irjet-versatile unidirectional ac-dc converter with harmonic current and reactive power compensation...

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056

Volume: 02 Issue: 04 | July-2015 www.irjet.net p-ISSN: 2395-0072

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VERSATILE UNIDIRECTIONAL AC-DC CONVERTER WITH

HARMONIC CURRENT AND REACTIVE POWER COMPENSATION

FOR SMART GRID APPLICATIONS

Sunil Patnaik 1 Hitesh Lade 2

1. Research Scholar . RGPV, Bhopal 2. Asst. Prof. Surabhi College of Engg. and Technology

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Abstract— This paper introduces a versatile unidirectional ac-dc converter with harmonic current and reactive power compen-sation. Since numerous unidirectional ac-dc converters can be connected with ac power systems, existing commercial converters possess the ability to improve substantially the stability of ac power systems by compensating harmonic current and reactive power. In this paper, the feasibility and limitations of the unidirectional ac-dc converter are explained when it is employed for harmonic current and reactive power compensation, and a control strategy for such functionalities is proposed. A MATLAB /Simulink model and a 1 kW dual boost PFC prototype board controlled by a digital signal processor are implemented to demonstrate the effectiveness of the proposed control method for improving power quality of the grid.

Key Word: Matlab , ac-dc converter , PFC prototype

1. INTRODUCTION Power factor correction (PFC) is a mandatory functionality of electronic products in the industrial and commercial market in order to mitigate grid harmonics and operate a power system economically [1], [2]. Since the load characteristics of most PFC applications such as home appliances, battery chargers, switched mode power supplies and other digital products support unidirectional power flow, the general ac-dc boost converter with step-up chopper is considered a popular topology. This is because they are low cost, simple, and their performance is well-proven. Its main task inside the system is to maintain dc-link voltage constantly in order to feed loads at different power ratings. In addition, it is necessary to control input current with a pure sinusoidal waveform in phase with input voltage [3]-[5]. Active power filters (APF) are another approach capable of improving grid power quality. Many research

endeavors have included APFs in their circuit topologies and control strategies [6]-[8]. Unlike PFC circuits, the APF is a system Conventionally, topologies with bidirectional power flow are used for APF applications. Despite their excellent perform-ance, they may not be the best solution to improve the power quality of an entire power system due to high capital and operating costs related to space and installation, as well as their intrinsic power losses. Since numerous unidirectional ac-dc converters are connected with ac power systems, existing unidirectional ac-dc boost converters can possess the ability to improve substantially the stability of ac power systems if they can compensate harmonic current and reactive power while fulfilling their basic function of furnishing constant dc-bus voltage. A small number of papers have presented these functionalities using unidirectional PFC converters [9], [10], but only either harmonic current compensation (HCC) or reactive power compensation (RPC) were considered and detailed analyses to validate their potential were not performed. It is inarguable that the unidirectional ac-dc converter has inferior functionality in the way of HCC and RPC, compared to bi-directional ac-dc converters. In this paper, the feasibility and limitations of the unidirectional ac-dc converter are explained when it is employed for HCC and RPC, and a control strategy for such functionalities is proposed. This paper starts with descriptions of control modes and analysis of local loads for the proposed system in Section II. The overall control scheme with HCC and RPC is presented in Section III. MATLAB/Simulink simulation results are shown in Section IV. Experimental results using a 1 kW dual boost PFC converter are presented in order to validate the proposed approach in Section V. Finally, Section VI concludes the paper.

2. CONTROL MODES



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The unidirectional ac-dc boost converter in this paper is realized as a bridgeless PFC converter. There are a few commercial power modules including IGBTs, gate circuits and protection circuits already available in the market, which accelerates the application of this topology to home appliances and digital products [11]-[13]. It is worthy to note that the control algorithms are almost the same as the conventional ac-dc converter using a diode rectifier and step-up chopper, except that the bridgeless converter controls ac input current while the conventional one controls rectified ac current. Fig. 1 shows a prevalent application of unidirectional ac-dc boost converters. The local loads can be considered separately as a non-linear load with harmonic current and a linear load with a poor power factor, which is indicative of mediocre power quality. Conventional converters consider the input current to be a purely sinusoidal waveform in phase with the input voltage. The proposed control method can ameliorate harmonic current and reactive power for improved grid power quality as well as regulation of dc-bus voltage. Even though the amount of HCC and RPC is limited compared to APFs, this control strategy can contribute to a more stable power system as more converters capable of HCC and RPC are available at the point of common coupling (PCC) without extra cost. The proposed unidirectional ac-dc converter has three operation modes i.e., PFC, HCC and RPC. Also, both HCC and RPC can be simultaneously used to improve the distortion and the displacement factors of the grid current.

