7. STATIC EXCITATION SYSTEM / Stage-I7.1. INTRODUCTION.

Till recent years, it was the practice to have a D.C. generator for the excitation of an alternator. Use of high frequency A.C. and static and brushless systems have come to be adopted in modern practice.

With the advent of high capacity generating units, the design of excitation systems, warrant a fast field current regulation to maintain system stability with large synchronous machines. The excitation system have not only to maintain field current for steady state stability limits. The static excitation system has come to be preferred over the conventional excitation system in this background.

In this system, the A.C. power is tapped off from the generator terminal stepped down and rectified by fully controlled thyristor bridges and then fed to the generator field thereby controlling the output voltage of the alternator. A high control speed is achieved by the electronic circuits. Any deviation in the alternator retards or advances the firing angle of the thyristors thereby controlling the field excitation.7.2. DESCRIPTION OF THE SYSTEM.7.2.1. SPECIFICATIONS.

The static excitation system consists of an excitation transformer, thyristor converter, field breaker, field suppressor and an alternate auxiliary supply for initial starting. The excitation transformer TE is fed from the 15 KV bus duct.

Capacity. : 2650 KVA. Dry type Natural cooling.Voltage. : 15 2 x 2.5% / 0.6 KV.Current ratio. : 102 A / 2550 A.Type of connection. : Y, D – 11.Thyristor converter is a completely controlled 3-phase bridge rectifier

and it supplies the excitation current according to the requirements of the generator operating conditions.Specification of the exciter:

No load excitation current. : 1032 A.Rated excitation current. : 2854 A.Rated excitation voltage. : 340 V.

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Field resistance at 750 C. : 0.119 Ohms.Specification of the converter:

Rated feeding voltage. : 600 V.Positive ceiling voltage. : 680 V.Negative ceiling voltage. : 545 V.Rated continuous D.C. : 7200 A.

7.2.2. PANEL ARRANGEMENT.The 600 V AC feeder from the excitation transformer is connected to

the copper bus bars of the excitation panel. The static excitation system consists of three convertor cubicles a regulation cubicle, an auxiliary cubicle, a field breaker cubicle; a field suppress cubicle and a AC power cubicle. (Fig-1) Each converter cubicle contains two completely independent totally controlled thyristor rectifier bridges, each bridge with its own firing circuits and ventilation system. The maximum D.C. rating of each bridge is 1100 A at 40 0C.

The regulation cubicle accommodates all the regulation and logic cards (pcc). The auxiliary cubicle incorporates all the auxiliary supplies and the related cards. The field breaker cubicle contains the field breaker 41G. This breaker is of type MAG. II. It is provided with spring closing, free trip mechanism in which the closing of the breaker is effected by using the energy stored in a spring system. I.e. when the operating latch is released the charged springs, act through a cam on the operating mechanism and cause the closing of breaker. The pre-charging of the spring can be done either electrically or manually.

The field suppressor cubicle contains a 0.2 resistance RSF. For discharging, the energy stored in the field winding when the field breaker trips preventing damage to the field and to the static excitation system.

The AC circuits, relating to the protective devices of the thyristors bridges are accommodated in the AC power cubicle and transformers which also for the supply of exciter auxiliary services.

7.3.0. DESCRIPTION OF THE POWER CIRCUIT.

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7.3.1. CONVERTOR.Each thyristor convertor bridge consists of six thyristors, six fuses,

three current shaping reactors and three current transformers. Each bridge has got a cooling fan (380 V AC) to cool the thyristors (Fig.2 and 3) RC protective networks are provided for all thyristors. Both the converter bridges of a cubicle are supplied from the 600 V AC bus through a draw out type breaker 52. The DC output from both the bridges of a cubicle are connected in parallel and fed to the DC bus through a draw out type disconnector 29.7.3.2. FIELD FLASHING DEVICE. (Fig. 4&5).

The DC power required for the field winding of the generator is tapped from this DC bus through the field breaker when the generator terminal voltage is more than 30% of the rated value. During initial building up of voltage for a small interval of time the field winding will be supplied from an auxiliary transformer rectifier unit through a breaker 31 and field breaker. This auxiliary DC supply is termed as field flashing supply. The above auxiliary transformer is supplied from the respective 415 V unit service bus through a breaker 52F. This transformer is of capacity 14KVA and the voltage ratio is 415/150 (YD-11).7.3.3. POWER SUPPLY FOR THE AUXILIARIES. (Fig. 6, 7, & 8).

The excitation system auxiliaries can be fed from the respective unit service bus or excitation transformer TE. The following transformers are connected to the transformer TE.

1. Potential transformer – TC (500/100 V) through circuit breaker 52V. A voltmeter is connected to the secondary of P.T. through a four position switch 29V.

2. Synchronising transformers TSA. And TSM. Of 50 VA capacity each 600/20 V (DY-11) fed through breakers 52 SA and 52 SM. These transformers supply the gate pulse generators of Auto and Manual regulators respectively.

3. Auxiliary transformer TC of 1 KVA capacity, 600/160 V (DY-11) fed through breaker 52C feeds the DC control bus through a rectifier ‘DC’.

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4. Auxiliary transformer TS of 8 KVA capacity, 600/380 V (DY-11) fed through breaker 52S. This feeds the 380 V auxiliary bus for thyristor cooling fans through changeover contactor 52-1.

5. Auxiliary autotransformers TS-I of 900 VA capacity 415/381 V fed from the 380 V auxiliary bus (reserve supply) for thyristor cooling fans through changeover 52-2.

