VOLTAGE;
• FIELD VOLTAGE;
• STATOR CURRENT;
• FIELD CURRENT;
• CONVERTER INPUT VOLTAGE.
MEASURES OF:
• STATOR VOLTAGE;
• FIELD VOLTAGE;
• STATOR CURRENT;
• FIELD CURRENT;

35
COMPANY
CONFIDENTIAL
One-line Diagram
Excitation System

36
COMPANY
CONFIDENTIAL
Cooling System
Excitation System
• The bridges are cooled by forced air circulation in open cycle. Each bridge
has its 2 own fans.
• A failure on the operating fan generates an automatic switch to the stand-by
fan, without interruption of service. A second failure on the operating fan
generates an automatic switch to the stand-by channel (Bridge + Fan),
without interruption of service.
• Each bridge is equipped with a Differential Pressure Sensor. Whatever is the
cause of failure of the ventilation (electrical power, mechanical impediment,
clogged filters, engine failure), the differential manometer detects a drop in
the pressure difference upstream and downstream of the fan and causes
different commutations.
Threshold Set

37
COMPANY
CONFIDENTIAL
Back View (Door Opened Views)
Cubicle Outline
Discharge
resistance
Crow-bar
Shunt

38
COMPANY
CONFIDENTIAL
Back View (Door Opened Views)
Cubicle Outline
Rotor earth
fault
protection

39
COMPANY
CONFIDENTIAL
Measure of the Rotor Current
Excitation System
• The “ Shunt “ is a Resistance with a
very low value.
• Each Controller measures the
voltage at its ends in order to obtain
the armature current.
• The current measurement is used
for Automatic and Manual
Regulation.
• Also the measurement is sent to the
Steam Turbine Control System for
display in the graphics page (4÷20
mA signal).
SHUNT

40
COMPANY
CONFIDENTIAL
Crow-Bar
Excitation System
The De-Excitation device, called ‘Crow-bar’ is a static circuit made of 2 thyristors and a
resistance, and is used for 2 duties:
A. De-Excitation
B. Overvoltage Protection
EXCITATION
TRANSFORMER
THYRISTOR
BRIDGE
DISCHARGE
RESISTANCE
CROW-BAR
THYRISTORS
GENERATOR
ROTOR
WINDING
FIRING
CIRCUIT VP
VN
FP
FN
FROM AVR2
FROM AVR1
EXC ON-OFF
+
-

41
COMPANY
CONFIDENTIAL
Crow-Bar: De-Excitation Device « A »
Excitation System
0 time
Stator voltage
• In case of de-excitation (due to a Trip or a Shut-Down), Crow-Bar makes Rotor Current recirculate into a
dedicated circuit in order to dissipate energy stored in the Generator Field until Stator Voltage drops down
to zero.
• Field Current is dissipated by a passive resistance circuit, composed by Rotor Resistance and an
Auxiliary Discharge Resistance connected in series.
• Discharge Resistance Value is about twice Rotor Resistance Value; so complete Discharging Time is
approximately equivalent to the no-load time constant T’do.
• That operation is obtained by using “Crow-Bar” positive (CB positive), which can be fired by the Logic.
Only the “Crow-Bar” positive has two voltage thresholds: the lower used when Crow-bar is actuated as
De-Excitation Device, the higher used when Crow-Bar is operated as Overvoltage Protection,.
• Generator De-Excitation reduces Stator Voltage to about zero (except the effect of the magnetic residual).
• Its ON state allows the Field Current flow through RES and consequent Electromagnetic Flux discharge.
G
CB positive RES
MAIN
GENERATOR
+
_
I excitation
Stator
Voltage

