Description:This invention relates to the field of power electronics, specifically to a Unified Active Power-Quality Compensator (UAPQC) device designed to improve power quality in grid-connected Electric Vehicle (EV) charging stations by mitigating both source-side and load-side power-quality impacts. The UAPQC device utilizes a feasible control algorithm to manage distorted load current and deviated grid source voltage, enhancing power-quality issues in the grid-connected EV charging systems.
Background of the invention:
The accelerated adoption of Electric Vehicles (EVs) in recent years has been driven by their numerous benefits, including lower maintenance costs, environmental friendliness, and reduced dependency on fossil fuels. While the shift towards electric transportation is essential to combat global warming and improve air quality, it has also introduced new challenges, especially in the field of power quality in the electric grid.
With the rise in the number of EVs, the demand for EV charging stations has also significantly increased. These charging stations often employ power-electronic converters such as Diode-Bridge Rectifiers (DBRs) for charging the EV batteries. While these converters are efficient in their function, they also introduce power quality issues in the grid-connected system. They are responsible for injecting harmonic distortions into the grid, thereby deteriorating the power quality. The power quality issues that arise from the charging stations can severely impact the utility-grid system, causing problems such as voltage-sags, voltage-swells, harmonic current distortions, reactive-power demand, and load unbalance.
Many power-system engineers and scientists have sought to mitigate these power quality issues by developing various compensation techniques that adopt Custom-Power (CP) methodology. Among the notable CP methods include Active Power Filters, Distributed Static Compensators (DSTATCOM), Dynamic Voltage Restorer (DVR), and Unified Active Power-Quality Conditioners (UAPQC). While these methods have their merits, none can singularly address both source-side and load-side power-quality impacts effectively.
Amidst these various compensation techniques, the Unified Active Power-Quality Compensator (UAPQC) has shown great promise. The UAPQC is a significant compensation device for addressing both voltage-current power quality issues. However, existing UAPQC devices have been limited in their effectiveness. There remains a need for a more effective UAPQC that can deliver enhanced power quality by effectively managing both load-side and source-side power quality issues.
Furthermore, current UAPQC devices require complex and often expensive control algorithms to manage the power quality issues. This makes them less feasible for widespread adoption. Therefore, a need exists for an improved UAPQC device that not only mitigates the power quality impacts but also uses a feasible control algorithm to manage the issues.
This invention was developed in response to the identified deficiencies and needs in the current state of the art. It presents an advanced version of the UAPQC device designed to mitigate both load-side and source-side power-quality impacts effectively, thereby significantly improving the power quality in grid-connected EV charging stations. The proposed UAPQC device also incorporates a feasible control algorithm, making it a more accessible and economical solution to the prevailing power quality issues in EV charging stations. Some patent prior art related to proposed invention mentioned below.
Patent No. US8902115B2: "Unified power quality conditioner (UPQC)" - This patent discloses a unified power quality conditioner (UPQC) system designed to simultaneously correct power quality problems, including reactive power, harmonic currents, and unbalanced loads in a power system. The system comprises a series compensator, a parallel compensator, and a control system that controls the compensators.
Patent No. US8803177B2: "System and method for charging electric vehicles" - This patent describes a system for charging electric vehicles that helps to manage and distribute power loads, reducing power quality issues associated with charging multiple electric vehicles simultaneously.
Patent No. US9325629B2: "Active power filter for power quality improvement" - The patent focuses on a device that can eliminate harmonic distortions in power systems. The device uses an innovative technique for harmonic identification and a control algorithm for active power filtering.
Patent No. US9822723B2: "Distributed static compensators (DSTATCOM) for voltage stabilization" - The patent discloses a DSTATCOM device used for regulating voltage and improving power quality in a power distribution network, particularly useful in handling voltage sags and swells.
Patent No. US20180284390A1: "Dynamic voltage restorer (DVR) for power quality correction" - This patent provides a DVR-based system for power quality improvement. The DVR is designed to mitigate voltage sags, swells, and other power quality issues.
Patent No. US8536903B2: "Power electronic converter for EV charging stations" - The patent describes a power electronic converter designed to charge electric vehicle batteries efficiently. It also includes features to reduce power quality problems caused by the converter.
