Power factor correction of three-phase electrical power supply by using of thyristor controlled reactor VAR compensator

. Low power factor (abbreviated as pf), particularly lagging pf, brings a lot of negative effects to electrical power supply system. Lagging pf is caused by inductive load. Inductive load with low pf draws higher current for the same active power demand. Higher current results in larger kVA rating and size of all electrical equipment connected to the electrical power system, larger conductor size used to deliver electrical power to the load, higher loss hence poorer power distribution efficiency and higher voltage drop hence poorer electrical power system voltage regulation. Considering all these negative effects, reactive power (VAR) compensation is needed. This paper analyses characteristics of a VAR compensation method called Thyristor Controlled Reactor (TCR) applied in three-phase electrical power system supplying an inductive load. A TCR consists of three sets of a capacitor connected in parallel with an inductor. The capacitor will supply a certain amount of reactive power needed by the load. The inductor is connected in series with an electronic static switch named as TRIAC to control the amount of reactive power absorbed by it. When reactive power supplied by the capacitor is higher than reactive power demanded by the load, the switching angle of TRIAC is controlled in such a way that excess of reactive power supply is absorbed by the inductor. In this work, three capacitors in which capacitance of each of it is equal to 50 µF as well as three inductors in which inductance of each of it is equal to 200 mH are used to compensate an inductive load which has power and pf vary from 1.5 kW to 4.6 kW and 0.46 to 0.84 respectively. By controlling the switching angle of TRIAC, pf of the three-phase electrical power system is successfully maintained close to unity at 0.95 following changes of the load. However, further analysis shows that operation of the TRIAC results in increase of current harmonics. Average of total current


Introduction
In general, loads in an electrical power system can be classified as resistive, inductive, and capacitive.However, most commonly electrical power loads are inductive such as transformers, induction motors, fluorescent lamps, and other electronic loads.Inductive loads consume both active and reactive power.Excessive use of inductive loads can result in decrease of pf (cos ϕ) of electrical power system to less than unity.The higher reactive power consumption, the lower is the pf [1].Inductive load with low pf draws higher current for the same active power demand.Higher current results in larger kVA rating and size of all electrical equipment connected to the electrical power system, larger conductor size used to deliver electrical power to the load, higher loss hence poorer power distribution efficiency and higher voltage drop hence poorer electrical power system voltage regulation [2].
To increase pf of an electrical power system, a method known as reactive power (VAR) compensation is applied [2].VAR compensation has become a common practice in addressing issues related to the quality of the electrical power supply system.Typical basic kind of VAR compensation is involving the use of fixed capacitors or fixed inductors to either supply or consume reactive power [3][4][5][6][7].These fixed capacitor and inductor could be placed in series or in parallel with the electrical power system.Series compensation requires capacitor and inductor with higher current capacity while parallel compensation requires capacitor and inductor with higher voltage capacity [8].Another kind of VAR compensation is popularly known as Static VAR Compensator (SVC) [9,10].The difference between using fixed capacitors or fixed inductors and SVC is that fixed capacitors or fixed inductors operate passively.In contrast, SVC operates actively, and active compensation generally has more rapid response.With the installation of SVC, voltage fluctuations can be improved by controlling the injection of reactive power into the system.SVC includes several methods, namely Thyristor Switched Capacitor (TSC) and Thyristor Controlled Reactor (TCR).TCR VAR compensator has been reported to be able to control reactive power flow in an industrial inductive three-phase power system [10,11].Other analyses show that TCR VAR compensator capable to overcome load unbalance and voltage stability of an electrical power distribution system [12,13].
In this paper, application of TCR VAR compensator to improve pf of a three-phase electrical power system is analysed, not only regarding to its capability to maintain high pf by controlling reactive power flow but more importantly regarding to harmonics generated.Harmonics is most probably generated when using TCR VAR compensator due to operation of TRIAC static switches in controlling reactive power flow [9,14].Harmonics then becomes one negative effect of using TCR VAR compensator.Harmonic gives negative impacts to electrical power system such as reducing efficiency of electrical power equipment and system, failure of logic circuit operation, overload on transformer, abnormal operation of motor and generator, neutral current flows and maloperation of circuit breaker [15,16].Therefore, study on harmonics generated by using of TCR VAR compensator is necessary.

