7.4 CLASS -III POWER SUPPLY SYSTEM

7.4.1   415 V SWITCHGEARS

  

415 V class-III switchgear panel in sub-station shall comprise of  :

i)  2 Nos. of 415 V, 2500 A/46 MVA incoming air circuit breakers.

ii) 2 Nos. of 415V, 1600 A/46MVA incoming air circuit breakers.

iii)2 Nos. of 415V, 2500 A/46 MVA bus section air circuit breakers

iv)2 Nos. of 415 V, 1000 A/46 MVA spare air circuit breakers.

v) 28 Nos. of outgoing feeders comprising of ACBs and MCCBs.

vi) 2 Nos 630A  MCCBs for SFSF.

vii)2 Nos. 630A MCCBs for AWTF.

Technical specification of PCC (NPCC and EPCC) panels is enclosed vide Table-VIII.

7.4.2  DIESEL GENERATORS

Two numbers of diesel generator sets of capacity 1010kVA each conforming to ISO 3046 shall be installed in sub-station for providing 415V, 50Hz grounded AC power supply to the plant's safety related loads during normal power failure/ outage conditions.  The DG sets shall have the capability to start from cold condition, reach synchronous speed and be ready to take the load, without undue stress, in about 10 seconds from the initiation of start impulse.  The generators shall

be capable of accepting their maximum rated load in single or multi steps.  When the machine is running in no-load condition at the rated voltage and speed, the transient voltage drop at its terminals on starting the exhaust fan motor drawing a huge starting current at 0.3 power

factor shall not exceed 10% of the rated voltage. When the machine is running at 80% load at 0.8 power factor at the rated voltage and speed, the corresponding voltage drop on starting the same motor shall not exceed 15%.  The response of the regulator and excitation system shall be  such

that following a step  application or rejection of rated load at rated power factor and subsequent to the drop or alternator rise of sub-transient voltage, the generator will recover its set voltage within 0.5 second.  The alternator shall be capable of taking momentary overload.  Rating of the two DG sets shall be so chosen that the entire emergency load of the plant can be continuously fed even if one of the sets fails.

The control panel design shall include microprocessor based fast and reliable control system.  The DG sets shall be provided with an independent cooling tower.  Each DG set shall also be provided with an independent day tank of 2500 ltr. capacity for storing the fuel. On consideration of fire safety, these tanks shall be  located outside of the room in which the DG sets are installed. In

addition to the local storage of fuel, supply of fuel for 7-day continuous operation of one DG set, from the bulk storage facility shall also be made available.

In the event of normal power failure, the DG set will start automatically.  The entire essential base loads like plant lighting, health & safety instruments, plant communication equipments etc., shall be fed within 30 seconds.  The design envisages manual connection of bulk loads like exhaust and off-gas fans one by one in a pre-set sequence to protect the generator from the effects of surge currents arising immediately after connecting a bulk load.

Technical specification of DG set is enclosed vide Table-IX.

The Safety related power supply system is categorized into two groups, Group-A and Group-B. Power Supply System belonging to Group A Safety Related System and Group B Safety Related System are identified in the following table:

Group-A Safety Related Systems	Group-B Safety Related Systems
CLASS-III EPCC PANEL (GROUP-A) MCC –1 (E2 FAN)	CLASS-III EPCC PANEL (GROUP-B) MCC –2 (E2 FAN)
MCC –1 (OFF GAS FAN)	MCC –2 (OFF GAS FAN)
MCC-E (FHA) MCC  (E- COMPRESSOR)
HIGH BLOCK-MLDB-E & 6 NOs. LIGHTING PDBs
UPS SET – 1	UPS SET – 2
UPS DB- 1	UPS DB – 2
DC SWITCHBOARD-1 1 No. LIGHTING PDB	DC SWITCHBOARD-2

7.4.3 OPERATING PHILOSOPHY OF CLASS-III POWER SYSTEM

7.4.3.1 In case of loss of power supply on one bus-section of EPCC (Emergency Power Control Centre), power supply to the affected bus-section shall be restored by starting the DG set of that section automatically/ manually.

7.4.3.2 In case of failure of incomer breaker of an MCC or the cable incomer of MCC or upstream breaker of corresponding EPCC, standby motor shall be started from the corresponding group of MCC.

7.4.3.3 In case of failure of any motor-starter or the outgoing feeder of an MCC, standby motor shall be started from the corresponding group of MCC.

