Internship PEL

1Muhammad Rizwan

BS Electrical EngineeringPIEAS

SWITCHGEARThe importance of electric supply in everyday life has reached such a stage that it is

desirable to protect the power system from harm during fault condition and to ensure maximum continuity supply. For this purpose, means must be provided to switch on or off generators, transmission lines, distributers and other equipment under both normal and abnormal conditions. This is achieved by apparatus called switchgear. A switchgear is essentially consists of switching and protecting devices such as switches, fuses, circuit breakers, relays, instrument transformers, ammeters, voltmeters, pushbuttons etc.

In normal operation, switchgear permits to switch on and off generators, transmission lines, distributors and other electrical equipment. On the other hand, when a failure (e.g. short circuit) occurs on any part of power system, heavy current flows through the equipment, threatening damage to the equipment and interruption of service to the customers. However the switchgear detects the fault and disconnects the unhealthy part from the system. In this way switchgear protects the system from damage and ensures the continuity of supply.

The apparatus used for switching, controlling and protecting circuits and equipment is known as switchgear.

Switchgear panels are classified into three different categories depending upon the voltage they handle.

Panels which are used at or below 1,000 volts are called LT panels

Panels which can handle voltage from 1,000 to 36,000 volt are classified as MT panels.

Panels which are operated at or above 36,000 volts are called HT panels.

PEL is fabricating MV/ MT (medium voltage or medium tension) and LV/LT (low voltage or low tension) switchgears which are totally enclosed in steel sheet housing, floor mounting type and vermin and dust proof. PEL Switchgears are designed by highly qualified engineers according to the standards of IEC, BSS, VDE and SGS and are tested at each step of fabrication.

Each switchgear panel consists of steel sheet housing and electrical circuitry. Steel sheet housing is fabricated at fabrication and paint workshop and electrical circuitry is fabricated at LV/LT or HV/HT workshop accordingly.

Fabrication and Paint WorkshopCubical housing of switchgear is fabricated from 2-3 mm steel sheet at fabrication and

paint workshop in following steps

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Pressing Cutting Punching Bending Painting

PressingLarge pieces of 2-3mm thick steel sheet are received in rolled form. First of all this

steel sheet is unrolled and passed through the press. This process straightens the sheet and removes dents.

CuttingSteel sheet is then cut into small pieces of desirable size according to the design

specifications for body, doors and partitions.

PunchingHoles and windows are punched according to the design specifications at CNC

punching machine.

BendingNC pressing are bending machine is used to give the desired shape to the sheet

pieces.

PaintingPEL is using Powder painting technique. Powder coating has today established itself

as the most effective means of metal surface finishing. It has replaced liquid painting and various methods of electroplating due to the many advantages this process as given below

Substantial improvement to chemical corrosion Substantially improved resistance to mechanical abuse – better scratch resistance,

abrasion resistance and mechanical wear and tear Cleaner process for improved environment Faster process for better productivity through the paint-shop Requires less skills for high quality coatings Guarantees uniform and consistent results Substantially improved aesthetics for higher sales value of the finished product Requires lower energy per meter square of surface coated Requires lesser care in storing and packing due to the toughness of the coat

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Procedure of Powder paintingPowder painting is carried out in following steps as shown in block diagram.

PretreatmentPretreatment removes foreign matter including soil, welding splatter, scale, grease

and oil etc. from the surface, conditions the surface for optimum adhesion of the coating

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Pretreatment

Dry-off

Cooling

Powder Coating

Curing

Cooling

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film to the surface, obtains uniformity throughout the entire treated surface and makes it suitable for optimal quality coating.

First of all sheet is dipped in Sodium Hydroxide solution for 15-20 minutes at room temperature which removes grease, oil and dust from the surface of article.

Rising at room temperature removes the alkali from the surface of metal. Then article is dipped in Hydrochloric Acid solution for 20-25 minutes at room

temperature which removes rust from the surface of article. Rinsing at room temperature removes acid from the surface of article. Then article is dipped in Phosphoric Acid solution at 70-80℃ for 15-20

minutes. This conditions the surface of article for optimum adhesion of the coating film to the surface of article and also avoids rusting of the surface.

Then article is rinsed with water at 70-80℃ because rinsing with cold water causes rusting.

Then metal sheet is dried and cooled.

