Compact Substation Installation and Operation Guide

Introduction

Compact substations (also known as package substations or transformer compact stations) are integrated power distribution units that combine medium-voltage switchgear, distribution transformers, low-voltage switchboards, interconnections, and auxiliary equipment within a single enclosed assembly. These factory-built, type-tested units provide a turnkey solution for stepping down medium-voltage power to low-voltage distribution levels, serving as an essential link between primary power sources and end users in residential, commercial, industrial, and renewable energy applications.

Designed for outdoor installation in public areas, compact substations offer significant advantages over traditional built-in-place substations. Their pre-fabricated, modular construction reduces on-site civil works, shortens installation time from weeks to mere days, lowers overall project costs, and minimizes the land footprint required for power distribution infrastructure. Additionally, compact substations are type-tested in accordance with international standards such as IEC 62271-202, which ensures high levels of safety for both personnel and equipment through features including internal arc classification and fully lockable, grounded enclosures.

The purpose of this guide is to provide a comprehensive reference for qualified personnel involved in the installation, commissioning, operation, and maintenance of compact substations. The information presented herein synthesizes industry best practices, equipment manufacturer recommendations, and applicable regulatory standards into a unified, practical framework.

2.1 Site Selection

The selection of an appropriate installation site constitutes the foundation of a successful compact substation deployment. Several critical factors must be assessed prior to finalizing site selection.

2.1 Site Selection

The selection of an appropriate installation site constitutes the foundation of a successful compact substation deployment. Several critical factors must be assessed prior to finalizing site selection.

• Accessibility. The installation site must remain accessible at all times for maintenance personnel and emergency response vehicles. Sufficient clearances must be maintained around all sides of the substation to allow personnel to work safely and equipment to be maneuvered in and out as required.
• Ground Conditions. The site shall be located on elevated ground, avoiding low-lying areas susceptible to flooding or water accumulation. The soil bearing capacity must be adequate to support the weight of the substation, which can range from approximately 10 to 17 metric tons depending on configuration and included equipment. Unstable marshy areas, locations prone to landslides, and soils with high liquefaction potential shall be avoided.
• Drainage. Adequate natural or engineered drainage is essential to prevent water from pooling around the substation foundation. Standing water can compromise electrical insulation, accelerate corrosion of metallic components, and create safety hazards for personnel. The installation area should be graded to direct surface water away from the substation perimeter.
• Proximity to Load and Network Connection. The substation should be located as close as practically possible to the load it serves to minimize power losses and voltage drops along the distribution feeders. Simultaneously, the site must facilitate reasonable access for medium-voltage cable connection to the utility network.
• Clearance from Hazards. The substation must be situated at a safe distance from fire hazards, explosive environments, sources of chemical corrosion, and heavy vibration zones. A minimum safety clearance of 5,000 mm from flammable objects is typically required. The site shall also maintain adequate clearance from trees, buildings, and other obstructions that could interfere with ventilation or create falling hazards.
• Environmental Considerations. Ambient conditions at the installation site must fall within the operating parameters specified for the equipment. Typical compact substations are designed for operation in ambient temperatures ranging from −20°C to +45°C, with relative humidity of 15% to 95% (non-condensing), and at altitudes not exceeding 1,000 meters above mean sea level.

2.2 Foundation Requirements

Once the site has been selected, a suitable foundation must be constructed to support the substation securely and durably. The foundation design must account for the following factors.

