Excitation

ANSALDO

SAGEM s.r.l. TRAINING MANUAL

EXCITATION SYSTEM

\\Ternavasio\Estensione\Samalk \\Ternavasio\Estensione\Samalkot-electric\eltrain ot-electric\eltrainindia-05A.doc india-05A.doc

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MANUAL CONTENTS:

SAFETY INSTRUCT I NSTRUCTIONS IONS

sh.2

SKILL OF THE ASSIGNED STAFF

sh.3

EXCITATION SYSTEM GENERAL

sh.4

SCHEDULED MAINTENANCE

sh.25

TROUBLE SHOOTING

sh.26

CARD REPLACEMENT

sh.35

IMPACT OF EXCITATION SYSTEM WITH GRIDE



sh.37

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MANUAL CONTENTS:


sh.2

SKILL OF THE ASSIGNED STAFF

sh.3

EXCITATION SYSTEM GENERAL

sh.4

SCHEDULED MAINTENANCE

sh.25

TROUBLE SHOOTING

sh.26

CARD REPLACEMENT

sh.35

IMPACT OF EXCITATION SYSTEM WITH GRIDE



sh.37

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All the activities regarding the Excitation System and the auxiliary equipments equipments can be dangerous for the staff. The The assi assign gned ed staf staff, f, who who in any any case case must must have have tech techni nica call trai traini ning ng,, qualifications and experience adequate to the work they have to carry out, must follow the instructions instruct ions given below. Scant attention and/or the lack of the necessary degree of knowledge of the safety safety instr instruct uctio ions ns can cause cause even even fatal fatal lesio lesions ns to peopl people, e, failu failures res or malf malfun unct ctio ion n of the the mach machin ines es and and equi equipm pmen ents ts or a redu reduct ctio ion n of the the components life. The following list does not include all the necessary precautions, but it is limited to some precautionary measures that can be very important: All the activities regarding the system and the auxiliary equipments must be carried out in observance of the laws in force in the matter of safety and industrial hygiene; •

The staff who operates on the excitation system must have carefully read this manual, must know how to operate and follow the instructions, phase by phase, as they are described in this manual; •

Carry out every possible operations (measures, checks, components replacem replacement, ent, etc.) on power power parts and hot circuits with full knowledge knowledge,, taking taking into account all the safety and accident prevention rules that regulate such manoeuvres. •

SKILL OF THE ASSIGNED STAFF The operating staff, assigned to the use of the equipment, must have at least the following requirements: Knowledg Knowledge e of the operating operating principl principles es of the generato generatorr system system + • exciter and of the plant; Knowledge Knowledge of all the ways and st atuses of operation of the excitation

•

system; Knowl Knowledg edge e of the meanin meaning g of every every manoeu manoeuvre vre-me -membe mbers rs of the

•

exciter; Knowledge of all the manoeuvres that can be carried out acting on the exciter and their effect on the plant;

•

Knowledge of the meaning of every signal of the exciter and their

•

possible causes. The technical staff assigned to the functional maintenance of the equipment must have at least the following requirements:


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Basic technical knowledge;

•

Good electro technical knowledge; Ability to interpret interpret the functional diagrams; diagrams;

• •

Experience about power-systems;

•

Experience about relay logic-systems;

•

Experience about microprocessor digital system;

•

To be skilled with the use of MS DOS personal computer;

•

Knowledge of the excitation system operation;

•

Knowledge of the exciter auto diagnostic diagnostic system.

General The plant in SAMALKOT includes a turbo-unit constituted by gas turbines type V94.2 coupled with alternators type T Y 10546 built by Ansaldo Energia. The alternator has its own static exciter (EXC) and one variable-frequency static starters (SFC). The start of gas turbo-units is carried out by t he static starter.

EXCITATION SYSTEM GENERA G ENERAL L

•

Synchronous Synchronous machines need a D.C. current supply for the field winding. The “source” of this current is the excitation system that can be: • •

Rotating Static

Rotating Exciter : is basically directly directly coupled brush brush less exciter Static Static Exciter Exciter : is an electron electronic ic apparat apparatus us (rectif (rectifyin ying g bridg bridge) e) fed by an external source via a power supply transformer. The excitation transformer can be connected to the generator terminals or to an auxiliary bus. In case of static excitation, the current is transferred to the field winding through sliding contacts (slip rings and brushes) •

Definition


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In accordance with standard IEEE 421.1-1986, Excitation System is the device that supplies the excitation current to a synchronous machine, complete with all the feeders, regulators, controls and protection devices. The system is called Static because, adopting semiconductor rectifying parts, it is in contrast with the philosophy of the electric rotating excitation systems. The system consists of two parts functionally distinct (Fig. 1.1): The power section that supplies the excitation current to the rotor windings directly keyed to the generator shaft; •

The control section that guarantees the remarkable electric

•

quantities setting.

3

main

52G

feed EXCITATION SYSTEM

1

G

3

~

power section ALARMS SIGNALS MEASURES COMMANDS

EXCITATION current and voltage

control section

MACHINE current and voltage LINE voltage

Figure 1.1 Excitation system for synchronous machines •

• is greater;

Considerations The most significative advantages of the static excitation systems, in comparison with the rotating ones, can be summed up in the following points: In terms of efficiency, the ratio given power/absorbed power ( ∼ 98%)

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Maintenance problems are smaller: there is a remarkable reduction in the hours of stop due to maintenance, mainly because there are not mechanical parts in motion; •

The system reliability is higher: having the static type a greater MTBF (medium time between failures), the number of stops due to failure is considerably reduced; •

The higher rapidity of response to sudden load changes eliminates the drawbacks due to main falls; •

• The system better accuracy (≤ 0,5% in comparison with 4% or 5% that can be obtained with the rotating type) brings about considerable general improvements of the system. The digital control systems, in comparison with t he analogue ones, allow:

• A dead times reduction in carrying out the reparations (MTTR=Medium Time To Repair), thanks to the use of a smaller number of electronic parts; A better configuration capability, allowing to specify gains and time constants, to enable or to forbid functions without altering the hardware; •

An easy human interface, using keyboards and monitors/displays with extremely “user - friendly” modalities; •

A complete process control, being able to have access to the main logic and analogue variables statuses through the user interface; complete diagnostics, obtained integrating the “external” information from luminous control units and lamps with the information summed up in the proper messages shown on monitor/display. •

Exciter connection Equipment power electronics is fed by the machine mains (Fig. 2.1a) that supplies power to TRE (excitation transformer primary), providing the adjustment of voltage levels; the transformer is sized in such way to assure the excitation performances specified for the machine, taking into account greatest shrinking and copper leakages, and is mechanically structured in accordance with plant features.

Figure 2.1 Converter Power Supply


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3

line line

52G 52E

3

TRE crow bar

3

G

~

converter


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Double control channel and double power converter configuration Diagrams relevant to the acquired configuration are provided below:

converter 1

52G

converter 2 shunt

crow-bar

Vac

G ~

Vdc Rd

=

= =

control 1

= =

= =

= =

= =

=

control 2 F64

Figure 2.2 considered configuration


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Thyristor bridge

•

The Totally Controlled Unidirectional Three-Phase Bridge configuration (Fig. 4.1) is adopted for the power converter: each of the six branches of the bridge is equipped with thyristors (SCR) properly driven to conduction, to supply the excitation to the synchronous machine field.

+

Vac

Vdc

-

Figure 4.1 SCR (Silicon Controlled Rectifier) bridge The thyristors of each branch are sized on the basis of the no load TRE secondary voltage value (Fig. 2.1) and of the converter rated current value. The overall safety is further highlighted by applying over-dimensioning coefficients in the choice of thyristors compared to the minimum calculation size. •

Fuses Fuses are provided to protect thyristors’s bridge in case of internal failure and short circuit between dc buses; they are not requested to operate in case of over current induced by a three-phase short circuit on alternator terminal lugs. They are connected in series on thyristor’s bridge input phases (fig. 4.2).


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Vac

Vdc

Figure 4.2 Fuses

•

Snubbers In parallel to every thyristor of each bridge branch there is a static snubber circuit and a dynamic snubber circuit (fig. 4.3).

Figura 4.3 Snubbers The first one, consisting of a RC filter, is sized to protect the thyristor from over voltages that can occur during its switchover; the second one is necessary when the field current, deviated in the crow bar (Fig. 4.6), passes through the discharge resistor applying between the converter dc terminals a continuous voltage that can takes significant values (kV). The ohmic values requested are obtained by means of two resistors in series, in order to have a certain degree of redundancy in case of short circuit of one of them. Without its snubber, the thyristor may short-circuit owing to an overvoltage, but anyway it would be a controlled failure, limited by the fuse and confined within the power bars.

•

Crow bars This device consists of two thyristors in anti-parallel and connected in series to a discharge resistor. The positive crow-bar CBP guarantees a quick de-


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energizing of the synchronous machine field and operates as a protection for continuous side direct over voltages, whereas the negative one CBN operates as a protection for continuous side inverted over voltages permitting the recirculation of currents with negative polarity, during line or machine failures (e.g. loss of step) (Fig. 4.6). Two thyristor sizing ensures, through the perforation of the crow-bar, the bridge and the field winding protection against over voltages, even in case of firing circuit failure. Moreover the de-energization of the machine is guaranteed, as from the conditions of maximum current excitation, following a three-phase short circuit of its terminal. The crow bar thyristor ignition is carried out in this way: Intervention of the B.O.D. (Break Over Diode) circuit for side dc over voltages, of positive and negative polarity; •

Lowering of the B.O.D. intervention threshold (main electromechanical control) and control of the pulse train (redundant control of thyristor firing). •

The pulse train for the crow-bar firing is generated by the digital control software. The Rd discharge resistor has a value such that it causes, at its terminals, a voltage drop corresponding to the negative “ceiling” (maximum forcing), when the maximum excitation current flows through it.

+ CBP CBN

G

Vdc Rd

-

~

Figure 4.6 Crow bar and discharge resistance •

Discharge resistor The Rd discharge resistor (Fig. 4.6) is connected in series to the thyristors of the crow-bar; it has a value such that it gives rise, at its terminals, to a voltage drop corresponding to the negative “ceiling” (maximum forcing), when the maximum excitation current flows through it.


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The thermal dimensioning must consent the quick de-energization in case of three-phase short circuit at the machine terminals starting from the condition of maximum excitation current. In order to foster its dissipation in thermic energy during the conduction of the crow-bar, it is positioned on the upper wall of the locker. Control section General In the figure 5.1 the main parts of the control section are summed up. For “external electric system” is meant everything that is not included in the exciter and in the synchronous machine but that is electrically connected to them. The digital control device (regulator and phase shifter) carries out the operations provided for by the software, on the basis of information coming from the external system; it accepts like input logic signals and digitalized analogue quantities, giving output logic signals, converted from digital to analogue values and firing pulses for each thyristor power section.

operator interface

control section

digital control feed relay logic feed

power section rgulator &

G

impulse amplifier

~

Phase shiftter

relay logic feed

signals

analogic interface

a b c

Alarm annunciator signals local instrument

signals

feed

EXTERNAL ELECTRICAL SYSTEM

Figure 5.1 Control Section Block Diagram


net

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The main control logic parts are the relays, whose electrical and structural features depend on the plant that they are destined for. Controls utilized and/or produced by the control logic permissive must be electrically adapted to ensure compatibility with the relay circuitry. The de-coupling is carried out through relays or opto-coupler devices. The analogue quantities coming from the electrical system external to the board (e.g. machine, relay...) cannot be dealt as a rule by the digital elaboration system directly, because they have too high values and need to be de-coupled by the control. For alternate quantities tool transformers with a low voltage secondary are used, whereas continuous quantities are drawn by appropriate shunts through dedicated transducers. The synchronous machine excitation control is carried out by the regulator and phase shifter functions realized through software with advanced programming languages, that use the digital technology with a micro controller as hardware support. Regulator and phase shifter are implemented with three electronic cards only:

