Excitation systems for brush generators and compensators of up to 1000 MW

The SVTG series excitation systems (SVGG, SVGD, SVG, SVSK) (hereinafter the ES) are static, silicon-controlled, high-speed excitation systems.

General characteristics of ES Series

• covered range of generator rated power:
  – up to 1000 MW;
• types of generator excitation:
  – brush;
  – brushless;
• range of rated excitation current and voltage:
  – excitation current: up to 7500 A;
  – excitation voltage: up to 700 V;
• excitation transformer types:
  – oil- or dry-immersed transformer;
  – indoor or outdoor 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;
  – no redundancy – single channel with automated and manual controllers;
• power converters redundancy circuits:
  – standby according to 1+1 circuit: dual-channel full redundancy of converters;
  – redundancy according to N-1 circuit: single-channel redundancy via one redundant converter;
  – no redundancy;
• number of parallel thyristors on the converter arm:
  – 1;
  – 3 or 4;
• number of parallel converters in one ES channel:
  – 1;
  – 2 or more;
• power converter cooling:
  – natural air cooling;
  – forced air cooling;
  – 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);
• completeness:
  – full completeness – without using former ES elements;
  – partial completeness – using former ES elements:
  – power transformer (one-group and two-group);
  – field suppression discharging resistance;
  – field suppression device (FSD);
  – devices for connection to standby station excitation system;
• logging tools:
  – recorder embedded into the Automatic Excitation Controller;
  – stand-alone recorder (not included into Automatic Excitation Controller) for independent control over the Automatic Excitation Controller run;
  – event log file built into the AEC4 controller.
• instruments for ES control, monitoring and diagnostics:
  – operator touch pad;
  – Computer 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 specifications

ES functions:

• automatic excitation and suppression during generator start and shutdown;
• initial excitation from the field flashing unit and from the generator residual voltage;
• automatic adjustment of generator voltage to line voltage;
• soft start with initial excitation;
• cutting into mains by precision synchronization method in normal modes of the power system:
  – from the unit control desk/main 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;
• cutting in the network by the self-synchronization method in emergency modes of the power system;
• excitation switching to a standby station excitation system according to standard rules without changing the generator operation mode;
• rating of the generator short-circuit and no-load (when power is supplied from an external source for self-excitation system);
• adjustable parameters in Automated Excitation Control:
  – generator voltage (main mode);
  – generator reactive power;
  – cos (?) generator.
• control structure Automatic Excitation Control – “ARV-SD (stabilizing)”;
• back-up manual controller of excitation current:
  – switching to a manual controller in case of automatic controller feedback malfunction;
  – switching to a manual controller with maintaining excitation setpoint;
  – excitation control through “More/Less” commands;
• rotor electromechanical oscillations damping, securing stability of the generator and the power system;
• droop (compounding) and voltage drop compensation in unit transformer for generator full and reactive current;
• excitation boost;
• electric braking mode;
• line charging mode;
• re-synchronization mode upon loss of field;
• excitation winding temperature calculation;
• 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 parameter 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” with the following control channels (see figure down):

• 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;

Note: the control structure for generators of up to 60 MW can be switched from the “Power System Stabilizer” mode to the PID-controller mode.

Additional functions of Automatic Excitation Control for Generator/motor at Pumped Storage Power Plants:

• supported starting circuits:
  – direct-on-line and inductor start;
  – auxiliary motor start;
  – start with a soft starter;
• start with a frequency converter and excitation current control over the “4-20mA” channel from frequency converter.

Structural design of ARV-SDPK control system

Automatic excitation control limiters:

• minimum and maximum generator voltage;
• minimum and maximum excitation current;
• maximum no-load excitation current with power off and generator overfrequency;
• minimum excitation – limitation of generator reactive power subject to active power («Q=f(P)»);
• generator overvoltage during its frequency drop («V/Hz=const»);
• stator overload limiter by time-dependent characteristic;
• rotor overload limiter by time-dependent characteristic;
• forcing time by rotor overvoltage characteristic;

Protection system

Protection types for operation on thyristor excitation system:

• generator overvoltage;
• generator overcurrent;
• loss of excitation;
• internal short circuits in thyristor converters;
• short circuits in excitation circuit;
• overvoltage in arrester excitation circuit;
• loss of conduction on the converter arm;
• loss of conduction in parallel thyristors on arms;
• no signal from voltage transformer during field flashing;
• no signal from voltage transformer and current transformer for the generator in operation mode with switching to manual mode maintaining the excitation setpoint;
• against forcing excessive duration limiter failure and excitation overcurrent limiter failure;
• against the “P/Q” minimum excitation limiter failure and transfer to a manual mode with set excitation that ensures over-excitation mode;
• excitation circuits insulation resistance decrease protection:
  – built-in insulation control device with resistance indication on the console terminal, operator touch-panel, AWS;
  – protection unit “BENDER” with resistance display panel on the UPS cabinet door or on AWS (optional);
• against resistance increase of excitation circuit “cable-brush rig-rotor”;
• design excitation winding temperature protection;
• protection against the stator circuit breaker a-contacts malfunction through redundancy on the stator current;
• overcurrent and overtemperature protection of power rectifier transformer:
  – basic current protection – ensured independently from ES by using a block incorporate into the generator protection panel;
  – basic temperature protection – secured independently from the ES by using tinternal protection block built into the transformer;
  – backup current and temperature protection – provided by the ES built-in protection;

Protection for operation on the backup station ES:

• protection against excitation circuits insulation resistance decrease;
• protection unit “Rotor Protection Relay (RPR)”:
  – protection types:
  • against forcing excessive duration and excitation overcurrent;
  • for short circuits in the 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.
  • De-excitation relay – a signal informing about the request to decrease the excitation upon overload time expiration;
  • Shutdown relay – a signal to shut down if excitation overcurrent fails to decrease upon overcurrent time expiration or upon the short-circuit current.

Note: The RPR unit serves as the basic protection for operation on the backup excitation system and as the backup protection for operation on the thyristor excitation system.