ELECTRICAL SYSTEM (LT) INSTALLATION , PRE-COMMISSIONING , COMMISSIONING OF DIFFERENT LTMCC -ABC OF THERMAL POWER PLANT

Besides HT cubicles other common electrical items are transformer , LTMCC ( Low tension Motor eontrol center ) , Different Power distribution board coming under lighting packages etc. In this dispatch we will discuss only about LTMCC. There are numbers of LTMCC in a power station which can be classified as LTMCC or the name of the system like soot Blower MCC, Valves MCC etc. LTMCC is having operating voltage of  415 V.

To appreciate the construction feature of LTMCC one must study the IS ( Indian standard ) codes  for items . Few  of them is listed below for ready reference.

375         Specification for Marking & Gen Arrangement of Swgr, Busbars, Main Connection & Aux. Wiring

513         Cold Rolled Low Carbon Steel Sheet & Strips

722         Specification for A. C. Electricity Meters. (Integrity Meters)

1248       Specification for Direct Acting Electrical Instruments.

1822       AC Motor Starters of Voltages not exceeding 1000

1841       Specification for EC grade Aluminium Rod Produced By Rolling

1897       Specification for Copper Strips for Electrical Purposes

1901       Visual Indicator Lamps.

2147       Degree of Protection provided by Enclosure for Low Voltage Switchgear

2208       Specification for HRC Fuse Links up to 650 Volts

2516       A.C. Ckt Breaker Requirement for Voltages not Exceeding 1000 V (Part - I, Sec - I + Part - II, Sec-II)

2705       Specification for Current Transformers.

3231       Electrical Relays for Power System Protection.

3247       Switchgear General Requirements

3427       Metal Enclosed Switchgear & Control gear

4064       Specs for Heavy Duty Air Break Switches Fuses for Voltages not exceeding 1000 V.

4237       General Requirement for Switchgears & Control gear for Voltage not exceeding 1000 V

6875       Control Switches / Push Buttons

8623       Factory Built Assemblies of Switchgears & Control Gear.

8828       Miniature Circuit Breaker

13947    LV Switchgear and Control Gear.

General Information on LT MCC  ( only a few ones  to understand the construction features.)

-the enclosure shall provide a degree of protection of not less than IP-52. The height of all the panels shall not exceed more than 2200 mm & depth shall not be more than 500 mm for Single Front & 800 mm for Double Front, except for the Circuit Breaker panel, where the depth could be 1000 mm.

- CRSP (t=1.4mm and above) ,  angle , channels are used for cubicle construction.

-The Panel shall be fully compartmentalized with all doors on the front only.

  The panel should have cable entry point , Bus bar mounting system , shrouding of bus bar etc

- There will three live bus bar and one neutral bus bar running horizontally and vertically .

-Earth bus bar will run all around the panel . The size will be same as neutral . Earth will be ultimately connected with station earth at the end of the panel.

-Bus bars shall be of high conductivity Aluminum with current density of 800 A Inch² .

- As per requirement and to facilitate easy transportation panels are sent as per transportation size which are then assembled at site .

WARNING: MAJOR CONTRACTOR ( LIKE l&t ML12,10 ML 8 ETC) ARE HAVING MOVING CONTACTS WHICH CAN BE EASILY REMOVED WITH SPECIAL TOOLS . AT SITE THEFT OF MOVING CONTACT IS VERY COMMON THEREFORE YOU SHOULD BE VERY CAREFUL IN THIS ASPECT OTHERWISE DURING CHARGING YOU WILL FIND INSPITE OF ALL  REQUIRED CONDITIONS YOUR PANEL IS NOT GETTING CHARGED. MOVING COST MORE THAN 60% OF THE MAIN CONTRACTOR COST.

CONTINUED TO PART-II 

PRE CHARGING TEST AND COMMISSIONING OF HT SWITCH GEAR (TYPICAL) ( photoes) -ABC OF THERMAL POWER STATION

HT switchgear getting erected in the power plant.

A portion of single line diagram of HT line at thermal power station.

PRE CHARGING TEST AND COMMISSIONING OF HT SWITCH GEAR (TYPICAL) -ABC OF THERMAL POWER STATION

CONCLUDING PART

PRE – ENERGISING CHECKS & TESTS

-Inspect any physical damage in the busbar, insulation, panel doors, power & sliding contacts etc. and to be rectified if found. Special attention to be given on porcelain insulators holding the live line

-Making the panel inside dust free & free from foreign materials

-Checking the tightness alignment & clearance of bus bars ,smoothness in racking in & racking

out of modules, proper making of limit switch contacts(test and service), tightness of input power cables and outgoing  power & control internal wirings.

-Inspect all relays and make sure that any blocking used for shipping purpose is removed and the

armature  moves freely.

-Check all doors for proper mounting with respect to the switch handle.

-Check door interlock mechanism and connection.

-Test the pts,cts & relays involved. keep the recommended settings for the relays like over

current,  under voltage & overload protections, calibrate the meters (ammeters, voltmeters, power

meters & energy meters etc) and keep proper records for future reference.

-  Test the insulation of boards with 5000 v dc megger & the value should be as per the relevant

is specification (more than 50 mega ohms).