1.1 Power Factor Correction Control PFC control strategies for single phase ac-dc converters are very common in the literature, especially those utilizing the feedback controller and feedforward controller, e.g. [5], [14] shown in Fig. 2. The duty ratio equation with an input inductor, L and its parasitic resistance, R can be obtained as

....1

where, d is the average on -time duty ratio of the switches, v dc is the output voltage in the dc-link, vg is the grid voltage, and is is the input current of the converter, respectively. Theoretically, the duty ratio in (1) should be generated for the ideal switch voltage as accurately as possible through adequate converter compensators to yield pure sinusoidal input current. In order to classify the duty ratio, d of the system in (1), the feedback duty ratio, dFB and the feedforward duty ratio, dFF can be considered

Fig -1: Proposed unidirectional ac-dc converter system connected to linear and nonlinear loads

separately. dFB produces the exact phase difference between the source voltage and the average switch voltage. dFF produces the inverse of the source voltage waveform as the average switch voltage, as shown in Fig. 3. Hence, the input current tracking is improved and the frequency range for which input admittance acts as a resistor can be extended to higher frequencies owing to this feedforward duty

1.2 Harmonic Current Compensation Nonlinear loads can be defined when the current is drawn in abrupt short pulses rather than in a smooth sinusoidal manner due to the physical properties of the loads such as adjustable speed drives, electronic lighting ballasts or solid state rectifiers. Thus, the current waveform is non-sinusoidal, and is called “distorted current” [15]. This distorted current can be decomposed into a weighted sum of sinusoids whose frequencies are integer multiples of the fundamental frequency via the Fourier series. These component frequencies are called harmonics. Harmonics cause disturbances and interferences to other electric facilities, resulting in malfunction or conductor heating. Therefore, it is desired to reduce these harmonics in power systems. Fig. 4 shows the current waveform of a typical nonlinear load using a single-phase diode rectifier. Generally, the distorted load current, inon can be written in terms of its fundamental, ifn and harmonic, ihn components as,




Fig -2 : Conventional control algorithm for unidirectional ac-dc converters

Fig -3: Duty waveforms of feedback and feed forward controllers

Fig -4: Example for non-linear load current (THD: 80%, PF: 0.705, P: 550W, Q: 200 Var)

(a)

(b) Fig -5: Current flow diagram at PCC, (a) without HCC, (b) with HCC

where, ω1 is the line angular frequency and θn is the phase difference between the source voltage and input current. Assume that the input current from the unidirectional ac-dc converter operating in PFC mode is a purely sinusoidal waveform. The grid current, ig, includes the harmonics, ihn, from a non-linear load as shown in Fig. 5(a) . These harmonics are undesirable and should be removed. If the unidirectional ac-dc converter can generate the harmonic current capable of canceling the harmonics of the nonlinear load, the grid current will be comprised of only fundamental components of the converter current and load current as shown in Fig, 5(b). Therefore, the new current reference for the current controller of the converter can be expressed as

..........(2) where Is* is the magnitude reference from the dc -bus voltage controller. The harmonic current of the load can be obtained by subtracting the measured load current from the fundamental load current which can be estimated by employing a band pass filter in real implementations.

.................(3) where Is* is the magnitude reference from the dc -bus voltage controller. The harmonic current of the load can be obtained by subtracting the measured load current from the fundamental load current which can be estimated by employing a band pass filter in real implementations. C. Reactive Power Cmpensation Unlike non-linear loads, the current waveform of a linear load is sinusoidal at the frequency of the power system [15]. The power factor is almost unity when loads behave as a pure resistor. In contrast, the power factor can be significantly exacerbated when the load is capacitive or inductive. Reactive power oscillates between the ac source and the capacitor or reactor, at a frequency equal




to two times the rated value (50 or 60 Hz) [8]. This reactive power increases the total current unnecessarily in power systems, which causes increased conduction loss or deteriorated performance of voltage regulation at the PCC. Therefore, reducing reactive power is required. Fig. 6 shows the current waveform of a typical inductive load such as a single-phase induction motor. The current flow, consisting of the converter current with RPC and the load current consuming reactive power, is shown in Fig. 7 and can be written respectively as

