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This research work brings out the unique predictive current control method for attaining an efficient grid connected Photo Voltaic (PV) system by Shunt Active Power Filter (SAPF) as grid connected converter. The major objective of the research work is to address the presence of Direct Current (DC) component, frequency improvement, quicker theta response, voltage magnitude estimation in the input signal of the Phase Locked Loop (PLL) which is challenging. This work focuses on tuning the PLL block (
K_{p}
,
K_{i}
,
K_{v}
and
K_{o}
)
through Artificial Bee Colony (ABC) optimization algorithm. The proposed ABC based modified three-phase PLL method is based on adding a new loop inside the PLL structure. In power converters, ABC algorithm is used to select the optimal switching states. The voltage vector which minimizes a cost optimization function is selected. Simulation is carried out for both balanced and unbalanced system and the results validate that the performance of the proposed approach is better in terms of harmonic compensation as per the IEEE standards within ±5%, power factor improvement of the system, quicker theta tracking and suppression of frequency jump with the interconnection of PV system.

In recent years, power quality at end user’s terminal is affected greatly and it has become a great concern in many industrial applications. Therefore, it is very essential to identify novel and efficient system that would mitigate the disturbances in the electrical systems, improving their power quality [

Shunt active power filter has the growing popularity than series active power filter since most of the industrial applications require the compensation towards current harmonics. It has the ability to keep the current in the source as balanced and sinusoidal after compensation, irrespective of whether the load is nonlinear, balanced or unbalanced [

In recent decades, interconnection of Renewable Energy Sources (RES) with SAPF has become one of the attractive research topics. But, there is a certain level of influence and impact that renewable generation has on power quality on account of its nonlinearity, especially as both solar generation plants as well as wind power generators have to be connected as such with the grid using high power static Pulse Width Modulated (PWM) converters [

Many types of active power filter configurations have been proposed in the last two decades to achieve the required harmonic compensation level. The main techniques in the active filter are to generate the reference current and the control method applied to inject the required compensation current into the line, which decides the performance of an active filter. There are several methods for reference current generation for the shunt active power filters. In 1984, H. Akagi stated instantaneous active and reactive power theory control method that was quite efficient method for three phase loads in the balanced system, being later worked by Watanabe and Aredes for three phase four wire systems; later F.Z. Peng proposed zero sequence currents. Synchronous Reference Frame (SRF) concept is simple algorithm having good dynamic responses. The SRF could compensate the harmonics in the current and reactive power component from the distorted load currents.

In 1995, Bhattacharya proposed dq component’s calculations of the instantaneous three phase currents and created synchronous reference frame concept. A dq based current reference generator scheme is used to obtain the reference current signal of the active power filter. This scheme presents a fast and accurate signal tracking capability, which avoids voltage fluctuations that deteriorate the current reference signal affecting compensation performance [

To implement the dq based current reference generator, some kind of synchronizing system should be used. PLL has been widely used for the synchronization purpose. SRF-PLL has been widely used in power system applications due to its simplicity and considerable performance. But it suffers from double frequency error when the input signal is unbalanced. Moreover, it also has drawbacks due to the presence of DC components. The DC component can be an intrinsic component of a signal (e.g., the DC link voltage in a single phase rectifier that has a DC and a second order harmonic), and can be generated by measurement devices (e.g., due to the saturation phenomenon in a current transformer [

In this paper the author proposed a method to address the DC component in the input signal of the PLL and notch filter algorithms for filtering and synchronization applications. This paper takes the motivation of the abovementioned work and focuses on improving the performance of the overall system through the utilization of swarm intelligence optimization algorithm.

