Compensated Medium Voltage 6-Pulse CSR Using Shunt Active Power Filters : Three Different Configurations MS

The 6-pulse controlled AC/DC converter produces harmonics. The input current total harmonic distortion and the input power factor, which is firing delay angle dependent, are major drawbacks, and a compensation technique is mandatory. This paper introduces a compensated 6-pulse current sourcecontrolled rectifier with a shunt active power filter (APF) in different configurations. The shunt APF with predictive current control is coupled to the 6-pulse systems in three different compensation configurations. The APF is connected either directly to the front-end transformer primary or secondary side or via a transformer to reduce the filter side voltage. The comparison between these configuration is introduced; each configuration has merits and demerits. The comparison cannot be genuine. Simulation results are presented for a medium voltage converter which is scaled to allow low-voltage experimental confirmation.


Introduction
The 6-pulse phase controlled AC/DC converter is often employed in low voltage (LV) and medium voltage (MV) systems for domestic and industrial applications (Wu 2006;Freitas et al. 2007;Rice 1994).Although controlled converters could be a good choice for higher power applications, due to their high efficiency and reliability (Wiechmann et al. 2008) they have major drawbacks in terms of AC side power quality, input current total harmonic distortion (THD) and power factor (pf) (Bose 2009).The AC side harmonics occur at 6p ± 1 times the fundamental (p = 1, 2, 3…etc.).The magnitude of these harmonics and the overall THD do not meet input current THD standard guidelines (Williams online).Moreover, the power factor depends on the current THD and the thyristor firing delay angle ( ) (Rashid 2001).
(1) Where, I s is the root mean square (RMS) supply current and I s1 is the RMS fundamental supply current.The power factor can be calculated as: (2) Where, DPF is the displacement power factor, given by (3) Higher pulse configurations are possible (Choi and Cho 2000;Hamad et al. 2012), but the system cost and complexity are increased and the problem of power factor dependency on the firing delay angle remains (Wu 2006).A compensating technique is therefore mandatory to compensate for both the reactive and harmonic currents in order to improve the input current THD and pf (Bose 2002).APF avoids passive filter drawbacks; moreover, it is durable and reliable (El-Habrouk et al. 2000;Akagi 1996;Akagi 2005;Rahmani et al. 2010;Singh and Solanki 2009).
In this paper, the 6-pulse converter is fed from a star/star front-end step-down transformer.Three APF connection configurations are considered, all with the objective of producing sinusoidal supply currents with a near unity power factor.In reference (Akagi et al. 1986), the APF is connected across the primary side.In the other configurations, the APF is connected via a high-bandwidth step-down transformer (Cheng et al. 1999).The APF is connected across the secondary side of the transformer (Tenca and Lipo 2004).For MV applications, reducing the filter side voltage affects the system size, losses, and cost, and permits a higher switching frequency (Akagi 2005).The three compensation connections to be compared are: -The APF connected to the primary side, -The APF connected directly to the secondary side, and -A secondary side connection via a high-bandwidth step-down transformer.
The APF is controlled to compensate the main current THD and pf.Simulations for a MV system plus a scaled LV practical system are used to enable performance comparison of the three configurations.
The voltage source inverter (VSI-APF) is used as it is more convenient for APF applications since it is lighter, cheaper, and expandable to a multilevel configuration to improve the performance at high power compensation with lower switching frequencies (Routimo et al. 2007).Also, in this paper, APF operation is based on the control strategy (Hamad et al. 2012;Massoud et al. 2004a) where harmonic and reactive current extractions are achieved using capacitor voltage control (Anuradha and Kothari 1998;Huang and Wu 1999), and the current control is achieved using predictive control.This control technique is simple, suitable for DSP implementation, and provides a constant switching frequency (Massoud et al. 2004b).The design details of APF are introduced in (Hamad et al. 2012).
After the introduction, the paper is organized as follows: Section 2 explains the compensation of a 6-pulse converter using a shunt APF.Section 3 introduces configuration #1.Section 4 introduces configuration #2.Section 5 introduces configuration #3.Finally, there is a discussion and conclusion.

