Substation building services
This technical article introduces five main principles involved with substation control building and switchyard services that make a lot of trouble to substation crew if poorly designed. The article covers basic internal and external lighting design, heating, ventilation and air-conditioning (HVAC) as well as fire detection and suppression practice.
Any of mentioned substation services could trigger disastrous effects like human casualties and million-dollar equipment damage. The real question is why services like these lack of proper design.
I suppose that a tight time-frame for the substation construction is responsible, as well as the lack of professionals and experts in the design and/or construction. Sometimes the lack of funds is the reason for such situations. But all in all, that’s not good.
- Substation lighting
- Substation earthing arrangements
- Heating, ventilation and air-conditioning
- Fire detection and suppression
- Cables, control panels and power supplies
Lighting schemes may be necessary for the following substation areas:
- Indoor and outdoor schemes for control buildings and indoor switchrooms under both normal and emergency (loss of alternative current (AC) supply) conditions.
- Floodlighting and emergency schemes for outdoor switchyards and door or gate access.
- Security/access road lighting together with any supplementary lighting for video camera surveillance.
- Enclosed transformer pen lighting.
The type of light source chosen for an unmanned substation control building or switchyard will be less influenced by aesthetic than technical characteristics such as colour rendering, glare, efficacy, life and cost. Without getting into details of luminaire types, let’s just mention that nowadays, LED lights are the the most common type.
Typical substation lighting levels are shown below.
Table 1 – Typical substation lighting levels
|Substation area||Standard illuminance|
|Limiting glare index|
|Cable tunnels and basements||50||—|
|Entrance halls, lobbies, etc.||200||19|
|Corridors, passageways, stairs||100||22|
|Lavatories and storerooms||100||—|
|Outdoor switchyard (floodlighting)||20||—|
|Exterior lighting (control buildings, etc.)||15||—|
Note that flameproof lighting fittings may be necessary in the battery room because of fumes given off from unsealed batteries.
See Figure 1 – an example of lighting design of the UPS room:
The characterization of LV building services AC distribution systems may be described by the type of earthing arrangements used. Effective earthing is essential:
Reason #1 – To prevent the outer casing of the apparatus and conductors rising to a potential which is dangerously different from that of the surroundings. Where there is an explosive risk there may be a danger from very small voltage differences causing sparking.
Reason #2 – To allow sufficient current to pass safely in order to operate the protective devices without danger.
Reason #3 – To suppress dangerous earth potential gradients.
Earthing methods are defined by a three letter coding:
First letter – defines the state of the supply system in relation to earth:
- T = Directly earthed system at one point.
- I = Either all live parts are insulated from earth or one point connected to earth through an impedance.
Second letter – defines the state of the exposed conductive parts of the installation in relation to earth.
- T = Exposed conductive parts connected directly to earth, independent of any earthing of a point on the supply system.
- N = Exposed conductive parts connected directly to the earthed point of the supply system, normally the supply transformer neutral point.
Third letter – defines the earthing arrangement of the system conductors.
- C = Combined neutral and earth conductors.
- S = Separate neutral and earth conductors.
Five common arrangements are shown in Figure 2. The TN-S system, using separate neutral and protective conductors throughout the network, is the recommended method for installations in hazardous areas such as substations feeding chemical plants.
The neutral and protective conductors may also be combined into a single conductor on part of the system (TN-C/S) or combined into a single conductor throughout (TN-C).
In the TT system, with exposed metal bonded directly to earth, quick acting sensitive earth leakage protection is required.
Irrespective of what earthing system is used it will not be effective unless it is frequently checked to ensure that all earth bonds are mechanically strong and free from corrosion. Furthermore, earth impedance should be monitored and recorded so that any change can be detected and the appropriate action taken.
Learn substation earthing best practices in details here.
The correct air circulation or number of air changes per hour is essential to ensure comfort of substation operations and maintenance personnel. The number of air changes depends on the number of personnel and size of the room but a minimum of four fresh air changes per hour is recommended.
In addition, it is necessary to prevent the build up of dangerous gases such as may occur in a battery room using vented cells. Typical air changes per hour for different substation building areas are listed below:
Table 2 – Typical air changes per hour for different substation building areas
|Substation area||Air changes per hour|
|MV and/or HV switchrooms||4 to 8 (30 to 60 for smoke removal)|
|LV AC and/or DC switchrooms||4 to 8|
|Control and relay rooms||4 to 8|
|Battery rooms||6 to 10|
|Control and communication rooms||Control and communication rooms 4 to 8 (overpressures via a filter may be specified to prevent ingress of dust into sensitive equipment partly depending upon the equipment enclosure protection (IP) rating and need for adequate heat dissipation)|
|Offices||4 to 8|
|Toilet and wash rooms||10 to 12|
|Mess room||10 to 12|
|Corridors||3 to 6|
All ductwork must be designed in conjunction with the fire safety engineers in order to ensure that the fire zoning is reflected in the ductwork.
