Active Power Exchange Research Articles

Capability Chart for Distributed Reactive Power Resources

The ubiquity of synchronous generation should not be assumed in highly renewable power systems. Various problems may consequently arise, not least of which are the difficulties entailed in maintaining regional reactive power balance. Offering a potential solution to such problems, modern renewable generator technologies offer controllable reactive power resources. As many of these generators will be embedded in distribution networks, their incorporation into transmission system operational and planning activities appears challenging. An extension of the capability chart concept offers insight here: for a given active power exchange between the transmission system and a distribution network section, the range of controllable reactive power typically available is of interest. This aggregate capability depends on the innate machine capabilities of the distributed generators and on the prevailing conditions within the distribution network. Novel optimisation techniques are useful in addressing the latter point, offering a means to identify the combination of power flow profiles within the distribution system most restrictive to reactive power provision. The capability chart thus derived gives the dependable range of reactive power available, under the assumption that each generator is operated to locally maximize its own reactive power contribution. Such a description can be applied in transmission system planning or to quantify the effects of modifications to the distribution system.

Design and Implementation of Sliding Mode and PI Controllers based Control for three Phase Shunt Active Power Filter

This paper presents a simulation and experimental comparative study of Sliding Mode Controller (SMC) and Proportional Integral (PI) regulator based the control of the DC bus voltage of three phase shunt Active Power Filter (APF). The capacitor that feeds the active filter plays the role of a voltage source. This tension must be kept constant, so as not to degrade the filter performances, and not to exceed the voltage of semiconductors. The main cause of the variation of this voltage is the change in the pollutant load, which creates an active power exchange with the network (if the inverter provides power active, then the average voltage across the capacitor will decrease and if the inverter consumes power active, then the average voltage across the capacitor will increase). The algorithm used to identify the reference currents is based on the Self Tuning Filter (STF). This study was verified experimentally, using a hardware prototype based on dSPACE-1104.

Compensation range calculation in an interline dynamic voltage restorer (IDVR) encountering different voltage sag types

SUMMARY This paper is concerned with determining the compensation capability of an interline dynamic voltage restorer (IDVR) under various balanced and unbalanced voltage sag scenarios. An IDVR consists of two or more DVRs with a common dc link which enables active power exchange among DVRs. This feature increases the compensation range of an IDVR as compared with separate but otherwise similar DVRs with no energy storage. The steady-state modeling and analysis of an IDVR is briefly explained in this paper. The expressions governing power requirement for compensating various voltage sags are derived. Consequently, the constraints associated with an IDVR operation are obtained and an algorithm is developed to calculate the compensation range of an IDVR and two similar storage-less DVRs. Illustrative graphs demonstrate the operating characteristics of an IDVR and compare these characteristics with those of two similar storage-less DVRs. Copyright © 2013 John Wiley & Sons, Ltd.

Voltage-Based Control of a Smart Transformer in a Microgrid

For the islanded operation of a microgrid, several control strategies have been developed. For example, voltage-based droop control can be implemented for the active power control of the generators and the control of the active loads. One of the main advantages of a microgrid is that it can be implemented as a controllable entity within the electrical network. This requires the ability of the utility grid to control or influence the power exchange with the microgrid by communicating with only one unit. However, little research has been conducted on controlling the power transfer through the point of common coupling (PCC). This paper addresses this issue by introducing the concept of a smart transformer (ST) at the PCC. This unit controls the active power exchange between a microgrid and the utility grid dependent on the state of both networks and other information communicated to the ST. To control the active power, the ST uses its taps that change the microgrid-side voltage at the PCC. This voltage-based control of the ST is compatible with the voltage-based droop control of the units in the microgrid that is used in this paper. Hence, the microgrid units can automatically respond to changes of ST set points and vice versa. Several simulation cases are included in this paper to demonstrate the feasibility of the ST concept.