6. Auxiliary transformer TS-2 of capacity 2 KVA, 415/220 V fed from the incoming supply of TS-1. This transformer feeds the space heaters of all cubicles through 52R breaker and 72R contactor.

7.4.0. DESCRIPTION OF THE REGULATION CIRCUIT. (Fig. 9)The regulation part of the exciter has the function to maintain the

output voltage of the alternator at constant value within the capability range. The variation of the excitation current required to maintain constant output voltage is obtained by controlling the firing angle of the thyristors, the reference voltage signal is supplied by a solid state circuit which replaces the conventional motor operated rheostat. Regulation with this circuit can be achieved from the local panel or control room. The feedback signals are obtained from the line and the terminals of the alternator though voltage and current transformers. The operating range is between 80% and 110% of the rated voltage. The accuracy of the regulator is 5%.

The regulating system consists of the following components.1. Power supplies.2. Input intelligence panel.3. Voltage regulator.4. Additional Operational devices.

7.4.1. POWER SUPPLIES. (Fig. 10).Two voltage-regulated inverters of 48 volts output with rectangular

wave shapes will supply to all the regulation and logic circuits. The inverters are fed from the station batteries. The two inverters feed two totally independent buses (48 volt) from which power is fed to the various regulations circuits. There are four groups of independent feeders, which supply power to the voltage regulator, logic and protection circuits. The above arrangement is so connected that in case of fault in one of the feeders

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or inverters the other feeder or inverter will supply power to the concerned circuits, such that logic and protection circuits never starve for supply.7.4.2. INPUT INTELLIGENCE PANEL.

This panel has dual function i.e. to obtain galvanic separation between the generator and the regulator and to adopt the current and voltage signals to adequate levels for the logic and protection circuits.7.4.3. VOLTAGE REGULATOR.

A manual regulating channel and an automatic regulating channel are available in the excitation system for regulating the voltage. The above channels are completely independent. An auto/manual commutation switch is provided and in case of fault in the automatic regulation circuit, automatic changeover to manual regulation mode is also available.7.4.3.1. AUTOMATIC VOLTAGE REGULATOR.

This consists of the following elements.1. Voltage sensor and compounding circuits.2. Maximum excitation current limiter.3. Minimum excitation current limiter.4. Static voltage reference.5. Voltage regulator.6. Excitation system stabiliser.7. Signal mixer.8. Adapter for ceiling calibration.9. Automatic firing circuits.10. Auto-manual follow up.

7.4.3.1.1. VOLTAGE SENSOR AND COMPOUNDING CIRCUIT. (VS. card)

This circuit (Printed Circuit Card) has a function to provide a signal proportional to the generator voltage and suitable to be compared to the voltage reference signal generated in the static reference. Besides, the circuit is to provide a compounding action of modifying the generator voltage signal as a function of the generator current. The range of such compounding action is from zero to 15% in the generator mode and from zero to –30% in the motor mode.

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7.4.3.1.2. MAXIMUM EXCITATION CURRENT LIMITER.This device has the function to limit the excitation current within the

working range of the field current of the alternator as fixed by the capability curve. This device also has the function to introduce a time delay enough to prevent the instant actuation of protection of the generator caused by temporary overloads. In this condition, the generator can be over excited without exceeding the thermal limit of the winding.7.4.3.1.3. MINIMUM EXCITATION CURRENT LIMITER. (MEL card)

The function of this device is to maintain the excitation current above the minimum values as fixed by the capability curve in the under excitation regime. When the excitation current reduces below this limit, it may result in the generator falling out of synchronism.7.4.31.4. STATIC VOLTAGE REFERENCE. (SR. card).

This device gives reference voltage signal to the voltage regulator. It will also transmit whether this reference signal has reached maximum or minimum value.7.4.3.1.5. VOLTAGE REGULATOR. (VC card).

The voltage regulator receives the signal from the CT. and PT. of the generator through the voltage-sensing device. It determines the difference between the reference voltage (from static reference) and the average of the 3-phase voltage at the alternator terminal. The signal coming out is therefore an error signal proportional to the deviation between the required conditions and the actual conditions. Furthermore, the regulator receives a signal of stabilisation to adjust the delays introduced by the time constants of the alternator and regulator.7.4.3.1.6. EXCITATION SYSTEM STABILIZER.

The corrective action taken by the regulator takes relatively larger time by the alternator to be responded to, due to the inherent inertia of the electro mechanical system when compared to the electronic system. To overcome this difference in speed of response a negative feedback signal is given to the regulator from the stabilizer.7.4.3.1.7. SIGNAL MIXER.

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This device mixes various signals from voltage regulator and other logic and protection circuits. This includes a summing up circuit to add the inputs and deliver an output equal to their sum, a selection circuit whose output equals the maximum input signal and a limitation circuit.7.4.3.1.8. ADAPTER. (OI. card).

This device allows converting the regulation signals to match the parameters of the field.7.4.3.1.9. AUTOMATIC FIRING CIRCUITS. (GPG, LR and GPA. cards).

The gate pulse generator tranduces the regulation signals into pulse trains for the thyristors ignition. Pulse transformers to drive the thyristors are connected just at the pulse generator terminals to guarantee a correct rectifier ignition with a firing sequence taking into account, the different phases. The characteristic of the output of the pulse generator ensures thyristor ignition with AC. Voltage drop up to 20% of the rated voltage under a three-phase short circuit at the HT side of transformer.7.4.3.1.10. AUTO MANUAL FOLLOW UP. (AML. Card).

