Precautions and troubleshooting for installation of box type substation

Prefabricated substations (commonly known as “box substations”) are widely used in residential community power distribution, industrial parks, municipal infrastructure, and temporary construction power supply scenarios. They offer advantages such as a high degree of integration, short installation cycles, and a small footprint. However, numerous on-site incidents indicate that most defects in box substations do not stem from the equipment itself, but rather from inadequate foundation construction, non-compliant grounding, missing seals, and subsequent lax inspection practices. 

I. Site Selection and Foundation Requirements Prior to Installation

1. Environmental Control for Site Selection

The prefabricated substation should be located in a dry, well-ventilated area free from severe vibration and impact, away from low-lying areas prone to flooding, flammable or explosive locations, areas with highly corrosive gases or liquids, and sources of strong electromagnetic interference. The substation must be installed horizontally, with an overall tilt not exceeding 5°. Sufficient space must be reserved around the unit for operation and maintenance access; louvers and ventilation openings must not be obstructed by shrubs or weeds.

2. Concrete Foundation Construction (The Most Often Overlooked Step)

• The concrete strength grade of the foundation should not be lower than C25, and the surface must be level to prevent deformation and cracking caused by uneven stress on the unit later on;
• Drainage holes must be provided at the base of the foundation, and water accumulation in the cable trench is strictly prohibited—moisture traveling up the cables can easily cause flashover and discharge in insulating components;

• The flat grounding steel embedded in the foundation must be securely welded. A visual inspection must be completed before the equipment is installed to ensure: the enclosure has no dents or deformation, the sealing strips show no signs of aging or cracking, and the parameters on the nameplate match the design specifications.

II. Mandatory Standards for Grounding Systems

Grounding is the first line of defense for lightning protection and personal safety in prefabricated substations; it is absolutely unacceptable to simply “insert an angle iron to pass inspection”:

Item Standard Requirements Grounding Conductor Material Copper conductor, cross-sectional area ≥ 30 mm²Burying Depth Horizontal grounding electrodes must be buried at a depth of ≥1 m; vertical grounding electrodes must be installed if necessary Grounding Resistance≤4 Ω; if the limit is exceeded, additional grounding electrodes must be installed or the soil must be replaced Equipotential Bonding The enclosure, the transformer neutral point, and the surge arresters must each have separate down conductors connected to the main grounding grid, and these must be securely welded

If the grounding resistance remains above 4 Ω for an extended period, the discharge of direct lightning strikes on equipment during thunderstorms will be impeded, significantly increasing the risk of insulation breakdown.

III. Cable Installation, Sealing, and Commissioning Tests

1. Cable Pre-treatment: After high-voltage cables have been installed and prior to connection, insulation resistance tests and AC withstand voltage tests must be performed; connection is strictly prohibited if the cables fail these tests.

2. Sealing of Cable Inlets and Outlets: All cable entry and exit holes in the enclosure must be tightly sealed with fire-resistant sealing compound. This step is often omitted; however, small animals such as rodents and snakes entering the high-voltage compartment and gnawing on insulation are a common cause of phase-to-phase short circuits in prefabricated substations.

3. Mandatory Pre-Commissioning Tests (No item may be omitted):

• Verification of transformer DC resistance and winding connection group;
• Insulation resistance testing on both high- and low-voltage sides, and ratio verification;
• AC withstand voltage test on the high-voltage side;
• Power-on drive test on the low-voltage side;
• Only after verifying that all protection settings are correct may power be officially energized.

IV. The “Look, Listen, Measure, and Check” Method for Routine Inspections

Simple criteria commonly used in front-line operations and maintenance:

• Look: Check for oil leaks or condensation droplets on the casing, and ensure the louvers are unobstructed;
• Listen: Check whether the transformer’s “humming” sound is uniform and listen for any unusual hissing discharge noises;
• Measure: Perform infrared temperature scans of busbars and cable joints; if joint temperatures exceed 70 °C, immediately arrange for a power outage to tighten the connections;
• Check: Verify that the condensation controller and temperature-controlled fan’s start/stop logic are functioning normally; in summer, clogged air filters are the primary cause of overheating-induced tripping.