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NaOH solution

(For 15-20 minutes)

Rinsing

HCl solution

(For 20-25 minutes)

Rinsing

H 3PO 4 solution

(For 15-20 min. @ 70-80℃)

Rinsing (with hot water)

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Powder CoatingAfter pretreatment articles are loaded to a frame and are taken to the sprat booth

where articles are grounded and desired powder paint is sprayed by using an electrostatic gun, or corona gun (electrostatic spray unit). The gun imparts a negative electric charge to the powder, then sprayed on the grounded object by compressed air and then accelerated toward the work piece by the powerful electrostatic charge this results in uniform and even distribution of powder on the surface. Powder paint booth is used to conserve the powder paint.

CuringThen articles are moved to the oven where are heated at 200℃ for 15 minutes.

When a thermoset powder is exposed to elevated temperature, it begins to melt, flows out, and then chemically reacts to form a higher molecular weight polymer in a network-like structure (because powder is thermoplastic or thermoset polymers). This cure process is called crosslinking. The curing schedule could vary according to the manufacturer's specifications.

CoolingThen the articles are cooled down to make tough shiny layer of powder on the

surface. After that steel sheets are sent to the corresponding workshops.

Low Tension (LT) WorkshopPEL is fabricating floor mounted metal enclosed air insulated LT switchgear panels at

LT workshop for various industries inside and outside the Pakistan. LT panels are classified into incoming and outgoing panels according to their location in the distribution system. First of all Plates from Fabrication and paint shop are bolted to form housing of the panels(housing of different panels are of different size),after that bus bars are installed and other electrical equipment are installed step by step and are checked by quality control department at each step.

LT panels usually contain following components

1. Bus bars: for connecting PFI panel in the circuit. The dimensions of copper alloy bus bar are selected according to the current rating. Insulation (PVC sleeve) and gap between bus bars is selected according to the voltage level. According to IEC for LT phase to phase air break down distance is 25mm.

2. Insulators: to insulate bus bars from the earthed housing of panel.3. Microcontroller base control unit: to automatically control specific function

according to the panel.4. Relays: to detect, identify and initiate protective measures e.g. short circuit relay,

earth fault relay, thermal overload relay, under voltage relay, over voltage relay, DC supervision relays, auxiliary relays etc.

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5. Circuit breaker: to disconnect the panel from main power supply in case of fault in LT ACBs (for large current), MCCBs (for medium current) and MCBs (for small currents) are used.

6. Timer: to set the delay time for on and off of generator or other circuit.7. Magnetic contactors: to control the contacts of high voltage circuit by low voltage

signals.8. Current transformer: to step down high currents to a level suitable for measurement

with ammeter.9. Meters: to measure different electrical quantities e.g. ammeter, voltmeter, power

factor meter and energy meter.10. Selector Switches: to select one function from multiple options.11. Pushbuttons: to turn on or turn off particular features of panel.12. LED indicators: for indication of state, functioning or malfunctioning of components

of panel or circuit.13. Terminal blocks: for multiple connections of controlling and metering devices.14. Connecting wires and cables: for connections. Wires and cables are selected

according to the current requirement.

All of the above mentioned components or some of them are present in a panel. Also additional components are added according to the functionality required by the customer.

PEL is fabricating following types of LT switchgear panels

1. Distribution Panels2. Power Factor Improvement Panels3. Motor Control Centre

a. Auto Star Delta Panelsb. Direct on line Panels

4. Auto Transfer Switch5. Relay and control panels for 132/ 220KV Grid Stations6. AC/DC Auxiliary Services Panels7. Piano type control desks8. Distribution boards for power as well as for lighting9. Feeder Pillars

Distribution PanelsLT Distribution Panels are utilized to dispense dc current from one power supply to

other electrical devices. The foremost function of distribution panels is to monitor and prevent any possible damage to dc systems. The major function of these electricity distribution panels is to monitor the amount of electricity being transferred to the specific area in any industry.

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Power Factor Improvement PanelsAs we know that inductive or capacitive load in the circuit introduce phase difference

between voltage and current which introduce reactive power and increases apparent power in the circuit resulting in increase in copper losses in the circuit. So it is desirable to reduce this phase difference between voltage and current. Since inductance dominates the capacitance in most loads so this causes voltage to lead current in the circuit and results in decrease of power factor. If the power factor falls below the specified tolerable value, power supply company charges penalty to the consumer. That is why it is desirable to keep the power factor equal to or neat to 1 for this we need a capacitor bank so that we adjust capacitance in the circuit according to our requirement and this is achieved by power factor improvement panel.

