Goran Strbac

Contributions of individual generators to loads and flows

Because of the introduction of competition in the electricity supply industry, it has become much more important to be able to determine which generators are supplying a particular load, how much use each generator is making of a transmission line and what is each generators contribution to the system losses. This paper describes a technique for answering these questions which is not limited to incremental changes and which is applicable to both active and reactive power. Starting from a power flow solution, the technique first identifies the busses which are reached by power produced by each generator. Then it determines the sets of buses supplied by the same generators. Using proportionality assumption, it is then possible to calculate the contribution of each generator to the loads and flows. The applicability of the proposed technique is demonstrated using a 30-bus example.

Control of DFIG-based wind generation for power network support

This paper addresses the design and implementation of a novel control scheme for a doubly fed induction generator (DFIG), of the type employed with wind turbines, to provide support to power system operation. It is shown that this controller provides a DFIG-based wind farm with operational and control compatibility with conventional power stations, the ability to contribute to voltage support and recovery following network faults, the ability to provide a power system stabilizer capability that improves overall system damping, and the capability of contributing short-term frequency support following loss of network generation. A simple but realistic test network that combines synchronous and wind farm generation has been modeled and used to assess dynamic performance. Simulation results are presented and discussed that demonstrate the capabilities and contributions of the new DFIG controller to network support.

Trading wind generation in short term energy markets

Even with state-ofthe-art forecasting methods, the short-term generation of wind farms cannot be predicted with a high degree of accuracy. In a market situation, these forecasting errors lead to commercial risk through imbalance costs when advance contracting. This situation is one that needs to be addressed due to the steady increase in the amount of grid-connected wind generation, combined with the rise of deregulated, market-orientated electricity systems. In the presence of imbalance prices and uncertain generation, a method is required to determine the optimum level of contract energy to be sold on the advance markets. Such a method is presented here using Markov probabilities for a wind farm and demonstrates substantial reductions in the imbalance costs. The effect of market closure delays and forecasting window lengths are also shown.

A power system stabilizer for DFIG-based wind generation

A power system stabilizer (PSS) for a wind turbine employing a doubly fed induction generator (DFIG) is presented. It is shown that this PSS can significantly influence the contribution that a DFIG-based wind farm can make to network damping. A simple, generic test network that combines synchronous and wind farm generation is used to demonstrate system performance contributions. The results of both eigenvalue analysis and time response simulation studies are presented to illustrate contributions to network dynamic and transient performance that the DFIG controller with its PSS can make. Performance capabilities superior to those provided by synchronous generation with automatic voltage regulator and PSS control are demonstrated.

Microgrids and virtual power plants: Concepts to support the integration of distributed energy resources

This article presents the concepts of the microgrid and the virtual power plant (VPP) as vehicles to facilitate cost-efficient integration of distributed energy resources (DERs) into the existing power system. These concepts were designed to enhance the system value and the controllability of DER and to provide frameworks for the development of interfaces among energy and ancillary service resources, system operators, and energy market participants. Through aggregation, DER access to energy markets is facilitated, and DER-based system support and ancillary services can be provided. By enabling this additional functionality, it is envisaged that system performance measured in the form of energy efficiency, power quality, security, and economic operation can be improved. In this paper, the technical and commercial functionality facilitated through the microgrid and VPP concepts is described. The paper concludes with case studies demonstrating the application of the concepts on a test system.

A method for computing the value of corrective security

This paper describes a method for computing the optimal balance between preventive and corrective security actions. This method relies on a three-stage Benders decomposition and is capable of handling the mixed integer linear nature of this problem. A number of cases studies are presented to demonstrate the effectiveness of this method and to illustrate the consequences of introducing demand side corrective actions. In particular, it is shown that using corrective actions can significantly reduce the total cost of security and that the reliability of the transmission system affects the optimal mix of preventive and corrective security. The proposed method provides a basis for negotiations between the transmission operator and potential providers of demand side corrective actions.

Investigation of regulatory, commercial, economic and environmental issues in microgrids

Concepts of microgrids are proposed to address primarily various issues related to integration of small scale renewables and increased demand of reliable electricity supply. With an active management control approach and ability to operate in islanding mode, a cluster of micro generators, electricity storage and electrical loads can be operated within the microgrids framework to provide higher supply reliability to highly value customers. Solutions are required not only to make these concepts technologically feasible and safe to operate but also to be commercially viable and attractive, economically efficient and supported by electricity regulations. This paper summarises the results of investigations on various economic, regulatory and commercial issues faced by the development of microgrids in MICROGRIDS project. The potential economic benefits and contributions to environment from applications of microgrid technologies are also presented and described in this paper

Assessing the competitiveness of demand-side bidding

Demand-side bidding (DSB) provides a very flexible way of meeting the peak load in a production schedule. This paper demonstrates that, in a competitive electricity pool, DSB and other forms of flexible generation can cause sharp and unwarranted price increases if the production schedule is established on the basis of a minimization of the total production cost. Furthermore, the competitiveness of DSB is artificially inflated if the load recovery periods which accompany these load reductions are not taken into consideration by the scheduling program. It is argued that the actual value of DSB should be re-examined in the light of these findings.

Active management and protection of distribution networks with distributed generation

The extent of distributed generation (DG) that can be connected to a distribution network is often limited by the permissible voltage limits. The DG not only raises voltage but also modifies network fault current and may cause problems on existing distribution network protection schemes. Voltage control by active management can increase the level of DG penetration but requires an assessment of the voltage at each point on the network. Active management of the distribution network (DN) must also be practicable for the generators and the network operator. The paper presents possible solutions to some of the problems of active management and protection of distribution networks with higher penetration of DG. The paper also addresses the issues of costs and benefits of active management and DG islanded operation.

Influence of Tower Shadow and Wind Turbulence on the Performance of Power System Stabilizers for DFIG-Based Wind Farms

The aim of the paper is to demonstrate the way in which mechanical power variations, due to tower shadow and wind turbulence, influence control performance of power system stabilizer (PSS) loops for doubly-fed induction generators (DFIGs). The PSS auxiliary loops are applied on a specific DFIG control scheme, the flux magnitude and angle controller (FMAC). However, since the PSS signal is applied at the output of the basic controller, the PSS performance characteristics displayed are deemed typical for DFIG control schemes in general. The relative capabilities of PSS controllers based on stator power, rotor speed, and network frequency, when the DFIG turbine is subjected to aerodynamic torque variations, are investigated via simulation studies. A two-generator aggregate model of a wind farm is introduced, which enables the influence of tower shadow and wind turbulence on both an individual turbine and on the overall wind farm itself to be assessed.