Power Flow Control Method Research Articles

Operation and control of an insulated gate bipolar transistor‐based current controlling device for power flow applications in multi‐terminal high‐voltage direct current grids

One of the main problems that need to be solved to allow the realisation of multi‐terminal high‐voltage direct current (HVDC) transmission systems is the absence of a practical power flow control method. Industry leaders and researchers have proposed a few methods of power flow control based on either the control of converter station or the connection of new power electronic equipment to the grid. This study presents the operation and control of a three‐port insulated gate bipolar transistor‐based current flow control (CFC) device suitable for multi‐terminal HVDC systems. Key features and functionalities of the proposed controller including the balancing of cable currents, limiting the magnitude of cable current and current nulling are demonstrated. The three‐port CFC was simulated using power system computer aided design (PSCAD)/electromagnetic transient and direct current (EMTDC), network simulation software to evaluate its steady state and dynamic performance. Furthermore, low‐power prototypes are implemented for a two and three‐ports CFC to experimentally validate their different functionalities. Simulation and experimental studies explore the fast dynamic response and the results show that the CFC studied may have a significant role to play in the control of power flows in multi‐terminal high‐voltage DC systems.

Simple Power Flow Control Method for Reducing the Power Source Capacity and Managing the Storage Energy for Hybrid Power Source Electric Vehicles

SUMMARYElectric railway vehicles require high electric power when powering and return high electric output when regenerating. Therefore, the fall in vehicle performance due to a voltage drop and the restriction on regenerative power due to a load shortage become problems in a dc‐electrified railway. Therefore, energy storage devices (ESDs) are mounted on vehicles to assist the power of powering operation and to absorb the power of regeneration. In recent years, various hybrid vehicles have been studied because the performance of ESDs has become good enough for them to be mounted on vehicles. Generally, the design of the controller is a complicated problem because the vehicle operates under various running conditions. In this paper, a simple controller for ESDs is designed in the frequency domain. This controller aims at smoothing the input power and at managing the ESD energy. The proposed controller is examined under various running conditions with numerical simulations.

A Power Decoupling Method Based on Four-Switch Three-Port DC/DC/AC Converter in DC Microgrid

This paper presents a power decoupling method based on a four-switch three-port dc/dc/ac converter in dc microgrid. The proposed topology has three interfaces which can be connected with a dc source, a capacitor, and an ac port. With a corresponding power flow control method, this converter can decouple the low-frequency ripple power which is necessary for the ac port and undesirable for the dc source with small capacitor. The proposed topology is compact and cost-effective because, compared with the H-bridge inverter, no extra switches or devices are required. With the model of the proposed dc/dc/ac converter, a power control method with two inductors' current control is presented for power decoupling. In addition, the control parameters are designed as well. Experimental results on the laboratory prototype verified the feasibility of the proposed topology and the designed power control method.

Modeling, Control and Seamless Transition of Bi-directional Battery-Driven Switched Reluctance Motor/Generator Drive based on Integrated Multiport Power Converter for Electric Vehicle Applications

This paper proposes a generalized control method for battery-driven switched reluctance motor (SRM) drives based on integrated multiport power converter (IMPC) with small-ripple bidirectional power flow ability in electric vehicles. Large power ripple caused by the current commutation is a challenge issue for SRM drives, which would lower the system efficiency, requires extra heat dissipation in the battery and shortens its lifetime. The detailed model of the SRM drive and IMPC is developed which covers both motoring and generating modes of operation. Based on this model, an advanced multiobjective power flow control method with repetitive controller is proposed to restrain battery current ripple. Because of the unified model, a seamless transition between various modes can also be achieved. Finally, simulation and experimental results demonstrate the feasibility and superior performance of the proposed control method.