Patent No. US9136878B2: "Compensation technique for improving power quality in electric vehicle charging" - This patent presents a compensation technique for improving power quality in electric vehicle charging systems, which effectively reduces harmonic distortions in the system.
Patent No. US20190268251A1: "Control algorithm for power quality improvement" - This patent discloses a control algorithm for managing and improving power quality in electric power systems. The method provides real-time control over power quality parameters.
Patent No. US20180332581A1: "Voltage source inverters for power quality enhancement" - This patent outlines a voltage source inverter for enhancing power quality. It features a unique control system and inverter design that optimizes power transfer while reducing harmonic distortions.
Patent No. US20200231896A1: "Method and system for improving power quality in grid-connected electric vehicle charging stations" - This patent describes a method and system for improving power quality in grid-connected EV charging stations using a combination of filtering and compensation techniques. It specifically targets the reduction of voltage and current distortions caused by non-linear loads.
Summary of the proposed invention:
The proposed invention pertains to an advanced Unified Active Power-Quality Compensator (UAPQC) device aimed at mitigating power quality impacts in grid-connected Electric Vehicle (EV) charging stations. The unique aspect of this invention is that it targets both source-side and load-side power-quality impacts, thus delivering an enhanced overall power quality.
The UAPQC device, as proposed, comprises multiple Voltage Source Inverters (VSI's) connected in a shunt-series manner. These VSIs are powered by a common DC capacitor with a desired voltage level, which aids in enhancing the existing power-quality issues. By mitigating both load-side and source-side power-quality problems in grid-connected EV charging stations, the UAPQC device can improve the overall performance and reliability of the power grid.
Additionally, this proposed UAPQC device introduces a feasible control algorithm for its operation. This control algorithm is designed to extract reference voltage and current signals by sensing the source voltage and non-linear (DBR) load currents. This innovative combination of hardware and software solutions results in more effective mitigation of power-quality issues while maintaining the efficiency of the charging process.
The performance of this proposed UAPQC device is verified under conditions with multiple EV chargers integrated. The verification process uses the Matlab/Simulink software tool, and the results are compared to existing solutions, demonstrating the effectiveness and superiority of this novel device.
In essence, this invention offers a more efficient, reliable, and economical solution to the power-quality issues prevalent in grid-connected EV charging systems, bringing significant improvements to the EV charging infrastructure and contributing positively to the widespread adoption of electric vehicles.
Brief description of the proposed invention:
The proposed invention is a Unified Active Power-Quality Compensator (UAPQC) device designed to enhance power quality in grid-connected Electric Vehicle (EV) charging stations. The block diagram of the proposed UAPQC is depicted in Fig.1 (not provided).
The UAPQC device is connected to a three-phase utility-grid source that powers the batteries in the EV charging station through a three-phase Diode-Bridge Rectifier (DBR) with a charging control. The non-linear DBR in the EV charger tends to produce significant harmonic distortions in the source current, adversely affecting the point of common coupling (PCC) of the grid-connected distribution system. The UAPQC device, therefore, serves to maintain a constant voltage profile by mitigating voltage-related power-quality issues such as voltage sags and voltage swells.
The UAPQC consists of dual Voltage Source Inverters (VSIs) connected in a series-shunt manner, a control algorithm, a DC-link capacitor unit, sensing devices, gate-drive circuitry, and a line-interface filter. The dual VSIs of the UAPQC are energized by the DC-link capacitor (Cdc) for compensating for both voltage and current-related power-quality issues.
The shunt-VSI of the UAPQC acts as active compensation for harmonic current distortions, managing source currents to achieve harmonic-free currents, load balancing, reactive power control, and power-factor correction. It injects the necessary compensation currents into the PCC of the distribution network to mitigate all current-related power-quality problems while also maintaining a constant voltage level at the PCC.
The series VSI of the UAPQC operates as a series-active power filter for the compensation of various voltage-related power-quality problems caused by the grid source voltage. It manages load voltages for compensation of voltage sags-swells and load voltage balancing.
