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Whether harmonic mitigation is needed for EV charger

I. Harmonic Issues of EV Charger

(I) Sources and Characteristics of Harmonics

As equipment that converts electrical energy into a form suitable for charging electric vehicle (EV) batteries, charger generate harmonics during operation. This is particularly true for AC slow-charging piles: their internal rectifier circuits (which convert alternating current to direct current for battery charging) are non-linear loads, the primary source of harmonic generation. When AC power passes through the rectifier circuit, the current waveform becomes severely distorted and no longer maintains a standard sine wave, resulting in the production of a large amount of harmonic current. Among various harmonic components, odd-order harmonics (such as the 5th and 7th harmonics) are the most prominent.

From actual measurement data, a single 7kW AC charging pile typically has a Total Harmonic Distortion of Current (THDi) ranging from 30% to 40% when operating at full load. Among these harmonics, the 5th harmonic current often exceeds the limit specified in national standards (5.75A). For example, in a charging pile cluster of a residential community, a test on a single 7kW AC charger showed that its 5th harmonic current reached 6.5A at full load.

In contrast, DC fast EV charger (with relatively high power, ≥100kW) are usually equipped with a Power Factor Correction (PFC) circuit. This circuit optimizes the current to a certain extent, keeping the THDi of DC fast EV charger generally below 15%. Compared with AC slow EV charger, DC fast EV charger have a smaller harmonic impact on the power grid.

(II) Hazards of Harmonics

1. Equipment Malfunctions

In a three-phase four-wire power supply system, harmonic currents superimpose in the neutral line. When harmonic currents are excessively large, the current in the neutral line may exceed its rated current-carrying capacity, triggering overcurrent protection actions.

A residential community in Guangdong once encountered this issue: 35 units of 7kW AC charger were installed, and during simultaneous operation, the distribution switches tripped frequently. Testing and analysis revealed that harmonic currents generated by the EV charger superimposed in the neutral line, causing the neutral line current to exceed the rated current of the switches (the neutral line terminal showed signs of blackening and carbonization), leading to repeated tripping.

2. Reduced Efficiency

Harmonic currents intensify heat generation in power equipment such as transformers and cables. Taking transformers as an example: harmonic currents create additional losses in transformer windings, which not only increases the transformer’s energy consumption but also raises its operating temperature. Relevant studies indicate that harmonics can increase transformer losses by 15% to 20%. When transformers operate under such high-temperature and high-loss conditions for a long time, their insulating materials age acceleratedly, shortening the transformer’s service life.

Similarly, for cables, harmonic currents increase their resistance, leading to higher power losses during energy transmission. To ensure the normal operation of EV charger, the transformer’s idle capacity must be maintained at over 20%.

3. System Interference

Harmonics interfere with other equipment in the power system. For instance:

II. Harmonic Mitigation Requirements in Different Scenarios

(I) Factory Scenarios

Take a factory with 2 installed 120kW DC EV charger + 25 installed 7kW AC EV charger as an example:

1. Core Contradiction

In this factory scenario, harmonic issues become prominent when 25 units of 7kW AC EV charger operate concurrently. Due to the large number of AC EV charger, their individually generated harmonic currents superimpose when operating simultaneously, leading to a significant increase in total harmonic current. In particular, the 5th harmonic current can reach a peak of 300A or even higher. Such high harmonic currents far exceed the current-carrying capacity of the neutral line in the factory’s power distribution system, posing a serious threat to the safe and stable operation of the distribution system.

2. Key Focus of Mitigation

Since DC EV charger are equipped with built-in filtering modules that can suppress their generated harmonics to a certain extent, the focus of harmonic mitigation in this factory scenario should be placed on the 25 AC EV charger. Effective mitigation of harmonics generated by AC EV charger can significantly reduce the harmonic content in the entire factory’s power distribution system, ensuring the normal operation of power equipment.

3. Necessity

If harmonics in the factory’s power distribution system exceed standard limits, it will cause huge losses to factory production. For example, when harmonics lead to failures in key equipment on the production line, the entire production line may shut down. Statistics show that in cases where production lines are shut down due to harmonic issues, the economic loss from a single shutdown is often no less than 100,000 yuan. This includes not only equipment maintenance costs but also indirect losses such as delayed production orders and breach-of-contract compensation caused by production halts.

