Power Flow Control Capability Research Articles

A Multiport Embedded DC Power Flow Controller for Meshed DC Distribution Grids

Ring and meshed dc distribution grids have attracted increasing interest lately. Their multipoint and multiterminal structure facilitates flexible operational control and reliable power supply across the network. However, the dc grids implemented with existing power converters lack sufficient control freedoms for proper power flow adjustment; therefore, it is necessary to develop additional dc power flow controllers (DCPFCs) to be added to these grids for improved power flow control capability. Specific to multiterminal dc networks built with modular multilevel converters (MMCs), this article proposes an embedded DCPFC (e-DCPFC) directly integrated in the MMC topology. Unlike previously proposed solutions, the e-DCPFC configuration is derived from the conventional MMC and does not need any isolation transformers or external power supply. Its other main features include modular construction, convenience for multiport extension and bidirectional power flow adjustment capability. In this article, the topology, working principle, and the coordinated control strategy for e-DCPFC and its host MMC are elaborated. Experimental results from a lab prototype verify the proposed topology and its performance under typical working conditions.

Improving the resilience of large-scale power systems using distributed static series compensators

Natural disasters such as hurricanes can cause severe damages at the transmission level, resulting in widespread power outages and renewable energy curtailment. To reduce such power outages and renewable curtailment, this study takes advantage of the power flow control capability of distributed static series compensators (DSSC) and proposes a model to optimally deploy DSSC in face of natural disasters. The model is based on stochastic optimization and can determine the optimal locations and quantities of the DSSC modules in power systems considering transmission contingencies. With a high computational efficiency, the model can meet the time frame requirement for DSSC redeployment after natural disasters to improve the resilience of large-scale, real-world power systems. The model was implemented on a modified Texas 2000-bus test system under different conditions, and results show that the proposed model can optimally allocate DSSC to mitigate transmission congestions in both normal operations and emergent situations after natural disasters. By optimally redeploying DSSC using the proposed model after natural disasters, load shedding and renewable energy curtailment can be reduced up to 100% and 90%, respectively, improving the resilience of the power system.

Dc-MMC for the Interconnection of HVDC Grids With Different Line Topologies

DC-DC converters are needed for the future development of high voltage direct current (HVDC) grids, as they allow to interconnect lines with different voltages and topologies. The dc-dc converters can increase the grid controllability adding power flow control, voltage regulation and/or fault blocking capability. The dc modular multilevel converter (dc-MMC) is a non-isolated solution proposed to interconnect HVDC systems with the same line topology. This paper proposes a new dc-MMC with a control strategy, which allows the converter to interconnect different line topologies (e.g., rigid bipole connected to a symmetric monopole). The paper presents the different line topologies in HVDC installations. Then, a mathematical model with a variable transformation is proposed for the new dc-dc converter. A control structure is proposed and implemented in Matlab/Simulink using an average arm model and simplified dc grids. The results validate the control in normal operation, fault blocking capability and post-fault scenario (degraded mode).

A current‐limiting DC circuit breaker with power flow control capability

Power flow controller (PFC), fault current limiter (FCL) and DC circuit breaker (DCCB) are important equipment in the DC grid. There are a large number of power electronic components in these equipment, resulting in high cost. In order to reduce the use of additional components, a current-limiting DC circuit breaker with power flow control capability is proposed here, which integrates the PFC, FCL, and DCCB into one equipment. Further, the main breaker in the proposed circuit breaker is shared by all lines connected to a DC bus to reduce cost. The topology and operating principle of the circuit breaker are introduced. Theoretical analysis and parameter design are carried out. Besides, the proposed circuit breaker is verified in PSCAD/EMTDC. Finally, the performance and cost are compared with existing schemes. The simulation results show that the proposed circuit breaker can effectively realize power flow control, current limiting and fault isolation at a lower cost.

Power flow control capability analysis of unified power flow controller

In the field of power system restructuring, Flexible Alternating Current Transmission System (FACTS) technology has become indispensable for alleviating the challenges of load flow control, voltage control, transient stability, and dynamic stability. The Unified Power Flow Controller (UPFC) is the fastest, most flexible, and most capable FACTS device because it has the full advantage of providing simultaneous and independent real-time control of voltage, impedance, and phase angle, which are the main power system parameters that affect system performance. This paper uses a Newton-Raphson load flow that includes UPFC to analyze how UPFC can control the flow of power.