Power factor improvement panel contain following components

1. Insulated bus bars2. Insulators3. Relays: auxiliary relays4. Microcontroller based control unit5. Circuit breaker: air circuit breaker (ACB)6. Magnetic contactors7. Meters: voltmeter, ammeter and power factor meter8. Selector Switches9. Pushbuttons: to select required capacitance 10. LED indicators11. Terminal blocks

Motor Control CentreAs we know that in start a motor draws 6 to 7 time more current than it rated value.

This high value of current can damage the motor winding. Hence we use motor control (starter switch) which limits the starting current to protect the winding of motor. A motor controller is a device or group of devices that serves to govern in some predetermined manner the performance of an electric motor. A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and faults.

Motor control centres are classified into following two classes

a. Direct on line (DOL) starter panels (for small motors with power rating less than 37 kW)

b. Auto Star Delta (ASD) starter panels (for large motors with power rating greater than37kW)

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a. Direct On Line (DOL) Starter PanelsA direct on line (DOL) or across the line starter applies the full line voltage to the

motor terminals. This is the simplest type of motor starter. A DOL starter can be used if the high inrush current of the motor does not cause excessive voltage drop in the supply circuit. The maximum size of a motor allowed on a DOL starter may be limited by the supply utility for this reason. DOL starting is sometimes used to start small water pumps, compressors, fans and conveyor belts.

DOL motor control panels contain following components

1. Insulated bus bars2. Insulators3. Relays: auxiliary relays4. Circuit breaker: Motor control circuit breaker5. Magnetic contactors: for larger motors magnetic contactors are used (small motors

are operated manually) 6. Current transformer: to step down high currents to a level suitable for ammeter.7. Meters: voltmeter, ammeter and power factor meter8. Selector Switches: Selector switch is used meters to measure different quantities like

current and voltage9. Pushbuttons: to turn on and off the DOL starter panel10. LED indicators11. Terminal blocks

b. Auto Star Delta (ASD) starter PanelsIn the case of an asynchronous motor, such as the 3-phase squirrel-cage motor, the

motor will draw a high starting current until it has run up to full speed. This starting current is typically 6-7 times greater than the full load current. To reduce the inrush current, larger motors will have reduced-voltage starters or variable speed drives in order to minimize voltage dips to the power supply. For this first we operate motor in star or Y- configuration this results in less voltage across the winding of motor and this implies less current through the windings of the motor when the motor attain certain speed connection are changed from star to delta this applies maximum rated three phase voltage across the terminals of motor hence protects the motor windings form damage due to high current at the start. ASD starter panel contains following components

1. Insulated bus bars2. Insulators3. Relays: auxiliary relays, thermal protection relay, short circuit protection relay and

under voltage protection relay 4. Circuit breaker: air circuit breaker (ACB), molded case circuit breaker(MCCB)

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5. Magnetic contactors: for control the motor with the help of low voltage signals6. Timer: to select the delay time for shifting from one mode to another.7. Current transformer: to step down high currents to a level suitable for ammeter.8. Meters: voltmeter, ammeter and power factor meter9. Selector Switches: to select the mode of measuring meter10. Pushbuttons: to turn on and off the panel and manually control the motor.11. LED indicators: to indicate different states of motor12. Terminal blocks: for connection of control system.

Auto Transfer Switch (ATS)When a consumer has a connection from a power supply company and a backup

generator for continuous power supply. In case of fault in power line or load shedding ATS turn on generator and transfers load from line to generator. On restoring line power ATS transfers load from generator to power line and also turn off the generator. Used manually or in automatic mode. ATS consists of following components

1. Insulated bus bars2. Insulators3. Relays: auxiliary relays4. Microcontroller base control unit: to turn on and off generator automatically. 5. Circuit breaker: ACB(air circuit breaker)6. Timer: to set the on and off delay time of generator. Timer is important because it

ensures that power supply has established hence avoid jerks. 7. Magnetic contactors8. Meters: voltmeter, ammeter and power factor meter9. Selector Switches10. Pushbuttons: to on and off panel, generator and supply line manually.11. LED indicators12. Terminal blocks13. Connecting cables