-Check closing & tripping interlocks ,overload relay setting in the panel.

-Measure busbar resistance , vcbs contact resistance (should be less than 50 milli ohms), closing

& tripping time if test report is not available at site.

-Check the protection circuit wiring connection by conducting primary injection current.

-Carry out high pot test for 75% of voltage mentioned in the test report for ht buses,100 % for ht

incoming power cables and find out the leakage current.

-Check  the input power cable terminations from transformer secondary

-Ensure  the phase sequence of incoming pt wirings and circuit closing of cts are proper before

charging the panel.

-Check closing & tripping mechanism operates smoothly both manually and electrically.

-Space heaters & thermostat shall be in good condition Space heaters to be ON a day before charging.

-Ensure all covers are provided and no conducting part is earthed. All the gland holes are covered

and temporarily sealed to minimize the short circuit of live terminal through rodents.

CHARGING PROCEDURE:

-Ensure tie breaker in associated station switchgear is in withdrawn position.

-Ensure all feeder breakers of unit  Swtichgear  are in withdrawn position.

-Check and record the final IR of supply cables for the DC control bus and the AC heater supplies :

-Energise both the supply cables for DC control bus and AC heater supplies.

-Put the tie breaker  in Test position.

-Put  all control fuses for the Tie breaker in position and switch on its control supply.

-Select LOCAL/REMOTE selector switch to local and close and trip the tie breaker once check  its operation.

-Check  tie isolator.  Interlock  in  closing circuit of tie breaker.

-Check that tripping relays for Tie breaker and main incomer breaker are not operated and are reset.

-Select Tie breaker Local / Remote switch to REMOTE.

-Give closing impulse to Tie breaker from UCB and check that the breaker closes.

-Short Terminals (tripping contact of E/F and O/C Relay) and check that tripping relay energises and tie breaker trips.

-Switch off the control supply and reset the tripping relay and remove the shorts  as detailed earlier.

-Put the incomer breaker  in Test position.

-Put control fuses of Incomer breaker in position and switch on control supply.

-Select Local/Remote switch of Incomer breaker to local and close and trip the incomer breaker once to check its operation.

-Short the terminals  in Incomer breaker cubicle to bypass the “Generator Breaker closed permissive”.

-Select Incomer breaker Local/Remote switch to Remote.

-Give closing command by control switch in UCB and check that the breaker closes.

-Switch on the control supply to tie breaker.

-Short terminals  in incomer breaker and check that :

a.            Incomer breaker trips

b.            Tie breaker closes automatically

-Reset all alarms and relays and remove shorts.

-Give trip command to incomer breaker control switch in UCB to bring switch position corresponding to breaker position.

-Put check/manual switch for incomer breaker in UCB in MANUAL position.

-Give closing command to incomer breaker and check  that :

a.            Incomer breaker closes.

b.            Tie breaker trips automatically.

-Reset all alarms and relays.

-Give trip command to tie breaker control switch in UCB to bring switch position corresponding to

breaker position.

-With incomer breaker ON and check/manual switch in manual position, give closing command to the tie breaker and check  that :

a.            Tie breaker closes;

b.            Incomer breaker trips.

-Reset all alarms and relays.

-Trip the Tie breaker.

-Switch off the control supplies to both incomer and tie breakers.

-Remove all test connections and check that any disconnections made for the purposes of testing have been correctly reinstated in tie and incomer panels.

-Keep  incomer breaker in withdrawn position (It will remain in withdrawn position till unit auxiliary

transformers are commissioned).

-Check ‘danger/warning notices’ are fixed at relevant points to ensure safety.

-Ensure effective communications between  UCB and switch board room are established.

-Check and record the final IR value of the following equipments :

a Isolator/Tie breaker of associated station switchgear.

b.Tie breaker of  Unit switchgear

c. Unit switchgear bus bar

d.Tie bus duct/cable megger used ___________ KV

-Ensure VT is  in service position with all its fuses put on.

-Check all the relay settings in TIE breaker panel are as per their protection schedule.

ENERGIZING PROCEDURE

-Put the Tie breaker of associated station switchgear in service position and close the tie breaker to charge the Tie cable.

-Observe the cable for any abnormality.

-Put the tie breaker in service position.

-Switch on the control supply to the tie breaker.

-Inform all concerned parties about the intention of charging the unit switch board.

-Put local/remote switch of tie breaker in REMOTE position.

-Give closing command from the UCB to tie breaker to charge the unit switch board.

-Check and record the unit bus bar voltage for all the three phases.

-Observe the unit switch board for any abnormality.

-Put the synch. Check/manual selector switch to CHECK.

-Check permissive contact in synchronising check relay closes.

-Check the phase sequence at  unit bus VT secondary terminals as detailed in following table.  (It shall be  same as that of associated station board).

MEASUREMENT IN VT CUBICLE

-Do the phasing out between VT secondaries of station and unit board.

-Keep the board charged for 24 Hours and  observe it for any abnormality.

-Inspect all equipment externally for  any sign of abnormality after being charged for 24 Hrs.

-It will  be the duty of the testing team leader to ensure that when the system is operating under load conditions that all equipment are externally inspected for any sign of stress or abnormality. 

Few photographs of drawing is given below

balance phographs in final part