Simulation of a Wind-Power Plant Linked to a Transmission Grid:Part II: Capacitive Compensation and Pitch Control

In the first part of this paper discussed the modeling and simulation of the components that make up a wind farm, the following is a basic study on two aspects that arise in a wind farm, the capacitive compensation and blade control. The review of the compensation done at static and dynamic problems is presented with some possible solutions. The concepts of blade control are presented and the simulation of two types of control, fixed gain and variable gain are given. The exchange of active power and reactive power between the grid and the wind farm is analyzed and the effects and solutions to various contingencies wind are shown. It shows the ability of the package ATP (Alternative Transients Program) to conduct studies of Stage I, Stage II and Stage III requested by the Compañía Administradora del Mercado Mayorista Eléctrico (CAMMESA), especially those that refer to electromagnetic transients and power quality.

Sliding mode approach to torque and pitch control for an wind energy system using FPGA

Abstract Wind energy, being a fluctuating resource, requires a tight control management to ensure stability when integrated with the grid system. This has triggered interest towards developing advanced controllers. Hence this paper presents the study of a variable speed wind energy conversion system that uses a Double Fed Induction Generator (DFIG). Above rated wind speed, pitch control has been applied and below the rated speed torque control has been adopted. Generator torque control is able to reduce the effects of the pitch actuator limitations. Sliding mode control is applied for torque and pitch control in WECS and it has been implemented in MATLAB SIMULINK and FPGA to achieve control of active and reactive power exchange between the stator of the DFIG and the grid. Performance parameters like pitch angle, active, reactive power, turbine speed, and DC voltage has been compared by using SMC, Hill Climbing (HC) Algorithm and Perturb and Observe (P&O) Algorithm and performance for these three methods has been simulated and implemented in FPGA. Total Harmonic Distortion for all the performance parameters has been reported. Hardware implementation of developed algorithm was accomplished with the help of Xilinx system generator and Xilinx Tool Kit.

Power Transfer Capability & Reliability Improvement in a Transmission Line using Distributed Power- Flow Controller

The Distributed Power Flow Controller (DPFC) is derived from the Unified power-flow controller (UPFC).The DPFC is a solution to control the power flow in a single transmission line. By eliminating the common DC link and distributing the three phase series converters of the UPFC, a new concept of the Distributed Power Flow Controller (DPFC) is achieved. The active power exchange between the two converters, which is through the common dc link in the UPFC, is now through the transmission lines at the third-harmonic frequency in the DPFC. It inherits the advantages of the UPFC and the DFACTS concept, which allow power flow control for multi-line systems with relatively low cost and high reliability without additional cost. This paper presents two types of DPFCs; one is one three phase shunt converter and four three phase series converters. Another model is one three phase shunt converter and nine single phase series converters. By using three phase series converters, no common dc link between shunt and series converters but cost will increases. By using single phase series converters, no common dc link between shunt and series converters and cost also decreases. Detailed simulations are carried out on two- machine systems to illustrate the control features of these devices and their influence to increase power transfer capability and every series converter consists of D-FACTS concept so Reliability also improves because failure of series converters does not effect.

Design and experimental verification of a dead beat power control strategy for low cost three phase PWM converters

As a cost-effective and reliable alternative to standard three phase PWM converters, the low cost converters have attracted great attentions today. While the research trends mainly focused on using these converters for AC motor drives, some successful efforts in grid connected applications are also reported. These works use the voltage oriented technique to regulate the active and reactive power exchanges with the electric grid. This paper presents a dead beat direct power control strategy for grid integration of low cost three phase PWM converters. While keeping the advantages of fast and accurate power control associated to the VOC technique, the proposed strategy offers a considerably simpler algorithm. Extensive simulation and experimental results are provided which confirm the validity of the proposed technique.

Robustness of an energy storage system-based stabiliser to suppress inter-area oscillations in a multi-machine power system

In order to understand how an energy storage system (ESS)-based stabiliser suppresses a power system inter-area oscillation in a multi-machine power system, this study proposes a tie-line power oscillation-related ESS linearised model. Based on the linearised model, a general conclusion is established to assess the capability of the ESS-based stabiliser to affect the damping of the inter-area power oscillation. Analysis derived from the general conclusion explains how and why the robustness of the ESS stabiliser is different to the variations of the loading conditions of the tie transmission line when the stabiliser is realised by controlling the exchange of either active or reactive power between the ESS and the rest of the power system. It is analytically concluded that when the stabiliser is implemented via controlling the exchange of active power, the stabiliser is of better robustness. The study presents an example multi-machine power system installed with a battery energy storage system (BESS)-based stabiliser. Results of eigensolutions computation and non-linear simulation demonstrate and confirm the analytical conclusion about the robustness of the ESS-based stabiliser in suppressing the inter-area power oscillation.