Simplified hybrid reliability simulation approach of a VSC DC grid with integration of an improved DC current flow controller

In this paper, different reliability simulation approaches of an improved DC current flow controller (CFC) integrated voltage-sourced converter (VSC) based DC grids are investigated with the following contributions: (1) In order to achieve bidirectional power flow control and fault current blocking capability, an improved DC CFC topology with a set of reversed switches is analysed to enhance the controllable path and control margins; (2) Two different reliability simulation approaches of the improved DC CFC integrated meshed multi-terminal DC (M2TDC) grid are analysed. In Approach 1, the VSCs and their AC sides are modelled without simplification. In Approach 2, the VSCs and their AC sides are represented by ideal DC voltage/current sources to simplify the system modelling; (3) Synthesizing the merits of both approaches while avoiding the deficiencies, a hybrid reliability simulation approach is proposed with high efficiency of simulation while maintaining the accuracy; (4) In order to further enhance the simulation efficiency, a different reliability simulation approach of the ideal DC current source, namely improved ideal DC current source, is proposed. Simulation systems are established in PSCAD/EMTDC for verification.

Power flow control capability of the power transistor‐assisted Sen transformer and the unified power flow controller: a close comparison

Currently, transmission grids are subject to experience more operation and control difficulties. The use of power flow controllers (PFCs) can ease such difficulties. However, most competent PFCs need to be used. Cost-wise, the power transistor assisted Sen transformer (TAST) is attractive as compared to the unified PFC (UPFC). However, while the UPFC has three control parameters, the TAST has only two. The UPFC can regulate the voltage of its shunt bus while the TAST cannot. The main objective of this study is to investigate the extent to which the TAST and UPFC are comparable when used for power flow control. Power flow control is performed in this study for congestion management using a TAST/UPFC in a modified IEEE-118 bus system. A TAST/UPFC is optimally located using sensitivity analysis. For the sake of validation, results are obtained using both load flow analysis and voltage vector analysis. With only two control parameters, the performance of the TAST is found very closely comparable to that of the UPFC. Accordingly, the TAST can competently replace the UPFC for application of power flow control, at a far lower cost. Nonetheless, appropriate limited angle TAST is a suitable substitute for the TAST at a further lower cost.

Flexible Converters for Meshed HVDC Grids: From Flexible AC Transmission Systems (FACTS) to Flexible DC Grids

Flexible Alternating Current Transmission Systems (FACTS) have achieved to enhance the flexibility of modern AC power systems, by providing fast, reliable and controllable solutions to steer the power flows and voltages in the network. The proliferation of High Voltage Direct Current (HVDC) transmission systems is leading to the opportunity of interconnecting several HVDC systems forming HVDC Supergrids. Such grids can eventually evolve to meshed systems which interconnect a number of different AC power systems and large scale offshore wind (or other renewable sources) power plants and clusters. While such heavily meshed systems can be considered futuristic and will not certainly happen in the near future, the sector is witnessing initial steps in this direction. In order to ensure the flexibility and controllability of meshed DC grids, the shunt connected AC-DC converters can be combined with additional simple and flexible DC-DC converters which can directly control current and power through the lines. The proposed DC-DC converters can provide a range of services to the HVDC grid, including power flow control capability, ancillary services for the HVDC grid or adjacent grids, stability improvement, oscillation damping, pole balancing and voltage control. The present paper presents relevant developments from industry and academia in the direction of the development of these converters, considering technical concepts, converter functionalities and possible integration with other existing systems. The paper explores a possible vision on the development of future meshed HVDC grids and discusses the role of the proposed converters in such grids.