• Load Bearing Capacity. The foundation shall be engineered to support the full weight of the substation, including all internal components such as the transformer, switchgear, and auxiliary systems. The bearing capacity of the foundation must be at least 1,000 Pa, though specific requirements vary based on soil conditions and substation weight.
• Soil Conditions. The type and bearing capacity of the soil at the installation site directly influence foundation design. In areas with competent soil conditions, a gravel bed or reinforced concrete slab may suffice. Where expansive clays, loose sands, or challenging geotechnical conditions exist, deeper foundations such as concrete support beams or pile foundations may be required. In difficult terrain, the foundation should be set on piles to prevent differential settlement.
• Frost Depth. The foundation must be constructed to a depth sufficient to prevent frost heave, which can cause uneven settlement, misalignment of doors, and structural stress. Local building codes typically specify the required frost depth for the geographic region.
• Cable Entry Provisions. The foundation shall include pre-formed recesses, ducts, or trenches to accommodate incoming and outgoing power cables and control wiring. These cable entries should be positioned to align with the cable entry points on the substation enclosure as specified in the manufacturer’s general arrangement drawings. For medium-voltage switchgear, cable trenches of appropriate dimensions allow cables to be rolled into position before termination.
• Foundation Types. Several foundation configurations are available depending on site conditions and requirements.
• For installations on stable, well-drained ground, a layer of leveled gravel can serve as the foundation. The gravel bed should extend at least 500 mm beyond the substation footprint in all directions, with a total gravel area of the substation length plus 1,000 mm by the substation width plus 1,000 mm.
• For more demanding conditions, a reinforced concrete slab of suitable thickness (approximately 150 mm) cast directly on-site provides a stable, level mounting surface. The slab should be constructed with openings for cable entries and shall have a levelness tolerance within ±1 mm over 1 meter.
• In situations requiring elevated installation or where ground settlement is a concern, concrete support beams cast in two trenches at frost-proof depth, or concrete piles placed at each corner of the substation, offer enhanced stability.
• Sealing and Barrier Protection. After cable installation is complete, the foundation openings should be sealed to prevent ingress of rodents, insects, and moisture. Options include backfilling with sand or gravel followed by a thin cement topping, or installing purpose-made cover plates supplied with the substation.

2.3 Pre-Installation Equipment Inspection

Before commencing installation, the delivered substation and its components shall undergo thorough inspection to verify completeness and identify any damage sustained during transport.

• External Inspection. Examine the enclosure for visible damage to the exterior finish, dents, deformations, cracked welds, or compromised seals. Verify that the anti-corrosion coating remains intact, paying particular attention to corners, edges, and lifting points.
• Internal Inspection. Open all compartments and inspect the internal components including switchgear, transformer, and auxiliary equipment for signs of damage, loose parts, or foreign objects. Verify that electrical connections remain firmly secured and that wiring is properly routed and labeled.
• Documentation Verification. Confirm that all required documentation is present, including installation manuals, circuit diagrams, the bill of materials, and warranty information. Verify that the nameplate data matches the ordered specifications, including rated power, voltage levels, serial number, and date of manufacture.
• Accessory Check. Ensure that all supplied accessories are accounted for, including operating handles, cable glands, protective gloves, fire extinguisher, and any optional components specified in the order.

Transport and Handling

Proper transport and handling procedures are critical to prevent damage to the compact substation during movement from the factory to the final installation location.

3.1 Transport Considerations

Compact substations are typically transported from the factory without complete packing, though protective measures such as wooden corner protectors and plastic film may be applied for longer distances or export shipments.

During road transport, the substation must be firmly secured to the truck or trailer to prevent shifting, tipping, or impact damage. Transport support beams of approximately 75 mm in height and 150 mm in width should be placed beneath the substation at the partition walls. Removable strapping eyes mounted at the upper corners of the enclosure provide secure attachment points for restraining straps. Straps shall not be passed over the roof of the substation, as this may damage the roof structure and surface finish.

The center of gravity of the substation is not necessarily located at the geometric center of the unit, particularly when a transformer is factory-installed within the enclosure (which may be the case for certain configurations). Transport equipment and securing methods must account for this offset weight distribution.

3.2 Lifting Operations

Lifting a compact substation from the transport vehicle onto its foundation demands careful planning and execution to ensure personnel safety and equipment integrity.

Most compact substations are provided with lifting eyes or holes affixed to the roof or structural frame specifically for hoisting operations. All lifting holes shall be used when transporting the device-no lifting point should be omitted, as uneven load distribution can cause the substation to tip or twist.

During lifting, the maximum angle of the lifting straps at the crane hook shall not exceed 60 degrees. The length of each lifting strap should be adjusted so that the substation does not tilt more than 3 degrees during lifting. Properly adjusted lifting straps ensure that the center of gravity remains directly below the crane hook, preventing uncontrolled swinging or binding.

