Improvement of Voltage Profile by Series Active Filter in Grid Connected PV-System
Rudranarayan Senapatiα, Byomakesh Dashσ, Rajendra Narayan Senapatiρ & Jagat Jyoti RathaѠ
Voltage fluctuation and harmonics in voltage has been most significant issue since the inception of renewable energy system into the grid network. Besides this the fluctuating pattern of load also makes worse the voltage profile. Under such a situation use of custom power device in the form of Series Active Filter becomes the answer to the problem. The main objective of this work is to improve the voltage profile through Series Active Filter implementing Sinusoidal current control strategy. The strategy is a time domain control strategy based on instantaneous p-q theory. The control strategy has been elaborated here in details and has been implemented using MATLAB 2016A. The results have been given and described in details explaining efficacy of the above control strategy. Since sinusoidal current control strategy is a simple and effective control strategy it has tremendous potential for application in the Distributed Generation oriented system.
Keywords: Power quality, series active filter, sinusoidal current control strategy.
Author α: School of Electrical Engineering, KIIT. Deemed to be University, Patia Bhubaneswar, Odisha, India.
σ: Department of Electrical Engineering,
SOA, Deemed to be University, Bhubaneswar, Odisha, India.
ρ: Bhubaneswar, Odisha, India.
Ѡ: PDF, LAMIH UMR CNRS 8201, University of Valenciennes, France.
A usual assumption for most utilities is that the central generation utility produces sinusoidal voltage. In the transmission system, voltage variation is less and may possibly be kept within the specified limit. But in distribution systems due to unbalanced loading, at a large number of locations, the voltage distortions is significant. At several load points, the current waveform rarely seems to be a sine wave. This anomaly gives rise to the concept of harmonics, for the description of distortion in waveform leading to the deterioration of the quality of electrical power with a decrease in the efficiency of the system.
The followings are the main causes of voltage related issues.
- Microprocessor and Microcontroller-based faster islanding and isolation.
- High-efficiency adjustable-speed motor drives raising the level of harmonic over power systems.
- Deregulation of utilities with reduced awareness of harmonic control and lower reliability.
- Highly interconnected network, where the failure of any component jeopardizing the system stability.
- Introduction of Distributed Generation (DGs) into the power with enhanced harmonic levels.
The common threat that runs through all the above reasons for rising stress over the power quality (PQ) is due to the continuous drive from the manufacturer side for the increase in productivity through faster, more productive and efficient machinery for all utility customers who encourage the effort to make their customers more profitable. The installed machineries and equipments suffer the most from common power disruptions as well as they are the source of additional PQ issues. During the entire processes of automation, the competent operation of machineries and their control moreover depends on the quality of power.
The traditional distribution system passes the electrical energy from an individual source of power to multiple loads. In the present scenario with the increase in demand, the DGs are integrated with the traditional distribution system. The DGs use smaller-sized generators as compared to the exemplary central power plants and are scattered throughout the power system adjacent to the loads. Now, the DG has evolved as more crucial to figure out the assimilation of these systems by the interaction of power electronics with the current power system circuitry. As the DG operations are controlled through the use of power electronic devices, the excessive use of these controllers result in severe PQ issues. Hence implementation of multiple DG sources with the traditional system may lead to the severity of PQ issues.
Power Quality (PQ), the term nowdays has become one of the most prolific buzzword as the electric utilities and end users of electric power are day by day more and more concern about the quality of power. It is just like an umbrella concept for a horde of individual power system disturbances.
There may be various definitions for PQ, depending on different frame of references, e.g., from utilities point of view it may be reliability demonstrating the system as almost cent percent reliable, similarly from the manufacturer of load equipment point of view it might be the characteristic of the power supply enabling the equipment to work properly. But pertaining PQ as a consumer driven issue, it may be defined as to suit the end users: Any power issues manifested in voltage, current or frequency deviations that results in failure or malfunction of customer equipments. However PQ cannot be ever quantify and there is no such single accepted interpretations for quality of power but the ultimate measure of PQ may be determined by the productivity and performance of the end user equipments .
The PQ puts the boundaries of the deviation levels of voltage, frequency and waveform shape of power supply for the proper functioning of the equipment. Though day-by-day the power electronics devices are making the system compact and reliable, these are the primary cause of the production of harmonics in the system. The use of such kind of filters shields electrical apparatus which can be exaggerated by deprived/low PQ and avoids the propagation caused instabilities in the power systems. Many control strategies have been realized, out of which the most effective is sinusoidal current control strategy for mitigation of the harmonics and also others PQ issues as generated due to unbalanced or unstable system owed to the non-linear loading condition.