Bidirectional Power Flow Control of a Multi Input Converter for Energy Storage System

The objective of this paper is to propose a multi-input DC-DC converter with bidirectional power flow control capability. Compared to the traditional power converter, the multi-input converter (MIC) can save on the number of components and the circuit cost. Under normal conditions, the MIC is able to transfer energy from different input sources to the load. However, if the battery module is adopted, both the charging or discharging features should be considered. Therefore, the bidirectional power flow control of the MIC is necessary. On the other hand, because of the inconsistency characteristics of batteries, unbalanced circuit operation might occur whereby the circuit and the battery might be damaged. Therefore, dynamic current regulation strategies are developed for the MIC. Consequently, the proposed MIC circuit is able to achieve the bidirectional power flow control capability as well as control the input currents independently. Detailed circuit analysis and comprehensive mathematical derivation and of the proposed MIC will be presented in this paper. Finally, both simulation and experimental results obtained from a 500 W prototype circuit verify the performance and feasibility of the proposed bidirectional multi-input converter.

Suggestion of a New Protection Scheme for a Transmission System Equipped with a Thyristor-Controlled Series Capacitor

A thyristor-controlled series capacitor (TCSC) is employed to a transmission line in order to enhance the usable capacity of the present as well as upgraded lines, improve system stability, reduce losses, and improve power flow control capability. However, in an abnormal situation, the TCSC may transit from the existing operation mode to the other mode according to its control system and protection strategy. There is much difference in the impedance of the TCSC between each mode. This threatens the reliability of the conventional protection system, especially the distance relay, that works based on the measurement of line impedance. In this paper, we suggest a new protection scheme for a distance relay of a transmission line equipped with a TCSC. In the suggested method, in order to mitigate the effect of the TCSC in the fault loop, the TCSC injected voltage is subtracted from the measured phase voltage before supplying the voltage signal to the distance relay. The suggested scheme was verified by a real time digital simulator (RTDS)-based closed-loop test bed of a protective relay. The effect of the TCSC in the fault loop was completely mitigated. The distance relay works properly with the suggested scheme.

Power Flow Control in Multi-Terminal HVDC Grids Using a Serial-Parallel DC Power Flow Controller

Multi-terminal HVDC (MT-HVDC) grids have no capability of power flow control in a self-sufficient manner. To address this important issue, utilization of dc–dc high power and high-voltage converters is motivated. However, proposing suitable partial-rated dc–dc converters as well as their suitable modeling and control in both primary and secondary control layers as well as the stability analysis are the existing challenges that should be alleviated beforehand. This paper addresses the control of power flow problem through the application of a power converter with a different connection configuration, namely, serial parallel dc power flow controller (SPDC-PFC). The SPDC-PFC input is the transmission line voltage, and its output is transmission line current. Therefore, employing a full-power dc–dc converter is avoided as a merit. Additionally, in this paper, the common two-layer MT-HVDC grid control framework comprised of primary and secondary layers is efficiently modified in order to integrate the SPDC-PFC. A differential direct voltage versus active power droop control scheme is applied to the SPDC-PFC at the local control layer, guaranteeing dynamic stability, while an extended dc power-flow routine—integrating the SPDC-PFC—is developed at the secondary control layer to ensure the static stability of the entire MT-HVDC grid. The proposed control framework enables the SPDC-PFC to regulate the flow of current/power in the envisioned HVDC transmission line. From the static and dynamic simulation results conducted on the test CIGRE B4 MT-HVDC grid, successful operation of the proposed SPDC-PFC and control solutions are demonstrated by considering power flow control action. In more detail, the SPDC-PFC successfully regulates the compensated lines’ power to the desired reference both in static and dynamic simulations by introducing suitable compensation voltages. In addition, good dynamic performance under both SPDC-PFC power reference and wind power-infeed change is observed.

Improved Control Strategy for Bidirectional Single Phase AC-DC Converter in Hybrid AC/DC Microgrid

This paper presents the improved control strategy with optimal switching scheme to improve the bidirectional converter's power flow control capability and to allow for different modes of operation in hybrid AC/DC microgrid. Also, this improved control strategy eliminates the need of PI controller for independent reactive power control. With the improvements in digital controllers, it is possible to implement complex control circuitry easily. The performance of bidirectional converter with improved control strategy is verified first in simulation environment and then on lab scale experimental set-up with the control circuit is implemented using hardware in loop in the real-time digital simulation platform. The simulation and experimental results for all the cases show that the controller is highly capable for power transfer between AC and DC grid with very fast response and also control reactive power without additional PI controller. Also, it maintains the current total harmonic distortion (THD) within standard limits.