This device carries out a trouble free auto-manual change over without any hunting. A centre zero voltmeter in the excitation panel monitors the difference between the two circuits. This circuit is fed from the logic and protection circuit power supply to ensure the operation of the circuit in case of failure of power supply to the regulation circuit.7.4.3.2. MANUAL VOLTAGE REGULATION.

This device is a backup to the automatic regulation and its use is limited to failure conditions. In order to trace the faults in automatic regulating circuits manual regulation circuits can be taken into service. This circuit has its own supply, static reference, a current controller (c.c.card) and a pulse generator with its own pulse transformers. Here the current instead of voltage is controlled and kept at the desired value by the controller.

7.4.4. ADDITIONAL OPERATIONAL DEVICE.7.4.4.1. SYSTEM STABILIZER. (PSS CARD).

This device acts on the automatic control loop to improve stability of the machine. The stabilisation circuit output is proportional to the active

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power and also frequency variations due to disturbances on the generator and the line. The output is intended to oppose these variations. In case of load throw off conditions or due to sudden load reduction, this device is automatically disconnected by a switching circuit at a stage when the active power reduce below a preset minimum value.7.4.2.2. RESISTIVE AND REACTIVE LINE VOLTAGE DROP COMPENSATOR.

This device is necessary whenever a transmission line terminal voltage is to be regulated. It compensates for the reactive and resistive voltage drops along the line. The signals required for the compensator are tapped from the CT and PTs of the line or bus.7.4.4.3. VOLTAGE/FREQUENCY LIMITER. (VHT CARD)

In certain conditions, the machine may be required to run at frequencies below normal range. In such cases, the voltage regulation is modified to control over fluxing of the cores leading to saturation of the core and consequent over heating.7.4.4.4. MAXIMUM STATOR CURRENT LIMITER. (LC. and I2 t CARD) This time delay circuit prevents instantaneous reactive overload or tripping due to

less of synchroniousm. Hence, the machine may be temporarily overloaded without exceeding

the I2 t limit of the stator windings.

7.4.4.5. VOLTAGE EQUALIZER. (VE/RPD. CARD)This circuit compares the line voltage signal with generator terminal

voltage and drives the static reference itself. Equalising action stops when the line and generator voltage signals match. It also ensures the discharge of the power stored in the rotor before opening of the generator breaker 54G.7.4.4.6. ROTOR LOAD ANGLE LIMITER. (LAL CARD)

This circuit ensures the synchronism of the generator with the grid. It intervenes into the voltage regulator function to raise the excitation current when it tends to go low.7.5.0. CONTROL, LOGIC AND PROTECTION CIRCUITS.

The logic circuits of regulation system govern, the controller operation with relation to the signal coming from the regulator and from external protection. A series of internal circuits makes a thorough diagnostic check up of all internal systems and operating conditions and govern their operation.

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Input/Output interfaces of the excitation system, the external alarm and the annunciation panel are also connected to the logic circuits.7.5.1. CONTROL CIRCUIT OF FIELD BREAKER. 41G.

Field breaker is a draw out type breaker, which is operated by spring mechanism. A DC motor is available to charge the closing springs of breaker. Control supply for these circuits is tapped from 220V DC control bus. This control bus is formed by paralleling the 220V DC supply from the battery system and the rectified DC output from 3-phase rectifier DC through a breaker 52B. This rectifier is fed by a 1KVA 600/160V, 3-phase transformer TC, which is fed from the output of the excitation transformer TE, through a breaker 52C. The closing circuit and the spring charging motor circuit are fed from the control bus through a breaker 52BC; where as the tripping, circuit is fed by another breaker 52BA.7.5.1.1. CLOSING CIRCUIT.

When ‘start’ push button is pressed for closing the breaker, the crowbar circuit will be switched on when the following conditions are satisfied.

1. Emergency local trip push button not pressed.2. No fault in generator (86G de-energised).3. Remote ‘stop’ push button not pressed.4. Start enabling relay ROK should be energised. This relay will energise

when the following conditions are satisfied.a) Closing spring of 41G is charged (OK 41G).b) Auxiliary excitation supply AC breaker 51F closed.c) Either auto or manual regulator switched on AUX MAX with their

corresponding limit switches (RA min and RM min) in minimum position.

d) Start lockout relay 86S de-energised.e) Breaker 52-1 for the thyristor cooling fan supply should be

switched on.f) The remote/local switch should be in Remote position (REM).g) 86E relay energised.

5. 86E relay Energised.

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Crow bar circuit relay E will energise EX relay, which in turn will energise relay EX1. EX1 will energise the closing coil (BC) of the field breaker. The auxiliary contacts of 41G will change over when the breaker is closed, to make ready the tripping circuit, to energise the auxiliary relay 41GX and trip circuit supervision relay CBA. The spring charging motor will stat through the limit switch FC41G. An antipumping relay 41GY is also provided.

When EX1 de energises trip relay of the breaker BA will energise to trip the breaker. When 86E relay drops out, crow bar circuit will be switched off, there by de energising EX and EX1 relays. When the field breaker trips, the space heaters for the cabinets will be switched on.7.5.1.2. FIELD FLASHING CIRCUIT.

As soon as EX relay is energised EX2 will pick up and will hold on through its own contact.

Through the voltage relay (V <) which will pick up at 30% of the rated voltage, EX2, and 41GX, the field flashing contactor 31 will energise which will feed excitation flashing supply. This supply will be out off automatically after 1 sec. (31T timer). By this time, the excitation current would have reached 216A and generator voltage 30% of rated value.7.5.1.3. TRIP CIRCUIT.

