S.K. Salman

Improvement of fault clearing time of wind farm using reactive power compensation

Generation of electricity using wind power has received considerable attention worldwide in recent years. In order to investigate the impacts of the integration of wind farms into utilities networks, various windmill models have been developed. One such impact is related to the Critical Clearing Time (CCT) of the Wind Power Based Embedded Generators (WPBEGs). The CCT determines the stability of an integrated wind farm. It has been found that it is much lower than the time setting of the protection devices normally installed on networks feeders. The work in this paper therefore describes an attempt to improve the CCT. It is based on reactive power compensation (RPCs) techniques. Results from several case studies aiming at examining those factors, which influence the stability of WPBEG during network fault are presented. The effect of shaft modelling on the CCT is also discussed.

Comparative Study of Protection Requirements of Active Distribution Networks Using Radial and Ring Operations

Configuration, operation and management of existing distribution networks are based on passive networks, whereby the role of such networks is confined to transferring electricity from generation and transmission system to load centres. Under this situation, the main function of distribution network is delivering good quality electricity with minimum amount of network control to the load centres at different voltage level regardless of the most critical case. Most distribution networks are designed to operate in radial mode without having generation unit accommodate on them. Continued increase of distributed generation (DG) penetration into existing distribution networks in recent years has resulted in bi-directional power flow and changes in network voltage profile. These in turn caused important impact on the operation of conventional feeder protection schemes that are originally designed based on radial network operation assumption. The protection issues that had been identified in the presence of DG include (i) malfunction of protective devices due to increase of network fault current level, (ii) unnecessary tripping of non- directional relays due to significant fault contribution by DG, (iii) reduction of fault detection sensitivity of overcurrent relays due to fault contribution of DG, (iv) fault clearing time of graded overcurrent relays is affected and (v) operation of re-closer is affected with introduction of LOM relay. It has been reported that future distribution networks are likely to be operated in ring mode rather than radial in order to optimise the integrated capacity of DG. There are advantages in adopting ring mode operation but it could complicate the protection systems of networks with integrated DG. This paper reports an investigation into the impact of distributed generation on the setting of protective devices of distribution networks that originally operated in radial mode and then switched to ring mode and to compare the protection requirements for the two modes of operations.

Critical clearing time of doubly fed induction generator

A detailed model of grid-connected doubly-fed induction generator (DFIG) suitable for fault analysis is presented. 5th order model of wound rotor induction machine together with a two-mass model of the mechanical drive-train is employed to investigate transient voltage stability of integrated wind turbine. Converter system is represented in such a way to contain adequate model of the rotor- and stator-side converters as well as the DC-link components. PWM converters, respective controllers and the switching schemes are represented in the same reference frame as the machine to realize a combined machine-converter model. Applying a balanced three-phase network fault to the simulated system, case studies are conducted to determine the value of the critical clearing time (CCT) for the DFIG as well as the fixed-speed induction generator (FSIG). Further investigations are also made to evaluate the impact of the shaft system parameters on the CCT of a grid- connected FSIG and DFIG.

The Effect Of Private Generators Ohf The Voltage Control Of 11 KV Network And On The Operation Of Certain Protective Relays

The addition of private generators to an 1 IkV distribution network car1 affect the operation of the network in two ways. First it may cause the deterioration of the performance of AVC relays due to the changes of active and reactive power flows. Consequently this will adversely affect the control of the voltage of 1lkV network. Second, the sudden reduction or complete disconnection of one private generator may cause an undesirable tripping of the other generators on the system due to the mal-operation of their corresponding undervoltage relays.

Assessment and enhancement of grid fault-induced torsional oscillations for induction generator-based wind turbines

This paper investigates the torsional levels of the Doubly-fed Induction Generator (DFIG) and the Fixed Speed Induction Generator (FSIG) initiated by network faults. The inclusion of the turbine-generator mutual damping is investigated for the DFIG, and FSIG. For the DFIG the significance of a detailed representation of the grid-side input filter and the DC-link capacitor are studied. A method is proposed to mitigate the torsional oscillations of the turbine-generator shaft system using a Minor Control Loop (MCL) strategy. PSCAD-EMTDC studies demonstrate that the proposed controller can alleviate the risk of torsional oscillations. A conventional Power System Stabilizer (PSS) requires identification of four parameters, however the proposed controller requires only two parameters (MCL gain and washout time constant).

