Electrical Substations and Switchyard Design

Electrical substations and switchyard design
Technology training that works

Power network
TS – Terminal substation
ZS – Zone substation
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Substations
• Terminal substations Zone substations Distribution
substations
• Terminal substations
− High voltage transmission lines interconnected around the network at Terminal
substations
− Step down transmission voltage and supply power to zone substations. (Secondary
transmission)
− Secondary transmission voltages -132kV, 66kV and 33kV
• Zone substations
− Further step down voltage and supply to distribution networks
− Typical output voltage - 6.6kV ~ 33kV
• Distribution substations
− Receive power from zone substations, supply power to distribution network and
end users after stepping down voltage (MV or LV)

Functions of a substation
• Supply electric power to consumers continuously
• Supply of electric power within specified voltage limits
• Shortest possible fault duration
• Optimum efficiency of electrical network
• Supply of electrical energy to consumers at lowest cost

Substation types
Based on service
• Transformer Substation – Transform power from one voltage to another voltage
• Switching Substation – Switching of power lines without transforming voltages
• Converting Substation – Conversion of AC - DC – AC (for HVDC transmission)
Based on voltage
• High voltage Substation - 11kV and 66kV
• Extra high voltage Substation - 132kV and 400kV
• Ultra high voltage Substation – Voltages above 400kV
Based on installation
• Outdoor Substation
• Indoor Substation (Air insulated or gas insulated)
− Usually for < 66kV
− Heavily polluted areas
− Adverse climatic conditions
− Need for high reliability
− Space constraints

Air insulated substation vs Gas insulated substation
Air insulated substation
•Popular where space constraints and environmental restrictions are
not severe
•Exposed to environment
•Not completely safe from electrical shock hazards
Gas insulated substation
•Limited space requirements
•Protected from environment
•Higher reliability than air insulated substation
•Provides safety from electrical hazards – Shock, arc flash
•More expensive than air insulated substation

Substation types
Outdoor
Indoor – Air insulated
Indoor – Gas insulated

Location of key assets
• Key assets refer to major electrical installations
– Generating plants
– Switching/Transformer substations
– Transmission and distribution corridors
• Correct location makes a substation optimal
– In terms of investment
– In terms of recurring losses
– In terms of maintaining proper voltage profile
– In terms of maintainability and expandability

Planning criteria
• Load forecast (average load, peak load, base load, etc.)
• Security of supply (frequency and duration of outages)
• Power quality (harmonics, voltage flickers, BIL, etc.)
• Voltage limits (permissible voltage drop, etc.)
• Site conditions (IP rating, soil resistivity, etc.)
• System safety
• System flexibility
• System reliability
• System earthing
• System protection

Principles of design
General requirements
Installation and equipment shall be capable of withstanding
electrical, mechanical, climatic and environmental influences
anticipated at site
Design should take into account:
• Purpose of the installation
• End customer’s requirements such as power quality, reliability, availability
• Ability to withstand effects of transient conditions – Switching large loads,
short power outages and re-energisation
• Safety of substation personnel and the public
• Ease of extension (if required) and maintenance

Approach to substation design
• Collect field level data
• Projection of future growth
• Analyse data for optimised location of substation
• Decide on basic system parameters
• Decide substation configuration
• Arrive at equipment ratings/sizing
• Select appropriate equipment
• Ensure maintainability
• Ensure expandability

Data collection
• Make assumptions based on existing consumption
patterns
• Check for new developments
• Assess new load requirements (E.g. A new industry
being planned)
• Check for any major demographic patterns in the offing
• Assess seasonal change patterns
• Assess possible growth of loads
• Use economic indices for growth planning

Data collection
• Overall energy requirement and power demand
– Indices from similar locations
• List of connected loads and locations
• Pattern of loading (variations due to requirements)
– Loads of highly fluctuating nature to be considered separately
• Assess load factor and diversity factor
• Separate critical loads from non-critical
• Assess future growth plans

Projection of future growth
• A difficult and uncertain exercise
• Needs constant re-evaluation
• Getting right inputs early
• Provide additional capacity (say for a 5 year period)
– Higher investment
• Provide flexibility in the system for additions
– Lower investment
• Avoid the need for total equipment replacement to
accommodate growth by proper initial planning

Distribution voltage options for industries
• Receiving and distribution at same voltage
– Avoids need for transformer
– Difficult to regulate voltage
– Independent earthing of plant system not possible
• Receiving at higher voltage
– Involves transformer
– Internal voltage independently variable through OLTC
– Can choose system earthing independent of Utility
– Transformer acts as buffer (harmonics, voltage dips)

Single transformer, single bus
configuration

Industrial supply with duplicate feeds
• Industries prefer duplication at different levels
• High availability requirements
• Expensive but flexible
• Multiple bus sections with couplers
• Duplicate bus systems
• Duplication of feeders as well as transformers