Power transistor-assisted Sen Transformer: a novel approach to power flow control

The importance of flexible power flow control in electric power transmission networks is increasing owing to many factors that have arisen. Sen Transformer (ST) is one of the attractive power flow controllers. It has a wide control range, independent active and reactive power flow control capability, and is economically attractive. However, it operates in stepwise mode, has limited operating points, relatively slow response rate, and suffers from compensation error. Motivated by the limitations of the ST, power transistor assisted ST (TAST) as a novel power flow controller is proposed in this paper. The TAST consists of a highly-rated ST and a lowly-rated AC chopper based transistorized ST (TST). This paper first points to the importance of transmission lines’ power flow control and reviews available power flow control devices. It then introduces the proposed TAST, determines the ratings of the TST and the switching pattern of its choppers. Next, it demonstrates the steady-state performance of the TAST, determines its control limits, compares the TAST to the ST and the unified power flow controller (UPFC), and the different control strategies of the TAST. In this paper, the TAST is modeled in MATLAB/SIMULINK and tested in an equivalent two bus system and in the IEEE-14 bus test system. The work also demonstrates improvement of the response rate of the ST. Cost analysis of the TAST is also done and is compared to that of the equivalent UPFC.Based on the results, the TAST realizes continuous, error-free, more flexible operation, improved response rate, and low cost. Moreover, the TAST provides 14.62% wider power flow control range. In conclusion, the TAST's operational characteristics are closely comparable to that of the UPFC with the advantage of lower cost and extended control area.

Power flow controlling using SSSC based on matrix converter via SA-PSO algorithm

Abstract: The static synchronous series compensator (SSSC) is one of the series flexible AC transmission system (FACTS) devices that can be used in power flow control and reactive compensation in transmission lines. In this paper, a new configuration of SSSC based on a matrix converter is presented. The proposed SSSC has some advantages such as elimination of the DC link, bidirectional power flow control capability, fast dynamic response, and simple control system. By presenting a switching algorithm for the AC-AC matrix converter, an almost sinusoidal series compensation voltage is injected to the transmission line with low total harmonic distortion. In this paper, a modified particle swarm optimization (PSO) algorithm is used to obtain the optimal parameters of a power flow controller. The modified PSO algorithm is simulated annealing-PSO. Application of the proposed SSSC in different operating points is simulated by MATLAB/Simulink software. Simulation results verify that the designed control system enables the proposed SSSC topology with independent control of active and reactive power flow in transmission lines. Presented simulation results confirm the validity and effectiveness of the suggested SSSC in power flow controlling system.

Fuzzy Control of Supercapacitor Current in Hybrid Diesel Generator/Fuel Cell Marine Power System

This paper presents a power control strategy for a marine power system made up of a hybrid diesel generator, a fuel cell, and an energy storage unit. For this purpose, a self-tuning fuzzy control is designed to manage the power generation between power sources during different maneuverings and voltage disturbances (both balanced and unbalanced) in an AC system. As a solution, a current control strategy using a voltage source converter is presented. Simulation results show the response of the whole system under a test driving cycle and this variety of voltage disturbance conditions. They illustrate the performance, including power flow control and voltage disturbance ride-through capability, of the proposed control strategy.

Sensitivity Model to Power Flow Control Capability Using Controllable Series Compensators

The series compensator, such as the thyristor controlled types, is often applied to high-voltage transmission to control power flow distribution and avoid overload/overvoltage. Various power flow schemes are needed to quantify its control capability which is related to network topology and load level. Sensitivities of bus voltages and power flows to the controlled line reactance, current magnitude, and active power flow, are given to quantify the control effect of the controllable series compensation under different operation conditions. Feasible setting to the controlled parameters within the adjustable range of the series reactance is estimated. The proposed model is applied to the ultra high-voltage (UHV) transmission, whose line parameters are lumped from the distributed line model. The numerical results on WSCC 9-bus system are proposed to validate control effect and error of the proposed model.

