A photovoltaic integrated unified power quality conditioner with a 27-level inverter

Abstract This paper presents a Unified Power Quality Conditioner (UPQC) with a 27-level inverter based on an asymmetric H-bridge topology. Each phase of the inverter is composed of three H-bridges, supplied by three DC sources scaled in the power of three. The output of the multilevel inverter is connected directly to the point of common coupling (PCC) without the need to a transformer or a filter. The calculation of the Shunt Active Power Filter (SAPF) compensation current is based on the generalized theory of synchronous frame (d-q theory) while the calculation of a series active filter voltage is based on Instantaneous Reactive Power (p-q theory). The control of the SAPF is achieved by using a closed-loop vector control followed by a new multilevel modulation technique. In addition to the capability of harmonic elimination of both current and voltage drawn from the source, the UPQC can produce real and reactive power to feed the loads during prolonged voltage outages or source shortage. Batteries pack are used as a dc link, which is charged from photovoltaic array connected to the battery through a maximum power point tracker and charge controller. The injection of real and reactive power depends on the state of charge (SOC) of batteries, the frequency of the system, real and reactive power of the load, and power factor at the point of PCC. The proposed UPQC strategy is simulated in MATLAB SIMULINK and the results have shown a significant improved in Total Harmonics Distortion (THD) of both the voltage and currents.

a multilevel converter if the number of levels is three or more [19]. Multilevel converters are advantageous for medium voltage applications since they are capable of providing higher output voltages than the voltage that power semiconductors have to withstand. Besides, multilevel converters produce less dv/dt and have a better performance in terms of Total Harmonic Distortion (THD) when compared to two-level converters [20]. There are three classical topologies of multilevel converters, namely the neutral point clamped (or diode clamped) [21], the flying capacitor (or capacitor clamped) [22], and the cascaded multilevel converter [23]. Nevertheless, several topologies have been proposed in the literature, aiming to achieve higher numbers of levels with reduced switch counts [24][25][26][27][28][29]. Accordingly, control and modulation techniques for multilevel converters have been also widely investigated [30,31]. The objective of this paper is to show the advantages of using UPQC that is using multilevel inverter and battery storage. Compared to conventional PWM techniques, this converter, in any of its functionalities, able to produce current waveforms with negligible harmonic content. To produce active power, the system uses a battery pack kept charged using photovoltaic arrays. Furthermore, the batteries can also be charged from the mains when required. The paper describes the topology of the system and show simulations with a small 13-kVA prototype. The results demonstrate the ability of active filters to compensate harmonics in voltages and current in addition to compensate reactive power. Moreover, the paper shows the ability of such a filter to be used as a renewable energy source to support the grid in high-demand hours.

Basic Principle
The power circuit of the three-phase PV-UPQC system is shown in Figure 1. The APF is using a 27-level asymmetrical inverter. Each phase of this inverter consists of three H-bridges supplied by three independent photovoltaic arrays. A 125 w polycrystalline PV modules (Mitsubishi Electric PV-EE125MF5F) have been used to design all photovoltaic arrays. These photovoltaic arrays are delivering 12.5 kva to each phase, 10 kva for shunt active filter and 2.5 kva for the series active filter as shown in Table 1. Battery packs are used as dc link and charged from photovoltaic array connected to the battery through a maximum power point tracker and charge controller. Table 2 shows the details of the battery packs used for the series and shunt filters. The output of the shunt active filter is connected directly to the transmission line at a PCC while the series active filter is connected to the transmission line through a coupling transformer. In addition to the capability of harmonic elimination of both current and voltage drawn from the source, the combined system can produce real and reactive power to feed the loads during prolonged voltage outages or source

Multi-level Inverter
A symmetric cascaded multi-level inverter is used in this research [31]. Each leg of the inverter consists of three H-bridges connected in series and supplied by three independent photovoltaic arrays scaled in the power of three as shown in Figure 2. This configuration of the multi-level inverter helps to generate 27-level at the output of the inverter instead of generating 7 levels when using symmetric sources. The output of each H-bridge is shown in Figure 2. It can be noted that the switching frequency of the main H-bridge, which manages more than 80% of the total power, is the same frequency of the system, in this case, only 50 Hz. The frequency of the auxiliary H-Bridges are also low but increases as the voltage level of the inverter become lower in the chain. The total harmonic distortion of the output waveform of the multilevel inverter is only 1.8% while it is in the case of using 2-level inverter is 5.4%. This low THD at the output of the multi-level helps to get ride of the need for any kind of filtering at the output of the active filter.