42
COMPANY
CONFIDENTIAL
Crow-Bar: Overvoltage Protection « B »
Excitation System
IMPORTANT
3 possible methods guarantee the de-excitation in any situation:
1) Direct command from the AVR to the Crow-Bar Thyristor
2) Any time an external trip or shut-down command arrives to the Excitation
System, the overvoltage threshold is lowered to zero, so the crow-bar
thyristor is switched on, without need of AVR Intervention.
3) ‘Natural’ de-excitation. If the crow-bar thyristor fails, the excitation current
keeps on flowing through the thyristor rectifier and through the
secondary windings of the excitation transformer. The time requested to
extinguish the current is slightly longer, but the process is carried out as
well in a safe way. 0 Volt
Crow bar firing
voltage
1st threshold
for Overvoltage
Protection
2nd threshold for
De-Excitation
backup
• The two Thyristors (of Positive and Negative Crow-Bar) are used to cut the Rotor Overvoltages.
• Overvoltage Protection of the D.C. circuit is based on the Polarity of the same overvoltage; only the Thyristor,
that receives overvoltage on its anode will be switched on.
• The Overvoltage itself is used to trigger the Gate of the Thyristor involved, without need of the regulator.
• Overvoltage Protection is used also as a Back-Up for the de-excitation function.
Ceiling Voltage
Rated Voltage

43
COMPANY
CONFIDENTIAL
Protection against Line Side Overvoltages
Excitation System
• Varistors V1-V2-V3 act against overvoltages coming from the transformer supply (caused for example by
the circuit breaker opening, or by any other anomalous transient condition).
• Varistors are dimensioned for intervention above the repetitive peaks coming from the converter, caused
by thyristor commutations, in order to avoid aging problems.
• Varistors are protected by fuses.

44
COMPANY
CONFIDENTIAL
Regulators and HMI
Control and Regulation
Control System includes two identical
Digital Regulators (one operating and
the other in stand-by); they are
provided, for a 100 % redundancy.
Both programs are running, but only
one Regulator activates the Switching
Commands to the Thyristor Converter
Each of the 2 Digital Regulators is
composed by 4 Electronic Cards.
On the front of the panel, an HMI
(Human Machine Interface) allows the
operator to be informed about the
operating status, the alarms, the trips,
and to give the commands to the
system
The operator can read the variables in
real time and modify the regulator
parameters. In case one regulator
fails, the display will automatically
show the cause of the failure
In case of failure of one of the Digital
Regulator operating, there is an
automatic switching to the other in
stand-by.

45
COMPANY
CONFIDENTIAL
Operating Diagram
REGULATOR 1
(AUTOMATIC AND
MANUAL)
AVR 1
REGULATOR 2
(AUTOMATIC AND
MANUAL)
AVR 2
PLC
MONITORING LOGICS
CONVERTER 1
CONVERTER 2
INTERNAL FEEDER 1
INTERNAL FEEDER 2
FAN 2/2
FAN 2/2
FAN 1/2
FAN 1/2
MEASURES OF:
• STATOR VOLTAGE;
• FIELD VOLTAGE;
• STATOR CURRENT;
• FIELD CURRENT;
MEASURES OF:
• STATOR VOLTAGE;
• FIELD VOLTAGE;
• STATOR CURRENT;
• FIELD CURRENT;

46
COMPANY
CONFIDENTIAL
Digital Controller
21ALID1 (POS01)
power supply
21ANSD1 (POS 06)
acquisition and protection
21MCGF1 (POS 11)
microprocessor, memory, watchdog
21IOMD1 (POS 16)
interface
4 CARD OF CONTROL

47
COMPANY
CONFIDENTIAL
Control and Regulating
Operating Modes
• MANUAL MODE:
• Rotor current regulation
• Direct set of thyristor switching angle (only for test pourpose, not in operation)
• In the first case Operator can choose the set point of Excitation Current
• Manual Mode is used only locally (front panel of the cubicle) by the operator during special
tests or commissioning.
• AUTOMATIC MODE
1. Stator voltage regulation
2. Reactive power regulation
3. Power factor regulation
• Automatic Stator Voltage Regulation: Generator Voltage is automatically regulated to a
defined value by comparison with the Stator Voltage Feedback.
• Automatic Reactive Power Regulation: Generator Reactive Power is automatically regulated
to a defined value by comparison with Reactive Power calculated from the Stator Voltage
and Stator Current Feedback.
• Automatic Power Factor Regulation: Generator Power Factor is automatically regulated to a
defined value by comparison with the Power Factor calculated from the Stator Voltage and
Stator Current feedback.

