BUU:TS / DS

Transmission &
Training Module-II Distribution
Substation
 Recap of Module-I BUU:TS / DS

Following areas were covered in module-I

• Type of substations
• Busbar switching scheme
• Content of substation
• Layout engineering
• Design & Engineering tools
• Business development strategy
• Important references
 Contents of Module -II BUU:TS / DS

• Frequently Asked Questions (FAQ) on substation engineering covering the

following
– Busbar material
– Earthing
– Auxiliary system
– Cable engineering
– Illumination
– Structural engineering
– Civil engineering
– Installation activity

• Competitors and our positioning

• More on sales/marketing analysis

FAQ : Busbar Material BUU:TS / DS

What are the types of Busbar?

• Rigid bus comprising of AL

tube

• Strung bus comprising of

A C S R / A A A conductor

What are the types of support?

• Bus post insulator for rigid bus

• String Insulator for strung bus

FAQ : Busbar Material BUU:TS / DS

How busbar sizing is done?

• Busbar sizing is done to check withstand ability of
– Thermal Stress
– Mechanical Stress
– Deflection
What are the input parameters?
• Continuos current
• Short circuit current & duration
• Ambient temperature
• Wind load on the conductor
• Cantilever strength / Ultimate tensile strength of the support
• Minimum clearance
• Span of the conductor
 FAQ : Busbar Material BUU:TS / DS

Thermal Stress:
• Flow of current causes thermal stress.

• Temperature rise is checked for

– Normal load current
– Fault current.
Note: Sizing based on governing criteria

• Steady state temperature depends on

– Radiation heat loss
– Convective heat loss
– Solar heat gain by the conductor
 FAQ : Busbar Material BUU:TS / DS

Mechanical Stress

• Mechanical stress on the busbar material is due to

Dynamic force

– Electrodynamic force during short circuit condition

– Wind force

Static force

– Static conductor tension

– Self weight

• Adequacy of busbar material is decided when it’s bending stress /

tensile stress are within the limiting value.
FAQ : Busbar Material BUU:TS / DS

Deflection

• Deflection of busbar happens

At Dynamic condition

– Due to combination of electromagnetic force & Wind force

At Static condition

– Due to self weight and sag

• Check for deflection in case of rigid busbar is mechanical property

which decides the limiting value

• In case of strung busbar it is minimum electrical clearance decides

the limiting value.
Purpose of Grounding system BUU:TS / DS

Two main design goals to be achieved by any substation ground

system under normal as well as fault condition

• to provide means to dissipate electric current in to the earth

with out exceeding Operating and Equipment limit.

• to Assure that a person in the vicinity of the grounded facility is

not exposed to the danger of electric shock.

Terms and Definitions BUU:TS / DS

Ground Electrode : A Conductor imbedded in the earth and used for

collecting ground current from or dissipating current in the earth.

Ground Grid : A closely spaced horizontal and vertical bare

conductor that are connected to each other at junction point and often
placed in shallow depths.

Grounding System : Comprises all interconnected grounding facilities

in a specific area.
 BUU:TS / DS

Basic of
Grounding system
Design
Design Philosophy BUU:TS / DS

ðFirst step is to calculate the tolerable touch and step values based
on IEEE 80 guidelines, which ensures the safety of person working in
the substation

ðCalculate attainable step and touch voltages based on preliminary

design of grounding mat. Attainable value in the individual meshes
should be less then the tolerable values.

ðThe attainable step and touch potentials depend on fault current,

earth resistivity and grounding mat configuration.
Design Procedure BUU:TS / DS

Field data
A, ρ

Tolerable Touch
and Step voltage -
IEEE80, IEC479
Attainable step and Touch voltage ∝ ρ, Ig, D

Grid
current
Ig

Grid Attainable Touch Safety Final

Lay out and step voltage check Design
D,n,L, h
 IEEE80 based grounding system design BUU:TS / DS

• Simple method based on empirical equations.

• Gives only approximate peak attainable touch and step voltage.

