Distributed PV microgrids face random power fluctuations, creating heavy impacts on power transformers. Conventional distribution transformers fail to handle bidirectional current and harmonics, threatening transformer power lines. Chuanli develops dedicated transformers for PV microgrids. This article analyzes operational risks, introduces adaptive technologies and summarizes standards to guide the selection of power line transformers.

PV Microgrid Power Fluctuation Impacts on Power Transformers

Power swing risks for power line transformers

The output of distributed photovoltaic power varies greatly with the intensity of light, cloud cover, and day-night changes, and the line transformer will experience a wide range of load fluctuations within a day. At noon, when the sunlight is sufficient, the output of the photovoltaic reaches the peak value, and the transformer bears the load of high-power reverse transmission to the public network. When there is no light at night, the transformer bears the entire positive load of the user side, and the heavy and light loads form a huge contrast in a short period of time. Frequent alternating load fluctuations will cause transformer windings and iron cores to repeatedly expand and contract, accelerating insulation cracking and aging. At the same time, the instantaneous power mutation will cause the voltage of the transformer power line to fluctuate violently, resulting in the flicker of the user’s equipment, the misoperation of the control relay, and the abnormal monitoring signal of the microgrid. In a microgrid without energy storage, the daily power fluctuation of the line transformer can reach 0- 100% of the rated capacity, far exceeding the load change range of the conventional power grid transformer, which will greatly shorten the service life of the equipment in the long run.

Bidirectional power stress of transformer for power

Unlike the traditional unidirectional power transmission of the distribution network, the photovoltaic microgrid realizes bidirectional power transmission through the inverter and energy storage PCS, which brings the unique bidirectional current stress to the power transformer. When the photovoltaic power generation is greater than the local load, the excess power is reverse-fed from the low-voltage side to the high-voltage side; When the power generation capacity is insufficient, the public network will supply power to the microgrid load, and the bidirectional alternating current will generate alternating magnetic flux in the core of the transformer, increasing the iron loss and magnetic vibration noise, and the alternation of the positive and negative current will also aggravate the hidden danger of the loose terminal of the winding. Ordinary transformers are only designed according to unidirectional current flow, without bidirectional current tolerance optimization, and long-term bidirectional operation is prone to local overheating and increased terminal contact resistance, creating a permanent fault hazard for the equipment. The following table visually compares the differences in operation between ordinary distribution transformers and special transformers for solar microgrids in bidirectional fluctuating power conditions.

Chuanli micro-grid transformer reserves a 20% overload margin on the basis of rated capacity, which can cope with the instantaneous peak load of photovoltaic without triggering an over-temperature alarm. The ordinary transformer will overheat if the load exceeds 100% for a short time.

Core Adaptive Designs of PV Microgrid Power Transformers

Harmonic resistance design of power transformer

A large number of photovoltaic inverters and energy storage bidirectional converters generate 3, 5, and 7 high-order harmonics, injecting distorted current into the transformer and power circuit, causing the copper and iron losses of the ordinary transformer to rise and local hot spots in the winding. The special power transformer for photovoltaic power adopts a double-layer harmonic suppression design to solve the problem from the root: the core is made of low magnetostriction and high magnetic permeability silicon steel sheet, which can reduce the harmonic magnetic loss by more than 22% compared with the ordinary core. Add an interlayer electromagnetic shielding layer to the primary and secondary windings to isolate the interference of high-frequency harmonic magnetic fields. The whole machine adopts K=12 heavy harmonic tolerance standard, and can operate stably without abnormal temperature rise under the condition of 12% total current distortion rate; The insulating oil of the oil-immersed type adopts a high-temperature anti-oxidation formula to avoid the carbonization of the oil caused by the heat accumulation of harmonics; SC (B) The dry product is filled with high thermal conductive epoxy resin, the winding heat dissipation is uniform, and the local hot spot induced by harmonics is eliminated.

Wide-load adaptive winding structure

In order to adapt to the power fluctuation of the photovoltaic microgrid from zero load to peak overload, Chuanli optimized the winding structure of the power transformer. The primary and secondary windings adopt the structure of multi-strand transposed copper wire, and the current is evenly distributed under bidirectional current, which prevents local current collection from overheating. The winding ends are matched with high-temperature elastic insulating pads, which buffer the mechanical stress caused by the expansion and contraction of the coil due to the fluctuation of the load, and avoid vibration and friction damage to the insulation. The iron core adopts a full slant seam process to reduce the magnetic vibration noise under variable load conditions, and the noise of the dry micro-network transformer is controlled within 65dB, which conforms to the official product parameter standard. In terms of heat dissipation, the oil-immersed transformer supports the dual cooling mode of ONAN/ONAF, and automatically starts forced air cooling when the PV peak temperature rise occurs; The dry type machine is equipped with multiple axial flow fans, which adjust the heat dissipation in real time with the change of load, ensuring the stable operation temperature of the whole machine under the fluctuating load in the whole range.

