Harmonics Measurement in Computer Laboratory and Design of Passive Harmonic Filter using MATLAB

In this paper the harmonics measurement for computer loads is analyzed and passive filters are designed for mitigating those harmonics. The filters are designed to meet the IEEE standard 519-1992 which is recommended for harmonic current limits. Personal computers are non-linear loads that generate harmonics due to non-sinusoidal current present at entrance of power supply. In this work the personal computers in laboratory are taken as domestic load and harmonics generated by them cannot be ignored which are simulated using MATLAB/Simulink. The purpose is developing analytical method for the design of the passive harmonic filter that absorbs current harmonics caused by computer loads. The findings of this study would be supportive to make the source current free from harmonics thereby reducing the THD. Simulation results of proposed design method of passive filters shows attractive results for harmonic reduction with profit of upgrade of power factor. Design of passive harmonic filters by using non active power can be simple cost effective solution for systems. Keywords—Power Quality (PQ); personal computers; Total Harmonic Distortion (THD); passive filter; MATLAB/Simulink


I. INTRODUCTION
In AC power supply systems, current waveform is distorted and harmonics are created due to usage of non-linear loads that reduces the quality of power [1], [2].That causes enormous fault occurrence and monetary losses toward power supply system.Harmonics effect in AC power system is increasing day by day [3], [4].Harmonics due to personal computers loads has particular attention to power quality (PQ) because personal computers are frequently concentrated in a massive group and produces harmonics [5], [6].

A. Personal Computers(PC's)
Personal computers are non-linear loads to AC system.Switch Mode Power Supply (SMPS) used in computers generate harmonics.For nonlinear load, non-linear flow of current is created because of dc-link capacitor in power supply [7].Fig. 1 shows personal computer equivalent SMPS circuit model.

B. Relation between Active Power, Reactive Power, Apparent Power
We use the following terms to describe power flow into a system, active power P [Watt], reactive power Q [Volt Ampere reactive] and apparent power S=|S| [VA].For sinusoidal waveform P, Q and S can be written as =√ . Apparent power S is the product of rms values of current I and applied voltage V [8].
It is significant as it represents the total power that must be available to provide power to the load, though just a part of this is of useful power S=VI.For harmonic supply S = P + jQ = V I (cosφ + j sin φ), In this active power P = V I cosφ and the reactive power Q = V I sin φ.The cosine of the phase angle φ between the applied voltage and the current is called the power factor.Pf =cos = P/S.In case of non-sinusoidal signals the influence of so called distortion power H or D is included this consists of unequal harmonics of current and voltage [9]- [12].The powers active, reactive and distortion are connected by a relation =√ .Geometric sum of reactive and distortion powers is equal to non-active power = √ √ .Distortion power has raised increasing interest with the expansion of non-linear loads [13].The function representation of mentioned powers is shown in Fig. 2 and 3.The paper is organized as follows: Section I presents the introduction of non-linear load and harmonic distortion, Section II presents the methodology of research work, Section III presents the result of measurement of harmonic distortion for computer laboratory, Section IV present the simulation of system for proposed filter design and discussion about obtained results and Section V concludes the paper.

II. METHODOLOGY
Harmonic distortion depends upon electronic elements used in appliance"s circuitry.Numbers of nonlinear residential loads are increasing day by day; therefore harmonics caused by these loads cannot be neglected.There are many techniques to reduce harmonics but economic and maintenance points of view there have necessity to select best compensation method.The design attempts to comply with the IEEE Standard 519-1992 recommended harmonic limits applied to the current harmonic limits [14]- [17].
In this section, measurement of harmonics values and power quantities obtained from power quality analyzer (PQA) device for computer laboratory.In this work total 24 PC"s are joined to the AC supply system are shown in Fig. 4 and 5 and all obtained measurements are written in Tables I and II   The features of PQA are as shown in Fig. 6 to 8.

IV. SIMULATION OF EQUIVALENT SMPS OF PC
The parameters used for this system are given in Table III and equivalent SMPS of PC is simulated using MATLAB software by Simulink library as shown in Fig. 14.The 3 rd , 5 th , 7 th , 9 th and 11 th harmonic component are high in magnitudes.So these harmonic components are to be mitigating during this research work.Fig. 15 and 16 show the supply current and supply voltage prior to installation of passive filter, the current and voltage harmonic distortion (THD i and THD v ) are found to be 35.79% and 0.38 % respectively.So IEEE limits for THD i distortion limits are violated while THD v is found under standard limit.

A. Design of Single-Tuned Passive Filter
Quantities required for the filter design: 1) Non active power N in VAr 2) Supply frquency f in Hz Here to avoid from using a trial-and-error approach for single tune filter design, non-active power is considered as design parameter and proposed MATLAB/Simulink model for doing this calculation is as shown in Fig. 17 and various power obtained from this proposed model is written as shown in Table IV.Similarly follows the same steps ,in this paper four single tuned passive filter has been designed such as 3 rd ,5 th ,7 th and 9 th and the values of all passive tuned filter can be shown in Table V.    Fig. 18 show simulation model of equivalent computer power supply using designed filters.Fig. 19 and 20 show the source voltage and source current after passive filter installation.THD v was reduced to 0.07% from 0.38% after passive filter installation, whereas THD i was reduced to 4.41% from 35.79 % after passive filters installation according to IEEE limit.
It can be noticed that before compensation, the input supply current is not sinusoidal (distorted) with r.m.s.(root mean square) value of 14.26 ampere.The input source current after compensation by passive filter becomes nearly sinusoidal from the distorted wave.After compensation the source current becomes sinusoidal with r.m.s value of 37.45 and inphase with the supply voltage.

V. CONCLUSION
It is concluded that the harmonic distortion depends upon electronic elements used in appliances circuitry.Numbers of nonlinear residential loads are increasing day by day; therefore harmonics caused by these loads cannot be neglected.Produced harmonics due to residential loads should be minimized.There are many techniques to reduce harmonics but economic and maintenance points of view there have necessity to select best compensation method.Design of passive harmonic filters by using non active power can be the better and simple solution and cost effective systems.By means of the simulation of proposed design method of passive filters showed good results for harmonic reduction with profit of upgrade of power factor too.

Fig. 4 .
Fig. 4. View of computer laboratory in electrical department QUEST Nawabshah.

Fig. 17 .
Fig. 17.Simulation of proposed model for passive filter design.

3 )
Supply voltage (rms) Vs in volts 4) Harmonic order h Number 5) Quality Factor QL Number Steps to calculate values of C, L and R for single tuned filter.

TABLE III .
CIRCUIT PARAMETERS USED FOR SIMULATION OF EQUIVALENT COMPUTER POWER SUPPLY th ,7 th ,9 th Lsys 1mH Fig. 14.Simulation of equivalent SM.

TABLE IV
Now we can calculate values of required passive harmonic component filter as follows.