The UAPQC of series-connected VSI is employed to compensate for all voltage-affiliated PQ issues at the PCC of the grid connected distribution network. The efficient appearance of the series-connected VSI of the UAPQC is dependent on the generation of an effective reference voltage signal by using an SRF controller with a suitable value of the exact supply voltage. Using Park's conversion method, the exact supply voltage in an ordinary abc-frame (VL.abc) is transformed into a rotating dq-frame (Vdq0.act) in a direct and quadrature axis.
[■(V_(d.act)@V_(q.act)@V_(0.act) )]=2/3 [■(cosθ&cos[θ-2π/3]&cos[θ+2π/3]@sinθ&sin[θ-2π/3]&sin[θ+2π/3]@1/2&1/2&1/2)][■(V_(L.a)@V_(L.b)@V_(L.c) )] (1)
θ=∫▒〖ω.dt〗 (2)
Where θ represents the displacement factor, ω represents the angular velocity, VL.abc is the exact load voltage in the ordinary abc frame, and Vdq0.abc is the converted direct and quadrature frames, accordingly. The voltage used as a reference is kept constant while operating, and the voltages of the dq-frame remain constant at 1p.u. and 0p.u. The actual voltage (Vdq.act) can be distinguished from a reference voltage (Vdq.ref) after the conversion to generate excellent compensation voltages along with certain error sequences. These sequences are reduced by employing a PI controller with proportional (Kpa) and integral (Kia) gain values that are appropriately tuned. The PI controller's transfer function is as follows:
U_err (s)=k_pa+k_ia/s E_err (s) (3)

for the series-connected VSI of the UAPQC. Because the error sequences are minimized, the PI controller's output is treated as a voltage reference signal (Vdq.ref*). subsequently, the reference signals in dq-frame are re-converted into the exact ABC by employing the inverse-Park's conversion method, as shown in Eqn. (5),
V_(dq.ref)^*=(V_(dq.ref)-V_(dq.act)) (4)
[■(V_(a.ref)@V_(b.ref)@V_(c.ref) )]=2/3 [■(cosθ&sinθ&1@cos[θ-2π/3]&sin[θ-2π/3]&1@cos[θ+2π/3]&sin[θ+2π/3]&1)][■(V_(d.ref)^*@V_(q.ref)^* )] (5)
Overall, the UAPQC device enhances the power-quality features in the PCC of a grid-connected distribution network while complying with IEEE standards through the use of feasible control algorithms. This solution leads to a significant improvement in power quality in EV charging stations, effectively handling the challenges posed by the grid integration of EV chargers.
, Claims:1. A Unified Active Power-Quality Compensator (UAPQC) device, configured to mitigate both source-side and load-side power-quality impacts on grid-connected Electric Vehicle (EV) charging stations.
2. The UAPQC device of claim 1, wherein said device includes multiple Voltage Source Inverters (VSIs) connected in a shunt-series manner and powered by a common DC capacitor with a specified voltage level.
3. The UAPQC device of claim 2, wherein said VSIs are configured to mitigate both load-side and source-side power-quality issues in grid-connected EV charging stations.
4. The UAPQC device of claim 1, further comprising a control algorithm designed to extract reference voltage and current signals by sensing the source voltage and non-linear Diode-Bridge Rectifier (DBR) load currents.
5. The UAPQC device of claim 1, wherein the device operates by extracting the distorted load current and deviated grid source voltage to improve power quality.
6. The UAPQC device of claim 1, further configured to inject the necessary compensation currents into the distribution network's Point of Common Coupling (PCC) to mitigate all current allied power-quality issues while also maintaining a constant voltage profile at the PCC level.
7. The UAPQC device of claim 1, wherein said device includes a series VSI that operates as a series-active power filter for the compensation of various voltage-related power-quality issues caused by the grid source voltage.
8. The UAPQC device of claim 7, wherein said series VSI manages load voltages for the compensation of voltage sags-swells and load voltage balancing.
9. A method for mitigating power-quality impacts in a grid-connected EV charging station, the method employing the UAPQC device of claim 1 to enhance power-quality features in the Point of Common Coupling (PCC) of a grid-connected distribution network.
10. The method of claim 9, wherein said method adheres to IEEE standards and uses feasible control algorithms to achieve effective mitigation of power-quality issues.