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Home / Technical Articles / Procedures and activities during the design and tendering phase of HV project engineering

Project definition for a start

Before getting into the project engineering phase, power system planning with the focus on basic design aspects of main power system components must be performed. Based on recommendations resulting from various studies, the project engineering phase, mainly covers the design, tendering and contracting/project implementation phases. This technical article tackles the tendering procedure from a technical point of view.

Procedures and activities during the design and tendering phase of HV project engineering
Procedures and activities during the design and tendering phase of HV project engineering

In this technical article the focus is set upon the general procedures and activities during the high-voltage project engineering phase up to contracting/project implementation, after the project has been approved for realization and execution.

The creation of projects in the field of power and energy may be required for various reasons, such as:

  • Construction of new substations, overhead transmission lines or power cable systems as a result of power system planning
  • Extension of existing power systems due to expansion of the supply areas, increase in power demand, addition of new supply point(s)
  • Re-configuration of existing power network(s) to cope with developments and requirements in cities, regions or countries
  • Interconnection between power systems
  • Development of new power supply network(s) for new industrial complexes
  • Addition of new or extension of existing power plants
  • Improvement of existing power supply or electrical networks to increase reliability, operation flexibility, reduce network losses
  • Rehabilitation or refurbishment of existing plants, substations, lines or components to meet the increasing requirements of power system development and expansion.
The prior studies and planning work provide the basis for the definition of parameters and design criteria for the project material and components. Further engineering activities include development and preparation of technical concepts, specifications, layout plans, route plans, project implementation schedules and estimation of project costs.

The engineering activities required to realize the planned project are carried out by the engineering divisions of the client (e.g. a utility), or are assigned to an engineering company.

Table of Contents:

  1. Terms of Reference (TOR)
    1. Scope of Engineering Activities
  2. Project Funding
  3. Form of Tendering
  4. Planning and Design Requirements
    1. Substations
    2. Overhead Lines
    3. Electrical Distribution Network
    4. Environmental Aspects
  5. Tender Structure
    1. General
    2. Tender Setup
    3. General Technical Specifications
      1. General Rules and Provisions Related to the Design
      2. High Voltage Metal-Clad SF6 Gas-Insulated Switchgear
      3. Common Features
      4. Circuit Breakers
      5. Isolators
      6. High-Speed Earthing Switches
      7. Measurement Transformers
      8. Interlocking and Control
      9. Gas System
      10. Medium Voltage Switchgear
      11. Control, Supervision and Protection
      12. Overhead Lines
      13. Power and Auxiliary Cable
      14. Telecommunication System
      15. Transformers and Reactors
      16. Auxiliary Equipment (for AC and DC supply)
      17. Station Lighting
      18. Lightning Protection and Earthing System
      19. Civil Works for Substations
  6. Scope of Work and Supply (Example)
    1. 380 kV Switchgear
    2. 123 kV Switchgear
    3. Transformers and Reactors
    4. Telecommunication System

1. Terms of Reference (TOR)

Terms of reference for a defined project are issued by clients/authorities and describe the services, supplies and work requested for the execution of the project under its terms and relevant regulations. Eligible or short-listed companies are invited to offer the work and services.

The terms of reference ( TOR ) are normally prepared by the client.

The TOR is a very important document. It gives a comprehensive overview of the client and its organization, the nature and status of the project, the requirements for services, engineering services, hardware and software, implementation schedule and commercial conditions and to outline the objective (such as to construct new high-voltage substations and overhead lines to meet the continuing growth in power demand, or to investigate the power interconnection with an industrial company to improve the reliability of power supply).

The functions and areas of responsibility of the client and the organization of the power sector are outlined, for example, responsibility for generation and transmission of bulk power throughout the country, or responsibility for distribution of electric power. Data and information relevant to the power system and system composition are stated, for example, radial distribution and transmission system.

Download an example of Terms of Reference (TOR) in PDF format:
Download (PDF)


Under this technical article three examples of projects are selected which are defined e.g. by utilities or power supply companies under development or power system expansion programs, or part thereof, with the aim to cope with the requirements of increasing power demand or changes in the power network structure.

Example

The client (e.g. in south-east Asia) plans to expand the existing 380/123kV supply system with new substations and switching stations in order to cope with the continuing growth in demand. Engineering services are needed to design the substations and switching stations and prepare turnkey solicitation followed by the tendering procedures.

Engineering activities shall cover the engineering and design as well as the preparation of tender documents followed by the tendering procedures.


1.1 Scope of Engineering Activities

The required engineering activities range from feasibility studies to project monitoring during the implementation stage. For project execution, two main project phases are usually defined: the preconstruction phase and the construction phase.