Simple Power Flow Control Method for Reducing the Power Source Capacity and Managing the Storage Energy for Hybrid Power Source Electric Vehicles

SUMMARY Electric railway vehicles require high electric power when powering and return high electric output when regenerating. Therefore, the fall in vehicle performance due to a voltage drop and the restriction on regenerative power due to a load shortage become problems in a dc-electrified railway. Therefore, energy storage devices (ESDs) are mounted on vehicles to assist the power of powering operation and to absorb the power of regeneration. In recent years, various hybrid vehicles have been studied because the performance of ESDs has become good enough for them to be mounted on vehicles. Generally, the design of the controller is a complicated problem because the vehicle operates under various running conditions. In this paper, a simple controller for ESDs is designed in the frequency domain. This controller aims at smoothing the input power and at managing the ESD energy. The proposed controller is examined under various running conditions with numerical simulations.

Transient Stability Improvement with Unified Power Flow Controller Using Fuzzy Logic and ANFIS Approach

Transient stability is an important aspect in planning and operation of an electric power system. The application of Fuzzy logic and Adaptive Neuro Fuzzy Inference System (ANFIS) approach to power system problems has received increased attention in recent years. This paper investigates the fuzzy logic based Unified Power Flow Control (UPFC) methods namely in-phase voltage control and quadrature voltage control in improving transient stability of power system using energy function method. The potential of the two control methods in transient stability enhancement is identified in Single Machine Infinite Bus System (SMIB) system. UPFC is connected in SMIB system in middle of the transmission line. Results indicate that significant reductions in the transient swings are obtained with UPFC. The result shows that the transient stability margin is substantially improved with in-phase voltage control higher than the quadrature voltage contro1.

Study on Dynamic Performance of UPFC Based on PSASP

An improved power flow control method of UPFC was proposed with consideration of the dynamic performance of dc capacitor: the line power flow was controlled based on current prediction in the series side, while the bus voltage and the dc capacitor voltage were controlled by the modulation ratio and phase shifting angle, respectively. The transient model of UPFC was established by Power System Analysis Software Package (PSASP) using its User Program Interface (UPI). The simulation results indicate that the model shows a good dynamic performance of UPFC.

Arctan Power–Frequency Droop for Improved Microgrid Stability

The term microgrid is usually reserved for a modest sized, local distributed generation network that will largely operate standalone (i.e., without a grid connection.) The most common power flow control method utilized in a standalone microgrid is a technique known as power-frequency droop. This paper introduces the concept of utilizing an arctan function for the power-frequency droop profile. The use of this arctan function improves the small signal stability of the two-inverter microgrid, provides natural frequency bounding, and is flexible in its application. SABER simulations are performed to obtain the operating points about which the system is linearized for the stability analysis. Experimental results obtained from a dSPACE-controlled, low-voltage, two-inverter hardware system are presented to verify the theoretical and simulation results.

Application of the direct Lyapunov method for robust finite-time power flow control with a unified power flow controller

Unified power flow controller (UPFC) is one of the most versatile and complex flexible AC transmission system devices to have emerged with a proven capability of instantaneous control of transmission line parameters. This study presents an approach based on the direct Lyapunov stability theory with finite-time convergence and chattering free characteristics to improve the power flow control in transmission lines using a UPFC. A state variable control strategy is derived and implemented to tackle the problems of reference tracking, robustness against parameter uncertainty and external disturbances. The main goal of the presented control system is power flow control with finite-time convergence of system states'. The chattering phenomena and discontinuity of the controller that is common in finite-time controllers are also removed to obtain a continuous and smooth controller. Simulation results are given to illustrate the effectiveness of the proposed algorithm. It is shown that the settling time of the system enhanced with the proposed controller is significantly less than the conventional non-linear controllers. As the most simply measurable states of the system are used in the suggested controller, there is no need to design a state space variable observer system. The proposed controller is investigated on the UPFC connected to a twobus power system.

A New Power Flow Control and Management Method Based on Multi-Line Transmission/Distribution Networks

A new concept of using a PV solar power plant as interline PV system is introduced in this paper. As the name suggests, the interline PV system interconnects two (possibly more) transmission/distribution lines by reconfiguring existing PV solar plant inverters. This newly developed system thus can act as a FACTS device providing a flexible control over both active and reactive powers on multiple lines simultaneously.

