Shunt active power filtering for smart appliances

TL;DR: This paper investigates the harmonic content generated by some household appliances and suggests a solution to minimize harmonics by means of active filtering.

Abstract: Due to the increasing trend towards energy saving white goods appliances and the commercial viability of power electronic components, there has been an expansion in the use of solid state electronics and variable frequency drive motors in these applications. However, a major drawback of using such energy efficient loads is the introduction of current harmonics onto the local distribution grid. Furthermore, the proliferation of such devices elevates the supply harmonic content and can potentially impact residential distribution networks. This paper investigates the harmonic content generated by some household appliances and suggests a solution to minimize harmonics by means of active filtering. An active filter circuit was simulated to compare filter performance when used as an active front versus a feeder input compensator.

Summary

3.1 Active Filtering Introduction

• Also, an active filter could be part of a hybrid filter (a filter that consists of active and passive filters to improve efficiency) [18].
• Series active filters are located in series between source and nonlinear loads, they are meant to compensate voltage harmonics of the load.
• If the authors were to have the filter in series with the load, then they will have the total current passing through the filter which would require higher ratings for the circuit power electronic switches [18] [19].
• There are two kinds of SAPF; Current Source Active Filter (CSAF) and Voltage Source Active Filter (VSAF).
• The difference between these two topologies is the type of the storage element as shown in Figs. 3.1 and 3.2. VSAF is less expensive, lighter, produces less losses and is easier to control compared to the CSAF [18], hence it was selected as the circuit topology for the filter design in this thesis.

3.2 Filter Operation

• The concept of shunt APF is based on harmonic cancellation by the act of injecting equal but opposite harmonic current to that of the non-linear load.
• The only downside of using SAPF is the injection of high switching frequency in the system.
• Note, the operation of the switches will be described later.
• 25 - A capacitor, 𝐶𝐷𝐶, to store the energy required for the filter operation. - A line inductor, 𝐿𝑓, to smoothen the filter current and prevent high current ripples. - A line resistor, 𝑅𝑓, representing the losses in the circuit (switches and inductor coil resistance losses).
• The gates of the IGBT switches are controlled precisely in conjunction with the measured load current.

3.2.1 Capacitor (Energy Storage) Operation

• The capacitor energy is represented by: 𝐸 = 1 2 𝐶𝐷𝐶𝑣𝐷𝐶 2 (3.4) The DC capacitor acts as a voltage source and an energy storage element [20].
• The filter circuit has to be run in extreme caution, to avoid a short circuit situation in the filter circuit; the capacitor has to be pre-charged by the supply prior to the filter activation.
• The VSC circuit boosts up the voltage of the energy storage capacitor through the use of the inductor.
• On the other hand, the inductor shouldn’t be too small since this would cause the filter current to have a high ripple content [22].

3.2.3 Reference Extraction Methods

• There are numerous methods to extract the reference current.
• It can be decomposed mainly to two techniques, frequency domain and time domain [20].
• The following diagram depicts the methods found in the literature research.
• 33 Assuming a non-distorted power supply, PI Control of Capacitor Voltage was chosen for this research, since this method represents an indirect way of obtaining the desired current and is the easiest to implement in hardware and requires the least calculations.
• Where, 𝑖𝑠 ∗ is the minimum sinusoidal current required from the supply.

3.2.4 Control Techniques

• Numerous current controlling techniques were researched in the literature [21]; Fig. 3.10 shows different control methods.
• Regardless of the controlling method, the APF must have the capability to track sudden slope variations in the current reference [21].
• Hysteresis 34 current controller (HCC) possesses certain advantages namely, quick response, easy implementation, maximum current limit and insensitivity to load parameter variations [20].
• One drawback using the HCC is that it is difficult to limit the minimum and maximum switching frequencies [23].
• The load current, 𝑖𝑙, will be subtracted from the calculated source current (𝑖𝑠 ∗), to get the reference filter current (𝑖𝑓 ∗).

3.3 Summary

• A literature review on active power filters has been conducted.
• Active power filters rely on detection of reference current from the load circuit.
• Hence, it minimizes research time and effort to run the APF simulation.
• Simulink’s SimPowerSystems library was utilized to run and test the aforementioned simulations.

4.1 Fridge as a Load

• The fridge current measurements captured in Chapter 2 were used to represent the non-linear load in the simulation.
• The data from the Signal Builder block was then fed to a Controlled Current Source block to emulate the non-linear current of the fridge.
• The load current THD was analyzed and results are depicted in Fig. 4.2. 40 Fig. 4.3 shows the results obtained at the steady state operation of the filter.