This is especially important where migration of smoke from one zone to another could be a substantial hazard (See Figure 3). The need for zonal control also applies to cable trenches running through different fire zones within the substation building.
The internationally recognized standard used for the design of Heating, Ventilation and Air-Conditioning (HVAC) is that produced by:
- In US by ASHRAE (American Society of Heating, Refrigeration and Air-Conditioning Engineers)
- In UK by CIBSE (Chartered Institute of Building Services Engineers)
Since this is a very specific subject, only a brief introduction is given here. Areas of substation buildings should be air-conditioned in the following ways:
Table 3 – Areas of substation buildings and HVAC requirements
|Substation area||Air changes per hour|
|Switchgear rooms||Ventilation with air tempering to prevent the formation of condensation on cooled surfaces is the ideal approach. The most controllable solution is to temper the supply air using electrical or low pressure water coils. Alternatively, the switchgear should be specified to operate at ambient temperatures up to 40°C and 90% relative humidity with ratings calculated accordingly and panel heaters should be installed to prevent condensation and freezing.|
|LV AC and/or DC switchrooms||As for MV and HV switchgear rooms.|
|Control and relay rooms||HVAC. If because of economies switchgear and control and relay panels and operators’ desks are all provided together in a single room then airconditioning will be required.|
|Battery rooms||Extract ventilation with high quality acid fume resistant fans.|
|Control and communication rooms||Manufacturer’s standard light current equipment often demands a stringent environment. Such sensitive equipment|
should be maintained typically at 20°C and 50% relative humidity.
|Offices||HVAC to between 20°C and 25°C with relative humidity maintained between 40% and 60% for comfort.|
|Toilet and wash rooms||Extract ventilation fans with supply air drawn in through transfer grilles to ensure smells do not enter the working area.|
|Mess room||Extract ventilation discharging to the outside with air transfer grilles as described for the toilet and wash rooms. Possible HVAC depending on budget.|
|Corridors||Air-conditioning if in a manned substation where people are frequently moving between rooms.|
Heating is not normally required in climates where the minimum ambient temperature does not fall below 15°C although units may be installed where close control of temperature and humidity is necessary.
The following methods are available:
- ‘Air heater’ batteries using electrically heated elements operated in stages, or low pressure hot water coils, located in air-conditioning ductwork.
- ‘Electric space heating’ from room heaters.
- ‘Low pressure hot water’ (LPHW) space heating using radiators fed by circulating hot water.
Anti-condensation heaters with ratings of a few tens of Watts are often specified for installation within switchgear and control panels.
- Heated building;
- Transformer chamber
- Window with externally mounted panel type heat reflecting screen
- Ventilation unit
- Adjustable shutters with electric drive
- Air inlet screen
- Extract duct
- Digital transformer
- Heating devices (electric heaters)
- Relay panels
- Maintenance personnel
Heating is not envisaged during the cold season in buildings housing power and measuring transformers (see Figure 4) and in reactor chambers, due to large heat release.
Let’s remind ourselves of the basics. Heat, fuel and oxygen are required together for a fire to exist. Removal of any of these components will extinguish the fire. The fire safety philosophy (you should always remember) is to: safeguard personnel and to maintain the functional state of the substation.
Further, special precautions have to be taken concerning the spread of fire caused by oil leakage from transformers or the oil-filled types of switchgear.
Let’s describe general fire detection and suppression schemes for substation installations covering fires originating from transformers, switchgear, control and protection equipment and cables.
Hand or trolley mounted fire extinguishers are the cheapest form of manual fire extinguishing. Such extinguishers should be mounted in the substation control or switchgear building as well as in the switchyard.
The types of commonly available fire extinguishers and their usage under different conditions are as shown in Table 7.6.