86E relay will be in energised condition normally and when it drops out the field breaker will trip. 86E relay will drop out for the following reasons;

1. When the generator voltage not developed to normal value within the specified time (31T timer) during field flashing.

2. When the generator breaker is in closed condition and 41G in open condition (86 AI). This ensures that the field breaker cannot be closed when the generator breaker is in closed position.

3. Both the power supplies to thyristor cooling fans fail or and DC supply to both the invertors 1 and 2 fail (86T) (86AL).

4. If the winding temperature of the excitation transformer increases to 1550C or load in the excitation transformer primary (86TE).

5. When the excitation current drops below the minimum permissible value (CBX).

6. No of conducting thyristors reduced to minimum (86FT).

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7. Any of the fuses (FU7 to FU12) blown out in filter circuit (FRC) over voltage protection.

8. Auto manual lockout (86AM).9. Minimum frequency trip (86MF).10. Over excitation (86IM).

7.5.2. LOGIC CIRCUITS.The logic circuits include the following card circuits. These cards are

provided with LED lamps to indicate the different actions being carried out.7.5.2.1. PRESENSE DETECTION CARD (PD)

This is to detect the conduction of thyristors. When conduction ceases in a thyristor, an unbalance is created on the AC side, and this variation is sensed to indicate the condition of the thyristor.7.5.2.2. FAULT DETECTION CARD (FD)

This is to transform the above PD signal to a continuous signal and to send it on to an adder circuit.7.5.2.3. ADDER PANEL CARD (AP)

This is placed one in each cabinet. This determines the some of the correctly operating thyristors for each bridge.7.5.2.4. TIMING AND LOGIC CARD (TL)

This card ensures pulse suppression in the firing circuits in case of a bridge fault. It allows the pulses to reach the auto or manual regulator according to auto-manual logic. It gives information about bridge readiness or bridge fault.

7.5.2.5. FAULTY BRIDGES CARD (FBC)This intended is used to check whether the numbers of operating

bridges are lower or higher than or equal to the required number of bridges. 7.5.2.6. P / Q. TRANSDUCER AND COMPOUND CARD (TRD)

This card is used for measurement of active power P and reactive power Q exchanged between alternator and grid.7.5.3. PROTECTION OF THE EXCITATION SYSTEM.

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The following protections are provided for the excitation system, which will trip the field breaker and consequently the generator. Whenever the generator trips the 86G relay will trip the excitation system also.

1. Over current protection on AC side.2. Over voltage protection on AC side.3. Crow bar protection.4. Current unbalance protection.5. Excessive ceiling duration protection.6. Over excitation protection.7. Rotor earth fault protection.8. Minimum thyristor protection.

7.5.3.2. OVER CURRENT PROTECTION ON AC SIDE.This circuit causes instantaneous tripping of the excitation system on

internal short circuit of excitation transformer or shorts in AC power circuits of the excitation transformer. A circuit for causing of the above tripping in case of rise of winding temperature to 1550C is also linked with the above protection. For the former protection, an over current relay ‘I >’ is connected to the HV side CT of ratio 120/5A, which picks up at 2.5 times the rated current. A contact of a temperature instrument ‘O’ affords the latter protection. Either of the above occurance energises the relay 86TE to cause tripping.7.5.3.2. OVER VOLTAGE PROTECTION ON AC SIDE.

When breakers opened in the 230 KV system voltage surges occur in the system and these surge disturbances should not be allowed to reflect on the AC side of the excitation circuits. The above surge variations imposed on the system are smoothened by a series of RC filters provided on the AC side of thyristor convertors. Failure of these filters would mean an impermissible limit of over voltage and blowing of protection fuses, as a consequence of which the excitation system is tripped.7.5.3.3. CROWBAR PROTECTION.

This protection is avoid over voltage, on the field circuit. It will protect field circuit from excessive transient voltages due to improper synchronisation and falling out of step. The crowbar circuit will sense the value and direction of the current through the de-excitation circuit.

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7.5.3.4. CURRENT UNBALANCE PROTECTION.It has to be ensured that the firing pulses for all thyristors are present

and all convertor bridges are functioning properly. Hence, the CT wave from in the AC side is monitored by this protection circuit. Be measuring the difference between the wave shapes, unbalance if any is sensed by this circuit and the operation mode is changed from auto to manual or from manual to trip.7.5.3.5. EXCESSIVE CEILING DURATION PROTECTION.

This protection will intervene if the duration of ceiling voltages (both positive and negative) is excessive (over 1.5 sec.) Normally such condition may arise due to any of the following reason.

a) Malfunction of automatic regulator.b) Interruption within the measuring circuit of the generator voltage.c) Fault in voltage regulator card.d) Loss of static reference signal.

Protection does not operate when the voltage at generator terminal falls below 50%.7.5.3.6. OVER EXCITATION PROTECTION.

This is an inverse time dependant protection, which prevents the increase of excitation current beyond the allowed rotor I 2t value. This protection will trip the excitation system only when the regulator is in manual mode.7.5.3.7. ROTOR EARTH FAULT PROTECTION.

The rotor earth fault relay (ASEA make, type RXNB-4) monitors the excitation circuit and provides early detection of a ground fault or insulation deterioration in any part. A single ground fault in the field circuit will not cause any immediate damage. A second ground fault will result in extensive machine and exciter damage. It is therefore important to detect the first ground fault to enable unit shut down for repairs. The protection provided is based on injection principle, which allows ground fault detection irrespective of field voltage level and location of the fault. The rotor shaft must be kept grounded for detection of the first earth fault, since the current that flows from the excitation winding to the ground has to measured. The relay will

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give an annunciation in case of the above earth fault and the machine should be taken out of service.7.5.3.8. MINIMUM THYRISTOR PROTECTION.

