Control house at HV/EHV switchyards and substations (construction, layout and functions)

Control house

Nowadays, the control house is one of the main parts in every modern HV switchyard and substation. Why? Simply, because substations have grown in complexity and equipment such as circuit breakers, switch disconnectors, CT, VTs and others must be controlled, monitored and protected from some common point in the field.

Necessity for supplemental equipment such as protection relays, controls, batteries, communications equipment, and LV distribution equipment also increases. And all that equipment must be placed somewhere in the field.

That’s why substation needs a control house. In short.

For small distribution substations, this equipment can usually be contained in weatherproof enclosures or control cabinets. For larger substations, separate equipment housing is necessary.

MV and LV switchgear can also be contained within control houses, or this equipment may be contained within weatherproof enclosures dedicated to that purpose.

Contents:

1. Control house construction:
   1. Foundation
   2. Floor
   3. Control house structure
2. Control house layout:
   1. Control and relay protection panels
   2. DC equipment
   3. AC equipment
   4. Cableways
   5. Cable entrance
   6. Lighting
   7. Control house HVAC systems
   8. Control house plumbing
   9. Communications

1. Control house construction

This section discusses general aspects of the control house construction.

1.1 Foundation

The control house foundation typically consists of a spread footing with either masonry blocks or cast-inplace walls. The footing is designed for an allowable bearing capacity based on soil data. If soil data is not available, a maximum bearing of 48 kPa can be used.

The footings are installed below frost depth and in accordance with local building codes and practices.

Damp-proofing of foundation walls is desirable, especially if concrete block is used. If a basement level is constructed, damp-proofing should be provided.

Footing drains are usually provided when a basement level is constructed. All foundation walls should be insulated with a 5.1-cm (2-inch) thickness of rigid insulation for energy conservation.

It is preferable to install the insulation on the inside of the walls, although the outside is acceptable.

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1.2 Floor

The control house floor is typically a floating concrete slab 12 to 16 cm thick reinforced with welded wire fabric, deformed steel bars, or a combination of both.

The finished floor elevation is usually 10 to 20 cm above the finished grade outside the control house.

The base beneath the floor slab should be 10-12 cm of compacted sand or gravel, thoroughly mixed and compacted sand or gravel, or thoroughly mixed and compacted natural soil. A 0.15 mm thick plastic film vapor barrier should be installed between the floor slab and the base.

The method for cable routing in the control house has to be considered before finalization of the floor slab design. Cable trenches can be formed into the floor slab, or false floors can be installed providing access to large areas below the finished floor.

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1.3 Control house structure

The control house structure must be constructed from fire-resistant, low-maintenance building materials. Most control houses presently being designed and constructed are of the pre-engineered metal or masonry block type.

Figure 3 show example of control building structure type.

Masonry buildings constructed of block masonry are most economical when masonry module dimensions are used to size the building and the building openings.

Two types of roof systems are commonly used for masonry buildings: pre-cast, pre-stressed concrete panels; and steel joists and steel decks. A sloping roof is recommended for both systems and can be obtained by pitching the roof deck or installing tapered roof insulation.

The roof membrane has to be compatible with the slope. For the slopes of 8.3 cm/m (1 in./ft) and less, built-up pitch and slab is commonly used. For greater slopes, gravel is used.

The control house should be equipped with at least one double door, possibly with a removable transom, conveniently located to facilitate equipment entry and removal. In certain circumstances a second exit needs to be installed in the control house.

The National Electrical Safety Code (NESC) states:

If the plan of the room or space and the character and arrangement of equipment are such that an accident would be likely to close or make inaccessible a single exit, a second exit shall be provided.

The National Electrical Code (NEC) also defines specific requirements for a second exit. The doors should include locking devices, astragals to prevent water from entering, and adequate weatherstripping and hardware to permit a rapid exit from the control house.

Locate and mark adequate fire-extinguishing equipment in the control house. Windows can be provided, if desired, in office and lavatory areas. Battery rooms and control and metering areas do not need windows.

Consider adequate methods for building insulation. These methods include use of insulated wall panels, ceiling insulation, storm doors, and windows, and weatherstripping around all openings.

Metal buildings are shop painted and require only minor field touch-up after erection. Masonry buildings may be left unpainted or may be painted with portland cement or latex paint. Tint all prime coats to match the finished coat.

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2. Control house layout

2.1 Control and Relay protection Panels

Most protection relaying, metering, and control equipment is mounted on fabricated control and relay panels installed within the control house. A variety of panel types is available to suit individual requirements.

Figures 4, 5 and 6 show examples of various panel layouts.

Single vertical panels can be used, particularly for distribution circuits where space requirements are minimal. The relaying, metering, and control equipment can all be mounted on one panel, allocating a separate panel for each circuit. In some instances, two circuits may share the same panel.