The activities in the preconstruction phase include planning and design as well as technical studies as may be required for determination of design parameters or to confirm power system configurations. The preconstruction phase generally covers the engineering and design work until award of a contract to the successful contractor(s).

The main engineering activities required and defined by the client or utility company are of the sort described below.

  • Collection of data and review of previous studies/reports (design parameters and requirements, standards, environmental aspects, technical risks).
  • Review of design practices, preparation of design for the network components (e.g. substation and associated equipment, overhead lines, communication equipment).
  • Preparation of tender documents with all associated administrative, commercial and technical sections, schedules and drawings: for example, invitation, general conditions, technical specifications, data sheets, price schedules and drawings.
  • Preparation of prequalification documents as required.
  • Assistance during the tendering period.
  • Tender evaluation work.
  • Assistance during the contract award procedure.
  • Detailed power system studies, for example, power system optimization studies for an industrial power system.

The main engineering activities during the construction phase focus on the following aspects:

  • Approval of the contractor’s drawings and documents, submitted for the defined project and forming the basis for manufacturing, testing and execution of works.
  • Tests on equipment in the contractor’ s factory or at independent testing sites for type tests.
  • Construction supervision and monitoring.
  • Site tests, commissioning and taking over of completed facilities.
  • Review and compilation of “as-built” drawings and documents; review of operations and maintenance manuals.

Suggested Guide – Best practice in HV substation construction projects

Best practice in HV substation construction projects

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2. Project Funding

When planning a power system project, for example, power transmission/power distribution, interconnection of power systems, extension of telecommunication system, apart from the technical and economic feasibility it is of great importance for successful implementation and timely completion to secure the necessary funding and financing for the technical equipment, work and services in connection with the project.

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3. Form of Tendering

After the design parameters and equipment specifications have been established in the preceding stages, preparation of the tender documents follows as the next step. The whole project may be divided into component packages or lots as may be defined between the client and the funding agency if this is required because of the project’ s size and varying project financing or to allow contractors to quote in their special fields only (for example, switchgear, substations or overhead lines), or for the whole project as general contractor.

For assistance and support in the engineering activities, in the majority of cases the services of external engineering firms or institutes are engaged by the client according to its own policy or/and in line with the regulations of the funding agency.

Upon approval of the tender documents by the client and higher authorities, the tender procedure can commence. A tendering period of up to 3 months for normal size overhead line or substation projects is the standard time allowed for the contractor to prepare the tender. Further steps are described in the following subsections.

The procedures for tendering and awarding of contracts for international construction projects are generally based on guidelines and recommendations by FIDIC (Federation Internationale des Ingenieurs). Provisions are made to assist the client or the assigned engineering company to receive complete, sound and competitive tenders in line with the tender documents.

Download FIDIC Tendering Procedures in PDF format:
Download (PDF)

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4. Planning and Design Requirements

Diligent planning, design and dimensioning of electrical power systems and their related components are important engineering activities and form the basis for the technical specifications of equipment, layouts and work to be prepared for the tender documents. The activities include the review and updating of existing specifications, data, layouts, routing for overhead lines and cables, power system configurations, and determination of parameters in connection with the use in the defined project.

Relevant system and network studies are to be performed, where necessary, to confirm the planned configuration, design data or parameters, or to recommend measures for improvement or selection of higher ratings for equipment and so on.

Specialized engineers from the different engineering disciplines are assigned to the specific tasks: for example, electrical and mechanical engineers for power system planning, substation design, including switchgear and associated components and equipment, protection design, auxiliaries, power transformers, power cables and overhead lines; civil engineers for the design and construction of buildings, foundations and structures.

Detailed design specification is not covered in this technical article.

The aim of the planning and design work is to reach the best technical standard and coordination between requirements for the equipment and work under the definition of the project and integration into existing power systems such as to ensure maintenance and operational compatibility.

Design requirements and activities to be dealt with are presented for the main components, such as substations, overhead lines and distribution network.

Suggested Reading – Medium voltage networks – Load flow calculation & network planning

Medium voltage networks – Load flow calculation and network planning

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4.1 For Substations

  1. General technical substation requirements
  2. Basic electrical data, insulation coordination
  3. Layout plan and general arrangement of complete substation
  4. Drawings including electrical single-line diagrams
  5. Detailed description and requirements for indoor/outdoor type high-voltage substations, including switchgear, circuit breaker, isolator, current and voltage transformers, earthing switch and control units
  6. Detailed description and requirements for MV metal-clad switchgear, including withdrawable circuit breaker, instrument transformers, metal-clad cubicles and earthing switch
  7. Control equipment, including interlocking, measuring and metering instruments, alarm indication and voltage regulation
  8. Protective and control systems
  9. Station supply, power and auxiliary cables and lighting system.

