7. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV – Velenje 2005
CIGRE ŠK A1 - 10
Excitation systems for high power synchronous generators
with redundant configurations
Zvonimir Jurin, Blaženka Brkljač, Marin Kolić
KONČAR – Elektronika i informatika
Fallerovo šetalište 22 , Zagreb, Croatia
E-mail: uzbude@koncar-inem.hr
tel. 00385 1 365 5768
Mladen Kajari, Vinko Ćesić,
KONČAR – Institut za elektrotehniku
Fallerovo šetalište 22, Zagreb, Croatia
E-mail: mkajari@koncar-institut.hr
tel. 00385 1 365 5301
Abstract – In the article the solutions of synchronous generator excitation system with thyristor converter, digital voltage
regulator and field discharge equipment are presented with attention paid to achieve increased reliability and availability
by applying redundant configurations. The excitation system with thyristor converter in three-phase fully controllable
bridge connection and 1+1 configuration, twin channel digital voltage regulator and field discharge equipment for big
and important generators in parallel operation are shown. Field results verify the project-foreseen characteristics and
qualities of presented solution.
I. INTRODUCTION
To achieve high availability of synchronous generators for electric energy production, attention has to be
paid to the increase of availability of all generator
subsystems and among them of the synchronous
generator excitation system during a very long-term
service. Requirements on availability of excitation system,
beside its technical functions, have been always high
and improvements are still being implemented.
The excitation system consists of exciter (the excitation power source) and voltage regulator.
In the article, the solutions of thyristor rectifier
exciter and digital voltage regulator are discussed with
the aim of achieving reliable and redundant configurations with increased availability in synchronous
generators operation.
voltage shaft generator and static thyristor rectifier. The
compound-source controlled-rectifier exciter that utilizes
the potential and the current of synchronous generators, as
an excitation power source, together with thyristorcontrolled rectifier is also used for special purposes.
The actual and world most common solutions of
high power synchronous generator excitation system,
is potential source thyristor rectifier exciters with
digital voltage regulator.
The basic approach to achieve high reliability and
availability is application the proper ratings for components and usage of high quality components. To
avoid production failures comprehensive during and
after production tests and commissioning have to be
carried out. Beside that it is not possible to guarantee
that no failure can occur.
The actual solution of an excitation system consists
of a big number of electrical, electronic and mechanical
components and software products, working together
under heavy technical conditions.
The basic technical environmental requirements are
extreme working and storage temperatures, their deviations and rate of change, combined with humidity,
dust and oil vapour, vibrations and shock strains and
electromagnetic interference.
The requirements on excitation power source depend on
machine characteristics and its role in power system.
There are potential-source and controlled rectifier exciter
(static excitation system), alternator supplied rectifier
excitation system (brushless excitation system) that use
mechanical power on generator shaft and auxiliary constant
Fig. 1: Block diagram of a static excitation system
with thyristor converter in 1+1 configuration, the twin
channel digital voltage regulator and discharge equipment
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7. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV – Velenje 2005
CIGRE ŠK A1 - 10
To minimize risks of failure of synchronous generators excitation system and outage of production of
the generator the hardware and software redundancy
is applied:
1.
Thyristor converter configuration n-1 and 1+1,
2.
Twin channel digital voltage regulator
3.
Field discharge equipment and field overvoltage protection
significant disturbance of generator operation. Converters have redundant cooling fans.
The block diagram of a static excitation system
with thyristor converter in 1+1 configuration, the twin
channel digital voltage regulator and discharge equipment is in Fig 1.
II. THYRISTOR CONVERTER
CONFIGURATIONS
N-1 AND 1+1
The standard solution of excitation system thyristor
converter is a three-phase bridge converter with 1
thyristor in series and n thyristor in parallel per bridge
arm. Fast fuse is in series with each thyristor, Fig 2.
The number of parallel thyristors per arm is n>3, to
maintain selectivity of fuse protection. Current ratings
of thyristor are so that n-1 thyristor per arm are
capable to withstand all static and transient ratings
and one additional thyristor per arm is for security
reasons. The converter has redundant fans.
Fig. 3. Thyristor converter in 1+1 configuration
Behaviour of generator and excitation voltage after
simulated failure on one thyristor in the operating
thyristor bridge is shown in Fig. 4. Transfer to the
second bridge is carried out in app. 50 ms.
