Electrical Operations In Sub-stations
Types Of Sub-Stations:
Transmission Sub-Station
Distribution Sub-Station
Open Arc OR Metal Clad Substation
Transmission Sub-Station (Converts High Voltage
To Low Voltage :
Starts at 69KV or 69000 Volts and Typically 765KV In
The United States
Typically Used to Step-Up the Voltage at 500KV and sends
it to distribution sub-station where the process is reversed
Standing Switching Procedures are required
High Voltage LOTO
o 3 Point Test
o Visible Open
Over-Current Protection
o Transfer-Trip Relay
Schemes o Differential Relays
Distribution Sub-Station (Converts High Voltage
to Low Voltage)
Typically transforms transmission voltages
(>69KVA) to Distribution Voltages (12.4k to 24.9k)
Standing switching procedures are required
Identical High Voltage LOTO Requirements
Over-Current Protection
Oil Circuit Breakers
o Fuse Disconnects
(Distribution Sub-Station)
Open Air or Metal Clad Sub-Station
The open air or metal clad substation receives the voltage
from the distribution substation and further reduces the
voltage from 12,470 or 13,800 to something like 480 Volts
Open Air Substation:
Following is the picture of an open air sub-station:
It is a gas type open air substation
The terminals are usually charged to 289,000 volts to ground
They are Bare and Exposed
Metal-Clad Sub-Station:
Following is the picture of metal clad Sub-Station:
Typically transforms the distribution voltages (12.4k to
24.9k) to 480 Volts
Standing Switching Procedures are required
Both high voltage and low voltage LOTO requirements apply
Overcurrent Protection
It is usually Metal Encased
You can Easily Stand Close To It
Fused Disconnects
o Molded case circuit breakers
o Low Voltage Power Circuit Breakers
Personal Protective Grounding In Substations:
Key-Points:
o Trip Grounds vs Personal Protective Ground
o Substations have ground grids, connect to
them o Use same methods as for overhead lines
o Induction can be a serious problem is substations
o Remember to isolate the bus protection transformer
Sub-Stations Equipment:
Safety Switch vs Load-Rated Switch
Comparison
Safety Switch (Isolator) Load-Rated Switch
Designed for high Designed to pick-up or
voltage break-out shut-off electrical
systems
Arc-Quenching Not Arc Quenching Is
Available Available
Cannot break load Can Break Load Current
current
Cannot Interrupt Fault Cannot Interrupt Fault
Current Current
Images:
Safety Switch (Isolator):
Load Rated Switch:
Oil Circuit Breaker (OCB) / Gas Circuit Breaker (GCB)
Fused Disconnects
Air Disconnects
Station Power Transformers
Battery Banks
Primary Metering
Tap Changing Transformers
Relay Packages
Places Where not to place grounds:
There are some places in grid stations and transmission
lines where grounds should not be places at any cost
Corona-Rings:
Corona rings are metallic, torus-shaped conductors
mounted on high-voltage equipment like bushings,
insulators, or terminals.
Their purpose is to distribute the electric field evenly and
reduce corona discharge, especially at high voltages.
Grounding changes the electric field distribution around
high- voltage terminals, leading to uneven stress.
Instead of reducing corona, grounding the ring may
cause intense corona or arcing at unintended spots.
Grounded corona rings can cause voltage concentration
across insulators, increasing the risk of flashover.
Grounding interferes with the equipment’s
designed performance and may lead to insulation
failure or equipment burnout.
Safety Hazard Can create unexpected touch and
step potential, putting personnel at risk.
Conduction Spacers:
Conduction spacers (also called aerial spacers or spacer
dampers) are mechanical devices used in overhead
high-voltage transmission lines, especially bundle
conductors (e.g., double, triple, quad conductors).
They are installed to:
Maintain spacing between conductors in a bundle.
Prevent clashing or galloping due to wind or other
forces. Dampen vibrations and maintain system
integrity.
Why Grounding Them Is NOT Allowed:
Reason Explanation
Floating Potential Disturbance : Spacers are installed on
live conductors, and grounding them can short the phase
to ground, causing faults.
Arc Risk : Introducing ground on spacers may cause
arcing or flashover due to high voltage differentials.
Interference with Line Function : They are purely
mechanical and not meant to conduct current —
grounding disturbs the intended insulation and operation.
Equipment Damage : May cause damage to conductors,
insulators, or even transformers by creating
unexpected fault paths.
Personnel Safety Risk Accidental grounding can make the
line unexpectedly live at other points, risking
electrocution or fire.
Bus Ports:
Bus Pots (often referred to as bus ports, bus bushings, or
bus entry terminals) are:
Insulated termination points where bus-bars (high-
voltage conductors) enter or exit metal-clad equipment
like transformers, circuit breakers, or switchgear.
They allow the safe passage of HV conductors
through grounded enclosures without electrical
contact.
Why Grounding Bus Pots Is NOT Allowed:
Insulator Breakdown: Bus pots contain high-voltage
insulation, and grounding them can cause
insulation failure or internal flashover.
Short Circuit Risk: Since bus pots connect to live
conductors, grounding them may result in a
direct phase-to-ground fault.
Equipment Damage: Can lead to catastrophic failure
of switchgear, CTs, transformers, or breakers due to
arcing or internal heating.
Electric Field Distortion : Alters the designed
dielectric strength, leading to partial discharges or
corona inside the equipment.
Personnel Danger: May result in unexpected
energization of grounded surfaces, risking shock
or explosion.
Currently ,there are two kinds of control centers
National Power Control Center (NPCC)
Regional Power Control Center (RPCC)
1. Isolation (De-Energization)
Meaning:
Isolation means disconnecting a portion of the electrical
system from the live (energized) parts so that it
becomes completely dead and safe to work on.