Suggested Course – The Power Substation Fundamentals Course

The Power Substation Fundamentals Course: Theory, Equipment, Earthing, and Design

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4.2 For Overhead Lines

  1. Line route profile
  2. Configuration/design of towers and tower earthing in view of insulation coordination
  3. Applicable standards
  4. Line design regarding conductor size, loading parameters, maximum/minimum and average spans and foundation types
  5. Mechanical and electrical requirements for all line components such as phase-conductors, earth-conductors, insulator strings and hardware
  6. Soil classification and foundation design standards
  7. Requirements for construction, erection and maintenance
  8. Environmental aspects of line routing.

Suggested Handbook – Handbook on EHV overhead lines and underground cables

Handbook on EHV overhead lines and underground cables

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4.3 For Electrical Distribution Network

  1. Review of existing loads
  2. Review of actual network topology with view to adaptation to new requirements
  3. Review of design philosophy; for example, conductor cross-section, concrete/wooden poles in MV/LV network
  4. Review of technical network data with view to adaptation to new requirements, for example, the need for upgrading to overcome constraints
  5. Updating of the single-line diagrams, layout plans, tables with revised data.

Suggested Reading – Power Distribution Network Explained To Electrical Engineers

Power Distribution Network Explained To Electrical Engineers (Beginners)

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4.4 For Environmental Aspects

Environmental regulations and requirements need to be considered. In some cases the tender documents ask for an environmental impact study to clarify the impact on hydrological conditions, dewatering, animal life, noise-emission, and so on, and suggest suitable countermeasures.


5. Tender Structure

5.1 General

The design and specifications developed or updated during the engineering phase, especially the design phase, for the equipment and work involved in the implementation of the defined project form the basis for the preparation of the tender documents.

The tender documents contain all necessary information, instructions, commercial procedures, technical specifications, scope of supply and work, data and drawings enabling the tenderer to offer the desired equipment and services in line with the client’s and the bank ’s guidelines.

The tender documents are made up of technical parts, containing specifications, technical requirements regarding the engineering, supply of material, installation and putting into operation of equipment and plants, and the administrative, commercial part. In case of large projects or as determined by the authorities or for reasons of control, the tender documents often are split into lots , also called packages, to differentiate individual project components or groups or equipment.

For example:

  • LOT 1: HV (400 kV) transmission line from location A to substation B
  • LOT 2: Substations
  • LOT 2A: New HV (400/132 kV) grid station(s) (with conventional control, and supervision or computerized control and supervision)
  • LOT 2B: Extension of HV (400 kV) grid station(s)
  • LOT 2C1: New HV/MV (132/33 kV) substation(s)
  • LOT 2C2: Extension of HV/MV (230/33 kV) substation
  • LOT 3: Underground cables (HV and MV)
  • LOT 4: Substations and overhead lines (33 kV).

Generally the tender documents are prepared for international competitive bidding taking into consideration local regulations/stipulations as well as the guidelines of funding agencies.

Download an example of Tender Document in PDF format:
Download (PDF)

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5.2 Tender Setup

A standard has been developed regarding set-up and arrangement of the tender documents. Typically, the tender documents consist of documents or sections common to all lots or packages and documents specific to each individual lot or package subject to the nature of the project as described in the sections below.


5.2.1 General, Common Sections

The administrative, commercial and financial aspects are compiled in four sections as indicated below:

  • Section 1: Tender invitation and instructions to tenderers
  • Section 2: Tender forms
  • Section 3: Price tabulation sheets and time schedule
  • Section 4: General conditions of contract.

In Section 1, guidelines and instructions are laid down for tenderers concerning information: for example, project location, routes, limits of work, climatic and environmental conditions, preparation and submission of tenders, time schedule, bid opening and evaluation procedure and criteria, set-up of the tender documents, filling-in of form sheets, currency, certificates, references and documents from the tenderers to prove their eligibility, qualification and experience to carry out the work according to international standards.

In Section 2, instructions are given to be observed by the tenderer as regards acknowledgement of contract conditions, confirmation of prices, completion of works, tender and performance bond to be filled or provided by the tenderer, company profile and form of contract agreement.

Section 3 contains the price tabulation sheets and time schedule. The price sheets are prepared for the tenderer to quote for supply of material and equipment as well as for erection. Foreign and local currencies are differentiated. Key dates are to be indicated in the time schedules for the main activities, such as design work, production, transport, erection work and commissioning.

Table 1 shows a sample price sheet.

Table 1 – Sample price sheet for an HV grid station (click to zoom)

Sample price sheet for an HV grid station
Table 1 – Sample price sheet for an HV grid station

Section 4 deals with the general conditions to be observed in the execution of a contract. In general the “Conditions of contract for electrical and mechanical works” prepared by FIDIC are taken as a basis.

