Unified Interphase Power Controller Research Articles

A new robust nonlinear control strategy for UIPC in isolated hybrid microgrids

This paper focuses on the control of unified interphase power controller (UIPC) in isolated hybrid microgrids (HmGs), where the UIPC is used as the main component for interconnecting of AC and DC sub-grids. The investigated HmG encompasses two AC sub-grids and one purely storage DC sub-grid (PSDC) and multiple adjustable loads. A robust nonlinear model reference adaptive control (NMRC) strategy is offered for controlling the UIPC, keeping an acceptable two-way power exchange control with uncomplicated topology. Further, a novel configuration for the DC link power converter is suggested using the dual-active-bridge topology. Also, a harmonic-based modeling method is used to model the UIPC's power converter. The proposed configuration provides bidirectional power flow, increased power density, straightforward adaptation of zero-voltage switching, and direct access to cascading and parallelism to increase the ratings and reliability of UIPC. The simulation results illustrate the feasibility of the suggested topology for isolated HmGs.

Isolated UIPC-intertied AC-DC microgrids operation with novel dynamic flexible loads model

This work discusses the islanding operation of hybrid microgrids (HµGs) interconnected by a unified interphase power controller (UIPC). It is illustrated that considering islanding operation of HµG, the problem of power dispatch among intertied AC and DC µGs, as well as voltage and frequency control, can be handled by UIPC. The power dispatch among various AC and DC µGs using UIPC is analyzed and formulated through defining a simple demand condition function (DCF). A new control structure is proposed for the series power converter of the UIPC which enjoys the robustness of sliding mode control (SMC) concept and adopts a new energy management system (EMS) with embedded decision-making. This provides a fast, robust, and simple power flow control unit which acts as a general supervisory system for the UIPC and islanding operation of HµG. Also, a novel generalized dynamic model for flexible loads has been proposed. These loads have been directly controlled through UIPC supervisory system here. The analytical response of the proposed flexible load model is discussed and further developed using the nonlinear autoregressive-moving-average artificial neural network (ANN) model. The Levenberg-Marquardt learning algorithm is used to train the network based on the model estimation error. Besides, a new model predictive-based optimal control scheme is proposed for the flexible load. Simulation results using MATLAB support the usefulness of the proposed UIPC-based islanded HµG platform.

Affine projection‐like optimal control of UIPC in networked microgrids with controllable loads

AbstractThe upcoming generation of power systems would be outlined as a system of systems in which many AC and DC microgrids are interconnected to exchange power in a controlled manner. In this work, a new platform for the interconnection of multiple microgrids using a unified interphase power controller (UIPC) is presented. The networked microgrids operate in an islanded mode where voltage control in DC microgrids, and voltage and frequency control in AC microgrids are demanding. Therefore, we develop a new structure for the UIPC according to a current‐controlled four‐leg power converter configuration which can satisfy voltage and frequency support at the AC side. Due to many power and voltage oscillations, UIPC control is challenging in this heterogeneous environment. Therefore, the UIPC is equipped with a new affine projection‐like optimal control (APLOC) strategy which enables the microgrids to safely exchange power when necessary. Further, each AC or DC microgrid contains a new controllable load model which is commanded by the UIPC according to the power management paradigm. Simulation results proved the sufficiency of the proposed networked microgrids model in both controlling the exchanged power among microgrids and improving power quality problems.

Improving Transient Stability of a Synchronous Generator Using UIPC with a Unified Control Scheme

In this paper, the Unified Interphase Power Controller (UIPC) is utilized to protect the synchronous generator in case of faults occurring in the transmission system. The UIPC not only maintains the generator’s stability by keeping its load angle within safe operational limits but also prevents high-amplitude currents from flowing through the stator windings. This also allows for more loading on the generator without compromising the system’s stability. Moreover, utilization of the UIPC improves the LVRT capability of the generator by injecting reactive power at the faulted location. Additionally, a novel unified control scheme is proposed for the UIPC that enhances its performance by omitting the necessity of fault detection algorithms. To evaluate the performance of the proposed controller and the efficacy of the UIPC in protecting the synchronous generator under the faults, simulations have been conducted in a MATLAB/Simulink environment. A test grid was developed comprising a synchronous generator, transmission line model, UIPC, and an infinite grid representing the Point of Common Coupling (PCC), and three fault scenarios have been implemented in the transmission system. The comparative analysis of simulation results demonstrates the capability and efficacy of UIPC in isolating the synchronous generator from the faulted location, which in turn not only enhances transient stability of the generator, but also protects generator windings from detrimental faults currents. Moreover, according to the results, UIPC also contributes to recovering the voltage dip of the fault location via injecting reactive power.