4.2 Multi-Appliance Test

• The filter as a feeder compensator was simulated to compensate harmonics generated by all of the appliances mentioned in Table 2.3.
• Thus, the filter’s parameters need to be changed to properly filter the harmonics of a bigger load (Multi-appliance).
• The results for the smaller 5mH inductance looks much better than in the case of the 20mH inductor.
• The compensated source current was analyzed in the time and frequency domains as depicted in Fig. 4.9.

4.3 Summary

• A SAPF was simulated using MATLAB/Simulink library.
• Measured non-linear loads data from laboratory tests were imported and modeled using the Signal Builder block of Simulink.
• It was obvious that the size of the line inductor played a big role in altering the filter performance.
• It can be noticed that the performance of the feeder compensator when 𝐿𝑓 =5mH rendered a better result than when 𝐿𝑓 =20mH as in Figs. 4.8 and 4.6 respectively.
• The filter’s performance will be assessed based on THD reduction and power consumption.

5.1 Filter Prototype

• To prove the results obtained from simulations in Chapter 4, a hardware prototype was built to attest the operation of the filter circuit and control algorithm.
• Since the filter current theoretically should compensate for harmonics of the load current, the authors can refer to equation (2.6) to calculate 𝑖𝑓.
• To cope with component availability in the laboratory, a 60 mH inductor was chosen for the prototype.
• The theoretical precharge value of the capacitor should almost equal to the peak of the supply voltage (𝑉𝐷𝐶𝑝𝑟𝑒 ≈ 𝑉𝑃𝐸𝐴𝐾).

5.1.1 Measurement Boards

• A Tamura 3FD-210, which is a regular low frequency (60 - 400Hz) step-down isolated voltage transformer (PT) was used to monitor the supply voltage.
• The PT’s frequency range is optimal since the authors are only interested in the fundamental component (i.e. 60 Hz).
• The stepped-down voltage was fed to a signal conditioner to offset the signal above 0V.
• Fig. 5.6 shows the supply voltage monitor circuit.
• The load and the filter currents were measured using an LEM LA 55-P, a high frequency current transducer (CT).

5.1.2 MCU

• There are many available microcontrollers that are fast and reliable.
• For this application, it was determined that it was preferable to use a microcontroller that allows for automatic conversion of Simulink model to C-code, then download the C-code to a stand alone MCU.
• This procedure significantly reduces the burden of programming a lengthy code and eliminates possible errors generated from line coding.
• With a core frequency operating at up to 168 MHz, the Cortex-M4 core features a Floating Point Unit (FPU) single precision which supports all ARM single-precision data-processing instructions and data types.
• The Analog inputs of the MCU are rated to sample voltages between 0-3.3V.

5.1.3 Operating the Filter

• 58 - The filter algorithm can only be run, if the aforementioned steps has been conducted.
• The code for the MCU was first created using Simulink.
• The four IGBTs get switched based on the control signal coming from the gate driver.
• An ION 7650 power quality meter and a TPS2014B Tektronix oscilloscope was utilized to measure the supply voltage, load current, filter current and source current.

5.2 Filter Performance and Results

• While running the filter, measured waveforms displayed by TPS2014B Tektronix oscilloscope were also recorded using the ION 7650 power quality meter as shown in Figs. 5.4 and 5.11.
• The hardware results showed a superb improvement of the source current as shown in Fig. 5.12.
• For the first half cycle, the filter current is acting in a way such that it reduces the magnitude of the load current.
• Fig. 5.12 shows the time domain capture of the source, load and filter currents.
• The figure not only shows a sinusoidal waveform for the source current, but also shows a great reduction of the peak value of the load current.

5.2.1 Power Consumption

• To determine the feasibility of implementing the filter, power consumption by the filter need to be calculated.
• Knowing the sampled data of source voltage, 𝑣𝑠, and source current, 𝑖𝑠, the apparent power (S) can be obtained: 𝑆(𝑛) = 𝑣𝑠(𝑛) 𝑖𝑠(𝑛) (5.1) where, 𝑛 is the sample number.
• The calculations were repeated using equation 5.2 to determine the power consumption with the filter in parallel with the load.
• This time, the source current consists of the load and filter current.

5.3 Challenges Implementing the Filter

• Since increasing load current (including harmonics amps) is inversely proportional to inductor size, if any load current demand changes, then this will greatly impact the performance of the filter.
• A concern that may arise is the high switching frequency of the compensated source current as in Fig. 5.19.
• Reducing the ripple on the filter current can be achieved by increasing the switching frequency.
• Yes, the switching frequency can be increased to decrease losses, but this would add more computational burden and would require higher switching 67 frequency rating for the switches (i.e. IGBTs or MOSFETs).