This protection circuit has a device to count the no of thyristor bridges in operation by checking the fuses in service and also the working of ventilation system. It also checks the conducting and non-conducting thyristors. When the number of thyristors in service had come to the minimum the annunciation “minimum no of thyristors” is caused. Further failure of thyristors trips the excitation system.7.5.3.9. PROTECTION FOR THYRISTORS.

For individual thyristors, the following are provided to protect them from damage.

a) Extra rapid (fast acting) fuses in series.b) RC protective circuits with separate fuses to protect them from high

voltage surges.7.6.0. INSTRUMENTATION AND ANNUNCIATIONS.

All measuring signals necessary to monitor the operation of the excitation system are taken by means of suitable transducers. This provides galvanic separation from the power circuits and the signals are transmitted to the local panel instruments and to the control room instrument. The excitation system has a series of device to check the correct operation of the system and to give annunciation in case of any abnormality.7.6.1. INSTRUMENTS.

Transducer CV3, CV4 and CV5 are provided to measure rotor current, rotor voltage and rotor temperature respectively. CV3 and CV4 are connected to the following instruments at local panel and control room.

a) Rotor ammeter (0 to 4000A).b) Rotor voltmeter (0 to 600V).

CV5 is connected to rotor temperature indicator, which is provided at the control room. There is another transducer CV6 which is fed by Auto-manual follow up logic circuits. CV6 is connected to balance zero voltmeters (5-0-5V) one at local panel and another at control room to measure the different between the auto and manual circuits. Generator voltmeter and generator ammeter are also provided in the local panel.

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The following control equipments are provided in the local panel.1. Remote/local change over key.2. Auto/manual selector push buttons.3. Increase/decrease push buttons for auto & manual modes.4. Emergency stop push button.5. Push button for testing the lamps.6. In the control room, all the above control equipments except

emergency stop push button are provided.7.6.2. INDICATIONS AT LOCAL PANEL.

These circuits and the related circuits are fed from +24V supply from the established power supply. The following indications (LED lamps) are provided in the local panels.

1. Auto reference limit switch maximum.2. Auto reference limit switch minimum.3. Manual reference limit switch maximum.4. Manual reference limit switch minimum.5. Start enabling (starting circuits alright).6. Automatic regulation.7. Manual regulation.8. Emergency stop.9. Field breaker open.10. Field breaker closed.11. Maximum excitation limit.12. Minimum excitation limit.13. Stator current limit.14. Volts/hertz limit.15. Rotor load angle limit.16. Local control.17. Voltage equalizer and reactive power discharge on.

A test push button (TLPB) is provided to test all the above lamps.7.6.3. ANNUNCIATIONS IN THE ALARM PANEL AT LOCAL.

The following annunciations are provided in the alarm panel.1. Untimely intervention SP-EX. (This will appear when the field breaker is

in open position when the generator breaker is in closed position).

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2. Supply lockout. (This will appear when both the power supplies to the cooling fans of converters fail (or) DC supply to both the inverters 1 and 2 fails).

3. Filters failure R-C. (This will appear when any one of the fuses F7 to F12 in the RC filter circuit is blown out over voltage protection).

4. Intervention crowbar (This will appear when the excitation current drops below the minimum value under excitation).

5. Excitation transformer lockout (This will appear when the winding temperature of excitation transformer TE increases beyond maximum value or over load on the primary side of excitation transformer).

6. Auto-manual lockout. (When either the modes have not been switched on this annunciation will remain).

7. Minimum number ‘Active thyristors’. (No of active thyristors comes down to minimum).

8. Flashing time relay. (When field-flashing supply is cut out or when breaker 31 is opened this annunciation appears).

9. V/Hz. Protection over fluxing protection.10. Maximum excitation current (Over excitation protection).11. Fault auto regulator.12. Fault manual regulator.13. Bridge and thyristors fault.14. Thyristors sold out reserve (i.e. No reserve thyristor available).15. Supply stabilised fault.16. DC supply fault. (This will appear if the 220 V DC supply to any of

the control circuits or alarm panel fails or AC breaker 52C in the rectifier circuit trips).

17. AC supply fault.18. Opening coil 41G fault.19. Unbalanced current.20. Ceiling voltage maximum time.21. Voltage feed back fault. (When any one of the fuses in the

primary side of generator PT blows out this annunciation appears).22. Start lockout.

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23. Excitation transformer high temperature (When the winding temperature increases to 145 0C).

7.7.0. OPERATING INSTRUCTIONS.7.7.1. PRE-COMMISSIONING CHECKS TO BE CARRIED OUT WHEN THE GENERATOR IS NOT RUNNING.

With the brushes raised up from the collector rings proceed to make following checks and tests in the order given.

1. Verify all the external wirings.2. Verify the operation of all relays, breakers and adjusters.3. Check the connection, to the test source and check the operation of AC

side breakers.4. Check for correct operation of convertor power bridges and gate pulse

generators.5. Check static logics and regulating circuits using input signal simulation.6. Check the protective circuits using simulated intelligence source.7. Check the excitation power transformer (Insulation ratio etc.).8. Check the polarities and interconnections of excitation transformer to

the switchgear.9. Check for proper phase sequence.10. Check all controls for proper operation from the control room.11. Feeding from auxiliary services source and moving the links (fig.

6,7 & 8) it is possible to energise the generating and amplifying pulse circuits of thyristor bridges without energising power circuits. With this, verify the firing pulse appearance by checking ‘Test Points’.