Table 4 – Commonly available fire extinguishers and their usage
|Type||Color code bands||Application||Extinguishing action|
|Water||Red||Fires involving wood and other solid, organic or carbon based material.||Cools fuel to below the temperature at which sustained flaming occurs.|
|CO2||Black||Mainly electrical equipment fires.||Cools and inert the atmosphere.|
|Dry powder||Blue||Flammable liquid fires/electrical fires.||Inhibits the chemical reactions in the flames.|
|Halo (BCF)**||Green||Flammable liquid fires, electrical fires and fires in carbon based solids.||Same as dry powder but also has a cooling effect.|
Power Plant Fire Suppression System
The exits from substation control and switch rooms must ALWAYS be kept clear. Panic release bars should be fitted to the doors such that the doors will quickly open outwards from the inside by pressure against the release bar. Doors should be sized greater than 750 mm × 2000 mm and areas at the rear of switchboards should not be less than 12 m from an exit.
Doors between equipment containing bulk oil should have a 2-hour fire resistance rating.
Emergency lighting fittings should be installed over each exit. Fire and emergency signage should be installed in accordance with the appropriate national standard.
Basic first aid kits (first aid dressings, medication, eyewash, blankets, etc.) and safety barriers to screen off work areas MUST be available on the substation site.
Signage describing the actions to be taken in the event of electrical shock should also be clearly on display.
Manual call points
The fire may be detected by personnel manning or working on the substation site at the time of the fire (See Figure 7). The alarm may therefore be raised by the breaking (or lifting) of glass at a manual call point.
These should be mounted at approximately 1.4 m above floor level and located on exit routes both inside and outside the substation building so that no person need travel more than approximately 30 m from any position in the building in order to raise the alarm.
Two types of detector or sensor are found in substation applications:
1. Heat detectors
Depending upon the type these sense changes in the thermal environment very locally or in the immediate vicinity of the unit. Bimetallic strips and thermistors are commonly used devices in such sensors.
2. Smoke detectors
These sense small particles of matter or smoke in the air which are the result of a fire. Ionization detectors work on the principle that the current flowing through an ionization chamber reduces when smoke particles enter the chamber.
Electronic alarm circuitry detects this change and initiates the alarm.
3. Radiation (flame) detectors
These detect ultraviolet or infrared radiation and are mainly suitable for supplementing heat and smoke detectors or as a general surveillance of a large switchyard area.
It is important to avoid anomalous operation of a fire detection and suppression system. Therefore a ‘double knock’ system is usually employed where two sensors have to detect the alarm before the suppression system is activated. Radial circuits are used with the detectors effectively cabled in parallel together with an ‘end-of-line’ resistor.
The circuit is monitored for both short circuit (typically less than 1000 ohms) and open circuit conditions and a maintenance alarm raised if the circuit is out of tolerance. Sensor circuits are arranged on a ‘zonal’ basis in order to isolate the fire into certain areas.
The zone where the fire has occurred is indicated on the fire detection control panel. The panel sends signals to alarm sounders to alert personnel and to send signals to the automatic fire extinguishing systems or to shut down the HVAC plants which could spread the fire. Both inert gas and CO2 gas systems require the rooms to be enclosed.
‘Fire stopping’ is the term used to describe the sealing of small openings in fire barriers.
Ventilation louvres should be fitted with temperature sensing or remote controlled closing devices.
Gas bottles are suspended from the ceiling in the room being protected, or a central set of cylinders with a piping system to ceiling mounted nozzles in the different rooms may be employed. The required concentration of gas to extinguish the fire, while small, is considered dangerous to personnel.
It is therefore considered necessary to avoid personnel being in the zone during gas discharge.
Water sprinkler systems may be employed in cable basements. The normal sprinkler has a liquid filled glass bulb valve which is activated by the expansion of the liquid and shattering of the glass. This is not sufficiently fast for cable fire protection.
Therefore the glass ampoule is fitted with a ‘percussion’ hammer which is activated electronically from the smoke or heat detectors.
It makes sense to ensure that the cabling associated with the substation fire detectors is both flame retardant and flame resistant if it is to operate successfully and reliably initiate an alarm.
A fire alarm must take precedence over any other indication that the control panel may be giving. Key switch isolation facilities must be available for maintenance and test functions.
The audible alarm generated within the control panel may be both locally and remotely sounded.
- Transmission and Distribution Electrical Engineering by Dr C. R. Bayliss CEng FIET and B. J. Hardy ACGI CEng FIET
- IEEE Guide for Substation Fire Protection
- Dynamic heating and ventilation of transformer substation buildings with adjustable resistance to heat transfer in windows by Andrey A. Yablokov, Nikolay N. Smirnov, Vladimir V. Tyutikov, Vladimir A. Gorbunov at Ivanovo State Power Engineering University, Russia