7.6    CLASS-I POWER SUPPLY SYSTEM

7.6.1    BATTERY BANK :The batteries form part of the essential, uninterrupted class-I, 110V DC power

 supply system. When the AC supply is not available to the battery-charger through class III power supply,

the batteries will take over the function of feeding DC power to plant's critical loads like switch gear controls,

indications & annunciation and the emergency lighting along the evacuation routes in active areas. Two battery

banks, each of 110V (nominal), 425 AH, 56 cell YKP 35 Plante type, shall be installed in sub-station . The

advantage of Plante type batteries are:

a) Higher discharge voltage and discharge capacity.

b) Low gassing rates.

c) Long life.

d) Capacity for fast recharging without affecting life.

e) High performance (low internal resistance).

f) High energy output.

g) Low maintenance.

h) Compact and light weight.

i) High reliability for standby applications.

j) Lesser fall in capacity with time.

k) Continuous renewal of active material in service.

l) Minimum float currents & corrosion rate.

m) Higher scrap value.

n) Transparent SAN containers allow for visual monitoring & prompt remedial action.

Each battery bank shall be designed to supply the entire class I power requirement of 350 amps for a period of 30 minutes at high discharge rates without affecting the life of the batteries.  All the control equipment connected to the DC bus shall operate satisfactorily from 93V to 121V (85-110% of nominal voltage).  The trip coils of breakers of 33KV/433V switchgears shall operate

between 55V and 137V (50-125% of nominal voltage).  The batteries shall be capable of feeding DC power within the required voltage limits. The brief specification of battery bank system is given in Table-XI. During boost charging of a set of batteries through ‘Float cum Boost charger’, all the DC loads will be supplied through standby Float charger/ other ‘Float charger’. The brief specification of battery-charger system is given in Table-XII.

All such loads will be transferred to the other battery-charger / battery set in order to protect them from high boost charging voltage.  Sizing of the batteries shall be done as per IEEE-485.

7.6.2 OPERATING PHILOSOPHY OF CLASS-I POWER SYSTEM

7.6.2.1 During normal condition, 110V class-I loads of ROP facility will be fed through Battery charger.

7.6.2.2 In case of non-availability of class-III supply to Battery charger, class-I loads of facility shall be fed through 110V DC Battery Bank of ROP substation.

7.6.2.3 In case of failure of battery bank of a group, the loads of this group shall be fed by closing the bus coupler breaker of DC PDB of ROP substation.

7.7   DISTRIBUTION NETWORK

    This system shall deal with power, control wiring and earthing system network for entire plant. It is envisaged to upgrade the electrical control system by installing drawout type MCCs/ PCCs, FRLS cables and by segregating class-IV, class-III and control cables/ panels inside the plant for enhancing safety and reliability of electrical system

7.7.1  POWER DISTRIBUTION BOARDS (PDBs)  AND MOTOR CONTROL CENTRES 

          (MCCs)

NON DRAWOUT TYPE PANELS (GENERAL FEATURES)

The AC  motor  control centres  (MCCs)  and  power distribution  boards  (PDBs)  are  designed  for 415V, 3Phase, 4Wire, 50Hz supply.  The PDBs and the MCCs  are self standing cubicle type made of 2/2.5mm thick  sheet steel.  The boards are single front/ double front  type with  front doors having concealed hinges and  neoprene gaskets.   The cubicles are dust tight, vermin and insect proof IP51 enclosure with ventilation hood.  Six to eight cubicle module, stacked one over  the other, form  one vertical  panel.   Each module is an outgoing SFU or a starter or an incomer or a  buscoupler. The  aluminium  busbars are located in horizontal and vertical bus  bar chamber.  Proper insulation barriers have been provided  to eliminate all dangers of personnel contact with live bus.  The panels are provided with cable alley and  the cables enter the alley either from top or bottom of the alley  as specified.  Ground bus of adequate  size  has been  provided along the entire length of  the  boards.  Apart from the ground bus, a scrape earth has also been provided  for  drawout modules.  Two nos.  of  earthing   bolts  have been provided for connection  with  plant's ground  grid.   Space heater with thermostat  has  also been provided to prevent condensation inside the pan.

The lighting and power distribution boards are  non-drawout  type  located  in   various  electrical rooms  distributed  over  the  entire  plant.  

The MCCs shall conform to IS 13947 (Part 4 : Low voltage electromechanical contactors and motor starters. The various operational facilities provided on the starter modules are :

   a)   Local or remote control facility through  lockable local/remote selector switch.

   b)  Stopping facility at both remote and local station  irrespective of L/R selector switch position.