5.4 Variable Inductor Discussion

• From the challenges specified in the previous section, the choice of the inductor size can very well affect the filter performance.
• A separate control algorithm needs to be utilized to accurately tune the inductor based on the load THD and load current requirement.
• The separate control algorithm would be based off of equation 3.22.
• A separate MCU with low processing capability can be utilized to calculate the inductor size.
• The actuator could be driven by a drive circuit triggered by the MCU outputs.

5.5 Summary

• The filter was tested with a non-linear high THD load that draws about the same RMS current to that of the fridge.
• The hardware results showed consistency with the simulation conducted in this chapter (simulation result in Fig. 5.16); the THD of the line current were reduced significantly.
• As a rule of thumb, the following can be concluded from the results obtained.
• If an inductor is too large given a specific load current requirement, the filter cannot effectively compensate the THD in 𝑖𝑠.
• Several challenges have been encountered but overcame while designing and operating the filter, some are MCU based and some are equipment based.

6.1 Research Outcomes

• A design of a SAPF has been proposed, and filter results from simulation have been validated experimentally.
• Results have shown in both cases the reduction of the supply current THD (refer to Fig. 4.4 and Fig. 4.7).
• The compensation is minimal in the case of feeder compensation and the filter rendered a better performance when used as an active front end.
• The topic of APF is vast and versatile; there are many scholars who have presented state of the art work in this area.
• Few papers focus on limitations of filter design.

6.2 Further Research

• This type of SAPF utilizes a variable line inductor and a second MCU to be able to determine the required inductor size based on the load THD and current demand.
• It will be challenging to design a variable inductor with many taps/selections, since this will increase the cost of such a device and will impose computational complexity on the MCU.

Figures

Fig. 3.5 Capacitor pre-charge.

Fig. A.6 Emergency stop of filter operation.

Fig. A.5 PI controller.

Fig. 4.4 Analysis results for source current, 𝑖𝑠, after compensation of fridge current harmonics.

Table 4.1 Simulation circuit parameters.

Fig. 5.16 Simulation of 𝑖𝑠, after filtering of a 21Ω half wave rectifier load.

Fig 2.3 LG fridge studied as part of this thesis.

Fig. 4.8 𝑣𝑠, 𝑖𝑠, 𝑖𝑙 and 𝑖𝑓 waveforms after filtering of appliances current when 𝐿𝑓 =5 mH.

Table 5.1 Hardware prototype parameters.

Fig. 5.10 Experimental setup showing gate driver on the left, measurement and data acquisition boards at the bottom and the MCU at the top right.

Fig. 5.11 Full laboratory experimental setup.

Fig. 4.7 Analysis of 𝑖𝑠 after compensation of appliances current harmonics.

Fig. 5.23 Proposed design of variable inductor.

Fig. 5.22 Proposed SAPF with a variable inductor.

Fig. 3.1 Current source active filter (CSAF) circuit.

Fig. 3.2 Voltage source active filter (VSAF) circuit.

Fig. 5.6 Supply voltage monitor circuit.

Fig. 5.7 Load current measurement circuit.

Fig. 2.1 shows a distorted waveform which has a third harmonic with a magnitude of 1/3 and a phase shift of 0º superimposed on the 60 Hz fundamental frequency. The distorted waveform in Fig. 2.1 can be written mathematically as:

Fig. 2.10 Fridge supply voltage without fridge light, measured with the FLUKE Scopemeter.

Fig. 5.1 Supply, load and main power circuit components for SAPF (MCU unit and measurement boards are not shown).

Full text

SHUNT ACTIVE POWER FILTERING FOR
SMART APPLIANCES

SHUNT ACTIVE POWER FILTERING FOR SMART
APPLIANCES
BY
LAITH AL-MUSAWI, B.Tech.
A THESIS
SUBMITTED TO THE DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING
AND THE SCHOOL OF GRADUATE STUDIES
OF MCMASTER UNIVERSITY
IN PARTIAL FULFILMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
MASTER OF APPLIED SCIENCE
© Copyright by Laith Al-Musawi, April 2016
All Rights Reserved

ii
Master of Applied Science (2016)
McMaster University
(Electrical & Computer Engineering)
Hamilton, Ontario, Canada
TITLE:
Shunt Active Power Filtering for Smart Appliances
AUTHOR:
Laith Al-Musawi
B.Tech., (Energy Engineering Technology)
McMaster University, Hamilton, Canada
SUPERVISORS:
Dr. N. Al-Mutawaly
Dr. N. Schofield
NUMBER OF PAGES:
xv, 86

iii
I would like to dedicate this thesis to my role model and dear friend, my father. He is a
great man, who has always put my interests ahead of his. A special feeling of gratitude goes
to my loving mother, whose words of encouragement opened up the doors of success in my
life. Also, I would like to dedicate this thesis to my siblings for their endless support
throughout my studies. Finally, to my lovely wife, thank you for always being by my side!

