Documentation

# Three-Phase Harmonic Filter

Implement four types of three-phase harmonic filters using RLC components

## Library

Simscape / Electrical / Specialized Power Systems / Fundamental Blocks / Elements

## Description

Three-phase harmonic filters are shunt elements that are used in power systems for decreasing voltage distortion and for power factor correction. Nonlinear elements such as power electronic converters generate harmonic currents or harmonic voltages, which are injected into the power system. The resulting distorted currents flowing through the system impedance produce harmonic voltage distortion. Harmonic filters reduce distortion by diverting harmonic currents in low impedance paths. Harmonic filters are capacitive at the fundamental frequency, so they are also used for producing reactive power required by converters and for power factor correction.

To achieve an acceptable distortion, several banks of filters of different types are connected in parallel. The most commonly used filter types are:

• Band-pass filters, which are used to filter lowest order harmonics such as 5th, 7th, 11th, 13th. Band-pass filters can be tuned at a single frequency (single-tuned filter) or at two frequencies (double-tuned filter).

• High-pass filters, which are used to filter high-order harmonics and cover a wide range of frequencies. A special type of high-pass filter, the C-type high-pass filter, is used to provide reactive power and avoid parallel resonances. It also allows filtering low-order harmonics (such as 3rd), while keeping zero losses at the fundamental frequency.

The Three-Phase Harmonic Filter is built of RLC elements. The resistance, inductance, and capacitance values are determined from the filter type and from the following parameters:

• Reactive power at nominal voltage

• Tuning frequencies

• Quality factor. The quality factor is a measure of the sharpness of the tuning frequency. It is determined by the resistance value.

The four types of filters that can be modeled with the Three-Phase Harmonic Filter block are shown below:

The simplest filter type is the single-tuned filter. The following figure gives the definition of the quality factor Q and practical formulae for computing the reactive power QC and losses (active power P). The quality factor Q of the filter is the quality factor of the reactance at the tuning frequency Q = (nXL)/R. The quality factor determines the bandwidth B, which is a measure of the sharpness of the tuning frequency as shown in the figure.

 Tuned harmonic order n = fn/f1 = $\sqrt{{X}_{C}/{X}_{L}}$ f1 = fundamental frequency Quality factor Q = nXL/R = XC/(nR) ω = 2πf1 = angular frequency Bandwidth B = fn/Q where fn = tuning frequency Reactive power at f1 QC = (V2/XC)·n2/(n2 – 1) n = harmonic order = (fn/f1) Active power at f1(losses) P ≈ (QC/Q)·n/(n2 – 1) V = nominal line-line voltage XL = inductor reactance at fundamental frequency = Lω XC = capacitor reactance at fundamental frequency = 1/(Cω)

The double-tuned filter performs the same function as two single-tuned filters although it has certain advantages: its losses are much lower and the impedance magnitude at the frequency of the parallel resonance that arises between the two tuning frequencies is lower.

The double-tuned filter consists of a series LC circuit and a parallel RLC circuit. If f1 and f2 are the two tuning frequencies, both the series circuit and the parallel circuit are tuned to approximately the mean geometric frequency ${f}_{m}=\sqrt{{f}_{1}{f}_{2}}$.

The quality factor Q of the double-tuned filter is defined as the quality factor of the parallel L, R elements at the mean frequency fm: QR /(L · 2πfm).

The high-pass filter is a single-tuned filter where the L and R elements are connected in parallel instead of series. This connection results in a wide-band filter having an impedance at high frequencies limited by the resistance R.

The quality factor of the high-pass filter is the quality factor of the parallel RL circuit at the tuning frequency: QR /(L · 2πfn).

The C-type high-pass filter is a variation of the high-pass filter, where the inductance L is replaced with a series LC circuit tuned at the fundamental frequency. At fundamental frequency, the resistance is, therefore, bypassed by the resonant LC circuit and losses are null.

The quality factor of the C-type filter is still given by the ratio: Q =R / (L · 2πfn).

The following figures give R, L, C values, and typical impedance versus frequency curves obtained for the four types of filters applied on a 60-Hz network. Each filter is rated 315 kV, 49 Mvar.

Single-Tuned, 315 kV, 49 Mvar, 5th Harmonic Filter; Q = 30

Double-Tuned, 315 kV, 49 Mvar, 11th and 13th Harmonics Filter; Q = 16

High-Pass, 315 kV, 49 Mvar, 24th Harmonic Filter; Q = 10

C-Type High-Pass, 315 kV, 49 Mvar, 3rd Harmonic Filter; Q = 1.75

## Parameters

Type of filter

Select one of the four filter types: `Single-tuned`, `Double-tuned` (default), `High-pass`, or `C-type high-pass`.

Filter connection

Select the connection of the three filter branches.

 `Y(grounded)` Neutral is grounded. `Y(floating)` Neutral is not accessible. `Y(neutral)` Neutral is made accessible through a fourth connector. Default. `Delta` Three phases connected in delta.

Nominal L-L voltage and frequency

The nominal phase-to-phase voltage of the filter, in volts RMS (Vrms) and the nominal frequency, in hertz (Hz). Default is `[315e3 60]`.

Nominal reactive power

The three-phase capacitive reactive power QC, in vars. Specify a positive value. Default is `49e6`.

Tuning frequencies or Tuning frequency

The tuning frequency of the single frequency filter (single-tuned, high-pass or C-type high-pass), or the two frequencies of the double-tuned filter, in hertz (Hz). Default is `[11*60 13*60]` when Type of filter is `Double-tuned` and `[5*60]` when Type of filter is `Single-tuned`, `High-pass`, or `C-type high-pass`.

Quality factor (Q)

The quality factor Q of the filter defined as explained in the above Description section. Dimensionless positive number. Default is `16`.

Measurements

Select `Branch voltages` to measure the three voltages across each phase of the Three-Phase Harmonic Filter block terminals. For a Y connection, these voltages are the phase-to-ground or phase-to-neutral voltages. For a delta connection, these voltages are the phase-to-phase voltages.

Select `Branch currents` to measure the three currents flowing through each phase of the filter. For a delta connection, these currents are the currents flowing in each branch of the delta.

Select `Branch voltages and currents` to measure the three voltages and the three currents of the Three-Phase Harmonic Filter block.

Default is `None`.

Place a Multimeter block in your model to display the selected measurements during the simulation. In the Available Measurements list box of the Multimeter block, the measurements are identified by a label followed by the block name.

Measurement

Label

Branch voltages

Y(grounded)

`Uag:`

Y(floating)

`Uan:`

Y(neutral)

`Uan:`

Delta

`Uab:`

Branch currents

Y(grounded)

`Iag:`

Y(floating)

`Ian:`

Y(neutral)

`Ian:`

Delta

`Iab:`

## Examples

The `power_harmonicfilter` example illustrates the use of the Three-Phase Harmonic Filter block.