Electrical Engineering Substation Design

This article is for design, construction and installation of main substations. Main keywords for this article are Substation Layout Diagram, Substation Installation, Electrical Engineering Substation Design, Basics of Designing Power Substations, Substation Design and Engineering.

Substation Design and Engineering References

American National Standards Institute (ANSI)
C2 National Electrical Safety Code
Institute of Electrical and Electronic Engineers (IEEE)
80 Guide for Safety in AC Substation Grounding
979 Guide for Substation Fire Protection
980 Guide for Containment and Control of Oil Spills in Substations
National Fire Protection Association (NFPA)
70 National Electrical Code
72 National Fire Alarm Code
850 Recommended Practice for Fire Protection for Electric Generating Plants and High Voltage
Direct Current Converter Stations
2001 Standard on Clean Agent Fire Extinguishers Systems

Basics of Designing Power Substations

Substation Layout Diagram

Electrical Engineering Substation Design

Substation Design and Engineering Definitions

For the purpose of understanding this standard, the following definitions apply.

Clean Agent. Electrically non-conducting, volatile, or gaseous fire extinguishing agent that does not leave a residue upon evaporation.
Main Substation. A substation which provides the interface between the source of power (power grid, utility, or generating plant) and the plant electrical distribution system.
Substation. A grouping of equipment for the supply and control of electrical power either to an electrical distribution system or directly to utilization equipment. The equipment usually, but not always, contains one or more transformers which reduce the supply system voltage to a distribution or utilization voltage.
Vital Equipment. The main power supply to the plant, and certain other facilities, are designated as vital equipment and are provided with special protection. The emergency generator and power supply to the security lighting system, plant communications and emergency systems are likewise classified as vital equipment.  

Substation Power Supply

Incoming Power

• Supply voltage shall be a function of the utility grid, plant load, and size of large  motors. When feasible, electrical service shall be taken from the utility at highest required utilization voltage.
• Services shall be installed as required to meet the reliability requirements of the specific facility, by mutual agreement of the electrical utility. For any facility, the number of services shall be minimized, and single point metering shall be utilized wherever possible.
• General Arrangement and Location of Main Substations  
• The preferred arrangement shall be to install and maintain the incoming  service transformers and high voltage (HV) inside the facility boundary.
• Main substation shall consist of HV switchgear, HV to medium voltage (MV) transformers and MV switchgear for distribution of power within the plant to unit substations.
• Main substation shall be located in an unclassified area between the utility’s incoming service and the concentration of loads to be served, and shall be close to the concentration of loads to be served. 
• Emergency generator(s) shall be provided to meet the requirements of SSD 13.
• Main substation shall be located inside the plant security fence, as this will avoid the need for a separate security fence, and will keep the feeders from the emergency bus to the security lighting within the secured area.  
• Main substation shall be far enough away from the process areas to be outside of the hazardous classified area.
• When selecting location for main substation on the plot plan, the following factors shall also be considered: routing for HV supply cables; location of large motors and load concentrations; and routing of plant distribution feeders. 

Substation Design and Engineering Criteria

Facility Description

Main substation shall consist of a building to house switching equipment, an outdoor  transformer yard, and the plant emergency power facility. Switching equipment may include HV gas insulated switchgear (GIS) for control and protection of circuits to the main transformers; MV switchgear for the plant distribution system; additional 4.16 kV switchgear for supply of utilization equipment in the utility area or nearby process units; and low voltage (LV) switchgear to provide station service to the main substation and nearby utility loads. In this case there might be a number of transformers in the outdoor yard, grouped around the building to minimize bus duct lengths for connection to the switchgear.

Bus and System Configuration

Main substation shall be designed to provide reliable service, facilitate  maintenance and allow for future expansion. Initially, a minimum of 20 percent spare capacity shall be provided in all equipment. Complexity of main substation, and extent of redundancy, shall be dictated by cost of lost production, risk to personnel and plant equipment that would be caused by loss of electrical power to the facility, and reliability of utility power supply.

 Where incoming voltage is supplied from HV transmission lines, indoor GIS shall be used for control and protection of transmission circuits. The more common bus arrangements for this switchgear are main-tie-main, ring bus, breaker and one-half and double bus. In most cases, the main-tie-main arrangement should be used.

If specified in purchase order and line diagram redundant transformers (or utility feeders) shall be provided, so at least 2 sources of power are available at MV level. MV circuits shall supply a double ended (main-tie-main) lineup of indoor MV switchgear. Deviation from this configuration shall be considered by Company when specific plant conditions warrant it. 