The crane and lifting equipment must be rated for a load sufficient to handle the full weight of the substation, which ranges approximately from 10 metric tons (without medium-voltage switchgear) up to 17 metric tons (fully equipped with switchgear and transformer). Sudden, jerky movements during lifting must be avoided.

Personnel must not stand beneath the load during lifting operations, nor should any person walk or stand on the roof of the substation at any time. Lifting straps or hooks shall not be thrown onto the roof, as this can damage the paint and surface finish.

3.3 Positioning on Foundation

When the substation has been lifted and positioned over the prepared foundation, it shall be lowered slowly and guided into its final position. The foundation boundaries must be observed to ensure proper alignment of cable entries and door openings.

After the substation is set down, levelness shall be verified. Adjustments can be made using steel packing plates beneath the corners of the base framework. Anchor bolts of appropriate size and type shall be installed to secure the substation to the foundation, with tightening torques applied as specified by the manufacturer.

Installation Procedures

4.1 Enclosure Positioning and Securing

With the substation placed on the foundation and adjusted to level, it shall be anchored into position. Holes in the base frame may serve as templates for drilling anchor bolt holes into the concrete foundation. Anchor bolts shall be torqued to the values specified in the installation documentation.

If the substation is supplied with a pre-installed roof, no roof removal is required for installation. In configurations where the transformer is not factory-installed, the roof must be removed to allow transformer placement before proceeding with connections.

4.2 Cable Installation and Connections

Cable installation constitutes a major portion of the on-site electrical work and requires careful attention to detail to ensure reliable, long-term operation.

Cable Entry. Cables for medium-voltage connection, low-voltage distribution, internal power supply, protective grounding, and control wiring shall be routed through the designated cable entries in the foundation and the substation base plate. Cable entries are typically located in the bottom of the substation to facilitate direct termination into switchgear and transformer compartments.

Cable Laying. Between the low-voltage switchgear and connected inverters, three-phase cables require particular attention to arrangement. Where three separate cable runs exist for AC cables (for example, separate cable channels), the spacing between cable runs must be at least twice the diameter of an AC cable to prevent current imbalances. One line conductor from each phase (L1, L2, and L3) should be placed in each run to ensure balanced current distribution.

Cable Termination. Medium-voltage cable terminations shall be performed using appropriate termination kits that are compatible with the switchgear bushing type. Termination methods include cold shrink, heat shrink, and reusable insulating shrouds. For cold shrink terminations, the termination body is factory-expanded onto a removable core; when the core is withdrawn, the silicone rubber body shrinks securely around the prepared cable end, creating a water-tight seal without requiring heat or special tools.

For reusable insulating shrouds, the sleeving included with the termination kit must overlap the cable box bushing by at least 50 mm on each phase. The overlap section of the bushing must be thoroughly cleaned, preferably using an abrasive material, to remove any silicone release agent that may be present on the bushing surface. This step is essential to ensure water-tight sealing after termination is complete.

When terminating cables to transformer bushings or switchgear sockets, proper torque values shall be observed. As an example, fixing bolts for ring switch bushings (typically M16 hex bolts) require a tightening torque of approximately 95 Nm.

Cable Clamping and Support. Cables shall be properly clamped or supported before termination to avoid imposing mechanical stress on bushings and terminations. Improper support can cause bushing damage and eventual failure. For cables routed to the medium-voltage switchgear, pre-mounted cable fixing clamps attached to the compartment walls provide mechanical retention.

Protective Screen Earthing. The metallic protective screen of each medium-voltage cable shall be earthed at the switchgear end only. Earth grounding on both ends can create circulating currents that cause heating and reduce cable ampacity.

4.3 Transformer Installation

If the compact substation is delivered without a factory-installed transformer, the transformer must be installed after the enclosure is positioned. The roof shall be lifted off carefully to provide access to the transformer compartment. Lifting devices stored within the transformer room may be used for roof removal, attaching to the middle of each roof gable.

The transformer shall be lifted into the transformer compartment and set down onto the provided mounting points. Where an oil collection pit is present (standard in certain configurations), the transformer should be placed on wooden beams positioned within the pit. These beams serve two purposes: they distribute mechanical loads to protect the bottom of the transformer, and they act as vibration dampers for the entire substation.

