A Simulink Model to Improve Total Harmonic Distortion (THD) using Shunt Active Power

Filter

By,Ranganath VallakatiMaster of Science in Electrical EngineeringUniversity of North Dakota

Introduction

• Loads: Types of Loads1. Linear loads – sinusoidal current with sinusoidal voltage2. Non-linear loads – non-sinusoidal current with sinusoidal voltage

• Harmonics and it’s effects1. Overheating2. Voltage distortion and flickering3. Interference

• Different solutions:1. Capacitors2. Compensators3. Passive filters4. Active filters

Filters

• Active power filter

1. Series active power filterI. SeAPF is connected in series with the T.LII. Acts as a controlled voltage source

2. Shunt active power filter:III. ShAPF is connected in parallel with the T.LIV. Acts as a controlled current source

3. Unified power quality controllerV. Combination of passive, series and shunt active filters

Shunt active power filter

• The principle of the shunt filter is to produce harmonic currents equal in magnitude but opposite in-phase to those harmonics that are present in the grid.

• Phase shift of the harmonic current is 180 degrees.

• Non-linear load with SAPF becomes a Linear load.

• SAPF is a closed loop structure

• SAPF can compensate reactive power and can also mitigate harmonics and distortions.

• I(comp) = I(load) - I(source)

Shunt Active Power Filter

Reference [1]

Different blocks in SAPF

• PWM Converter

• Instantaneous power calculation block

• Reference currents calculation block

• DC voltage regulator

PWM Converter

1. Responsible for power processing.

2. Consists of VSC or CSC

3. To force the PWM converter act as a controlled voltage or current source.

4. VSC is made up of PE devices(GTO, IGBT…)

5. PE devices are fired based on the APF currents.

6. APF currents can be calculated using Hysteresis Controller method.

Hysteresis Controller

• The method of controlling a VSC in such a way that the output current will be generated based on reference current values.

• A reference value is kept and is compared with the two input of the controller.

• Based on the error between the 2 inputs and the reference value, signals are generated.

• Inputs to the controller can be taken in two different ways:1. Direct control method.2. Indirect control method.

Instantaneous Power Calculation

• The crucial part of SAPF which calculates the compensation currents.• These currents are calculates using “P-Q theory.”• This Constant power control strategy was the first strategy developed for Active power

filters by Akagi et al. in 1983.• This theory uses Clarke’s transformation which consists of real matrix that transform three

phase ‘V’ or ‘I’ into αβγ stationary reference frames.

Clarke’s Transformation and It’s Inverse

• For a 3-phase system without a neutral/ground, we can neglect the zero sequence component to make the matrices as

Cont.…• Using the Clarke’s transformation, we can convert the currents or voltages into αβ

frame(removing the zero sequence components) and again back to abc frame using the inverse Clarke’s transformation.

• We separate the apparent power(S) into real and imaginary parts using the p-q theory. = *

Real/active power (p) = + Imaginary/reactive power (q) = +

Average and Oscillating components• : Mean value of Instantaneous real power that is transferred from source to load

in a balanced way through a-b-c coordinates. This is the fundamental energy source to load.

• : Alternating value of Instantaneous real power which is exchanged between power source and load. As this not contribute in energy transfer, this component must be compensated. This is due to harmonic currents.

• .

• : Average value of instantaneous imaginary power exchanged between phases and load. This is also must be compensated.

These currents and voltages are taken as inputs to the filter from the line or load.

Through transformation, we get the real and imaginary power values

The power to be compensated must be selected i.e. etc.

The powers to be compensated are given input. The compensator should draw exactly the given amount of current that produces the inverse of powers that are drawn by the load.

By applying Inverse Clarke's transformation, we get the actual abc coordinates which can be applied to the line again.

Actual Implementation of p-q Theory in SAPF

Reference [1]

DC Voltage Regulator• It consists of a energy storage element such as Capacitor.• Main aim of having a capacitor is to supply real power difference between load and source

during the transient periods.

PI Controller• PI controller is one of the most widely used controllers in applications for feedback

mechanisms.• This is used to generate the signal which is added to the real power calculations. This loss can

be the filter losses.

Simulation

Ideal grid voltage V 40 V

Grid frequency f 50 Hz

Grid resistance R 3 mΩ

Grid Inductance I 0.1 μH

DC Capacitor(SAPF) 3.5 μF

Constant DC voltage 120 V

DC Load1 Resistance 60Ω

DC Load1 Inductance 10 mΩ

DC Load2 Resistances 2Ω,4Ω, 6Ω

Simulation time 0.5 seconds

SAPF connection time 0.1 seconds

MATLAB simulation

Shunt Active Power Filter block

Compensation Currents Calculation block

Hysteresis Controller and PI Controller blocks

Simulation results-1

𝐼 𝑠𝑜𝑢𝑟𝑐𝑒

𝐼 𝑐

𝐼𝑏

𝐼𝑎

Simulation results-2

𝑉 𝑆𝑎𝑏𝑐

𝑉 𝑑𝑐

𝐼𝑎𝑝𝑓

Simulation results-3

𝑉 𝐿𝑎𝑏𝑐

𝐼𝐿𝑎𝑏𝑐

𝐼𝑆𝑎𝑏𝑐

FFT Analysis and Total Harmonic Distortion

The left side graph shows the THD calculated Using the currents on Load side. THD is around30.90%

The right side graph shows the THD calculated Using the currents on Source side. THD is around2.79%

Conclusion• From our MATLAB/SIMULINK simulation process, we have designed a power system which

consists of source, non-linear load and SAPF which is connected to the actual transmission line after 0.1 seconds.

• The initial peaks in the THD are during the time period when the SAPF has not been connected to the power systems.

• The maximum allowable limit of Total Harmonic Distortion as per IEEE 519_1992 regulations is below 5% for bus voltages less than 69KV

• Using our Shunt Active Power Filter, we have reduced the THD remarkably from 30% to 2.79% on the simulated power systems circuit.

References[1]. “Instantaneous Power Theory and Applications to Power Conditioning” by Hirofumi Akagi, Edson Hirokazu Watanabe and Mauricio Aredes.

[2]. MATLAB and Simulink R2013b (www.mathworks.com)

[3]. H. Akagi, Y. Kanazawa and A. Nabae, “Generalized Theory of Instantaneous Reactive Power and It’s Applications,” Transactions of the IEE-Japan, Part B, vol. 103, no.7, 1983

[4]. E. Clarke, Circuit Analysis of A-C Power Systems, Vol.1—Symmetrical and Related Components, Wiler, 1943.

[5]. SimiPowerSystem, for use with Simulink, by MATLAB

Thank You…

• Questions…?• Comments…?