•

CONDEE micro controller card

•

DAUXEA I/O expansion card

•

DPSEx •

pulses generation card

CONDEE Micro controller card

The card with the greatest number of parts is the CONDEE card. In order to keep its dimensions reasonable, it was made in Surface Mounted technology. Through three flat cables it is interconnected with the other cards (Fog. 5.2a, 5.2b); another flat cable connects the card with the diagnostic terminal (LCD). The micro controller integrated circuit (INTEL 80C 196 Kc), the memory units (EPROM where is the control software, and EEPROM that records the setting and featuring parameters of the specific plant), some logic and analogic interface circuits, the protection logic and the watchdog are all on the CONDEE. The watchdog controls the correct carrying out of the processing loop. On the CONDEE card there is a serial port (RS232) that allows the optional connection with a PC for diagnostic purpose and to personalize the system.


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•

DAUXEA I/O expansion card

The DAUXEA card increases the already high resources of the CONDEE card analogue and digital I/O. The Sample & Hold, 12bit ADC, 12-bit DAC, opto-isolated I/O circuits and the excitation voltage trasduction find place on this unit. Special high-safety outputs and a watchdog circuit complete the hardware equipment. It gets the inputs though flat cable from the DPSEx card and the control signals through another flat cable connected with the CPU CONDEE card (Fig. 5.2a, 5.2b). The digital and analogue I/O has its terminations on an extractable connector with screw-tightening terminal lugs. The synchronous machine Vrs and Vst concatenate voltages and Ir and It analogue currents signals are sampled, held and then read by ADC to obtain, after having been processed by CPU, the VM voltage feedback, the IM current, the PM active power and the QM reactive power, according to Park transform based algorithm. From the Vrs voltage the period measure and then the synchronous machine frequency are drawn. A special circuit coupled to a programmable timer, allows the accurate measurement of the excitation voltage mean value, without introducing considerable delays on the reaction line. On board of this card the crow bar command pulse train is generated too. DPSEx pulses generation card There are two hardware versions of a same printed circuit: DPSEA and •

DPSEB (Fig. 5.2a; 5.2b). The only difference between them is the existence aboard of pulse transformers for galvanic de-coupling as to the converter bridge thyristors gates. In particular, the DPSEA is the standard solution for Brush-less machines or low-power machines excitation. The DPSEB is the standard solution for ring synchronous machines excitation. The DPSEx generates the supply voltages at + 5 V (switch mode) and ± 15 V, obtaining them from a ± 24 Vdc input.


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The three synchronism signals for the thyristors starting, the excitation current feedback and the supply voltages are brought to the CPU CONDEE card through flat cable. The excitation voltage feedback measure and level adaptation is sent to the DAUXEA card; from that comes the pulse train, that, amplified, will be brought to the terminal block for the connection with the Crow Bar external pulse transformer. The pulse trains, that enlarged will cause the thyristors firing, are received by the CPU CONDEE card. On board card there are also the maximum instant current protection (protection short circuit field side) and the ± 24 Vdc power voltages surveillance circuits.


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•

• •

Regulation diagrams Voltage regulator can be in two different modes: Automatic (A.V.R.) Manual (M.V.R.) Automatic The “AVR” keeps the voltage level at the generator terminals at the right value, adjusting the excitation current. It is an electronic device, of digital type. The turbine governor obtains the frequency regulation. This is the normal mode of operation. The voltage regulator should be settled in such a way that on one hand an adequate stability is attained, and on the other hand the turbo generator cannot be continuously subjected to overloads. This is achieved partly by the speed of regulation and field forcing and partly by the excitation current limiters (see “Generator capability curves). The regulating range for the generator voltage is designed to suit the allowed voltage range for continuous operation (see “Generator Data Sheets”). Manual Manual operation of the excitation system is possible as an emergency operation if automatic regulators fail, or to perform test on the generator. It should be noted that in manual operation, the excitation must be manually controlled in order to follow any change in load demand and voltage. For any particular description of voltage regulation system functions, refer to the relevant manuals 1,2,3. The excitation control is carried out by the digital control functions based on the current state of the regulated system, represented by its more significative analogue quantities measured. See diagram (control section).


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line

feeding converter

52G Line voltage Synchronism voltages

power section crow bar

alpha

converter control section

= = = = = =

G

~

excitation current exicitation voltage crow bar current machine voltage machine current

Figure 5.3 Analogue quantities measurements


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The phase shifter (Fig. 5.4) carries out the interface between the excitation digital control and the power converter functions: it must fire the thyristors, automatically synchronizing with the main and ordering starting angles for rectifiers between 5° and 150°, according to the various cases. The right firing order check is carried out keeping the excitation (voltage or current) under control.

amplifier

Vcon + -

converter

Phase shifter

G

Vecc

software Figure 5.4 Excitation control


~

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The phase shifter is designed to form an as much as possible linear “block” with the complex pulses amplifier-converter: the ratio between the excitation voltage (Vecc) and the control voltage (Vcon) produced by the regulator must have the shape indicated below (Fig. 5.5.); furthermore, the maximum positive field forcing (“ceiling”) (angle of thyristors firing equal to 5°) must correspond to the linear maximum Vcon value, whereas the minimum Vcon value must cause the maximum negative forcing (angle of thyristors firing equal to 150°) (v. Par. 5.3).