Grid-tie Three-phase Inverter With Active and Reactive Power Flow Control Capability

This paper proposes a methodology for the active and reactive power flow control, applied to a low voltage grid-tie three-phase power inverter. The control technique is designed by means of feedback linearization and the pole placement is obtained using Linear Matrix Inequalities (LMIs) together with D-stability concepts. Through multi-loop control, the power loop uses adapted active and reactive power transfer expressions, in order to obtain the magnitude of the voltage and the power transfer angle to control the power flow between the distributed generation (DG) and the utility grid. The state-feedback linearization technique is applied at the whole control system in order to minimize the nonlinearities of the system, improving the controller's performance and mitigating potential disturbances. The methodology main idea is to obtain the best controllers with the lowest gains as possible placing the poles in the left-half s-plane region specified during the design procedure, resulting in fast responses with reduced oscillations. Demonstrating the feasibility of the proposal a 3kVA three-phase prototype was experimentally implemented. Furthermore, experimental results demonstrate anti-islanding detection and protection against over/under voltage and frequency deviations.

Unified Power Flow Controller (UPFC) Integrated with Electromagnetic Energy Storage System for System Stability Enhancement

In this paper analysis the UPFC integrated with energy storage devices for improvement of system stability compensation. Before that, used traditional UPFC is to control all transmission line parameters simultaneously or selectively but don’t have appropriate control for throughout system. During large transients; traditional UPFC have restricted capability of power flow control. In this paper to do reduce or eliminate that negative aspect of established UPFC using substantial energy storage devices adapted with UPFC. In this proposed system Integration of Superconducting Magnetic Energy Storage (SMES) into UPFC is described. SMES is connected to UPFC through an interface with DC-DC chopper. UPFC with SMES system will inject or absorb real and reactive power to or from an influence system at really quick rate on a repetitive beginning of stability problem. Here Comparative Analysis of the two types, one is integration of electromagnetic energy storage into UPFC and another is integration of electrochemical energy storage into UPFC is done by means of MATLAB/SIMULINK software package.

Matrix converter for static synchronous series compensator using cooperative bacteria foraging optimisation

Static synchronous series compensator (SSSC) is utilised for power flow control and reactive power compensation in transmission lines. In this paper, a new configuration of SSSC based on matrix converter is presented. The proposed SSSC has various advantages over traditional one. It is recognised by bi-directional power flow control capability, elimination of the DC-link, fast dynamic response and simple control system. Using the introduced switching algorithm for ac/ac matrix converter, an approximately sinusoidal series compensation voltage is injected to transmission line with low total harmonic distortion. The optimal parameters of power flow controller are achieved by using cooperative bacterial foraging optimisation. The proposed SSSC is tested at different operating condition by using MATLAB/Simulink software. Results show that the designed control strategy with the new SSSC topology is robust against sudden changes in active and reactive power flow. As well as, it copes significantly with symmetrical and asymmetrical faults conditions. Finally, measurements of system performance validate the proposed technique and emphasise its feasibility.

Two-Level Dynamic Stochastic Optimal Power Flow Control for Power Systems With Intermittent Renewable Generation

High penetration of intermittent renewable energy imposes new challenges to the operation and control of power systems. Power system security needs to be ensured dynamically as the system operating condition continuously changes. The dynamic stochastic optimal power flow (DSOPF) control algorithm using the Adaptive Critic Designs (ACDs) has shown promising dynamic power flow control capability and has been demonstrated in a small system. To further investigate the potential of the DSOPF control algorithm for large power systems, a 70-bus test power system with different generation resources, including large wind plants, is developed. A two-level DSOPF control scheme is proposed in this paper to scale up the DSOPF algorithm for this 70-bus system. The lower-level area DSOPF controllers control their own area power network. The top-level global DSOPF controller coordinates the area controllers by adjusting the inter-area tie-line flows. This two-level architecture distributes the control and computation burden to multiple area DSOPF controllers, and reduces the training difficulty for implementing the DSOPF control for a large power network. Simulation studies on the 70-bus power system with large wind variation are shown to demonstrate the effectiveness of the proposed two-level DSOPF control scheme.