                                                           

c)      Interlock by-pass facility provided for pre-commissioning trial runs, should be dis-

 connected prior to  plant operation.

   d)  Overload   reset  facility  through  push   button without opening the module door.

   e) On/off and auto-trip indications are also provided on all starter module.

Protection scheme envisages following devices for all motors of rating above 25 kW:

Short circuit protection through fuse.

Overload protection through bi-metallic overload relay.

  

Overload protection for high inertia motors through saturable reactor core overload relays.

Single phasing protection through negative phase sequence single phasing preventer.

Protection against high winding temperature through thermisters and thermister relays.

DRAWOUT TYPE PANELS (GENERAL FEATURES)

Drawout  type  MCCs  have been  provided  for  all  utility,  air-conditioning  and  ventilation           equipment which  have  motors  of rating 15 KW  and  above.   For access   gallery motors where  the  level  of  radioactivity is high, the MCCs are installed in general corridors for ease of maintenance and  interchangeability.   All  squirrel  cage motors of fractional HP rating  to  90 kW rating  are fed through Direct-On-Line starters  except  some  90  KW exhaust system  motors  which    are    on class-III  power  supply. The  transformers  are so designed  that the system can take the  inrush  current load during starting of any motor without a dip in  the system  voltage.  The 90 KW exhaust system motors  have star  delta starters/ VVVF drives  as they are to  be  started  and operated  on  DG set when normal power  supply  is not available. Running hour meter shall be provided for  all the utility & services system motors. In drawout modules  the  outgoing  and incoming contacts  are drawout type  and  the  control  terminals  are sliding type.  The trolleys  have  three positions   i.e.,  "service",  "test"  and "isolate".  Outgoing  power  and control wires  are terminated  in  cable  alley  or  in marshalling  box.  The  power distribution  details of all major MCCs is listed in  Table XIII. Technical particulars of MCCs are listed in Table-XIV. High Inertia motors shall be provided with suitable protections to avoid operation of the overload relays during  its pro-longed starting time. Such motors may be provided with suitable pulleys to avoid long starting time. Microprocessor based motor protection relays are envisaged for motors of rating 90kW and above.

7.7.2 PROTECTION & PROTECTION COORDINATION

 The protection scheme and protection co-ordination for the entire electrical systems shall be provided as per IEEE-242, 1986 and current industrial practices. The following protections will be provided:

For 415V System

(a) Incoming ACB/MCCB feeders to MCCs

• All incoming ACB/MCCB feeders to MCCs shall be provided with Over current, Short circuit and Earth fault Releases.

·         All feeders from PCCs to MLDBs / PDBs and MCCs are provided with over load and short-circuit protections. Feeders to MCCs will be provided with Breaker with microprocessor/ thermo-magnetic based releases depending on their ratings.

(b)  Motor feeders

·      For protection of motor of rating below 25 kW, fuses and thermal bi-metal relay will be provided.

·      For protection of motor of rating between 25 kW and 90kW, fuses, thermal bi-metal relay, single phasing protection relay, thermistor relay, and space heater will be provided.

·      For protection of motor of rating ≥ 90 KW, fuses, motor protection relay (M.P.R.) and space heater shall be provided.

·      Type-2 coordination will be provided for all motor starters.

7.7.3  H.T./L.T. Cables

H.T./L.T. cables conforming to IS 7098/1554, ASTM-D 2843, 2863 and IEC-754(I) shall be used. EPR insulated, CSPE sheathed Radiation resistant cables shall be used for ROP cell cranes, cell lights/plug sockets, CCTV systems etc. The radiation resistant cable can withstand a cumulative dose of 100 MR.

415V SYSTEM

The cabling system is divided into two categories as follows:

            i) Cables connected through circuit breaker

            ii)Cables connected through fuse.

The cables protected by circuit breakers are designed on the basis of short circuit duty.  But the cables which are protected by current limiting HRC fuses, shall not be designed on the above basis as the fault cleaning time of HRC fuses is less than a cycle resulting in the limiting action of let through current.  These cables are sized only on the following considerations:

            i.          Continuous current carrying capacity considering 100% load factor.

            ii.         Derating factors has been considered as per IS 1961 and leading cable

                        manufacturers’ recommendations.

           

iii.        Maximum conductor temperature as per IS: 1961

iv.                Voltage drop in the cables between the transformer and switchgear and between switchgear and motor control centre is limited to 2% under full load  condition. The  voltage drop  in the cables between motor control centre and motor terminal is limited to 3% under full load conditions.

            Voltage dip during starting is limited to 15% at motor terminals.

(a)        110 V DC POWER CABLES

110 V DC power cables are on the normal current carrying capacity and voltage drop not exceeding 2.5% under normal conditions.  The feeders are protected by

fuses/MCCBs.

