Skip to content 
Photovoltaic plants produce heavy harmonic currents from nonlinear electronic loads, threatening transformateur de puissance, transformer power lines, power line transformer and other transformers and power facilities. Without filtering equipment, transformer for power will suffer overheating and insulation degradation. As a specialist supplier, Chuanli develops K4/K9/K13 anti‑harmonic power transformer and supporting filter solutions complying with IEEE and IEC standards. This article offers complete matching guidance for PV‑plant electrical designers and EPC engineers.
Nonlinear converter equipment in the photovoltaic power plant continuously injects into the power grid 3, 5, 7, 11 and other high-order harmonics, the distorted current flows through the entire transformer and power circuit, forming two types of irreversible damage mechanisms for each level of line transformer, laying hidden dangers for the long-term stable power generation of the power station, the whole passage is fluent and integrated, without fragmentation and splitting.
Harmonic currents will significantly increase the iron core and winding internal eddy current loss and stray magnetic loss of the line transformer, and the high-frequency harmonic magnetic flux penetrates the winding insulation layer, accelerating the aging of epoxy or oil paper insulation, shortening the service life of equipment by 30% to 50% compared with the linear load condition. When the outdoor transformer power line is long-distance overhead laid, the harmonic voltage distortion will induce corona discharge on the surface of the cable insulation tube, resulting in creep aging and line flash fault. At the same time, serious harmonic distortion will cause the neutral line of the transformer to overload, burn out the neutral busbar, cause the overcurrent protection of the distribution room to trip, and lead to the sudden power outage of the thermal storage and power generation unit. The series of K-class anti-harmonic transformers with reinforced winding transposition structure and iron core silicon steel sheet design, which weakens the response of harmonic loss, is the basic premise for safe matching with harmonic filter.
Under the condition of harmonic pollution, the total loss of the empty load and load of the special distribution transformer increases significantly, and the additional loss is positively correlated with the total distortion rate of the current THDi.
| Total Current Distortion (THDi) Level | Transformer Additional Load Loss | Average Winding Temperature Rise | Annual Additional Power Consumption Loss |
| ≤5% (Filtering Compliant) | Additional Loss ≤8% | Temperature Rise ≤25K | Negligible |
| 10% Uncontrolled Harmonics | Additional Loss 22% | Temperature Rise 42K | Annual Additional Power Consumption 160,000 kWh |
| 20% Severe Harmonic Distortion | Additional Loss 47% | Temperature Rise 68K | Annual Additional Power Consumption 380,000 kWh |
When the photothermal power station is not equipped with a harmonic filter, a single 10MVA transformer will generate hundreds of thousands of extra electricity losses every year, and high temperatures will force the unit to reduce power generation during the summer peak.
Excessive loss not only raises the operating cost of the power station, but also makes the transformer run at an over-rated temperature for a long time, which is very easy to cause oil leakage, gas relay alarm and other faults, and in severe cases, it will cause winding short circuit. The matching adapter harmonic filter can control THDi within 5%, reduce additional losses to the lowest range, and balance equipment protection and power generation revenue improvement.
The photothermal transformateur de puissance and the harmonic filter should not be matched only by capacity matching, but need to comprehensively consider the four dimensions of transformer K’s harmonic resistance grade, short-circuit impedance, filter compensation current, and system impedance coordination, to avoid resonance, insufficient filtering, or over-compensation problems, and the paragraph should be continuous without splitting into small paragraphs.
The rated compensation current of the harmonic filter needs to be calculated comprehensively according to the rated capacity of the transformer, the actual load rate of the power station, and the measured THD value. The core calculation formula is: filter compensation current = transformer secondary rated current × measured THDi × 1.25 safety margin.The THDi of the main transformer of the large-scale solar thermal power plant with a capacity of more than 50MW can reach up to 20%, and it needs to be equipped with a multi-tuned branch passive filter group; Small and medium-sized heat storage distribution transformers with severe load fluctuations are matched with active harmonic filters with dynamic compensation function.