Equipment Ratings

Unless the utility provides HV to MV transformation, power shall be received at  115 or 230 kV or higher voltage level . Power shall be distributed at 34.5 or 13.8 kV from main substation to unit substations. Higher voltages can be considered subject to company written approval when specific plant conditions warrant it.

Both primary and secondary switchgear shall be selected to provide adequate short-circuit capacity, including a minimum 20 percent allowance for future growth under worse case conditions. For main-tie-main switchgear, shortcircuit current shall be based on tie breaker closed. Normal operation of switchgear shall be closed bus tie at HV switchgear, and open bus tie at MV and LV switchgear.  

Transformer size will be limited by available ratings of switchgear for continuous current and short-circuit duty. Each transformer supplying a double ended lineup shall be able to carry the total load on the lineup without exceeding its forced cool FA rating. Main substation shall have a minimum of 20 percent spare capacity for future growth and shall be designed for expansion to accommodate additional growth.

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Building Substation Installation

Security and Safety Directives

Main substation equipment is classified as vital equipment. Substation buildings shall  conform to SSD 9 and ANSI C2.

Air-Conditioning and Pressurization

Substation buildings shall be air conditioned. HVAC systems shall conform to HVAC System Design Criteria.

Interior Electrical Design

Wiring for station service and building utilities shall be in accordance with Building Electrical Design Notes and Guidelines. Cabling within the building shall be routed in cable tray. Use of conduit and wire shall be minimized. Cable penetrations of walls between rooms, and outside walls and floors, shall be sealed with approved fire rated cable windows. A minimum of 20 percent spare capacity shall be provided in all cable tray systems.

Lighting shall comply with Lighting Design Requirements in Process Industry , and shall meet the minimum illumination levels specified in Electrical System Design Criteria.

Equipment grounding shall be in accordance with Bonding and Grounding.  

Cable trays shall be installed parallel and at right angles to building walls and  ceilings. Sufficient space shall be provided around cable trays to permit adequate access for installation and maintenance of cables. Bottom of lowest cable tray shall be a minimum of 2.67 m above main substation floor. A minimum of 460 mm shall be maintained between top of a cable tray and ceiling. Clearance requirements for HVAC ducting and lighting shall also be taken into consideration. Clearance shall be measured to bottom of lowest roof
beam. Where cable trays are vertically stacked, a minimum of 300 mm shall be maintained between trays.

For cable tray installation under substation building, the same clearances shall be maintained, except that elevation of bottom of lowest cable tray may vary due to space limitations. A minimum clearance of 230mm (9 inches) shall be maintained between the top of a tray and tray supports, beams, piping etc., to facilitate installation of cables in the tray.

Raised/Access floor is not acceptable for electrical rooms/switchgear floor. Unless otherwise specified, cable cellar/room height shall be minimum 2 m above grade to allow cabling exit from the bottom of the building into cable trays.

 

Windows, Doors, Platforms and Stairs

• Substation buildings shall not have windows.
• Substation buildings shall have at least one single and one double exterior door, which shall open outward. Each room containing electrical equipment shall be provided with double doors, to avoid dead end aisles. Doors shall be double wall steel, and shall be provided with cylinder locks, inside stainless steel panic hardware, hydraulic door closures and stops. Single doors shall be at least 900 mm wide. Exterior doors shall be provided with weather stripping.
• Each room in substation buildings shall be provided with 2 means of exit.
• Provisions shall be included for moving the largest equipment module into the  building with the least possible disturbance to any other equipment in the building. Double doors shall be sized to permit passage of the largest assembled unit of equipment. Removable transoms shall be provided over equipment doors, to facilitate movement of large equipment.  
• Platforms and stairs shall be provided, as required, outside each exterior door. At equipment doors, platform shall be large enough for staging of equipment when it is moved in or out of the building.  

HV Switchgear Building

If 115 or 230 kV switchgear is required, a separate building shall be provided  for HV switchgear. HV building shall consist of the main switchgear room containing the GIS switchgear; a separate control room to house control and relay panels; a separate room to house station battery; and a mechanical room to house air conditioning equipment. The utility may require a separate room to house line protection panels, SCADA panels and metering.