• Limited to Square, Rectangular, T or L configuration of the substation.

• Uniform spacing of conductors.

• No need of sophisticated computers.

 Limitation of IEEE80 design BUU:TS / DS

• Uniform spacing of conductors.

• Not possible to analyze the grid performance in Multi layer soil.

• Not possible to consider the effect of the long Electrodes.

• Not possible to consider the effect of the non energized component.

• Inefficient design

• Leakage current current density is non uniform over the conductor

 BUU:TS / DS

How to analyze the ground

grid performance ?
How to analyze the ground grid performance ? BUU:TS / DS

ðFrom the safety point of view, touch and step voltages must be
investigated over the entire grid area. Conventional analysis ( IEEE80)
can calculate touch and step voltages only at the corner mesh where the
voltages are likely to be highest.

ðThe most vulnerable part of the substation from the safety point of view
is the fence because potential gradient are high near the periphery of the
grid. Person standing outside the substation touching the fence should be
safe. Touch and step voltages should be investigated over the entire
periphery for safety point of view.
 How to analyze the ground grid performance ? BUU:TS / DS

Cont…

ðPipe lines buried in the ground may carry the transfer potential during
the fault on the substation, this has to be investigated in the vicinity of the
substation.

ðIn some cases, the substation grid is connected with the other auxiliary
grounding mat. This is usual practice in power/industrial plants. For this
type of study we have to investigate safety aspects at all the points in
both the grids as well as the interconnecting grids.
 How to analyze the ground grid performance ? BUU:TS / DS

ðIn conventional analysis with the irregular configuration of substation

analysis for the equivalent rectangular area is performed then the
analysis of the rectangular area is superimposed on to the actual area.

ðWith improved knowledge and computational

facility, we can go to micro level analysis and
improve the reliability from safety point of view.
The step and touch voltages can be investigated
at all the point in the substation.
ðLocation of the highest step and touch voltages are also very
important to design the efficient and safe Grounding grid.
FAQ : Earthing specific to HV substation BUU:TS / DS

Why Earthing?
Earthing system is a key element of electrical system. It is critical to
ensure personnel safety as well as to provide protection for equipment
and to minimise interruption of service.
Guideline?
Earthing is carried out based on IEEE-80, IS:3043 and also by finite
element method.
Input required?
– Fault current & Duration
– Ground current and fault clearing time
– Soil resistivity
– Area of the switchyard
FAQ : Earthing specific to HV substation BUU:TS / DS

Soil Resistivity Measurement

• Resistivity test should be made to determine any important variation of

resistivity with the depth of soil.

• Resistivity test should be made at number of places within the site. The
wenner’s four pin methods is the most commonly used technique. Four
probes are driven in to the earth along the straight line, at equal distance
(A) apart, driven to the depth (B). The voltage between the two inner
electrodes is then measured and divided by the current between the two
outer electrodes to give a value of mutual resistance R. Then,
4∏AR
ρ=
2A A
1+ −
A 2 + 4B 2 A 2 + B2
FAQ : Earthing specific to HV substation BUU:TS / DS

Soil Resistivity Measurement

Where,
ρ= Resistivity of soil in Ω-m
R= resistance in ohms resulting from dividing the voltage between the
potential probes by the current flowing between the current electrodes.
A=Distance between adjacent electrodes.
B=Depth of electrodes in meter.
FAQ : Earthing specific to HV substation BUU:TS / DS

Soil Resistivity Measurement

• If B is small compared to A, as is the case of probes penetrating the

ground a short distance only, the above equation can be reduced to

ρ = 4∏ A R
• The current tends to flow near the surface for small probe spacing,
whereas more of the current penetrates deeper soils for large spacing.
Thus the resistivity measured for a given probe spacing A represents
the apparent resistivity of a soil to a depth of A.

