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MINISTRY OF EDUCATION – MINISTRY OF TRANSPORT HO CHI MINH CITY UNIVERSITY OF TRANSPORT CHAU VAN BAO IMPROVING THE POWER QUALITY USING THE HYBRID ACTIVE POWER FILTER BY INTELLIGENT CO

MINISTRY OF EDUCATION – MINISTRY OF TRANSPORT

HO CHI MINH CITY UNIVERSITY OF TRANSPORT

CHAU VAN BAO

IMPROVING THE POWER QUALITY

USING THE HYBRID ACTIVE POWER FILTER

BY INTELLIGENT CONTROL TECHNIQUE

Major: Control Engineering and Automation

Code: 9520216

SUMMARY OF DOCTORAL DISSERTATION

Science supervisor: 1 Assoc., Dr Vo Cong Phuong

2 Dr Chau Minh Thuyen

HCM CITY - 2019

The works have completed in: Ho Chi Minh City

Scientific supervisor: 1 Assoc., Dr Vo Cong Phuong

2 Dr Chau Minh Thuyen

The thesis can be found at the library:

- Library of Ho Chi Minh City University of Transport

INTRODUCTION

1 Reasons for choosing the topic

Along with the development of industry, the loads are increasing and the majority of nonlinear loads are the cause of harmonics Harmonics cause a lot of harmful problems for electrical systems and electrical devices, this is the cause of poor power quality

Today, power quality issues are very much concerned by many countries in the world One of the methods to eliminate harmonics, reactive power compensation Q in the electrical system is using an active filter circuit (APF) APF has the advantage of working online with electrical systems, no resonance occurs, regardless of the feature of the load However, its capacity is limited, its working efficiency is not high and it is not used in medium and high voltage electrical grids

Currently, in our country often use the static compensation capacitor

to improve power quality However, the method of using capacitor is ineffective, because only compensating Q without canceling harmonics is the nonlinear load In order to solve these problems, the hybrid active power filter (HAPF) model is a necessity, it can compensate for the integration of different harmonic sources and solve disadvantages of the capacitor Therefore, research on design, calculation and control for HAPF has an important meaning contributing to improving the working efficiency of filter circuit and improving power quality Therefore, the

topic: "Improving the power quality using the hybrid active power filter

by intelligent control technique" is necessary

2 Research purposes

− Theoretically: Find out the method of determining harmonic currents more accurately; Determine HAPF parameters by multi-objective optimization algorithms in considering the stability of the system; Find out the new control method for HAPF so that it minimizes errors, reduces transient time; Find out the new DC bus voltage stabilization method

− Application: The results of the thesis can be applied to construction

of hybrid active power filter models to compensate reactive power Q and eliminate the harmonics in the electrical system

3 Object and scope of the research

− Research object: The study was conducted on HAPF model and applied to low voltage grid

− Scope of research: Only research to improve the power quality in terms of total harmonic distortion (THD) and compensating reactive power Q

4 Research tasks

Using the methods, calculation, data and results of previous studies as

a basis for research and evaluation Since then: Improve the p-q harmonic detection method; Determine multi-objective optimization of HAPF parameters; Control methods for HAPF; DC bus voltage stabilization method Application of Matlab software to simulate the above problems

5 Research Method

− Analysis of harmonic detection methods, thereby improving its shortcomings by improved harmonic detection method with more accuracy and wider application scope Analyse methods of determining HAPF parameters From there, propose a multi-objective optimization method to determine HAPF parameters Analysis of DC bus voltage control methods, from which draw defects and give a method to stabilize

DC bus voltage in the direction of adaptive control Provide control strategies and control methods to solve problems such as wide application range, flexibility, efficiency in filtering harmonic and reactive power compensation

− Use Matlab to simulate for methods

6 The scientific and practical significance of the thesis

− Scientific significance: The thesis is a scientific work of theoretical

and practical significance, contributing to systematizing and clarifying problems of harmonic filtering From that, proposes the method of determining harmonics, the method of determining parameters of HAPF,