The 6-Pulse Converter Compensated for with a Shunt APF
For MV voltage applications, reducing the filter side voltage greatly affects system size and cost, and allows a higher switching frequency.The lower phase voltage requirement of a star connected primary is exploited, which is important in MV applications.Typical parameters for the MV transformer and the operating system parameters used in the simulations are listed in Appendix 1.A 2 kVA, 415 V, 6-pulse scaled prototype converter was used to investigate the performance of the different APF configurations.The operating conditions are common for all tested configurations; v s = 170 V, I dc = 1A (switching devices are rated to 3A), cos DPF f sw = 3.6 kHz and C = 3.2 mF.The star/star windings of the prototype front-end transformer with parameters listed in Appendix 2 has a turns ratio of N 1 / N 2 = 2.The operational environment is: -A three-phase, three-wire system -A balanced and sinusoidal voltage supply -A negligible source impedance, and -A balanced harmonic current-producing load.
All simulations and practical results are recorded for the representative phase 'a', as the system is a balanced three-phase one.

Configuration # 1
In Fig. 1, the shunt APF is connected to the primary side of the front-end Y-Y transformer.The objective was to produce a sinusoidal supply current with near unity power factor.The APF inverter DC side voltage (V dc ) determines the voltage rating of the shunt APF switches, where V dc is greater than the line peak voltage at the point of common coupling (PCC).For a MV system, the inverter requires semiconductor devices with high voltage ratings, possibly involving the series connection of switching devices.

Simulation Results
A MATLAB/Simulink (MathWorks, Inc., Natick, MA, USA) model of the MV compensated 6-pulse converter was used to study the effect of the shunt APF on the system performance.The simulation models the supply as sinusoidal, balanced, or having negligible impedance.The capacitor voltage was controlled at 7.6 kV. Figure 2 shows the simulation results of parts a-f when the 6-pulse converter operates at zero delay ( = 0 ) and the APF is connected to shown PCC.The supply phase voltage (v s ) is shown in part a.The primary current (i p ) is shown in b, while c shows the compensating current (i c ) injected by the APF.The supply current (i s ) shown in d became sinusoidal after activation of the APF as the compensating current cancels the current harmonics in ip produced by the 6pulse converter.Parts e and f show the supply current frequency spectra before and after compensation.No triplen components arise in a balanced three-phase system.

Practical Results
The LV prototype system representing configuration #1 was tested experimentally, with a 12 mH, 3-phase interfacing inductor and the capacitor DC-voltage controlled at 400 V.The test was performed with the converter operating in the rectifier mode ( = 0-90 o ).Parts a-c of Fig. 3 show the experimental results when the converter operates with = 45.Part a shows that the harmonic and reactive current components of the front-end transformer ip were compensated for by the filter current ( i f ).The compensated is sinusoidal and in phase with the v s .The spectra of the practical recorded supply current before and after activating the APF are shown in b and c, respectively.The THD improved from 28.5% to 13.4%, and the power factor improved from 0.6 to 0.912.

Discussion
Configuration #1 achieved the target of sinusoidal supply current with a near unity power factor, but may not be suitable for MV applications because the APF was connected directly to the front-end MV transformer primary.Consequently, the MV inverter capacitor is large and costly.The filter bandwidth was limited as was the switching frequency due to the high voltage, possibly comprised of series connected semiconductor devices, which increases control complexity, filter size and cost.To overcome these voltage problems, the APF can be coupled to the PCC via a highbandwidth, MV transformer, but the costs would be high (Corasniti et al. 2008).The transformer ratings increase due to the transmission of the harmonics and reactive current.An alternative solution would be to connect the APF to the front-end transformer secondary side.

Configuration # 2
APF is connected to the front-end transformer secondary as shown in Fig. 4. The same control concept is applied to all configurations, while it is detailed here for configuration #2.The transformer secondary phase voltages and currents were measured and used in the control system.The turns ratio of the front-end transformer is N 1 / N 2 = 2; thus, the voltage at the PCC was half that of configuration #1.This means that the capacitor voltage can be controlled at a lower voltage; consequently, semiconductor devices with a lower voltage rating can be used and, therefore, the operating switching frequency limit and the filter bandwidth can both be increased.The filter-side current was doubled.
A device voltage rating reduction only can be achieved if a step down transformer is used.