Traditional PI controllers are widely used in the control loop of the voltage source converters to produce the switching states of the converters because of its simplicity in control design. Perfect tuning of the controller is needed to have the satisfactory operation of the converters and several traditional tuning rules, such as Ziegler and Nichols, Astrom and Hagglud, Sain and Ozgen cohen and coon etc. are adopted for this purpose. Though they are enough for the first order system, they produce large overshoot and required repeated design process for any variations in the system model. But in general the power system has many disturbances, parameter variations and has many nonlinear loads which lead to the degradation of the quality of the power. Hence the continuous tuning algorithm is needed to have satisfactory operation in its dynamic nature [

The main contribution of this paper is to tune the PLL block parameters such as proportional gain (K_{p}), integral gain (K_{i}) , change in the instantaneous frequency (K_{o}) and voltage magnitude gain (K_{v}) . The settling time and the steady state response of the frequency are tuned by K_{i} and K_{p} gain values. Similarly, elimination of DC component and identification of the voltage magnitude are tuned through K_{o} and K_{v} gain values through the ABC optimization approach. Moreover, the control loop of Voltage Source Inverter (VSI) is proposed with the predictive current control approach. This method predicts the future load current values for the voltage vectors generated by the inverter. The voltage vector which minimizes a cost optimization function is selected. The cost optimization function used in this work evaluates the error current at the next sampling time. In order to select the optimal switching state that must be applied to the power converter, ABC optimization is used in this work which selects the best minimal optimal value “g”.

This paper is organized as follows. Section 2 deals with the proposed methodology. Section 3 discusses about the three leg converter model. Section 4 describes about the proposed ABC based predictive current control model. Section 5 narrates about current reference generation. Section 6 validates the simulation results to proof the performance of the proposed system.

_{s}, output ripple filter impedance Z_{f} of the converter and the impedance Z_{L} of the load. When RES generates excess power than the load demand, then the excess power is given to the grid to maintain the load voltage of frequency constant.

The main element of this system is a voltage source inverter, as it interfaces the renewable energy source to the grid and delivers the generated power. In this work, grid interfacing inverter can effectively be utilized to perform the following important functions: 1) non linear load current harmonics mitigation; 2) unbalanced current compensation in case of 3 phase 3 wire system; 3) ABC based Predictive control technique for cost optimization 4) ABC based PLL tuning approach under unbalanced load conditions. Moreover, with adequate control of grid interfacing inverter, all the four objectives can be accomplished either individually or simultaneously.

An illustration of converter’s power circuit employed here is presented in _{u}, S_{v} and S_{w} , as follows in Equation (1):

and Equation (1) can be expressed in vectorial form as in Equation (2).

where

The output voltage space vectors created by the inverter are defined as in Equation (3)

where

where

Taking in to consideration gating signals combinations of

where R_{eq} and L_{eq} are the 3L-VSI output parameters expressed as Thevenin impedances at the converter output terminals Z_{eq}. Hence, the Thevenin’s equivalent impedance is calculated by a series connection of the ripple filter impedance Z_{f} and the arrangement in parallel between the system equivalent impedance Z_{s} and the load impedance Z_{L} as shown in Equation (6).

For this model, it is assumed that _{eq} = R_{f} and L_{eq} = L_{s} + L_{f}. For complete understanding the PWM switching time calculation of the inverter switches are given in

Sector | Switching time vector |
---|---|

1 | |

2 | |

3 | |

4 | |

5 | |

6 |

The value of dc link capacitor is chosen from the reference dc voltage, (

where ω = 2p ´ 50 = 314.159; I_{c}_{(rated)} = rated current,

The amount of filtering inductance to be added to the SAPF is

where (

It is derived and designed so that the required current reference may be generated and the same may be utilized for compensating any undesirable load current components. With respect to this one, system voltages, DC link voltage of converter as well as the load currents are measured. During the unbalanced current/voltage conditions, PLL tuning is required. But, in order to improve the conventional PLL tuning, an intelligent ABC optimization algorithm is used in this approach

Converter model is employed for prediction of output current of the converter. Implying that considering a given sampling time

In [

Step 1: Generate the initial switching state

Step 2: Calculate the voltage for each switching state (particle) to obtain output voltage for each switching state V (P_{i}), and select the best _{i}.

Evaluate the fitness (output voltage for each switching state) (f_{i} = P_{i}) of the switching state

Step 3: Use the iteration formulae of ABC to get a new routing path.