DC bus voltage stabilization method and HAPF control methods to improve power quality

− Practical significance: The thesis has evaluated the situation,

demonstrated out the advantages and disadvantages of the harmonic filters The thesis is quite comprehensive and systematic, with practical significance to the issue of improving power quality

7 Structure of the thesis

The thesis consists of 143 pages and the order of parts is as follows: Introduction; content (including 6 chapters); conclusions and suggestions; list of published scientific works related to the thesis (including 10 papers and 01 applied scientific research); there are 119 references and appendices

Chapter 1: OVERVIEW OF FILTER

1.1 Issues of power quality

Non-linear loads are the cause of harmonics, which reduces power quality Harmonics cause many different problems in both the grid and the load such as: overheating equipment, overheating transformers, deviation control devices, power factor of the load decreases, causing losses in the electrical system, increasing the cost of the customer and affecting the stability of the grid Therefore, power quality has become an increasingly important issue for Electricity and electricity consumers

𝐼𝐼 1 100% (1.1)

1.3 Effect of harmonics on power quality

Although the sinusoidal source voltage is not distorted, but the nonlinear load causes harmonics and undesirable effects on power quality such as increased line losses, changing the voltage on the grid and grid frequency

1.4 Methods for harmonic filtering

1.4.1 Passive Power Filter (PPF)

This is a common solution to remove harmonics in electrical systems PPF is the simplest solution to minimize harmonics [27], [32], [36], [40] PPF has a simple structure consisting of the three elements R, L, C It is low cost, easy to implement However, it has disadvantages such as easy resonance, instability, low reliability

1.4.2 Active Power Filter (APF)

From the disadvantages of PPF, the APF was born to overcome the disadvantages of PPF, it is very effective in improving power quality, it has advantages such as flexible compensation, no dependent on property

of load, high efficiency, no occurs resonance with grid impedance APF is widely used to compensate Q and harmonic filtering [7], [25], [91], [96] The basic principle of APF is based on harmonic currents of the load to create a harmonic signal to compensate on the grid However, the disadvantage of APF is its high cost, low capacity, and difficult to apply

to high-voltage grids

1.4.3 Hybrid Active Power Filter (HAPF)

To improve the efficiency of APF, the HAPF model was born and developed [16], [26], [42], [62], [79] HAPF's structure is a combination

of PPF and APF Therefore, it has the advantages of both APF and PPF The most outstanding advantage of HAPF is its ability to work at high voltage and high power grids with a relatively small capacity of APF

Chapter 2: HARMONIC CURRENT DETECTION METHOD 2.1 Introduction

There are many methods of determining harmonic currents of nonlinear loads such as: using low-pass, high-pass filter circuits [13], it has the disadvantage of slow response and just a small change in frequency will make these filters ineffective The most common method is the p-q harmonic detection method [17], [89], [104] It has the advantage of being simple and easy to implement However, it also has the disadvantage of slow response to fast changing loads and large amplitudes [29-30] In this chapter, we propose an improved method of the p-q harmonic detection method using the fuzzy controller integrated into the pq method to automatically adjust the DC components of P and Q to close to the desired value, keeping the amplitude of the source current is not overshot when the load changes large and the transient time is reduced

2.2 p-q and i p -i q harmonic current detection method

2.2.1 The transformation from a-b- c coordinate system to α-β

coordinate system

The transformation from a-b-c coordinate system to α-β coordinate system is implemented by Clarke [97]

2.2.2 p-q harmonic detection method

p-q harmonic detection method is proposed by Akagi [7], in Figure 2.2

p q

Figure 2.2 Principle diagram of p-q method

The harmonic components determinated are:

1 23 2

1

(2.13) 3

2.2.3 i p -i q harmonic detection method

Figure 2.3 ip-iq harmonic detection method

The fundamental components are:

23 2 1

1 3

af

bf

n cf

overshoot and reduce

the dynamic response

LPF

dt d

1

(2.27) 3

Laf

p

q Lcf

i

p K

q K U

Table 2.5 Response of q

During the period (0÷0.2s) During the period (0.2s÷0.4s) Transient

of the system

Chapter 3: MULTI-OBJECTIVE OPTIMIZATION DESIGN FOR

HYBRID ACTIVE POWER FILTER

3.1 Introduction

Currently, the parameters of HAPF are mostly determined based on basic formulas such as studies [24], [70], [98] Therefore, the achieved results may not satisfy the system stability condition Multi-objective studies such as Gen algorithm application for PPF design [20], [43]; using the PSO algorithm [18], [95] for PPF design In summary, previous multi-objective studies mainly computed for PPF, and APF parameters had little research and multi-objective optimization studies without considering the stability of the system

To overcome this drawback, in this chapter, we perform a stable analysis for HAPF to find the stability of the system Then, use the SSA multi-objective optimization algorithm to determine the best set of parameters for HAPF

3.2 Stable analysis for hybrid active power filter

Control block diagram of HAPF is shown as Figure 3.3

Lh

+ +

inv

U

Figure 3.3 Control block diagram of HAPF

Transfer function of the load harmonic current I Lh according to the

supply harmonic current signal I sh:

( )

) ( ).

( ).

( 1

1

s G s G s G I

I s G

out inv c Lh

From (3.4), the characteristic equation of the control transfer function:

7 0 6 1 5 2 4 3 3 4 2 5 1 6 0

)

(s a s a s a s a s a s a s a s a

In order for the system to be stable, the formula (3.6) must be satisfied

3.3 Multi-objective optimization design

for HAPF

− System stability constraints:

The HAPF system is stable when the

conditions in Equation (3.6) are satisfied

3 0 2 1

2 1 3 0

3 0 2 1

2 1 3 0

3 0 2 1

c c c

c b c

b b b

b a a b

a a a a

(3.6)

− Constraints on resonance conditions in PPF: L and C values in a

branch must resonate at a certain frequency

1

n n

L C

ω ω

= (3.7)

− Constraints of R, L, C: Values of R,

L, C must be positive and satisfy the

condition (3.8) and resonance condition

max max max

0 0 0

i i i

− Maximum capacity compensated by

PPF but not over-maximized

− Constraint of controller parameters:

Parameters of controller must be positive and satisfy the system stability condition (3.6)

s bi

THDi Q Error





3.4 Simulation results

3.4.1 Traditional design

According to the article

[24], [46] we have the

parameters given in Table

3.2 Figure 3.6 shows the

waveforms in the traditional

design The THD of i s

decreases from 27.65% to

1.897%, while Q decreases

from 4820VAr to 1490VAr,

which means Q compensated

is 3330VAr Compensation

error in steady-state

decreases to ± 8A Figure 3.6 The waveforms in steady-state

of the traditional method Table 3.2 HAPF parameters with traditional design methods

3.4.2 Multi-objective optimization method using SSA

The multi-objective optimization method will find all HAPF parameters including power circuit parameters and control circuit parameters

Table 3.4 HAPF parameters with SSA method

error decreases from

± 100A to ± 3A Figure 3.8 The waveforms in steady-state

of the SSA method

3.5 Conclusion

In this chapter, a new approach in multi-objective optimization design for HAPF was provided This approach allows us to calculate all the parameters of both the power circuit part and the control circuit part of HAPF The achieved results are globally optimal and satisfying the system stability condition This study has practical implications in determining all HAPF parameters that contribute to improving power quality in the electrical system

4.1 Introduction

This chapter presents an overview of the methods of stabilizing the DC bus voltage used for HAPF On that basis, a DC bus voltage stabilization method is proposed The simulation results demonstrate that the proposed method has better results in reducing voltage ripple on the DC-bus, compensation error and total harmonic distortion of the supply current in steady-state Especially, this method is able to stabilize the DC bus voltage when the load changes

