220 VOLT DC SYSTEM-BATTERY CHARGER -COMMISSIONING PART -I I-ABC OF THERMAL POWER PLANT.

COMMISSIONING PROCEDURE:

     TESTING OF BATTERY CHARGERS:

The insulation value shall be checked with 500 V DC Megger. During this test, all low voltage circuits  & Relays etc, shall be  bypassed ( shorting)

The panel shall be switched on at no load. For this, first make input switch ON & check the voltages at all phases indicated by lamps  & voltmeter by selector switch . Now the voltage is connected to the primary of the Main transformer which steps the voltage down as required for the specific DC output Voltage & isolates AC Input line from Battery circuits. The transformer's secondary is connected to the Full wave rectifier assembly.

The bridge is standard  three phase full wave half controlled silicon bridge. The voltage control is accomplished by the application of a positive pulse to the gate terminals of the SCR at the desired time. Before the pulse is applied , the SCR is open  and no current flows in the circuit.  As soon as SCR is fired, it operates as a standard silicon rectifier until the forward current is reduced to almost zero.

Check the no load output voltage at the output terminals.

Ensure the output voltage is steady without any trouble for 15 minutes.

Check for annunciation interlocks  etc.

If load is available, the equipment shall be loaded progreesively upto rated value

Check for voltage regulation, free from oscillation, ripple etc.

Check for dropping characteristics at higher loads.

Run the equipment for 4 hours and observe of stability and check for local heating.  When unit is loaded the ripple voltage level (peak to peak) may be observed . Follow the advice of Service engineer.

II          FIRST CHARGING OF BATTERY:

 Thumb rule says DC  Voltage needed for battery charging will be 2 times the no of cells in battery bank. The initial charging will take approximately 16 to 24 hours since the batteries are already charged and sent as maintenance free ones.

k board is firmly coIt is of utmost importance that the terminal of the battery is connected to +ve load of charging source. So polarity should be ascertained.

Commence charging not later than 24 hours after connecting all battery cells.

Give the first portion of charge at a rate of 100 Amps for 10 Hours

Discharge it using Water resistance or halogen lamps as recommended by supplier

Give the second charging at the rate of approximately 20 Amps till the signs of completion  of charge observed.

Select any one cell (except the end cell or regulating cell) as pilot cell out of every rack as given in the layout of the battery bank.

Voltage and specific gravity reading from pilot cells will indicate the state of charging of the whole battery system. Follow supplier’s recommendation

All cell terminals & connections are to be applied with pertroleum jelly to reduce contact resistance. No adjustment should be made which will disturb charging.

When all the cells have been greasing freely and when voltage and specific gravity of pilot cell has been constant over 3 successive hour readings, the cells can be deemed to be fully charged and the charging may be terminated. Then wipe down outside of all cells and clean the floor.

Put the battery system on trickle charging when charging completes.

The trickle charge should be adjusted to give optimum battery voltage of 1.53 - 1.67 volts per cell.

III         ENERGISING OF DISTRIBUTION BOARD:

Take final I.R. value of the board. Check ea
ethi ected with earth through wire or strip as shown in the drawing.

Ensure that all outgoing circuits are locked in OFF position.

Ensure that all safety procedures have been observed.

Check that polarities of D.C switch board battery charger and battery are matching.

Close the incomer switch of board to charge it.

Measure D.C Voltage at the Input.

Observe D.C board for any abnormality.

If board condition is satisfactory, check that all remote end fuses of outgoing circuits are removed.

Switch ON outgoing feeders one by one. Record the following for each circuit.

i.)         Polarity  at both ends of circuit

ii.)        Voltage at both ends of circuit.

IV       EARTHFAULT RELAY ON LOAD TEST:

To carry  out on load test for the Battery E/F relay, connect variable resistance box of the range of 15 to 150 K Ohms between +ve  pole of the board and earth

Keep selectivity setting plug of E/F relay  at 3 ms setting.

Cut out resistance slowly to simulate an E/F  and observe the current in milli ammeter when the relay operates.

Record resistance and current in mA

Carry out this test on each available setting on relay.

While doing test observe and record the direction of deflection in the ammeter. Check whether it confirms the pole on which the fault is simulated.

Repeat  once a again on -ve pole of the board.

The test is satisfactory if the operation of the relay confirms with the setting.

Note : Electrical commissioning demands cool approach not rushed to wards the job. Any mistake may cause permanent damage and that was witnessed many a time.

All instrument need to calibrated so that during dispute you can rely. This is also a common problem.

All hand tool should be insulated one and value checked before using.

