For after-sales maintenance teams, harmonics and heat are rarely separate problems in a solar plant. Harmonic current adds losses in windings, structural parts, leads, and connections; higher ambient temperature and restricted cooling make those losses harder to remove. The result may be an apparently normal transformer that slowly develops abnormal oil temperature, accelerated insulation ageing, or repeated thermal alarms during high-production hours.

A high voltage transformer for solar power plants must operate under conditions that differ from the steady, largely sinusoidal loading assumed in many conventional distribution applications. Solar output ramps quickly, inverter switching introduces waveform distortion, and the most demanding electrical period often coincides with the hottest part of the day. Good maintenance is therefore less about reacting to a single temperature alarm and more about understanding whether the transformer’s thermal behaviour still matches its actual loading and power quality.

Why solar transformer heating deserves a different diagnostic approach

In a conventional load profile, a transformer may see predictable morning and evening peaks. In a photovoltaic plant, output can rise sharply after cloud movement clears, then fall just as quickly. The transformer does not cool at the same speed that electrical loading changes. Oil and winding temperatures have thermal inertia, so a short period of heavy harmonic loading can continue influencing the temperature trend after active power has already dropped.

This is why a single spot reading is often misleading. A top-oil temperature that looks acceptable at 4:00 p.m. may have followed a much higher winding hot-spot condition at noon. Conversely, a warm transformer at low output may be showing poor radiator performance, blocked airflow, incorrect fan sequencing, or lingering heat after a previous loading event. The maintenance question is not simply “Is it hot?” It is “Is the temperature rise reasonable for this load, ambient condition, cooling state, and harmonic content?”

Harmonics complicate that assessment because transformer losses are frequency-sensitive. Current harmonics can increase eddy-current and stray losses, especially in winding conductors, tank walls, clamps, and other metallic structural components. A transformer can remain below its nameplate kVA while experiencing more heating than the load level alone would suggest. That is the practical risk when inverter-side measurements are reduced to average current and power factor without reviewing waveform quality.

Start with the operating pattern, not the alarm threshold

When an overtemperature event occurs, the first instinct is often to inspect the cooling equipment. That inspection matters, but it should not be the only step. Pull at least several days of synchronized data from the transformer monitoring system, plant SCADA, inverter records, and weather station where available. Compare active power, reactive power, phase currents, top-oil temperature, winding temperature indicators, fan or pump status, ambient temperature, and inverter operating mode.

Look for patterns rather than isolated peaks. For example, a temperature rise that occurs only at high irradiance may point to loading, harmonic losses, or insufficient cooling margin. A rise that occurs on one phase while plant output is balanced deserves a closer look at terminals, bushings, CT connections, lead joints, or localized winding issues. If temperature is elevated at modest load and during cooler weather, cooling degradation or an abnormal loss mechanism becomes more likely.

The same principle applies after inverter replacement or firmware changes. A plant may produce the same MW output while presenting a different harmonic spectrum or reactive-power operating profile to the transformer. Maintenance records should note these changes. Otherwise, later thermal behaviour can look mysterious when it is actually linked to a known modification in the power conversion system.

Measure harmonics where they affect transformer loading

A power-quality survey should be planned around the transformer rather than treated as a generic grid-compliance exercise. Measurements at the inverter output, collector bus, transformer low-voltage terminals, and grid-facing side can reveal whether distortion is being generated locally, amplified by resonance, or influenced by the wider network. The exact test arrangement depends on the plant design and applicable grid requirements, but location matters: a clean reading at one point does not prove that every transformer connection is lightly stressed.

Do not rely only on total harmonic distortion. THD is useful as a summary indicator, yet it may conceal individual harmonic components that are relevant to loss and resonance concerns. Review voltage and current spectra, phase balance, loading level, and the timing of harmonic peaks. If capacitor banks, harmonic filters, STATCOM equipment, or long cable runs are present, investigate whether operating changes shift the system response. A filter that is unavailable, detuned, or poorly coordinated can change transformer stress without triggering an obvious mechanical alarm.

For recurring thermal events, temporary high-resolution recording is often more valuable than a short manual check. Solar plants can look electrically calm during a routine inspection while the problematic condition appears only during a narrow combination of high irradiance, inverter dispatch, and reactive-power demand. Capture enough operating variation before concluding that the transformer is undersized or that the cooling system has failed.