Based on converters power filters are connected in series or shunt to the distribution system. Therefore, according to the connection, there are three types of active filters i.e. shunt active power filter (ShAPF), series active power filter (SAF) and hybrid power filter (HPF).
SERIES ACTIVE FILTER (SAF)
The SAF is connected in series with the power supply which is used as voltage booster as SAF compensates for voltage as a constant voltage source (CVS) [2-3]. A single-phase SAF has been developed based on SPMC. It has been designed in such a manner that it can operate bidirectional without any use of dc capacitor and additionally its implementation is simpler in terms of hardware control. The SAF is usually used to solve any deviation in voltage and other PQ related issues. They are more competent than shunt compensators as they are able to compensate current issues.
The SAF has two components, i.e., PWM voltage control and active filter controller.The series active filter voltages are incorporated by the converter with a dc capacitor. The set voltage for this converter is calculated by the active filter controller, which has the input signal as load voltage and load current.
Active filter controller will be processing the signal to define the real-time instant compensating voltage values to be passed in a continuous manner to the PWM converter. The SAF works in a closed loop method sensing the voltage and computes the instant values of the compensating/series voltage reference value for the PWM controller. A voltage source inverter (VSI) has been used in SAF for its higher efficiency and compact size. A capacitor as an energy storing component linked at dc end of the converters for SAF to perform like a compensator. In supplement, the exchange of average energy should remain zero among the power filter and the power system.
V. SINUSOIDAL CURRENT CONTROL STRATEGY
The design of the controller for SAF for non sinusoidal as well as irregular supply voltages, compensation of load voltage is carried by the SAF assuring for a single optimal compensation only. Hence, according to the choice of preferences the design of the controller for a SAF is made which is the major cause of inferring sinusoidal current control strategy.
Sinusoidal current control strategy (SCCS) is based on instantaneous power theory, which is based upon the transformation from abc- frame to αβ0- frame. But the reason for not adopting control in abc- reference frame is that in 3-φ system the 3-phases are mutually dependent on each other, so independent control of the quantities is difficult. To make the control simple, 3-φ quantities are converted into 2-φ mutually independent quantities, so that easier control is possible in pq-domain, which is a stationery reference frame as proposed by H. Akagi. The purpose for choosing this control strategy is its simplicity in implementation. So far several applications on this strategy have been seen in different literatures. For a system with multiple renewable energy systems integrated, implementing a robust control becomes cumbersome, as the control of renewable itself requires a lot of complexities [4-5].
The control involves 3-φ quantities first converted into 2-φ quantities. Then these 3-phase quantities are used to evaluate the instantaneous powers in time domain, both instantaneous active as well as reactive power can be estimated. By using low pass filter, the harmonic power can be extracted, which can be used to generate the compensating current once the zero sequence power is known. The neutral point clamped capacitor voltage can be used to evaluate the zero sequence power. Hence the above mentioned method is termed as SCCS due to sinusoidal nature of the compensating current.
Advantages of pq-Theory over all other compensating theory are it can be valid for both steady state as well as transient state. Instantaneous power can be defined on αβ0- frame i.e. in three phase form. So, three phase system can be considered as a single unit but not the addition of three individual 1-φ circuits. abc- frame to αβ0- frame transformation is also known as Clarke transformation.
For appropriate expression, zero sequence power, p0 is expressed in terms of αβ0- frame of reference, as the instantaneous watt power, p and instantaneous VAR power, q are known from the instantaneous power theory.
Here, the 3-φ instantaneous active power is defined by both instantaneous active power with the instantaneous zero sequence power. In case of 3-φ 3-wire (3P3W) system, instantaneous zero sequence power does not exist so for this type of system, P3-ϕ can be treated as p only which may be noted as:
From (3), it can be observed that instantaneous active power can be divided into two parts i.e.,and,where is referred to the average value or dc value of active power which implies total energy transfer in the system anddefines the oscillating component of active sequence power and the instantaneous imaginary power can be defined as:
Where,andare the instantaneous current and voltage in frame, whereas and are the instantaneous current and voltage in frame.
As the converters used now a days are basically acts a non-linear load, the energy flow between the systems has a boundary condition. Comparing to the response of the converter and the generation of harmonic components and reactive power with the conventional approaches the analysis of different type of power is not sufficient using average or rms value as variables. So in a nonlinear circuit, time domain analysis has to be carried out for analysis of energy flow.