Control Scheme
Many techniques were introduced in the literature to calculate the reference currents and voltgaes for active filters [14,15,17]. These currents and voltages will have all the components of the current and voltage that needed to be compensated by the active filter to achieve unity power factor and minimum distortion in both voltage and current of the source.

Reference Current Calculation for Shunt Active Filter
Synchronous Reference d-q method was adopted in this research to calculate the current reference for active filter [14]. The algorithm of this method is illustrated in Figure 3. This method based on transforming the load current into the load voltage synchronous frame d-q frame. In such a case the current will have two components Id and Iq. The Id current is in the direction of the load voltage and so it is the real power current component. The Iq current will be perpendicular to the load voltage and hence it is the reactive power current component.
These d-q current components will have two components. The first component is the dc component that represents the fundamental component of the real and reactive components. The ac component represents the harmonics in the current. Moreover, the injection of real power from the PVs can be done by adding a component to Id as shown in Figure 3.

Refrence Voltage Valculation for Series Active Filter
The instantaneous real and reactive method (PQ method) is adopted in this research to calculate the reference voltages for the series active filter [17]. This technique is illustrated in Figure 4. The output components Vα_ref and Vβ_ref are the harmonic components in the voltages that are needed to filter out using the series active filter.

Control of Active Power Injection for Shunt Active Filter
The injection of the active power (P) injected to the point of common coupling (PCC) depends on the SOC of all battery packs and the system frequency to ensure the stability of  Figure 5. The control system will measure the state of charge (SOC) of all batteries and the network frequency then output a signal to a multilevel inverter controller contains the optimum value of real power need to be injected. If the SOC of any battery less than or equal 0.28, this state called empty or fault battery state, then the system will charge the batteries at a maximum rate to push the state of charge (SOC) from an empty region to charging region. And If the state of charge (SOC) of all batteries greater than 0.35, and the frequency of the system is normal i.e. between 49.5 and 50.5, this state is called normal state and the system will charge the batteries at a rate that guarantee fully charged batteries at the end of sunset. During the charging process, the excessive power from PV arrays will be injected to the electrical distribution network directly by setting the real power reference current (id) according to the excessive power from the charging process.
When the state of charge (SOC) is greater than 0.5 and the frequency of the system is less than 49.5, this state called unstable network state. The system will stop charging batteries and inject all power that comes from PV arrays, also discharges the batteries at rated discharging current, by setting the real power reference current (id) according to the maximum power available. If the frequency of the system is greater than 0.5, or no real power load at distribution network then this state called non-real power load state, the system will stop power injection and charges the batteries only, by setting the real power reference current (id) to zero. These reference currents Id_ref and Iq_ref will be used then by the controller.

Control of Reactive Power Injection for Shunt Active Filter
The injection of reactive power (Q) is only necessary when the load draws reactive power from the source. In the case of empty batteries, the system will disconnect the batteries from the dc link and continue reactive power compensation by inserting of capacitor banks at the dc link of each "H" bridge inverter. So the reactive power injection did not depend on the state of charge (SOC) of batteries, this will increase the reliability of the hybrid active filter to support the reactive power load even if no power generated from photovoltaic arrays. The flowchart diagram of the control algorithm is shown in Figure 6. When there is no load connected to the source the reactive power current reference (iq_ref) set to zero and no reactive power injected to the point of common coupling (PCC). And if reactive load is connected to the distribution network, the system will automatically generate iq-ref required for all reactive power loads connected, and the source will see a unity power factor independent on the type of contaminating the load. The speed of compensation of reactive power depending on the power factor of the electrical network, so if the power factor (PF) is less than 0.85 (lagging), the system compensates at maximum available reactive power, else the system will inject the excessive power from charging process. According to the above explanation, the system works like a synchronous machine, where the reactive power is controlled through the excitation coil. In this case, it is controlled through iq_ref.