12. Reconnect the feeding circuits and prepare to feed power circuits of power bridges. With the gate pulse generator signal at the minimum level and with the power unit test load circuit closed, close the feeding circuit breaker and check minimum and maximum firing pulse angles of the convertor bridge. Check the correct firing of the bridge by using first the automatic GPG and Thyristor Bridge

13. Step 12 is to be repeated for each thyristor bridge.14. Check with CRO that six firing pulses per cycles in the output

waveform are present.15. Reconnect all the locking circuits.

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7.7.2. CHECK TO BE CARRIED OUT ON REGULATOR COMPONENTS.(WHEN THE GENERATOR IS NOT RUNNING).

1. Check the correct operation of internal logic by simulating actual operation conditions of the machine.

2. Check the overall operation of the logic circuits, firing circuits and the generator field as follows.

a) With regulator control switch in ‘Manual’ position close field breaker 41G and adjust the reference at minimum to give generator field current.

b) Maintain the regulation reference at minimum value with a source of simulated machine potential applied to the input intelligence panel, by the voltage feed back adjuster and check whether the balance zero meter moves to zero value.

c) Now change the regulator control to ‘Auto’. There should be no bump or change in the generator field voltage.

d) Raise the voltage adjuster by increasing the static reference and check to ensure that the generator field voltage increases sharply. Lower the voltage reference adjuster and check to ensure that the exciter field voltage decreases sharply.

3. Test the various protective circuits by simulating various parameters.4. Reconnect the entire regulating system and then trip the AC supply

and field breaker. Remove the power load resistors.5. Check to ensure that the signal limiters and protective equipments are

set for predetermined values.7.7.3. START UP PROCEDURE.

1. Ensure 400 V AC 3 Ø Aux. Supply for the thyristor cooling fans.2. Charge the TS.1 (415 / 380 V) transformer by closing the automat at

the primary side 52 S1.3. Put the changeover switch to 52-2 position.4. Start the thyristor cooling fans from the respective thyristor cabinets

(52 VE) and ensure the fans are running normally.

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5. The TS.2 transformer (415/220V) should be kept normally energised by closing the automat 52-S2 and 52R, and supplies the space heaters when the field breaker is in tripped condition.

6. Close 52B breaker at the crowbar panel and close the 52C breaker at AC power feeder cabinet and read the battery voltmeter at the local auxiliary panel.

7. Close the following breakers: 52BA, 52BC, and 52CM for field breaker, 52AL for alarm panel and 52 I1 and 51 I2 for inverter supplies.

8. Check the SPST switches at the blocks AS1, AS2, AS3, AS4, AS5, AS6, AS7 and AS8 at the regulation panel in ‘ON’ position.

9. Close the 29S switch for providing supply to local alarm panel.10. Close the AC input breaker 52 and DC isolating switch 29 of each

thyristor bridge.11. Rack in the field breaker 41G.12. Ascertain the unit is running at synchronous speed.13. Put the remote/local key actuator in remote position, same key

should be used for control desk operation as well as for local panel operation.

14. Keep the auto/manual switch at the control desk in manual position and observe the desk light.

15. Keep the manual reference potentiometer at local in minimum position, and observe light indication in local panel.

16. Ascertain that the crowbar is in operating position.17. Ensure 400V AC 3 Ø supply to the field-flashing transformer by

closing 52F automat and observe the ‘Start enabling’ indication light at the desk and at the local panel.

18. Press the ‘Start’ push button and observe the built in light at the control desk. Now the field breaker will close and ‘Field breaker closed’ indication to be observed at the desk and ‘41G closed’ at the local panel.

19. Observe rotor voltmeter, ammeter and Generator voltmeter. In one second the generator voltage would have to 30% of rated value and rotor current to 216A.

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20. Increase the reference minimum setting by giving ‘increase’ impulses at the local panel.

21. Give ‘increase’ impulse at the generator desk by operating the manual mode push button; now observe increase in generator voltage, rotor voltage and current.

22. Now observe the balance zero ‘voltmeter’ at the control desk and bring it to zero position by operating the increase/decrease push button. Now the excitation system is ready.

23. Put the regulator in auto position.7.7.4. SHUTDOWN PROCEDURE.

1. Ascertain generator breaker is in open condition.2. Bring the regulator to manual position.3. Decrease the manual reference to minimum position.4. Decrease the manual regulator by giving decrease impulses at control

desk.5. Observe the rotor voltmeter.6. Press the ‘stop’ push button and check for tripping of 41G.7. 41G breaker can be racked out now.8. The cooling fans for the thyristors are to be switched off.9. The auxiliary supply breakers are to be switched off.10. Open 52 breakers and 29 disconnectors.

7.8.0. NORMAL OPERATION CODE. For detecting the faults easily in the static excitation system, each

card, having regulation, protection and logic functions is provided with number of signalling LEDs, which permit checking the correct operation of the system.

In the following tabular columns are listed, the meaning of the glowing of each signalling LEDs for various cards.

The indication (*) immediately after the LED number means that the particular LED will glow when the situation opposite to the one indicated occurs.

CARD LED DESCRIPTIONII 1. Closed 52 Ve 1.

2 Closed pulse on.3. Closed 52 Ve 2.

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4. Gate pulse reset.7. Cabinet circuit breakers closed.8. Cabinet Disconnector Drawn-in.

II

1(*) PTHY 1 Fuses intervention.2 (*) PTHY 2 Fuses intervention.3 (*) PTHY 1 R.C. fuses intervention.4 (*) PTHY 2 R.C. fuses intervention.