Double or duplex panels are commonly used for higher voltage circuits, necessitating additional space for equipment mounting. Normally, these panels are arranged in two parallel rows with the panel backs facing each other. In this configuration, operating, instrumentation, and control equipment for a circuit is installed on the front of one panel, and the corresponding relaying equipment for the same circuit is installed on the front of the panel directly to the rear.

In some instances, two circuits may share the same control and relaying panels.

Some equipment such as static relaying systems and communications equipment is available mounted in racks. Consequently, separate relay and/or control panels are not required for this equipment.

The trend is toward more compact equipment arrangements that often reduce overall control house size.

Individual three-phase microprocessor relays can replace three single-phase electromechanical relays and associated voltage, current, and power meters, all in one case.

Compact relay and programmable logic controller designs can be mounted on 48.26-cm (19-inch) racks.

To facilitate operation, panels are located in an arrangement that conforms as closely as possible to the actual equipment and circuit layout in the substation yard. To assist in circuit location and operation, mimic buses are sometimes used on the control panels, particularly for large complex substations.

The mimic buses identify the bus and circuit arrangements.

Mimic buses may be implemented on screens viewed from a PC monitor. When practical, position meters at eye level and switches at a convenient operating level. Locate recording meters for ease of viewing and chart replacement.

Provide ample space for relay installation, removal, operation, and testing. Panel construction can include removable front plates for device mounting.

Panels may also include 19-inch rack mounting facilities. Many of the newer relays and items of accessory equipment are designed to fit into 19-inch racks. Cover plates may cover space reserved for future use. In this way, only a new predrilled plate is required when changing out a device or modifying the configuration.

Cutting, drilling, or covering openings in the panels is eliminated.

Panel wiring is accomplished on the backs of the panels. Devices are interconnected and wired to terminal blocks, as required, for operation and connection to devices on other panels.

Panels can include small sections perpendicular to the main section at each end for installation of terminal blocks, fuse blocks, or small auxiliary devices.

Cable connections from the equipment in the substation yard can be made directly to terminal blocks mounted on the panels or to strategically placed terminal cabinets. Interconnections between the terminal cabinets and the panels can then be made with single conductor wire.

Anchor panels to the floor in such a way as to facilitate relocation to coincide with yard equipment and circuit relocations.

Panel arrangement in the control house should permit ready accessibility to the backs of the panels. Some vendors of pre-engineered buildings can provide completely wired and tested control and relay panels and auxiliary AC/DC power systems as part of the building package.

In this way, the entire panel line-up can be witness-tested in the factory. The complete building system is shipped to the site, fully tested. Only the external wiring from the building to the outdoor equipment has to be field-installed.

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2.2 DC Equipment

Substation DC equipment located in the control house normally consists of the battery, battery charger, monitoring and control devices, and distribution panelboard.

The battery should be located in a separate room where practical. If the battery cannot be located in a separate room, it should be located so that electrical switching devices and receptacles are not in the immediate vicinity, ventilation is adequate to prevent gas accumulation, and live parts are protected from accidental contact.

The battery charger, monitoring and control devices, and distribution panelboard are normally located in the control and relay room to facilitate cable routing and equipment maintenance.

Detailed design requirements and procedures for the substation DC system can be found in this technical article.

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2.3 AC Equipment

A low-voltage AC system is provided in the substation for lighting, convenience outlets, heating, ventilating, air conditioning (HVAC) equipment and miscellaneous control functions.

Convenience outlets should be strategically located throughout the control house to provide adequate accessibility. Also, if necessary, workshop and testing area with a high-capacity AC source and a three-phase source should be provided.

For greater reliability, two separate sources may be provided for the AC system service. These sources are often fed through a manual or automatic transfer switch so that ac system power can be restored if one source fails.

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2.4 Cableways

Cable routing can be accomplished by using any of several methods.

2.4.1 Cable Trenches

Cable trenches are formed into the concrete floor slab and are covered with metal plates. The covers should be flush with the finished floor when in place.

With duplex panels, it may be desirable to use the entire space between the front and rear panels as cable trench, depending on circuit quantities.

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2.4.2 False Floors

False floors are useful when large open areas are desirable for cable routing. Lightweight removable floor panels installed on adjustable pedestals are positioned in areas requiring extensive cable interconnections or where future plans dictate a large amount of cable rerouting.

The top of the removable panels should be flush with the finished floor.

If circuits in one area are damaged, other undamaged circuits in the other parts of the building are likely to keep the substation in service.

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2.4.3 Conduits

Conduits can be used for cable routing in floors, along walls, and for cable entrance into the control house. Conduits are available in plastic, aluminum, and steel. Each of these types may be used in control houses for wire containment to convenience outlets, lighting fixtures, and other control house auxiliary power equipment.