48
COMPANY
CONFIDENTIAL
Automatic Regulation
Mode: closed loop with
the stator measures
GEN
Regulation
& Excitation
systems
Stator
voltage
Electric
feed-back
Set-point
selection
Automatic
correction
Manual Regulation
Mode: open loop
GEN
Stator voltage
indicator
Human visual feed-back
Continuous human
Supervision is necessary!
Regulation
& Excitation
systems
Automatic and Manual Regulation Modes

49
COMPANY
CONFIDENTIAL
PSS or Power System Stabilizer
PSS is a Stabilizer that operates against the Electromechanical
Oscillations of different types:
• Oscillations of Generator with the Grid, typically in a range between 0.8 and 3 Hz.
• Oscillations among Generators of the same Power Plant.
• Oscillations among different Power Plants.
• Oscillations among Parts of Electrical System in different geographical areas, typically in a
range between 0.1 and 0.8 Hz.
-1 0 1 2 3 4 5 6 7 8
0
0.5
1
1.5
P
-1 0 1 2 3 4 5 6 7 8
0
0.5
1
1.5
P
ACTIVE POWER
WITHOUT PSS
ACTIVE POWER
WITH PSS

50
COMPANY
CONFIDENTIAL
Compound
REACTIVE CURRENT
VOLTAGE
REFERENCE
0 ÷ - 22%
0 ÷ 22%
SHARE
0
OFFSET
This function adds a positive or negative contribution (tunable, max 22%) to the reference of the Stator Voltage,
proportionally to the actual measured Reactive Power.
It has two possible applications:
• Positive slope (blue lines): in this case Generator has connected to its own Step-up Transformer, before being
connected to others units. The purpose of the compound function is to compensate a part of the voltage drop
across the external reactance of the main transformer.
• Negative slope (orange lines): in this case Generator is directly connected to other Generators, without the
interposition of any transformers. The purpose or the function is to prevent recirculation of Reactive Power
between Generators.

51
COMPANY
CONFIDENTIAL
Over-Excitation and Under-Excitation
Generator “ G “ synchronized on the Grid, can be represented as follows, in a simplified way
and from the Reactive Power only point of view.
The interconnecting reactance “ X “ allows Reactive Power exchange between Generator and
Grid.
Just after the Synchronization Va = Vr and no Reactive Current Ia is flowing into the reactance:
I = (Va-Vr) / X è I = 0 = Q
With Vr constant, changing Va, a Reactive Current rises:
• if Va > Vr I and Q flow from Generator to the Grid (Over-Excitation).
• if Va < Vr I and Q flow from the Grid to Generator (Under-Excitation).
G
X
Grid
Va Vr

52
COMPANY
CONFIDENTIAL
G
X
Grid
Va Vr
Ia
∆Vt
Under-excitation
+
_
G
X
Grid
Va Vr
Ia
∆Vt
Over-excitation
+
_
In Over-Excitation condition, Ia is flowing from Generator to the Grid and the Voltage Drop on
the Reactance X is with the + at Generator side.
In Under-Excitation condition, Ia is flowing from the Grid to Generator and the Voltage Drop on
the reactance X is with the + at the Grid side.
In Over-Excitation Generator sees an Inductive Load, while in Under-Excitation Generator sees
a Capacitive Load.