• Incoming transmission lines and circuits to main transformer primary shall be solid dielectric cables run below grade. GIS switchgear shall have bottom entry for cables. A cable vault shall be provided below the GIS switchgear, or GIS switchgear room shall be elevated to allow routing of HV cables in cable trays, below the switchgear room. Unless otherwise specified on purchase order and related drawings, height of finished building floor shall be 2 m above grade.
• As a minimum, building shall be arranged to allow addition of one GIS bay to each end of a main-tie-main lineup. Cable tray space shall be provided for routing of associated cables. After addition of future bays, there shall be adequate clearance remaining around switchgear to comply with applicable code requirements. As a minimum, the following clearances shall be provided:
  a. 2000 mm from all sides and ends of switchgear to walls of building
  b. 4000 mm from all sides and ends of switchgear to any other  equipment
• Clearance requirements for GIS vendor’s HV test equipment shall be  determined before finalizing layout of HV switchgear building. Equipment layout shall be adjusted as required to provide necessary clearance.
• Building ceiling height shall be set to maintain a minimum vertical clearance of 1000 mm above highest piece of equipment, taking into account requirements for HVAC ducting and lighting. If cable tray is to be installed above equipment, see 7.3.4 for interior electrical cable tray design requirements.  
• An overhead crane shall be installed to facilitate movement of GIS switchgear.
• At least two 230 V receptacles shall be provided in switchgear room and control room of HV switchgear building.

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MV Switchgear Building

• MV switchgear building shall contain the MV switchgear supplying power to plant distribution feeders, station service transformers and other MV or LV transformers required for utility or process loads in close proximity to the main substation, and as a normal supply to the emergency power system switchgear. HVAC equipment shall be located in the same building and separate mechanical room is not required. A separate room shall not be required in this building for batteries. Hoods with vent shall be installed over batteries. Hood vents shall be equipped with an automatic damper, so building positive pressure will cause air flow out the vent.
• Power shall be supplied from transformers to switchgear by bus duct or cables through building side walls. Cables leaving the building shall be routed below grade or above grade in cable trays. Cable trays shall be installed above the switchgear or in a cable vault below the switchgear.  
• As a minimum, the building shall be arranged to allow addition of one vertical section to each end of each switchgear lineup. Cable tray space shall be provided for routing of associated cables, plus an additional 20 percent spare space. After addition of future sections there shall be adequate clearance remaining around switchgear to comply with applicable code requirements. As a minimum, the following clearances shall be provided:
  a. 1200 mm from the non-operating side and ends to nearest obstruction
  b. 2000 mm from the operating side to nearest obstruction
• Building ceiling height shall be set to maintain a minimum vertical clearance of 1000 mm above the highest piece of equipment, taking into account
  requirements for HVAC ducting and lighting. If cable tray is to be installed above equipment, see 7.3.
•  Unless otherwise specified in purchase order or related drawings, height of finished building floor shall be 2 m above grade.

Detection, Alarm and Security Systems

• Each room containing electrical equipment shall be provided with the following  fire detection and extinguishing equipment:
  a. Fire extinguisher certified for type A, B and C fires
  b. Optical flame detectors (number as required). A minimum of two  (redundant) detectors shall be provided per equipment room.
  c. Manual break glass fire alarm station
• Each substation building shall be provided with the following fire alarm equipment:
  a. Fire alarm control panel with outputs to local alarm light and horn shall be connected with the plant common system and to remote fire alarm
  control panel(s) located in security control room(s). Horn shall have a distinctive sound pattern.
  b. Local alarm light and horn mounted on exterior of building, for alarm on detection of fire or combustible gas. Light shall be visible from the
  nearest road.
• Each substation building shall be provided with an annunciator panel having visual and audio alarms for building systems and electrical equipment alarms. A contact shall be provided for remote general alarm.
• Each substation building shall be provided with a telephone connected to the plant telephone system. Where required, a hot line direct telephone connection
  shall be provided for communication with the serving utility and shall be in conformance to SSD 12.

Clean Agent Fire Suppression Systems

General Description
a. Each room containing electrical equipment may be provided with a
b. clean agent fire suppression system. The clean agent fire suppression  system shall be designed and engineered for total flooding for the areas outlined on the fire protection drawings.
The system shall be activated by a cross-zoned type detection system of ionization smoke detectors. Activation of the first smoke detection zone shall:
(i) Energize per-discharge bell(s) in the protected area
(ii) Illuminate the ‘alarm’ LED on the control panel
(iii) Illuminate an LED on the activated detector
(iv) Annunciate an alarm condition to fire and gas control panel
c. Activation of a second smoke detection zone shall:
(i) Continue to operate alarm devices
(ii) Sound evacuation horn(s)
(iii) Shut down the air conditioning and ventilation systems and close the dampers to affected area (via clean agent panel)
(iv) Shut down non-emergency electrical power to electrical building
(v) Activate an adjustable time delay that will delay the release of agent for up to thirty (30) seconds. The agent shall be released at the end of this time period unless a manual release is pulled. If a manual release is pulled, the agent shall be released immediately, by-passing any remaining time delay.
d. Discharge of the system may be delayed by activation of a dead-man type abort station, resulting in an audio and visual trouble alarm indication. Release of the abort station after both detection circuits have activated, shall result in a ten (10) second discharge delay period. Any time during the delay that the panel is reset with no reoccurring alarms will result in termination of the release sequence of the clean agent system. 

