Земля active harmonic filter working principle uses a closed control loop: it measures distorted load current, calculates selected unwanted components, commands a power converter, and injects compensating current in parallel at a defined bus. At that boundary, load current and filter current determine the source current, linking sensing, reference generation, current tracking, and shunt injection in one operating chain.
What a Shunt Active Harmonic Filter Changes in the Power System
A shunt active harmonic filter supplies compensating current in parallel with the load, reducing how much of the selected unwanted current must travel through the source side of the chosen bus.
A nonlinear load does not draw current in direct proportion to a sinusoidal supply voltage. A rectifier front end in a VFD, for example, can draw current in pulses, so the load-current waveform contains a fundamental component plus harmonic components.
Harmonic current is the portion associated with waveform components above the fundamental. The shunt active filter supplies a controlled counterpart of selected components instead of asking the upstream source to supply all of them.
The shunt AHF is a controlled current source connected beside that load. It does not carry the entire load current in series; it adds another current path at the compensated bus.
Ан active power filter, or APF, uses controlled power electronics rather than a fixed passive network; this article focuses on the shunt-current form.
Choose the sign convention before reading the current balance. Define source current and filter current as entering the compensated bus, with load current leaving toward the load.
i_source + i_filter = i_load
Equivalently, i_source = i_load - i_filter. For a targeted load component, this branch-reference convention assigns the filter component the same algebraic sign as the load component; the subtraction represents the filter supplying that component locally.
Consider several VFD rectifiers connected to one plant bus. The drives continue to draw the current their processes demand, while the AHF supplies selected harmonic and reactive-current components locally; upstream conductors then carry the remaining sum at that bus.
| Quantity or functional job | What it represents | Why it must stay distinct |
|---|---|---|
| Measured load current | The current signal observed by the sensor at a defined boundary | It is an input to the controller, not the correction itself |
| Compensation-current reference | The controller’s time-varying target | It says what the converter should produce next |
| DC-bus regulation | The job of maintaining the converter’s usable DC energy state | It supports current synthesis but is not the injected AC current |
| Actual filter current | The current produced by the switching converter | It must track the reference closely enough to affect the bus |
| Resulting source current | The sum seen on the supply side of the chosen boundary | This is where the compensation objective is evaluated |
Measurement, command, converter current, and source current remain separate quantities throughout the AHF working principle.
Read as an active harmonic filter diagram, the sequence runs from the measured load-current signal to the controller reference, actual filter current, and resulting source current.
Step 1: The CT Captures the Load-Current Signal
The current transformer gives the controller a representation of current at a defined electrical boundary; the CT itself does not remove harmonics or inject corrective current.
Current-transformer placement decides what the controller can see. A feeder CT and a main-bus CT represent different compensation boundaries even when the sensor hardware is identical.
Polarity and phase correspondence preserve the intended signal direction. The active harmonic filter CT placement and wiring guide connects those decisions with ratio, routing, and commissioning evidence on the one-line diagram.
For a newcomer reading a vendor block diagram, the clean distinction is simple: the arrow from the CT to the controller is information. The heavy-current path from the converter to the bus is the compensating action.
Step 2: The Controller Builds a Compensation-Current Reference
The controller processes the measured electrical quantities and builds a compensation-current reference: the target waveform the power stage should supply next.
In hardware terms, a digital signal processor or another real-time controller turns the sensor input into reference current. That reference is a command, not yet the physical current flowing in the filter branch.
That job is more precise than “detect harmonics.” The controller must distinguish the current components tied to its configured objective, form a reference with the required sign, and keep updating the command as the load changes.
Different designs may use synchronous reference frame methods, instantaneous p-q methods, spectral analysis, or another control architecture. Those algorithm names describe ways to calculate the target; they do not change the common functional sequence of measurement, reference generation, and current tracking.
An engineer comparing one block diagram labeled dq0 with another labeled p-q should therefore ask two separate questions. What current objective is being calculated, and how does the converter track that reference under the expected load and supply conditions?
Compensation-signal estimation, DC-bus regulation, and converter current control are related but distinct jobs. Together they form the active power filter working principle even when the internal algorithm and hardware implementation differ.
Step 3: The DC Bus and IGBT Converter Produce Filter Current
DC-bus control supports a switching converter that turns the digital reference into real AC filter current on the compensated bus.
The DC bus is the converter’s regulated internal energy buffer. It gives the power stage the electrical headroom needed to shape current, while a separate control function keeps the DC-link voltage near its operating target.
The IGBT inverter is not merely an electronic on/off switch for the cabinet. Its semiconductor legs switch under current control so the converter output follows the changing compensation-current command.
This is also why saying an AHF “stores harmonics” gives the wrong mental model. The controller estimates a desired current, and the converter continuously exchanges electrical energy with the AC system while synthesizing that current; there is no warehouse of captured harmonic waveforms waiting to be emptied.
Current tracking closes the loop between command and action. The controller observes the electrical response, adjusts converter switching, and updates the command as the nonlinear load changes, so the actual filter current follows the reference rather than remaining a fixed waveform.
Step 4: Filter Current and Load Current Add at the Selected Bus
With all branch arrows directed as defined above, cancellation occurs through current summation at the selected bus: the filter reproduces selected load-current components with the same algebraic sign, so subtracting filter current reduces their contribution to source current.
Suppose the load current contains a fundamental component plus several harmonic components. The source still supplies the fundamental power demanded by the process, but the filter branch can carry a controlled counterpart of selected unwanted components.
The familiar “equal and opposite” phrase assumes load and filter currents are plotted using one common global reference direction. Under the branch-reference convention used here, the two component values have the same algebraic sign because the load and filter branch arrows point in opposite directions relative to the bus.