53
COMPANY
CONFIDENTIAL
Active
power
Reactive
power
Over-
excitation
Active
power
Reactive
power
Under-excitation
+
_
G
X
Grid
Ia
Va Vr
∆Vt
G
X
Grid
Ia
Va Vr
∆Vt
+
_
Va
Vr
∆Vt
Ia
OVER-
EXCITATION
Ia is 90° delay on Va = INDUCTIVE LOAD
Va > Vr
UNDER-
EXCITATION
Va
Vr
∆Vt
Ia
Ia is 90° ahead on Va = CAPACITIVE LOAD
Va < Vr
LAGGING LEADING

54
COMPANY
CONFIDENTIAL
« Capability » Diagram
Generator Operating
ϕ
Rated Working Point
Inaccessible area due to
the Turbine Power
Limitation
Rotor Size of Generator
Stability Problems and
Overheating of some part in
correspondence of Stator
Core and Winding Ends
Max Apparent
Power of Generator
Active Power
Positive Reactive
Power
Negative Reactive
Power
Under-
Excitation
LEADING
Over-
Excitation
LAGGING
0
Motor
Under-Excitation
Limit
Max Rotor Current
Limit
Max
Turbine
Power

55
COMPANY
CONFIDENTIAL
Generator Operating
• Vertical Axis represents “Active Power” and Horizontal Axis represents “Reactive
Power”. Capability Diagram shows all Generator Working Points at a specific Stator
Voltage Value.
• Capability Diagram represents the Generator connected to the Grid or to a load and is
the result of the Turbine and Generator sizes; that takes into account some restrictions
that limit the theoretical possible operating area.
• Limitations are mainly due to materials temperatures limits and dynamic performances
(stability). When limit values are exceeded, system tries to reduce the exceeded
value by increasing or decreasing the excitation current.
• The following slides show a sequence of main considerations that give, as conclusive
result, the construction of the final “capability area” for the Turbine-Generator Set.

56
COMPANY
CONFIDENTIAL
Generator Operating
Q
-Q
P
A2 = P2 + Q2
A=1
A=0.8
A=0.6
A=0.4
A=0.2
A= V • I
Curves with the same
Apparent Power
(and also with the same
Stator Current)
On “Active Power / Reactive Power Plan”, it is only shown area corresponding to positive Active
Power [we are speaking about a “Generator”, a machine that delivers Active Power to the Grid
(positive sign), not about a “Motor”, a machine that absorbs Active Power from the Grid
(negative sign)].

57
COMPANY
CONFIDENTIAL
Generator Operating
Q
-Q
P
cosϕ=0.8
cosϕ=0.6
cosϕ=0.9
cosϕ=0.95
cosϕ=0.85
cosϕ=0.4
cosϕ=0.8
cosϕ=0.6
cosϕ=0.9
cosϕ=0.95
cosϕ=0.85
cosϕ=0.4
Lines with the same
Power Factor

58
COMPANY
CONFIDENTIAL
Generator Operating
• Generator Size (in Apparent Power, MVA) defines on this plan the half-circle corresponding
to all Machine Working Points, having less or equal MVAs then Rated Size. The boundary of
that half-circle represents the maximum allowed MVA in different conditions.
• Rated Power Factor and Rated Apparent Power define in the Diagram the Rated Working
Point of the Turbo-Group (Turbine + Generator).
ϕ
Rated Apparent Power
(MVA) of Generator
Acitive Power
Positive Reactive
Power
Negative
Reactive Power
Under-
Excitation
LEADING
Over-
Excitation
LAGGING
0
Motor
(+)
(-)
Generic Working Point
(MVA, corresponding to
MW / MVAR)
MVAR
MW
MVA
ϕ of Rated
Power Factor
Rated Working Point
(at Rated Apparent Power and
at Rated Power Factor)