The main topics include:

  • Definitions and interpretations
  • General instructions, such as language, approval procedures and standards
  • Documents to be submitted
  • General obligations, such as performance bond, customs duties and taxes
  • Quality of material, equipment and workmanship
  • Transport
  • Erection work, completion and acceptance of work
  • Responsibility and liability of contractor, insurance
  • Force settlement of disputes
  • Terms of payment
  • Damage penalties.

Suggested to Study – Technical specification for construction of 33/11kV 2×31.5MVA stations

Technical specification for construction of 33/11 kV 2×31.5 MVA power substations

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5.2.2 Sections Specific to Each Lot or Package

The technical specifications, scope of work and supply, technical data sheets, tender drawings are specific for each individual project, depending on the nature of the project: for example, substation project, overhead line project, communication project. The required papers are compiled in the sections detailed below.

For the individual project lot or package, the “technical” sections are combined with the common sections (Sections 1–4 above), to form the complete tender document.

  • Section 5: General technical specifications
  • Section 6: Particular technical specification, scope of work and supply
  • Section 7: Technical data sheets
  • Section 8: Tender drawings.

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5.3 General Technical Specifications

Technical specifications as applicable for the electromechanical equipment, material and work are compiled and included in the tender documents. It must be assured that the last valid versions of the technical specifications and data are being used. The relevant specifications are to be applied subject to the nature of the power project.

For example:

  • General criteria for the design, including climatic and ambient conditions such as temperature, rainfall, storms, relative humidity, and so on
  • High voltage metal-clad Gas-insulated switchgear (SF6)
  • Medium voltage switchgear (gas-insulated type)
  • Control, supervision and protection
  • Overhead line
  • Power and auxiliary cable
  • Communication system
  • Transformers and reactors
  • Auxiliary equipment
  • Spare parts, tools and test equipment
  • Site services and civil works.

Taking a substation project as an example, the following component specifications are applicable, as expanded upon in the subsequent text.

  • General criteria for the design
  • High voltage metal-clad SF6 gas-insulated switchgear or high voltage outdoor switchgear
  • Medium voltage switchgear (gas insulated type) or air-insulated metal-enclosed switchgear
  • Control, supervision and protection
  • Auxiliary equipment and spare parts
  • Site services and civil works for substations

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Suggested Reading – Instructions for making specifications and selecting the main components of an HV substation

Instructions for making specifications and selecting the main components of an HV substation


5.3.1 General Rules and Provisions Related to the Design

General parameters and information are outlined related to prevailing local conditions and practice to be observed in the design work and for tendering. The equipment must be suitable in all respects for use and operation within the defined power systems.

For example:

  • Location, site levels, climatic conditions (information on wind velocity, storms, etc.)
  • Ambient temperatures, relative humidity and meteorological data
  • Soil conditions (soil thermal resistivity in km W−1)
  • Power system data and characteristics, such as system operating voltages:
    • HV level: For example, 380/420 kV, 220/245 kV;
    • HV and MV levels: For example, 132/145kV, 33/36 kV, 11/12 kV;
    • Low voltage system: For example, 400/240V (+10 – 15%), number of phases, 3 (3-phase, 5-wire system); frequency 50 Hz; neutral earthing.
The technical specifications, as part of the tender document, are the basis for the technical design of the equipment and work required for the defined project. The client’s standard technical specifications may have to be adapted to the individual project needs in the course of completing the tender documents.

Tenderers are responsible for including everything required and/or necessary to complete the entire work properly, irrespective the fact that not every item may be specifically mentioned in the specifications.

Interesting Reading – The story of designing the electrical part of MV/LV power substation

The story of designing the electrical part of MV/LV power substation

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5.3.2 High Voltage Metal-Clad SF6 Gas-Insulated Switchgear

In the case of a high voltage substation project, the high voltage switchgear is the main project component. From the two basic designs, air-insulated outdoor switchgear and gas-insulated switchgear, the metal clad SF6 gas-insulated switchgear is taken as an example for the switchgear specification.

The specification of the gas-insulated switchgear and switchgear components include principal aspects such as common features, circuit breakers, isolators, high-speed earthing switches, measuring transformers and transducers, interlocking and control and the gas system itself.