After the transformer is positioned, the medium-voltage cables from the switchgear and the low-voltage connections to the switchboard shall be terminated to the transformer bushings. For transformers with open terminals, cable lugs connect directly to the bushings. For larger ratings requiring higher current capacity, separate terminal connectors may be provided.

The transformer rating plate must remain visible from the inspection door to facilitate identification and maintenance scheduling.

4.4 Substation Earthing

Proper earthing (grounding) constitutes one of the most critical safety features of any electrical installation, and compact substations demand particular attention to earthing system design and implementation.

• Purpose of Earthing. An effective earthing system provides a low-impedance path for fault currents to return to the source, limits voltage rise during fault conditions, stabilizes circuit potentials with respect to ground, and ensures that the enclosure and exposed conductive parts remain at or near zero potential during normal operation. Proper earthing protects both personnel and equipment from dangerous voltage conditions that could otherwise cause shock, fire, or equipment damage.
• System Grounding. The neutral point of the transformer secondary winding shall be grounded. This system grounding limits voltage rise during single-phase-to-ground faults and provides a reference voltage essential for proper operation of protective relays and metering equipment.
• Equipment Grounding. All metallic enclosures within the substation, including the medium-voltage switchgear housing, low-voltage switchboard, transformer tank, and the substation enclosure itself, shall be bonded together and connected to the main earthing terminal. Equipment grounding primarily serves personnel safety: when an insulation failure occurs within a piece of equipment, the fault current flows through the grounding conductor to ground rather than through a person who might be in contact with the equipment.
• Earthing Conductors. The cross-sectional area of earthing conductors must be selected based on the maximum expected fault current and the allowable temperature rise. Typical minimum cross-sectional areas for copper earthing conductors within a compact substation include 30 mm² for main earthing connections, 35 mm² for medium-voltage switchgear and transformer earthing, and 95 mm² for the connection to the low-voltage neutral busbar.
• Earthing Electrodes. Connection between the substation earthing system and the earth itself is achieved through ground electrodes such as copper-clad steel rods driven into the soil, ground plates buried horizontally, or buried copper tapes. Ground rods are typically driven to a depth of 2.5 to 3 meters. Multiple rods are spaced at least 5 meters apart to maximize effectiveness. The earthing grid is generally constructed by driving rods at the four corners of the foundation and interconnecting them with a buried conductor loop.
• Grounding Resistance. The target grounding resistance for a substation earthing system is typically 5 ohms or less, though some utilities design their systems to achieve a few tenths of 1 ohm. Additives such as resistivity-lowering agents may be required in high-resistivity soils to achieve acceptable grounding resistance values.
• Connection Requirements. The substation enclosure must be reliably connected to the earthing grid at two separate points. The main earthing terminal, typically located in the low-voltage compartment, serves as the interface between the internal substation earthing system and the external system earthing conductor. In TN-C systems, the protective earth and neutral conductor (PEN) are combined and connected to the PEN busbar. In TN-S systems with separate neutral and protective conductors, each conductor connects to its respective busbar, with the neutral and protective earth bonded only at the source.

4.5 Final Installation Checks

Following the completion of all mechanical installation, cable terminations, and earthing connections, a final inspection shall be performed before proceeding to commissioning.

Verify that all doors open and close smoothly without binding or excessive force. Confirm that all bolted connections tightened during installation remain secure, paying particular attention to busbar connections, cable terminations, and earthing bonds. Factory-tightened fasteners are often marked with paint as a visual indication of proper torque; check that these marks have not shifted.

Inspect all cable entries to ensure they are properly sealed against moisture ingress and rodent penetration. Confirm that warning labels are present and affixed correctly in all required locations. Remove any foreign objects, including loose screws, nuts, tools, or debris, from all compartments.

Commissioning

5.1 Pre-Energization Checks

Before the compact substation can be energized and placed into service, a systematic series of checks and tests must be performed to verify that the installation is ready for operation and that all safety systems function as intended.