The point of common coupling, or PCC, is the agreed electrical boundary where the team evaluates the relationship among source, load, and filter currents. In plain language, it is the bus where the question becomes: what current does the supply now see?
The current balance also answers a common community question about where the harmonics go. The filter does not move a stored packet of distortion somewhere else; it produces a controlled current whose selected components reduce the corresponding components in the source-side sum.
When the load changes, the measured waveform changes, the reference is recalculated, and the converter output follows. Dynamic updating is the defining difference between a controlled active current source and a fixed tuned network.
Why Active Cancellation Is Different From Passive Filtering
Active and passive harmonic filters act through different physical mechanisms: an AHF measures and generates changing current, while a passive filter uses inductors, capacitors, and resistance to create frequency-dependent impedance paths.
That distinction resolves the familiar question, “Are harmonic filters also capacitor banks?” A capacitor bank may supply reactive power, and a tuned passive filter may include capacitors, but neither automatically becomes the controlled converter described above.
| Comparison point | Shunt active harmonic filter | Tuned passive harmonic filter |
|---|---|---|
| Connection and action | Connects in parallel and injects commanded current | Provides a frequency-dependent impedance path through passive components |
| Response to a changing load | Recalculates and tracks a changing current reference | Its tuning is set by component values and system interaction |
| Main controlled quantity | Converter current follows a calculated reference | Network impedance shapes the path of selected frequencies |
| Primary design dependency | Measurement boundary, controller objective, current capacity, converter dynamics, and integration | Tuning frequency, component ratings, network impedance, resonance, and switching conditions |
Active and passive equipment may also be coordinated in one facility because each acts through a different electrical mechanism.
For a full selection decision—including tuning, resonance, changing-load behavior, and installation trade-offs—use active harmonic filter versus passive harmonic filter.
What “Equal and Opposite” Does Not Guarantee
“Equal and opposite” describes the physical component relationship when both currents share one global reference direction; it is not the algebraic rule for the branch arrows used above. Residual distortion still depends on sensing, available compensating current, controller response, network conditions, and load state.
A meter reading changes with the measurement point and operating state. A result at one panel therefore cannot settle a different PCC or a different load condition.
Важно: A lower harmonic-current reading at one panel is not proof that CT placement, filter capacity, or a PCC acceptance target has been settled—tie the result to the defined measurement boundary, representative operating state, available compensating current, and agreed acceptance method.
Total harmonic distortion, or THD, is a useful summary of waveform harmonic content. It is not a stand-alone answer to where the CT belongs, what current rating is required, or whether a contractual limit has been met.
Используйте CT placement and wiring guide to define the measurement/control boundary before treating a reading as representative. Once that boundary is fixed, the APF sizing calculation guide converts measured demand into capacity inputs, and the Руководство по выбору AHF applies those inputs to equipment and acceptance criteria.
A correct control-loop model does not provide the site data needed for equipment selection. Representative logs, a one-line diagram, installation constraints, and a defined acceptance point complete that next decision.
How the CNBYG AHF/APF Product Family Implements the Chain
The CNBYG AHF/APF product family follows the same functional chain described above: external CT sensing, digital extraction of the target current components, and IGBT-based generation and injection of compensating current.
Земля CNBYG AHF/APF product family presents wall-mounted and rack-mounted forms. Across those forms, external CT input feeds digital processing, which commands the inverter current injected at the bus.
Before comparing a CNBYG series or rating, define the electrical boundary, representative harmonic-current profile, system voltage and configuration, load states, existing compensation equipment, installation conditions, and acceptance target.
Земля Система качества электроэнергии hub organizes the measurement and system context used before choosing equipment. The CT, sizing, and selection guides provide the next level of detail once that record is complete.
Часто задаваемые вопросы
How does an active harmonic filter work step by step?
It measures current at a defined boundary, processes the signal into a compensation-current reference, uses a controlled converter to produce filter current, injects that current in parallel, and updates the command as the load changes. With source and filter currents defined toward the bus, the source supplies the load current minus the filter contribution.
What is the difference between a passive and an active harmonic filter?
A passive filter uses tuned inductors, capacitors, and resistance to shape network impedance at selected frequencies. An active filter measures the electrical state and commands a power converter to generate changing compensating current.
Why is the compensating current called equal and opposite?
The phrase uses one common global reference direction, under which the physical filter component is opposite to the targeted load component. With the branch arrows used in this article, the filter component is a same-sign copy of the targeted load component, and i_source = i_load - i_filter subtracts it from source current.
Does an AHF eliminate all harmonics?
No universal result follows from the principle alone. Residual current depends on the measurement boundary, targeted components, available current capacity, controller and converter response, load state, and system integration.
Where is an active harmonic filter connected?
The common shunt AHF connects in parallel with the load or bus so it can inject current. The exact connection point and CT location must match the intended compensation boundary and approved project design.
What are the four types of active filters?
A common high-level taxonomy is shunt, series, hybrid, and unified power quality conditioner. Shunt filters primarily inject current in parallel; series filters inject voltage, hybrid arrangements combine active and passive functions, and a UPQC coordinates series and shunt functions.
Can an AHF also correct power factor?
Some active-filter products can include reactive-current compensation or power factor correction as a configured function. Whether that function is available and how capacity is shared must be confirmed for the selected product and application.
How is an AHF sized and what does it cost?
Sizing begins with representative measured harmonic-current demand at the intended boundary, then considers voltage, load state, topology, installation, thermal conditions, and the acceptance objective. Cost follows the required equipment capacity and project integration rather than the working-principle diagram alone; use the linked sizing and selection guides for that decision.
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