Electrical signboard /safety signboard should be displayed .

220 VOLT DC SYSTEM-BATTERY CHARGER PRE-COMMISSIONING PART -I -ABC OF THERMAL POWER PLANT.

220 volt dc system with battery charger commissioning

Till last dispatch I have discussed about the basic requirement of electrical system in apower plant . In continuation with that commissioning of DC 220v system will be described briefly.

The system consist of :-

i.          Battery Bank

ii.         Battery Charger - Main and Standby

iii.        DC Distribution board.

The battery is installed in the battery room in the HT switchgear room.  This provides a 220 V DC supply to the relay and control circuits in the Generator. Generator transformer and Unit transformer protection and control panels, 11KV  and 3.3 KV HT switchgear and 4.5 V LT switchgears Unit and station service boards and aux. Control panels. As told you earlier 220 volt DC system is backbone of the electrical system .To energise AC system you need DC system.

Battery Cells - Data Sheet ( Typical 500 MW power plant

            Guaranteed AH Capacity at 10 Hrs.discharge

            Rate 1 Volt / Cell at 27 C                                -           990 AH

            Designation                                                     -           KPH 990 P

            Recommended range of floating/trickle

            Charging voltage                                            -           1.4 -- 1.42 V/Cell

            Recommended range of boost charging

            Voltage                                                            -           1.53 -- 1.67 V/Cell

            Trickle charging current                                 -           1 to 2 MA /AH

            Boost charging current                                   -           198 A

             Open circuit voltage of cell when completely

            Discharged at 27 C at 5 hrs discharge rage   -            1.0 V

            Dimension (mm)                                           -            522 L x 192 W x 405 H

            Amount of specific gravity or electrolyte

            Per cell  required for first filling at 27 C         -          18 Lit / cell, 1.19 +- 0.01

            Specific gravity of cell when fully charged                1.19+- 0.01

            Max. electrolyte temp. that cell can withstand

            Continuously                                                  -           50 C

            Net dry cell weight                                        -           45 Kg

            With electrolyte                                             -           68 Kg

            Cell container material                                  -           Polypropylene

            No of cells / Bank                                         -           169

             Charger - I & II

            Input Voltage                                                  -           415 V+- 10%

            Output Voltage                                               -           220 V DC

            Float Voltage                                                   -           236 V DC

            Boost Voltage                                                  -           287.3 V  DC

            Rectifier current limit                                     -           400 A

            Battery current limit                                       -           198 A

            Auto charge over current                               -           40 A

            Steady state output voltage Regulation (For +- 10% I/P

            Voltage variation                                            -           +-1%

            0 - 100 % load variation

            at a time).

The above data was given from a data sheet of particular power station but this may vary marginally  power station to power station depending on system design and integration for which I am not expert.

Pre -commissioning  

- Battery,  battery charger and distribution boards are installed as per drawing.

- Cabling between the battery / battery charger and the distribution boards completed .

-     

      Basic testing .quality checks of components like insulation testing ,continuity testing , dimension checking etc are over. Charge the distribution board live at 220 V DC in  a safe and orderly manner without load and keep it charged for 24 hours.

- To carry tests and checks to ensure that all the components on the boards are operating satisfactorily including cables to all the remote points in the 400 KV substation and control room.

-  AC distribution board for input supply to be charged and commissioned.

-  Each module will be checked individually

-  Each outgoing supply will be tested up to the fuses at the remote end.

-  Battery E/F will be simulated for both the poles and E/F relay sensitivity will be

    checked.

-  Following  services will be  required during ,testing and pre –commissioning

 - 415 V,  3 Phases, 50 Hz supply ,Adequate running water, Suitable voltmeter, ammeter,

  

   thermometer and hydrometer,Variable resistance or water load for discharging the battery.

    Electrolyte for cells as per IS-266, Pure battery grade distilled water, Variable resistance box

    of 15 to 150 K Ohms, 1 No.  500 VDC megger, Multimeter , hand tools etc.

-Standard safety equipment for handling acid and battery .  Also fire prevention equipment like

extinguishsers , Exhaust fans to take out fumes if any , etc should be made available. Different

precautionary sign board in and around the panel battery room etc.

continued part -II battery charger commissioning.

ELECTRICAL SYSTEM 220 V DC SYSTEM FOR MAIN PLANT- ABC OF THERMAL POWER STATION

220 volt DC system

The 220V DC system supplies direct current as source of operating power for control, signaling, relays, tripping and closing of switchgears, emergency motors of most important auxiliary systems.