Suggested Course – GIS Design, Installation, Operation and Maintenance, Testing

Gas-Insulated Switchgear Course: Design, Installation, Operation and Maintenance, Testing

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5.3.3 Common Features

General arrangement of switchgear and accessories for installation in switchgear rooms shall consider the following:

  • The switchgear construction shall be of suitable material and thickness to withstand the mechanical and thermal stresses due to short circuits. The rated duration of short circuit is 3 seconds.
  • Rupture diaphragms shall be provided in each compartment to allow for pressure relief.
  • Future extension shall be enabled.
  • The latest modern engineering practice shall be followed.
  • The switchgear shall be supplied complete with all auxiliary equipment necessary for operation, routine maintenance, repairs or extensions.
  • The design shall be compact, fully metal-clad and of the SF6 sulfur hexafluoride-insulated type.
  • Equipment shall be constructed for the indicated busbar system and include all necessary switches, current and voltage transformers, as indicated in the single-line diagrams.
  • Components shall have interchangeability as far as possible.
  • The arrangement of the switchgear shall be such that any part can be removed without interruption or disturbance to adjacent feeders or circuits.

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5.3.4 Circuit Breakers

Generally, the circuit breakers must fulfill the service operation conditions, for example, making and breaking of fault current. The circuit breaker operating mechanism must be capable of storing energy for the operation sequence as specified in the technical data sheets.

The three-phase circuit breakers shall  incorporate SF6 gas as an insulating as well as an arc-quenching medium (restrike-free switching must be guaranteed). Switching conditions are defined.

Suggested Course – Learn to Read and Analyze Circuit Breaker Schematics and Control Wiring Diagrams

Learn to Read and Analyze Circuit Breaker Schematics and Control Wiring Diagrams

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5.3.5 Isolators

Isolators must switch under zero-current condition and keep the switch position during short circuit.

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5.3.6 High-Speed Earthing Switches

High-speed earthing switches are required at the outgoing ends of every feeder and for the busbars. They shall be constructed to withstand an accidental switching onto a live part, that is, they shall be of the make-proof type.

Learn More – Major components you can spot while looking at opened LV and MV switchboards

Major components you can spot while looking at HV/EHV GIS (Gas-insulated switchgear)

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5.3.7 Measurement Transformers (Cts, VTs)

Measurement transformers include current transformers (CTs) and voltage transformers (VTs) according to the applicable standard IEC 60044. Main design particulars include:


5.3.7.1 Current transformers
  • Ring-core design of current transformer, secondary windings embedded in cast resin.
  • Requirements as to short-time primary rating to be not less than that of the associated switchgear.
  • Thermal rating of the current transformer such as to allow, under site conditions, a 20% continuous overloading referred to nominal rating of the current transformer.
  • Requirements regarding CT-cores for measuring and protection, magnetizing curves, rated output (30 VA minimum), class of accuracy, rated accuracy limit factors, rated primary current, turns ratio, knee-point voltage and resistance of the secondary windings.

Learn More – IEC & NEMA/IEEE ratings of current transformers (CTs) in MV applications

IEC and NEMA/IEEE ratings of current transformers (CTs) in medium voltage applications


5.3.7.2 Voltage Transformers
  • SF6-insulated voltage transformers shall be of the inductive type, encapsulated, the gas compartment to be segregated from the adjacent compartments.
  • Minimum rated output shall be 100 VA.
  • Busbar-VT shall be connected through hand-operated isolators.
  • Construction and testing in accordance with IEC 60269.

Learn More – The Essentials Of Voltage Transformers (Advanced Theory and Practice)

The Essentials Of Voltage Transformers (Advanced Theory and Practice)

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5.3.8 Interlocking and Control

The interlock system is defined, including circuit breakers, isolators, earthing switches and bus-couplers, to prevent any incorrect operation of the circuit breakers and switches and to at least fulfill the general requirements according to the detailed specification.

In case of key-operated switches, the operator is responsible for all switching operations.

Learn More – Learn how to interpret interlocking schemes between MV cubicles

Learn how to interpret interlocking schemes between MV cubicles (single line and wiring diagrams)

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5.3.9 Gas System

Principal features are as follows:

  • Due to the requirements for maintenance, the switchgear requires individual compartments, each having its own overpressure relief device. Extension of the busbar system shall be possible without de-gassing the existing part; that is, gas-tight bushings shall be provided at each busbar end.
  • The individual compartments shall be supervised via gas-density monitors with temperature compensated pressure gauges.
  • Gas losses shall be guaranteed less than 1% per year.
  • Further components form part of the specification, such as SF6-terminals, bus ducts, outdoor wall-mounted bushings, metal-oxide surge arresters.
  • SF6-gas losses shall be capitalized.

The maximum gas losses per switchgear compartment are defined not to exceed 1% per year. This value is to be guaranteed by the manufacturer.

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5.3.10 Medium Voltage Switchgear

For to medium voltage switchgear, standard designs and models from the manufacturer’s switchgear program are generally specified under the condition that the equipment meets the requirements of the specifications and serves the intended purpose. The minimum quality and performance requirements must be fulfilled.

