Excitation system for brushless generators of 800-1200 MW

The SVTG series excitation systems (hereinafter the ES) are static, silicon-controlled, high-speed excitation systems for turbogenerators.

General characteristics

  • covered range of generator rated power:
    – 1000 MW and above;
  • type of generator excitation:
    – brushless;
  • range of rated excitation current and voltage:
    – excitation current: up to 320A;
    – excitation voltage: up to 230 V;
  • excitation transformer types:
    – oil-immersed transformer;
    – indoor transformer;
    – one-group or two-group transformer;
  • power supply circuits:
    – one-group and two-group;
    – shunt-wound and separate excitation;
  • Automated Excitation controllers:
    – automatic (3 types) + system stabilizer (PSS);
    – manual;
  • Automated Excitation Control redundancy circuits – redundancy according to 1+1 circuit: dual-channel full redundancy;
    power converters redundancy circuits:
    – redundancy according to 1+1 circuit: dual-channel full redundancy of converters;
    – redundancy according to N-1 circuit: single-channel redundancy via one redundant converter;
  • number of parallel thyristors on the converter arm – 1;
  • number of parallel converters in one ES channel – 1;
  • protection against internal faults in converter:
    – automatic circuit-breaker;
    – fuses.
  • power converter cooling – natural air or combined air cooling;
  • excitation switching to standby station excitation system:
    – with excitation switching devices to the standby ES;
    – without excitation switching devices to the standby ES;
    – with digital “field-protective relay” (FPR);
  • scope of supply:
    • full completeness – without using static part elements of the former ES;
    • partial completeness – using static part elements of the former ES:
    – power transformer (one-group and two-group);
    – field suppression discharging resistance;
    – field suppression device (FSD);
    • additional equipment:
    – rotor protection cubicle;
    – brushless exciter (BE) control cabinet;
    – BE sensors;
  • logging tools:
    – recorder embedded into the Automatic Excitation Controller;
    – stand-alone recorder for independent control of the Automatic Excitation Controller operation;
    – event log file built into the AEC controller.
  • instruments for ES control, monitoring and diagnostics:
    – operator touch pad;
    – Automated workstation (a human-machine interface);
  • network interfaces for communication with Automated process control systems:
    – Profibus DP, Modbus RTU, Modbus TCP (Ethernet), CAN, etc.

Basic technical characteristics

The ES power supply circuit is made according to a single-group shunt-wound circuit – with power supply to ES power transformer from the generator voltage and with the initial excitation device (Figure 1).
The redundancy circuit for ES converters is made according to a dual channel 100% redundancy circuit “1+1”.
Automatic Excitation Control redundancy is full, two-channel redundancy.

ES structural design

ES consists of:

  • TCC1, TCC2 – thyristor converter cabinets with control systems, autpmatic excitation control and protection of channel 1 and channel 2;
  • UPSC – cabinet for power connection of channels 1 and 2 and UPS for excitation windings;
  • KSC – cabinet with knife switch for ES power supply;
  • RPC – rotor protection cabinet;
  • ECC – excitor’s control cabinet;
  • CC – cabinet (panel) of local control and alarm.

ES functions:

  • automatic excitation and suppression during generator start and shutdown;
  • initial excitation from the field flashing unit and from the generator residual voltage;
  • soft start with initial excitation;
  • automatic adjustment of generator voltage to line voltage;
  • cutting into mains by precision synchronization method in normal modes of the power system:
    – from the unit control desk synchronizer;
    – from automatic excitation controller (AEC):
    * adjustment of generator voltage to the network voltage;
    * turbine frequency adjustment;
    * generator switch closing with control of level and difference in phases of generator and network voltages;
    * out-of-phase reclosing independent relay;
  • rating of the generator short-circuit and no-load;
  • steady-state reactive power distribution between twin generators;
  • operation in joint reactive power regulation circuit;
  • adjustable parameters in Automated Excitation Controller:
    – generator voltage (main mode);
    – generator reactive power;
    – cos (?) generator.
  • embedded staircase generator for adjustment of AEC coefficients;
  • relay excitation forcing;
  • rotor electromechanical oscillations damping, securing static and dynamic stability of the generator and the power system;
  • idle current droop for load division between twin generators;
  • idle and total current droop for voltage drop compensation in unit transformer;
  • excitation boost;
  • electric braking mode;
  • line charging mode;
  • re-synchronization mode upon loss of field;
  • excitation winding temperature calculation;
  • calculation of rotor current by Potier method;
  • generator active and reactive energy accounting;
  • password-protected change of settings.

The ES parameters control accuracy:

  • generator voltage – 0.2%;
  • generator reactive power – 0.5%;
  • cos (?) generator – 0.5%;
  • excitation current in the manual mode – 0.5%.