Vecc [p.u.]

1

1

Vcon [p.u.]

Figure 5.5 Ratio between Vecc and Vcon


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The regulator carries out the excitation control using the machine voltage as a feedback signal (Fig. 5.6).

amplifier

Vrif

+ regulator

Vcon

-

converter

Phase shifter

VM

Vecc

software

G

~

Figure 5.6 Machine voltage control

The function configuration parameters are such as that the phase shifter excitation loop is faster than the regulator machine voltage loop: from the point of view of the last one, the phase shifter acts like an unitary block (Fig. 5.7).

Vrif

+ -

regulator

Vcon

1

G

VM

~ Figure 5.7 Machine voltage control


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•

Thyristors firing Every thyristor is in the conduction status when it is subject to a direct polarization voltage between anode and cathode (V AK > 0) and when the current flowing through it, driven from a positive pulse applied to the gate electrode, reaches at least the “lachting” value; the conduction automatically stops when the V AK voltage changes sign. The pulses that command the thyristors conduction are generated from the digital control. They are amplified from a MOSFET stage and later filtered in order to be used to drive the power electronics. At any time the firing signal of the two thyristors involved in the conduction must have a total duration of 6.6 msec (with a three-phase main with frequency equal to 50Hz, 20 msec is the corresponding period and 20 msec/3 = 6,6 msec is the conduction time of the couple of SCR involved) for a single stage; it consists of a burst of pulses (20 µsec) repeated every 40 µsec (Fig. 5.8)

20µsec

40µsec

20µsec

40µsec 6.6msec

Figure 5.8 Thyristors control signal

Using the burst of pulses the value of the “latching” current is surely discontinuous, but the probability of the thyristor switch on is increased in case of inductive loads and the mechanical overall dimensions of the pulses transformers are reduced; the transformers have an 1:1 transformation ratio and they realize the galvanic de-coupling between the power electronic circuits and the control circuits.


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The pulses “moulder” circuit consists of a resistance in parallel with a capacity (Fig. 5.9); the signal featuring shape is graphically represented in Fig. 5.10.

C

amplifier

impulse transformer

R

K

G A

circuit moulder

Figure 5.9 Amplifier and pulses moulder stage

3A

1A

1µsec

20µsec

Figure 5.10 SCR gates input signal form


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The phase shifter can drive the conduction of each one of the six branches of the bridge (Fig. 5.11). Since a period of the synchronism voltage corresponds to 360° electric degrees, starting from a certain moment every 360°/6=60° a couple of branches will be in conduction and it will be different depending on phase sequence (Fig. 5.12).

+ 1

3

5

4

6

2

R S T

-

Fig. 5.11 Converter thyristors and a.c. phases

Direct phase sequence

..→ R → S → T → R →..

•

phases sequence

•

ignitions ..→ 1+6 → 1+2 → 3+2→ 3+4 → 5+4 → 5+6 → 1+ 6 →..

Reversed phase sequence

•

phases sequence ..→ R → T → S → R →..

•

ignition

.. → 1+ 6 → 5+ 6 → 5 + 4 → 3 + 4

→


3+2

→

1+2

→

1+ 6

..

→

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Excitation voltage graphics are shown in Fig. 5.13.

Mean value of Vecc>0

Thyristors firing with 30° angle

Thyristors firing with 90 ° angle Mean value of Vecc=0

Thyristors firing with 150° angle Mean value of Vecc<0

Figure 5.12 Excitation voltage (converter Output)

24

24

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•

SCHEDULED MAINTENANCE General information

•

To ensure the correct and continuative operation of the machinery it is necessary to carry out some scheduled maintenance operations. Mechanical controls

•

The following operations 1, 2 must be carried out monthly, while the other ones at least during scheduled group stops: 1. Check and, if necessary, clean or replace filters on the vents. 2. Examine visually the parts to single out possible wear and/or burning, dust and/or foreign particles accumulated inside the board. WARNING: OPERATE WITH MACHINE OFF WITH UNFED BOARD. 3.

Carry out the necessary replacements of damaged parts. Remove dust and possible foreign matters accumulated inside the board. Dust can be removed with a duster or with dry compressed and not lubricated air (4 ÷ 6 atm) avoiding to direct the air wash on fragile parts (ex: electronic cards). 4. Disjoin connectors, verify their wear condition and restore connections. 5. Check the terminal block contacts status. 6. Check the tightening of the terminal blocks and all the terminal electric contacts of the inside board parts. 7. Check the tightening of the power bars nuts, the converter, etc. 8. Check the wear condition of all the mechanical and electromechanical moving parts.

•

Electric controls

The following operations must be carried out at least during the scheduled group stops: WARNING: OPERATE WITH SIMULATED SIGNALS, OR WITH EXCITATED MACHINE IN MANUAL MODE. 1. Measure the control electronics and relay logic feeding voltages; adjust the levels if necessary. 2. Verify the efficiency of the measure transformers (C.T. and P.T.). 25

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3. Verify the efficiency of the measure transducers. 4. Check the local indications on the panel-instruments and the indications on the local terminal operator interface display (Variables Menu). 5. Verify the coherence of the local panel signalling and the operator interface local terminal display (Flags Menu) signalling with the relay logic command status. 6. Carry out some local operations that involve the relay contacts status alteration (selectors movement, pressing buttons) and check the logic operation. 7. Check the functionality of all electromechanical moving parts.

TROUBLE SHOOTING •

General: The excitation system distinguishes between ALARMS and TRIPS: while the events that cause the first ones do not trip the generator operation, when the second ones occur it is necessary to stop supplying energy to the field, in order to avoid or limit permanent damages to the machinery and/or to the synchronous machine. The events causing alarms and trips can be inside the board or arise from outside the board.