Preliminary Inspections. Conduct the following preliminary checks with all switchgear in the de-energized condition:

Verify that all switch-disconnectors and circuit breakers are in the OFF position before proceeding. Confirm proper operation of mechanical and electrical interlocks, which are designed to prevent unsafe operating sequences. For SF6-insulated switchgear, check the gas pressure indicator to confirm that the pressure remains within the specified range (typically the pointer should be in the green zone on the gauge).

Ensure that the control power supply is available and that all protection relays and meters are correctly set and functional. Perform an operational test of the ring main unit and low-voltage panel control and protection systems, observing correct operation of circuit breaker opening and closing functions.

Insulation Testing. Measure the insulation resistance to earth of each phase conductor. Standard practice requires insulation resistance readings not less than 200 mega-ohms. Lower readings may indicate damaged or contaminated insulation, and further investigation is warranted before proceeding.

Dielectric Testing. If required by local regulations or project specifications, a power-frequency withstand voltage test may be performed. The test voltage to be applied at site is typically 80% of the factory test value specified in IEC 60694, applied for a defined duration.

Phase Sequence Verification. Confirm that the phase sequence of the incoming medium-voltage supply matches the required rotation for the transformer and downstream equipment. An incorrect phase sequence can cause motors to run backward and may affect the operation of some protective relays.

5.2 Step-by-Step Commissioning Sequence

With all pre-energization checks satisfied, the substation may be commissioned using the following sequence.

Step 1-Isolation Verification. Before proceeding, confirm that all switch elements are in their OFF or OPEN positions, including the low-voltage switchgear, medium-voltage switchgear, and any miniature circuit breakers controlling auxiliary circuits.

Step 2-Medium-Voltage Energization. Only duly authorized personnel trained in electrical safety may connect the medium-voltage supply to the transformer. The medium-voltage switchgear shall be switched on in accordance with the manufacturer’s operating instructions. At the moment of energization, personnel should stand clear and observe the equipment for any abnormal sounds, arcing, or other signs of malfunction.

Step 3-Low-Voltage Checking. After the transformer has been energized, check the low AC voltage present at the low-voltage switchgear. Measure the voltage difference between line conductors and record these values in the commissioning report. The measured low voltage should be within ±5 volts of the inverter nominal voltage. If the deviation exceeds this tolerance, the transformer taps must be adjusted using the on-load tap-changer, then the voltage re-measured and verified.

Step 4-Internal Power Supply Connection. If the substation includes an internal power supply transformer, the internal power supply for the station itself should be connected by closing the main circuit breaker on the station sub-distribution board. Then, close individual miniature circuit breakers for station lighting, the earthed outlet (ensuring the integrated residual current device is functional), ventilation fans, metering equipment, and communication devices.

Step 5-Inverter Power Supply Connection. Close the circuit breaker labeled “Voltage supply SC1” for the first inverter. If two inverters are installed, close the corresponding circuit breaker for the second inverter.

Step 6-Low-Voltage Switchgear Energization. With all checks complete, the low-voltage switchgear may be switched on to supply power to the downstream distribution network. The specific procedure for energizing low-voltage switchgear is detailed in the switchgear manufacturer’s documentation.

Step 7-Inverter Commissioning. Following successful energization of the substation and verification of proper voltage levels, the connected inverters may be commissioned according to the inverter manufacturer’s instructions.

5.3 Commissioning Documentation

The commissioning process must be thoroughly documented. For warranty and guarantee claims to remain valid, certain requirements apply. Specifically, the initial start-up may need to be carried out by the equipment manufacturer or their authorized representative, or a fully completed and signed commissioning report must be filed with the manufacturer prior to placing the equipment in service.

Commissioning documentation should include:

• The substation serial number and type designation
• Installation site address
• Recorded voltage measurements
• Results of insulation resistance testing
• Verification of correct phase sequence
• Confirmation of proper protective relay settings
• Acknowledgment that all mechanical and electrical interlocks function correctly
• Final sign-off by the responsible commissioning engineer

5.4 Final Acceptance

After the substation has been energized and all functional tests verified as satisfactory, the installation is accepted for service. The commissioning documentation shall be filed with the system documentation for the substation, readily accessible for future reference by maintenance personnel.