The ES parameters control range (adjustable):

  • generator voltage:
    – no load mode – from 5% to 110%;
    – network mode – from 95% to 110%.
  • excitation current manual control:
    – no load mode – from 5% to 110% of the generator rated no-load current;
    – network mode – from the minimum limit of 20 … 40% to 110% of the rated generator current.

Control system

All CS tasks are fulfilled by a software and hardware approach.

The control structure of the Automatic Excitation Control – “ARV-SDPK” (Figure 2) with the following control channels:

  • PID voltage controller:
    – by voltage deviation – dUs (proportional-integral section);
    – by the generator voltage derivative – Us’ (differential section);
  • PSS – power system stabilizer:
    – by the excitation current derivative – If`;
    – by the generator frequency derivative – Fs`;
    – by the generator frequency deviation – dFs.
  • DF – direct feedback on rotor voltage for brushless excitation systems;

Structural design of ARV-SDPK control system

Automatic excitation control limiters

  • generator voltage limiters:
    – maximum voltage;
    – minimum voltage;
    – maximum voltage in direct proportion to its frequency decrease “V / Hz = const”;
  • minimum excitation – limitation of generator reactive power subject to active power («Q=f(P)»);
  • maximum rotor voltage limiter;
  • rotor current (calculated by Potier method) with time delay of up to 0.2 sec:
    – maximum current of forcing;
    – maximum current in case of no conduction on any arm of the rotating rectifier brushless exciter or blowing of 3 (4 for unit rotating rectifier) or more fuses on the same arm;
    – overload and forcing time by time-dependant characteristic;
    – minimum current for operation in the mains;
  • BE excitation current limiter:
    – maximum forcing current;
    – maximum no-load current in inverse proportion to the generator frequency increase (“A * Hz = const”);
    – maximum current in case of converter overtemperature and no backup ES channel;
    – maximum current in case of no conductivity of any converter arm;
    – overload and forcing time by time-dependent characteristic;
    – minimum current for operation in the mains;
  • stator current overload by time-dependent characteristic.

Protection system

The activity of all types of ES protection is displayed on the terminal of each ES channel, on the AWS screen and recorded to the Automatic Excitation Controller and AWS logs.

  • generator overvoltage protection;
  • generator overcurrent protection;
  • excitation overcurrent (with time delay of up to 0.2 sec);
  • no-load excitation overcurrent protection;
  • loss of excitation;
  • internal short circuits in thyristor converters;
  • overvoltage in rotor winding circuit (protection via thyristor arrester);
  • loss of conduction on the converter arm – ES operation continues with current limitation;
  • protection against the stator circuit breaker a-contacts malfunction through redundancy on the stator current;
  • against forcing excessive duration limiter failure;
  • protection against excitation overcurrent limiter failure;
  • protection against forcing excessive duration limiter failure;
  • protection against overcurrent limiter failure and rotor voltage limiter failure;
  • invert conversion failure;
  • design excitation winding temperature protection;
  • BE maximum humidity;
  • protection against decrease in insulation resistance of BE excitation circuit and AC circuit of ES (two settings, indication of resistance on the Automatic Excitation Controller);
  • protection against decrease in insulation resistance of rotor field – protection unit “BENDER” (two settings, resistance display panel on the Power Input Cabinet (PIC) door and AWS, output 4-20mA);
  • overcurrent and overtemperature protection of power rectifier transformer (high voltage):
    – basic protection – ensured independently from ES by using a protection block incorporate into the generator protection panel;
    – backup protection – provided by the ES built-in protection;
  • protection against internal faults in rotating rectifier;
  • independent from Automatic Excitation Controller two-channel protection unit “Rotating rectifier controller” (in a separate cabinet):
    – protection types:
    • against blowing of fuses on parallel paths of rotating rectifier brushless exciter (RRBE) with indication of respective numbers:
    – for non-unit type RRBE – one fuse – a signal, two fuse – forcing inhibition, three and more – rotor current limitation;
    – for unit-type RRBE – two fuses – forcing inhibition, four and more – rotor current limitation;
    – indication of protection activity:
    • on the AWS screen;
  • independent from Automatic Excitation Controller protection unit “Rotor Protection Relay (RPR)” (joint for two channels of ES):
    – protection types:
    • against forcing excessive duration and excitation overcurrent for BE;
    • for short circuits in the BE excitation circuit;
    – the RPR block consists of:
    • RPR controller;
    • Warning (If> 1.05 * Inom) relay – combined signal of overload starting point and RPR non-readiness (normally closed relay).
    • De-excitation relay – a signal informing about the request to decrease the excitation upon overload time expiration;
    • Shutdown indicating relay – a signal to shut down if excitation overcurrent fails to decrease upon overcurrent time expiration or upon the short-circuit current.

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