Excitation Systems
THYRIPART® Compound-Excitation System for Synchronous Generators
Power Generation
Operating Characteristics Load dependent Short circuit supporting Low voltage gradient dv/dt Black start capability Digital open and closed loop control High reliability and availability Extraordinary flexibility Easy maintenance Best control capabilities The THYRIPART® excitation system is a static excitation system supplied by voltage and current from the generator and incorporating a transistorized voltage regulator. It supplies the energy required to excite the generator directly via slip rings. The outstanding flexibility of the standard system and the modular construction make the THYRIPART® system suitable for application in industrial-, steam- and hydro power plants and particularly suitable for the modernization of existing excitation systems. THYRIPART®-compartments (examples)
Standard circuitry This basic circuit with its excellent dynamic properties also features a transistorized controller which provides a parallel circuit to the excitation winding. The transistorized controller is a DC chopper regulator, a standard product of DC drive technology. It operates in parallel with the rectifier, i.e. it is connected in parallel to the excitation winding. The transistorized chopper can both add and subtract current to the field current supplied by the basic THYRIPART® circuit, thus increasing or reducing the field current. This control feature permits a relative voltage stability of ± 0.5%. In island mode it is even possible to operate the excitation system without closed loop control. The hardware components transformer, ‚Harz circuit’ and bridge converter are capable to control the generator voltage to an accuracy of (depending on generator characteristics) approximately ± 10 %. The modular construction the graphically supported flexible software of the digital open and closed loop control as well as the little space requirement makes the system ideal for the modernization of existing excitation systems. For new systems adaptation to specific requests is possible.
Mechanical Design
The functional principle is based on a ‘Harz circuit’ which uses an oscillating circuit to convert a voltage source into a current source. The no load component of the excitation is supplied from the excitation voltage transformer and converted into a current source by the ‚Harz circuit’ The load component is supplied by the excitation current transformer. The generator current flows through the primary winding. The no load component from the ‚Harz circuit’ flows through an additional winding and is added by this to the load component. A third winding supplies the resulting excitation current which is rectified in a bridge converter and fed to the generator field.
The THYRIPART® excitation system is installed in a standard cubicle specially designed for equipment comprising open loop and closed-loop controls and power electronics devices. Modular design and easy accessibility of all components facilitate all setting and optimizing operations. The standard cubicle group is 1500 to 2400 mm wide and 600 mm deep at a height of 2200 mm. The adaptability to all kinds of local conditions and existing plant may necessitate deviating cubicle dimensions, especially in the case of modernization projects. As the generator lead or neutral point has to go through the current transformer and the voltage transformer will be connected to the generator lead the transformers should be assembled close to the generator. Therefore the transformers are supplied bulky for erection into a separate room of MV compartment.
The current-dependent component also helps to boost or maintain excitation in the event of voltage dips or short circuits. The oscillating circuit is supplied by the voltage generated from residual magnetism during run-up to speed or also at rated speed. This enables the generator to operate as a self-excited machine without an external power source.
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Main components are: The voltage transformer (1) supplies the no load part of the excitation. The oscillating circuit consisting of the reactor (3) and the capacitor (4) transforms the no load part into a current source. The current transformer (2) supplies the load part of the excitation and adds it to the no load part. The B6U converter bridge (7) rectifies the excitation current. The components (1)-(7) alone control the generator voltage with an accuracy of max. ± 10 % The digital open and closed loop control (8) receives the actual values from the measurement transformers (5) and controls the transistor chopper (9). This chopper adds or subtracts a part of the excitation current. This results in an accuracy of ± 0,5 % for the generator voltage.
In manual mode, the actual value excitation current is detected via a shunt resistor (10) and controlled by the closed loop control. The over voltage protection (11) prevents from over voltages in the field circuit. It is self restoring so that excitation can go on after over voltages caused by load steps or short circuits. To de-excite the generator, the de-excitation contactor (6) shorts the feeding current source. Additionally the shorted current source prevents the generator from self-excitation out of the residual magnetism if the generator is at speed or being turned. Optionally it is possible to speed the de-excitation up by the fast de-excitation circuit (12) consisting of the de-excitation switch and the de-excitation resistor.