A power-flow control method for hybrid active front-end converters

The hybrid active front-end converter, which is composed of a capacitor and a voltage source converter in series connection, has been presented recently. Compared with grid-connected converter, the hybrid converter could be operated with reduced dc voltage as well as small-sized passive filters due to series-connected capacitor. This paper presents a power flow control method for a hybrid active front-end converter. Real power of the converter is controlled by the output voltage vector perpendicular to the grid voltage, whereas reactive power of the converter is determined by the output voltage vector parallel to the grid voltage. By dynamically adjusting voltage vector, power conversion between low-voltage dc side, such as PVs or batteries, and high-voltage grid side could be accomplished with no low-frequency transformer required. In addition, a harmonic resistance is emulated in the proposed method to avoid inrush current flowing into the converter due to unintentional voltage spike coming from the power system. Operation principles are explained in detail, followed by both computer simulations and experimental results validating effectiveness.

Preliminary Design of Power Controller Devices Using the Power-Flow Control and the Ideal Phase-Shifter Methods

This paper introduces a new method for the preliminary design of power controllers (PCs) in the electric power grid. The method, which is denoted the ideal phase-shifter (IPS) method, utilizes the concept of the power controller plane where the active power of the PC line is plotted versus the difference in voltage angle between the PC terminals. The power controller plane makes it possible to graphically visualize the working area of a PC in a power grid and thus determine the grid situations which are dimensioning for the PC. The IPS method offers the possibility of plotting the grid characteristics in the power controller plane which are unbiased with respect to the reactive properties of the PC. This makes the method suitable for comparison and preliminary design of PCs of different types and with different characteristics by simple geometrical considerations. In this process, the IPS method uses the power-flow control method for deriving the PC characteristics. This paper includes an application example of the method where it is used for dimensioning of two different PCs in a 26-bus test system.

Multiple soft-switching operating points-based power flow control of contactless power transfer systems

This study proposes a new power flow control method of a resonant converter used for contactless power transfer applications based on a multiple soft-switching operating points (MSSOP) approach. The essential idea is to switch the converter to hop among different zero-current-switching (ZCS) operating points. The proposed MSSOP controller consists of two closed loops. The inner loop achieves soft-switching by detecting the zero crossing instants of the current and controlling the semiconductor switches synchronously. The outer loop controls the power flow by selecting proper ZCS operating points according to the error between the measured and the reference voltages. Simulation and experiment results have verified that the proposed control method can regulate the output voltage of the system to be constant against wide load variations.

단상 계통연계 인버터의 SRF 전력제어 방법

화석에너지의 고갈과 환경오염 문제를 해결하기 위한 대안인 신재생 에너지를 이용하는 분산발전 시스템은 핵심 구성 요소인 PWM 인버터의 전력제어가 요구된다. 본 논문에서는 단상 계통연계 인버터 시스템의 전력제어를 위해 계통 임피던스를 고려한 SRF(synchronous-reference-frame) 전력제어 방법을 제안한다. 제안한 SRF 전력제어 방법은 복잡한 계통 임피던스의 추정 없이 계통 임피던스의 공칭값(nominal value)에 기반해 단상 인버터에 전압 기준값(reference)을 제공하여 독립운전 모드와 계통연계 모드에서 모두 운전이 가능함을 나타내었다. 또한, 유효전력과 무효전력의 독립적인 제어가 가능한 장점을 가진다. 계통을 포함하는 시뮬레이션을 통해 제안한 SRF 전력제어 방법의 타당성을 검증한다. 시뮬레이션 결과는 제안한 SRF 전력제어 방법을 통해 단상 계통연계 인버터 시스템의 전력 흐름을 적절히 제어할 수 있음을 보여주고 있다.