The manufacturer must prove at least 5 years of successful service in the field.

The specification covers the following: Design, ratings, testing, shipping, installation and commissioning of factory-assembled, type-tested switchgear of different characteristics, for example, air-insulated switchgear, metal-clad type, single or double busbar system, draw-out section, vacuum circuit  breaker and SF6-insulated switchgear, triple-pole or single-pole metal-clad type, single or double busbar system, draw-out section, vacuum circuit breaker.

The voltage levels extend up to 36 kV; the maximum busbar rated current at this voltage level is stated as 2500 A and the maximum rated short-time current 31.5 kA.

Detailed and specific data required are contained in the drawings and data sheets of the tender documents.

Learn More – Lessons I learned during the start-up of an MV switchgear

Lessons I learned during the start-up of an MV switchgear

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5.3.11 Control, Supervision and Protection

The specification covers the control, supervision and protection of the substation. For control and supervision, the microprocessor-based substation control and monitoring system is the preferred technique rather than the conventional technique.

General features and design requirements related to the various components are specified in detail. For the protection systems, transformer tap changers, transformer supervision, meters, recorders and alarm indication panels shall be provided and installed in the control room.

The panels must be arranged in at least the following sections:

  • Switchgear control,
  • Transformer on-load tap-changer (OLTC) control/parallel interlocking,
  • Alarm annunciation,
  • Fault monitoring system,
  • Event recorder,
  • Protection panel(s),
  • Synchronization,
  • Communication system,
  • Load dispatch center (interface),
  • Air conditioning and ventilation,
  • Fire protection,
  • AC supply system, and
  • DC supply system.

Suggested Reading – Control system equipment that supervises, protects and controls the substation operation

Control system equipment that supervises, protects and controls the substation operation

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5.3.12 Overhead Lines

For high voltage (HV) and extra high voltage (EHV) overhead transmission lines, a large number of different configurations is available, using self-supporting lattice steel towers. For low- and medium- voltage lines, concrete, steel or wood poles are used.

The specification for the overhead line is set up according to the nature of the project and covers the design and requirements concerning manufacture, factory testing, delivery, transport, installation, site testing of towers, insulators, phase-conductors, earth-conductors, fittings, and so on as well as the associated civil works, access roads, foundations.

All required equipment, material and work for the overhead line, whether specified or not, shall be included to achieve a safe and reliably designed system. The main components and aspects dealt with in the specifications include the following.

Suggested Reading – The art of the switchyard design: Handpicked details you must consider without fail

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5.3.12.1 Soil Investigations and Tests

Soil investigations and tests are done to determine the necessary soil mechanical parameters for the foundation design of towers. The soil conditions and characteristics shall be ascertained, using approved methods.

After determining the specific soil characteristics, the applicable type of foundation shall be defined.

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5.3.12.2 Foundations

Foundation types are, for example, normal foundations and pile foundations for towers, which may be employed where special ground conditions exist. A number of other types of foundation can be required depending on the tower or pole type, the soil investigations and results of laboratory tests.

Prior to the selection of foundation types, all relevant calculations, data (design criteria, uplift criteria, safety factors) and drawings are to be submitted by the contracting firm for approval.

Suggested Reading – Substation and switchyard support structures for electrical equipment

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5.3.12.3 Towers

Towers for a high voltage transmission line project are assumed in the following. The specification covers the main features and design data for construction and materials as well as for phase-and earth-conductors. The different tower types, such as suspension, tension and angle towers, shall be of standard construction.

Single-circuit, double-circuit or multi-circuit towers are defined. The tower shall be designed as self-supporting lattice type steel frame with square base. Tower outlines shall be as shown on the tender drawings. The members of the lattice structure are to be of hot-rolled steel angle sections, factory made and hot-dip galvanized.

Existing proven tower design may be used, if equaling or exceeding the design loading and clearances required by the specification.

Criteria such as design loading and design unbalanced loading (broken wire conditions), safety factor and overload capacity are to be considered in the design, as well as wind conditions. Permissible values according to standards must not be exceeded. No damage or permanent distortion of any members, bolts, connections of fittings or elongation of bolt holes shall be permitted for these design conditions.

Other aspects include:

  • Tower grounding
  • Solar-powered aviation obstruction lighting
  • Workmanship equal to the best modern practice in the manufacture and fabrication of materials covered by this specification
  • Tower locations based on detailed survey work performed by the contracting firm to determine the tower locations in the map and longitudinal profiles and prepare tower lists with main data for each tower including wind span and weight span.
  • Tower testing by load tests to be applied to the tower to specify that each tower shall withstand the test loads for at least 5 minutes without failure or permanent distortion of any member, fitting, bolt or part and without elongation of bolt holes.