7.5    CLASS-II POWER SYSTEM

Two nos., 230V, single phase, 50 KVA each UPS systems (UPS A & UPS B) are provided for class-II loads of ROP project. Both UPS systems are seismically qualified. The class-II load includes control & instrumentation loads, nuclear instruments, Public address system and control room lighting. Maximum demand of all Class-II loads connected to the UPS is 50 KVA. The class II system has 100 percent standby capacity, as each UPS unit is adequate to cater to entire class –II i.e 50 KVA load. Uninterruptible Power Supply is On-line, Three Phase Input, Single Phase Output, IGBT based System. The UPS maintains continuity of electrical power within the defined limits (5 milli second) on failure of AC source. The system consists of a solid-state converter (rectifier) with Battery Charger; PWM IGBT based inverter, 30 nos of SMF batteries block of 12V each providing 60 minutes back up, Static Bypass switches (2 nos in each system) and manual maintenance switch. Technical specification of UPS system is enclosed vide Table-X.

The scheme is shown in the drawing A3/NRG (P)/E&I/ESS/06/01-R0 (CLASS II POWER SUPPLY SCHEME) enclosed. There are three nos of class-II switchgear panels viz IPDB-1, IPDB-2 and PDB. UPS A feeds IPDB-1 and IPDB-1 in turn feeds IPDB-2. UPS B feeds PDB. The UPS system A is backed up by UPS B through static bypass switch (VTT2A) and UPS B is backed up by its bypass line through static bypass switch (VTT2B). These switches operate automatically in order to maintain the continuity of the power supply. The loads of PDB connected to UPS B are those, which are not affected even when inverter of UPS B fails, as these loads have built in batteries as a back up source of power.

GENERAL DESCRIPTION:

 1. UPS system -A

It feeds I&C system, emergency control panel, nuclear instruments, PA system etc. The system is installed in UPS room along with power distribution board (IPDB-1 & IPDB-2) at third floor, high block building of ROP.

2.  UPS system -B

It feeds loads like fire alarm system, Tel exchange, Control room lighting, High block lighting,  etc.  The system is installed in the UPS room at third floor, high block building of ROP.

SYSTEM DESCRIPTION:

Each UPS system consists of following major components:

A) 12 Pulse Rectifier with buck boost converter.

B) PWM Inverter utilizing IGBT as switching device

C) Electronic Static Bypass Switch.

D) Manual Maintenance Bypass Switch.

E) Microprocessor based Control and Data Monitoring System.

FUNCTIONAL DESCRIPTION:

The UPS supplies stable and conditioned AC power to the class II loads. The Rectifier converts AC Input voltage to DC voltage. This DC voltage is used for inverter input as well as it is fed to DC-to-DC converter (IGBT based float cum boost Charger) for charging the battery bank. With the inbuilt charger control circuit, the batteries are always maintained in fully charged  condition.

When the AC input voltage drops below the specified limits (+10%) or in case of power failure, the Inverter will continue to supply power to the equipment load up to one hour utilizing the battery as power source. The continuity of power to safety related loads can further be maintained through static bypass in case any UPS fails / trips. The transfer of loads from UPS A to UPS B takes place without any interruptions.  Loads connected to UPS B are automatically transferred through static bypass switch to isolation transformer, in case of failure of UPS B. Provision for manual by pass (MB) operation also exists for facilitating maintenance of each UPS by transferring the load to the by pass line.

OPERATING PHILOSOPHY:

UPS – A feeds IPDB-1 & IPDB-2 and UPS – B feeds PDB loads. The inverter output of UPS – A is connected to the inverter output of UPS-B through static switch (STATIC BYPASS SWITCH VTT2A). Inverter output of UPS – B has its own bypass line connected through static switch (Static Bypass Switch VTT2B) to class III power through transformer. There is also class III bypass line through 3 phase to single phase transformer connected to UPS A through Manual Bypass Switch (MBS).

The scheme is shown in the drawing A3/NRG (P)/E&I/ESS/06/01-R0 (CLASS II POWER SUPPLY SCHEME) enclosed annexure I. The operating philosophy of UPS-A & UPS-B systems under different conditions is described below.

1.      Normal operation:

              UPS -A feeds IPDB-1 & IPDB-2 and UPS – B feeds PDB. Circuit breakers CB 1, CB2, CB3, CB-4, CB5, CB-6, static Bypass Switch (VTT1A), static Bypass Switch (VTT1B), switch   (S8A & S8B ) and out put circuit breaker (Q3A & Q3B) remain closed. Static Bypass Switch (VTT2A), Static Bypass Switch (VTT2B), CB7 and CB8 remain opened. MBS shall be at X position.  Circuit breakers (Q21A & Q21B) are kept closed to keep 3Ø to1Ø transformer warm. MBS will be operated under administrative control. Audio – Visual annunciators are provided for showing ON and OFF status of UPS A and UPS B in ROP main control room.