As a result, the grid power factor at the PCC can be improved by injecting reactive power from the converter as shown in

Fig. 6. Example for linear load current (THD: 0.5%, PF:0.8, active power: 1500W, Qreactive power: 1100 Var)

Fig -7: Current flow diagram at RPC in PCC

Fig -8: Phase diagram of the grid voltage and current during RPC

Fig. 8. However, it should be considered that the input current of the converter is distorted as shown in Fig. 9. Uncontrolled regions exist where the signs of the input voltage and current reference are opposite due to the natural commutation of diodes which causes zero-current distortion. Also, the capacitive current is more distorted due to the extended cusp distortion [4], [16]. Hence, the amount of reactive power from an individual converter should be restricted due to the current distortions. Since the current waveform of the converter in RPC mode is not sinusoidal, the phase angle of the current cannot be deduced by a simple reactive power equation. Thus, the phase angle of the converter current can be generated by using a proportional integral (PI) compensator with the error signal between reactive power command and estimated reactive power as

where Kpc and Kic are proportional gain and integral gain of the reactive power compensator, respectively and θcom is the desired phase to be shifted from the original current reference. As an alternative method, the look-up table for generating a proper phase angle can be applied in an open loop manner, but RPC performance may decline.

3. CONTROL STRATEGY FOR ACTIVE POWER FILTERS The proposed control strategy of the unidirectional ac-dc boost converter including a feedforward controller, HCC, and RPC is shown in Fig. 10. Two control blocks for HCC and RPC have been added to the conventional control




algorithm. Thus, the current reference of the converter from (2) and (7) can be expressed as,

The new current reference should be limited within its power rating and consider the harmonic distortion from the converter current. Also, it is worthwhile to mention that functionalities of HCC and RPC in unidirectional ac-dc boost converters are available only when the converter supplies power to its dc load. Thus, the current reference able to be used for HCC and RPC is highly dependent on its power rating and its existing loads. As well as the control method for an individual converter under given harmonic current and reactive power references, the supervisory control strategy when multiple unidirectional converters are available as shown in Fig. 11 can be suggested as follows.

Step 1: Calculate the compensation amount for harmonic-producing components and reactive power to improve the grid power quality.

Step 2: Obtain the available capacities from an individual converter in order to compensate HCC and RPC. Step 3: Determine and distribute commands to individual converters.

Step 4: Measure THD of the grid current. If the grid current is below 5%, the commands of RPC need to be reduced. Otherwise, the commands of RPC can be increased up to their maximum to achieve unity power factor.

Step 5: Repeat Step 1 through Step 4. Using these steps, the grid power quality can be enhanced as long as the available capacities of converters for HCC and RPC remain. Fig -9: Current waveforms with reactive power in unidirectional ac-dc converter, (a) capacitive current, (b) inductive current

Fig – 10:Proposed HCC and RPC control block diagram

Fig -11: Configuration of multiple unidirectional ac-dc converters

4. SIMULATION RESULTS In order to investigate the effectiveness and performance of the proposed control method for a unidirectional ac -dc boost converter, a 2kW bridgeless PFC converter model, a nonlinear load with 80% THD and a linear load with 0.8 PF are implemented in MATLAB/Simulink. For the evaluations of performances, the three converter operation modes are simulated: 1) HCC mode, 2) RPC mode, 3) combined operations of HCC and RPC.

4.1 Harmonic Current Compensation




Fig. 12 shows the simulation results in HCC mode when a single-phase rectifier as a nonlinear load with 80% THD of the current is connected to the unidirectional ac-dc boost converter at the PCC. The PFC operation begins with a 200V dc-bus voltage reference while the current THD is 3% and the PF is unity. However, the grid THD is polluted with the harmonic current from the nonlinear load, resulting in 17% THD. At 0.2s, the operation mode of the converter is changed from PFC to HCC. It can be observed that the grid current is a nearly sinusoidal waveform with 3% THD as a result of canceling the load harmonic current, but this also causes distortion of the converter current. 4.2 Reactive Power Compensation Fig. 13 shows the simulation results in RPC mode when a single -phase induction motor connected to the unidirectional ac-dc boost converter at the PCC is used as linear load with a poor PF of 0.8. It can be observed that the power factor of the grid is improved from 0.950 to 0.985 when the converter generates 500 Var in RPC mode. However, the THD of the grid current increases from 1.3% to 8% due to inherent distortions of reactive power current in unidirectional ac-dc converters. Thus, as explained in previous sections, the amount of reactive power used for compensation should be limited to maintain low THD of the grid current.