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10 12 11
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THYRIPART® Standard Circuit
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Mode of Operation Base Excitation The excitation current supplied by the THYRIPART® consists of the voltage-dependent no-load component and the current-dependent load component. The no-load component is generated from an oscillating circuit according to the Harz circuit principle. The oscillating circuit consisting of reactor L and capacitor C is connected to the generator lead and the voltage transformer respectively. At a frequency near resonance, the oscillating circuit produces a current at the leads between the reactor and the capacitor. This current is proportional to the voltage applied (Harz principle). By adjusting the reactor inductance (by varying the air gap) or changing the tap on the current transformer this current can be set to correspond to the no-load field current when the generator is at nominal voltage. Based on the Harz principle this current is independent of ambient conditions as e.g. the rotor temperature. At rated speed the rated voltage is reached automatically. As this current flows through the reactor its phase is offset by 90°el from that of the current. During load operation a current component decoupled directly from the generator current is added to this no-load component. Decoupling and addition are performed in the current transformer (T). The generator current flows through the primary winding of the CT, which only has a few turns, and the current from the oscillating circuit flows through the first secondary winding. The sum of the two currents (adapted according to the phase angle and the transformation ratio) is picked off the second secondary winding, rectified, and fed to the field winding. This base excitation controls the generator current to an accuracy of min. ±10 %.
-L
-C
U(G) (400 V)
If -T I(G)
Principle diagram of the basic excitation
Current- and Voltage transformer (example)
Voltage built up The oscillating circuit operates near resonance frequency. The result is a voltage rise which supplies a little excitation current even if the supply voltage generated from residual magnetism is low. By this the generator voltage rises and therefore also the excitation current. This results in a voltage built up to rated generator voltage.
Comanche Peak nuclear power plant, USA: THYRIPART® systems qualified to American standard 1E
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Closed loop control The closed loop control controls a transistor chopper, which adapts the excitation current from the ‘Harz circuit’ by current addition or subtraction. This results in a control accuracy of ± 0,5 % for the generator voltage. Automatic Voltage Regulator (Automatic control system) The actual value of the generator voltage is compared with an adjustable generator voltage setpoint. The resulting signal is compared with the output of the excitation limiter and taken to the input of the PI voltage controller. The PI voltage controller with adjustable gain and time response provides an output signal which is applied to the setpoint input Iref of the secondary field current controller. The output of this controller governs the generation of the frequency-modulated driving pulses for the power transistors of the associated output stage, which is operated with a clock frequency of about 2,5 kHz. The DC current flows through the phase U and W of the transistor power circuit. Automatic cos ϕ or reactive power regulator at the generator leads (Automatic control system) The cos ϕ regulator compares the actual value with an adjustable cos ϕ reference value. In case of a deviation the reference value of the voltage regulator is adjusted until the deviation is reduced to zero. In isolated or no-load operation of the generator the operation mode of the automatic control system is switched over from cos ϕ regulation to voltage regulation.
Options V/Hz-limiter (over fluxing limiter) Cos - ϕ - or reactive power regulator at the supply point VAR joint control of several generators
Commissioning mode The manual control system is designed as excitation current regulator which permits generator characteristics to be recorded during commissioning and inspections, and also short-circuit operation of the generator to be carried out for setting the protective relays. When the automatic voltage regulator is faulted, it can also be used for the operational adjustment of the generator excitation.
Follow-up control The setpoint value of the field current controller is continuously updated during operation on the automatic control system, thus ensuring rapid and nearly bump less changeover to manual control in the event of a fault. Automatic switchover takes place when certain fuses or protective circuit breakers operate or in case of failure of the automatic control system.
