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POWER SYSTEM 2015

Category Archives

Data-Mining-Based Intelligent Differential Relaying for Transmission Lines Including UPFC and Wind Farms

Abstract:

This paper presents a dataminingbased intelligent differential relaying scheme for transmission lines, including flexible ac transmission system device, such as unified power flow controller (UPFC) and wind farms. Initially, the current and voltage signals are processed through extended Kalman filter phasor measurement unit for phasor estimation, and 21 potential features are computed at both ends of the line. Once the features are extracted at both ends, the corresponding differential features are derived. These differential features are fed to a datamining model known as decision tree (DT) to provide the final relaying decision. The proposed technique has been extensively tested for single-circuit transmission line, including UPFC and wind farms with in-feed, double-circuit line with UPFC on one line and wind farm as one of the substations with wide variations in operating parameters. The test results obtained from simulation as well as in real-time digital simulator testing indicate that the DT-based intelligent differential relaying scheme is highly reliable and accurate with a response time of 2.25 cycles from the fault inception.


Control strategy for Single-phase Transformerless Three-leg Unified Power Quality Conditioner Based on Space Vector Modulation

Abstract:

The unified power quality conditioner (UPQC) is known as an effective compensation device to improve power quality for sensitive end-users. This paper investigates the operation and control of singlephase threeleg UPQC (TL-UPQC), where a novel space vector modulation method is proposed for naturally solving the coupling problem introduced by the common switching leg. The modulation method is similar to the well-known space vector modulation widely used with threephase voltage source converters, which thus brings extra flexibility to the TL-UPQC system. Two optimized modulation modes with either reduced switching loss or harmonic distortion are derived, evaluated and discussed, in order to demonstrate the flexibility brought by the space vector modulated TL-UPQC. Simulations and experimental results are presented to verify the feasibility and effectiveness of the proposed space vector modulation approach as well as two mentioned modulation modes.


Modeling and Nonlinear Optimal Control of Weak/Islanded Grids Using FACTS Device in a Game Theoretic Approach

Abstract:

A nonlinear discrete-time model along with an optimal stabilizing controller using a unified power quality conditioner (UPQC) is proposed for weak/islanded grids in this paper. An advanced stabilizing controller greatly benefits islanded medium-sized grid and microgrid due to their relatively small stored energy levels, which adversely affect their stability, as opposed to larger grids. In addition, a discrete-time grid model and controller are preferred for digital implementation. Here, the discrete-time Hamilton-Jacobi-Isaacs optimal control method is employed to design an optimal grid stabilizer. While UPQC is conventionally utilized for power quality improvement in distribution systems in the presence of renewable energy, here, the stabilizing control is added and applied to the UPQC series voltage in order to mitigate the grid‘s oscillations besides UPQC’s power conditioning tasks. Consequently, the UPQC can be employed to stabilize a grid-tie inverter (GTI) or a synchronous generator (SG) with minimum control effort. When controlling the GTI associated with renewable energy sources, a reduced UPQC structure is proposed that only employs the series compensator. Next, a successive approximation method along with neural networks is utilized to approximate a cost function of the grid dynamical states, the UPQC control parameters, and disturbance, in a two-player zero-sum game with the players being UPQC control and grid disturbances. Subsequently, the cost function is used to obtain the nonlinear optimal controller that is applied to the UPQC. Simulation results show effective damping behavior of the proposed nonlinear controller in controlling both GTI and SG in weak and islanded grids.


Zero-sequence voltage injection for DC capacitor voltage balancing control of the star-connected cascaded H-bridge PWM converter under unbalanced grid

Abstract:

This paper presents a dc capacitor voltage balancing convtrol method for the starconnected cascaded Hbridge PWM converter in the static synchronous compensator (STATCOM) applications. The proposed control utilizes the zerosequence voltage injection to accomplish the dc capacitor voltage balancing, and the this operation of zerosequence voltage does not affect the original reactive power control. The proposed control method also works for the low-voltage ride-through operation. The control algorithm is verified with a 220-V 1kVA STATCOM based on starconnected cascaded PWM converter, and the test results verify that all the dc capacitor voltages are still regulated at the commanded value even as the grid voltage sag occurs.