Agent

The fire suppression agent shall be FM-200 manufactured by great lakes chemical  corporation, or SABIC approved equal. The agent supply shall be located indoors, as
indicated on fire protection drawings.

Agent storage

a. Clean agent shall be stored in Underwriters Laboratories (UL) listed alloy steel containers.
b. Each container shall be equipped with suitable lifting attachments, discharge valve, safety relief device, and an anti recoil device for shipping and handling.
c. Discharge valve shall use a fast-acting scored rupture disc, to give the fastest and least restrictive means of operation.
d. A 24 Vdc, electric operated control head with local manual lever operation shall be used for automatic operation of the system.
e. Cylinders with mechanically or solenoid-operated valves shall not be acceptable.
f. Each container with storage capacity of 90 kg or greater shall be equipped with a liquid level device, to determine the amount of agent in the container without the need for removal.
g. Each container shall be mounted against the wall and secured in place.

Discharge Piping

a. Distribution piping and fittings shall be in accordance with  manufacturer’s guidelines and NFPA 2001.
b. Piping and fittings shall be galvanized.
c. Reductions in pipe size shall be made using concentric reducing fittings. Reducing bushings shall not be acceptable.
d. Piping system shall be securely supported in accordance with NFPA 2001.
e. Unless otherwise specified by manufacturer, for ceiling levels over 3.7 m in height, a second row of nozzles shall be installed, and a third row if over 7.3 m high.

Control Panel

a. Control panel shall operate from the central uninterrupted power  supply (UPS), with a battery back-up system for providing supervisory power for the extinguishing and alarm system for a 24 hour period, and be capable of supplying 100 percent of all alarm, detection and extinguishing functions for a minimum of 5 minutes.

b. The microprocessor based control panel shall be Kidde Sorpio,  approved equal fire control panel. It shall be UL listed as a releasing panel suitable for use in clean agent systems.
c. Control panel shall have a transfer switch for main/reserve operation.
d. Control panel shall have a fully supervised container disable switch. Operation of this switch shall cause a trouble signal and shall prevent discharge of clean agent.
e. Control panel shall have cross-zone, pre-discharge, manual release, time delay, abort, and shutdown logic features. Panel shall have general alarm/trouble contacts, and main/reserve indication (for  example pressure switches) to interface with the fire and gas control panel.

Miscellaneous Requirements  

a. Fire alarm and detection equipment shall be UL listed.
b. Smoke detectors shall be installed in accordance with NFPA 72.  Smoke detectors shall be spaced to cover no more than 23 m2 of floor area each.
c. Audio/visual devices shall consist of 150 mm diameter, 24 Vdc predischarge alarm bells, and 24 Vdc evacuation horn/strobes labeled ‘fire’. Placement of devices shall be as shown on fire detection drawing(s).
d. Manual discharge stations shall be supplied as shown on drawing(s). Manual discharge station shall be made of aluminum, and shall be of the dual action type with a keyed reset.  
e. Instructional signs shall be provided at all points of egress, manual discharge stations, and audio/visual devices.

f. Wires shall be identified at every termination point, in accordance with NFPA 70, Article 760-3 and 760-22.
g. LV control wiring shall be 18 gage solid copper as a minimum for detection circuits, and 14 gage for audio and solenoid valves.
h. Fire alarm circuits from electrical devices shall be run in conduit back to the panel, where the return wiring shall be landed on separate terminals as required. Wiring runs shall be continuous, point to point, with no splicing between terminals.

