electrical substations and switchyard design

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Electrical substations and switchyard design

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Power network

TS – Terminal substationZS – Zone substation


• 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)

Substations


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 typesBased 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


• 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

Planning criteria


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

Principles of design


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


• 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

Load forecasting


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


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

Security of supply


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

Single 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

Double busbar configuration


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 configurationAdvantages•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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