Under normal conditions of station generation, the storage battery units are kept floating in DC bus bars by means of the trickle chargers (also known as float chargers). The trickle chargers of each battery unit, which is a rectifier with AC input, is normally made to take all DC requirements of the power station without allowing the battery to discharge. This is achieved by maintaining the DC output voltage of trickle charger a few volts higher than the voltage of the battery.

A typical 500 MW Main DC Plant is described below. In a power plant Number of DC system are running parallel in different BOP packages like Air compressor ,CHP ,AHP etc. Low voltage DC system is also sunning as per requirement of the system like FSSS,DCS,ATRS etc. However we are describing only main plant. Switchgear DC system at switchyard also not considered.

Main Plant consists of:

i.                     Battery Bank

ii.                   Battery Charger - Main and Standby

iii.                  DC Distribution board.

The battery is installed in the battery room in the HT switchgear room.  This provides a 220 V DC supply to the relay and control circuits in the Generator. Generator transformer and Unit transformer protection and control panels, 11KV  and 3.3 KV HT switchgear and 4.5 V LT switchgears Unit and station service boards and aux. Control panels.

Battery Cells - Data Sheet

                Guaranteed AH Capacity at 10 Hrs.discharge

                Rate 1 Volt / Cell at 27 C                                                -              990 AH

                Designation                                                                     -              KPH 990 P

                Recommended range of floating/trickle

                Charging voltage                                                            -              1.4 -- 1.42 V/Cell

                Recommended range of boost charging

                Voltage                                                                            -              1.53 -- 1.67 V/Cell

                Trickle charging current                                                 -              1 to 2 MA /AH

                Boost charging current                                                   -              198 A

              Open circuit voltage of cell when completely

                Discharged at 27 C at 5 hrs discharge rage                    -              1.0 V

                Dimension (mm)                                                            -              522 L x 192 W x 405 H

                Amount of specific gravity or electrolyte

                Per cell  required for first filling at 27 C                     -                 18 Lit / cell, 1.19 +- 0.01

                Specific gravity of cell when fully charged                   -              1.19+- 0.01

                Max. electrolyte temp. that cell can withstand

                Continuously                                                                   -              50 C

                Net dry cell weight                                                          -              45 Kg

                With electrolyte                                                               -              68 Kg

                Cell container material                                                    -              Polypropylene

                No of cells / Bank                                                            -             169

   Charger - I & II

                Input Voltage                                                                   -              415 V+- 10%

                Output Voltage                                                                -              220 V DC

                Float Voltage                                                                   -              236 V DC

                Boost Voltage                                                                 -              287.3 V  DC

                Rectifier current limit                                                     -              400 A

                Battery current limit                                                       -              198 A

                                Auto charge over current                                             -              40 A

                Steady state output voltage Regulation (For +- 10% I/P

                Voltage variation                                                              -              +-1%

                0 - 100 % load variation

                at a time).

Above is standard specification of DC voltage system  followed by BHEL in 500 MW EPC contract. Cell voltage sometimes changed into 2 V +/- 5% instead of 1 volt .

Following installation checks to be carried out during erection of the DC system

1.  Room to be checked thoroughly about level , acid resistance tiles etc

2. Check the battery about damage , condition of battery i.e dry or wait.

3. If it is dry then prepare the electrolyte for filling.

4.Check all the cable connection from battery bank to DCDB

5. Check the installation of Battery charger and cabling.

6. Follow all safety precaution listed in safety manual

7. Use calibrated instruments  for measurements of parameters.

8. As we will be handling acid solution extra precaution is required for safe handling.

9. Check the ventilation system including exhaust fans are working properly.

Continued part –ii  for commissioning of battery charger

ELECTRICAL SYSTEM OF A THERMAL PLANT CONCLUDING PART- ABC OF THERMAL POWER PLANT.

     Earth pit checking

Each earth pit is checked as mentioned in the standard check list of 

Switch yard earthing system

Check that the depth of the earth pit is 3.5 Mtr in case of pipe 

electrode 3 Mtr incase of rod electrode.

Incase of pipe electrode, earth pit must be having the layers of sand, gravel, salt, and coal with the thickness  as mentioned in the separate detail.

Thickness of the gravel must be 150mm  and the gravel must be spread over the switch yard area.

Main earthing and subsidiary conductors are to be ensured that 40/50 mm dia rod is used for the same.

GENERAL CHECKS :

Minimum spacing between electrodes shall be 6 Mtrs.

Earthing conductor shall be laid at least 300 mm below cable trenches and under ground service ducts, pipes, tunnels, railway tracks etc., if crossed.