Enhancement of fault ride-through capability and damping of torsional oscillations for a distribution system comprising induction and synchronous generators

The dynamic interaction of a Distributed Synchronous Generator (DSG) with Fixed-speed (FSIG) and Doubly-Fed induction generator (DFIG)-based wind turbines during network fault conditions is presented in this paper. The objective is to enhance the damping of the wind turbine torsional oscillations, and the transient stability margin of the distributed generators measured by their respective Critical Clearing Times (CCTs). Power System Stabilizer (PSS), Static Var Compensator (SVC), Transient Gain Reduction (TGR) and high-speed exciters including static and solid-state exciters are investigated as candidate solutions to the system subjected to multiple-fault conditions. Impact of various factors such as fault location, SVC location, SVC size, PSS gain, exciter type and increased ceiling voltage are discussed using the results obtained from the PSCAD/EMTDC simulation.

Fuzzy Logic-Based AVC Relay for Voltage Control of Distribution Network with and without Distributed/Embedded Generation

Voltage control is considered one of the basic operational requirements of electrical power systems both at distribution and transmission levels. The most popular voltage control equipment includes On-Load Tap Changer (OLTC) transformer controlled by Automatic Voltage Control (AVC) relay, which is mostly used in 11 kV networks. In recent years, a growing number of distributed/embedded generators are connected to distribution networks. Conventional AVC relay usually equipped with compounding whose setting are chosen to compensate for the voltage drop along the feeder(s) emanating from source substation. However, the integration of Distributed/Embedded generators (DGs/EGs) including wind farms into distribution systems causes voltage regulation problems due to the interference with the performance of AVC relay. Therefore, the development of a new voltage control scheme of supply system with DGs/EGs is required. This paper presents an attempt to design an AVC relay based on Fuzzy Logic. The structure of the proposed Fuzzy Logic based-AVC relay is presented and the results that show its performance with and without integrated distributed generation into distribution network are also presented and discussed.

Impact of embedded wind farms on voltage control and losses of distribution networks

Voltage magnitude of a distribution system is usually controlled using an on-load tap changer transformer with AVC relay. However, the trend of integrating embedded generation such as wind farms into distribution networks makes it difficult for the AVC relay to maintain voltage magnitude within the statutory limits, particularly if the generator is connected at a long distance from the substation where the relay is located. This paper investigates the use of reactive power injected by a capacitor as a means for assisting the AVC relay for maintaining network voltage magnitude within the required limits. The paper also investigates the effect of the location of such a capacitor on the network voltage profile and network losses.

Investigation of the torsional vibration and transient stability margin for doubly-fed induction generators

This paper presents the results obtained from a PSCAD/EMTDC dynamic fault simulation of a grid connected Doubly-fed Induction Generator (DFIG). The transient stability performance of the DFIG is investigated; case studies which demonstrate the possibility of wind turbine generator torsional vibration, and the concept of Critical Clearing Time (CCT) for the DFIG are presented and the results are compared to those of a Fixed Speed Induction Generator (FSIG).

Dynamic behaviour of integrated multiple wind farms during fault conditions on the hosted distribution network

Generation of electricity from wind power, as a renewable energy source, is continually attracting the attention of investors, researchers and electrical utilities. It has been predicted that the annual growth of wind power between 1998 and 2040 would be between 20% and 30%. Consequently, it is expected that wind power would supply at least 20% of the worlds electricity by 2040. These figures show the likely future dramatic increase in the level of wind power based embedded generation penetrating into existing utilities networks which occur mainly at distribution voltage levels. Such penetration may take the form of integrating several wind farms into the same network. This in turn raises the importance of understanding the behaviour of wind farms following fault conditions that may develop at any point on the hosted network and consequently may affect the security as well as the quality of electrical supply. This investigation therefore conducted on an electrical supply system with two integrated wind farms which led to learning several aspects about the dynamic behaviour of the two wind farms. It has been found that in a two-wind farm situation the wind farm with the large rated power has predominate effect on the stability of the smaller wind farm. It has been found that the CCT of multiple wind farms can be improved using var injection technique. It has also been found that the best improvement can be achieved by connecting static shunt capacitors to the system at the instant of time that corresponds to that of fault inception.