2.      Rectifier failure :

            If rectifier section of UPS A fails, UPS-A feeds IPDB-1 & IPDB-2 through its battery & inverter. Subsequently batteries start discharging and on “LOW BATTERY” (87.5 % of rated voltage) signal, automatically static Bypass Switch (VTT1A) gets open and static Bypass Switch (VTT2A) gets closed within 5 milli seconds. UPS – B starts feeding load of UPS -A in addition to PDB. The rectifier failure is displayed on LCD through 4 lines of 16 characters along with audio – visual annunciation provided in the mimic of UPS Panel (inverter cubicle). The same annunciation for failure is also duplicated in ROP substation through substation PC. Circuit breakers positions remain same as described under point no 1.

            If rectifier section of UPS B fails, their batteries of UPS-B feed PDB through its inverter. Subsequently batteries start discharging and on “LOW BATTERY” (87.5 % of rated voltage) signal, static bypass switch (VTT1B) gets open and static bypass switch (VTT2B) gets closed automatically within 5 milli seconds. The PDB is fed through the bypass line of UPS –B. The rectifier failure is displayed on LCD through 4 lines of 16 characters along with audio – visual annunciation provided in the mimic of UPS Panel (inverter cubicle). The same annunciation for failure is also duplicated in ROP substation through substation PC. Circuit breakers positions remain same as described in point no 1.

            If rectifier sections of UPS A and UPS B fail simultaneously then automatically loads will get transferred to their respective batteries. Subsequently batteries of UPS A and

            UPS B will start discharging. On “LOW BATTERY” (87.5 % of rated voltage) signal of UPS A, the corresponding loads will be transferred to UPS –B. On “LOW BATTERY” (87.5 % of rated voltage) signal of UPS B, the loads will be transferred to bypass line of UPS B.

The corresponding loads can also be fed through class III bypass line under administrative control through Manual Bypass Switch (MBS) , if felt necessary.

3.      Inverter failure :

            If Inverter of UPS A fails and Inverter of UPS B is healthy, automatically Static bypass switch (VTT1A) gets opened and Static bypass switch (VTT2A) gets closed within 5 milli seconds. UPS – B starts feeding load of UPS -A in addition to PDB. The inverter failure is displayed on LCD through 4 lines of 16 characters along with audio – visual annunciation provided in the mimic of UPS Panel (inverter cubicle). The same annunciation for failure is also duplicated in ROP substation through substation PC. Circuit breakers positions remain same as described under point no 1.

            If Inverter of UPS B fails, automatically static bypass switch (VTT1B) gets opened and static bypass switch (VTT2B) gets closed within 5 milli seconds. The PDB is fed through the bypass line UPS –B. The inverter failure is displayed on LCD through 4 lines of 16 characters along with audio – visual annunciation provided in the mimic of UPS Panel (inverter cubicle). The same annunciation for failure is also duplicated in ROP substation through substation PC. Circuit breakers positions remain same as described under point no 1. 

            If inverter sections of UPS A and UPS B fail simultaneously, the loads will be transferred to bypass line of UPS B automatically.

The corresponding loads can also be fed through class III bypass lines under administrative control through Manual Bypass Switch (MBS) if felt necessary. 

4.      Failure of battery or Battery circuit breaker of UPS :

In such cases, loads of UPS A and loads of UPS B will continue to be fed by input class – III power supply through rectifier and inverter. In this case if class III power or rectifier or inverter of corresponding UPS A fails, then the loads gets transferred to UPS B automatically within 5 milli second.  If the circuit breaker Q1B or rectifier or inverter of corresponding UPS B fails, then the loads gets transferred to bypass line of UPS B automatically within 5 milli second as mentioned in point no 2 and 3. The failure of battery or battery circuit breaker is displayed on LCD through 4 lines of 16 characters along with audio – visual annunciation provided in the mimic of UPS Panel (inverter cubicle). The same annunciation for failure is also duplicated in ROP substation through substation PC. 

5.      UPS –A is under maintenance and UPS – B is feeding  loads of UPS A 

When the UPS A is taken under maintenance, circuit breaker CB-1 is opened manually. The mean time to repair (MTTR) is 8 hours. Under this scenario, if

            Rectifier section of UPS B fails, and then Battery – B will feed loads automatically. Since battery back up is for one hour, all-necessary plant shut down operations related to load of UPS - A and UPS - B will be carried out within this time. Subsequently, the entire loads get transferred to by pass lines of UPS B automatically.

            Inverter section of UPS B fails, automatically Static Bypass Switch (VTT1B) gets opened and Static Bypass Switch (VTT2B) gets closed within 5 milli seconds. The entire load is fed through the bypass line UPS –B.

            Above 5.1 and 5.2 conditions are considered as total class –II power failure and plant    operations are to be stopped.