•

Diagnostics

On the digital excitation systems control, diagnostics takes place: •

Through the luminous signals on dedicated “control units” (alarms and

trips); With the help of synthetic messages, appearing on the operator interface local terminal display. •

The Information’s drawn from the examination of the “control unit” and of the display reading are Complementary: both contribute to single out the possible cause of malfunction.

WARNING: Through diagnostics, only malfunctions (inside and outside the excitation system) envisaged during the project step can be singled out.

26

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Machinery reset

•

The operative procedure allowing the operator to reset machinery disabled because of any malfunction is the following: 1.

Single out the cause of malfunction with the help of diagnostics;

2. Eliminate the cause of malfunction; 3.

Restore the functionality of every digital regulator simultaneously pressing

the <> and < mode> KEYS on the operator interface local terminal; 4. Reset the alarm and block control units. If at the end of phase 3 the regulator does not return to availability status and/or in phase 4 the luminous signaling relevant to a certain event does not switch off, it will be necessary to go back to phase 1.

•

Alarms and trips In this paragraph the main alarms and trips regarding the machinery operation are listed. Their signaling is shown on the internal panel called T-LINE and take place in the compartment called Control. See following 3 alarm tables.

Alarms

27

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Alarms (sheet 1 of 3) Events

Description and actions

A.V.R.1 FAULT.

Check the =M2-AK02-K3 relay status (energized means OK).

A.V.R.2 FAULT.

Check the =M2-AK02-K4 relay status (energized means OK). .

CONVERTER CHANGE-OVER FOR CURRENT UNBALANCE

Converter changeover request for current unbalance detected in the working converter. Check the ex-working converter, the protection rack, =NI-PROCO the =D1-UA10 and =D1-UA11 current transformer.

AC/DC POWER SUPPLY GA01 FAULT

AC/DC POWER SUPPLY GA02 FAULT

CONVERTER 1 FUSES INTERVENTION

Faulty -GA01P/N power supplies or AVR1 aux power supply loss. Check the =F2-GA01P/N devices, the sensitive to voltage relay. Faulty -GA02P/N power supplies or AVR2 aux power supply loss. Check the =F2-GA02P/N devices, the sensitive to voltage relay. -FU11/13 power fuses intervention on -GS01 converter. Check the converter bridge and the =L7-AK09K7 -K8 relay (OK when energized).

CONVERTER 2 FUSES INTERVENTION

-FU21/23 power fuses intervention on -GS02 converter. Check the converter bridge and the =L8-AK10K1K2 relay (OK when energized).

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Alarms (sheet 2 of 3) EVENTS AC/DC POWER SUPPLY GA03 FAULT AC/DC POWER SUPPLY GA04 FAULT CROW-BAR OPEN

Description and actions Faulty=F3 -GA03 power supply, redundancy loss for the power supply. Check the sensitive voltage relay. Faulty =F4 -GA04 power supply, redundancy loss for the power supply. Check the sensitive voltage relay. This alarm means that the static disexcitation system is open. The current do not flow through the discharge resistor =H2-RD01 Check CROW-BAR thyristors-

MOTOR FAN 1 CONV.1 FAULT

Converter 1 motor fan 1 fault Check the power supply circuit: fuse =D2-FU12, Switch =G1 - QMO1 Thermal relay =G1-FR01 then check=G1-SP01 SP02 air flow sensor and =L7-AK09K1-K2 relays if energized when the converter GS01 is operative and fan1 is selected.


Converter 1 motor fan 1 fault Check the power supply circuit: fuse =D2-FU12 Switch =G1-QMO2 Thermal relay =G1-FR02 then check=G1-SP01, -SP02 airflow sensor and =L7-AK09K1-K2 relays if energized when the converter. GS01 is operative and fan 2 is selected.


Converter 2 motor fan 1 fault Check the power supply circuit: fuse =D2-FU22, Switch =G2-QMO3 Thermal relay =G2-FR03 then check=G2-SP03SP04 airflow sensor and =L7-AK09K3-4 relays if energized when the converter GS02 is operative and fan3 is selected.


Converter 2 motor fan 1 fault Check the power supply circuit: fuse =D2-FU22, Switch =G2-QMO4, Thermal relay =G2-FR04 then check=G2-SP03SP04 airflow sensor and =L7-AK09K3-4relays if energized when the converter GS02 is operative and fan4 is selected.

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Alarms (sheet 3 of 3) EVENTS AUX SUPPLY 110V CC

Description and actions Check that auxiliary power supplies 220vca and 400vca are present. Check aux breaker =F1-QE11 and =F1-QE09

OUT OF RANGE

EXTERNAL TRIP

This alarm means that the excitation system has been locked for an external reason.

OVERVOLTAGE

This alarm means that one fuse of the

PROTECTION FUSE CONTROL RACK COOLING FANS FAULT

Over voltage protection system is faulty. Cooling fans =F3-MV01, 02,03 for control boards faulty. Check fuse =F3-FUMV Check if auxiliary power supply is present.

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•

Trips

Trips (sheet 1 of 2) EVENTS FAULT INPUT/OUTPUT PLC CONVERTER FUSES

BOTH REGULATORS FAULT

Description and actions Same of more important inputs and output to PLC are duplicated. When trip occurs, that means the two signals disagree. Intervention of one or more converter phase fuses due to a thyristor or firing circuitry failure. The failure of firing circuits may derive also from a power supply failure; should such an important anomaly occur, other alarms should operate accordingly. Check the first cause through control unit acquisition system. System general anomaly. - Lack of machine voltage retroactions; - Lack of UPS power supply; - Wrong interventions on fed electronic parts Check Sys-error on terminal.

REGULATORS CHANGE FAULT System logic requires an automatic switchover owing to an anomaly of regulator in service, but stand-by regulator is not ready having no power supply or being blocked as a consequence of previous anomalies. CONVERTER PROTECTIONS AUX. POWER SUPPLY

The converter protections rack is without power supply. Check the + 24Vdc power source. Check if the protections relay for the supply monitoring is energized.