For each employed bee Do

Produce new solution

Calculate the value

Apply greedy selection process

Calculate the probability values P_{i} for the solutions

For each onlooker bee

Select a solution _{i}

Produce new solution

Calculate the values

If there is a discarded solution for the scout then replace it with a new solution which will be randomly produced.

Memorize the best solution so far.

The algorithm iterates to the preferred cycle number and the sources having the best nectar in mind gives the possible values of the variables. The obtained nectar amount denotes the solution of the objective function. The 8 possible output current predicted values can be obtained as in Equation (10)

As shown in Equation (10), in order to predict the output current at the instant

Selection of optimal switching state applicable to power converter requires, comparison of 8 predicted values obtained for

Here

Therefore, the main goal of the cost optimization is to achieve the g value close to zero. Voltage vector

DQ based reference current generation scheme has been employed here to attain active power filter current reference signals. The scheme offers quick and precise signal tracking capabilities. This particular feature negates voltage fluctuations deteriorating reference current signal that influence compensation performance [

Distortion (THD) of the load

where the value of

DQ based scheme is operational as part of the rotating reference structure, hence measured currents need to be multiplied using

Eliminating of tracking errors is feasible as SRF-PLLs are designed so that phase voltage unbalancing is evaded, harmonics (i.e., less than 5% and 3% in fifth and seventh, respectively) and resultant offset generated on account of nonlinear load conditions and measurement errors.

Thus, for the complete removal of DC component and sudden response of frequency ABC is used in this work to tune the PLL block

Equation (13) shows the relationship between the real currents

The DC component of the phase currents

A significant advantages that dq based current reference generator scheme offers is that it facilitates a linear controller implementation in DC voltage control loop. Though, an obvious disadvantage that arises from dq based current reference frame algorithm, deployment for producing reference current generation is of second order harmonic components in

Traditionally PI controller is used to control the DC voltage of the converter. This is an important issue in the evaluation, since the cost function according to the Equation (9) is designed using only reference currents, in order to avoid the use of weighting factors. In general, these weighting factors are obtained experimentally, and they are not well defined for different operating conditions. In addition to that, the slow dynamic response of the voltage across the electrolytic capacitor will not affect the current transient response. For this reason, the PI controller represents a simple and successful way for the DC voltage control. The DC voltage remains constant (with a minimum value of

The absorbed active power by the converter is controlled by adjusting the amplitude of the active power reference signal

In the conventional tuning of PLL there is a frequency jump in the grid voltage and a delay in theta tracking. Both will be varies with respect to the type of load used. Moreover the varying nature of RES produces ripples in the output which affects the grid voltage [

For a three phase balanced set of input signals is as shown in the Equation (15)

and the d component is given as in the Equation (16)

The loop regulates this quantity to zero and, thus, regulates φ to θ. Assume that the input signal has a DC component of

fundamental frequency. The proposed ABC based modified three phase PLL is shown in

is equal to

This section discusses about the tuning of gain values to attain the optimal performance of PLL.

where

Manual tuning of the controller parameters in removing the performance becomes more difficult and consumes more time. To overcome this major drawback, ABC based optimization approach is proposed here to auto tune the controller parameters and also to improve the efficiency of the control strategy in extracting reference currents for SAPF under non linear load conditions also.

1) Initialize the population (

2) Initially, the controller parameter are generated which is of size N

3) Produce new solutions of DC components through employed bees and then apply greedy selection process

4) Probability values are calculated for the above solution

5) Produces new set of population for the parameters of the controller

6) Update the best values achieved so far, and sort the obtained values

7) Based on probability values again the new solutions are obtained through onlooker bees and then apply greedy selection process

8) Determine the left out solution for the scout, if exists, and replace it with a new randomly produced solution

9) Update the best value achieved so far

The proposed structure of

where

The variables V, ω and

This work has been simulated in MATLAB SIMULINK r2011a. This section clearly discusses about the performance evaluation of the proposed model in which the voltage source converter is controlled to achieve minimized current harmonics under non linear load conditions at unity power factor. Moreover, this proposed model is verified for the RES interconnection with inverter to attain balanced sinusoidal grid currents under unbalanced load conditions through ABC optimization technique. The other simulation scenario wherein the PLL is tuned to eliminate DC component, minimizing the peak overshoot and improving the settling time of frequency and theta is considered. In this simulation, the