6.      UPS –B is under maintenance and UPS – A is feeding  IPDB-1 & IPDB –2 and load of UPS B is fed through the bypass line of UPS B

Under this scenario, If

            Rectifier section of UPS A fails, then automatically its battery feeds load. Since battery back up is for one hour, all-necessary plant shut down operations related to load of UPS A and B will be carried out within this time. Subsequently, the entire loads get transferred to by pass lines of UPS B automatically.

            Inverter section of UPS A fails, automatically the load of UPS A is fed through the bypass line UPS –B.

By pass lines of UPS A can be energised by closing Manual Bypass Switch (MBS) under administrative control, if felt necessary.

Above condition is considered as total class –II  power failure and plant operations are to be stopped.

7. Class-II bus fault on anyone of class – II distribution (IPDB’s) panel:

     When there is a fault (short circuit) on any bus, the outgoing breaker CB-3, CB-4 of corresponding UPS (A or B) trip on over current. Failure is displayed on LCD through 4 lines of 16 characters along with audio – visual annunciation provided in the mimic of UPS Panel (inverter cubicle). The same annunciation for failure is also duplicated in ROP substation through substation PC. Then power supply to the respective loads connected to the affected bus will not be available. Under this condition all process transfers will get automatically stopped, all pumps will stop, solenoid valves will get de-energized/ closed and plants will go under safe shut down state as analyzed under black out condition. Fire Alarm System, Telephone exchange has their own batteries as a back up source of power for 4 hours.

      

8    Failure of class-III power supply

8.1  Failure of DG-1

     On the failure of DG-1, UPS-A feed IPDB-1 & IPDB-2 through its battery & inverter.  Subsequently batteries start discharging and on “LOW BATTERY” (87.5 % of rated voltage) signal, automatically static Bypass Switch (VTT1A) gets opened and Static Bypass Switch (VTT2A) gets closed within 5 milli seconds UPS- B starts feeding load of UPS- A in addition to PDB. The Incomer of the UPS A failure is displayed on LCD through 4 lines of 16 characters along with audio – visual annunciation provided in the mimic of UPS Panel (inverter cubicle). The same annunciation for failure is also duplicated in ROP substation through substation PC.

8.2  Failure of DG-2

     On the failure of DG-2 the batteries of UPS-B feed PDB through its inverter. Subsequently batteries start discharging and since battery back up is 1 hour, so by opening CB-4, CB5 and by closing CB-7, CB-8 manually, PDB is fed by UPS-A through tie line between IPDB-1 and IPDB-2. This will lead to temporary interruption of UPS power supply to the loads connected to PDB, including Fire Alarm System and Telephone Exchange. The FAS has a built in battery having capacity of operating upto 4 hours, hence the interruption of UPS supply will not affect its performance. Also an independent Public address system is provided for the plant so short interruption does not affect the performance of the system. The Incomer of the UPS B failure is displayed on LCD through 4 lines of 16 characters along with audio – visual annunciation provided in the mimic of UPS Panel (inverter cubicle). The same annunciation for failure is also duplicated in ROP substation through substation PC.

9        Failure of input circuit breakers

9.1 Failure of CB-1 of UPS A :

     On the failure of CB-1, UPS-A feed IPDB-1 & IPDB-2 through its battery & inverter.  Subsequently batteries start discharging and on “LOW BATTERY” (87.5 % of rated voltage) signal, automatically static Bypass Switch (VTT1A) gets opened and Static Bypass Switch (VTT2A) gets closed within 5 milli seconds UPS- B starts feeding load of UPS- A in addition to PDB. The CB1 failure is displayed on LCD through 4 lines of 16 characters along with audio – visual annunciation provided in the mimic of UPS Panel (inverter cubicle). The same annunciation for failure is also duplicated in ROP substation through substation PC.

9.2 Failure of CB-2 of UPS B :

     On the failure of CB-2 the batteries of UPS-B feed PDB through its inverter. Subsequently batteries start discharging and since battery back up is 1 hour, so by opening CB-4, CB5 manually and by closing CB7 & CB8,  PDB is fed by UPS-A through tie line between IPDB-1 and IPDB-2.

     Failure of CB-2 is displayed on LCD through 4 lines of 16 characters along with audio – visual annunciation provided in the mimic of UPS Panel (inverter cubicle). The same annunciation for failure is also duplicated in ROP substation through substation PC. This condition is similar to condition 8.2 as mentioned above and it will be applicable.

10    Failure of input supply to both UPS A & B simultaneously:

     On the simultaneous failure of incoming supply to both UPS, both UPS will work on their respective battery for 1 hour and plant has to be shut down in the planned manner. In this condition both UPS will be out of synchronism. On the failure of inverter of UPS A, loads get transferred to healthy inverter of UPS B with a break of about 5 m sec.