A 3-φ sinusoidal voltage which consists of only positive and zero sequence voltages are considered for the realization of zero sequence power. Symmetrical component in frequency domain only is applicable for steady state operation. Hence it can be converted into time domain for analysis of both steady state and transient state. For voltage, (5) is used as follows:
Whereas (6) used for current is as follows:
In order to obtain zero sequence components, the above equation is required to be converted into αβ0-frame by using Clarke transformation. For voltage, (7) is used as follows:
Whereas (8) used for current is as follows:
From (7) and (8), the instantaneous zero sequence power can be obtained as:
So the instantaneous zero sequence power can be divided into two parts which consists of average power and oscillating component of power which is at double the line frequency. Here is unidirectional energy flow as conventional active power andrepresents the oscillating component whose average value is zero. Interesting fact about zero sequence power is cannot be obtained alone without the oscillating component . Hence the total zero sequence components always associated with both average as well as oscillating component [6-7].
In the case of pq-theory, the instantaneous power so obtained is in time domain without consideration of the rms value of voltage and current. It also comprises of conventional frequency-domain concept for which the theory is not the contradictory theory rather it is the complementary theory in frequency domain.
The instantaneous zero sequence power components in the fundamental voltage and current or in harmonics do not add any impact on instantaneous real power and imaginary power. The total instantaneous active power is always same as the addition of instantaneous true power and instantaneous zero sequence power which includes both average and oscillating components. The instantaneous reactive power reveals the energy exchanged between system even in the harmonic and unbalance condition.Imaginary power shows the energy exchanged between the phases not the energy transferred from one phase to other phase. The active and reactive current components are derived from the instantaneous abc voltages and currents are represented as:
With the use of Inverse Clarke Transformation abc real and imaginary current may be obtained as follows:
Also and are the real and imaginary current components which generate real and imaginary power respectively.
The line voltage does not contain any zero sequence component as: .
As the line voltage is free from zero sequence components, hence (12) and (13) can be re-written as:
Among the two classifications of instantaneous power theory, one of them has already been described, i.e., instantaneoustheory. The other one is instantaneous theory where use of Clarke transformation (to transformation) is avoided. In this process instead of calculating real and imaginary power, active and non-active current may be calculated from phase voltage and currents .
5.1 SCCC for SAF
Apart from the above, SAF compensates the current distortions resulted due to non-linear loads with an appointment of high impedance path (by generating a voltage of same frequency as that of the current harmonics to be waived) enforcing the high frequency current to pass through the passive filter in parallel.
Based on the above discussions the proposed work is once again carried out with the simple control strategy on instantaneoustheory on SAF, which is applied on a 3P3W system. The basic block diagram of a 3P3W SAF for compensation of voltage is illustrated in Figure 1.
The SAF is for compensation of voltage. The input for the control block meant for calculation of the instantaneous power is the phase voltages at the PCC and the line currents of the nonlinear load to be compensated.
Fig. 1: Basic block diagram of 3-P-3-W SAPF
Assuming no zero-sequence current, the relation among the source voltage, load voltage and active filter voltage is given by,
The basic SAF voltages are synthesized by threesingle-phase converters with a common dc capacitor. The reference voltage for these converters is calculated by the “Active Filter Controller (AFC)” as shown in the Figure 1, which has as input signals the load voltages and currents (equal to the source currents). Here the voltages are calculated by the dualtheory (assumed as the currents, and the real and imaginary powers are known and the voltage components should be calculated in case of presence of series voltage compensation which is the dual of shunt current compensation) as given in (15):
From (15), the oscillating real powerand the oscillating imaginary power,where the zero sequence powersandare assumed to be zero due to zero-sequence current. With these oscillating powers, the instantaneous voltages to be injected by the SAF for load harmonic voltage compensation by using:
A certain amount ofshould be added towith an objective to compensate the losses. The reference voltagesandcan be converted to the reference by:
The SAF generates the voltages mentioned in (17) for the harmonic voltage compensation in the load producing oscillating active and reactive power at the load end. This approach confirms the voltage from the source side have purely sinusoidal waveforms. The control block diagram of the SAF has been illustrated in the Figure 2.
Fig. 2: Control circuitry of SAPF
The voltageare the voltage that needs to be obtained to compensate the harmonic component of load, which produce the oscillating real and reactive power. The source and load voltages thus obtained are purely sinusoidal in nature.