Plastic conduit is easily installed and is available in a variety of sizes. Take adequate physical and thermal precautions when using plastic conduit to ensure safe operation.

Metallic conduits of aluminum and steel are widely used as control house cableways. Intermediate- and heavy-walled steel conduit provide excellent physical protection.

The installed costs, however, may be relatively high because of the extensive labor required for installation. The installed cost of rigid aluminum conduit may be somewhat less than that for steel.

A lower installed cost may be realized by using thin-walled steel conduit (i.e., electrical metallic tubing) since it is less expensive and easier to install.

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2.4.4 Wireways

Wireways are sheet-metal troughs used for routing groups of power circuits around a control house to feed various branch circuits. Conduit is used between the wireway and the devices.

Select and install wireway in accordance with the National Electrical Code.

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2.4.5 Cable Trays

Cable trays can be used for overhead routing of cables to and between control and relay panels. Expanded metal or ladder-type trays provide the best facilities for conductors entering and leaving the trays.

A large variety of types, sizes, and fittings is available to suit individual requirements. Cable tray should be selected and installed in accordance with the NEC and NEMA Standards.

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2.5 Cable Entrance

Control and power cables are brought into the control house through windows, sleeves, or cable pits. The windows are square or rectangular openings, usually through the foundation wall but possibly above grade.

The window openings enable many cables to be pulled without interference. To protect the cables during pulling, the windows should have smooth surfaces and beveled or rounded edges. After cable pulling, split sleeves can be installed around the cables and grouted into place.

Cable sleeves can be used above or below grade. The sleeves are usually cast into place during construction of the foundation wall or installed during construction of the superstructure.

Pitch the sleeves to drain out of the building. Provide covers over the cables. Install spare sleeves during initial construction.

Cable pits may be cast-in-place concrete or masonry openings through the control house foundation to permit access to the inside at floor level. Install a cover over the pit and provide a means to drain water.

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2.6 Lighting

Fluorescent lamps are generally used for lighting in control houses. The trend is toward energy-saving lamps and energy-saving electromagnetic or electronic ballasts.

Install lighting to eliminate, as much as possible, reflection and glare from meters, relays, and monitoring screens.

An emergency DC-operated incandescent system is recommended for most control houses. This system can be operated in case of failure of the ac system. It can be operated from battery-pack units or from the station battery system.

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2.7 Control House HVAC Systems

To maintain the functions and accuracy of electrical equipment installed in the control house, HVAC systems may be desirable.

In areas requiring heat only, unit electric space heaters are positioned throughout the control house for balanced heating. If both heating and cooling are required, electric heat pumps can be used. Several small units, or one large unit with a duct system for air distribution, can be used. Supplemental electric resistance heating coils may be required for heating in colder areas.

In some cases baseboard radiation heating units can be used in rooms not reached by the main heating system. These rooms include offices, lavatories, and storage rooms.

Temperature control levels may vary because of several requirements. Operating ranges of equipment have to be considered as well as economics. It is recommended that consideration be given to a dual control.

Most stations will be unattended and, therefore, a normal personal comfort level is not required. However, for maintenance reasons, comfort levels are necessary.

If the control house is to be heated only, it is usually desirable to install power ventilation equipment for air circulation. Size the system for three to five air changes per hour. Place power-operated, thermostatically controlled roof ventilators and manually operated wall louvers to achieve good air circulation.

Position wall louvers so that equipment does not interfere with air circulation. Provide fusible links to close the louvers in case of fire.

It is advisable to provide ventilation that will maintain a positive pressure within the control house at all times to prevent dust from entering through doors and other openings, and prevent accumulation of combustible gases.

The isolated battery room should be equipped with a gravity roof ventilator to remove corrosive and combustible gases. Do not use power-operated roof ventilators.

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2.8 Control House Plumbing

Control houses may require plumbing for stationary eyewash facilities. Additionally, very large, major locations may warrant a shower, lavatory, drinking fountain, and maintenance sink.

A water supply, when required, may be obtained from an existing system or a private well on the substation site.

Most substations with toilet facilities will require septic tank and drain field systems.

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2.9 Communications

A commercial telephone is usually installed in the control house for external communications. Additionally, system telephones or voice channels over carrier systems may be used for system communications.

Larger installations may include substation automation systems or SCADA for remote control and monitoring of substation equipment.

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References:

1. Design Guide for Rural Substations by United States Department of Agriculture
2. Drop-In Control House for a Large 230 kV Transmission Substation: A Case Study in Implementation by Douglas M. Arcure (Shaw Group) and Chris Clippinger (Schweitzer Engineering Laboratories, Inc.)