Further Study – HV Transmission Line Components (Towers, Conductors, Substations, ROWs and Roads)

HV Transmission Line Components (Towers, Conductors, Substations, ROWs and Roads)

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5.3.12.4 Phase-Conductors and Earth-Conductors

Particulars of the conductors to be supplied and of standards are set out in the schedule. The conductors shall be manufactured and tested in accordance with standards as indicated in the schedules. The conductor is supplied on reels. Further specifications include optical fiber ground wire (OPGW) to comprise an optical unit integral with the earth-conductor.

The OPGW design shall be mechanically and electrically compatible with design of the transmission line.

Suggested reading – Learn how to design an optical communication system for a transmission line network

Learn how to design an optical communication system for a transmission line network

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5.3.13 Power and Auxiliary Cable

The specification covers the design, manufacture, factory testing, supply, transport, laying and installation, and site testing of power cables, associated pilot, telephone and optical fiber cables, auxiliary cables and control cables, including all civil works, cable terminals, cable racks, cable fixing material, and so on.

All equipment must be covered, irrespective whether specified or not, to form a complete and reliable system. Typical power, control and communication cables used in electrical power systems are:

  • High voltage XLPE cables
  • Oil-filled cables of the low pressure oil-filled type
  • Medium voltage XLPE cables
  • Low-voltage power cables (nominal AC distribution voltage, 3-phase/5-wire system, PVC insulation)
  • Protection and telephone cables (e.g. 17-pair cable, 5 pairs for protection, 12 pairs for telephone usage)
  • Optical fiber cables (shall have fibers of the single-mode type, suitable for transmitting light signals).

Requirements are laid down for the different types of power, control and communication cables, including standards to be complied with, cable design and construction, sealing and drumming, tests at independent institutes (type tests) or manufacturer’s premises and at site, laying and routing.

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5.3.14 Telecommunication System

The specification covers the design, manufacture, testing, delivery, transportation and erection as well as the commissioning of all material and equipment required for the telecommunication systems and system extension. Requirements are defined such as extension of existing communication systems for telephony, teleprotection signaling and data/alarm transmission by use of fiber-optic transmission media.

Compatibility of equipment to cater for any upgrading without limitation shall be guaranteed.

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5.3.15 Transformers and Reactors

The specifications and requirements cover the design, manufacture, factory testing, delivery, transport, erection and commissioning related to:

  • Power transformers
  • Distribution transformers (power rating up to 2.5MVA)
  • Reactors, including shunt reactors, current limiting reactors and neutral grounding reactors.

Detailed specifications cover the mechanical and electrical design of the transformers and reactors, including the aspects listed below:

  • Ambient temperature, operation condition
  • Number and type of windings
  • Magnetic core
  • Transformer/reactor tank, oil conservator, cooling (natural air ONAN, forced air ONAF)
  • Tap changer (on-load, off-load), voltage regulation
  • Control and monitoring device
  • Terminals, bushings
  • Condition of parallel operation
  • Test requirements.

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5.3.16 Auxiliary Equipment

Specifications of the auxiliary equipment are part of the tender documents for substations. The features and requirements regarding construction and design, parameters and arrangement (e.g. panels) are detailed.

For example:

5.3.16.1 AC supply

AC supply from two independent power sources with specified voltage, for example, 400/230 V, with defined tolerance, including distribution panels of a self-standing cubicle type, constructed equivalent to the control and protection panels of the switchgear.

The continuous and short-time/short-circuit ratings of the switchgears shall be according to the specified transformer ratings and the expected short-circuit rating. Operation conditions are defined.

Suggested Reading – AC Auxiliary Systems In Power Substations (Design Requirements and Equipment)

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5.3.16.2 DC supply

DC supply with battery and charger, including two battery chargers, two battery banks, two voltage-control units for regulating the output voltage, two main switchboards 110V for distributing power to the various loads.

Batteries shall be either of the nickel-cadmium type or of the long-life, sealed lead-acid type.

The distribution panels shall be of a self-standing cubicle type, constructed equivalent to the control and protection panels of the switchgears. The
continuous and short-time/short-circuit ratings of the switchgears shall be according to the specified transformer ratings and the expected short-circuit rating. Operation conditions are defined.

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5.3.17 Station Lighting

A complete station lighting system, indoor as well as outdoor, is to be included.

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5.3.18 Lightning Protection and Earthing System

It is required to determine by calculations and measurements whether impermissible touch and step voltages occur at any place of the station that may be endangered. Design principles are stated in the specification (e.g. the HV and MV systems are solidly earthed at the neutral of the transformers).