• The above derivation for the soil measurement is based on the

assumption that the soil resistivity is uniform. Uniform soil resistivity
means, soil resistivity remains constant both laterally and with depth to
infinity.
 FAQ : Earthing specific to HV substation BUU:TS / DS

Earthing Design
• Following aspects are checked while designing earthing system
– Adequacy of earthing conductor cross section
– Grid size of the earthing conductor
– Step potential and Touch potential

Cross section of earthing conductor is to be sufficient to withstand the short

circuit current. Also while selecting the size corrosion allowance is taken
into consideration.

In the switchyard earthing conductors are laid forming a mesh. Usually the
grid spacing is uniform. But non-uniform grid spacing also can be adopted
by calculation earthing through finite element method.
FAQ : Earthing specific to HV substation BUU:TS / DS

Step Potential
Step Potential is the voltage
difference between a person's feet
cause by the dissipation gradient of
a fault entering the earth. Typically,
one meter from a fault entry point
voltage reduced by 50%. (e.g. a
1000A fault entering through ground
impedance of 5 ohms. will create
voltage of 5000V. At a distance of
less than one meter away a fatal
potential of 2500V will exist).
Earthmat installed in that area will
make the voltage gradient uniform
and less than the safety limit.
FAQ : Earthing specific to HV substation BUU:TS / DS

Touch Potential

Touch Potential is similar to step

potential except that the part of the
fault current passes through the
person's hand and body if he/she
touches any structure which is
carrying fault current. Touch potential
is eliminated by connecting structure
to the Earthmat so that least
resistance path for fault current is
established.
FAQ : Earthing specific to HV substation BUU:TS / DS

What are the critical issues for earthing design?

Earthing design becomes critical when…

• Soil resistivity is high in rocky areas ( > 150 ohm-m )

• High value of fault current and fault duration

• Switchyard area is too small

• Tolerable values of touch potential & step potential

FAQ : Lightning Protection for HV substation BUU:TS / DS

What is D.S.L.P.?

Direct Stroke Lightning Protection (D.S.L.P) for outdoor air insulated

substation is a very important aspect. Design of the D.S.L.P. is
probabilistic approach which ensures minimum probability of lightning.

How it is achieved?

It can be achieved by providing

– Lightning Mast

– Shield Wire

– Combination of above two

FAQ : Lightning Protection for HV substation BUU:TS / DS

Protective Zone
by LM
Typical DSLP Layout Shield wire

Lightning Mast

What are the methods for D.S.L.P. design?

• As per "High Voltage Engineering" by Razevig with help of LM
• As per IS:2309 which is also using LM and spikes
• As per electro-geometry method by using combination of LM and shield
wire.
 FAQ : AC Auxiliary System BUU:TS / DS

What is AC auxiliary system?

• System that provides auxiliary AC to
• Equipments
• Lighting
• HVAC system
• Firefighting

What are the configuration normally followed?

• One Incomer and required outgoing feeders
• Two Incomer with Sectionalized switchboard
• Dedicated LT Transformer/s
• Duplication of source to have redundancy
• Separate PCC (Power Control Center) and ACDB (AC Distribution Board)
• Diesel Generator as emergency source of power
 FAQ : AC Auxiliary System BUU:TS / DS

11KV 33KV FORM

This a typical FROM TERTIARY OF
S.E.B. 415V AC 315MVA
TARNSFORMER
AC system 250KVA
630KVA AUXILIARY
AUX. 1000KVA
DG SET
TRAFO.
SYSTEM AUX. TRAFO.
block single 415V MAIN SWDB-1A

line diagram.
This comprises FIRE 100KVA
FIGHTING LIGHTING
of two separate SYSTEM TRAFO.

415V ACDB-2A
incoming
VENTILA EMERGEN
LIGHTI
TION DB- CY
source and DG 5A LIGHTING
NG DB-
3A
DB-4A

as backup
220V 50V Typical AC System
BATTERY BATTERY
power supply. CHARGER CHARGER
SLD
 FAQ : AC Auxiliary System BUU:TS / DS

How feeders requirements are being engineered?