(II) Residential Community Scenarios

For example, in a residential community with 100 EV charger, long-term monitoring statistics show that during the peak night charging period, the average number of concurrently operating piles is approximately 25, with a simultaneous operation rate of 25%.

1. Characteristics

EV Charger in residential communities are characterized by scattered installation and concentrated charging time periods. Typically, residents’ electric vehicles are mostly charged at night, resulting in highly concentrated charging hours. However, despite the concentrated charging periods, the simultaneous operation rate of charging piles is relatively low (generally not exceeding 30%) due to differences in residents’ electricity usage habits.

2. Potential Risks

Although obvious problems such as tripping may not occur in the short term due to the low simultaneous operation rate of EV charger, the long-term impact of harmonics cannot be ignored. Persistent harmonics will keep the neutral line in an overloaded and overheated state for a long time, accelerating its aging. In addition, harmonics will increase transformer losses and reduce its service life.

3. Mitigation Threshold

Harmonic mitigation must be enforced when the number of concurrently operating EV charger in the community reaches 20 or more, or when the load rate of the community transformer exceeds 80%. This is because under these conditions, the impact of harmonics on the power distribution system becomes significant. Failure to implement timely mitigation will seriously affect the safe and stable operation of the community’s power system and may even trigger safety accidents.

III. Technical Design Schemes for Harmonic Mitigation

(I) Source Control (Design and Selection of Low-Harmonic EV Charger Equipment)

1. Technical Requirements

When purchasing EV charger, priority should be given to products equipped with built-in Power Factor Correction (PFC) circuits or LCL filters. These advanced technologies can effectively suppress harmonics generated during the operation of EV charger, reducing the Total Harmonic Distortion of Current (THDi) of a single charging pile to less than 15%. Currently, some domestic brands on the market have achieved low-harmonic output in their 7kW AC EV charger by adopting these technologies, meeting strict harmonic standards.

2. Cost

Although selecting low-harmonic equipment increases the initial procurement cost by 10%-15%, it is cost-effective in the long run. This is because it reduces subsequent investments in harmonic mitigation equipment and costs related to equipment maintenance and replacement caused by harmonic issues. For example, a charging station with 50 EV charger may incur an additional initial procurement cost of 50,000-75,000 yuan if all low-harmonic equipment is adopted. However, over the subsequent 10-year service life, it is expected to save 100,000-150,000 yuan in expenses caused by harmonic mitigation and equipment failures.

(II) Passive Mitigation (Two Economical Configuration Schemes)

1. Incoming Line Reactor Configuration Scheme

① Application Scenarios

Mainly applicable to clusters of low-power AC EV charger, such as scenarios where a large number of 7kW AC EV charger are installed centrally. The cost per unit is relatively low, generally ranging from 300 to 500 yuan. This makes incoming line reactors an economical harmonic mitigation option, especially suitable for small-scale projects sensitive to costs.

② Principle

Incoming line reactors, leveraging their inductive characteristics, generate high impedance to harmonic currents (such as the 5th and 7th harmonics), thereby suppressing the transmission of harmonic currents. Generally, a reactance rate of 3%-4% is considered appropriate. When harmonic currents pass through the incoming line reactor, the reactor impedes their flow, causing a certain voltage drop across the reactor and thus reducing the harmonic currents entering the power grid.

③ Effect

In practical applications, after installing incoming line reactors, the THDi of AC EV charger clusters can be reduced from 40% to 25%-30%. For instance, in a small charging station, after installing incoming line reactors, tests on a cluster of 7kW AC EV charger showed that the THDi decreased from 38% to 28%, achieving a certain degree of harmonic suppression (though significant harmonics still remain).

2. LC Passive Filter Configuration Scheme

① Application Scenarios

Suitable for DC fast EV charger areas with relatively single harmonic components. In DC fast-charging stations, if testing reveals that the main harmonic components are concentrated at several specific frequencies, LC passive filters can perform well. The cost per unit for a group of switched LC passive filters is approximately 8,000 yuan per group. Although the cost is relatively higher, it remains a feasible option for DC fast-charging stations that have high power quality requirements and single harmonic components.

② Principle

An LC passive filter consists of an inductor (L) and a capacitor (C) to form a resonant circuit. By adjusting the parameters of the inductor and capacitor, the filter creates a low-impedance path for harmonics of specific frequencies. When harmonic currents of specific frequencies flow through the filter, they are absorbed by the filter and form a loop within it, thereby reducing the injection of such harmonic currents into the power grid.