V. Common Typical Faults and Standardized Troubleshooting Procedures

Fault 1: Equipment Overheating, Excessive Temperature RiseDimension Details Symptoms Excessively high transformer oil temperature; low-voltage busbar/cable joints becoming hot and discoloredCommon CausesProlonged overloading; filter screens clogged with dust; cooling fan failure; excessive contact resistance at bolted joints Key Troubleshooting Steps① Verify the real-time load factor; reduce the load by transferring it elsewhere if necessary; ② Clean dust and verify fan start/stop functions; ③ After re-testing with an infrared thermometer, shut down the power and tighten connection points; ④ For oil-immersed transformers, simultaneously monitor oil chromatography and oil quality

Fault 2: Abnormal On-Load Tap Changer

Symptoms include failure to change tap positions, abnormal noises during switching, or internal arcing sounds. Causes typically include contact erosion, breakage of the transition resistor, mechanical jamming, or deformation of the switch rod frame due to prolonged operation, as well as fatigue fracture of the main shaft. This type of fault requires shutdown for repair; it is strictly prohibited to force a tap change while the fault persists. For new projects, strict control must be exercised over the brands of switch components and installation procedures.

Fault 3: Low-Voltage Side Circuit Breaker Trip

Trips generally result from two types of causes:

① Busbar faults;

② Protection misoperation or circuit breaker mis-trip.

Standard troubleshooting sequence:

1. First, disconnect all low-voltage feeder circuit breakers, then restore power one feeder at a time to quickly pinpoint the faulty feeder;

2. Verify whether any protection settings were omitted or whether setpoints have been altered;

3. If accompanied by an overcurrent signal on the low-voltage side of the main transformer, focus on verifying whether it is a “pure overcurrent” or “overcurrent + line protection operating simultaneously”—the latter can generally be determined as a feeder line fault, requiring a full-line inspection along the circuit.

Fault 4: High-Voltage Fuse Blowout / Over-Rated Trip

Critical Precaution: Do not simply replace the blown fuse and force the system back online when fuses blow frequently, as this will exacerbate the incident.

Correct Procedure: First, open the high-voltage isolator → test for voltage and ground the circuit → then systematically inspect the following:

① Check the cable terminations for surface flashovers caused by poor workmanship;

② Check the high-voltage compartment for signs of short circuits caused by small animals;

③ Check for abnormalities in the transformer’s DC resistance and insulation; restore power only after clearly identifying the cause.

Fault 5: Condensation and Insulation Flashover

When a prefabricated substation “sweats” or relative humidity remains persistently high, the effective creepage distance is reduced, making surface discharge highly likely. Countermeasures: Activate heaters/dehumidification modules, repair damaged seals, clear ventilation paths, and clean contaminated insulators.

VI. Fire and Explosion Prevention and Minimum Service Life Management Requirements

• Prohibit long-term overloading; for units in service for 5 years or more, closely monitor coil aging and insulation degradation;
• For oil-immersed box-type transformers, periodically sample and test the insulating oil; if test results fail to meet standards, filter or replace the oil promptly;
• Ensure that surge arresters in the high- and low-voltage compartments are fully installed and within their valid inspection period to prevent lightning反击;
• As box-type transformers are live high-voltage equipment, strictly enforce the “Two Permits and Three Systems” during maintenance; it is strictly prohibited to open cabinet doors while the equipment is energized.

VII. Conclusion

A prefabricated substation is not simply a turnkey system that can be considered complete just by “buying a metal enclosure and connecting the power lines.” Three key factors—foundation drainage, ground resistance, and cable sealing—determine the incidence of faults during the first three years of operation; meanwhile, a practical infrared inspection system combined with standardized procedures for troubleshooting trip events determines the level of stable service for the subsequent ten years or more. For operations and maintenance teams, the fundamental way to truly reduce unplanned outages is to maintain complete records of “hidden work” from the installation phase and to archive the protection messages for every trip.