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A battery energy storage system can satisfy the harmonic limits at its point of common coupling and still introduce a resonant frequency into the electrical network.
This is not a contradiction.
Grid compliance confirms that voltage and current distortion remain within defined limits under the conditions assessed. Harmonic resonance describes how the complete electrical system responds at different frequencies.
A resonant point may exist without immediately causing a compliance failure. However, if a BESS inverter, another converter or existing background distortion excites that frequency, a relatively small harmonic current can produce significant voltage amplification.
This distinction is becoming increasingly important as utility-scale battery energy storage systems connect to medium- and high-voltage networks.
BESS harmonic resonance occurs when the inductive and capacitive elements of a grid-connected battery energy storage system interact at a particular frequency.
A utility-scale BESS contains much more than batteries. Its AC connection typically includes:
Power conversion systems
Inverter output filters
Step-up transformers
Medium-voltage collector cables
Main grid-connection transformers
Auxiliary transformers
Reactive power equipment
Protection and switching equipment
The transformers and external network contribute inductance. The cables, inverter filters and capacitor banks contribute capacitance.
At a certain frequency, the inductive and capacitive reactances can interact to create either parallel or series resonance.
Parallel resonance creates a high network impedance at a particular frequency.
When harmonic current flows at or near this frequency, the high impedance can produce a disproportionately large harmonic voltage. This is one reason a small harmonic current can result in unexpected voltage distortion at the point of common coupling.
Series resonance creates a low-impedance path at a particular frequency.
This can allow excessive harmonic current to circulate through transformers, cables, capacitors or harmonic filters. The resulting current may increase equipment heating and electrical stress even when the harmonic voltage at the PCC appears acceptable.
A BESS harmonic resonance problem must therefore be evaluated using both voltage and current measurements.
Grid codes and harmonic standards normally specify voltage and current distortion limits at the point of common coupling.
Compliance demonstrates that the measured or calculated results satisfy those limits. However, the result is only valid for the operating conditions, network configuration and assumptions included in the assessment.
Resonance is different because it is a characteristic of the complete system impedance.
A resonant point can remain in the network even when harmonic distortion is below the relevant limit. It only becomes visible when sufficient harmonic energy exists near the resonant frequency.
The BESS may therefore pass its initial harmonic assessment but experience a problem later after:
The grid short-circuit level changes
A transformer is removed from service
A cable or overhead circuit is disconnected
A capacitor bank is switched
Another renewable plant connects nearby
Background harmonic distortion increases
The BESS changes operating mode
The number of energized inverters changes
Grid compliance should not be treated as proof that the installation can never experience harmonic resonance.
A conventional harmonic assessment may represent an inverter as a source of harmonic current. Although useful, this model does not describe the inverter’s complete interaction with the grid.
A modern BESS inverter has a frequency-dependent output impedance. Its behaviour is influenced by:
Current-control bandwidth
Phase-locked-loop settings
Digital control delays
Active damping
Voltage feed-forward
Switching frequency
Output filter design
Active and reactive power
Grid-following or grid-forming control
At some frequencies, the inverter can appear inductive. At others, it can appear capacitive. Its effective impedance may also change with its operating point and control mode.
The basic relationship between harmonic voltage and current can be expressed as:
Harmonic voltage = harmonic impedance × harmonic current
If the network impedance is low, a particular harmonic current may produce limited voltage distortion.
If the impedance increases sharply near a parallel resonance, the same harmonic current can produce a much higher harmonic voltage.
The resulting distortion is therefore determined by more than the harmonic current produced by the BESS. It also depends on the impedance through which that current flows.
Medium-voltage BESS plants often contain extensive cable networks connecting multiple inverter stations to a common switchboard.
These cables introduce capacitance into the system. When combined with transformer and grid inductance, the cable capacitance can create a parallel resonant point.
As more cable is installed, the total capacitance increases. This can shift the resonance towards a lower frequency and bring it closer to harmonic components produced by the inverters or already present in the grid.