(b)        24 V/48 V DC POWER CABLES

24 V/48 V DC power cables are sized on the normal current carrying capacity and voltage drop not exceeding 2.5% under normal conditions.  The feeders are protected by fuses.

            (c)        240/110 V DC AND 110 V, 24/48 V DC CONTROL CABLES

                        (i)         P.T. circuits

Voltage transformer leads shall be checked for voltage drop which shall be limited to within 0.5% for commercial metering and 3% for other purposes.

                        (ii)        C.T. circuits

Voltage drop criterian not applicable however, suitable size should be selected so as to avoid high burden on CT secondary.

                        (iii)       Control circuits

                                    Voltage drop need not be considered.

(d)  FIRE RESISTANCE, LOW SMOKE (FRLS) ELECTRIC CABLES

SELECTION CRITERIA

These cables have improved performance in case of fire as follows:

i) Lesser smoke generation.

ii) Lesser acid gas generation.

iii) Higher critical temperature.

iv) Higher oxygen index.

v) Increased flame resistance.

GENERAL FEATURES OF FRLS CABLE

Polymeric materials like PVC, XLPE are often modified with suitable additives to make them flame retardant and to restrict the smoke generation from them during burning with flame or smouldering. The flame retardancy of a polymeric material is measured by determining oxygen index and temperature index of the material. By oxygen index it is meant that part i.e. percentage of the oxygen needed in the ambient atmosphere that will support combustion of the material.  Materials which ignite in air have oxygen, index below 21 (Normal atmospheric air contains 21% of oxygen).  Therefore, higher the oxygen index better is the flame resistance.  By temperature index it is meant at what ambient temperature, the material will ignite in air i.e. at oxygen concentration of 21 parts.

(e)  HALOGEN FREE FLAME RETARDANT (HFFR) WIRES

GENERAL FEATURES- All electrical wiring of ROP project shall be done by HFFR wires. HFFR is a Poly-olefinic compound with small amount of Al(OH)₃ HFFR wires have following advantages over FRLS wires.

(i) The smoke emitted is almost negligible.

(ii) The smoke is transparent and non-toxic.

(iii)Unlike PVC, the smoke emitted is non corrosive.

(iv)The insulation does not burn readily and does not melt and drip to spread fire.

(v)Absence of smoke, toxic fumes and uninterrupted power supply enable safe evacuation of trapped people and efficient fire fighting.

(vi)Critical Oxygen Index is around 35%.

(vii)Temperature index is more than 300⁰C.

(viii)Smoke density (light absorption) is less than 10%.

(ix)Acid gas generation is less than 5%.

(x) Flammability test as per IEC-332-1 and IS 694:1990.

(xi)Enviro Care Compounds A/S (ECC) confirms that HFFR is environmental friendly, in that they do not contain neither heavy metals nor halogens. HFFR is based on copolymers of

(xii)Polyolefins and have high loading of inorganic filler.

(xiii)Not UV degradable.

(xiv) Could be recycled as most of the thermoplastic polymers.

7.7.4  Cable Support System And Cable Routing

Cables are laid in galvanised steel cable trays. Ladder type cable trays are generally used for power cables and perforated type cable trays are used for control cables. All safety related power and control cable trays are covered with GI sheet.

Diverse routes and physical separation have been followed for safety related Group-A and Group-B

cable trays.

                

Cable trays of ladder and perforated types and associated accessories such as coupler plates, tees, elbow etc. will be fabricated from Hot rolled 14 gauge (2 mm thick) hot dip galvanised steel. Cable tray cover will be fabricated from 16 gauge perforated (1.6 mm thick) galvanised M.S. sheets.

Cable trays will be supplied in standard length of 2500 mm and clear inside width of trays will be as follows:

                 Perforated type trays           :           150, 300, 450 and 600 mm

                 Ladder type trays    :                       150, 300, 450, 600 and 750 mm

7.7.4.1 Cable trays are supported by steel support structure attached to the building at suitable locations.

7.7.4.2 Cable trays are installed so that at least 250mm vertical clearance is available between bottom of bottom tray and bottom of top tray. Cable tray containing class-IV power cables are placed at the top followed by safety related power cables and control cable tray at the bottom. Separation between non safety and safety related cable trays and also between redundant circuits of safety related systems is maintained by suitable separation distance, barriers etc.