INSTANTANEOUS MAXIMUM CURRENT

It is due to short-circuit downstream the converter; the 4 times IN protection calibrated converter maximum current, has operated.

DELAYED MAXIMUM CURRENT It is due to a prolonged overload beyond the converter and rotor system endurance limit. The PROCO card in base of the excitation current reading calculates the function. At 1,25 In converter value a ± 5 dedicated transducer type TDBAS1 code - UA35 is operated after 30 Sec.

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Trips (sheet 2 of 2) Description and actions

EVENTS UNBALANCE CURRENT (THYRISTOR TRIP)

CONVERTERS FAN FAULT

Unbalance current protection intervention the measuring system (-UA10, -UA11 “hall” effect current transducers and -RR10, -RR11 burden resistors; see sheet nr. 030) supplies to the PROCO protection rack and analogue signal which magnitude is proportional to the working converter input current. The supply current unbalance can be due to the converter, working without one (or more) of its legs contribution, or to the main power supply source, or to the excitation transformer in case of internal failure: check the power fuses, the thyristor firing circuit and electronic command (firing pulses). Both the converter cooling systems are faulty checks each cooling system efficiency feeding the motor fans by the aux line (use -SA01 and -SA02 switch) Check each fan motor protection and the airflow detectors.

32

DE - EXCITER FAILURE

PERFORATION of one or more CROWBAR circuit thyristors. Prolonged current flow for a pitch (step) loss greater than 250mSec.

EXCITATION OFF WITH GENERATOR BREAKER CLOSED

Excitation field breaker is open or excitation is missing when generator breaker is closed. .

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•

Operator interface local terminal display

The digital system carries out controls about the existence of analogic signals necessary to set and check the congruence of some logical commands; besides, one’s own hardware status is checked. To incur one of the under mentioned events mean a SYSTEM ERROR: made exception for vm_err , which cause the commutation from local control automatic mode to manual mode (for single-channel system only), while the other ones cause the REGULATOR FAULT. System error

•

System error (Sheet 1 of 2) Nr MESSAGE Meaning

33

Suggestions

1

vm_err

vm, machine voltage Verify: measurement error: the system - The Vm_val measure, ≠ 0% in does not detect anymore the operation; feedback signal. - The measure scale factor, vm_scl (family Stat_val); - Machine URS and UST voltage signal wiring (TV, filters, connections)

2

e_fbk

Lack of “ve” excitation voltage Verify: feedback: the system does not - ve_val measure; detect this signal - the scale factor measure, ve_scl (family Rot_val); - the excitation voltage signal wiring (divider, transducer, connections)

3

e_min

Lack of parallel excitation: In case of premature interventions, system detect a current value ie verify: lower than 0,25*Le0 - The excitation current measure, ie_val: scale factor (ie_scl) and/or offset (ie_offs) can be inadequate (family Rot_val); - The ie current signal wiring (shunt, transducer, connection)

4

sinc

Lack or inadequate level of the Verify: synchronism voltages - The synchronism monitoring circuit functionality; - Synchronism voltage signal wiring (TT, filters, monitors, connections); - The wiring that brings the logical lacking synchronism signal from the monitor to the control

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System error (sheet 2 of 2) nr Message Meaning

34

Actions

5

k52e

Discrepancy between 52E and Verify: START excitation command - The status signals and the plant commands existence ; - verify the inside board relay logic

6

k52g

Discrepancy among 52E, Verify: START excitation command - The status signals and the plant and 52G commands presence; - Verify the inside board relay logic

7

ram

CONDEE card: test error on the Replace CONDEE card RAM memory condition

8

eep

CONDEE card: test error in the It appears if EPROM or EEPROM EEPROM memory status memory or CONDEE card have been replaced. Otherwise, replace the CONDEE card

9

± 15V

10

kp

11

ioc

12

wdog

CONDEE card: circuit error

13

HSO

CONDEE card: High Speed Replace the CONDEE card Output circuits anomaly

14

A/D

DAUXEB card: A/D converters Replace the DAUXEB card time-out

15

auxIO

CONDEE card anomaly on the ± 15V feeding voltages -

Verify digital control circuits; Replace the DPSEB card; Replace the CONDEE card

CONDEE card: lack of “external protection” input

kp For a normal operation, the input “external protection” (XM1-20 CONDEE) must be on the high logical level (P24/R). In case of untimely intervention replace the CONDEE card CONDEE card: max In case of untimely intervention: instantaneous excitation current - Replace the CONDEE card

DAUXEB card: circuits anomaly

watch-dog -

I/O

Replace the DPSEB card; Replace the CONDEE card

digital Replace the DAUXEB card

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•

CARD REPLACEMENT

WARNING: Before starting the replacement operation take the necessary precautions against Electrostatic Charges. Some basic precautions to take against electrostatic loads are: •

Use an antistatic arm-band in contact with the operator skin;

•

Use conductive or antistatic bags and containers;

•

Discharge possible static loads on the conductive work plan before

handling the parts; •

Keep the parts in their antistatic containers;

•

Minimize the parts handling;

•

Avoid to touch the reophores and to handle the parts touching only their

bodies; •

Avoid to keep plastic materials over the working areas, excluding

antistatic or conductive materials; •

Avoid loads generation;

•

Not subject semiconductors to wiping movements under each surface;

•

Operate only on workstations correctly connected to t he ground.

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Digital control cards

•

Preliminary operations: 1. Cut off power supply to the digital regulator. 2. Enter the digital control electronic cards; remove the cover or protection (if any). - Replacement of a CONDEE, DAUXEA, DPSEx electronic card: 1. Remove terminal connectors. 2. Remove flat cables and serial connection connectors. 3. Remove the electronic card. 4. Install the new electronic card. 5. Plug flat cables and serial connection connectors. 6. Plug terminal connectors.