5 Fan 1 pressure switch.6 Fan 2-pressure switch.

II

1. Sequence thyristors TH 1.2. Sequence thyristor TH 4.3. Sequence thyristor TH 34. Sequence thyristor TH 65. Sequence thyristor TH 56 Sequence thyristor TH 27 Manual regulator ON.8 Automatic Regulator ON.

OI1. Ready PTHY 1.2. Ready PTHY 2.

3 (*) Fault PTHY 1.4 (* Fault PTHY 2.

PD

1. Thyristor ON TH 4.2. Thyristor ON TH 1.3. Thyristor ON TH 6.4. Thyristor ON TH 3.5. Thyristor ON TH 2.6. Thyristor ON TH 5.

FD 1 (*) Fault Thyristor PTHY 1.2 (*) Fault Thyristor PTHY 2.

VS 1. Line droop compensation ON.

OI

3. Constant current function.4. Breaking.5. Stator current limit.6. V/HZ limit.7. Max. Excitation limit.8. Min. Excitation limit.

11 (*) Min Frequency.12. Limit switch Maximum.13. Limit switch Minimum.14. Rotor load angle limit.

II 1. Step current 1.2. Step current 2.3. Step current 3.4. Step current 4.5. Automatic Reference Increase.6. Automatic Reference Decrease.7. Line Voltage gradual Build-up ON.11. Excitation ON.12. Step current 5.

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13. Step current 6.14. Step current 7.17. Constant current.18. Line Motor function.

LC

1. Over load off.2. Start counting time for I 2t.3. VM < 20 + 80% Vmn limit off.4. Limit Intervention.5. Regulated limit intervention.

MEL 1. Limit Intervention.LAL 1. Limit Intervention.

SR7. Limit switch Maximum.8. Limit switch Minimum.9. Increase.10. Decrease.

TRD 1. Line Motor Function.2. Line generator Function.

LF

1. Static Reference Increase.2. Static Reference decrease.3. Auto-Manual regulator Available.4. Line Semi convertor Function.5. Unbalance synchronising voltage.6. Electronic gun.7. Voltage equaliser and/or reactive power Discharge ON.

LF8. Following up ON.10. Computer signal ON.9. J.V.C. ON.

CC 1. Current limit at 1 P.U. ON (No load excitation current).

SR7. Limit switch Maximum.8. Limit switch Minimum.9. Increase.10. Decrease.

OI

3. Line Voltage gradual build up ON.4 (*) Automatic regulator fault.

5. Automatic regulator ON.6 (*) Manual regulator fault.

7. Manual regulator ON.13. Maximum reference limit switch.14. Minimum reference limit switch.

VE/RPD

1. Equalization obtained.2. Zeroing ontained.3. Decrease.4. Increase.5. VE/RPD ON.

UC/CMT 1. Unbalanced current signalization.2. Maximum ceiling time signalization.

AML 1. Pulse suppression.2 (*) Auto-Manual Feed fault.

3. Auto-Manual lock out.

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4 (*) Fault Automatic.5. Automatic ON.

6 (*) Fault Manual.7. Manual ON.

LC1. Overload off.2. Start counting time for I 2t.4. Limit intervention.

II

1. PTHY 1 Ready. Thyristor Convertors Cabinet

1.2. PTHY 2 Ready.

3 (*) PTHY 1 Fault.4 (*) PTHY 2 Fault.

5. PTHY 1 Ready. Thyristor Convertors Cabinet

2.6. PTHY 2 Ready.

7 (*) PTHY 1 Fault.8 (*) PTHY 2 Fault.11. 52 GX Breaker.12 EX relay contact.

13 (*) Pulse suppression.14. TVX Relay contact.15. PTHY 1 Ready. Thyristor

Convertors Cabinet 3.

16. PTHY 2 Ready.17 (*) PTHY 1 Fault.18 (*) PTHY 2 Fault.

II

1. PTHY 1 Ready.Thyristor Convertor

Cabinet 4.2. PTHY 2 Ready.

3 (*) PTHY 1 Fault.4 (*) PTHY 2 Fault.

5. PTHY 1 Ready.Thyristor Convertor

Cabinet 5.6. PTHY 2 Ready.

7 (*) PTHY 1 Fault.8 (*) PTHY 2 Fault.11. Reactive power zeroing ON12. Voltage equalizer On.13. Automatic Regulator Selections.14. Manual Regulator Selection.15. PTHY 1 Ready.

Thyristor Convertor Cabinet 6.

16. PTHY 2 Ready.17 (*) PTHY 1 Fault.18 (*) PTHY 2 Fault.

FBC 1. Start lockout for Minimum number active thyristor.2. Thyristor sold-out.3. Normal number active thyristors.5. PTHY 1 Fault. Thyristor convertor

cabinet 1.15. PTHY 2 Fault.8. PTHY 1 Fault. Thyristor convertor

cabinet 2.6. PTHY 2 Fault.11. PTHY 1 Fault. Thyristor convertor

cabinet 3.12. PTHY 2 Fault.4. PTHY 1 Fault. Thyristor convertor

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cabinet 4.10. PTHY 2 Fault.9. PTHY 1 Fault. Thyristor convertor

cabinet 5.13. PTHY 2 Fault.14. PTHY 1 Fault. Thyristor convertor

cabinet 6.7. PTHY 2 Fault.

OI

2 (*) Maximum Excitation Current.3 (*) Minimum number active thyristor.

4. Fault Thyristor Bridges.5. Thyristor Bridges sold-out reserve.6. Start Lockout.7. Synchronization enabling.8. Opening enabling 52G.11. Ceiling Maximum time.12. Unbalanced current.13. Voltage equalizer ON.