The final resonant frequency depends on factors including:
Cable length
Cable construction
Number of parallel circuits
Transformer leakage impedance
Grid short-circuit level
Number of connected inverters
Existing filters and capacitor banks
Cable capacitance should therefore be represented accurately in every BESS harmonic study. Treating the collector system as a simple fundamental-frequency impedance can conceal important harmonic behaviour.
A utility-scale BESS may contain tens or hundreds of inverter modules operating in parallel.
Each inverter may comply with its equipment-level harmonic requirements when tested against a specified reference impedance. However, connecting many inverters to a common collector network changes the equivalent impedance of the plant.
The harmonic current observed from one inverter can include:
Current generated by that inverter
Current originating from other parallel inverters
Current caused by background grid voltage distortion
Circulating current between inverter units
Each inverter and its output filter can also introduce a resonant peak. As more units are connected, the combined impedance response becomes more complex.
This means that twenty individually compliant inverters are not always electrically equivalent to one inverter multiplied by twenty.
The full BESS must be studied as an interconnected system.
A recent study examined a 50 MW BESS connected to a 33 kV network. The plant contained 19 inverters and their associated step-up transformers.
Measurements taken during commissioning and operation found harmonic non-compliance across multiple charging and discharging conditions. The affected frequencies extended from approximately the 14th to the 65th harmonic orders.
Two main resonant regions were identified.
The first resonance occurred between approximately the 14th and 20th harmonic orders and was associated with the self-impedance of the BESS inverters.
The second occurred between approximately the 50th and 70th harmonic orders and was linked primarily to the capacitance of the 33 kV collector cables.
The investigation showed that the problem could not be explained by inverter harmonic emissions alone. The BESS had changed the network impedance and amplified particular harmonic components.
A validated system model was developed using measurements from idle, charging and discharging modes. A 5 MVAr C-type harmonic filter was subsequently designed and connected to the 33 kV BESS switchboard.
The filter damped the resonant regions and brought the installation within the applicable harmonic limits.
The case demonstrates why a BESS harmonic study must evaluate both harmonic sources and frequency-dependent network impedance.
It is often assumed that maximum charging or discharging power will produce the highest harmonic distortion.
This is not always correct.
The harmonic spectrum of a BESS inverter can change with its output power, reactive power and control mode. The number of operating inverter units may also change at partial load.
At low BESS output:
Fewer inverter modules may be operating
Harmonic cancellation between units may decrease
Current distortion as a percentage of fundamental current may increase
Control modes may change
The plant impedance may shift
Background voltage distortion may become more influential
The most severe BESS harmonic resonance can therefore occur during partial charging, partial discharging, reactive power support or standby operation.
A harmonic assessment based only on rated active power may miss the actual worst-case condition.
Network impedance is not fixed.
It changes as the transmission or distribution system moves between operating configurations. A strong grid with multiple transformers and circuits in service may have a different harmonic impedance profile from a weak grid during a planned outage.
Changes in network inductance can move the resonant frequency closer to or further from a significant harmonic source.
Important grid conditions include:
Minimum and maximum short-circuit levels
Normal and outage configurations
Transformer availability
Bus-section switching
Transformer tap positions
Nearby generation status
Capacitor and reactor switching
Future network reinforcements
A single short-circuit level and X/R ratio cannot always represent the complete harmonic behaviour of the external grid.
A robust BESS grid-connection study should use an impedance envelope or multiple frequency-dependent network cases.
A complete BESS harmonic study should examine the interaction between the inverters, collector network and external grid.
Impedance scans should be performed across the frequency range required by the applicable grid code.
The scans should identify:
Parallel resonant peaks
Series resonant troughs
Resonance movement between operating cases
Frequencies close to significant inverter emissions
Frequencies close to background grid harmonics
Interactions with existing filters and capacitor banks
The study should include:
Full-power charging
Partial-power charging
Full-power discharging
Partial-power discharging
Standby operation
Zero-active-power reactive support
Different numbers of operating inverters
Reduced plant availability
Transformer impedance, cable capacitance, filter parameters and reactive power equipment must be represented accurately.
Small errors in capacitance or inductance can shift the calculated resonant frequency and change the predicted filter performance.