Modelling and Control of Shunt Active Filter
Assume that the active filter is connected to the PCC through a resistance and inductance as shown in Figure 7. The following equations hold true: All (1), (2), and (3) are transformed in terms of the d-q variables using the reference frame transformation as follows: both (4) and (5) can be rewritten as:  (7) where vd = (Vinvd − Vpccd) − ωLf iq (8) vq = (Vinvq − Vpccq) + ωLf id (9) both (8) and (9) are used then to design a proportional-integral controller (PI) to control the id and iq components of the inverter currents as shown in Figure 8. The output of the controllers (Vdq) is then used to generate pulses for the IGBTs in the multi-level inverter as shown in Figure 9.

Results and Analysis
The proposed system is simulated in MATLAB SIMULINK environment to check the performance and the correct working of the complete system. The simulation of Unified Power Quality Conditioner (UPQC) is tested to insure the correct working of all system parts composed together. Figure 10 shows the source current before and post the operation of the active filter. At t = 0.4 s the active power filter starts working to mitigate the harmonics and the source current became pure sinusoidal. The total harmonic distortion (THD) reduced from 18% to 0.4%. Then at t = 0.7 s, the active power injection starts with 6 A current assistant as seen in Figure 10 and the total harmonic distortion remained very low (0.5%) during active power injection.

Simulation Results for Series Active filter
The three-phase source voltage signal is shown before and post the operation of the series active filter is shown in Figure 11. At t = 0.4 s the series active filter started to mitigate the voltage harmonics. Then At t = 0.7 s, the system steps up the source voltage to 230 v and made the electrical network ideal with total harmonic distortion 1% in source voltage. It is clear from the above results, the behavior of Series APF became better and the capability of harmonic mitigation and voltage compensation became ideal because of the positive effect of shunt APF. Figure 12 proves the capability of shunt active power filter (SAPF) of keeping a unity power factor at the source. At t = 0.4 s, the shunt active filter starts to inject reactive power and the power factor has been improved from 0.78 to 0.97. Then at t = 0.7 s the active filter starts to inject real power the results in Figure 12 demonstrate that the power factor correction process hasn't been interrupted by the real power injection process.

Real Power Injection
The  Figure 13 shows the changing in source current when the system starts active power injection at t = 0.7 sec. The injection of real power has been stopped at t = 0.9 s and the source current returns to its previous value before the real power injection. Figures 14 and 15 demonstrate the effect of the frequency variation on the amount of the real power injected by the filter. At t = 0.5 s When the network frequency between the normal limits (between 49.5 Hz and 50 Hz), the system adjusts id-ref to inject real power with a certain value depending on the state of charge (SOC) of batteries as shown in Figure 14. Figure 15 demonstrates the current injection when the frequency in the normal region, id-ref is set to 15% of the rated id. On the other hand, if the SOC is set to 0.9 (for example) and the network frequency down suddenly to 48 Hz at t = 0.7 s, the controller will adjust id-ref to its maximum value and push the network frequency to the normal region as shown in Figures 14 and 15. The total harmonic distortion (THD) is reduced from 16.5% to 1% and did not change when the controller injects different levels of current to the point of common coupling. Hence the system is trusted and stable for all its tasks. Figures 16 and 17 show the effect of changing the SOC on id-ref and the amount of real power injected by the active filter. Figure 16 shows that the state of charge has been declined from 0.8 to 0.25 at t=0.7 s. The three-phase source current waveforms are shown in Figure 17. At t=0.5 s the SOC was 0.8 so the controller set the value of id_ref to the maximum as shown in Figure 17. Then at t=0.7 s the SOC was declined to 0.25 and hence the controller set id_ref to 0 as shown in Figure 17.

Conclusion
This paper has introduced a Unified Power Quality Conditioner (UPQC) integrated with Photovoltaic and using a multi-level converter. In addition to the capability to inject reactive power and filtering the harmonics, the UPQC has the capability to inject real power depending on the frequency and the state of charge of batteries. The multilevel inverter that is used has the ability to generate 27 levels that help to reduce the THD in the source currents and voltages significantly with the need for any extra filters.