Output power smoothing of wind power plants using unified inter-phase power controller equipped with super-capacitor

Due to the growing concerns regarding environmental and economic issues in power systems, nowadays the application of renewable energy sources (RES), especially wind energy has been increased. However, the increasing penetration of wind power plants (WPPs) into the grid has raised numerous challenges including output power fluctuations as well as the significant role these sources play in low-voltage ride-through (LVRT) in case of short circuit fault conditions. Energy storage systems (ESSs) are commonly employed to enhance WPP output power smoothing. On the other side, flexible AC transmission systems (FACTS) can be utilized to enhance dynamic transients of the WPPs under short circuit contingencies. This paper presents the utilization of unified inter-phase power controller (UIPC) and the super-capacitor to fulfill the aforementioned challenges in connection of WPPs to the grid. To achieve this, a novel power control scheme is proposed to compensate for output power variations. The WPP model studied here is of doubly-fed induction generator (DFIG) type wind turbine. Simulations and comparisons are performed in the MATLAB/Simulink® environment to demonstrate the effectiveness of the proposed scheme in smoothening WPP output power as well as enhancing their LVRT capability.

Low Voltage Ride Through Controller for a Multi-Machine Power System Using a Unified Interphase Power Controller

In recent years, grid-connected photovoltaic (PV) power generations have become the most extensively used energy resource among other types of renewable energies. Increasing integration of PV sources into the power network and their dynamic performances under fault conditions is an important issue for grid code requirements. In this paper, a PV source as a unified interphase power controller (UIPC) is used to enhance the low voltage ride through (LVRT) and transient stability of a multi-machine power system. The suggested PV-based UIPC consists of two series voltage inverters and a parallel inverter. The UIPC injects the required active and reactive power to prevent voltage drop under grid fault conditions. Accordingly, a dynamic control system is designed based on proportional-integral (PI) controllers for the PV-based UIPC to operate in both normal and fault conditions. Simulations are done using Matlab/Simulink software, and the performance of the PV-based UIPC is compared with the conventional unified power flow controller (UPFC). The results of this study indicate the more favorable impact of the PV-based UIPC on the system compared to UPFC in improving LVRT capabilities and transient stability.

Quasi-Luenberger Observer-Based Robust DC Link Control of UIPC for Flexible Power Exchange Control in Hybrid Microgrids

This work illustrates the application of an observer-based robust control of unified interphase power controller (UIPC) for power flow control between interconnected microgrids in hybrid microgrids. Here, the challenges of UIPC control in heterogeneous structure of hybrid microgrid are addressed, and a new control scheme for UIPC is proposed. In the proposed control scheme, the states of the UIPC dc link nonlinear dynamics are estimated using a Quasi-Luenberger observer. Then, the estimated states are implemented in a robust sliding mode controller that alleviates the effect of disturbances on the dc link dynamics and effectively controls the dc side power converter of the UIPC. The simulation results and comparison studies are provided to validate the efficacy of the proposed control scheme for hybrid microgrid.

Power flow control of hybrid micro-grids using modified UIPC

This work introduces a replacement advent for power flow control of interconnected AC-DC micro-grids in hybrid micro-grids connected to grids. It also supports implementing an Adaptive Neuro Fuzzy Inference System (ANFIS) controlled modified Unified Inter-Phase Power Controller (UIPC). For study, a classic hybrid micro-grid connected to grid comprising of a AC micro-grid and a DC micro-grid is taken into account. These micro-grids are interconnected employing a modified UIPC, rather than using the power converters connected in parallel. As the first input of this paper is the standard structure of UIPC, which used three power converters in every phase. It was then modified such as number of power converters is used less and implemented for the control of the exchange of power between AC-DC micro-grids. In every phase there is one power electronic converter in the improved structure. It is called as Line Power Converter (LPC). Also there is Bus Power Converter (BPC) to regulate the voltage of the DC bus. The Line Power Converters links the AC micro-grid to the main grid. The DC buses are also linked with them. It can be operated in Inductance Mode (IM) as well as Capacitance Mode (CM). The control structure of LPCs has an Adaptive Fuzzy Logic Controller in it. For hybrid micro-grids, the capability of the suggested power flow control strategy is confirmed by the MATLAB simulation results.