Cooling checks that go beyond “fans are running”

A fan turning is not proof that cooling is effective. Inspect radiator banks for dust, vegetation, insect accumulation, bent fins, oil staining, and blocked air paths. In dry sites, fine dust can gradually reduce heat transfer without producing a dramatic visual change. In coastal or humid environments, corrosion and contamination may affect motors, terminal boxes, control contacts, and radiator surfaces. Thermal imaging can help identify uneven radiator performance, but readings should be interpreted carefully because sunlight and wind can distort surface temperatures.

Confirm that fan groups stage in the intended sequence and that automatic controls respond to the correct temperature inputs. A failed temperature sensor, poorly set control point, stuck contactor, or unavailable auxiliary supply can leave part of the cooling system idle. For oil-forced designs, verify pump operation, flow indication where provided, and any interlocks connected to pump failure. Never bypass cooling alarms as a temporary convenience during peak production; a bypass can conceal the condition that should trigger a controlled reduction in loading.

Clearance also matters. Heat rejected from radiator banks needs somewhere to go. Equipment installed too close to walls, enclosures, or adjacent transformers can recirculate hot air. For relevant compact oil-immersed equipment, installation guidance may require at least 1.5 metres from a wall; site-specific drawings and manufacturer instructions should always take priority. The same practical discipline applies to cable trenches and control cabinets: poor surrounding ventilation can turn a manageable temperature rise into a repeated summer issue.

Oil, insulation, and connections: where heat leaves evidence

If abnormal heating persists, move from external observation to condition evidence. Oil tests can support a broader diagnosis, particularly when compared against the unit’s earlier results rather than viewed as a one-time pass/fail exercise. Dissolved gas analysis, moisture assessment, dielectric strength, acidity, and other applicable tests should be selected and interpreted by qualified personnel in line with the transformer type and maintenance program. A changing trend is often more informative than one isolated laboratory value.

Pay particular attention to connections. Harmonic current does not create every hot joint, but it can increase heating at already compromised contact surfaces. During planned outages, inspect and torque accessible bolted connections to approved values, check bushing terminals for discoloration or tracking, and examine flexible links, cable lugs, grounding points, and neutral connections. A localized hot connection may not significantly change bulk oil temperature until the problem is well advanced.

Insulation ageing is cumulative. Repeated operation at elevated hot-spot temperature can reduce insulation life even if no immediate protection trip occurs. That is why after-sales teams should resist the temptation to treat every thermal alarm as a nuisance caused by a hot day. One event may be tolerable; a repeatable seasonal pattern deserves engineering review before it becomes a reliability event.

Practical operating choices when margins are tight

The correct response depends on the cause. If power-quality measurements show elevated harmonic current, work with the inverter and plant-control teams before changing transformer settings. Possible actions may include reviewing inverter switching behaviour, reactive-power commands, filter availability, cable configuration, and operating combinations that coincide with the thermal peaks. The objective is to reduce unnecessary loss at the source, not merely to tolerate hotter operation.

If cooling performance is the limitation, cleaning, repair, control calibration, or improved ventilation may recover margin without changing electrical operation. Where loading is genuinely beyond the thermal capability of the installed unit under real harmonic conditions, managed curtailment during extreme ambient periods may be safer than repeatedly accepting alarm conditions. Any decision to uprate, replace, or derate equipment should be supported by design data, measured power quality, and a review of insulation condition.

For collector systems and auxiliary distribution points, equipment selection should also reflect the actual renewable-energy duty. Jinshida Electric Power Technology Co., Ltd. focuses on transmission and distribution equipment for grid, industrial, infrastructure, and new-energy applications, where manufacturing quality and practical serviceability both influence long-term reliability. An oil-immersed solution such as the 33kV Oil-Immersed Power Distribution Transformer can be configured for 33 kV to 0.4 kV distribution duties and offers an FR3 vegetable oil option. It should not be casually substituted for a plant’s main step-up transformer without confirming the application, voltage arrangement, harmonic duty, cooling design, and protection coordination.

Build a maintenance record that makes the next fault easier to solve

The most useful transformer maintenance record is not a checklist stored after each visit. It is a timeline that connects events: inverter changes, filter outages, fan repairs, oil-test trends, thermal images, maximum daily load, ambient temperature, and protection alarms. Over time, this allows the team to distinguish a normal seasonal temperature shift from a developing abnormality.

For a high voltage transformer for solar power plants, reliable operation comes from treating heat as evidence and harmonics as a loading factor, not a distant power-quality statistic. When temperature trends, cooling condition, connection integrity, oil health, and inverter behaviour are reviewed together, maintenance teams can intervene while the issue is still manageable—before insulation damage or an avoidable outage forces the decision.