Risers of 40/50 mm  dia MS rod shall be brought nearby equipment foundations :  1.  2 Nos. for each equipment earthing.  2.  1 No. for each structural earthing.

Risers end near the foundation shall be brought 300 mm above the ground level near foundation.

Earthing conductors along their run on cable trench ladder columns, beams, walls, etc. shall be supported by suitable welding/cleat fixation at intervals of 750mm .

Galvanized iron sleeves shall be provided for the passage of the conductors where ever it passes through the walls. Both ends of the sleeves shall be sealed to prevent the passage of water through the sleeve.

Earthing conductor embedded in concrete shall have approximately 50 mm concrete cover.

Auxiliary mesh of 1500 x 1500 mm size shall be laid at spacing of 300 mm below operating boxes of circuit breakers, isolators-main switches and earth switches.

All joints  shall be welded as per the drawing and IS code.

LAs, CVTs, and Earth wire down conductor shall be connected to the main earth mat through rod electrodes, rod electrodes shall be made out of 40 dia MS rods at site.

Lightning Masts down conductor shall be connected to earth mat through 1 NO. pipe electrode.

Transformer neutral shall be connected to earth mat through 2  nos. pipe electrodes.

Earth mat shall extend 2 mtrs outside the switch yard fence through rod electrode.

All equipment shall be earthed to at least two points on opposite side which shall be connected to different conductors of the earth mat.

Testing of earth pits  :  Earth pit resistance is measured with earth megger and it should be in the range of 1 to 1.5 ohms. The continuity between two earth pits connected together is checked using a multimeter and it should show exact continuity.

Earth resistance of earth mat is measured and it should be in the range of  0 to 0.05 ohms.

Check overlapping of joints are welded with no gap between two joints.

The connection of earthing network is checked according to the layout drawing. 

We will discuss 220V DC System  as this system is required to start AC system breaker. Therefore  a little knowledge on DC system will help to understand the electrical system.

 continued

ELECTRICAL SYSTEM OF A THERMAL PLANT - ABC OF THERMAL POWER PLANT.

In my earlier post erection of heaters ,de aerators  were discussed . beside that in Turbine package there are other auxiliaries both static and rotary such as different coolers , CEP ( condensate extraction pump ) , vacuum pumps , CPU ( condensate polishing unit) etc. Except CEP erection of other auxiliaries need not require any special strategy. For CEP erection foundation checking and canister erection is slightly crtical. CEP is a vertical pump therefore canister is to be erected before erection of CEP. It is sometimes noticed that CEP foundation is full of water at bottom most area and prevent  canister erection . It is recommended to repair the foundation so that leakage can be stopped or draining the pit before erection.

It was decided that next topics will be steam blowing as we have covered all the major equipment . However i received mails from young engineers to cover electrical system erection and commissioning . I am an electrical engineer but did not work with the system. Therefore what i have seen at site is written down. Value addition is to be done by individual. 
Earthing system is the mother of electrical system. You can not run an electrical machine without earthing.

The  earthing system consists of  following. Earthing is the basic requirement of any  Good electrical system. This is essential to protect human lives and also help us to detect fault in the system . Good earthing  provides smooth electrical operation  and avoid spurious tripping.

i.              Earth Pit

ii.             Subsidiary conductors

iii.            GI Flats

1.            EARTH PIT :   Each earth pit must be made as per their design normally that will be either pipe or rod electrode type earth pits.  In pipe electrode earth pit the depth will be 3.5 Mtr . Where as in rod electrode the depth will be 3 Mtr only.  In pipe electrode,  the earth pit must be made with different layers of  sand, coal, salt  and gravel  whereas in rod electrode, the rod will be inserted into the earth soil. BTG area earth mat was laid after  doing the PCC of the area. Risers were raised along the column foundation from two sides. Rods are welded at least 250 mm overlap so that firm contact is made.  Special earth pits are made for electronic panels and skids which are separated from electrical earthing. 

2.            SUBSIDIARY CONDUCTORS :  The conductors connects between earth pit and various equipments are subsidiary conductors.  They are connected with main earth mat  which is  600 mm depth by means of   raiser  and will be connected to each earth pit through test links.  The other end of the earth pit will be connected to the equipment.

3.            GI FLATS :  All the test links and equipment connections of the switch yard are  done with GI flats of 75 x 12 mm .  Inside the control building, the earthing connection is provided by 50x6 mm GI flats. 

Testing

-             To test the individual earth pits of 400 KV switch yard

-              To test the continuity of the earth mat

-             To measure the resistance of  total earth mat.

Testing Equipment

-          Earth  Megger calibrated.