Phase-Shifted Carrier Modulation Techniques for Cascaded H-Bridge Multi-Level Converters

Abstract:

Phaseshifted carrier modulation is an industry standard in its application to multilevel Hbridge converters. The major advantage of this scheme over level shifted and space vector modulation schemes is its inherent ability to evenly distribute losses between semiconductor devices. However, until now its implementation has necessitated a capacitor voltage balancing scheme which degrades converter harmonic performance. This paper develops two phaseshifted carrier modulation schemes which avoid harmonic degradation by exploiting an extra degree of freedom in the available switching states. Simulation and experimental results are presented that confirm the correct operation of the modulation strategies.


Full-Bridge Reactive Power Compensator With Minimized-Equipped Capacitor and Its Application to Static Var Compensator

Abstract:

This paper discusses reactive power compensators from the point of stored energy in the capacitor, and proposes a single-phase fullbridge configuration of semiconductor switches to be used with reduced equipped capacitance for reactive power compensation. By applying this concept to shunt-type static var compensator, a static synchronous compensator can be achieved with reduced-sized capacitor. Additionally, the switching loss can be reduced due to its distinctive capacitor voltage waveform, which swings at the double of the line frequency. Modulation technique and capacitor voltage control method based on the reduced capacitance and high-voltage ripple in the capacitor are proposed. The concept and control method were confirmed with small-scale experiments.


Individual Phase Current Control Based on Optimal Zero Sequence Current Separation for a Star-Connected Cascade STATCOM under Unbalanced Conditions

Abstract:

This paper proposes an individual phase current control method based on optimal zero sequence current separation for starconnected cascade STATCOMs under unbalanced grid voltage. The Individual phase current control (IPCC) is adopted to get better adaptation to unbalanced conditions. Each cluster of the cascade STATCOM has its own phase current control loop and dc voltage feedback loop, and it can generate the necessary negative sequence current automatically to rebalance power. However the zero sequence current which is unnecessary and harmful in star configuration is introduced in the process. An optimal zero sequence current separation (OZSCS) is presented to remove the zero sequence current, by reconstructing the reactive current command. This improved IPCC enables starconnected STATCOMs adapted to unbalanced conditions without the need for any power balancing algorithms. Also the operation range of the proposed control method is analyzed. Simulation researches are performed to verify the operation range analysis, and experimental operation is tested on a 25-level ±10Mvar/10kV STATCOM product installed at a wind farm.


A New Control Strategy for Distributed Static Compensators Considering Transmission Reactive Flow Constraints

Abstract:

This paper presents a new control strategy for a distributed static compensator (also known as distributed STATCOM or DSTATCOM), configured to regulate the reactive (VAr) flow at a point in a transmission system. This new control strategy takes into account the operating VAr limits of that reactive flow in determining the steady-state output of the DSTATCOM. The new control strategy applies a slow reset regulator (SRR) to slowly bias the VAr set point of the DSTATCOM master controller to maintain its steady-state output within a target bandwidth. The operating result maintains an appropriate VAr reserve level from the DSTATCOM for dynamic events in the system. This paper also presents a new algorithm to calculate the operating constraints of the SRR that reflect the VAr flow at the local or remote point in the transmission system and the allowable VAr thresholds for that flow. These allowable thresholds can be utilized to the full extent to lower the steady-state output of the DSTATCOM, maximize its VAr reserve for dynamic events and reduce equipment and associated system operating losses. Modeling, implementation, and simulation of an engineering project show that the new control strategy and algorithm are functioning properly as expected.