③ Effect

If designed and installed properly, LC passive filters can reduce the THDi to below 5%, effectively improving the power quality of DC fast EV charger areas. For example, in a DC fast-charging station, after installing LC passive filters designed for the main harmonic frequencies, the THDi decreased from 12% to 4%, achieving a significant effect.

(III) Active Mitigation (High-Precision and High-Efficiency Scheme with Active Power Filter (APF))

① Application Scenarios

Suitable for large-scale charging stations (with more than 50 EV charger) or highly sensitive areas with extremely high power quality requirements, such as charging stations near hospitals. In these scenarios, due to the large number of EV charger, complex harmonic interference, or strict requirements for power stability, active power filters can give full play to their advantages and effectively ensure the power quality of the power grid. The cost is relatively high; a centralized active power filter typically costs more than 200,000 yuan. This is mainly due to the advanced power electronic technology, complex control algorithms, and high-quality components adopted inside. However, considering its excellent harmonic mitigation effect, it is still a worthwhile investment for large-scale projects.

② Principle

An active power filter detects harmonic currents in the power grid in real time. Using its internal power electronic devices, it generates a compensation current that has the same magnitude but opposite direction to the harmonic currents. This compensation current is injected into the power grid to offset the original harmonic currents, thereby eliminating harmonics. Its response time is extremely short (generally less than 10ms), enabling rapid and accurate compensation for dynamically changing harmonics.

③ Effect

Active power filters have an extremely high harmonic elimination rate, usually reaching over 95%. They can fully compensate for 2nd to 51st harmonics or compensate for specific harmonics according to actual needs. Meanwhile, APFs are also equipped with comprehensive protection functions, such as bridge arm overcurrent protection, DC overvoltage protection, and device overheating protection, ensuring their safe and stable operation in complex power grid environments.

(IV) Necessary Strategies for System Optimization

1. Three-Phase Load Balancing

Reasonably distribute the number of EV charger across three phases to balance the three-phase load as much as possible. Ensure that the number of EV charger is evenly distributed in each phase to avoid concentrated single-phase connection. When the three-phase load is unbalanced, the current in one phase may become excessively large, thereby intensifying harmonic generation. By evenly distributing charging piles across three phases, the 3rd harmonic can be effectively reduced by approximately 30%. For example, in a charging station with 30 charging piles, through reasonable adjustment of the three-phase load, the 3rd harmonic current caused by original three-phase imbalance was reduced from 15A to 10.5A.

2. Off-Peak Charging

With the help of an intelligent charging management system, set up batch charging during off-peak night hours (e.g., 23:00-7:00). This not only reduces the risk of harmonic superimposition during the same period but also allows users to enjoy lower electricity prices during off-peak hours, achieving a win-win situation for economic benefits and power quality.

3. Wiring Specifications

During the wiring of charging stations, a TN-S grounding system should be adopted to ensure that the grounding resistance does not exceed 4Ω. Meanwhile, armored shielded cables should be used, and the cable length should not exceed 50 meters. This can effectively reduce the amplification of harmonics during transmission and improve the stability of power transmission. For example, in the renovation project of a charging station, after replacing the non-compliant original wiring with a TN-S grounding system and armored shielded cables, harmonic interference was significantly reduced, and the operational stability of power equipment was significantly improved.

IV. Design and Analysis of Harmonic Mitigation Case Studies

(I) Factory Scenario (Taking 25 Operating 7kW AC EV Charger Installed in a Factory as an Example)

1. Grouped LCR Filter Mitigation Scheme

2. Centralized APF Mitigation Scheme

(II) Residential Community Scenario (Taking 100 Operating 7kW AC EV Charger Installed in a Community as an Example)

1. Low-Harmonic Equipment + Reactor Mitigation Scheme

2. Grouped LC Filter Mitigation Scheme

Note: LC filters and LCR filters are common passive filters in electronic circuits. Their core difference lies in whether a damping resistor (R) is included, which directly affects their frequency characteristics, power loss, and application scenarios.

  • LC filters: Composed of inductors (L) and capacitors (C), with topologies such as π-type and T-type. They achieve filtering by leveraging the characteristics of inductors (impeding high-frequency signals) and capacitors (passing high-frequency signals). Theoretically, there is no energy loss (for ideal components).
  • LCR filters: Add a resistor (R) to the LC structure, forming a series or parallel L-C-R configuration. The resistor (R) is introduced to provide damping, which suppresses the amplitude of the LC resonance peak and prevents circuit oscillation, but it causes energy loss. LCR filters are mainly used to suppress resonance spikes, such as damping resistors in π-type filter circuits.