A. Combined Compensation Mode Fig. 14 shows the simulation results for combined operations of HCC and RPC when the two emulated loads used in previous simulations are connected at the PCC. When the converter is operating in PFC mode, the grid PF and the THD of the grid current are 0.95 and 10%, respectively. HCC and RPC begin simultaneously, the resulting grid power quality improves to 0.982 PF and 5% THD. This means that

Fig -12: Simulation results in HCC mode

Fig

-13:

Simulation results in RPC mode

Fig -14: Simulation results in combined HCC and RPC mode




Fig. 19 – summery of simulation of result the grid power quality is enhanced as a result of the proposed control method. If more converters are available at the PCC and the total amount of RPC can be larger with smaller assignments of RPC of individual converters, the grid current will be more sinusoidal and in phase with the grid voltage. Simulation results are summarized in Table Fig 19.

5. EXPERIMENTAL RESULTS Fig. 15 shows the experimental test bench for validating the effectiveness of the proposed system. A 1kW single-phase unidirectional ac-dc converter is used. The passive and electronic loads used as linear and non-linear loads are connected with the grid and converter at the PCC. Table Fig. 20 lists some important experimental conditions. A. Harmonic Current Compensation

Fig -15 : Test bench set-up

Fig -20 :experimental setup




Fig -16: shows the experimental results in HCC mode when an emulated nonlinear load with 82% THD current is connected to the unidirectional ac-dc boost converter

operating at about 700W at the PCC. In PFC operation, the current THD is 2.7% and the PF is 0.994. However, the grid THD is polluted with the harmonic current from the nonlinear load, resulting in 15.5% THD with a peak- shape waveform. When the operation mode of the converter is changed from PFC mode to HCC mode, it can be observed that the grid current can be a sinusoidal waveform with 4.5% THD along with improved power factor as a result of canceling the load harmonic current as shown in Fig. 16(c).

B. Reactive Power Compensation

Fig -17: shows the experimental results in RPC mode when apassive load consisting of several resistors and capacitors connected to the unidirectional ac-dc boost converter at the PCC is used as linear load with a poor PF of 0.779 generating reactive power of 262 Var. Due to this capacitive load, the grid power factor decreases to 0.963 even under the unity power factor of the converter. When RPC mode is enabled to consume reactive power of 300Var by the inductive current, it can be observed that the power factor of the grid is improved from 0.963 to 0.992. However, the THD of the grid current increases from 1.89% to 3.93% as shown in Fig. 17(c) due to inherent distortions of reactive power current in unidirectional ac-dc converters as explained in previous sections. Thus, the




Fig -18: Experimental results in combined HCC and RPC mode, (a) before HCC and RPC, (b) after HCC and RPC, and (c) harmonic analysis of the grid current

amount of reactive power used for compensation should be limited to maintain low THD of the grid current. C. Combined Compensation Mode

Fig. 18 shows the simulation results for combined operations of HCC and RPC when the two loads used in previous experimental tests are connected at the PCC. When the converter is operating in PFC mode, the grid PF and the THD of the grid current are 0.960 and 11.2%, respectively. HCC and RPC begin simultaneously, and the resulting grid

Fig. 21- summary of experiment Resultspower quality improves to 0.992 P.F and 4% THD at the same time. It indicates that the grid power quality can be enhanced by using the unidirectional ac-dc converter with the proposed control method. Experimental results to all test conditions are summarized in Table 3.

6. CONCLUSION Since numerous unidirectional converters are connected with ac power systems, existing unidirectional ac-dc boost converters can possess the ability to improve substantially the stability of ac power systems by maximizing the function-alities of aggregated unidirectional ac-dc boost converters. In this paper, the control method of the unidirectional ac-dc converter has been presented to enhance the grid power quality through HCC and RPC. The effectiveness of the proposed control method was validated through simulation and experimental results showing improved power factor and total harmonic distortion of the grid. At the same time, it should be noted that due to the inherent limitations of the unidirectional ac-dc converter, the grid current can be distorted unintentionally when operating in RPC mode. Hence, the amount of reactive power injected from an individual converter to the grid should be restricted.

REFERENCES

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