Daniel S. Kirschen

Demand-side view of electricity markets

This tutorial paper discusses some aspects of electricity markets from the perspective of the demand-side. It argues that increasing the short-run price elasticity of the demand for electrical energy would improve the operation of these markets. It shows, however, that enhancing this elasticity is not an easy task. The tools that consumers and retailers of electrical energy need to participate more actively and effectively in electricity markets are discussed. The paper also describes how consumers of electricity can take part in the provision of power system security.

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.

Short-term generation scheduling with transmission and environmental constraints using an augmented Lagrangian relaxation

This paper proposes a new approach based on augmented Lagrangian relaxation for short term generation scheduling problems with transmission and environmental constraints. In this method, the power system constraints, e.g. load demand, spinning reserve, transmission capacity and environmental constraints, are relaxed by using Lagrangian multipliers, and quadratic penalty terms associated with power system load demand balance are added to the Lagrangian objective function. Then, the decomposition and coordination technique is used, and nonseparable quadratic penalty terms are replaced by linearization around the solution obtained from the previous iteration. In order to improve the convergence property, the exactly convex quadratic terms of decision variables are added to the objective function as strongly convex, differentiable and separable auxiliary functions. The overall problem is decomposed into N subproblems, multipliers and penalty coefficients are updated in the dual problem and power system constraints are satisfied iteratively. The corresponding unit commitment subproblems are solved by dynamic programming, and the economic dispatch with transmission and environmental constraints is solved by an efficient network flow programming algorithm. The augmented Lagrangian relaxation method enhanced by the decomposition and coordination techniques avoids oscillations associated with piece-wise linear cost functions. Numerical results indicate that the proposed approach is fast and efficient in dealing with numerous power system constraints. >

Estimating the Spinning Reserve Requirements in Systems With Significant Wind Power Generation Penetration

Spinning reserve (SR) allows system operators to compensate for unpredictable imbalances between load and generation caused by sudden outages of generating units, errors in load forecasting or unexpected deviations by generating units from their production schedules. As the proportion of power produced by wind farms increases, it becomes more difficult to predict accurately the total amount of power injected by all generators into the power system. This added uncertainty must be taken into account when setting the requirement for SR. This paper proposes a technique to calculate the optimal amount of SR that the system operator should provide to be able to respond not only to generation outages but also to errors in the forecasts for load and wind power production. Using a Monte Carlo simulation, the proposed technique for setting the SR requirements is then compared with the traditional deterministic criterion (i.e., the capacity of the largest online infeed), an approach to cope with wind imbalances and an approach that combines the traditional criterion with the approach to cope with wind imbalances. The results show that, contrary to what is commonly believed, an increased wind power penetration does not necessarily require larger amounts of SR.

A model of PV generation suitable for stability analysis

This paper describes a model of photovoltaic (PV) generation suitable for studying its interactions with the power system. Experimental results suggest that the maximum power point tracking part of the control system of the PV generator dominates the dynamic behavior of the system. These experimental results are used to develop and validate the proposed model. It is shown that the model accurately reflects the behavior of the generator following both small and fast changes in irradiance and AC grid voltage. The proposed model is designed to be integrated in a dynamic simulation program.

Decentralized Demand-Side Contribution to Primary Frequency Control

Frequency in large power systems is usually controlled by adjusting the production of generating units in response to changes in the load. As the amount of intermittent renewable generation increases and the proportion of flexible conventional generating units decreases, a contribution from the demand side to primary frequency control becomes technically and economically desirable. One of the reasons why this has not been done was the perceived difficulties in dealing with many small loads rather than a limited number of generating units. In particular, the cost and complexity associated with two-way communications between many loads and the control center appeared to be insurmountable obstacles. This paper argues that this two-way communication is not essential and that the demand can respond to the frequency error in a manner similar to the generators. Simulation results show that, using this approach, the demand side can make a significant and reliable contribution to primary frequency response while preserving the benefits that consumers derive from their supply of electric energy.

Optimal scheduling of spinning reserve

Effective control of spinning reserve can provide substantial cost reductions in large power systems. This paper suggests that this goal can he achieved by tightly integrating a probabilistic reserve assessment with the unit commitment (UC) function. This enables the reliability of individual units and the uncertainty of the load forecast to be considered for short-term scheduling. The generation can then be scheduled to meet a given risk index. It is further argued that the optimal value of this risk index should be selected on the basis of a tradeoff between the total schedule cost obtained from the UC solution and the expected cost of energy not served which can be derived from the probabilistic reserve assessment. Finally, the problem of over-commitment of reserve in Lagrangian relaxation based UC is reduced by means of a new unit decommitment technique incorporating a look-ahead feature.

On-Line Efficiency Optimization of a Variable Frequency Induction Motor Drive

The problems associated with the implementation of an optimal efficiency controller in variable frequency induction motor drives are examined. A simple method for minimizing, on-line, the global system losses is presented. This method is based on the adaptive control of the rotor flux in a field-oriented drive system. The effectiveness of this control strategy is verified using digital simulation.

Initial review of methods for cascading failure analysis in electric power transmission systems IEEE PES CAMS task force on understanding, prediction, mitigation and restoration of cascading failures

Large blackouts are typically caused by cascading failure propagating through a power system by means of a variety of processes. Because of the wide range of time scales, multiple interacting processes, and the huge number of possible interactions, the simulation and analysis of cascading blackouts is extremely complicated. This paper defines cascading failure for blackouts and gives an initial review of the current understanding, industrial tools, and the challenges and emerging methods of analysis and simulation.

Tracing active and reactive power between generators and loads using real and imaginary currents

In a competitive environment, usage allocation questions must be answered clearly and unequivocally. To help answer such questions, this paper proposes a method for determining how much of the active and reactive power output of each generator is contributed by each load. This method takes as its starting point a solved power flow solution. All power injections are translated into real and imaginary currents to avoid the problems arising from the nonlinear coupling between active and reactive power flows caused by losses. The method then traces these currents to determine how much current each source supplies to each sink. These current contributions can then be translated into contributions to the active and reactive power output of the generators. It is also shown that the global contribution of a load can be decomposed into contributions from its active and reactive parts. This decomposition is reasonably accurate for the reactive power generation. To determine the contributions to active power generation, the previously-described method based on the active power flows is recommended.