V. Analysis of the Issue Where EV Charger Operate Normally Without Harmonic Mitigation in Practice

In practice, some residential communities have installed a large number of EV charger without supporting harmonic mitigation solutions, yet there have been no occurrences of tripping or EV charger malfunctions. This phenomenon can be attributed to the following key reasons:

1. Low Simultaneous Operation Rate

Residents in residential communities mostly charge their electric vehicles at night. However, due to differences in residents’ living habits and vehicle usage patterns, the simultaneous operation rate of EV charger is relatively low, generally not exceeding 30%. As a result, the harmonic current injected into the power grid at any given moment is relatively small and has not yet reached the threshold that triggers the trip protection mechanism.

2. Power Distribution Redundancy

When planning and constructing new residential communities, considerations are usually given to potential future electricity demand growth. Therefore, the load rate of transformers is generally not too high, mostly at 50% or below. This power distribution redundancy provides transformers with a certain margin to withstand the harmonic current generated by charging piles, preventing overload tripping caused by harmonics to a certain extent.

3. New National Standards Regulating Power System Design

The new national standards have imposed strict requirements on the design criteria for neutral lines: the cross-sectional area of the neutral line must be equal to that of the phase line (whereas under the old standards, the cross-sectional area of the neutral line was usually half that of the phase line). This change significantly improves the current-carrying capacity of neutral lines, enabling them to better handle the additional current generated by the superposition of harmonic currents. Consequently, the possibility of tripping due to neutral line overload is reduced.

While no issues have occurred for a period of time, this is only temporary, and there are numerous hidden risk warnings, such as the following:

  1. Neutral Line Aging
    Although tripping may not occur in the short term, harmonic currents flowing through the neutral line for a long time will cause the neutral line to heat up continuously. According to relevant research, for every 10°C increase in the temperature of the neutral line, the service life of its insulating material is halved. When exposed to high temperatures for an extended period, the insulation performance of the neutral line will gradually deteriorate. Problems such as aging and damage may occur after 2–3 years, which could further trigger safety accidents like short circuits and electric leakage.
  2. Transformer Derating
    Harmonic currents generate additional losses in transformer windings, leading to an increase in the transformer’s temperature. When the transformer’s temperature rises by 15°C, its actual output power decreases by approximately 20%. This means that when the transformer endures harmonic currents for a long time, its load-carrying capacity will gradually decrease. It may no longer meet the community’s growing electricity demand, requiring early replacement or capacity expansion—which increases power supply costs.
  3. Metering Errors
    Harmonics affect the metering accuracy of smart electricity meters, causing the meters to overcharge. Studies have shown that in the presence of harmonics, the metering error of smart electricity meters may range from 3% to 8%. For community residents, this means they may pay extra electricity fees, increasing their electricity costs.

VI. Recommendations for Installation Design and Implementation

Through scientific mitigation, the harmonic issues of charging piles can achieve “controllable costs and zero risks”, which not only ensures the safety of the power grid but also extends the service life of equipment.

  1. Use professional power quality analyzers (e.g., FLUKE 435) to conduct on-site measurements of the power grid after EV charger are connected. Focus on monitoring the ratio of neutral line current to phase current: when this ratio exceeds 0.6, it indicates that harmonic issues have become relatively severe, and mitigation measures must be taken immediately. Meanwhile, closely monitor the 5th harmonic content—issue an early warning when the 5th harmonic content exceeds 180A. The monitoring period should preferably be during peak charging hours.
  2. For the selection of mitigation schemes:
    • Factories should prioritize the adoption of grouped LCR filters (5 piles per group) to ensure the Total Harmonic Distortion of Current (THDi) is less than 12%;
    • For new residential communities, deploy LC filters when the number of concurrently operating piles is ≥20;
    • For old residential communities, configure filtering devices directly according to 30% of the total number of EV charger to avoid the risk of compensation for power outages.
  3. Establish a harmonic monitoring ledger and re-test key indicators quarterly; use charging apps to implement off-peak scheduling and reduce the simultaneous operation rate to below 30%; require charging pile manufacturers to provide harmonic test reports certified by the China National Accreditation Service for Conformity Assessment (CNAS) (with THDi < 20%).

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