Impacts of High Penetration of DFIG Wind Turbines on Rotor Angle Stability of Power Systems

Abstract:

With the integration of wind power into power systems continues to increase, the impact of high penetration of wind power on power system stability becomes a very important issue. This paper investigates the impact of doubly fed induction generator (DFIG) control and operation on rotor angle stability. Acontrol strategy for both the rotor-side converter (RSC) and grid-side converter (GSC) of the DFIG is proposed to mitigate DFIGs impacts on the system stability. DFIG-GSC is utilized to be controlled as static synchronous compensator (STATCOM) to provide reactive power support during grid faults. In addition, a power system stabilizer (PSS) is implemented in the reactive power control loop of DFIG-RSC. The proposed approaches are validated on a realistic Western System Coordinating Council (WSCC) power system under both small and large disturbances. The simulation results show the effectiveness and robustness of both DFIG-GSC control strategy and PSS to enhance rotor angle stability of power system.


Application of PI and Super Twisting Drivers to Voltage Regulation of Wind farm via StatCom

Abstract:

In this paper, the steady-state and dynamic behavior of a wind farm with reactive power compensation using a StatCom devices is analyzed. An algorithm and a method for calculating the steady-state of the wind farm with SCIG and the random wind speed is proposed. A StatCom is used to minimize the effects of the wind speed variation. Two controllers are analyzed in the StatCom; one PI lineal type and the other is a non-lineal robust type known as SuperTwisting, to regulate the voltage magnitude in the connection node.


Doubly Fed Induction Generator for Wind Energy Conversion Systems With Integrated Active Filter Capabilities

Abstract:

This paper deals with the operation of doubly fed induction generator (DFIG) with an integrated active filter capabilities using grid-side converter (GSC). The main contribution of this work lies in the control of GSC for supplying harmonics in addition to its slip power transfer. The rotor-side converter (RSC) is used for attaining maximum power extraction and to supply required reactive power to DFIG. This wind energy conversion system (WECS) works as a static compensator (STATCOM) for supplying harmonics even when the wind turbine is in shutdown condition. Control algorithms of both GSC and RSC are presented in detail. The proposed DFIG-based WECS is simulated using MATLAB/Simulink. A prototype of the proposed DFIG-based WECS is developed using a digital signal processor (DSP). Simulated results are validated with test results of the developed DFIG for different practical conditions, such as variable wind speed and unbalanced/single phase loads.


Variable Forgetting Factor Recursive Least Square Control Algorithm for DSTATCOM

Abstract:

This paper proposes an implementation of distribution static compensator (DSTATCOM) for the three-phase distribution system. The functions of DSTATCOM are harmonics elimination, compensation of reactive power, and load balancing in power factor correction and voltage regulation modes. A variable forgetting factor recursive least square (VFFRLS)-based control algorithm is proposed for effective operation of DSTATCOM to estimate weighted values of active and reactive power components of load current. The proposed control algorithm is fast in convergence and has quick response. A prototype of DSTATCOM is developed in the laboratory using a digital signal processor (dSPACE 1104) and voltage-source converter and tested for various operating conditions under nonlinear load. The performance of DSTATCOM is found satisfactory in mitigating various power-quality problems with the VFFRLS-based control algorithm.


Operation and Control of an Improved Performance Interactive DSTATCOM

Abstract:

This paper proposes an improved performance interactive distribution static compensator (DSTATCOM) to address the limitations of conventional current control mode (CCM) and voltage control mode (VCM) operations. The proposed interactive scheme provides smooth transfer of modes of operation while remaining connected in the distribution system. In normal operation, the DSTATCOM operates in CCM to make source currents balanced, sinusoidal, and at a unity power factor. During voltage disturbances, the CCM operation cannot improve load voltage. In that case, DSTATCOM operation is changed to VCM, which maintains a constant voltage across the sensitive loads. Hence, this interactive DSTATCOM ensures continuous, flexible, and robust operation of the load. Moreover, the filter current requirements are reduced in the proposed scheme, which reduces the losses in the filter and feeder, improves inverter efficiency, and requires a reduced-rating inverter for sag mitigation. The principle of operation and control for both operating modes is analyzed. The detailed process of the flexible transfer between the two modes is derived. Deadbeat predictive control algorithms for CCM as well as VCM operation are developed for fast operation during mode transfers. The performance of the proposed scheme is validated by both simulation and experimental results.


