While energy storage technologies do not represent energy sources, they provide valuable added benefits to improve stability power quality, and reliability of supply. Battery technologies have improved significantly in order to meet the challenges of practical electric vehicles and utility applications. Flywheel technologies are now used in advanced nonpolluting uninterruptible power supplies. Advanced capacitors are being considered as energy storage for power quality applications. Superconducting energy storage systems are still in their prototype stages but receiving attention for utility applications. The latest technology developments, some performance analysis, and cost considerations are addressed. This paper concentrates on the performance benefits of adding energy storage to power electronic compensators for utility applications.

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Energy storage systems for advanced power applications

The impact of phase-locked loop (PLL) parameters on the dynamic and steady-state behavior of a voltage-source converter (VSC) in an HVDC transmission system is determined as a function of the system strength [parameterized by the short-circuit ratio (SCR)]. This is achieved by using a linearized small-signal model of the converter system and its controls. The model is validated via electromagnetic transients simulation of the fully detailed large signal model. An interesting result from this study is that the maximum power transfer capability of the VSC-HVDC converter is affected by the PLL gains, and that the theoretical limit (obtained from static voltage stability analysis) is approachable as the PLL gains become very small. This paper shows that gains of the PLL, particularly at low SCRs, greatly affect the operation of the VSC-HVDC converter and that operation at low SCRs approaching 1.3 is very difficult.

Impact of Short-Circuit Ratio and Phase-Locked-Loop Parameters on the Small-Signal Behavior of a VSC-HVDC Converter

In this paper, a new power frequency model for unified power flow controller (UPFC) is suggested with its DC link capacitor dynamics included. Four principal control strategies for UPFC series element main control and their impacts on system stability are discussed. The main control of UPFC series element can be realized as a combination of the four control functions. The supplementary control of UPFC is added for damping power oscillation. The integrated UPFC model has then been incorporated into the conventional transient and small signal stability programs with a novel UPFC-network interface. Computer tests on a 4-generator interconnected power system show that the suggested UPFC power frequency model and the UPFC-network interface method work very well. The results also show that the suggested UPFC control strategy can realize power flow control fairly well and improve system dynamic performance significantly.

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Application of unified power flow controller in interconnected power systems-modeling, interface, control strategy, and case study

Flexible AC transmission systems

The paper presents the establishment of a linearised Phillips-Heffron model of a power system installed with a unified power flow controller (UPFC). On the basis of the linearised model, the damping function of the UPFC is investigated. Basic issues on the design of the UPFC damping controller are discussed, regarding the effectiveness and robustness of the damping function of the UPFC, among which the selection of effective and robust input control signals of the UPFC to superimpose its damping function is specially addressed. An example power system is presented.

Damping function of unified power flow controller

A unified power flow controller (UPFC) is a typical FACTS device capable of instantaneous control of three power system parameters. This paper presents a basic control system which enables the UPFC to follow the changes in reference values of the active and reactive power supplied from the outer system controller. The analysis is based on the transformation of the three-phase power system to the rotating reference frame. As a step closer to a practical application of the UPFC, a modified control structure with a predictive control loop and precontrol signal for a DC-voltage control was designed. The new control system offers better stability and transient performance in comparison with the classical decoupled strategy, especially considering the harmonic distortion of the current being controlled. The derived basic control of the UPFC was tested with the NETOMAC program system.

Basic control of unified power flow controller

Increasing automation in modern industry and deregulation have changed the requirements on power quality. Computer and process control equipment as well as drive converters are sensitive to deviations of the line voltage from the ideal sinusoidal. Voltage sags, harmonic distortion, flicker and interruption of power supply are the most common problems. In an increasing number of cases, where conventional equipment cannot solve these problems, PWM converter-based shunt- and series connected power conditioners named DSTATCOM (distribution static synchronous compensator) and DVR (dynamic voltage restorer) have been introduced. With energy storage added to the power conditioner even more flexibility in system operation and planning is provided for utilities and industry. This paper describes such a 300 kW power conditioner with a chemical battery and its benefits to an industrial plant. If, for example, load demand exceeds the power rating of the supplying transformer or the negotiated power with the utility only at certain times of day, the excessive active power demand can be provided by the power conditioner. In addition, the power conditioner can be used for reactive power compensation or other tasks, if it is not fully used for charging or discharging. The new technology can therefore be used to postpone or avoid major investments and to optimize energy consumption costs.

P. Zunko

Papers

Basic control of unified power flow controller

300 kW battery energy storage system using an IGBT converter