AC/DC Auxiliary Services PanelsAC/DC service panels are outgoing panels used to distribute AC/DC electric supply in

controlled and protected way. As the name indicates AC panels are used to distribute, monitor, measure and protect AC circuits. Whereas DC panels are used to distribute, monitor, measure and protect DC circuit. There is miner difference between the construction of AC and DC panel. Panels are excited by main supply through bus bars and distribute electricity to different regions circuit breakers and then to each device or set of similar devices through MCB. AC/DC panels consists of following components

1. Insulated bus bars2. Insulators3. Relays: auxiliary relays

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4. Circuit breaker: MCCB or MCB(air circuit breaker)5. Meters: voltmeter, ammeter and power factor meter6. Pushbuttons: for manual on off of panel.7. LED indicators8. Terminal blocks9. Connecting cables

Distribution Boards for Power and LightingFloor or wall mounted distribution boards are outgoing panels used to distribute

electrical supply to subsidiary circuits, while providing a protective fuse or circuit breaker for each circuit, in a common enclosure. Normally, a main switch, and in recent boards, one or more Residual-current devices (RCD) or Residual Current Breakers with Overcurrent protection (RCBO), will also be incorporated. These are used in offices, school blocks and small residential buildings even in houses etc. Distribution boards are consists of following components

1. Buses2. Insulators3. Circuit breaker: Residual-current devices (RCD) or Residual Current Breakers with

Overcurrent protection (RCBO)4. LED indicators

High Tension (HT) work shopPEL is fabricating floor mounted metal clad air insulated HT switchgear panels at HT

workshop for WAPDA and different industries inside and outside the Pakistan. HT panels are classified into incoming and outgoing panels according to their location in the distribution system.(We classify a panel into HT when it handles 36,000 volts or more but when it handles voltage from 1,000 volts to 36,000 volts it is classified as MT panel, at PEL people call 11,000 volt panel as HT panel so I am also saying it HT panel) First of all Plates from Fabrication and paint shop are bolted to form housing (of dimensions of 2200*1600*900) of the panels, then bus bars are installed and then other electrical equipment are installed step by step and are checked by quality control department at each step.

Relays, metering and protection equipment are installed according to the customer’s requirement. HT panels almost contain the same components as given below,

1. Bus bars: for connecting the panel in the circuit. The dimensions of copper alloy bus bar are selected according to the current rating, insulation (PVC sleeve) and gap between bus bars is selected according to the voltage level. According to IEC standard for HT phase to phase air break down distance is 160 mm.

2. Insulators: specially designed insulators with radial grooves (to increase creeping distance) are used to support the bus bars and insulate bus bars from the earthed housing of panel.

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3. Microcontroller base control unit: to automatically control specific function according to the panel.

4. Relays: to detect, identify and initiate protective measures e.g. short circuit relay, earth fault relay, thermal overload relay, under voltage relay, over voltage relay, DC supervision relays, auxiliary relays etc. Both electromechanical and microcontroller based relays are in use. Electromechanical relays are simple but microcontroller based relays provide additional function of connectivity to the computer, memory record of type of fault, time of fault and user friendly interface for setting the parameters.

5. Circuit breaker: to disconnect the panel from main power supply in case of fault in HT circuit, vacuum circuit breakers (VCB) are used.

6. Magnetic contactors: to control the contacts of high voltage circuit by low voltage signals.

7. Current transformer (CT): to step down high currents to a level suitable for measurement with ammeter.

8. Potential transformer (PT): to step down high voltage to a level suitable for measurement with voltmeter.

9. Meters: to measure different electrical quantities e.g. ammeter, voltmeter, power factor meter and energy meter.

10. Selector Switches: to select one function from multiple options.11. Pushbuttons: to turn on or turn off panels or particular features of panel.12. LED indicators: for indication of state, functioning or malfunctioning of components

of panel or circuit.13. Terminal blocks: for multiple connections of controlling and metering devices.14. Connecting wires and cables: for connections. Wires and cables are selected

according to the current requirement.15. Male and female connectors: for connecting the devices which are to be removed

frequently.

All of the above mentioned components or some of them are present in a panel. Also additional components are added according to the functionality required by the customer. Door with VCB assembly is mechanically interlocked in such a way that we can’t remove VCB from the circuit in energies state.

