Generator excitation control operation

Increasing the output voltage of a generator is achieved by adjusting the magnitude of the excitation current. This happens because as DC current is increased, the rotating magnetic field increases thereby increasing the generator  voltage induced in the stator conductor.     As the voltage is increased, the generator will transfer more MVAR into the power system.

Instead of collector rings, suppose that a  brushless generator above  is used in our example. The exciter is provided from the DC winding which is wound in the stator. The rotor produces ac current that is feed into the rectifier built in the shaft. The rectifier converts AC to  DC and feed the rotor windings.

To control of the rectifier  output in the field windings of the excitation generator, voltage regulator receives its command signal from the AC voltage controller which monitors the ac voltage of the monitor.


The limitation of the generator due to current in the stator should follow within the machine capability curve. Increasing the excitation current of the generator to deliver MVAR will produce heating in rotor winding. alternatively, if the excitation is reduced voltage and VARS will fall. then the machine will have weak magnetic field .

Technical Data for Generator 60WX18Z-090 with Static Excitation 158,8 MVA 13800 V 60 Hz p.f. = 0,85 Tcg = 33 °C Temp.-Cl. = 130(B

In the capability curve, the horizontal axis is the  reactive power in per unit quantity, where 1 pu =159 MVAR and the vertical axis is the active power per unit quantity.

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Update: 2013
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Electronic Control of automatic Recloser

Electronic Recloser Contol is compromised of a number of programmable, solid solid-state electronic circuits that perform the command fuctions involved in automatic recloser operation. It is used to operate all electronically controlled reclosers.
the control panel of the control unit contains the programming and opening elements of the control.

Parts of the control panel of the electronic recloser:
Minimum trip resistors - Establish the minimum trip current levels for ground and each phase; cartridges are marked in primary amps and clamped in place.
Operation counter - record the cumulative trip operations of the control.
Sequence relay - steps the control through its operating sequence.
Ground- Trip Operation Switch -Blocks all ground in the BLOCK position: prevents unintentional tripping during single -phase switching operations.

Recloser Control panel

Manual Control Switch - In the TRIP position, it locks out the control, advances the sequence relay lockout, and disconnects the battery from the control circuits. In the CLOSE position, it moves the sequence relay to the home position, reconnects the battery and closes recloser. If held in CLOSE position, it will override cold- load inrush ; however, the control will lockout for permanent faults.
Control Fuse- Protects the closing solenoid coil (on reclosers that employ solenoid closing)  if closing voltage is too low. connected in series with the closing contactor in the recloser on motor- operated units; connected in series with the contactor rotary solenoid on reclosers that  employ solenoid closing
Non- Reclosing Switch - Sets the control  for one shot to lockout without disturbing the lockout setting of the operations selector.
Lamp Test Lockout Switch - Enables testing the signal  lamp and checking for lockout.
Lockout Indicating Signal lamp - Provides visual indication of control lockout
Battery Test Tetminals - Enable checking battery voltage, charging rate ,and quiescent current drain.
Reset -Delay Plug - Determines the delay interval before the control resets after a successful reclosure during an operation sequence. the delay value is determined by position of the plug in socket adapter.
Phase Trip Timing Plugs - Provide a variety of current integrated timing curves on individual plugs for coordinating the phase trip operation with backup and downline protective devices.
Ground Trip Timing Plugs -Provide a variety of current integrated timing curves on individual plug for coordinating the ground-trip operation with backup and downline protective devices.
Reclosing  Interval Plugs - Determines the delay interval for each closing operation. The delay value is determined by the position of the plug in the socket adapter. An instantaneous plug is available for the first reclose interval only.
Phase Trip Selector - Programs the number of fast phase trip operations as defined by the timing pug in Socket 1; the remaining (slower) operations to lockout are defined by the plug in phase trip socket 2
Lockout Selector - Programs the total number of operations to lockout.
Ground Trip Selector-  Programs the number of fast ground trip operations as defined by the timing plug in socket 1; the remaining (slower) operations to lockout are defined by the plug in ground trip socket 2

Automatic Circuit Recloser

Recloser is a device that is used in over head distribution systems to interrupt the circuit to clear faults. Automatic reclosers have its electronic control senses and vacuum interrupters that automatically recloses to restore service  if a fault is temporary. There are several attempts that may be made to clear and reenergize the circuit and if the fault still exist the recloser locks out. Reclosers are made in single-phase and three-phase versions, and use  oil or vacuum interrupters.

Operation
Systems where a SCADA control interface capability is required in the use of automatic reclosers. The controls for the reclosers range from the original electromechanical systems to digital electronics. The operating sequence of all the reclosers can  be all fast, all delayed or any combination of fast followed by delayed up. Fast operations clear temporary faults before branch circuit line fuses are weakened. Delayed operations allow time for down time protective devices to clear so that permanent faults can be confined to smaller sections of the system.

Three Phase Vacuum Circuit Recloser
image credit: www.abb.com  

A complete electronic recloser package consists of the recloser which interrupts the circuit, an electronic control which senses over-currents and actuates the recloser and an interconnecting control cable.

Tripping and Closing

Recloser tripping and closing are initiated by signals from the electronic control. When fault currents in excess of the programmed minimum-trip value are detected in one or more phases, a signal from the control actuates a low energy tripper in the operating mechanism of the recloser to trip the opening springs and open the interrupter contacts. Closing energy and the force required to charge the opening springs is supplied by a high-voltage closing solenoid momentarily connected phase-to-phase through a high-voltage contactor.  At the programmed reclosing time, the control energizes a rotary solenoid in the operating mechanism which mechanically closes the closing solenoid contactor to connect the closing coil to its power source.  The energized closing coil pulls a plunger down, charging the opening springs.

AC Circuit Phase Sequence

Phase sequence is the order in which the generated voltages in the phase winding of an alternator reach or  attain  their peak or maximum values. It is represented by the letters a, b, and c or the numbers 1, 2, 3 or by using double letter as ab, bc  and ca or  an, bn  and cn.

For instance , The three phase balanced voltages with their common magnitudes as K have sequence of a b c , then in complex form, 

Positive Phase Sequence 

ABC sequence  -  Va = Ka0˚ , Vb = a-120˚ and Vc = Ka-240˚

In this sequence Vb lags Va by 120˚ and Vc lags Vb by 120 or Vc lags Va by 240. The maximum value of Va comes first in the positive direction, next Vb and then Vc. 

ABC →BCA →CAB

AB - BC - CA → BC - CA - AB → CA - AB - BC

AN - BN - CN → BN - CN - AN → CN - AN - BN

Vector Representation

 Sequence ABC  

Sequence BCA

Sequence CAB

Negative Phase sequence 

ACB sequence- Va = Ka0˚ , Vb = a-120˚ and Vc = Ka-240˚

Voltage Vc lags Va by 120˚ and voltage Vb lags Vc by 120˚. 

ACB →CBA →BAC

AB - CA - BC → CA- BC - AB → BC - AB - BA

AN - CN - BN → CN - BN - AN → BN - AN - CN

Vector representation

Sequence BAC

Sequence  CBA

Sequence ACB

Assume a positive phase sequence if the phase sequence is not given . The three  phase alternators are designed to operate with positive phase sequence voltages.