Methodology
The basic principle of VAR compensation method is shown by Fig. 1.Improving pf is achieved by increasing the low of cos ϕ value by narrowing the angle ϕ1 to become ϕ2 so that ϕ1 > ϕ2.Effort to reduce the angle ϕ involves reducing the reactive power (VAR) component.The inductive reactive power component needs to be reduced, typically accomplished by adding a source of capacitive reactive power, often in the form of capacitors [2].From Fig. 1 we can determine the capacitive reactive power (QC) necessary to improve pf by following equation: in which QC is the total reactive power required to improve pf (VAR); P is active power of the load (Watt); tan 1 is angle in the initial reactive power; and tan 1 is angle in the resulting corrected reactive power.
To determine the size of the capacitor used to improve the pf, it can be calculated using the following equation: in which QC is the total reactive power required to improve pf, DC is the magnitude of the capacitor value; V is the operating voltage; and f is electrical frequency.
TCR is a kind of Static VAR Compensator (SVC) technique.It is an electrical power device designed to absorb reactive power of an electrical power supply system when there is an excess of reactive power.As shown by Fig 2 ., the main components of TCR are an inductor connected in series with static switch and a fixed value capacitor connected in parallel.In this figure, antiparallel connected thyristors are used as static switches to control the positive and negative parts of the ac sinusoidal three-phase voltage of the electrical power system.Alternatively, a TRIAC can replace the antiparallel connected thyristors.The current flowing through the inductor can be controlled by adjusting the switching angle of the thyristors or TRIAC [9].To determine magnitude of the inductor used to absorb reactive power supplied by the capacitor, the following equation is used: in which QL is the total reactive power to be absorbed by inductor (equal to QC), DL is the magnitude of the inductor value; V is the operating voltage; and f is electrical frequency.When reactive power supplied by capacitor is higher than reactive power demanded by load, the switching angles of thyristors/TRIACs are controlled in such a way that excess of reactive power supply is absorbed by the inductor.Controlling switching angles thyristors/TRIACs results in non-sinusoidal ac three-phase voltage and current waveforms.Non-sinusoidal voltage and current waveforms contain harmonics that give negative effect to the electrical power system.
To investigate the capability of TCR to improve pf of an electrical power supply as well as harmonics generated, two laboratory experiments has been set up.Block diagram of the first laboratory experiment set up is shown in Fig. 3.One can see that the system comprises of an ac three-phase electrical power supply and an induction motor representing inductive load.The motor is then loaded by a dc shunt generator.The dc generator is further loaded by resistive load (R).Before taking the VAR compensation measure, parameters such as pf, total harmonic distortion of current (THDi) and voltage (THDV), magnitude of voltage and current of the electrical power system are observed carefully.Fig. 4 shows block diagram of laboratory experiment set up after VAR compensation using TCR compensator.It looks alike to Fig. 3., but one can see that in Fig. 4 a TCR VAR compensator equipped with TRIAC trigger control circuit has been added.Subsequently, experiment is run to compare pf as well as THDi and THDV of the three-phase electrical power supply before and after reactive power compensation.This comparison is used to analyse performance of TCR compensator.

Initial data obtained before TCR installation
After collecting measurement data during the initial condition (before TCR installation), pf values of the electrical power supply for various loading condition are obtained.It was found that the lowest pf was 0.46 lag, while the highest pf was 0.84 lag.These pf values were used to determine the required capacitor and inductor values for reactive power compensation to increase pf closer to unity.

The required capacitor and inductor values
To determine the required capacitor and inductor values of TCR, data of the lowest pf, voltage and active power of the initial condition of three-phase electrical power system is used.With pf equal to 0.46, voltage equal to 386.25 V and active power equal to 1500.00 W then by using (1) the reactive power required to improve pf is calculated as 2401.95VAR.Subsequently, by using equation ( 2) and equation ( 3) values of capacitor and inductor of the TCR are calculated as 51.25 µF and 200 mH respectively.After installation of TCR compensator, pf of the three-phase electrical power system can be made close to unity.It indicates that TCR VAR compensator works properly.The objective of improving pf is to utilize the power supplied by the power utility (PLN) to its maximum capacity.Additionally, it helps to avoid penalties imposed by the power utility due to pf lower than the limit set by the utility, which is 0.85.Lagging pf may cause a drop in voltage in the network, while leading pf can result in voltage increase.Voltage fluctuations in electrical power systems can significantly disrupt the process of distributing electrical energy from the generators to consumers.Voltage changes are directly related to distribution losses such as heat losses and electromagnetic emissions that occur along the distribution network.

Data after TCR installation
Table 2 also shows that using TCR VAR compensator results in an increase of current total harmonic distortion (THDi).On average the THDi of the three-phase electrical power system is about 18.45% after TCR installation.Meanwhile, as shown by Table 1, on average the THDi of the three-phase electrical power system is about 15.13% before TCR installation.On the contrary, voltage total harmonic distortion (THDV) of the threephase electrical power system after TCR installation is slightly lower than before TCR installation.An increase in THDi values is less desirable because it can lead to losses such as heat generation, inaccuracies in electrical measurements, and other issues.The increase in THDi is due to the firing delay angle of TRIAC of the TCR VAR compensator [9].Switching angle indicates the amount of current that can pass through the TRIAC and flow into the inductor.Fig. 5 shows that the higher the initial pf (pf before installation of TCR compensator), the smaller the switching angle of TRIAC.For higher initial pf, reactive power absorbed by the inductor needs to be increased to balance reactive power supplied by the capacitor so that the load does not experience overcompensation.Overcompensation would make the load capacitive and result in leading pf.

Conclusion
Improvement of pf of three-phase electrical power system using TCR method has been successfully achieved.The TCR consists of three capacitors, each has a capacitance of 50 µF, and three inductors, each has an inductance of 200 mH.By controlling the switching angle of TRIAC of the TCR, pf of the three-phase electrical power system is successfully maintained close to unity at 0.95 when load varies from 1.5 kW to 4.6 kW and pf varies from 0.46 to 0.84.However, total harmonic distortion of current (THDi) of the threephase electrical power system increased from an average value of 15.13% before TCR installation to 18.45% after TCR installation.On the contrary, voltage total harmonic distortion (THDV) after TCR installation is slightly lower than before TCR installation.

Fig. 2 .
Fig.2.Arrangement of TCR VAR compensator for improving pf of an ac three-phase electrical power system.

Fig. 3 .
Fig. 3. Block diagram of laboratory experiment set up before VAR compensation using TCR compensator.

Fig. 4 .
Fig. 4. Block diagram of laboratory experiment set up after VAR compensation using TCR compensator.

Fig. 5 .
Fig. 5. Graph of pf as a function of thyristor/TRIAC switching angle.

Table 1 .
Pf and THD for initial condition (before TCR installation).

Table 2 .
Pf and THD after TCR installation.