4.2 Overview of DC bus voltage stability for HAPF

The paper [46] provides a method for stabilizing DC bus voltage using fuzzy logic applied on two models: TLSC (Three-Leg Split-Capacitor) and FLI (four-leg inverter) The paper [63] studied the DC bus voltage stability for HAPF in series using the PZP (Pole-Zero Placement) method Research [108] provides a closed-loop numerical control algorithm to control DC bus voltage for three levels APF using the space vector modulation method The paper [110] analyzed the effect of the DC bus voltage controller to stabilize DC bus voltage and the compensation efficiency of three-phase four-wire active filter circuit The paper [69] provides a DC bus voltage control method for APF using a PI controller combining fuzzy controller

In summary, all studies on DC bus voltage stability for HAPF only focus on stabilizing the DC bus voltage for APF and in case the load does not change In this chapter, a new DC bus voltage stabilization method is proposed, which can maintain the DC bus voltage even when the load changes

4.3 Analysis of DC bus voltage variation in HAPF system

According to [3], there are two main reasons that the DC bus voltage change: the source voltage is not ideal and the load is nonlinear

− Considering the case where the source voltage is not ideal: Suppose

the ideal source voltage is in the form:

0

u t =U ωt+ϕ (4.1) When voltage sag happens, the supply voltage in the form:

0 0

u t =ηU ωt+ +ϕ (4.2) The hybrid active power filter topology is shown in Figure 4.1 Source

U s

Z s

Non-linear load

Figure 4.1 The hybrid active power filter topology

Single-phase equivalent circuit of the injection circuit in Figure 4.2

t m

F

U

C C

− Considering the case of load is nonlinear:

Suppose the voltage applied to the fundamental frequency resonant circuit is ushn and the voltage at the inverter AC side output is uahn If considering the ratio transformer is 1: 1, we can replace the output of the fundamental frequency resonant circuit to the inverter output by an inductance L According to figure 4.1, we have: u shn =U shnsin(n tω )and

shn ahn n hn

P

n L

θω

= (4.6) From (4.6), we can see that: if 00<θn<1800 thenP hn >0, and the

active power of the nth harmonic transmits to the AC-side from DC-side

of the inverter, and as a result, the DC bus voltage will increase If

-1800<θn<00 thenP hn <0, and the active power of the nth harmonic transmits to the DC-side from AC-side of the inverter and as a result, the

DC bus voltage will drop

4.4 Proposed DC bus voltage stabilization method

The structure of the method is shown as in Figure 4.3

Figure 4.3 Proposed DC bus voltage stabilization method

Three-phase balanced power supply through the three-phase

unbalanced bridge rectifier to generate the DC voltage U b, this voltage oscillates from 1.5 to 1.73 times the amplitude of the source voltage To

reduce the voltage and current variations, we add a capacitor C b and

inductance L b at the output of the rectifier The voltage across the capacitor

C b is the input voltage of the Boost converter After passing the Boost

Converter, a DC voltage will be generated (voltage on the C capacitor) It

is greater than the voltage on the C b capacitor and with an output voltage

ripple is very small In order to stabilize the voltage on the C capacitor,

we must control the S switch to always keep a fixed voltage at the

DC-bus According to the above analysis, if 0

0 <θn< 180 then the power is transferred from the AC-side to the DC-side of the inverter, the voltage across the DC-bus is greater than the reference value, then the S switch

will close, the energy on the capacitor will is discharged through R b and

the voltage on capacitor C is reduced If the voltage on the capacitor C is less than the reference value, then switch S will open At this point, the capacitor C will be charged from the Boost circuit

400

800

UC

Figure 4.4 Waves with the methodology of the paper [3]

Table 4.2 Parameters of the HAPF with the proposed method

The simulation result of the proposed DC bus voltage stabilization method are shown in Figure 4.5

Table 4.3 Comparison of the effectiveness of the proposed method and the methodology of the paper [3]