iv
Abstract
Due to the increasing trend towards energy saving of white goods appliances and the
commercial viability of power electronic components, there has been an expansion in the
use of solid state electronics and variable frequency drive motors in these applications.
However, a major drawback of using such energy efficient loads is the introduction of
current harmonics onto the local distribution grid. Furthermore, the proliferation of such
devices elevates the harmonic content of the supply voltage and the ensuing potential
impact on residential distribution networks. This thesis investigates the harmonic content
generated by some representative household appliances and suggests a solution to minimize
current harmonics by means of active filtering. An active filter circuit is proposed and
simulations will be undertaken to compare filter performance when used as an active front
end versus a feeder input compensator. Further, a hardware design of the filter was
implemented to experimentally verify the filter operation.

Frequently asked questions

Q1. What have the authors contributed in "Shunt active power filtering for smart appliances shunt active power filtering for smart appliances" ?

In this paper, a parallel shunt active power filter ( SAPF ) is proposed to remove current harmonics from the load. 

Q2. What are the future works mentioned in the paper "Shunt active power filtering for smart appliances shunt active power filtering for smart appliances" ?

Further research needs to be conducted to improve the designs of SAPFs. It will be challenging to design a variable inductor with many taps/selections, since this will increase the cost of such a device and will impose computational complexity on the MCU. 

Q3. Why do many designs neglect the load impact in terms of power quality?

Since the primary motivation is load energy efficiency at low cost, many designs neglect the load impact in terms of power quality, because this adds to additional circuitry and hence cost. 

Q4. What is the drawback of using such energy efficient loads?

a major drawback of using such energy efficient loads is the introduction of current harmonics onto the local distribution grid. 

Q5. What is the effect of phase shift on the fundamental frequency signal?

6Phase shift of individual harmonics plays a major role in changing the pattern of distortion on the fundamental frequency signal. 

Q6. What was the purpose of building the laboratory?

The idea of building the laboratory was to study and estimate the effects of harmonics generated by such modern loads on a typical distribution transformer. 

Q7. What is the main reason why distorted currents cause overheating?

Research has shown that distorted currents have been known to cause overheating of neutral conductors, protection device malfunction [1][52], improper operation of metering devices, de-rating of distribution equipment and resonance [2]. 

Q8. What has led to the development of energy saving appliances?

Global climate change and the security, or insecurity of energy supplies has fostered a trend towards the manufacture of energy saving appliances in recent years [9]. 

Q9. Why is the frequency spectrum in Fig. 2.8 distorted?

It can be noted that the frequency spectrum in Fig. 2.8 only consists of odd harmonics, this is merely due to waveform symmetry [31]. 

Q10. What is the definition of power quality?

The term Power Quality encompasses all aspects pertaining to the quality of the electrical supply in terms of the magnitude and frequency of the voltage and current waveforms. 

Q11. Why is there a need to include the limits for residential loads in the IEEE 519 standard?

there is a need to include the limits for residential loads in the IEEE 519 standard, since it is the standard adopted in Canada [13]. 

Q12. What is the harmonic spectrum of the current waveform of Fig. 2.11.1920?

Fig. 2.12 shows the harmonic spectrum of the current waveform of Fig. 2.11.1920Compensating voltage harmonics is not widely addressed because power supplies have low impedance, as discussed in page 2 of [35]. 

Q13. Why are active filters not practical in residential homes?

due to the relatively large and bulky inductor required by such filters [18], they would not be realistic to apply in each residential home, if this were to be implemented. 

Q14. What is the role of utilities in ensuring reliable supply of electricity to consumers?

On the other hand, large loads customers (factories, large stores and etc.), have to ensure that they bind to electricity standards. 

Q15. What is the purpose of this chapter?

Chapter 3 will discuss the literature research conducted with respect to active filtering, circuit topologies and control methodologies. 

Q16. What are the common standards for preventing harmonics from distorting the supply of power?

In order to prevent harmonics from distorting the supply of power from utilities, many standards have been established and reinforced by utilities to force commercial and residential customers to abide by the harmonic limits presented in each standard. 

Q17. What is the phase shift of the harmonics in Fig. 2.2?

Fig. 2.2 again shows a distorted waveform which has a third harmonic with a magnitude of 1/3 superimposed on the 60 Hz fundamental frequency but now with a phase shift of 180º. 

Q18. What are the terms that quantify the harmonic content in a waveform?

There are primarily two terms that quantify the harmonic content in a waveform; total harmonic distortion (THD) and total demand distortion (TDD).