Fig. 2. Thyristor converter in n-1 configuration
The nowadays-applied configuration of thyristor converter is 1+1 Fig. 3, because high rating thyristors are
now available. This configuration consists of two equal
three-phase bridge converters with 1 thyristor in series
and 1 thyristor in parallel per bridge arm. Fast fuse is in
series with each thyristor. Each bridge is rated to
withstand all static and transient generator excitation
ratings. One bridge converter is in operation while the
other is hot stand-by. AC current of bridges are monitored and in case of a thyristor failure the changeover is
taking place using free wheeling thyristor without any
Fig. 4. Transfer of excitation Converter Bridge 1 to
Bridge 2 in the 1+1 configuration with the generator
in parallel operation to the grid
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7. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV – Velenje 2005
CIGRE ŠK A1 - 10
III. TWIN CHANNEL DIGITAL VOLTAGE
REGULATOR
The single channel digital voltage regulator in comparison to the analogue voltage regulator offers a lot
of benefits such as:
•
•
•
•
•
•
Performing of sophisticated control, protection,
supervision and logical and sequential control
functions,
Short response time,
Long term stability,
Self monitoring functions,
Reduced diversity of hardware modules,
Digital standardized communication to the superimposed control system.
Twin channel digital voltage regulator is a solution
with two identical digital voltage regulators working
in parallel in hot stand-by configuration. Each of two
channels has the same input signals and is executing
the same application program. Each channel has a
complete structure with automatic and manual voltage
control, reactive power regulation, limiters protecting
the generator of going-out the generator capability
diagram, compound function, V/Hz limiter and power
system stabilizer (PSS) functions. Every channel controls
both thyristor bridges Comprehensive monitoring
program monitors the function of both regulators and
in case of detection of an irregularity or exception the
changeover is performed to the healthy channel
without interfering normal generator operation.
The changeover between channels of two-channel
digital voltage regulator after simulated disturbance in
operating channel is shown in Figure 5.
IV. FIELD DISCHARGE EQUIPMENT AND
FIELD OVERVOLTAGE PROTECTION
There are two basic solutions of generator field
discharge equipment, one, which include DC field circuit
breaker with discharge contact, non-linear discharge
resistor and separate circuit for overvoltage protection
and second which includes AC circuit breaker with
common circuit for discharge and overvoltage protection. Besides, there is normal operative procedure for
generator deexcitation, the transfer of thyristor exciter to
inverter operation mode.
In classic solutions of discharge equipment the
thyristor converter is connected to synchronous
machine field winding via de- excitation D.C. breaker
with a discharge contact and a discharge resistor, Fig.
6. When instantaneous electrical fault arises in the
synchronous machine or in outside facilities, the
short-circuit current will appear and the field should
be suppressed as quickly as possible in order to limit
fault current damage. It is also necessary to ensure
safe de-excitation in event of a fault in the exciter
control circuit causing severe over-excitation. The
voltage dependent or linear discharge resistor is used
to achieve rapid field suppression with regard to the
highest permissible voltage across the field winding
and the field breaker.
The purpose of the rotor overvoltage protection is
to prevent the machine field and thyristor converter
from being exposed to an excessive overvoltages
induced in the machine field with rectifier excitation,
which could arise during certain transient condition
such as improper synchronizing, loss of synchronism
and asynchronous operation.
The main overvoltage protection circuit comprises
the antiparalell connected thyristor (crow bar) and
resistor in series. The thyristors are controlled by the
over-voltage sensing triggering unit with BOD
elements. When the voltage exceeds the preset
protection level the thyristor switch-on and conduct
the current caused by over voltage. The series resistor
is intended to limit the current in the protection
circuits. On the other hand, the resistor must be so
designed that the voltage drop caused by the highest
induced field current flowing through the resistor does
not exceed the protection level. The overcurrent relay
supervises of the protection and gives a tripping
impulse when the protection operates.
Fig. 5. Changeover of Channel 1 to Channel 2 with
generator in parallel operation to the grid
When thyristor converter is applied in configuration
1+1, freewheeling thyristor is used to short-circuit
field winding during emergency change over between
bridges under high currents.
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7. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV – Velenje 2005
CIGRE ŠK A1 - 10
Fig. 6 Discharge equipment with DC field circuit
breaker and separate circuit for rotor over voltage
protection
In the Fig. 6 the usual and most adequate solution for
the field circuit breaker and over voltage protection is
shown. The merits of this solution are the completely
independent over voltage protection circuit and field
circuit breaker that separates generator excitation on both
poles from thyristor rectifier. In case of failure on field
circuit breaker or non-linear deexcitation resistor, the rotor
over voltage protection could act as back-up deexcitation.