Simulation Results
The MATLAB model for the MV 6-pulse converter was modified to study the compensation technique using configuration #2.The voltage reduction at the PCC required a change to some of the operating parameters, as listed in Appendix 3. The simulation results for = 45 are shown in Fig. 5.The secondary phase voltage was measured then scaled to synchronize the secondary reference current (i * sec ).The supply phase voltage is shown in Fig. 5a.The converter current (i L ) is shown in Fig. 5b and the injected APF current is shown in Fig. 5c and compensates the harmonic and reactive currents.
The resulting sinusoidal transformer secondary current (i sec ) is shown in Fig. 5d.By transformer action, i sec was transformed to the primary side.The primary current was the supply current (i s ) as shown in e.The supply current was sinusoidal and in phase with the supply phase voltage.The spectra of the compensated i sec and are shown in Figs.5f and 5g, respectively.From the simulation results, it can be observed that the MV transformer had a negligible effect on the compensated system performance, as the THD of i sec and i s were virtually the same as what can be seen in configuration #1.The achieved input power factor (0.992 lagging) indicates that reactive power was absorbed by the transformer.However, the RMS filter current increases from 41.2 A, (configuration #1), to 66.5 A, (configuration #2).With the same inverter switching frequency, the same performance was achieved for both configurations, but with a lower voltage rated APF.Compensation on the secondary avoided har-  monic and the reactive currents passing through the transformer; consequently, the transformer harmonic burden was reduced.

Practical Results
Three-phase secondary side interfacing inductors of 5 mH were used and the capacitor voltage was con-

Configuration # 3
Filter side voltage reduction can be achieved by using the transformer coupling configuration shown in Fig. 7.As a case study, the turns ratio of the high bandwidth APF transformer was 2:1.This stepped down the PCC voltage to 825V and allowed the interfacing inductance to be reduced from 5 mH to 2 mH.The switching frequency was 3.6 kHz and the capacitor voltage was controlled at a reduced voltage of 1.8 kV (3.7 kV in configuration #2).The simulation results for = 45 are shown in Figs.8a-f.The frontend transformer secondary phase voltage was used to extract the i sec * .The supply phase voltage, the converter current, i L , and the i f , are shown in parts Figs.8a-c, respectively.The MV-side of the APF transformer i c is shown in Fig. 8d.The filter RMS current was 124.5 A. The i sec is shown in Fig. 8e.The compensated secondar current is shown in Fig. 8f and was sinusoidal and in phase with the supply phase voltage.The spectrum of the compensated supply current is shown in Fig. 8g.This control system achieves 29A of the RMS fundamental supply current and a THD of 13.2% with an input power factor of 0.991 lagging.The capacitor voltage was controlled at half the voltage of configuration #2 but the filter current was doubled as a result of utilizing the APF 2:1 high-bandwidth step-down transformer.
The matching transformer core material was required of being capable of transmitting the highest required harmonic compensating component frequency.

Conclusions
The APF used in three different configurations compensated both the supply current and input power factor of a 6-pulse converter system.Configuration #1 is restricted to MV applications as the operating voltage of the APF inverter semiconductor switches and the switching frequency are limited.Configurations 2 and 3 overcome the voltage problem as the compensation is on the secondary side of the front end transformer.All the configurations achieve virtually the same supply current THD and the pf is improved to near unity.Configuration 3 offers reduced voltage stresses on the semiconductor devices but the filter current is doubled with respect to configuration #2.Table 1 shows the configuration performances before compensation while Table 2 shows the configuration performance after compensation.The THD follows the standard at zero firing angle however, if the firing angle is bigger than zero the THD is increased by nature even the system is compensated.The selection of the APF position depends on voltage level and user.No clear conclusion can be made about which configuration is the best.If the voltage needs to be reduced than configuration #1 is not suitable.If there is no problem with switches current and voltage is required to be lower, configuration 3 is the preferred choice.

Figure 1 .
Figure 1.Configuration # 1 APF connected to the primary side.

Figure
Figure Configuration #2 for = 45 o ; (a) supply phase voltage, v s , (b) converter current, i t , (c) filter current i s , (d) compensated secondary current, i sec , (e) compensated supply current, isec, (f) spectrum of i sec , (g) spectrum of i s .

Figure 6 .
Figure 6.Configuration #2 practical results at =45 o : (a) current waveforms and (b) spectrum of compensated supply current i s .

Figure 7 .
Figure 7. Configuration #3 APF in the secondary side with a high quality stepdown transformer.

Figure 8 .
Figure 8. Configuration #3 at =45 o ; (a) supply phase voltage, v s , (b) converter current, i t , (c) filter current, i f , (d) compensation current, i c , (e) compensated secondary current, i sec , (f) compensated supply current, i s , (g) spectrum of compensated supply current.

Table 2 .
Configuration performance after compensation.