Analysis of integrated STATCOM-SMES based on three-phase three-level multi-pulse voltage source inverter for high power utility applications

This paper is aimed to investigate the operating characteristics of a static synchronous compensator (STATCOM) integrated with superconducting magnetic energy storage (SMES) for high power applications in the transmission network level. The STATCOM controller topology comprises multi-level multi-pulse neutral-point clamped-type (NPC) voltage source inverters (VSIs) using the harmonics cancellation technique, and incorporates a SMES coil. An innovative two-quadrant multi-level dc–dc converter is proposed to effectively interface the STATCOM with the superconducting coil using a buck-boost topology with neutral point voltage control capabilities; thus enabling to simultaneously control both active and reactive power exchange with the high voltage power system. A detailed analysis of major system variables is presented, including analytical results and digital simulations using the MATLAB/Simulink environment. Moreover, a three-level control scheme is designed, including a full decoupled current control strategy in the d– q reference frame with a novel controller to prevent the STATCOM dc bus capacitors voltage drift/imbalance and an enhanced power system frequency controller.

Design Strategy for Optimum Rating Selection of Interline DVR

This paper is concerned with calculating the optimum rating for two dynamic voltage restorers (DVRs) when used in an interline DVR (IDVR) structure. An IDVR consists of two or more DVRs which have a common dc link and, thus, can exchange active power. This can increase the compensation range of an IDVR compared with separate but otherwise similar DVRs. The basic operation of the DVR and IDVR is briefly explained. The limitations of IDVR operation in terms of active power exchange are explained and, based on that, the expressions governing the steady-state operation of IDVR are derived. The compensation range of an IDVR is compared with that of two separate DVRs. This paper also explores how the limitations in absorbing power from a healthy feeder can narrow the compensation range of an IDVR. After identifying and formulating various limitations in IDVR operation, a design procedure is presented to determine the optimum size (or rating) of the DVRs in an IDVR structure. In the proposed approach, all possible scenarios concerning healthy and faulty feeders are taken into consideration. Examples along with graphs and tables aid in conveying the proposed approach.

A FACTS Device: Distributed Power-Flow Controller (DPFC)

This paper presents a new component within the flexible ac-transmission system (FACTS) family, called distributed power-flow controller (DPFC). The DPFC is derived from the unified power-flow controller (UPFC). The DPFC can be considered as a UPFC with an eliminated common dc link. The active power exchange between the shunt and series converters, which is through the common dc link in the UPFC, is now through the transmission lines at the third-harmonic frequency. The DPFC employs the distributed FACTS (D-FACTS) concept, which is to use multiple small-size single-phase converters instead of the one large-size three-phase series converter in the UPFC. The large number of series converters provides redundancy, thereby increasing the system reliability. As the D-FACTS converters are single-phase and floating with respect to the ground, there is no high-voltage isolation required between the phases. Accordingly, the cost of the DPFC system is lower than the UPFC. The DPFC has the same control capability as the UPFC, which comprises the adjustment of the line impedance, the transmission angle, and the bus voltage. The principle and analysis of the DPFC are presented in this paper and the corresponding experimental results that are carried out on a scaled prototype are also shown.