14 (*) Automatic Manual Lockout.CT 1. Thyristor sold-out reserve.

2. Minimum Thyristor number lockout.

7.9.0. TYPICAL FAULT CONDITIONS.Some of the faults that may be arise during the operation of the excitation system are listed below in this section. The signals, (indications) that will appear for the particular fault and also the various causes for the fault are clearly indicated therein.7.9.1. THYRISTOR BRIDGE READY.

Signal: Glowing of LED on the output interface card of convertor cabinet.

Conditions:1. R.C. filter fuses O.K.2. 52 Va (supply to cooling fans switched on)3. ‘ON’ position of commutating switch for insertion of generator

gating pulses.4. 52 breaker closed.5. 29 closed.

7.9.2. THYRISTOR BRIDGE FAULT.Signal: of Led on output interface card of convertor cabinet not

glowing.Causes:

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1. RC filter fuse failure.2. Bridges fuse failure.3. 52 Ve tripped.4. Fan pressure switch for airflow in open position for 5 seconds.

7.9.3. CONVERTER BRIDGE GATE PULSE SUPPRESSION.Each thyristor bridge has an individual pulse suppression device which

operates within the pulse amplifier and allows that converter bridge to be excluded from operation when,

1. Thyristor Bridge is not ready.2. Failure of airflow. (for more than 5 seconds)3. Gate pulses switch in ‘OFF’ position.4. General request of pulse suppression due to excitation system

shutdown.7.9.4. FAILURE IN BRIDGE THYRISTOR GATE PULSE.

Signal: LED on fault detection card not glowing.

Causes:1. Thyristor fault or fuse fault.2. Lack of pulse transmission.3. Interruption of any one connections from line driver, line

receiver, gate pulse, amplifier cards and pulse transformer panel to the gate.

7.9.5. THYRISTOR BRIDGE FAULT.Signals:

1. Alarm local panel.2. Glowing of LED on output interface card of auto manual rack.

Cause:One or more thyristor bridges are faulty as sensed by bridges

anomalies counting card (FBC).7.9.6. OPERATING RESERVE BRIDGES AND/OR THYRISTOR OUT OF OPERATION.

Signals:1. Alarm at local panel.

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2. LED no. 5 pf output interface card in Auto-Manual rack glowing (count thyristor card)

3. LED no. 1 of CT card glowing.4. LED no. 2 of FBC glowing.

Causes: Numbers of thyristor bridges are not ready.

7.9.7. STARTING LOCKOUT.Signals:

1. Local panel alarm.2. Glowing up of LED No. 1 on FBC.3. LED No. 3 of output interface card in Auto-Manual rack, not

glowing.Causes:

1. Locks in starting circuit.2. No. of bridges ready, for operation is lower than minimum.

7.9.8. LOCK OUT FOR MINIMUM NUMBER OF OPERATING THYRISTORS.

Signals:Alarm at local panel along with ‘reserve Bridges sold out

annunciation.Cause:

All the available bridges/are in service, No reserve.7.9.9. FAULT IN AUTOMATIC REGULATOR.

Signals:1. Local panel alarm.2. LED No. 4 of output interface card of Gate pulse generator (GPG)

rack, not glowing.Causes:

1. Absence of ASC, ASP, ASN (± 24V) supplies to regulator.2. Absence of APSC and APSP supplies to logic circuits.3. Opening of 52 SA (Synchronism voltage to gate pulse generator).

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4. LED No. 3 of automatic and manual operation logic card (IF Card) in GPG rack not glowing.

5. Want of balance of synchronizing voltage to GPG for automatic regulation (LED No. 5 of LF card glowing).

6. Anomaly of voltage feed back signal TVX relay (or) voltage balance intervention (signal at alarm panel).

7.9.10. FAULT OF MANUAL REGULATOR.Signal:

1. Alarm at local panel.2. LED No. 6 of output interface card in GPG rack not glowing.

Causes:1. Absence of MSC, MSP and MSN supplies (± 24V).2. Absence of MPSC and MPSP supplies.3. 52 SM breaker open.4. LED No. 3 of LF card in GPG rack not glowing.5. Want of balance of synchronising voltage for manual pulse

generator (Glowing of LED No. 5 of LF card).7.9.11. AUTOMATIC SWITCHING BETWEEN AUTO AND MANUAL REGULATORS.

Causes:1. Fault of voltage regulator during operation in auto mode.2. Intervention of current unbalance protection while machine

voltage is more than 50 % of rated value (LED No. 12 or OI card in AMR rack glowing).

3. Intervention of maximum ceiling time protection of voltage regulator while machine voltage is more than 50 % of rated value (Glowing of LED No. 11 of input interface card in AMR rack).

7.9.12. LOCK OUT OF AM (36 AM).Signals:

1. Alarm at local panel.2. LED No. 15 of output interface card in auto-manual rack not

glowing.Causes:

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1. Absence of AM feeding voltage.2. Absence of AMSC, AMSP, AMSN, AMPSC and AMPSP supplies.3. Anomoly of both auto and manual regulators simultaneously.4. Current unbalancing for more than 5 seconds.5. With regulator in normal operation,a) Intervention of maximum ceiling time protection while machine

voltage is more than 50 % of the rated voltage.b) Anomoly of manual regulator.

-0-0-0-0-0-0-0-0--0-0-0-0-0-0-0--0-0-0-0-0-0--0-0-0-0-0--0-0-0-0--0-0-0--0-0--0-

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