59
COMPANY
CONFIDENTIAL
Generator Operating
Rated Working Point
Inaccessible Area due to Turbine
Power Limitation (max MW size)
Active Power
0
Max Power
(MW) of Turbine
(MVA) of Generator
(+)
Positive
reactive power
Negative
reactive power
Under-
Excitation
LEADING
Over-
Excitation
LAGGING
Motor
(-)
• “Rated Working Point” (MVA, MW, MVAR, cosφ) is the main reference for Turbine and
Generator sizing. Consequently, Turbine is designed to deliver, as its max power, the MW
correspondent to the Rated Working Point.
• This is the first limitation that “cuts” the upper part of the “Capability” Diagram. It will be
physically impossible to Over-Exceed that MW and, therefore, the upper highlighted area will
be erased because inaccessible (Forbidden Working Conditions of Turbo-Group).

60
COMPANY
CONFIDENTIAL
Generator Operating
Rated Working Point
Active Power
(MVA) of Generator and
Stator Size (+)
Inaccessible Area
due to the Rotor
Size of Generator
Limitation
due to the
Rotor Size
Positive
Reactive Power
Negative
Reactive Power
Under-
Excitation
LEADING
Over-
Excitation
LAGGING
Motor
(-)
• Generator has Stator Size designed on Apparent Power (MVA) of Rated Working Point;
similarly Rotor Size.
• Rotor is sized for the Steady Field Current corresponding to Rated Working Point and this
size limits Working Conditions up to the points corresponding to the red line.
• Consequently, the blue area (from the red line to right end of Diagram) is “cutted” and
inaccessible, because of Rotor Size.

61
COMPANY
CONFIDENTIAL
Generator Operating
Inaccessible area due to Stability
Problems and Overheating of
some parts in correspondence of
Stator Core and Winding Ends.
Active Power
Positive
Reactive Power
Negative
Reactive Power
Under-
Excitation
LEADING
Over-
Excitation
LAGGING
Motor
(-)
(+)
Limitation
Rated Working Point
• Last Limitation on Capability Diagram (blue area from the red line to left end) is due to Generator Stability
Problems and to Overheating of some parts at Stator Core Ends.
• Risk of Instability is mainly due to Weak Flux inside the Machine Air-Gap that makes feeble Magnetic
Link between Rotor and Stator, with consequent possible Sliding and Loss of Synchronization.
• Overheating of the extreme parts of Stator Core and Winding, due to a particular Magnetic Flux
disposition in that zone, is the second reason of limitation on this Under-Excitation Area of Capability
Diagram.

62
COMPANY
CONFIDENTIAL
Generator Operating
ϕ
Rated Working Point
the Turbine Power
Limitation
Rotor Size of Generator
Stability Problems and
Overheating of some part in
correspondence of Stator
Core and Winding Ends
Max Apparent
Power of Generator
Active Power
Positive Reactive
Power
Negative Reactive
Power
Under-
Excitation
LEADING
Over-
Excitation
LAGGING
0
Motor
Under-Excitation
Limit
Max Rotor Current
Limit
Max
Turbine
Power
All Permissible Working Points (at Steady State) for Generator are included
into the Green Area.

63
COMPANY
CONFIDENTIAL
Over-Excitation Limit
ACTIVE
POWER
REACTIVE
POWER
Kel3
Kel2
Kel1
Kel0
MAXIMUM
ROTOR CURRENT
MAXIMUM
ROTOR CURRENT
GENERATOR COOLING
LEVEL (HYDROGEN
PRESSURE)

64
COMPANY
CONFIDENTIAL
Under-Excitation Limit
ACTIVE
POWER
KU01
GENERATOR
COOLING LEVEL
(HYDROGEN
PRESSURE)
1 P.U.
0,5 P.U.
KU23
K5U3
K5U0
K1U3 K1U0
K1U2 K1U1
K5U2
K5U1
IF NECESSARY THE REDUCTION
COEFFICIENT Uel_KUR
REDUCES PROPORTIONALLY
ALL THE LIMITS, FOR EXAMPLE
DURING TESTS IN UNDER-
EXCITATION
REACTIVE
POWER

