Marek Zima

Design Aspects for Wide-Area Monitoring and Control Systems

This paper discusses the basic design and special applications of wide-area monitoring and control systems, which complement classical protection systems and Supervisory Control and Data Acquisition/Energy Management System applications. Systemwide installed phasor measurement units send their measured data to a central computer, where snapshots of the dynamic system behavior are made available online. This new quality of system information opens up a wide range of new applications to assess and actively maintain systems stability in case of voltage, angle or frequency instability, thermal overload, and oscillations. Recent developed algorithms and their design for these application areas are introduced. With practical examples, the benefits in terms of system security are shown.

Stability assessment and emergency control method using trajectory sensitivities

Very stressed power systems often lack ability to withstand certain disturbances, what might result in serious consequences. Therefore the tools mitigating such a situation have a crucial significance. This work presents a method for real-time emergency control of power systems based on the trajectory sensitivities. Out of the present conditions a nominal trajectory and its sensitivities is precalculated. If an incipient instability is detected, necessary stabilizing control measures are determined. A short review of trajectory sensitivities and model predictive control is included. A principal description of the method together with a theoretical foundation and examples with simulation results is provided.

Active Coordination of Thermal Household Appliances for Load Management Purposes

Abstract In this paper, a coordination approach for thermostat-controlled household appliances is developed. Under consideration are heating and cooling devices such as refrigerators, freezers, water boilers and heat pumps, which are usually characterized by an intermittent (duty cycle) operation. Without influence from the outside, an internal hysteresis switching controller toggles the “on/off” state of a device when its temperature boundary is reached. In order to realize the approach proposed here, the devices are connected to a central control entity by two-way communication. The coordination consists of a step-wise heuristic solution of a binary optimization problem, which serves to select a number of devices in each time step that are subject to compulsory switching, i.e. toggling the “on/off” state. It allows a large group of devices to track a setpoint trajectory with its aggregated power consumption, acting like a distributed virtual energy storage, while the individual temperature bounds of the appliances are not violated. This behavior can be used for grid-control purposes, such as the provision of active power reserves. It will be shown that the group of appliances can be characterized by an approximate aggregated dynamical model consisting of a first-order differential equation together with an approximate aggregated nonlinear cost function penalizing the control actions. The developed methodology is evaluated in a numerical simulation with a small appliance cluster corresponding to a residential housing area.

Potentials and applications of coordinated groups of thermal household appliances for power system control purposes

In this paper, a novel load management approach using thermal household appliances is presented, which provides extensive possibilities to support power system control tasks. A recently developed coordination algorithm for a high number of small thermostat-controlled appliances requires the connection of individual appliances to a central control entity through a two-way communication channel. The control method, which does not cause significant comfort losses for users, enables the group of appliances to act like a virtual distributed energy storage. While the coordination algorithm has been presented in detail before, the contribution of this paper lies in the analysis of the aggregated behavior of coordinated appliance groups. Apart from that, a methodology for estimating the coordinated load control potential in larger areas (e.g. countries) is presented. This is illustrated by exemplary considerations on the Swiss power system, which can also be adapted to other countries. Furthermore, different applications for the gained demand-side flexibility in a liberalized electricity market are discussed.

Sharing of Profit From Coordinated Operation Planning and Bidding of Hydro and Wind Power

Depending on market rules, namely congestion management and balancing management, coordination between wind and hydro producers may be mutually beneficial. In this paper we propose a new collaboration scheme and a fair and transparent method, based on the Shapley value, for splitting the extra value caused by a coordinated bidding and operation strategy.

Model Predictive Control Employing Trajectory Sensitivities for Power Systems Applications

Model Predictive Control (MPC) is a widely used method in process industry for control of multi-input, multi-output systems. It possesses some features, which make it attractive for applications in power systems. Power systems exhibit several features of complex systems, such as hybrid nature (mixed continuous and discrete dynamics), nonlinear dynamics and very large size. Since MPC involves optimization computations, it represents a big challenge to handle above listed properties in a reasonable time for large power systems. Therefore the reduction of the computational burden associated with MPC is a crucial factor. We describe in this paper a formulation of MPC for power systems based on trajectory sensitivities. Trajectory sensitivities are time varying sensitivities derived along the predicted nominal trajectory of the system. They refer to possible changes of initial conditions from which the nominal trajectory is predicted, or to a modification of system parameters including changes of discrete valued quantities, e. g. positions of transformers taps. Trajectory sensitivities allow an accurate reproduction of the nonlinear system behavior using a considerable reduced computational burden as compared with the full non-linear integration of the system trajectories.

On security criteria in power systems operation

In this paper a new way of minimizing the risk of cascading events is presented. A method to dispatch the system in such a way that the risk for subsequent line trips after the first initial event is minimized is presented. This method is based on the assumption that the probability of a line trip is a continuous increasing function of the line overloading. A slight overload implies a smaller probability of a line trip than a larger overload does. The new method is compared with the N-1 criterion and it is shown that the proposed method makes a system more robust against multi contingencies than the N-1 criterion.

An active coordination approach for thermal household appliances — Local communication and calculation tasks in the household

In this paper, an approach to the coordinated operation of a multitude of household appliances with thermal inertia is presented, which can be used for power system control tasks. Appliances under consideration are cooling and heating devices, e.g. refrigerators, freezers, or electric water boilers, which are characterized by an intermittent (duty cycle) operation. A recently developed coordination algorithm for a large group of these thermostat-controlled appliances equipped with a two-way communication interface uses centrally computed switching impulses based on an ”offer to be switched for a certain price” from the appliances. The price calculation on the local level requires an accurate prediction of the next switching instant triggered by the thermostat. This paper develops a framework for the communication within the household and to the outside, modeling and prediction approaches for the appliance duty cycles, and a switching price calculation method. Furthermore, the impact of the coordinated control on the appliances and requirements on the in-house communication system are discussed.

The Swiss Reserve Market: Stochastic Programming in Practice

This paper presents the evolution and design of the Swiss reserve market and describes its two-stage stochastic market-clearing model. In Switzerland, the reserve market comprises weekly and daily auctions. The decision-making problem is to determine the amount of reserves that should be procured in each market stage. The stochasticity stems from daily offers which are not available at the beginning of the week, when the first-stage decisions are made. The new market-clearing model minimizes expected procurement costs of reserves, while taking reserve dimensioning criteria and market properties into consideration. Since the last week of January 2014, this model has been clearing the reserve market in Switzerland. To our knowledge, this is the first real-world implementation of a stochastic market-clearing model in electricity markets.

Implementation and Applications of Wide-area monitoring systems

This paper discusses the design and applications of wide area monitoring and control systems, which can complement classical protection systems and SCADA/EMS applications. System wide installed phasor measurement units send their measured data to a central computer, where snapshots of the dynamic system behavior are made available online. This new quality of system information opens up a wide range of new applications to assess and actively maintain systems stability in case of voltage, angle or frequency instability, thermal overload and oscillations. Recent developed algorithms and their design for these application areas are introduced. With practical examples the benefits in terms of system security are shown..