| Solar thermal unit capacity | Chuanli transformer model | K harmonic resistance rating | Recommended filter type | Rated filter compensation capacity |
| ≤50MW Small solar thermal storage station | SC (B) 14 dry type / 6.3MVA oil-immersed | K4 | Single branch passive filter | 150–300kvar |
| 50–200MW Medium solar thermal power plant | S18 oil-immersed 10–35MVA | K9 | Multi-tuned passive filter group | 500–1200kvar complete set |
| ≥200MW Large solar thermal base | 110kV main power transformer | K13 | Active harmonic filter AHF | 800–2000A compensation current |
The K harmonic immunity level represents the transformer’s ability to withstand eddy current losses. The larger the size of the photothermal unit and the more inverters, the higher K value transformer needs to be matched.
When the capacity is matched, the filter must reserve 20%– 25% margin to deal with the instantaneous harmonic impact of synchronous operation of multiple thermal energy storage converters, and prevent the filter from being fully loaded and losing its filtering effect during the peak power generation period.
The short-circuit impedance Uk of the transformateur de puissance is the core parameter to avoid filter resonance in parallel with the power grid. The resonant frequency of the LC branch of the passive filter must be offset from the 5th, 7th, and 11th main harmonic order of the photovoltaic inverter, relying on the transformer impedance as a natural damping to suppress harmonic amplification.The K13 high-tolerance harmonic main transformer with Uk≥10. 5 must be connected in series with a 5%-7% misalignment reactor at the front end of each passive filter to shift the resonance point below the 5th harmonic frequency;Uk=6%– 8% Small and medium-sized branch transformers are equipped with 4.5% misalignment reactors to reduce the risk of harmonic amplification.If the impedance of the transformer and filter is not matched, the harmonic voltage at the common coupling point will double, causing the high-voltage protection of the transformer power line to trip repeatedly.
Combined with the characteristics of thermal storage and fluctuation of power generation load in photovoltaic power plants, there are clear applicable boundaries between the two types of passive and active filters and the various series of power transformers of Chuanli. Complex large-scale photovoltaic power bases can choose hybrid filters. The text is coherent and long without fragmentation.
Priority should be given to the use of multi-tuned passive filter groups for fixed large-capacity distribution transformers in photovoltaic power plants with a capacity of 50MW or above. Each LC branch is independently tuned to absorb fixed-order harmonics such as 5 and 7, with low manufacturing costs and strong long-term operation stability, suitable for photovoltaic and solar thermal units with stable daily power generation loads and small fluctuations. The passive filter is installed on the low-voltage side of the main transformer busbar, forming a low-impedance harmonic absorption channel with the transformer winding impedance, In the full load condition, the THDi of the power station can be controlled within 5%, and the K9/K11 oil-immersed transformer with anti-harmonic function should be matched, and the inductance parameters of the reactor should be marked according to the short-circuit impedance of the transformer before the filter is delivered, so as to eliminate the danger of resonance in advance. The shortcoming of the passive filter is that it has poor adaptability to the light and hot cloudy light-load operation, and the filtering effect will be reduced when the number of inverters in operation is reduced.
Active harmonic filter (AHF) is suitable for medium and small-sized photovoltaic thermal energy storage branch transformers and large base auxiliary distribution transformers with fluctuating loads. AHF real-time collects the distorted current signal of the transformer outlet, injects the dynamic compensation current to counteract the harmonics, and can adapt to the rapid fluctuation of the harmonic content caused by the change of light intensity. The K4/K6 dry-type line transformer with AHF can realize real-time dynamic filtering under 0-100% load variation, while also considering three-phase imbalance correction and reactive power compensation functions. In photovoltaic power plants with PCS converters that frequently start and stop, active filters can avoid the problem of insufficient filtering capacity of passive devices. The whole system is linked with the transformer temperature measurement system, and the compensation capacity is automatically increased when the harmonic accumulation causes the transformer to heat up.
The location of the filter installation directly determines the harmonic treatment effect of the entire transformer and power system. Even if the capacity parameters are matched correctly, unreasonable wiring and layout will greatly weaken the filtering effect. The paragraph logic is smooth and there are no small paragraphs.