Implementation and Experimental Verification of a Novel Control Strategy for a UPFC-Based Interphase Power Controller

Replacing the phase shifting transformers of interphase power controller with a reduced rating dual unified power flow controller (UPFC) results in a unified interphase power controller (UIPC) with potential for enhanced performance. However, in most cases, the UPFC is controlled as a static phase shifter (SPS), producing marginal improvements. A control approach that allows the minimization of the power ratings of the series voltage source converters (VSCs) of the UIPC was recently proposed. For that, one has to define four control parameters, as opposed to only two in the SPS-based UIPC. This can be done off-line, by means of an optimization procedure that splits the desired transmission line current among the capacitive and inductive branches of the UIPC. In this paper, a control scheme is presented for the implementation of the novel technique. The current sharing factors obtained from the optimization are stored in a lookup table. The gating signals for the VSCs are generated using Park's transformation so as to synthesize the optimal UIPC inductive and capacitive branch currents with proportional resonant controllers and sinusoidal pulse width modulation. The transmission line angle, as well as the magnitude and phase of the desired transmission line current with respect to the voltage at the receiving end voltage, is assumed to be provided by a transmission system operator. Experimental results that include steady state and transient conditions are provided to prove its feasibility.

Power Flow Control of Interconnected AC–DC Microgrids in Grid-Connected Hybrid Microgrids Using Modified UIPC

This paper introduces a new approach for power flow control of interconnected ac–dc microgrids in grid-connected hybrid microgrids based on implementing a modified unified interphase power controller (UIPC). A typical grid-connected hybrid microgrid including one ac microgrid and one dc microgrid is considered as studied system. Instead of using the parallel-connected power converters, these microgrids are interconnected using a modified UIPC. As the first contribution of this paper, the conventional structure of UIPC, which uses three power converters in each phase, is modified so that a reduced number of power converters is implemented for power exchange control between ac and dc microgrids. The modified structure includes one power converter in each phase (line power converter (LPC)), and a power converter which regulates the dc bus voltage (bus power converter (BPC)) here. The ac microgrid is connected to the main grid through the LPCs which their dc buses are linked and can operate in capacitance mode or inductance mode. A fuzzy logic controller is used in the control structure of the LPCs. The fuzzy inference system is optimized based on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${H}_{{\infty }}$ </tex-math></inline-formula> filtering method to reduce the errors in membership functions design. Through the BPC, the dc voltage of LPCs is supplied by the dc microgrid. However, since the dc microgrid voltage is provided here by a photovoltaic system, the dc link voltage of the LPCs is fluctuating. Thus, as the second contribution to stabilize the dc link fluctuations, a new nonlinear disturbance observer-based robust multiple-surface sliding mode control strategy is presented for dc side control of the BPC. The simulation results confirm the effectiveness of the proposed power flow control strategy of the improved UIPC for hybrid microgrids.

Application of UIPC to improve power system stability and LVRT capability of SCIG‐based wind farms

This study discusses the possibility of the connection of squirrel cage induction generator (SCIG)-based wind farms (WFs) to power system through unified inter-phase power controller (UIPC) to improve the low-voltage ride-through (LVRT) capability of WF and enhance the power system stability. The UIPC is based on inter-phase power controller (IPC), where its phase-shifting transforms (PSTs) are replaced by two series converters (SECs) and one shunt converter (SHC). The operation of the UIPC is divided into normal and LVRT operation modes. In normal operation mode, the UIPC injects the total active power generated by the WF to power system and controls the power factor of the UIPC connecting point (CP) by reactive power control. In LVRT mode, the WF which is connected through UIPC, acts as a flexible AC transmission system device with capability of active and reactive power controls at CP. In this study, the UIPC model is developed based on phase angles of injected SECs voltage and a unified control scheme including UIPC SECs, UIPC SHC and pitch angle control of wind turbine, is proposed to improve LVRT capability of WF and power system stability during faults. Also, the impact of the simultaneous application of the IPC and static synchronous compensator is studied and compared with the application of the UIPC for connecting the SCIG-based WFs to the power system. The PSCAD/EMTDC simulation results show that the UIPC presents an effective solution for connecting WFs to power system.