Reduced-Capacitance Thin-Film H-Bridge Multilevel STATCOM Control Utilizing an Analytic Filtering Scheme

Abstract:

In this paper, an analytic capacitor voltage filtering scheme for a cascaded Hbridge (CHB) multilevel converter-based static synchronous compensator (STATCOM) is proposed. The scheme is able to filter out a low-frequency ripple of any magnitude and introduces a negligible delay to the control system. Therefore, it facilitates using high-reliability film capacitors with low capacitance instead of the conventional electrolytic capacitors. The proposed analytic filtering scheme is implemented and verified on a single-phase seven-level CHB multilevel converter-based STATCOM. The parameters’ sensitivity evaluation is provided to demonstrate practicability of the proposed scheme. Simulation and experimental results are provided to compare the performance of the proposed filtering scheme with the performance of the most commonly used second-order filter-based approach to demonstrate its effectiveness.


Modulation and Control of Transformerless UPFC

Abstract:

In this paper, a modulation and control method for the new transformerless unified power flow controller (UPFC) is presented. As is well known, the conventional UPFC that consists of two back-to-back inverters requires bulky and often complicated zigzag transformers for isolation and reaching high power rating with desired voltage waveforms. To overcome this problem, a completely transformerless UPFC based on an innovative configuration of two cascade multilevel inverters has been proposed. The new UPFC offers several advantages over the traditional technology, such as transformerless, light weight, high efficiency, low cost and fast dynamic response. This paper focuses on the modulation and control for this new transformerless UPFC, including optimized fundamental frequency modulation for low total harmonic distortion and high efficiency, independent active and reactive power control over the transmission line, dc-link voltage balance control, etc. The new UPFC with proposed control method is verified by experiments based on 4160-V test setup. Both the steady-state and dynamic-response results will be shown in this paper.


Intelligent Islanding and Seamless Reconnection Technique for Microgrid With UPQC

Abstract:

A new proposal for the placement, integration, and control of unified power quality conditioner (UPQC) in distributed generation (DG)-based grid connected/autonomous microgrid/microgeneration (μG) system has been presented here. The DG converters (with storage) and the shunt part of the UPQC Active Power Filter (APFsh) is placed at the Point of Common Coupling (PCC). The series part of the UPQC (APFse) is connected before the PCC and in series with the grid. The dc link can also be integrated with the storage system. An intelligent islanding detection and reconnection technique (IR) are introduced in the UPQC as a secondary control. Hence, it is termed as UPQCμG-IR. The advantages of the proposed UPQCμG-IR over the normal UPQC are to compensate voltage interruption in addition to voltage sag/swell, harmonic, and reactive power compensation in the interconnected mode. During the interconnected and islanded mode, DG converter with storage will supply the active power only and the shunt part of the UPQC will compensate the reactive and harmonic power of the load. It also offers the DG converter to remain connected during the voltage disturbance including phase jump.


An Integrated Dynamic Voltage Restorer-Ultra capacitor Design for Improving Power Quality of the Distribution Grid

Abstract:

Cost of various energy storage technologies is decreasing rapidly and the integration of these technologies into the power grid is becoming a reality with the advent of smart grid. Dynamic voltage restorer (DVR) is one product that can provide improved voltage sag and swell compensation with energy storage integration. Ultracapacitors (UCAP) have low-energy density and high-power density ideal characteristics for compensation of voltage sags and voltage swells, which are both events that require high power for short spans of time. The novel contribution of this paper lies in the integration of rechargeable UCAP-based energy storage into the DVR topology. With this integration, the UCAP-DVR system will have active power capability and will be able to independently compensate temporary voltage sags and swells without relying on the grid to compensate for faults on the grid like in the past. UCAP is integrated into dc-link of the DVR through a bidirectional dc-dc converter, which helps in providing a stiff dc-link voltage, and the integrated UCAP-DVR system helps in compensating temporary voltage sags and voltage swells, which last from 3 s to 1 min. Complexities involved in the design and control of both the dc-ac inverter and the dc-dc converter are discussed. The simulation model of the overall system is developed and compared to the experimental hardware setup.


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