The SAF integrated to the system that is implemented to take care of power quality related issues and to inject grid as well as load desired power. It consists of a PV system whose voltage is stepped up by a boost converter.
To verify the effectiveness of the proposed sinusoidal current control strategy through SAF the simulation was carried out on a 3P3W system for non-linear load. The proposed strategy was simulated using MATLAB/SIMULINK 2016a with a system having Intel Core i5 processor with clock frequency 2.4 GHz, 8 GB RAM. The analysis of SAF was carried in different environments.
The load parameters considered for the simulation are given in the Table I.
Table 1: Load Parameter
230 V (RMS)
DC Link Capacitance
The simulation was carried out as per the control strategy explained in Figure 2.
The Figure 3(a) represents the performance of source voltage considering the SAF. This SAF is resulting source voltage characteristic in phase and the Figure 3(b) presents the performance of the source current of a 3φ3W system. It is observed that the sinusoidal current control strategy in series active filter maintains sinusoidal behavior of source voltage while load voltage harmonics is compensated.
Fig. 3: (a) Source Voltages, (b) Source Currents
Because of nonlinear load (i.e., RL rectifier circuit) the load voltage was found to be distorted and unbalanced in absence of compensation, but applying sinusoidal current control strategy, the characteristics shows the load voltage becomes balanced and smooth as shown in Figure 3 (a) and 3 (b).
The circuit breakers (CB) were a given a time delay of 0.1 sec to see the actual performance before the inception of the SAF into the circuit. The non-linear behaviour of the load giving rise to distortions can be seen during the first 0.1 second. After this, due to the effect of SAF on the system the balanced and distortion free voltage can be observed. These waveforms disclose the compensation of SAF for the disturbances at the source end.
The Figure 4 (a) and (b) show the load voltage and current waveforms respectively. The load voltage was found to have 231 V RMS value and load current is found to be having an RMS value of 3.53 RMS value.
Fig. 4: Load Parameters: (a) Voltage (b) Current
Due to the application of Sinusoidal current control strategy the load voltage and current waveforms are found sinusoidal. Further the strategy is applied to variable grid voltage condition to see the performance of series active filter compensation. The Figure 5 shows the grid voltage at different instances.
Fig. 5: Grid injected voltage
There is voltage sag between 0.5 sec to 1 sec and voltage swell in between 1.5 sec to 2.0 sec. But the series active filter able to maintain the voltage level fixed at 311V RMS. The injected voltage by the series inverter is shown in Figure 5.
The load voltage is fixed due to the voltage injected by a series active filter which can be observed in the Figure 6.
Fig. 6: Load voltage
The injected voltage adds up to grid voltage during 0.5 sec to 1.0 sec and nullifies during 1.5 sec to 2.0 sec. Here it is found that the response of the Series active filter is quite fast and it tries to maintain the desired load voltage by injecting the compensating voltage.
The series injected voltage at the series transformer terminals is presented in Figure 7.
Fig. 7: Series inverter injected voltage
There was an injection of voltage during voltage sag from 0.5 seconds to 1 second. From 1.5 second to 2 seconds there was an interruption, taken care by SAF by injection of voltage during the same period.
The harmonic analysis of load voltage is obtained as seen in the Figure 8.
Fig. 8: Harmonic analysis of load voltage
The THD in load voltage is found to be 0.74%. The RMS value of load voltage is 311.5 V. Presence of higher order harmonics are suppressed by SAF which can be seen from the THD analysis.
The grid current is shown in Figure 9 and its harmonic analysis is given in the Figure 10.
Fig. 9: Grid Current
Fig. 10: THD of Grid current
The THD in grid current was found to be 22.35%. The RMS value of grid current was 8.623 A. Presence of 5th order harmonics has been suppressed by SAF significantly. Its magnitude in respect of fundamental is below 20 %.
The SAF based on sinusoidal current control strategy offers assurance against voltage sag and swell. Besides this, it offers harmonic isolation to load voltage, as evident from the results. The THD in load voltage is found to be 0.74 percent which is quite satisfactory.
A concept-oriented study of SAF, using sinusoidal current control Strategy has been carried out for 3-P-3-W system using passive and non-linear load. Even though the voltage and current in three phases are subjected to the disturbance in the transient state, the control strategy leads to drawing of constant current by the load during steady state condition. The load voltage at load terminal was obtained sinusoidal and balanced irrespective of sag, swell.
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