The optimal installation position of all harmonic filters is the low-voltage output bus of the photothermal main transformateur de puissance, that is, the PCC point of the transformer and the common coupling of the substation distribution network, which intercepts the harmonic current from the source and prevents the harmonic current from flowing back to the transformer winding and the high-voltage input transformer power line. The large main transformer is equipped with a passive filter group, which is installed in a separate filter cabinet adjacent to the transformer room. The length of the filter to the transformer output cable is controlled within 10m to reduce harmonic attenuation loss. The small branch transformer is equipped with an active filter, which can be installed on the side of the low-voltage cabinet, close to the current transformer sampling point, to ensure accurate signal acquisition. Filters are strictly prohibited from being installed at the end of long-distance power distribution lines, as the harmonic losses in the lines will lead to incomplete treatment of the transformer body.
When the filter is matched with the transformer, three major resonance avoidance measures must be implemented: First, in the design stage, the LC resonance frequency of the filter branch is determined according to the short-circuit impedance of the transformer, and a misalignment reactor is used to offset the main harmonic frequency; In the light load state of the second transformer, the synchronous operation of multiple sets of passive filters is prohibited, and the number of input branches is automatically adjusted according to the real-time load rate of the transformer; The neutral point of the third transformer is reliably grounded, and the thick neutral busbar is added to prevent the superposition of neutral harmonic voltage, which may cause resonance oscillation in the transformer power line. After the filter is put into operation and put into trial operation, use the power quality analyzer to sweep the entire line. If there is a harmonic amplification phenomenon, adjust the inductance parameters of the reactor in time and match the impedance of the transformer again.
After completing the installation and connection of the harmonic filter and Chuanli power transformer, four key tests must be completed before connecting to the grid to verify the matching effect and optimize the parameters.First full load power quality test: record the THDi of the transformer at the PCC point and the content of each harmonic under the 100% rated load of the photothermal unit, and the total distortion rate must meet the IEEE 519 standard (THDv ≤ 5%); Second transformer temperature rise comparison test, record the transformer winding temperature under the two working conditions of filter switching, after commissioning, the temperature reduction is not less than 30K, confirm the harmonic loss suppression effect; Third light load resonant sweep test, simulate 20% unit low load operation, check whether the transformer power line has harmonic voltage amplification, if resonance occurs, adjust the filter branch switching logic; Fourth, 72-hour continuous operation tracking test, continuously monitor the transformer loss and power quality data, and optimize the filter compensation threshold according to the fluctuation law of light and heat load during the day and night. If the filtering effect is found to be insufficient during testing, increase the capacity of the single branch of the filter or upgrade the transformer to the K13 harmonic resistance level; If compensation occurs, the proportion of reactive power compensation of the filtering group will be reduced.
Q1: What types of harmonic-resistant power transformers does Chuanli provide for solar thermal power plants?
A1 : The small branch adopts K4, the medium-sized unit adopts K9, and the 110 kV large main transformer adopts K13 grade.
Q2: What type of filter should be recommended for large photovoltaic transformers above 100MW?
A1 : A set of multi-tuned passive harmonic filter with a series-connected misalignment reactor.
Q3: Where is the best place to install the harmonic filter in conjunction with the power transformer?
A3 : At the common coupling bus of the low-voltage side of the transformer, the harmonic intrusion is blocked from the source.
Q4: How to avoid resonance between transformer power lines and passive filters?
A4 : 5% to 7% misalignment inductors are used to offset the LC resonant frequency from the main harmonic order.
Q5: What is the standard limit of total harmonic distortion after filtering of the photothermal transformer?
A5 : Control THDi≤5%, comply with IEEE 519–2022 grid power quality specification.
Multi‑order harmonics from PV inverters continuously damage transformateur de puissance, transformer power lines, power line transformer and overall transformers and power system. Chuanli delivers graded transformer for power (K4/K9/K13) matched with passive and active filters. Project teams shall take transformer K‑class and station load fluctuation as core references, prevent system resonance and run post‑commissioning tests. Proper matching cuts extra loss, extends equipment life and guarantees grid‑compliant power generation.