Control accuracy The control accuracy of all control systems is ± 0.5 %
Manual control system (Excitation current regulator) The P-action control amplifier of this regulator receives a filtered setpoint signal that is compared with the actual value of the field current. The output signal controls the frequency-modulation drive circuit for the power transistors of the associated output stage. Of the three control systems the automatic one is normally in operation, even during starting and stopping of the electric set. The automatic control system includes the measuring and setpoint devices and the control and monitoring circuits for the following functions: Generator voltage regulation Fast secondary control and limitation of the output current of the field current chopper regulator and/or field-forcing limiter. Limiting controller for the under-excited range (under excitation limitation) Delayed high limiting control for the over-excited range (over excitation limitation) Delayed generator current limiter (stator current limitation)
Control board inside the transistor power part
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Limiting controllers When operating a synchronous generator, it is necessary to observe the permissible combinations of active and reactive power, which can be seen from the capability diagram. LMO Limit characteristic of the underexcited range OP Limit set by the stator temperature rise PQ Limit set by the rotor temperature rise
The stator current limiter ensures the delayed limitation onto working points, within the N-P range of the generator power diagram. The main task of the stator current limiter is to prevent the generator stator from thermal overload, which can be caused by a high reactive power at increased active power. The stator current limiter also permits increased excitation values for a limited period so that the generator can back up the system.
Similar characteristics with reversed active-power flow apply to motor operation of the generator. Active Power
The underexcitation limiter corrects the reactive power by raising the machine voltage as necessary to ensure that, in case of an excursion beyond the limit characteristic L-M-O, the operating point is returned to that characteristic before the machine is tripped by the underexcitation protection.
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Max. turbineoutput
N
Active Power O P
N
Max. turbineoutput
Underexcited M Underexcited L
M Overexcited
Q Reactive Power
Limit characteristics of a synchronous machine in generator operation
The overexcitation limiter ensures that, in the overexcited range, the operating point always keeps within capability curve section P-Q of the generator. In response to system voltage drops caused by high reactive power requirements, switching manipulations or faults, the voltage regulator raises the excitation level so as to keep the generator voltage constant. The overexcitation limiting device acts as a safeguard against thermal overloading of the rotor. The overexcitation limiter admits excitation current values between the maximum continuous current and the maximum excitation current (field forcing) for a limited period of time so that the generator can back up the system in response to short-time system voltage dips. The secondary excitation current limiter (field-forcing limiter), in contrast, has the task of limiting the excitation current to the maximum permissible value as quickly as possible.
L
Reactive Power
Minimum excitation limiting
t(s) 40
30
Overexcitation limitation
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10 Subordinate excitation current limitation
1 1.1
1.5
Excitation current If (Rated Value)
Overexcitation limiting and secondary field current limiting 5
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Open loop control
Power circuit
The open loop control system contains interlocks for switching the excitatioin on and off. The protective OFF signal on the other hand is applied directly to the coil of the excitation contactor. The open loop control also affects the changeover between the automatic and manual closed loop control systems.
The power circuit uses transistor chopper regulators, which provides the necessary excitation power via a DC link. The field voltage is adjusted by varying the pulse/pause ratio and the field circuit resistance causes the field circuit to vary accordingly. The field current is measured in the output stage and the signal is converted for the field current controller.
Every operating mode of the excitation system is monitored and displayed; slow control processes are carried out and evaluated. In addition to detailed internal fault indication, the internal monitoring routine makes the following alarms available at the cabinet terminals: Fault with Protective Off command Fault in automatic control system and switchover to manual control system Group alarm triggered by various internal fault signals. Group alarm triggered by various internal fault signals causing starting lockout.
Control
P L1 L2 L3 N
Rectifier
Smoothing
Adjuster
Process alarms are also made available for external indication: Output
Excitation is on Excitation is off Automatic Voltage Regulator is on Excitation Current Regulator is on Cos ϕ – or VAR Regulator is on Limiters are active Further more detailed signals are optionally possible.
Principle connection of the transistor power part
The four quadrant chopper allows positive as well as negative outpot voltages with positive or negative output currents. If the mean value of the output voltage is bigger than the excitation voltage of the base excitation, an additional current is fed into the field circuit and the excitation current rises. If the mean value of the voltage is lower, a part of the base excitation current is fed back into the DC link. The result is a lower excitation current.