65
COMPANY
CONFIDENTIAL
Stator Current Limit
ACTIVE
POWER
f
Cosf=0,73
MAXIMUM
STATOR
CURRENT
GENERATOR
COOLING LEVELS
(HYDROGEN
PRESSURE)
Kal3
Kal2
Kal1
Kal0
f
Cosf=0,73
REACTIVE
POWER

66
COMPANY
CONFIDENTIAL
Ceiling Forcing
Excitation System
• Ceiling Voltage is max Excitation Voltage (positive or negative) that Exciter is able to
deliver to Field Winding.
• During transients, Regulator can force Excitation Voltage far over its final desired
steady state value, so Rotor Current reaches its final value faster.
• It is applied only for few seconds, during transients after changes in Grid or by
Operator Command.
• Ceiling Voltage is applied by Converter in a very short time (< 40 ms), while Rotor
Current follows and increases following its slow time constant.
• Thypical Positive Ceiling Voltage is twice Generator Field Rated Voltage. Negative
Ceiling Voltage is typically 0,8 times the value of Positive Ceiling Voltage.

67
COMPANY
CONFIDENTIAL
Example of Ceiling System Intervention
Excitation System
• Blue = excitation voltage
• Red = generator voltage
• Yellow = excitation current
• Green = reactive power
100 ms

68
COMPANY
CONFIDENTIAL
Accuracy & Responce Ratio
Excitation System
• Control Accuracy is measured by the displacement, at steady state, between Actual Value
and Reference Value of Stator Voltage.
• Regulator Accuracy is 0,2%.
• The Control Accuracy of Stator Voltage depends on the complete chain of the control loop,
including transformers.
• It is a parameter that gives an idea of the promptness by which Excitation System responds
to the variations, mainly in Automatic Regulation Mode.
• Starting from Rated Excitation Voltage, Ceiling Voltage is imposed by Regulator and
Resulting Excitation Voltage Shape on the Field is a curve significant of the Exciter
Response.
• Response Ratio makes sense for Rotating Exciters only, because in the case of Static
Exciters that is directly proportional to the Ceiling and it is very fast.
• International Standards define this evaluation method as shown into the following image
(find the compensation line, mark the intersection point at 0.5 sec and apply the indicated
formula).
Accuracy
Response Ratio

69
COMPANY
CONFIDENTIAL
Responce Ratio
Excitation System
time
[sec]
Vexcitation
O
A
B
C
Vceiling
A
R = AB / (BC*OC)
0.5 s
Vrated excitation
Rotating Exc. è RED AREAS
Static Exc. è GREY AREAS
AC = compensation line
considering instant t = 0,5 s, area ABC must be equal
to the area under the real curve (the exponential for
rotating exciter and the step for static)
STATIC
BRUSHLESS

70
COMPANY
CONFIDENTIAL
Settling Time
Excitation System
t [sec]
Stator voltage
Settling Time t0 is defined as the time necessary for Stator Voltage Waveform to definitively
enter inside a specified range. For example 2%.
Vg2
Vg1
t0
Range (± … %)
O

71
COMPANY
CONFIDENTIAL
Logic
Excitation System
• Excitation ON command is allowed only if the Generator Circuit Breaker is opened, and if the
speed is higher than 95%.
• In Automatic Mode, when the excitation command is given, Regulator rises the voltage
following a gradual ramp (which slope is tunable by a parameter) up to a final value (which is
also specified by another parameter).
• PSS Stabilizer is automatically disabled if the active power is lower than a tunable threshold,
or if the main circuit breaker is opened.