Dynamic Adjustment Methods for Transformer Power Operation

Tap control to stabilize transformer power lines

The voltage deviation of the transformer power line under the fluctuation of photovoltaic power is the most frequent fault, and the photovoltaic micro-grid transformer of Chuanli is equipped with a wide-band load or no-excitation tap structure according to the capacity, so as to realize the dynamic voltage stabilization of the line.10–10,000kVA medium and small microgrid transformer configuration ±5 × 2.5% no excitation tap, can be manually adjusted according to the average output of photovoltaic power according to the season, to offset the long-term voltage drift of the line;1250-10MVA large and medium-sized models are equipped with intelligent on-load tap changer (OLTC), relying on the micro-grid SCADA system to collect voltage signals and automatically adjust the ratio in real time, so that the voltage fluctuation of the line can be controlled within ±3% of the rated value throughout the day. At noon, the reverse power transmission of photovoltaics will raise the voltage of the transmission line, and the reduction ratio of the time-division switch will be lowered; at night, when the load voltage drops, the reduction ratio will be raised; the voltage flicker and over-limit problems caused by the fluctuation of the power of the transmission line transformer will be solved from the root.

Smart monitoring for transformers and power balance

All of the transformadores de potencia for photovoltaic microgrids in Chuanli have reserved intelligent monitoring signal interfaces to realize full data linkage with the energy management system (EMS) of the microgrid. The iron core, high and low-voltage windings, oil tank (oil type), and multi-channel temperature sensor upload real-time temperature data to the monitoring platform. The current and voltage transformers on the primary and secondary sides collect bidirectional flow data and calculate the real-time power, power factor, and harmonic distortion rate of the transformer and the power system. The monitoring system sets multi-level warning thresholds for overload, over-temperature, and harmonic over-standard, and automatically sends alarm signals when the instantaneous power fluctuation of the photovoltaic exceeds the parameter, and the energy storage PCS adjusts the charging and discharging power to equalize the transformer load. In the isolated micro-grid with no public network backup, the intelligent monitoring module supports remote reading of parameters and fault recording, greatly reducing the frequency of on-site inspection by operation and maintenance personnel.

Fire-retardant dry transformers should be preferred for indoor closed micro-networks to eliminate the risk of oil leakage and fire; Large-scale outdoor ground-mounted photovoltaic microgrids adopt the oil-immersed type, which has stronger overload heat dissipation ability and lower comprehensive project cost.

Matching Tips for PV Microgrid Power Transformer

1. According to the peak output of photovoltaic power and the maximum reverse current, reserve a sufficient capacity margin; the rated capacity of the power transformer is taken as 1.1- 1.25 times the total peak power of the photovoltaic inverter, fully considering the instantaneous power superposition of strong light, and avoiding the long-term overload of the transformer during noon.

2. According to the number of inverters, choose the corresponding anti-harmonic grade: a large micro-network with more than 10 inverters must use the K=12 heavy anti-harmonic transformer of Chuanli, and a small distributed micro-network with less than 5 inverters can use the K=4 standard model, balancing cost and performance.

3. Reasonably select the distribution device according to the length of the grid-connected line: the overhead transformer power line with a long distance of the micro-grid is equipped with a loaded distribution switch, which should deal with the fluctuation of the voltage drop of the line. The short-distance indoor power distribution microgrid adopts an economical non-excitation tap structure.

4. Distinguish between dry and oil-immersed models according to the layout environment: SC (B) series power-specific transformers must be used in indoor and underground closed distribution rooms; Oil-immersed transformers with wider weather resistance and heat dissipation range can be selected for open-air industrial parks and mountain ground photovoltaic microgrids.

5. Isolated microgrids without public network backup need to be equipped with a complete set of intelligent monitoring modules, so that transformers can cooperate with energy storage devices to dynamically level out power fluctuations and prevent equipment damage caused by zero-load and full-load extreme conditions.

Conclusión

Intermittent PV output brings challenges to transformers and power systems. Chuanli’s dedicated transformer for power solves harmonic and bidirectional load issues. Proper capacity reservation and model selection help stabilize transformer power lines and guarantee long-term safe operation of PV microgrids.

Preguntas frecuentes

Q1: What kind of special adapter structure is used for the two-way current in the power transformer of the solar microgrid of Chuanli?

A1: Multi-strand bidirectional winding + elastic anti-vibration insulation fixed structure, forward and reverse power supply both maintain low-loss operation.

Q2: What is the standard anti-harmonic grade of the distributed photovoltaic microgrid transformer of Chuanli?

A1: The large micro-grid with multiple inverters is equipped with K=12 heavy anti-harmonic design, and the small distributed model adopts K=4 standard grade.

Q3: How to stabilize the voltage fluctuation of the transformer power line under the peak of photovoltaic?

A3: The intelligent on-load tap changer automatically adjusts the transformation ratio in real time according to the line voltage signal, and the voltage fluctuation is limited within ±3% of the rated value throughout the day.

Q4: What kind of power transformer should be used in the indoor rooftop photovoltaic microgrid?

A4: SC (B) series resin-insulated dry-type transformer, flame retardant and low noise, no oil leakage safety hazard.

Q5: Why can’t ordinary distribution transformers be used in photovoltaic microgrid projects?

A5: Ordinary transformers have no bidirectional load, wide fluctuating load, anti-harmonic special optimization, and are extremely prone to overheating and rapid aging under fluctuating photovoltaic conditions.