Dual incoming feeders with single transformer

Substation components and equipment
• Incoming transmission lines
• Circuit breakers, disconnectors, earth switches
• Power transformers
• Busbars and cables
• VTs and CTs
• Lightning arrestors
• Capacitor banks
• Protection relay and control apparatus

Loads and their characteristics
• Utility
– Dispersed loads
– Mix of large and small loads
• Residential (domestic)
– Small, perhaps single phase
– Daily and seasonal load cycle
• Buildings (commercial/business premises)
– Similar in nature to domestic but different daily cycle
• Industrial
– Larger loads concentrated in a small area
– Load fluctuations and load generated problems

Load forecasting
• Should determine following factors:
− Peak load
− Average load
− Connected load
− Maximum demand
− Load factor
− Demand factor
− Diversity factor
• Critical load, essential loads, general purpose/non-critical
loads
• Constant voltage supply without sags or surges
• Harmonic currents, need for filtering

Basic system parameters
• Voltage level selection
– Optimised cost/performance
– Availability (incoming supply)
– Standardisation
• Fault level
– Present
– Future system growth
• Nature of loads
– Critical loads
– Tolerance for interruptions
– Harmonics, power factor
– Steady, cyclic loads

Configuration
• Incomers and outgoing
– Number of incomers
– Incoming and outgoing voltage levels
• Redundancy planning
– Type of distribution (Radial, Ring etc.)
– Type of redundancy (Additional equipment, spare capacity etc)
– Integration of local generation if any (wind power, solar, co-generation)
• System Earthing
– Local statutory requirements
• Equipment for voltage/power factor/harmonic control

Design criteria
• Ensures all parties involved in design understand and
agree with rules and basic needs
• Ensures client and designer have considered all basic
aspects of project’s features
• Ensures client accepts operational and maintenance
needs
• Differentiates between requirements of specific project
and generalized requirements of standard specifications

System reliability
• Primarily relates to equipment outages and customer
interruptions
• In normal operating conditions, all equipment (except
standby equipment) are energized and all customers
receive power
• Scheduled outages (such as maintenance) and
unscheduled events disrupt normal operating conditions
and can lead to outages, interruptions
• Substation design plays important role in reliability of
the System

System flexibility
• Proper busbar configurations, connections - Loads can be added
and/or changed with ease
• Spare feeders even if breakers are not furnished
• Spare space for future transformer bays, incoming feeders if
substation is subject to significant load rise in future
• Enough ampacity and short circuit capability for conductors for
prospective load increases in future
• Adequate capacity for transformers so they are not overloaded
during future expansion projects

Substation layout
• Consists arrangement of number of switchgear in orderly fashion – Based on
function and separation space
• Earth clearance: Clearance between live parts and earthed structures
• Phase clearance: Clearance between live parts of different phases
• Isolating distance: Clearance between contacts of isolator
• Section clearance: Clearance between live parts and terminals of a work section
• Should provide security of supply - Very important
• Redundancy to ensure reliability in the event of fault, equipment breakdown or
maintenance
• However, not always practically feasible due to very high cost of
implementation

Security of supply
Types of substations giving varying securities of supply:
• Type 1: No outage required within substation for either
maintenance, fault conditions
• Type 2: Short outage necessary to transfer load to an alternative
circuit for maintenance/ fault conditions
• Type 3: Loss of circuit/ section of substation due to fault,
maintenance
• Type 4: Loss of entire substation due to fault, maintenance

Single Busbar configuration
Advantages
•Individual bus sections can either be connected together or
separated based on need
•Comparatively simple construction
•Comparatively smaller space requirements
•Comparatively lower costs
Drawbacks
•Failure of one bus section would result in failure of power to that
section
•Comparatively lesser flexibility when compared with
configurations discussed in coming slides

Double busbar configuration
Advantages
•Improved reliability of power supply
•Much more flexibility than single bus bar scheme
Drawbacks
•More complex system
•More space requirements
•More complex protection requirements
•Higher costs

Breaker and a half configuration
• Utilises legs with three breakers connected between two buses
• Two breakers connected in each leg – One and half breaker per circuit
• All breakers are closed and both main buses energised under normal conditions
• Two associated breakers must be opened to trip a circuit
• High reliability, maintainability and flexibility
• Protective relaying more complex compared with earlier discussed schemes
• Breakers and other components must be rated for sum of load of two circuits
• High security against loss of supply
• More complex
• More space requirements
• More expensive than other configurations

Mesh type configuration
Advantages
•Improved reliability of power supply
•Increased flexibility of operation
•More easier maintenance of switchgear without disrupting
supply
Drawbacks
•More complex than schemes mentioned earlier
•More complex protection systems
•More space requirements
•More costlier than previous schemes

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Electrical Substations and Switchyard Design

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