- XAI48B electronic card replacement: 1. Remove the connector. 2. Release the cables fixed to the terminal lugs. 3. Remove the electronic card.

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IMPACT OF EXCITATION SYSTEM WITH GRIDE (Reactive estimated about 0,1735 MVAR/KM on 230 KV systems)

PREFACE Find herewith attached some criteria and general guideline for operating the generators with their step up transformers and substations in the network. We give here some general theoretical information about electric networks and criteria how to share the active and reactive power between the generators. The major problems are found in very small networks (below the 2000 MW) and small networks (from 2000 up to 6000 MW) while in the medium (between 6.000 and 15.000 MW) or in the large (from 15.000 to 40.000 MW) or in the infinite networks (over 40.000 MW), the grid is more stable and reliable. It is very important that the reactive power is shared in equal parts between all generators of the same network. We suggest to run generators trying to achieve the condition of the same cos. ϕ as well of the medium network cos. ϕ . In order to determine the various network cos. ϕ it is necessary to read simultaneously the values of the active and reactive power of the system, in each power plants, in the more significant moments over 24 hours period: 1) At the maximum active power demand 2) At the medium active power demand 3) At the minimum active power demand The cos. ϕ of the system will vary continuously during the day mainly due to the active and reactive power at that moment. Automatic voltage regulators mainly control the voltage of the system while the governors of the generators control the active power.

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We have used the word “mainly” because in fact the pure generator voltage regulator controls only the generator voltage. For Samalkot the voltage is 15 KV. In fact, when step up transformers are loaded, they have a per cent “voltage drop” of their voltage impedance (13 % for SAMALKOT Power Station at 100% of load; or proportion of 13% according to the per cent of the transformer load: i.e. 6,5% at 50% of transformer load; 3,25 at 25% of transformer load and so on). Practically speaking if transformers have each a maximum output (100% of rated MVA), for instance, in an island network, considering no any other influence from the network, the line voltage of 230 KV will drop of 13 % means at about 200 KV. This kind of effect seen by point of view of the excitation of the main generator can be represented as a “compound negative effect” of 13 %. Negative compound system is a self-stabilising system to be adopted if more generators, without interposing step-up transformers, are in parallel in the same grid. It has to be mentioned that the excitation systems are provided to maintain the voltage constant at the output of the generator. A.V.R.’s of 230/6,6 KV transformers maintain the voltage constant in low voltage side (6,6 KV side) after perturbations.

GUIDELINES TO OPERATE THE 230 KV, AND 6,6 KV SYSTEMS FOR SAMALKOT POWER STATION The following rules have been written for the sake of clarification of the operation of the electrical system on Samalkot power Station. For this guideline simple descriptions and straightforward examples covering some of the more important cases has been preferred to the theoretical explanations given in the preface.

• Generators output voltages will be kept almost constant during the normal

parallel operating period with a set point on the excitation system to obtain 15 KV at the generator terminals. • Before synchronisation check the voltage before and after the synchronising

breaker. If the voltage difference is more than 5% reduce the difference by acting on the generator step-up transformer on load tap changer. IMPORTANT NOTE: It is necessary to select a proper tap positions of the on-load tap changer of unit transformers and a suitable operation tap of the Step-up Transformer , according to the grid average voltage or special line conditions (if any) of the network, to avoid sudden voltage build-up over the design values of transformers, (235 KV +10% -12,5% (one steps= 1,25%) for Samalkot P.S.) resulting in over flux, during network disturbance.

• During synchronisation the set point on the excitation system must be

changed (so the generator voltage) to match the grid conditions. Set point changes will be either done by operator (for manual synchronisation) or by Automatic Synchroniser (for automatic synchronisation). 38

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• After synchronisation adjust the excitation set point in order to bring

generator voltage to the nominal value of 15 KV. • If the voltage is higher, excitation set point must be decreased and at the

same time on the generator transformer on load tap changer number must be decreased to match the reactive power demand of that moment from the grid.

• If the voltage is lower, excitation set point must be increased and at the

same time on the generator transformer on load tap changer number must be increased to match the reactive power demand of that moment from the grid.

• With the generator connected to the grid and its voltage constant at about 15 KV, the reactive power (inductive or capacitive) demand will change. If inductive reactive power increases (large motors, air conditioners, refrigerators, etc.) the grid voltage (230 KV system) will decrease. To increase the grid voltage is necessary to increase the high voltage side of the generator transformer by using its on load tap changer. To increase the H.V. side of the transformer is necessary to decrease the on load tap changer number. Lower tap number means higher voltage (exporting). • If inductive reactive power decreases the grid voltage (230 KV system) will increase. To decrease the grid voltage is necessary to decrease the high voltage side of the generator transformer by using its on load tap changer. To decrease the H.V. side of the transformer is necessary to increase the on load tap changer number. Higher tap number means lower voltage (exporting).

• If the capacitive reactive increases (long transmission lines, our case) the grid voltage (230 KV system) will increase. To decrease the grid voltage is necessary to decrease the high voltage side of the generator transformer by using its on load tap changer. To decrease the H.V. side of the transformer is necessary to increase the on load tap changer number. Higher tap number means lower voltage (while exporting from generator). • If the capacitive reactive decreases the grid voltage (230 KV system) will decrease. To increase the grid voltage is necessary to increase the high voltage side of the generator transformer by using its on load tap changer. To increase the H.V. side of the transformer is necessary to decrease the on load tap changer number. Lower tap number means higher voltage (while exporting from generator). • The 6,6 KV boards are fed by the Owner. According to the configuration it

will be very important to do the best effort in order to keep as constant as possible the station services voltage. It will be probably necessary a routinely check of the station voltage. Voltage variation less than 5% over or under 6.6 KV is acceptable, but differences of more than 5% require prompt actions. If station voltage 6,6 KV is high, it is necessary to decrease the transformer tap number in order to decrease the voltage. Lower tap number means lower voltage. (while stepping down from grid).