Usually DC field circuit breaker has two coils for
switching-off, which, together with monitoring of
switching-off circuits, realize very reliable solution.
The alternative solution of discharge equipment is
implementation thyristors from thyristors over-voltage
protection (crow bar) and AC circuit breaker at
primary of thytistor rectifier Fig. 7. Deexcitation is
performed by switching-off of the AC circuit breaker
what causes the over voltage on the generator
excitation winding. This over-voltage will activate
over-voltage protection (crow bar), reduce the overvoltage and deexcite the generator. In this case the
same resistor is used for the de-excitation and for the
field circuit over voltage protection, and in a case of
its failure, there is no redundant deexcitation circuit to
prevent damage of rotor winding.
Fig. 8 Discharging of generator by DC field circuit
breaker and nonlinear discharge resistor
Site tests are carried out on excitation system type
SEM 11E, delivered and putted into operation for two
units on Hydro Power Plant Varaždin. Generator and
excitation system data are given in Table 1.
It is Potential source thyristor rectifier exciter (static
excitation system) supplied from the generator
terminals. It is controlled by twin (two) channel
digital voltage controller with separate hardware for
each channel. Each channel of digital voltage
regulator comprises Generator voltage regulator
(AVR), Excitation current regulator (SR), Trigger
pulse system and Follow-up control between main
and stand-by controller, and between AVR and SR
mode for each channel. Excitation power is derived
by: three-phase fully-controlled (6-pulse), air forced
cooled, thyristor converter in configuration 1+1, with
one thyristor per branch and two converter bridges in
parallel. One thyristor bridge is in stand-by and
blocked.
TABLE I
GENERATOR AND EXCITATION SYSTEM DATA
Rated power
Nominal voltage
Power factor
Speed
Fig. 7 Discharge equipment with AC field circuit
breaker and common discharge resistor wit rotor
over voltage protection
Sn
Un
cosϕn
nn
MVA
KV
min-1
Unit A Unit B
53.5
50
10.5
10.5
0.85
0.85
125
125
50
2920
890
50
2740
940
Frequency
Nominal current
Nominal excitation current
fn
In
IfN
Hz
A
A
Nominal excitation voltage
UfN
V
210
220
Nominal exciter current
IEN
A
1250
1250
Ceiling current
Ip
A
1780
1880
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7. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV – Velenje 2005
CIGRE ŠK A1 - 10
V. CONCLUSION
The presented excitation system with redundant
configuration with a thyristor converter in consisting
1+1 three phase thyristor bridges, classic discharge
equipment with field circuit breaker and separate
circuit for over-voltage protection, and microprocessor based twin-channel voltage regulator is suitable
for big and important synchronous machines in
parallel operation. The excitation system is characterized by increased safety, reliability and availability
because of build-in redundancy.
Field results have verified characteristics in exploitation and qualities of presented strategy.
REFERENCE
[1] Z. Jurin, M. Kolić, M. Kajari, V. Ćesić:
Redundant configuration in synchronous generator excitation system Hydro conference, Dubrovnik 2003.
[2] S. Marijan, M. Kajari, N. Perić: Microprocessorbased Control System for Electrical Machines,
Proceedings of the 8th International Conference
on Electrical Drives and Power Electronics,
EDPE’94, Pula, Croatia, pp 203 - 209, September
1994.
[3] Mladen Kajari, Siniša Marijan, Vinko Ćesić,
Nedjeljko Perić: Digital Control of a Synchronous Generator Excitation System, Proceedings
of the 8th International Conference on Electrical
Drives and Power Electronics, EDPE’94, Pula,
Croatia, pp 203 - 209, September 1994.
[4] N. Perić, S. Marijan, M. Kajari: Microprocessorbased control system for the control of electrical
machines and processes. 6th European
Conference on Power Electronics and Applications, EPE’95, Sevilla, pp (2546-2550)
[5] S. Marijan, M. Kajari, N. Perić: Microprocessorbased control system for electrical machines,
Automatic 1-2/1995, Zagreb, pp (19-26).
[6] V. Ćesić, M. Kajari, S. Marijan, Z. Jurin, M.
Kolić: Excitation system with microprocessor
based twin-channel voltage regulator for
synchronous machines, EPE-PEMC 2002.
Dubrovnik & Cavtat,
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7. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV – Velenje 2005
CIGRE ŠK A1 - 10
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