7.7.4.3 Independence between safety related systems and non safety related systems

Safety related electrical systems are designed to perform during and after design basis event whereas non-safety related systems are not normally designed for this condition.  Hence failure of non-safety related systems should not cause failure of safety related systems.  Independence between safety related systems and non safety related systems is achieved by means of 

a)         Electrical isolation

b)         Physical separation

7.7.4.4 Independence between redundant safety related electrical systems

Safety related electrical systems are grouped into two redundant groups (Group-A and Group-B). Independence between these redundant groups is maintained so as to prevent simultaneous failure of both the groups due to common cause failure. Independence between redundant safety related systems is achieved by means of the following:

a)         Electrical isolation

b)                  Physical separation

7.7.4.5 Physical separation

Physical separation is provided for equipments and also for circuits. In areas where both safety related and non-safety related loads/systems are located, physical separation is required between them. The following measures are planned in such cases in the order of preference as indicated below:

a)             Physical separation is maintained by keeping minimum 900 mm horizontal distance, or

b)      Vertical clearance of 250 mm is maintained between ‘safety related cable trays’ and ‘non safety related cable trays’, or

c)     Safety related cable trays are routed in covered solid bottom trays below non safety related cable trays,  or

d)    Fire barriers having 3 hrs. rating are provided between safety and non safety related circuits, or

e)     Where separation distances/barriers mentioned above are not possible due to layout constraints, following measures are taken:

*      The non-safety related cable tray systems are considered as safety related and designed to the same standards as applicable for safety systems.

*      The cables used for those non-safety systems are designed, manufactured, inspected and tested to the same standards as those applicable for safety systems.

*      Safety related cable trays are covered and located below non-safety related cable trays.

Any short circuits/open circuits in the non-safety circuits due to failure of connected non-safety equipment may result in damages to these non-safety circuits.  However any such damage will be limited to non-safety areas only as fire barriers are provided between safety and non-safety areas.

In view of the above, it is considered that faults/failures originating from non-safety related systems close to safety system will not have adverse influence on safety related systems.

7.7.5 Fire proof Sealing of Cable penetration

Cables/ cable tray openings in walls and floors or through pipe sleeves from one area to another or one elevation to another will be sealed by a fire-proof sealing system. The fire-proof sealing system (FPSS) will effectively prevent the spread of fire from the flaming to non-flaming side, in the event of fire.

7.7.6 Fire Barriers/Fire Stops And Fire Breaks For Cables

7.7.6.1 Fire barriers/fire stops and fire breaks are provided along the run  of cables in cable              vaults/galleries, cable shafts, cable tunnels/trenches and cable trays to prevent spread of fire from one area to other and to contain the damage. Indian standard IS:12459 – 1788 code of Practice for fire safety in cable runs is the basis for the design of fire barriers and fire breaks.

7.7.6.2 Fire barriers/fire stops rated for 3 hour fire rating as per IS-12458-1988. Method of fire

resistance test of fire stops¢ are provided at the following locations.

a)         At the openings in walls when cables cross from one building to other, one redundant area to other, and openings in floor when cable trays cross from one floor to other

b)         Entry and exit of cable trenches for safety related equipment

c)         And where the physical separation between redundant safety related open type cable trays is less than acceptable value.

d)         At every 30 meters length in a cable trench/tunnel

e)         Fire barriers will be provided above panels/MCCs to segregate them from cable trays carrying redundant group of cables.

7.7.7 Mode of Laying

           

7.7.7.1 Cables outside areas of ROP sub-station will be directly buried.

7.7.7.2 Cables inside sub-station will be laid  in trenches or overhead cable trays.

7.7.7.3 Power cables will be laid in GI ladder type trays. Control cables will be laid in GI perforated trays.

             Cable trays will be supported from slabs/walls as found suitable. Vertical trays and all outdoor

             cable  trays will be provided with removable covers. Power and control cables will be laid in

             separate trays and distinct route for different class of supply cables will be followed to the extent

             possible.

7.7.7.4 Street lighting cables will be directly buried along the street with appropriate cable route marking.

7.7.8  EARTHING SYSTEM/NETWORK

 The basic objectives of an earthing system are :

a) ensure avoidance of personnel exposure to electric shocks in the plant premises.

b) provide adequate current carrying capacity, both in magnitude and duration, to accept

    ground fault current of the system.

c) facilitate the operation of protective relays and equipment by providing low resistance

    path to earth return currents.

The earthing system shall be designed as per IS-3043 (code of practice for earthing) and IEEE-80 for the electrical sub-station and plant areas. Earthing network of sub-station shall be suitably extended to the plant areas for equipment earthing. Stranded copper conductor shall be used as the main earthing material Two distinct paths of earthing shall be provided for all the equipments. The required cross-sectional area of earthing conductor has been calculated for copper for 0.5 second duration for a fault level of 50 kA at 415 volts.