The following requires AC supply:
• Isolator motor operating mechanism
• Circuit Breaker compressor / spring charging mechanism
• On Load Tap Changer(OLTC) and Transformer cooling (Fan power supply)
• Space heater, Lamp, Power socket for: CB switch cubicle, RTCC, Indoor
Switchgear, Control & Relay panel, Bay MK, Junction box, Isolator drive box
etc.
• Switchyard lighting
• Control room lighting
• Transformer oil filtration
• Battery Charger
• Air Conditioning/Ventilation system
• Storm water drainage pump
 FAQ : AC Auxiliary System BUU:TS / DS

What else to decide a feeder?

The following aspects are to be considered:
• Load current.
• Type of duty (continuous, intermittent)
• Amount of overloading to be considered.
• Spare feeder requirement (e.g. 10% spare are considered as good
engineering practice unless specified).
• Typical Incomer for PCC is Air Circuit Breaker (ACB) with electrical tripping
form O/C & E/F protective release or from built-in release.
• Typical outgoing feeders are with Switch Fuse Units (SFU).
• Single phase distribution are usually through SFU or MCB.
• However Molded Case Circuit Breakers (MCCB) are provided for outgoing
feeders as specification requirement by the customer.
FAQ : DC Auxiliary System BUU:TS / DS

What is DC Auxiliary system?

• Auxiliary system which provides reliable DC source for control & protection
circuit of substation equipment. DC system backed up by Battery ensures
higher level of security over AC auxiliary system.
What are DC loads in a substation?
• Protection & interlocks
• Switchgear control
• Remote control & indication
• Annunciation system
• Control & instrumentation
• Power line carrier ( PLCC )
• Emergency DC Lighting
• Electric clocks
• Communication systems
• Data acquisition & logging system
FAQ : DC Auxiliary System BUU:TS / DS

What are the type of Battery?

Lead Acid Battery
• Tubular battery comprises of tubular positive plates where tubes of
terylene hold the active material with lead antimony spines.
• Plante positive plates are solid cast plates of 99.99 purity lead. Negative
plates are pasted grid made of lead.
• Maintenance free battery comprises of flat pasted positive and negative
plates made of lead calcium alloy tightly pressing against each other with
glass wool mat in between.
Ni-Cad Battery
• Ni-Cad cell consists of nickel in the positive plate and cadmium in the
negative plate. Electrolyte is potassium hydroxide in deionised water.
 FAQ : DC Auxiliary System BUU:TS / DS

Which type of Battery you need?

Tubular Battery Plante Battery

• Battery is simple construction • 40% better performance
• Life is 10 years. • Longer Life (20 years)
• Most Economical • Almost 2.5-3 times Costlier than
Tubular battery

Maintenance Free Ni-Cd Battery

• Does not require any topping up of • Longer life span (25 years)
water • More stable & reliable performance
• Less hazardous and no special • Less hazardous and corrosion free
room required • Cost is 4 times of tubular battery
• Needs control environment at 270C
• Cost is 2 times of Tubular battery
FAQ : DC Auxiliary System BUU:TS / DS

What you need to size the Battery?

Ampere Hour Capacity of battery is decided from the loading pattern or duty
cycle over a discharge period of the battery. Following load are calculated

* Continuous Load * Emergency Load * First & Last minutes Instantaneous Load
Current in Amp

Instantaneous Load
Last minute
Instantaneous Load
First Minute
Emergency Load
Continuous Load

Time in Hours
BATTERY DUTY CYCLE
FAQ : DC Auxiliary System BUU:TS / DS

What you need to size the Battery Contd..

• Instantaneous or Momentary Load

– Circuit Breaker Trip / Close, Protective Relays

• Continuous Load

– Annunciation system, DC supervision, Indication Lamps, Auxiliary

Relays, Semaphores

• Emergency Loads

– Emergency lighting
FAQ : DC Auxiliary System BUU:TS / DS

What is the Battery Charger?