Purpose:
To perform maintenance, inspection, or repair on
electrical equipment.
To prevent electric shock, arcing, or accidental
energization.
How it's done:
By opening circuit breakers, isolators, or
disconnect switches.
Grounding the isolated section using earthing switches
to safely discharge any residual or induced voltages.
Energization:
Meaning:
Energization means connecting and applying voltage to
a previously de-energized or new part of the grid.
Purpose:
To restore power after maintenance.
To commission new equipment or lines.
Steps
usually involved:
Inspection to ensure safety and readiness.
Removing grounds.
Closing circuit breakers and isolators in a
proper sequence.
Monitoring voltage, current, and other parameters.
Single Line Key Diagram Of 220/132 KV Grid
Station NKLP
It is called single line because all the three
phases are connected to a single phase
Main Components Of A Grid Station
Isolator
CT
PT
Bus-Bar
Lightning Arrestor
Phases Of Power Sector
There are three phases of power sector
Generation
Transmission
Distribution
Utilization
Initially all these phases were under the control and supervision of
WAPDA but now Wapda only has the generation sector and
overall sector is now under the control of NEPRA.
Generation :
o All the power plants comes under generation
sector like Thermal Plants, Coal Plants etc
o Dams comes under power plants
Transmission:
o Responsible for the transmission of power over
all the areas through transmission lines
o To Transmit More Power we need less
voltage Power Conversion is usually done as follows
o Auto-Transformer : converts 500KV to 220KV
o Power is Futher Stepdown to 132KV from
220KV o 132KV is further step-down to 11KV
o Now a days power transmission through long lines
is not in AC but in DC , because the I 2R losses in
DC is several times less than the I2R losses in
AC,that’s why now a days for , long power
transmission lines,HVDC Transmission is
used,but we cannot use that dc power in homes
that’s why converter stations are used,
o A converter stations receives AC power to DC
power,send that DC Power into transmission
lines,another comverther is placed at the
specific location which receives DC power from
transmission lines,converts it into AC power and
then send it into distribution companies
o Conversion Mechanism
500KV (AC) --- 660KV (DC)
660KV (DC) --- 550KV(AC)
Distribution:
All the electricity supply companies comes
under distribution sector. Following are some of
the Electricity Supply Companies
LESCO Lahore Electricity
Supply Company
FESCO Faisalabad Electricity
Supply Company
IESCO Islamabad Electricity
Supply Company
QUESCO Quetta Electricity
Supply Company
HESCO Hyderabad Electricity
Supply Company
SESCO Sukkur Electricity
Supply Company
TESCO For Tribal Areas
GEPCO Gujranwala Electricity
Supply Company
At initial level,Karachi electricity supply company also
cames under these category but now it is officialy privatized,
Ring Systems:
All 500KV grid stations are interconnected with each
other,This phenomenon is called Ring System
Power Operation And Purchasing/Selling:
o NPCC (National Power Control Center) :
Usually controls the operation of power in the
whole country and deals with the 220KV and above
o RPCC (Regional Power Control Center):
Deals with 132 KV and above
o PDC (Power Distribution Companies):
Usually controls and manages all the power
supply companies
o ISMO(Independent System Market Operator)
Manages Purchasing And Selling Of Power
Types of Transformers Typically Used At Grid Stations Mainly,at
grid stations,there are 2 types of transformers used
Auto Transfomer Power Transformer
o For 220KV-132KV o For 220KV to 11KV
conversion use auto conversion,we use
transformer power transformer
o Usually have single because there is a big
windings difference and heavy
o It is usually tapped out scale performance is
from same winding required for
conversion
Tertiary Windings:
o Used in big transformers to protect from
winding damage
o Usually have Delta Windings
o Harmonics gets trapped in delta windings and
gets absorbed
Protection Of Electrical Devices In Grid And Other
Household Equipmets
IP RATING:
o The word IP stands for Ingress Protection
so the term is Ingress Protection Rating
o The Ingress Protection (IP) rating is a
standard that measures the level of
protection in an electrical devices or the
enclosure provides against solid objects and
liquids
o Its crucial for ensuring the reliability and
safety of electrical equipment in various
environments including grids
Key Points:
1. Two Digit Code:
The IP rating consists of two digits
each representing a specific type of protection
2. First Digit ( Solid Particle Protection) :
Rates
Protection against solid objects such as dust and
debris
3. Second Digit ( Liquid Protection):
Rates protection
against liquid such as water and chemicals
IP Rating Scale (First Digit/Solid)
o 0: No Protection
o 1-4 : Protection Against Solid Objects
of Varying Sizes
o 5-6 : Dust-Tight or Dust Protected
IP Rating Scale (Second Digit / Liquid)
o 0: No Protection
o 1-4 : Protection against dripping or
splashing water
o 5-8 : Protection against low-pressure
jets,high pressure jets , or immersion
Examples
Regarding Grid :
o Circuit Breakers : have IP65 or IP67
ratings to protect against dust and
water splashing
o Disconnectors : have IP54 or IP65
ratings to protect against dust and
water splashing
o Surge Arrestors: have IP65 or IP67
ratings to protect against water
exposure and harsh weather conditions
o Control And Protection Panels: have
IP20 or IP54 depending upon the
installation location and exposure to
elements
Points To Remember :
o IP ratings are defined by International
Electrotechnical Commision (IEC) in the
IEC 60529 standard
o Devices with higher IP ratings are
generally more suitable for harsh
environments , such as outdoor and
industrial settings