All keys for door locks shall be removed from the locks and stored in a secure, designated location accessible to authorized personnel only. The substation enclosure shall be locked to prevent unauthorized access, as required by safety regulations and standard operating procedures.

Operation

6.1 Normal Operating Conditions

Compact substations are designed for continuous operation under specified service conditions. The operator must ensure that these conditions are maintained throughout the substation’s service life.

• Electrical Parameters. The applied voltage and load current must not exceed the rated values specified on the substation nameplate. Continuous operation above rated values causes overheating, accelerated insulation aging, and may trigger protective devices.
• Ambient Conditions. The substation shall be operated within its specified ambient temperature range, which is typically −20°C to +45°C for standard configurations. Relative humidity must be maintained between 15% and 95% (non-condensing). Operation at altitudes above 1,000 meters requires derating of voltage withstand capability.
• Ventilation. The substation relies primarily on natural air circulation through louvers in the compartment doors for cooling. Transformer compartment doors must not be obstructed, and vegetation growing around the substation shall be trimmed as needed to maintain adequate airflow.

6.2 Operating Procedures

• Normal Switching Operations. When operating ring main units or other switchgear within the substation, the correct operating sequence shall be observed. For safety, before opening a cable compartment cover, the associated feeder must be earthed and verified de-energized.
• Mechanical Interlocks. The substation incorporates mechanical interlocks designed to prevent unsafe operating sequences, such as opening a cable compartment door before the circuit is earthed. The interlock must be correctly disengaged before each operation is attempted; forcing past an interlock indicates an incorrect operating sequence that could result in hazard.
• Insulated Operating Tools. An insulated operating rod shall be used whenever manual switch operations are required. The operating handle supplied with the substation is generally suitable for this purpose.
• Visual Indicators. Before performing any operating procedure, capacitive voltage indicators should be checked. A lit indicator signals the presence of line voltage; an unlit indicator indicates voltage absence, though caution is still required to verify de-energization by other means before touching conductors.

6.3 Routine Operational Monitoring

During normal operation, personnel should periodically observe the substation for signs of abnormal conditions.

• Audible Indicators. Listen for abnormal transformer noises such as humming that has changed in pitch or volume, crackling sounds that could indicate partial discharge, or mechanical rattling from loose components.
• Visual Indicators. Observe voltage and current readings on instruments and meters, watching for unusual fluctuations or readings outside expected ranges. Check that all status indicators and warning lights display correctly and that no alarm indicators are lit.
• Environmental Monitoring. Periodically check the interior temperature and humidity levels within each compartment. Excessive temperature may indicate overload conditions or ventilation blockage. Condensation inside compartments may indicate that heating or dehumidification equipment is not functioning properly.

7.1 Maintenance Philosophy

Regular, systematic maintenance is essential for ensuring the continued safe and reliable operation of a compact substation throughout its intended service life. A well-planned preventive maintenance program detects developing issues early, allowing corrective action before failures occur, thereby preventing unplanned outages, safety incidents, and costly emergency repairs.

Maintenance activities are categorized into three levels:

• Routine Inspection. Performed externally while the substation remains in operation. Routine inspection includes verifying voltage and current readings, checking for abnormal noise or odor, and observing environmental conditions around the substation. This inspection is typically part of regular facility patrols.
• Periodic Inspection. Performed at scheduled intervals with the substation either in operation (ordinary inspection) or de-energized (detailed inspection). Ordinary inspection focuses on external conditions such as pollution accumulation and corrosion. Detailed inspection requires interruption of operation and encompasses protection functions, control systems, interlock mechanisms, and internal inspection of equipment.
• Servicing. Performed when inspection reveals abnormal conditions or when a specified number of operating cycles has been reached (for example, 1,000 operations for certain switching devices).

7.2 Recommended Maintenance Intervals

Maintenance intervals vary based on environmental conditions, operational duty, and equipment criticality. The following schedule provides general guidance:

• Visual Inspection: Monthly, or more frequently under harsh conditions such as coastal environments, high-industrial pollution areas, or regions with heavy dust or sand.
• Routine Maintenance: Every 6 to 12 months, depending on operating environment and load conditions.
• Comprehensive Overhaul: Every 3 to 5 years for most equipment, or in accordance with manufacturer-specific recommendations.
• Detailed inspection of operating mechanisms: Approximately every 6 years for circuit breaker mechanisms and other frequently operated devices.