+220 V
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-220 V -
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positive
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zero
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negative
Principle of the pulse wide modulation
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PC Tools The operator friendly software tool SIMOVIS/DriveMonitor guarantees simply commissioning of the THYRIPART®. Through a serial interface the voltage regulator can be connected with the PC for easy configuration.
Customer friendly Configuring The SIMOVIS/DirveMonitor software for Microsoft Windows 9x/2x/NT allows the complete parameterization of the power circuit. Actual values can be monitored in the parameter list and
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parameters can be changed easily by selecting the corresponding parameter in the parameter list. It is possible to choose between a complete parameter list, pre-defined parameter lists with selection of parameter for a special application (e.g. input/ output) or a user defined parameter list by entering the interesting parameter numbers. A complete upread of parameters allows easy documentation.
THYRIPART®
Trace Trace is an add-on for SIMOVIS/Drive Monitor that permits visualization of recorded data. You can also store the data read out of the device and open it again later. It is also possible to import such data into text processing programs, such as Microsoft Word, or into spreadsheet programs, such as Microsoft Excel. You can perform simple measurements of amplitudes and instants using two moveable cursors.
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The trace recording function contains following features: Monitoring of up to 8 analog signals Monitoring of 16 binary signals per unused analog signal (e.g. 32 binary and 6 analog signals) Maximum recording time of 280s at a maximum sampling rate of 280ms or 1.4s at minimum sampling rate of 1.4ms Freely adjustable sampling rate between 1.4 and 280ms in steps of 1.4ms Fault recording is automatically triggered by programmable fault signals (triggered by binary signals e.g. faults or by comparing an analog value (condition: /=/<>) with a predefined value) Adjustable pre-trigger between 0% (no pre-trigger, only future) and 100% (only past, no future)
THYRIPART®
Configuring with D7-ES The complex voltage regulator with its limiters and calculations as well as control functions such as interlocking is integrated inside the programmable T400 circuit board. The T400 can be configured with the graphic D7-ES configuring tools, based on Windows 95/NT. Thus, it is very easy to implement even complex customized supplementary functions inside the THYRIPART®. Complete standard software packages are available for functions and applications which are frequently required. The picture below shows an example of a software structure and a selection of available functions out of the function block library.
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Technical Data THYRIPART® The range of application for THYRIPART® excitation systems are low and middle voltage generators with small and medium power when load dependence and short circuit support is required or the condition of the rotor insulation does not allow the high voltage gradient caused by thyristor converters. This mainly applies for older generators. Other reasons are the black start capability and the high availability. The standard application range is (extended application range on demand): Rated generator voltage U(GN) max. 13,2 kV Rated generator current I(GN) max. 2300 A Rated excitation current I(fN) max. 960 A Auxiliary supply from station battery 250 V:
For power circuit control Power consumption:
< 0,1 kW continuously < 0,2 kW short-time
For signal and controller power supply (24V DC) Power consumption: Instrument transformers:
< 0,2 kW continuously
Potential transformers: connected to 3 phases of the generator voltage. Power consumption < 5 VA per phase; Secondary voltage rating 100 V to 120 V. Current transformers: Two-phase (L1, L3), for measuring the generator current. Secondary current rating 5 A or 1 A, < 5 VA (plus CT cable losses). The transformers are not included in the scopeof supply of the voltage regulator .
Standards:
The THYRIPART® excitation system is rated and designed according to IEC-, EN-, DIN-, VDE-, IEEE-421- standards. Service and maintenance of the excitation system THYRIPART® can be executed according the VGB4- instructions.
Atucha nuclear power plant, Argentinia: four THYRIPART® systems 7.2 MVA for emergency generators of Atucha 2.
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THYRIPART®
Excitation Systems
Published by and copyright 2004: Siemens AG Power Generation Freyeslebenstrasse 1 91058 Erlangen, Germany fax: 0049-9131-18-4369 www.siemens.com/powergeneration Subject to change without prior notice The information in this document contains general descriptions of the technical options available which do not always have to be present in individual cases. The required features should therefore be specified in each individual case at the time of closing the contract.