Power Management and Power Flow Control With Back-to-Back Converters in a Utility Connected Microgrid

This paper proposes a method for power flow control between utility and microgrid through back-to-back converters, which facilitates desired real and reactive power flow between utility and microgrid. In the proposed control strategy, the system can run in two different modes depending on the power requirement in the microgrid. In mode-1, specified amount of real and reactive power are shared between the utility and the microgrid through the back-to-back converters. Mode-2 is invoked when the power that can be supplied by the distributed generators (DGs) in the microgrid reaches its maximum limit. In such a case, the rest of the power demand of the microgrid has to be supplied by the utility. An arrangement between DGs in the microgrid is proposed to achieve load sharing in both grid connected and islanded modes. The back-to-back converters also provide total frequency isolation between the utility and the microgrid. It is shown that the voltage or frequency fluctuation in the utility side has no impact on voltage or power in microgrid side. Proper relay-breaker operation coordination is proposed during fault along with the blocking of the back-to-back converters for seamless resynchronization. Both impedance and motor type loads are considered to verify the system stability. The impact of dc side voltage fluctuation of the DGs and DG tripping on power sharing is also investigated. The efficacy of the proposed control arrangement has been validated through simulation for various operating conditions. The model of the microgrid power system is simulated in PSCAD.

An advanced method for active power flow control in electric power interconnections

This paper presents an advanced method for active power flow control by use of static phase shifting transformer. It is based on non-standard load-flow model, which enables more accurate evaluation of the relevant technical effects of phase shifting transformer installation. For solution of the power flow control problem defined, the special fast decoupled procedure is developed. The high numerical efficiency and simplicity of this procedure has been established on the example of real interconnection formed by the Electric Power System of Serbia and Montenegro, Romania, Bulgaria, Former Yugoslav Republic of Macedonia, Greece and Albania (ex Second UCTE synchronous zone).

Improved Power Flow Control for Contactless Moving Sensor Applications

An improved power flow control method for contactless moving sensor applications is proposed. The method allows the design of a system where sensors with different power ratings or a wide range of load variations can be implemented. A phase-controlled variable inductor is used to tune the resonant circuit of the power pickups of an inductively coupled power transfer (ICPT) system according to the actual power requirements of the sensors, thereby, helping to reduce the power losses without affecting the maximum power transfer capacity. Soft switching is achieved in the variable inductor control, and the effect of the equivalent tuning parameters on the power flow is analyzed theoretically. Simulation results show that a significant improvement of the existing controllers is achieved at no load or very lightly loaded conditions.

An efficient decoupled power flow control method by use of phase shifting transformers

This paper presents an efficient fast decoupled power flow control method e.g. method for automatic adjustment of a phase shifting transformers (PST) for specific line flows. The effects of a PST are represented by injection model and the corresponding extension of conventional load flow equations is made. Furthermore, the load flow control by means of PST is modeled by additional equations. For solution of the power flow control problem defined the special fast decoupled procedure is developed. The high numerical efficiency and simplicity of this procedure has been established on the example of real interconnection formed by the power system of Serbia and Montenegro, Romania, Bulgaria, Former Yugoslav Republic of Macedonia, Greece and Albania (Second UCTE synchronous zone).

A New Instantaneous Power Flow Control Method Using Variable Inductance Realized by Variable Active-Passive Reactance (VAPAR)

Rapid power flow control of power systems is the key concept of Flexible AC Transmission System (FACTS). The power flow is essentially restricted by a line inductance. The authors have previously proposed the Variable Active Passive Reactance (VAPAR)(1) which can generate a virtual variable inductance. In this paper, a new instantaneous power flow control method using variable inductance is proposed. The theoretical analysis shows that the power flow follows in inverse ratio of the line inductance even in a transient state assuming the simple power system with two voltage bus connected by an inductive power line. Simulations and experiments were carried out using the feedfoward-based power controller to verify the effectiveness of this method. Then, the characteristics of the power control were analyzed for different line impedances by simulations. The results show that the power flow can be quickly controlled using variable inductance in the case of a small line resistance, while transient oscillations are observed in the power flow only when a line resistance is rather large.

Comparison Studies of Unified Power Flow Controllers with Static Var Compensators and Phase Shifters

This paper reports on the comparison of ratings and control flexibility of three major FACTS devices. The locations of UPFC installation is determined by 'contingency analysis.' After power flow modelling of SVC, PS, and UPFC is introduced, a new power flow control method for UPFC is described. Four practical indices are used to evaluate the behavior of power systems. The test results on a test network clearly illustrate the effectiveness of the proposed approach.