7.3 Maintenance Procedures

• Visual Inspection. Conduct a thorough walk-around inspection of the substation exterior. Check the enclosure for rust, dents, deformation, or damage to the anti-corrosion coating. Verify that all warning signs and labels remain present and legible. Ensure that doors and compartments are properly sealed and that louvers remain unobstructed.
• De-Energized Cleaning. When performing detailed maintenance, the substation must be de-energized and locked out in accordance with applicable safety procedures. After isolation, dust and debris shall be removed from all compartments. Insulators, bushings, and other insulation surfaces should be cleaned using a vacuum cleaner or dry compressed air. Sticky or greasy deposits on insulators may be removed with a lint-free cloth dipped in an approved cleaning agent.
• Connection Tightening. Due to thermal expansion and contraction cycles, terminal connections can loosen over time. During periodic maintenance, all busbar connections, cable terminations, and earthing bonds shall be retightened to the specified torque values using a calibrated torque wrench.
• Mechanical Component Check. Operating mechanisms for circuit breakers, disconnectors, and switches should be operated approximately 10 to 12 times to verify mechanical function. Check for smooth movement of rollers, pins, and sliding parts. Lubricate mechanical components using approved lubricants as specified in the manufacturer’s documentation; old grease should be removed before new grease is applied.
• Electrical and Thermal Testing. Perform insulation resistance testing of main circuits to verify dielectric integrity. Thermographic scanning (infrared inspection) shall be conducted to detect hotspots that indicate loose connections, overloaded circuits, or impending failures. Functional tests shall be performed on protection relays, fault passage indicators, and any remotely controlled switching devices.
• Gas Pressure Monitoring. For SF6-insulated switchgear, verify that the gas pressure indicator remains within the green zone (typically 0.25 to 0.5 bar relative pressure at 20°C). If the pressure approaches the alarm threshold, the condition shall be investigated. Gas density monitors with alarm and trip contacts provide automatic notification of low pressure conditions.
• Grounding System Verification. Check continuity of the earthing system at all accessible points. Measure ground resistance periodically to confirm that it remains within acceptable limits (typically 5 ohms or less). Visual inspection of ground connections should verify that all bonds remain tight and free from corrosion.
• Auxiliary System Checks. Verify proper operation of compartment lighting, door-operated switches, temperature and humidity controllers, fan operation, and any installed fire or smoke detection systems. If fans do not start automatically, the temperature controller should be adjusted to a setting below the current ambient temperature to manually start fans for testing purposes.

7.4 Special Considerations for Specific Components

• Ring Main Units. The RMU is designed to be largely maintenance-free, requiring only generic cleaning and periodic inspection. However, the gas-sealed sections are continuously monitored by gas pressure indicators and do not require periodic overhaul. A detailed inspection of operating mechanisms is recommended approximately every 6 years.
• Transformers. For oil-immersed transformers, oil temperature and winding temperature shall be monitored. The transformer protection unit should be configured with proper temperature settings. Periodically check for oil leaks, particulary around cemented joints, gaskets, and bushing seals. The bushing condition should be inspected for signs of tracking, cracking, or contamination.
• Low-Voltage Switchgear. Inspect circuit breakers for signs of overheating, including discoloration of terminals or melting of insulation. Verify that residual current devices (RCDs) function correctly by testing according to manufacturer instructions.

7.5 Maintenance Safety

All maintenance work shall be performed only by qualified personnel familiar with the specific characteristics of the switchgear and associated equipment.

Before any work is performed on the substation, the equipment must be de-energized. The Five Safety Rules of Electrical Engineering must be strictly observed:

• Isolate (disconnect from all supply sources)
• Secure against reclosing
• Verify safe isolation from supply
• Earth and short-circuit
• Cover or barrier adjacent live parts

When work is performed on energized equipment (e.g., during troubleshooting), proper safety equipment including insulated gloves, insulating mats, and other appropriate personal protective equipment must be used.