PEL is fabricating following types of HT switchgear panels

1. Incoming panel2. Bus Coupler panel3. Outgoing panel4. Power Factor Improvement Panels

Electrical power distribution system of Pakistan.

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Incoming PanelAs the name indicates this panel is installed at the output of power transformer

which step down voltage from 132 kV to 11 kV at the substation. There are two or more incoming HT lines at a substation and there are as many incoming panels as there are incoming HT lines. Output of incoming panel is fed to outgoing panel. Incoming panels are used for distribution, protection and metering purpose.

Bus Coupler PanelIn between two incoming panels there is a bus coupler panel which couples two

incoming panels in case of discontinuity or fault in one incoming HT line to transfer the load of faulty incoming HT line to the other incoming HT line. To avoid discontinuity of electricity to one area which was being fed by incoming HT line which is gone faulty now. Bus coupler panels are used for coupling of incoming panels, protection and metering purpose.

Outgoing PanelThere are 4, 6 or 8 outgoing panels attached to a single incoming panel. Output of

outgoing panel is the feeder which feeds a particular area of the city. Outgoing panels are used for distribution, metering and protection purpose.

Consumer PanelConsumer panel is an outgoing panel installed at the substation to feed a single

industry. This is dedicated panel to a single industry and is used for protection and metering purpose.

Industrial PanelWhen more than one industry are being fed on a single outgoing panel or feeder

then WAPDA installs an industrial panel outside the industry for protection and metering purpose. But the industry doesn’t have an authority to open and operate it.

Owner PanelAs we know that industry can’t operate industrial panel in case of tripping or for

maintenance and other purposes whit out the permission of WAPDA so to avoid production lose at industry, another panel is installed inside the industry after the industrial panel in such a way that output of industrial panel is fed to owner panel. Owner panel is under the custody of industry and industry has the authority to operate it for maintenance and other purposes.

Power Factor Improvement PanelsAs we know that inductive or capacitive load in the circuit introduce phase difference

between voltage and current which introduce reactive power and increases apparent power in the circuit resulting in increase in copper losses in the circuit. So it is desirable to reduce this phase difference between voltage and current. Since inductance dominates the capacitance in most loads so this causes voltage to lead current in the circuit and results in decrease of power factor. If the power factor falls below the specified tolerable value,

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power supply company charges penalty to the consumer. That is why it is desirable to keep the power factor equal to or neat to 1 for this we need a capacitor bank so that we adjust capacitance in the circuit according to our requirement and this is achieved by power factor improvement panel which contain a capacitor bank.

PFI panel can also be installed at the substation to reduce line loses. PEL is also fabricating PFI panels for WAPDA and other power generation companies.

Note: In the distribution system we install a circuit breakers in each panel to avoid the propagation of fault.

Testing Of SwitchgearMaterial and product is checked strictly under the supervision Quality Control.

Switchgears are fabricated and tested according to the following national and international standards

1. WAPDA P44962. IEC2983. IEC6944. IEC60-15. IEC54

Tests are categories into of following two basic categories

1. Type tests2. Routine tests

Type TestsTests which are performed once on the prototype of switchgear are called type tests.

These tests are performed on the prototype to study the feasibility of the design of switchgear.

Following type tests are performed on prototype of each switchgear

Temperature rise TestWe know that when current flows through a conductor electrical energy is dissipated

in the form of heat due the resistance of the conductor and the temperature of conductor rises. High temperature of the conductors may damage them. So a current of 630A is passed through the panel and temperature rise at different places is measured. Temperature rise should be 60℃ at buses, 65℃ at male and female contacts and 75℃ joints.

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Impulse TestImpulse test is carried out to check Basic Insulation Level (insulation between two

live parts and between live and earthed parts). In This test we apply 15 positive and 15

negative Impulses of 95kV for 120μs or

125μs. From 15 positive and 15 negative impulses 2

failures for each are tolerable.

Short Circuit Test

Routine TestsTests which are performed to find out the faults in electrical and mechanical

components on each switchgear are called routine tests.

Following routine tests are performed on each switchgear

Power Frequency Test of Main Circuit

Power Frequency Test of Control circuit

Test of Auxiliary Electrical Devices

Mechanical Operation Test

Measurement of Resistance of Main Circuit

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