Suggested to Study – Earthing system calculation for 132/11 kV, 1×40 MVA substation of steel factory

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5.3.19 Civil Works for Substations

The civil works are part of the tender documents and cover all civil work in connection with the related substation project, including for example, design, manufacture, testing, delivery, transport, storage at site, erection, installation, commissioning, performance testing, and handing over in satisfactory operating condition of all civil work, such as switchgear building, control building and miscellaneous work.

Regarding the buildings, air-conditioning system, fire-fighting system, station lighting system for indoor and outdoor as well as lightning protection and earthing for the entire station shall be provided.

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6. Scope of Work and Supply

Section 5, specific to each lot or package, deals with the determination of the scope of supply of equipment and material and associated work required under the defined project. The main objective of the project, the nature of the project and general requirements are introduced.

For details of technical requirements and descriptions of the complete work, reference is made to the technical specifications and the related particular sections of the tender documents.

An example is given below for a substation project (only one lot is assumed). The tender documents call for the supply, delivery, erection, commissioning and handing over of a new 380/123kV grid-station at location A and the extension of the 380 kV/123 kV substations at location B.

Within the framework of supply and work the following shall be outlined.


6.1 380 kV Switchgear

The gas-insulated switchgear (GIS) shall consist of a double busbar system for five feeders, current rating 3150 A, short circuit current 63 kA – 3 s, general single-line diagram as per the tender drawing. Space for one future feeder shall be allowed for.

Each feeder bay for overhead line connection to be equipped with:

  • Three-pole circuit breaker
  • Three-pole maintenance earthing switches
  • Three-pole high-speed earthing switch
  • Three-pole line-disconnecting switch
  • Three-phase current transformers with separate cores for different protection and metering purposes as per tender drawings
  • Set (three-phase) of voltage transformers, SF6-insulated
  • Set (three-phase) of gas – insulated type surge arresters
  • Set (three-phase) of outdoor terminations, consisting of SF6 bus-ducts with outdoor bushings
  • Related control cubicle for each feeder.

The other bays are equipped analogously according to the specification.

Suggested Reading – Secrets and warnings in operation and maintenance of a Gas Insulated Substation (GIS)

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6.2 123 kV Switchgear

The GIS shall consist of a double-busbar system for ten feeders, current rating 3150 A, short-circuit current 40 kA – 3 s, general single-line diagram as per the tender drawing.

The feeders each shall be equipped with:

  • Two three-pole busbar selection isolators
  • One three-pole circuit breaker
  • Two three-pole maintenance earthing switches
  • One three-pole high-speed earthing switch
  • One three-pole line-disconnecting switch
  • Two three-phase current transformers with separate cores for different protection and metering purposes as per the tender drawings
  • One set (three-phase) of voltage transformers, SF6-insulated
  • One three-pole cable end unit suitable for connection of XLPE cable sealing ends.

The related local control cubicle for each feeder shall be equipped with the necessary interlocking unit, one amperemeter with selector switch or three amperemeters, one voltmeter with selector switch, one selector switch with positions: off – local – remote, transducer and annunciator block.

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6.3 Transformers and Reactors

This section covers the design, manufacture, factory testing, transport, erection, installation, commissioning and handing over in satisfactory operating condition of transformers/reactors, including all panels, auxiliary equipment and accessories.

The design of the transformers, for example 300 MVA, shall comply with the following main design data:

  • Rated power: 300 MVA
  • No-load voltage ratio: 400 kV ± 15%/142 kV
  • Vector group symbol: YNyn0(d)
  • Type of cooling: ONAN/ONAF/ODAF
  • Rated frequency: 50 Hz
  • Voltage regulation: On-load tap changer.

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6.4 Telecommunication System

The scope of work and supply covers the design, production, supply, transport, installation, cable laying, wiring, testing, commissioning and handing over in satisfactory operating condition of the telecommunication system, including all auxiliary equipment and accessories for multiplex equipment (optical terminals), telephone-alarm system, teleprotection, signaling equipment, power supply for communication system, fiber-optic cables, radio equipment and closed-circuit television systems.

The equipment shall be accommodated in cubicles/distribution racks. Further components included within the scope are power cables, control cables and communication cable, auxiliaries and civil works.

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Source: Planning, Design, and Operation of Power Systems and Equipment by Juergen Schlabbach and Karl-Heinz Rofalski

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

3 Comments


  1. Asaad hummeida Abd Alsamad
    Sep 27, 2022

    This is very beautiful, coordinating between all professions, because the work of electricity does not bear a line


  2. Steven Katopwa
    Sep 27, 2022

    Well elaborated power systems project management by management project approach.


  3. David Bowles
    Sep 26, 2022

    Very well developed format and thorough plan for considering a project of several sizes and complexities. Could be used to consider submission of a proposal for a complicated project.

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