Maintenance activities that require opening the transformer compartment door may be interlocked with the ring main unit; operating these interlocks incorrectly may produce hazardous conditions, so the proper sequence outlined in the operating instructions must be followed.

8.1 Common Issues and Corrective Actions

• Abnormal Noise from Transformer. If the transformer emits crackling, buzzing of changed pitch, or other unexpected sounds, possible causes include core loosening, winding movement, partial discharge, or severe overload. The load should be reduced and the transformer inspected.
• Excessive Temperature. High operating temperatures may result from overload conditions, poor ventilation, high ambient temperature, or internal equipment failure. Verify load levels, clear obstructions from ventilation louvers, and use thermal imaging to identify the source of heating.
• SF6 Low Pressure Alarm. A falling gas pressure may indicate a leak in the sealed tank. Immediate rectification is required, as continued operation at low pressure compromises insulation and arc-quenching capability. Leaks shall be located and repaired, and gas pressure restored to the rated level.
• Ground Fault Indication. When a fault passage indicator or protection relay indicates a ground fault, the affected circuit should be identified and isolated. Investigation should determine whether the fault is transient (e.g., caused by temporary moisture or wildlife) or permanent (cable insulation damage or internal failure).

8.2 When to Contact the Manufacturer

Some conditions require manufacturer support or factory-authorized service personnel. Contact the manufacturer when:

• The substation requires replacement of major components not intended for field service
• Internal arc or major electrical fault has occurred
• Complex repairs to SF6-insulated switchgear are required
• Unusual conditions are encountered that are not covered by this guide or the equipment-specific documentation
• The protective relay settings require recalculation or reconfiguration beyond standard adjustments

The following safety considerations are of paramount importance throughout the installation, operation, and maintenance of compact substations.

• Electric Shock Hazard. High voltages are present within the substation. Before any work is performed, the substation must be de-energized from all sources, including low-voltage switchgear, medium-voltage switchgear, and any external voltage supply. After de-energization, verify absence of voltage before touching any conductors.
• Arc Flash Hazard. Compact substations designed to IEC 62271-202 incorporate internal arc classifications that contain arc energy within defined areas. However, all work on or near energized equipment must still be performed with appropriate personal protective equipment and in accordance with applicable arc flash safety standards.
• Burn Hazard. Some components, particularly fuses and transformer bushings, can become hot during normal operation. Wear appropriate gloves when handling or working near these components, and allow adequate cooling time before performing maintenance.
• Fire Hazard. A safety clearance of at least 5,000 mm from flammable objects shall be maintained. The substation is typically supplied with a fire extinguisher as part of the standard delivery, which shall remain accessible and regularly inspected for serviceability.
• Environmental Safety. When handling SF6 gas from switchgear, environmentally compatible methods shall be used. Gas recovery equipment prevents release of SF6, which is a potent greenhouse gas.

Related Transformer Installation & Operation Guides

Compact substations integrate transformer and switchgear in a space‑efficient enclosure for outdoor distribution. For traditional transformer types with different mounting, cooling, or enclosure designs, please refer to the following dedicated guides.

• Substation Transformer Installation and Operation Guide
• Power Transformer Installation and Operation Guide
• Pole Mounted Transformer Installation and Operation Guide
• Pad Mounted Transformer Installation and Operation Guide
• Dry Type Transformer Installation and Operation Guide

References

Compact substations are type-tested to the latest edition of IEC 62271-202, which specifies requirements for compact secondary substations. Additional applicable standards include IEC 62271-1 for common switchgear specifications, IEC 62271-100 for circuit breakers, IEC 62271-200 for switchgear cubicles, IEC 62271-102 for disconnectors and earthing switches, IEC 62271-103 for load break switches, IEC 60529 for degrees of protection provided by enclosures (IP codes), IEC 60076 for power transformers, and IEC 61439 for low-voltage switchgear assemblies.

For any questions or technical concerns regarding the installation, operation, or maintenance of a compact substation, contact the equipment manufacturer’s service line with the following information readily available: the serial number of the substation, the type designation, and the installation site address.