72
COMPANY
CONFIDENTIAL
Rotor Current Overload
Excitation System
If this function is activated, Rotor Current limit is set to the value Iem (tunable), higher of the maximum continuous
excitation current limit ( Iel ).
Whenever the field current exceeds the threshold Iel, the regulator starts the following calculation, to evaluate the
overheating:
When the integral reaches the maximum value C (tunable by a parameter), the limit is lowered to the value Iel and the
overload is not allowed any more, until a preset time is elapsed.

73
COMPANY
CONFIDENTIAL
Rotor Earth Fault Relay
Excitation System
• This device guarantees a continuous monitoring of the
insulation level of the rotor winding.
• In normal conditions this insulation level must be very high
(MΩ), while, if it drops (some KΩ), it is significant of some fault
in the insulation of the DC circuit (for example a damage in the
insulation material of the rotor winding).
• This kind of fault, if occurred in one point only of the rotor
winding, does not produce any effect, but always it strongly
suggests to stop the generator operating as soon as possible,
in order to avoid any further fault in other different points.
• Infact this situation could produce injurious effects to the
system.
• This device can be chosen with one or two monitoring
thresholds on the rotor winding insulation level and the
corresponding operations are the following:
§ with one threshold its action is only an alarm;
§ with two thresholds its actions are alarm (step 1, i.e.
40kOhm) and trip (step 2: i.e. 5kOhm).
GENERATOR
Rotor
Winding
Rotor Earth
Fault Relay
+
-
Eventual Fault

74
COMPANY
CONFIDENTIAL
Rotor Temperature Measurement
Excitation System
GENERATOR
Rotor
Winding
+
-
Vexc
Iexc
Rotor
temperature
calculator
Iexc Vexc
Excitation board
Output signal
for remote
°C
This device guarantees a constant monitoring of Rotor Winding Temperature, during Generator Operating.
Its working principle is based on the continuous measure of Excitation Voltage and Current in order to calculate the
actual winding temperature by the ratio Vexc / Iexc at any time interval of some milliseconds (cycle time of the
digital program).
The function is programmed considering the high thermal inertia of Rotor Winding, compared with the variation
times of the excitation voltage and current (i.e. the ceiling voltage impulses are not affecting the calculations
because too fast for a possible influence on the temperature winding).
The temperature is sent to the control system by means of a 4-20 mA signal.

75
COMPANY
CONFIDENTIAL
Limiting Functions
Excitation System
The following Protecting Functions are accomplished by each of the 2 regulators:
• Under Excitation Limit, that prevents loss of step and stator core heating at the iron ends. The
action of Regulator will be to increase the Excitation Current, in order to lower the module of the
Reactive Power (direction: to the Over-Excitation Zone)
• Maximum Excitation Current Limit, that prevents the heating of the rotor winding by keeping the
excitation current under a safe threshold
• Flux Limit, that prevents heating of the generator, the step-up transformer and the unit transformer
due to high values of flux (=V/Hz). The action of the regulator will be to reduce Voltage, because it
is not possible to control the frequency with the Excitation System
• Stator Current Limit, that prevents heating of Stator Winding due to an High Current Value. The
action of the regulator will be to reduce the module of the Reactive Power, because it is not
possible to control the Active Power with the Excitation System
When limit values are exceeded, system tries to reduce the exceeded value by increasing or
decreasing the excitation current.

76
COMPANY
CONFIDENTIAL
Operator Interface
Excitation Board
• modify the regulator parameters (gains, time constants, configuration parameters, limits and protection
setting);
• view all the electrical quantities and parameters;
• show the cause of an AVR failure on the display;
• change the password;
• configure the “ black-box ”;
• perform tests.
• to command the system, when the local/remote switch on the door is in 'local‘ position;
• supervision and diagnostics (system information, alarms, trips).
Touch-screen HMI on the front door, that allows:
LCD display with small keyboard (one for each regulator),
which allows to:

77
COMPANY
CONFIDENTIAL
LCD display for interface with the regulators
Excitation Board

78
COMPANY
CONFIDENTIAL
Touch-Screen Operator Interface
Excitation Board

79
COMPANY
CONFIDENTIAL
Excitation Board

80
COMPANY
CONFIDENTIAL
Excitation Board

81
COMPANY
CONFIDENTIAL
Excitation Board

82
COMPANY
CONFIDENTIAL
Excitation Board

83
COMPANY
CONFIDENTIAL
Excitation Board
FIRST OUT’ INDICATION
IS PROVIDED BY AN
ARROW

84
COMPANY
CONFIDENTIAL
Excitation Board

85
COMPANY
CONFIDENTIAL
Excitation Board

86
COMPANY
CONFIDENTIAL
Excitation Transformer
Excitation System
• Power Supply of the Static Exciter is
provided by a 3-phase Transformer.
• The transformer can be dry type (easy
installation, less maintenance, no fire
prevention issues, no civil works for oil
gathering), or oil type (more suitable for
outdoor installation, higher protection
degree, allows installation under rain and
sun).
• Natural Air Cooling is preferred to obtain
reliable operation.
• In case of dry type transformer, the
protection degree must allow a good air flow
to dissipate the transformer losses.

87
COMPANY
CONFIDENTIAL
Dry Type Resin Insulated Transformer
RESISTANCE
TEMPERATURE
DETECTORS
PT100 TYPE
PRIMARY WINDING
CAST RESIN
INSULATED
OFF LOAD TAP
CHANGER
PRIMARY
TERMINAL
CORE
SECONDARY
WINDING RESIN
IMPREGNATED
SECONDARY
TERMINAL
Transformer is equipped with three
PT100 RTD (Resistance
Temperature Detector) to monitor
Temperatures in each Winding and
on the Core.
They are acquired by the Control
System.
If the First Temperature Threshold
is exceeded by any of the RTDs, an
alarm is given.
Only for the windings temperatures,
if the second higher threshold is
exceeded by 2 sensors located in
the same position (logic 2 out of 3),
the shut-down is requested with the
following sequence:
• lowering the active load to the
technical minimum
• reduction to zero of the reactive
power
• open the main circuit breaker
• deceleration

88
COMPANY
CONFIDENTIAL
Trasformer Enclosure
Excitation System
In case the Transformer is not installed in a
separate locked dedicated room, it is provided
with a Metallic Protection Enclosure

89
COMPANY
CONFIDENTIAL
Dry Type Resin Insulated Transformer
Excitation system
Cables and Bus Bars which are connected to
the Transformer must enter the enclosure only
through Suitable Flanges specified at the
ordering stage.

90
COMPANY
CONFIDENTIAL
Off Load Tap Changer
Transformer allows to slightly change Voltage Ratio,
by changing the position of a Tap Changer (to be
operated at no load).
The tap changer is on Primary Winding.
The Voltage can be changed in 5 different levels, in
a range between +5% and -5%: +- 2 x 2,5 %

91
COMPANY
CONFIDENTIAL
Oil Insulated Transformer
Excitation System

92
COMPANY
CONFIDENTIAL
Rating Plate
• Each Transformer has fixed in its
structure an Identification Plate
which reports the Main Technical
Data
• Due to the type of load, that is a
thyristor rectifier, the shape of the
current is not sinusoidal, but
rectangular. This results in
harmonics and additional heating,
so that the transformer must be
dimensioned more heavily than a
normal distribution transformer
• The transformer secondary voltage
is dimensioned to allow the ceiling
forcing requested at the DC side of
the converter, taking into account
the voltage drop on the thyristors
• The Tap Changer on the primary
winding must be operated with the
Transformer De-Energized, but
usually there is no reason to modify
the voltage.

93
COMPANY
CONFIDENTIAL
Description
Excitation System

10 03 Excitation System.pdf

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to 10 03 Excitation System.pdf

Similar to 10 03 Excitation System.pdf (20)

Recently uploaded

Recently uploaded (20)

10 03 Excitation System.pdf