Robustness of damping control implemented by Energy Storage Systems installed in power systems

An Energy Storage System (ESS) installed in a power system can effectively damp power system oscillations through controlling exchange of either active or reactive power between the ESS and power system. This paper investigates the robustness of damping control implemented by the ESS to the variations of power system operating conditions. It proposes a new analytical method based on the well-known equal-area criterion and small-signal stability analysis. By using the proposed method, it is concluded in the paper that damping control implemented by the ESS through controlling its active power exchange with the power system is robust to the changes of power system operating conditions. While if the ESS damping control is realized by controlling its reactive power exchange with the power system, effectiveness of damping control changes with variations of power system operating condition. In the paper, an example of power system installed with a battery ESS (BESS) is presented. Simulation results confirm the analytical conclusions made in the paper about the robustness of ESS damping control. Laboratory experiment of a physical power system installed with a 35 kJ/7 kW Superconducting Magnetic Energy Storage (SMES) was carried out to evaluate theoretical study. Results are given in the paper, which demonstrate that effectiveness of SMES damping control realized through regulating active power is robust to changes of load conditions of the physical power system.

Direct Power Control of Doubly-Fed-Induction-Generator-Based Wind Turbines Under Unbalanced Grid Voltage

In this paper, the behavior of a doubly fed induction generator (DFIG) is studied under unbalanced grid voltage conditions. It is shown that if no special control efforts are employed, the behavior of the generator is deteriorated, basically due to two reasons: electromagnetic torque oscillations and nonsinusoidal current exchange with the grid. These phenomena are first analyzed theoretically as a function of the stator active and reactive instantaneous power exchange by the stator of the DFIG and the grid-side converter (GSC). This analysis provides the main ideas for generation of the active and reactive power references for the rotor-side converter (RSC) and the GSC, controlled by means of direct power control techniques. Therefore, this paper proposes a new algorithm that generates the RSC power references, without the necessity of a sequence component extraction, in order to eliminate torque oscillations and achieve sinusoidal stator currents exchange. On the contrary, the GSC power references are provided by means of voltage and current sequence extraction. Finally, simulation and experimental results successfully validate the proposed power reference generation methods.

Damping torque analysis for DC bus implemented damping control

AbstractDamping torque analysis is a well‐developed technique for understanding and studying power system oscillations. This paper presents the applications of damping torque analysis for DC bus implemented damping control in power transmission networks in two examples. The first example is the investigation of damping effect of shunt voltage source converter (VSC) based flexible AC transmission systems (FACTS) voltage control, i.e. static synchronous compensator (STATCOM) voltage control. It is shown in the paper that STATCOM voltage control mainly contributes synchronous torque and hence has little effect on the damping of power system oscillations. The second example is the damping control implemented by a battery energy storage system (BESS) installed in a power system. Damping torque analysis reveals that when BESS damping control is realized by regulating exchange of active and reactive power between the BESS and power system, respectively, BESS damping control exhibits different properties. It is concluded by damping torque analysis that BESS damping control implemented by regulating active power is better with less interaction with BESS voltage control and more robust to variations of power system operating conditions. In the paper, all analytical conclusions obtained are demonstrated by simulation results of example power systems. Copyright © 2009 John Wiley & Sons, Ltd.

Methods for multi-functional converter control in three-phase four-wire systems

An active rectifier-based shunt compensator plays a vital role in present-day static power compensation. This includes the conventional compensation features such as power factor improvements, harmonic compensation and neutral current elimination in a three-phase four-wire system. Recently, in addition to conventional compensation, the compensator has also been considered for bi-directional active power exchange, simultaneously, with grid-connected systems, when used with adjustable static drives or wind generators. The authors aim to investigate the appropriate control methods for this kind of multi-functional compensation to accurately generate the reference current signals in time domain. In this context, two prominent instantaneous power theories, namely p-q theory and p-q-r theory are compared from a multi-functional perspective. The compensation based on instantaneous p-q-r theory is analysed comprehensively for the proposed compensator. A new approach to eliminate the neutral current completely from four-wire systems with p-q-r theory is suggested. A control system for multi-functional compensation is designed based on instantaneous p-q-r theory. The control algorithm is verified through simulation and experimentation for both positive and negative active power exchange across the DC bus. The experimental results are shown to verify the effectiveness of the proposed controller.