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If station voltage 6,6 KV is low, it is necessary to increase the transformer tap number in order to increase the voltage. Higher tap number means higher voltage. (while stepping down from grid). The philosophy of operation must be as follow:

• 6,6 KV Station Transformer A.V.R.’s – (if any) they should be always kept on automatic mode after their commissioning. (By Owner) Even if A.V.R. are operating in automatic mode, it is important to keep a routine checking by the operator to the system voltages to detect eventual malfunctions.

• One important thing is to remember that in case two or more transformers are in parallel (if any) they must work with the same tap position of the on load tap changer to avoid circulation of reactive current between them, thus reducing their capability.

• For the 415 V systems only routine checking must be done. Voltage

variations up to ± 5 % are acceptable. If some of the 415 V boards are continuously working on the lowest or highest side of the acceptable range it will be necessary to change the off load tap changer on the respective transformer and this is to be done at the first opportunity the board can be switched off. Find hereunder the main data of the step-up transformer. Generators Step-Up Transformers 235 KV +10% -12,5% (one steps = 1,25%) (+8 x 1,25% / -10 x 1,25%) / 15 KV Vcc = 13% • Excitation Transformer: Refer to manual • Static Starter Transformer: Refer to manual

GENERATOR EXCITATION SYSTEM (theory)

The excitation system Automatic Voltage Regulator (A.V.R.) is a device that keeps constant the generator output voltage within accuracy usually of ± 0,5 %.

One of the multiple features of the A.V.R. is to make generators share reactive power when several of them are parallel working on the same bus bar. Several names were used for that device: compound, voltage droop, quadrature droop, reactive current static,etc.

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X

X

X

X

GT1

GT2

GT3

GT4

When a generator is normally working (due to the A.V.R. action) the voltage at the generator terminals remain constant during the reactive power loading from 0% to 100% of the generator nominal reactive power output and up to reaching the OVER or UNDER-excitation limits. After reaching the OVER or UNDERexcitation limits the voltage will respectively decrease or increase depending on the reactive load demand (inductive or capacitive).

Q1 00 % Un

0%

When 2 or more generators are parallel working on the same bus bar and their A.V.R.’s have not a compound action, the generator with the highest set point will be the first to be loaded with the reactive power. It will continue to get loaded until OVER limit will appear.

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Q1 100% Un 95% Un

0%

Then its output voltage will decrease until reaching the second higher voltage generator.

100% Q1 00% Un 95% Un 50% Q

0%

0%

If the reactive load demand continues to increase, the second generator will go also on OVER limit and voltage will decrease further until r each the third higher voltage generator. In this way two out of three generators will be working at they’re OVER limits at their maximum capabilities, with only one controlling the network.

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The way to overcome this difficulty is introducing compensation in function of the reactive load level on the generator. The compensation is made on the generator A.V.R. set point, so the set point value decreases with a linear function when the reactive load increases. This compensation is also effective even if both A.V.R.’s set points are different. The reactive power is measured on the excitation system using the voltages and currents provided by the generator potential and current transformers. The level of influence or compensation can be set from 0 % (no compensation) to ± 10 % . Q1

100 % Un 90 % Un

0%

The effect when 2 or more generators are parallel working on the same bus bar with their excitation system A.V.R. having the compensation included is analysed hereafter. We suppose two identical generators (all the values will be expressed in percentages) running in parallel: Generator 1: A.V.R. set point is at 100 % Un Negative compound is set at 10 % Generator 2: A.V.R. set point is at 95 % Un. Negative compound is set at 10 % Network reactive power demand increases slowly from 0 % to 150 % of one generator rated reactive power. NOTE: The situation described with the example is not completely real, because two generators connected directly to a bus bar with so big difference in A.V.R. set point will produce a circulation of reactive power between them. The simplification is made for the sake of understanding the sharing of reactive power between generators and grid. Being higher the set point of A.V.R. generator 1, this generator will start to generate reactive power before generator 2. Because of the negative compound effect more reactive power will cause the decrease of A.V.R.’s setpoint; when generator 1 generate 50 % of the reactive power its set-point will fall to 95 % of Un that is the value for the A.V.R.’s set point for generator 2. In this condition also generator 2 will start to take reactive power and being both generators identical they will share each, half of the remaining reactive power demand 100 % (so 50 % each). Even if not equally shared, at the end generator 1 will take 100 % (out of 150 % demand) and generator 2 will take 50 %, even if they have so different A.V.R.’s set-point settings. In this case being generator 1 arrived to its maximum reactive output; the shift operator must take

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care of increasing A.V.R. set point of generator 2 for a complete sharing of reactive power. In the case of generators connected to the grid by means of step-up transformers the case is completely different because due to the transformer short circuit voltage we obtain downwards the step-up transformer, the same effect that introducing the linear function (negative compound) on the A.V.R.’s set point. So if a generator is connected to a grid by means of a step-up transformer and its A.V.R. has this negative compound compensation function, that function must be set to zero or deactivated. The stability obtained by this configuration is a little higher because the voltage decrease is due to the total transformer load (active + reactive). Only in case of big differences on transformer short circuit voltages the use of the compensation could be useful.

GENERATOR NOMINAL REACTIVE POWER OUTPUT

100 % Generator 1

Generator 2

65 % 50 %

50 %

100 %

150 %

REACTIVE POWER DEMAND

Let us have a look at our Samalkot configuration. On Samalkot each of the two generators 10MKA and 11MKA are feeding the 230 KV system through stepup transformers having a V.c.c. = 13 %. 44

Excitation

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