Methods ΔU C /U CBefore load is changed THDi L THDi s Sai số ΔU C /U CAfter load is changed THDi L THDi s Sai số Paper [3] 5% 28.06% 1.69% ±10 A 5.49% 30% 1.31% ±10 A Proposed 1% 28.06% 1.61% ±5 A 1.02% 30% 1.08% ±5 A

400

800

Figure 4.5 Waves of the proposed method

4.6 Conclusion

In this chapter analysed the cause for changing DC bus voltage and an overview of DC bus voltage stabilization methods On the basis of that, a new DC bus voltage stability method for the HAPF was proposed The results of the simulation proved that: The proposed method is capable of stabilizing DC bus voltage with an output voltage ripply on the DC bus is very small

equivalent circuit of HAPF

From figure 5.1, we can

1 0 1 0 2 1 0 1

Z Z Z Z K Z Z

Z K

+

= +

5.3 Control strategies analysis for HAPF

Table 5.1 Parameters of the HAPF

0 100

0

1.5 2

0 100 200 300

40000.5 1 1.5 2

pi K

0 0.5 1 1.5 2

0 100

0 0.5 1 1.5 2

0 100 200 300

40000.5 1 1.5 2

pi K

0 0.5 1 1.5 2

0 100

0 0.5 1 1.5 2

0 100 200 300 400 0 1 2 3 4 5

pi K

From the above analysis, we can be seen that: control strategy U C =

KI L In Figure 5.10 has a relationship between K and η is linear, when K increases then η increases and vice versa, this is a best control strategy

5.4 Proposed control method for HAPF

5.4.1 PI-fuzzy controller for HAPF

0 0.5 1 1.5 2

0 100

0 0.5 1 1.5 2

0 100 200 300 400 0 0.5 1 1.5 2

pi K

0 0.5 1 1.5 2

0 100

0 0.5 1 1.5 2

0 100 200 300 400 0 5 10 15 20 25 30

pi K

0 0.5 1 1.5 2

0 0.5 1 1.5 2

0 100 200 300

400020 40 60 80

pi K

First, K P and K I of PI controller are calculated offline and do not change during control The fuzzy controller will produce ΔK P and ΔK I values, according to which the K P and K I parameters of the PI controller

will be adjusted to match the load change:







∆ +

=

∆ +

P old P new P

K K

K

K K

K

(5.15)

The inputs of the fuzzy controller are e(t) and Δe(t): e(t)=i Lh -i Fh and

Δe(t)=e(t)-e(t-1) Membership functions of the inputs and outputs in

∆

Figure 5.19 Membership functions of the inputs and outputs Table 5.2 Fuzzy Rules of ΔKP Table 5.3 Fuzzy rules of ΔKI

Simulation results

Figure 5.21 Simulation results with the PI controller

Figure 5.24 Simulation results with the PI - mờ controller

Table 5.5 Result comparison table of the PI controller and PI-fuzzy controller

changed

After load changed

Fuzzy - neural controller

X + -

e

Hysteresis controller K

Figure 5.4 Structure of the proposed control method

The structure of the HAPF control method using adaptive Hysteresis – Fuzzy-Neural controller is shown in Figure 5.4 There are two proposed control modes for HAPF, one is the Hysteresis control mode and the other

is the adaptive Fuzzy-Neural control mode The choice between the two modes is determined by the switch K

+ If |e(k)| > threshold value, select Hysteresis control mode

+ If |e (k) | ≤ threshold value, select the adaptive Fuzzy-Neural control mode

b21

) 1 (s+

1 1

− Adaptive Fuzzy-Neural controller

+ The nodes at the input layer:

2 , 1 , 1 1 1 1

i e O

i e O

2 , 1

; 2 , 1

; 2

2 1

Ai layer

fuzzy

+ The nodes at rules layer

49 , , 2 , 1

; 7 ,

2 , 1

; 2 , 1 ,

i O

i

j Ai layer

k layer rules k

m

layer rules k network