A Multifunctional Four-Leg Grid-Connected Compensator

There has been a growing demand for using active power filters (APFs) for grid-connected converter systems based on nonconventional energy sources such as solar, wind, and fuel cells. In addition to power quality conditioning, the APF can also be used for bidirectional active power exchange with the three-phase four-wire grid and, therefore, acts as a multifunctional compensator. This paper presents a four-leg shunt multifunctional grid-connected power quality compensator. The compensator reference currents are generated using the instantaneous p-q-r theory. This paper proposes a simple modification to the conventional theory to eliminate the source neutral current from the three-phase four-wire power system. A digital controller is used to implement dead-beat current control. The outputs of the digital current controller are used as reference voltages for a 3-D space vector modulator (3D-SVM). A new approach for pulse width modulation waveform generation in 3D-SVM is proposed. All the intermediate steps are discussed from the viewpoint of implementation on a digital signal processor field-programmable gate array platform. The multifunctional compensator is analyzed, simulated, and tested experimentally for the most practical conditions in the power system, and the results are presented.

Control Lyapunov Function for Series and Parallel Compensators with Energy Storage Capabilities

Control actions, and among others control Lyapunov functions based approaches, for improving the dynamic behaviour of electrical power systems are traditionally derived using a classical differential-algebraic model for the system, which neglects transmission networks dynamics. In the paper, an energy function which is based upon a completely differential model for power system description, is employed for deriving stabilising control actions for FACTS devices complemented with active power exchange capabilities, which have been proposed to provide additional improvements to power system dynamic behavior compared with reactive-only compensators. The use of the extended invariance principle is proposed to circumvent the difficulty arising from non-negative definite terms in the time derivative of the proposed energy function. The improvement of the dynamic behaviour achieved with the proposed strategy is proven by means of numerical applications.

Development of a Conduction-Cooled HTS SMES

A 35-kJ/7-kW conduction-cooled high-temperature superconductor (HTS) superconducting magnetic energy storage (SMES) is developed. This paper presents the configuration of the HTS SMES and a part of important test results. The magnet of the SMES is a solenoid-type coil made of Bi-2223/Ag tapes, and cooled to about 20 K by conduction-cooled method. The SMES is assembled in the Electric Power System Dynamic Simulation Laboratory, Huazhong University of Science and Technology. A series of tests are successfully carried out to evaluate the performance of the SMES, including the current-carrying ability of the magnet, the active and reactive power exchange capability between the SMES and a simulated power system, as well as the power-compensating effect of the SMES to the simulated power system after short-circuit fault, etc.

Static synchronous compensator with superconducting magnetic energy storage for high power utility applications

Abstract Power systems security in the case of contingencies is ensured by maintaining adequate “short-term generation reserve”. This reserve must be appropriately activated by means of the primary frequency control (PFC). Because the generation is an electro-mechanical process, the primary control reserve controllability is not as fast as required, especially by modern power systems. Since the new improvements achieved on the conventional control methods have not been enough to satisfy the high requirements established, the necessity of enhancing the performance of the PFC has arisen. At present, the new energy storage systems (ESS) are a feasible alternative to store excess energy for substituting for the primary control reserve. In this way, it is possible to combine this new ESS with power converter based flexible ac transmission systems (FACTS). This allows an effective exchange of active power with the electric grid and, thus, enhances the PFC. This paper presents an improved PFC scheme incorporating a static synchronous compensator (STATCOM) coupled with a superconducting magnetic energy storage (SMES) device. A detailed full model and a control algorithm based on a decoupled current control strategy of the enhanced compensator are proposed. The integrated STATCOM/SMES controller topology includes three level, multi-pulse, voltage source inverters (VSI) with phase control and incorporates a two quadrant, three level, dc–dc chopper as the interface between the STATCOM and the SMES coil. A novel three level control scheme is proposed by using concepts of instantaneous power in the synchronous rotating d–q reference frame. The dynamic performance of the presented control algorithms is evaluated through digital simulation performed by using SimPowerSystems of SIMULINK/MATLAB™, and technical analysis is performed to obtain conclusions about the benefits of using SMES devices in the PFC of the electric system. Presently, a laboratory scale prototype device based on digital signal processors (DSP) is being implemented.