The nonlinear behavior of the three phase grid system with nonlinear balanced load condition and how the compensation is done with the help of the voltage source converter acting as SAPF for the grid connected PV system is clearly depicted in

The nonlinear diode bridge rectifier will draw a nonlinear load current as shown in

Now the compensating performance of the SAPF could be clearly understood from

The performance of the ABC tuned SAPF for both nonlinear and unbalanced load conditions could be appreciated by analysing the waveforms shown in

Here the unbalanced condition is considered from 0.2 seconds onwards and up to that the system operates the nonlinear load under balanced condition only. Load unbalancing is created at 0.2 seconds, only on two phases i.e., phase A and phase B having 20A as unbalanced load currents. Phase C is kept in balanced nonlinear condition with the current magnitude of 14 A. This is shown in

Till 0.2 seconds the system has nonlinearity alone as shown in

The performance of the proposed model for the unbalanced nonlinear system is evaluated by investigating the waveform shown in

The main objective of this paper which is known as ABC tuned PLL with predictive current control method of SAPF is for optimizing the control parameters of the PLL can be evaluated by comparing it with the conventional PLL model, based on the factors frequency response, theta tracking, elimination of DC component and maintain the voltage magnitude of the system.

With optimal tuning of

The design of ABC based PLL tuned SAPF has been proposed in this paper for a grid connected solar PV system. The dynamic performance of the proposed model in compensating current harmonics and reactive power consumption is evaluated through the Matlab simulation. In this research, ABC optimization is proposed to select the optimal switching states that must be applied to the power converter, by selecting the best minimal optimal

Grid current THD in % for nonlinear load at balanced condition | ||||||||
---|---|---|---|---|---|---|---|---|

Method | 5^{th} | 7^{th} | 9^{th} | 11^{th} | 13^{th} | 15^{th} | 17^{th} | THD |

SRF-PLL based SAPF (Conventional) | 0.40 | 0.66 | 0.14 | 0.96 | 0.40 | 0.09 | 0.70 | 1.85 |

ABC based PLL tuning of SAPF (Proposed) | 0.04 | 0.01 | 0.06 | 0.50 | 0.04 | 0.06 | 0.39 | 1.20 |

Grid current THD in % for nonlinear load at Unbalanced condition | ||||||||

SRF-PLL based SAPF (Conventional) | 2.10 | 1.95 | 1.69 | 1.56 | 0.94 | 0.81 | 0.67 | 4.62 |

ABC based PLL tuning of SAPF (Proposed) | 1.70 | 1.47 | 1.41 | 1.34 | 0.78 | 0.81 | 0.69 | 3.79 |

Note : In both cases nonlinear load current (without SAPF) %THD = 25.01 |

value “g”. Simulated results have proved that the proposed predictive control algorithm is a good choice to classical linear control methods and the compensation effectiveness of the proposed shunt active power filter with ABC based PLL tuning. The performance of the proposed approach has been evaluated under unbalanced load conditions and it is observed that the proposed modified PLL approach attains fast theta tracking as 0.15 seconds, quick settlement of frequency compared with ±0.2 Hz jumping of conventional model, maintaining of the DC voltage magnitude in 700 V within 0.4 seconds, zero error for the presence of DC component, improved settling time and minimal attainment of THD as 1.2% for balanced load and 3.79% for unbalanced load. In the future the work can be extended to improve the voltage quality of the system and the converter operation can be analyzed as a simple inverter for injecting the power derived from solar PV and as SAPF for current harmonic compensation. Instead of VSI, multilevel inverters may be used for harmonic reduction.

Rajmohan Rajalakshmi,Vairamani Rajasekaran, (2016) Improved PLL Tuning of Shunt Active Power Filter for Grid Connected Photo Voltaic Energy System. Circuits and Systems,07,3063-3080. doi: 10.4236/cs.2016.710261