Where available, inverter manufacturers should provide harmonic emission data and frequency-dependent impedance models.
If detailed control models cannot be shared, black-box Norton or Thévenin equivalents may provide enough terminal information for the harmonic interaction assessment.
Background voltage distortion should be measured before the BESS is energized whenever possible.
These measurements help distinguish between:
Harmonics generated by the BESS
Existing grid distortion
Harmonics amplified by the BESS impedance
Interactions between background distortion and the new installation
Post-commissioning measurements should be compared with the simulation model.
If the measured and calculated impedance peaks do not align, the model should be corrected before harmonic mitigation equipment is designed.
The correct mitigation method depends on the frequency range, network impedance, inverter behaviour and required reactive power.
Possible solutions include:
Passive harmonic filters
C-type harmonic filters
Damped high-pass filters
Active harmonic filters
Hybrid passive and active filters
Additional converter or line reactors
Active damping in inverter controls
Virtual impedance or admittance
Changes to transformer or cable arrangements
Coordinated operation of inverter units
Passive harmonic filters provide a low-impedance path for selected harmonic currents and can add damping to the network impedance profile.
They are commonly used at medium voltage because they can be economical, efficient and suitable for large power ratings.
However, every passive filter also changes the network impedance. A poorly designed filter can move an existing resonant point or create a new one.
C-type filters can provide harmonic damping while reducing fundamental-frequency losses in the damping resistor.
They are often suitable where broadband damping and reactive power compensation are both required.
The design must consider:
Tuning frequency
Quality factor
Reactive power rating
Capacitor voltage and current
Reactor linearity
Resistor energy duty
Manufacturing tolerances
Component ageing
Network contingencies
Future system changes
Active filters measure harmonic current or voltage and inject a compensating signal.
They can adapt to changing operating conditions, but their required rating, control bandwidth and medium-voltage interface must be carefully evaluated.
In some installations, harmonic resonance can be mitigated by changing inverter control parameters or adding active damping.
This approach may be cost-effective during the design stage, but it requires close cooperation with the inverter manufacturer and careful validation across all grid conditions.
A harmonic filter should not be selected using only the largest harmonic order shown in a measurement report.
The filter must be designed against:
The complete impedance profile
All credible network conditions
Charging and discharging modes
Existing background distortion
Future harmonic sources
Component tolerances
Thermal and dielectric limits
Fundamental reactive power requirements
The objective is not simply to reduce one harmonic percentage. It is to create a stable and adequately damped impedance profile across the relevant frequency range.
Yes. A resonance can exist without being strongly excited. Harmonic voltage and current may remain within limits until the network configuration, BESS operating mode or background harmonic spectrum changes.
No. The resonance risk depends on the inverter impedance, cable capacitance, transformer impedance, grid strength and existing compensation equipment.
Yes. Multiple parallel inverters change the equivalent impedance of the plant. They can interact with each other and with the grid even when every inverter individually satisfies its equipment-level requirements.
No. Partial load, standby or reactive power operation can sometimes produce higher percentage distortion or a less favourable impedance condition.
Yes. Additional capacitance can shift a parallel resonant frequency closer to an existing harmonic source. Capacitor banks should be included in the complete harmonic model.
There is no universal filter. The correct design depends on the measured harmonic spectrum, network impedance, reactive power requirements and operating scenarios. C-type, tuned, high-pass, active and hybrid filters may all be appropriate in different installations.
BESS harmonic resonance is a system interaction problem, not simply an inverter emission problem.
A battery energy storage system may satisfy harmonic limits at the point of common coupling while still creating or shifting a resonant frequency. If that frequency is later excited by the BESS, another converter or background grid distortion, harmonic voltage or current can increase significantly.
A reliable BESS harmonic study should therefore include frequency-dependent impedance scans, multiple charging and discharging modes, realistic network configurations, accurate cable and transformer data, background measurements and post-commissioning validation.
The key questions are not only:
Does the BESS comply with the harmonic limits today?
They are also:
Where are the system resonances, what can excite them, and how will they change tomorrow?
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