 -     Calibrated digital multimeter.

State  of the plant

Erection of 400 KV Switch yard earthing network completed.

Test links and connections to various equipments disconnected for test.

Clearance from erection department for carrying out various tests obtained.

continued part -ii

HEATERS, DEAERATORS , PLATE TYPE EXCHANGERS , COOLERS ETC PART II _ ABC OF THERMAL POWER PLANT

Part I  i have discussed about the heaters ,coolers heat exchangers etc. In this blog I will discuss about the erection of  

DE AERATOR .

On erection point of view erection of  De aerator needs detail planning and equipment for erection .De aerator consist of  de aerator and FEED STORAGE TANK. De aerator is in one assembled unit and FST consist of three units pre fabricated but 

not assembled. Once assembled it becomes a very big round shaped

drum/ tank .There are other accessories also like platform around de aerator , saddle at the bottom of FST as such FST rests on it , stand pipes for level measurement ,misc pipe connection, misc structural items etc. Two dished ends are welded with respective pieces. This De aerator need to be lifted on 37M floor CD bay structure.

Approach to CD can be achieved from end only. If there is one unit then approach any one end will serve the purpose. If there is two unit then approach will be required from both the sides. If there is 03 units the approach from middle is required.

Minimum crane capacity required is 250 MT with adequate length of boom. You may have to remove a few member of roof structure

to accommodate the boom length and the angle. 

If you are having luxury 600MT/1200 MT cranes then you place your crane away from CD bay end but still you may have to remove some of the  roof structure for lifting FST and De aerator.

But having 600MT/1200MT will cost you more.

A 5T electric winch and 50 M pair of rail CR100 will be required to lay on the floor for dragging of FST. One end of the FST is lifted first and placed on the rail then dragged it to its position . Middle piece and de aerators are trial assembled at he bottom and then dismantled .This is done to avoid any mismatching during lifting at a height.2nd piece then lifted and locked temporarily on the floor . De aerator is then lifted and placed on the 2nd piece and bolted securely. Assembly was dragged into position by winch. Then third pieces were lifted.  

Different stages of FST erection shown in the above pictures.

HEATERS, DEAERATORS , PLATE TYPE EXCHANGERS , COOLERS ETC_ ABC OF THERMAL POWER PLANT

I have detailed major equipment of Turbine Package and their method of erection generally followed in India. There may be some differences during execution due project specifics but base is the same.

Already I have discussed main turbine , Generator and MDBFP and TDBFP.

In this article I will cover HP Heaters , LP Heaters , Deaerator , Plate type Heat exchangers ,coolers etc.

Erection Method of static equipment.

< All coolers are erected as soon as foundation is available.

   Normal foundation checking i.e level ,co ordinate w.r.t. turbine axis is checked before placement . Coolers are placed after checking the orientation. 30 mm to 50 mm packer plates are required for placement and subsequent secondary Grouting. Front piping opens after that as one end is fixed now. Same method is also followed for other heat exchangers. All the coolers are coming AB bay structures i.e Turbine hall.

HP heaters and LP Heaters

< In earlier days and upto 500MW plant all the heaters are erected vertically with the help of the TG hall EOT crane. An erection device is supplied with HP heaters for erection as clearance between EOT crane hook bottom and 10 M Turbine floor is minimal.Therefore erection device is necessary. 

Higher capacity unit like 660 MW and above heaters are placed Horizontally at BC bay of 17 M floor. Counter flanges for the connection of pipes to be checked .

Structural members for vertical heaters erection are to be checked , Specially welding between the members. It has been experienced that some of the members are not welded . Those are to checked and welded before erection. Also Level and co ordination are to be  checked and mark it in nearest column. This will help during piping erection for drawing reference. Vertical heaters are erected by EOT crane with a few packer plate in the bottom .As no grouting is involved therefore leveling and alignment is done through packer plate of different thickness.Once alignment is over then the heat is locked with temporary structure and release for piping.

Heaters require to be erected horizontally requires pre planning . All the heaters are erected one by one ,one after another side by side . Therefore pre planning is required to push heaters one by one . Heaters floor is concrete floor therefore guide rail is a must. Sequential erection is must otherwise there will not be enough space on the floor for overtaking . Location of heaters on 17 M floor such that approach is not available by EOT Crane. Heaters are unloaded on 17M floor at designated place.Dragging on rollers are carried out. Heaters are heavy weight items dragging to be done through particular place otherwise it can damage the concrete floor.

Picture shows horizontal heaters and Plate type heat exchanger . Erected and piping under progress from one end. 

continued to Part II