Active and reactive power control of wind farm for enhancement transient stability of multi‐machine power system using UIPC

This study discusses the connection of wind farms (WFs) to power system through unified inter-phase power controller (UIPC) for enhanced transient stability of the power system. The power circuit of the UIPC is based on the conventional inter-phase power controller (IPC), which its phase-shifting transforms are substituted by two series converters and one shunt converter. During fault condition, the WF connected through UIPC acts as STATCOM with capability of the active and reactive power control at UIPC connecting point. Based on the UIPC model and low-voltage ride-through requirements of the new grid codes, a control system for active and reactive powers control is proposed for enhancement transient stability of power system. The proposed approach is validated in a four-machine two-area test system. Power systems computer aided design (PSCAD)/EMTDC simulation results demonstrate that the UIPC provides an effective solution for enhancement of transient stability of power system including WFs.

Power-Flow Control and Short-Circuit Current Limitation of Wind Farms Using Unified Interphase Power Controller

This paper presents analytical and simulation studies of wind farms (WFs) connected to power system through a unified interphase power controller (UIPC), which should control power flow and the short-circuit contribution of WF. The UIPC is based on an interphase power controller (IPC) in which the phase-shifting transforms are replaced by two series converters (SECs). In this paper, the UIPC model is developed based on phase angles of injected SEC voltage and a control scheme is proposed to control power flow and limit the short-circuit contribution of WF. The WF model is based on an equivalent aggregated variable-speed double-fed induction generator. The PSCAD/EMTDC simulation results show that the UIPC is an effective and novel solution for power-flow control and limitation of short-circuit contribution of WF in power systems.

Comparison of Static Phase Shifter and Unified Power Flow Controller-Based Interphase

The basic interphase power controller (IPC) presents the interesting features of simplicity and low cost but is less flexible than most flexible ac transmission system controllers. A compromise performance can be achieved by replacing the phase shifting transformers with a reduced rating dual unified power flow controller (UPFC) resulting in a unified IPC. The comparison of the basic, phase shifter-based, with a new UPFC-based, control logic is the main goal of this paper. The control variables of the dual UPFC are computed based on an optimization procedure to minimize a given cost function. The results show the higher capability of the proposed logic not only to control the current flow through the transmission line but also to achieve this task with lower branch currents that should lead to reduced power losses.

Performance analysis of distance relay in presence of unified interphase power controller and voltage‐source converters‐based interphase power controller

In recent years new FACTS devices based on voltage-source converters (VSCs) have been investigated. Unified interphase power controller (UIPC) is one of these novel FACTS controllers and the other one is VSC-based interphase power controller (VSC-based IPC).The purpose of this study is to investigate and compare the performance of distance relays on transmission lines in the presence of VSC-based IPC and UIPC, while working in power flow control mode. Simulation results show that the distance relay sees the higher apparent impedance in the presence of proposed FACTS devices. Detailed simulations for different faults types and locations are evaluated. Moreover, the impact of the FACTS device location on the apparent impedance seen by the relay is discussed.

Unified Interphase Power Controller (UIPC) Modeling and Its Comparison With IPC and UPFC

The conventional interphase power controller (IPC) can control the power flow of the transmission line. In this paper, a novel topology for IPC, called the unified interphase power controller (UIPC), is introduced. In this topology, the phase-shifting transformers (PSTs) are replaced by voltage-source converters. In this paper, the transient behavior of UIPC is modeled and investigated. The results are compared with the simulation results of IPC and UPFC. It is shown that UIPC has all of the capabilities of IPC and UPFC, such as power-flow control, fault current limitation, voltage isolation, and local bus voltage regulation simultaneously. The results also reveal that UIPC increases the variations range of control variables of IPC and, as a result, the range of variations of active and reactive powers.