Transformer Yard

Layout of Yard

• Transformers shall be located adjacent to the substation building and arrange in a way that the lengths of secondary bus ducts and number of bends are minimal. Local isolation switch shall be provided for the transformer if it is fed from a remotely located switchgear. The remote location is classified as the place which is not within the vicinity of transformer and also not easily approachable during emergency. .
• Transformer yard layout shall comply with IEEE 979. To avoid long bus duct runs, building wall shall be fire resistant, or a separate fire wall shall be provided between transformers and building wall. Substation building shall be elevated, so there shall be a firewall between transformers and cabling area under substation.
• Fire walls and minimum clearance between/around power transformers shall be considered in accordance with NFPA 70 and NFPA 850.
• Transformers shall be mounted on concrete pads. These pads shall extend beyond the mounting base at least 150 mm in all directions.
• The area around equipment pads in transformer yard shall be covered with a 100 mm layer of 20 mm graded stone. Stone shall be retained by a concrete curb. Top of curb shall be 150 mm above top of finished grades inside or outside transformer yard. For the main transformers (HV to MV), a pit shall be provided below or around the transformers, sized to contain the oil of one transformer. Installation shall conform to IEEE 980. Size of curbed area shall be increased if necessary.
• Clearance from nearest obstruction, or between transformers and curb, shall be 1000 mm minimum. Clearance from front (primary connection side) of transformer to curb or nearest obstruction shall be 1300 mm minimum.
• Clearances shall be measured from furthest major projecting parts of transformer, for example terminal chambers, secondary throats, or radiator banks.
• Protective cages shall be provided around the transformers in accordance with SSD 10.
• At least one 60 A, three phase, 480 Vac rated welding receptacle shall be installed in transformer yard.

Yard Lighting

Lighting shall be provided in accordance with Lighting Design Requirements in Process Industry  and Electrical System Design Criteria. Additional  fixtures shall be installed as necessary, to ensure that areas around transformers that require maintenance or adjustment, for example the control cabinet, junction boxes and fans.

 Wiring

Yard wiring installation shall comply with Underground Installation Requirements.

Grounding

A ground grid shall be installed for the substation and equipment, grounded in accordance with Bonding and Grounding & Installation of Grounding and Bonding Conductors and IEEE 80.

Station Service

AC Power

AC station electrical service shall be taken from a reliable power source. A double ended 480 V utility substation located within the main substation shall be the preferred source. One feed shall be from plant normal power and one feed from the emergency system. Motor control centers (MCC’s) with required feeder breakers and starter units shall be installed for building HVAC equipment, battery chargers and all other electrical loads.

DC Power

A station battery with redundant battery chargers in accordance with Batteries, Racks and Battery Chargers in Plant  shall be installed for the control of switchgear. Separate batteries shall be installed for HV switchgear. No loads other than switchgear control, relay panels and SCADA equipment shall be placed on station batteries. Separate batteries shall be installed for emergency generator. For battery room see Battery Room Design Requirements .

Wiring

Wiring for station service shall be in accordance with Building Electrical Design Notes and Guidelines.

Emergency Power

Generator Installation

One or more emergency generators shall be installed in a separate enclosure/building  near main substation building. Generators shall be diesel or turbine driven, depending upon method of operation and size requirements. This installation shall comply with SSD 11. The controls for generators and plant emergency power bus shall be in MV switchgear building.

Emergency Power Loads

Security lighting system and plant communication systems shall be served by the  emergency power system. Emergency power system shall be sized to power the plant stand-by lighting requirements, feeds to UPS systems, battery chargers for substation dc control systems, and any other power required for controlled plant shut down.  

Emergency Diesel Generator (EDG) Building

EDG building shall comply with the requirements of SSD 9, 10 and 11, and shall have  all necessary maintenance facilities including monorail hoist with horizontal travel beam and ventilation, louvers with sand trap etc.
The EDG building shall be equipped with a fuel day tank, water point, drainage facility, lighting and outlets as per the area classification, a monorail hoist to lift the heaviest part of equipment, a FM-200, Conforming to NFPA 2001 automatic fire suppression system with fire detectors, fire call points and alarms suitable for the area classification and integrated to the overall plant fire alarm system. Refer to NFPA 2001. Appropriate double doors shall be provided with lay down area to permit removal of engine and generator.

Fencing

When main substation is located within the plant security fence, an additional fence around main substation shall not be required. Transformers and generators shall be installed within the fence, in accordance with SSD 10. Fence doors shall be kept locked.

If main substation is outside the limits of plant security fence, substation fence shall comply with SSD 1 and be provided with lighting in accordance with SSD 13. Fence height shall be 2.4 m. Personnel and equipment gates shall be provided. Gates shall be provided with provisions for padlocks. Fence shall be grounded and bonded to substation grounding grid. HV warning signs shall be prominently displayed on fence.

We have discussed Main keywords for this article are Substation Layout Diagram, Substation Installation, Electrical Engineering Substation Design, Basics of Designing Power Substations, Substation Design and Engineering.

 ARAMCO STANDARD SAES-P-119 SUBSTATION