• Battery Charger is to
– Charge the battery when battery gets discharged (Boost Charging)
– Keep / maintain the battery charge (Float Charging)
– Cater station continuous DC load during normal condition
What are the Battery & Charger configuration?
• One float charger and one boost charger
• One float charger and one float-cum-boost charger
• Two identical float-cum-boost chargers each capable of performing both
duties
• Three charger combination (2 main & 1 standby)
FAQ : DC Auxiliary System BUU:TS / DS

What does a battery charger contains?

– AC unit comprising of rectifier transformer and AC switching unit
– Rectifier unit (uncontrolled / half controlled / full controlled)
– Control circuit for rectifier, current controller, monitoring & annunciation
system
– DC changeover with contactor, dropper diode or tap cell arrangement
and DCDB if it is integral part of charger
3-Ph 415V
AC Supply

Transformer

DC Changeover
Rectifier

Ckt. & DCDB

&
Unit
SFU/MCB
Load

Typical Block Rectifier

Diagram of Control Battery
Charger Ckt.
FAQ : Cable Engineering BUU:TS / DS

What is cable engineering?

• Cable engineering for substation is done to achieve
– Reliable performance of the system
– Optimized solution
– Easy access and identification
– Optimal efforts during erection/ commissioning/ maintenance
What are the areas of cable engineering?
• Power Cable
– LT Power Cable
– HT Power Cable
• Control Cable
FAQ : Cable Engineering BUU:TS / DS

What is in cable engineering?

• Power cable engineering
– Sizing of cross section and number of run of cable based on
• Load current
• Short circuit current
• Fault clearance time
• Voltage drop in cable
• De-rating factors
– Selection of insulation (especially for HT cable)
• HT cable insulation based on the type of system i.e. Unearthed or
effectively Earthed system
• Specific care to be taken where impedance earthing is done but
system behavior is near to Unearthed system
– Cable routing, numbering, grouping and type of laying
FAQ : Cable Engineering BUU:TS / DS

What is in cable engineering (contd.)?

• Control cable engineering
– Sizing of cross section and number of cores
• Type of signals control cable to carry
– Voltage (PT, CVT)
– Current (CT)
– DC Circuit (Closing, Tripping, Indication)
– AC Circuit (DC supervision, operating mechanism ckt.)
– Potential free contact circuit (Indication, Annunciation, logic
ckt.)
• Voltage drop due to longer cable route length
– Grouping of signals for deciding the number of cores
– Cable routing and marking and crossing
– Cable and core numbering
 FAQ : Illumination BUU:TS / DS

About illumination?
• Illumination for substation is an important requirement for making operation
possible at night.
• Scope of illumination for typical substation are:
– Outdoor Switchyard
– Control Room building
– Auxiliary building
– Street Lighting (service road within the switchyard)
• Lighting system for substation are considered as:
– Normal AC to cater lighting requirement during normal operating
condition
– Emergency lighting on failure of normal AC lighting source
• DC emergency for illuminating strategic locations
• Lighting from DG system for bigger substations
 FAQ : Illumination BUU:TS / DS

How much illumination level required for substation?

• Lighting shall provide enough light depending on nature of work to be

carried out. Operational lighting as per IS:3646 part-III.
 FAQ : Illumination BUU:TS / DS

What are the typical luminaries used in substation?

• Luminaries for outdoor switchyard:
– High Pressure Sodium Vapour (HPSV) luminaries with various
combination of rating with high beam & narrow beam used to achieve
desired illumination
– Ratings usually considered
• 2X400W HPSV, 1X400W HPSV for switchyard and outdoor
equipment
• 2X250W HPSV, 1X250W HPSV for switchyard and outdoor
equipment
• 1X150W HPSV for street lighting
– High Pressure Mercury Vapour / Halogen luminaries are also taken if
they specifically required by customer
 FAQ : Illumination BUU:TS / DS

What are the typical luminaries used in substation?

• Luminaries for control room:
– Indoor illumination is achieved by various types of luminaries
according to the requirement and utility of various rooms.
– Room with CP/RP, office, conference room with air conditioning and
false ceiling are provided with recessed mounted 2X36W fluorescent
lamp with mirror optic reflector.
– For LT room, DG, Store etc. where no false ceiling or AC provided,
industrial type 2x36W fluorescent lamp is used.
– For cable cellar bulk head or industrial type 2x36W fluorescent lamp is
used as per specification.
– Battery room is provided with corrosion proof industrial type 2X36W
fluorescent lamp.
– For DC emergency bulk head incandescent lamps are used.
 FAQ : Illumination BUU:TS / DS

How illumination designed?

• Input requirement
– Lux level to be achieved at what height
– Size of the area for outdoor and dimension of the room.
– Type of interior color in case of room
• Design Calculation
– Lighting design calculation are normally carried out by the
manufacturer with their in-house software.
– Calculation provides point by point illumination level by which total area
illumination can be analysed.
 FAQ : Structural Engineering BUU:TS / DS

Input for structural engineering?

• Drawings
– Plan & Section Layout
– Equipment drawings, Operating box / Junction box details
• Customers specification
– Minimum thickness of the member
– Factor of safety
– Type of structure ( Lattice / Pipe)
• Loading information
– Conductor tension (static and short circuit)
– Wind pressure on structure, conductor, insulator
– Equipment loading data
– Weight of conductor, insulator string, hardware
 FAQ : Structural Engineering BUU:TS / DS

Column & Beam loading details?

• Horizontal Loading
– T: Conductor Tension
– SWT: Shield wire tension
– Wind load
• Parallel to Beam
• Perpendicular to Beam
• Vertical Loading
– Insulator String
– Conductor
– Deviation component
– Self weight of
• Beam
• Column
FAQ : Structural Engineering BUU:TS / DS

Equipment loading details?

• Horizontal Loading
– Electrodynamic force of busbar
– Wind load
• on equipment
• on busbar
• on structure body
• Vertical Loading
– Weight of busbar
– Self weight of equipment
FAQ : Civil Engineering BUU:TS / DS

Input for Civil Engineering?

• Loading data
– Tower foundation loading
– Equipment foundation loading
– Foundation load data for
• Transformers
• Reactors
• NGR
– Cable trench length & sections
– Weights of cable in trenches
FAQ : Civil Engineering BUU:TS / DS

Input for Civil Engineering?

• Control room details

– Equipment Layout

– Cable trench or Cable cellar details

– Equipment mounting & inserts details

– Pipe inserts and opening for cables

– Cable rack and inserts details

– Light fittings layout and air conditioning ducting layout

– Erection / maintenance opening

FAQ : Civil Engineering BUU:TS / DS

Input for Civil Engineering?

• Input from customer
– Technical specification with
• Type of foundation
• Factors of safety
• Finishing schedules
– Soil investigation report
– Water Table
– Land contour map for levels
– Finish Ground Level (FGL)
– Out fall point for surface drainage discharge
– Type of roads and it’s construction specification
Installation Activities BUU:TS / DS

Installation activities for substation can be divided into following

groups:
• Civil Activity (outdoor and control room)
• Structural erection (column, beam, equipment support structure)
• Equipment erection (indoor and outdoor)
• Cable laying and termination
• Erection of auxiliary system (HVAC, Fire-fighting, Illumination etc.)
• Pre-commissioning testing (individual equipment / system e.g. stability
test)
• Commissioning (energization)
• Punch list / defect list finalisation
• Final commissioning certification (from customer)
• Handing over / taking over certification (from customer)
• Winding up
Installation Activities BUU:TS / DS

Following activities are also important for completion of site

installation work:
• Site leveling and land development
• Material checking and inspection
• Monitoring & control of sub-contractor agency
• Measurement and invoicing
• Electrical Inspectorate approval / clearance
• Material reconciliation
• As-built marking on document / drawings
• Safety
Installation Activities BUU:TS / DS
Thank You