Gregor Verbic

Local voltage-stability index using tellegen's Theorem

Summary form only given. In the paper, the Tellegens theorem and adjoint networks are used to derive a new, local voltage-stability index. The new approach makes it possible to determine the Thevenins parameters in a different way than adaptive curve-fitting techniques, from two consecutive phasor measurements. The new index was rigorously tested on different test systems. The results were obtained on a static two-bus test system and on the dynamic Belgian- French 32-bus test system that includes full dynamic models of all power-system components crucial to the voltage instability analysis. The results show advantages of the proposed index: it is simple, computationally very fast, and easy to implement in the wide-area monitoring and control center or locally in a numerical relay.

A new concept of voltage-collapse protection based on local phasors

A new algorithm for protection against voltage collapse is proposed. The algorithm makes use of the magnitudes and angles of the local phasors (i.e., bus voltages and load currents). The change in an apparent power-line flow during a time interval is exploited for computing the voltage-collapse criterion. The criterion is based on the fact that the line losses in the vicinity of the voltage collapse increase faster than the delivery of the apparent power and, at the voltage-collapse point, the losses consume all of the increased power. The selected criterion equals 0 when a voltage collapse occurs. The proposed algorithm could be easily implemented in a numerical relay. The information obtained by the relay can be used at two levels-for the coordinated system-wide control action or for automatic action on the local level. The algorithm is simple and computationally very fast. It was tested on the IEEE 118-bus test system.

Modified topological generation and load distribution factors

In this paper, a modification of the topological generation load distribution factor method of power flow tracing is reported. The modification is needed since the existing method introduces additional fictitious nodes on system lines in order to take into account the transmission losses. Due to the expansion of a system, the existing algorithm based on the augmented matrix equation requires more mathematical effort and memory and longer calculation time to obtain a solution. The new approach successfully avoids the matrix expansion by introducing matrix decoupling, which is its main improvement. The second novelty is an introduction of an equivalent model of a line that unites the nodal reactive power with the power produced by shunt admittances. Thus, the modified method can be also applied to the reactive power flows and transmission-loss allocation.

A Novel Strategy for Variable-Speed Wind Turbines' Participation in Primary Frequency Control

This paper proposes a novel strategy for participation of variable speed wind turbines in primary frequency control. The proposed strategy is based on the deployment of the kinetic energy of the rotating masses to reduce the need for deloaded operation while still being able to provide the required power reserve for the defined time frame. A steady-state deloaded operating point is optimized with respect to the amount of kinetic energy stored in the rotating masses. The parameters of the algorithm are calculated offline and stored in lookup tables. The proposed strategy is tested using a modified Nordic 32-bus test system for various operating scenarios. The results show that part of the primary frequency control power reserve of the thermal units can be replaced by the reserve provided by wind power, while retaining the desired frequency response following a disturbance. A comparison with the conventional deloaded strategy shows that between 1.16% and 2.79% energy savings can be achieved on a yearly basis, depending on the wind regime of the sites analyzed.

A novel concept for voltage collapse protection based on local phasors

Voltage collapse is basically a dynamic phenomenon with rather slow dynamics. Therefore mostly static methods based on the system Jacobian matrix were proposed for its analysis. Because of computational burden these methods are slow and not appropriate for estimating the proximity of power system to voltage collapse in real time. Therefore a new scheme for protection against voltage collapse based on local phasors is proposed. Detection of critical lines instead of critical nodes is used. The difference between apparent power flows on the receiving and the sending end of the line is used for computation of voltage collapse indicator. According to the developed protection scheme, the relay sends a triggering signal to reactive power sources to increase reactive power production. Because of the computational simplicity the proposed protection scheme can be used for on-line system monitoring as well as for off-line analyses.

Cost-based models for the power-reserve pricing of frequency control

One of the tasks of the system operator (SO) is to control the frequency of the system within defined limits. In order to do this, the SO has to provide enough power reserves which can be provided on the basis of bilateral contracts or on the relevant competitive market. In this paper, several methods for the power-reserve pricing of frequency control are presented. These methods are well suited to small power systems, where an insufficient number of potential bidders makes an ancillary-services market difficult to organize. The proposed pricing methods are meant for estimating the annual costs for power-reserves provision when these reserves are provided on the basis of bilateral contracts. The proposed pricing methods are applied to the Slovenian power system.

The Feasibility of Hydrogen Storage for Mixed Wind-Nuclear Power Plants

A novel methodology for economic evaluation of hydrogen storage for a mixed wind-nuclear power plant is presented in this article in a context of a ldquohydrogen economyrdquo. The simulation of the operation of the combined nuclear-wind-hydrogen system is discussed first, where the selling and buying of electricity and the selling of excess hydrogen and oxygen is optimized to maximize profits. This simulation is done in two phases: In the pre-dispatch phase, the system operation is optimized according to stochastic wind and price forecasts to obtain optimal hydrogen charge levels for the operational horizon. In the second phase, a real-time dispatch is carried out on an hourly basis to optimize the operation of the system to maximize profits, and to follow the storage levels of the pre-dispatch phase. Based on the operation planning and dispatch results, an economic evaluation is performed to determine the feasibility of the proposed scheme for investment purposes. The results of these studies demonstrate that hydrogen for the sole purpose of storage of electricity is not economically feasible at the current state of hydrogen technology development, unless hydrogen is sold to the market for other purposes such as transportation, as in the case in a hydrogen economy, or in the case of limited electricity transmission capacities, i.e., transmission congestion.

A healthy dose of reality for game-theoretic approaches to residential demand response

This paper addresses the assumptions underpinning many control schemes for residential demand response (RDR), with particular focus on those that adopt the framework of non - cooperative games. We propose four principal assumptions that we believe are necessary to give a realistic grounding to research on RDR, so that they might be more readily applied to the real problems faced by aggregators and households in a future energy network. These are that: (i) The energy use levels of households do not take continuous values, they take discrete and hybrid values; (ii) In addition to the system state variables, each household has a private state, representing the states of the goals it addresses in consuming electricity; (iii) Households have private preferences that are state-based, and therefore non-convex and combinatorial, and moreover, the monetary costs imposed by the system operator represents only part of their preferences for electrical energy use; and (iv) Household behaviour is strategic, both at the level of equilibrium analysis and algorithmic design. For each assumption we argue why it is necessary that a RDR scheme satisfy it, and illustrate the effects of violating our proposed assumption, with reference the existing literature on RDR schemes. We also provide several examples of techniques that satisfy each assumption, and illustrate our assumptions by developing a model that satisfies all four.

Demand response through smart home energy management using thermal inertia

In this paper, the value of thermal inertia in demand response to benefit customers is determined through a Mixed Integer Linear Programming (MILP) algorithm. Thermal models with different sophistications for a smart house are investigated. The energy consumption for cooling a smart house is optimized to minimize the expenditure of cooling load. One parameter and two-parameter thermal models are integrated into the optimization. The optimization of thermal load for maintaining the smart house within thermal comfort level is formulated as a MILP algorithm under the dynamic pricing policy. It is observed that the utilization of thermal inertia could potentially benefit both smart house owners and grid operators in the context of smart grid.

A Fast Distributed Algorithm for Large-Scale Demand Response Aggregation

A major challenge to implementing residential demand response is that of aligning the objectives of many households, each of which aims to minimize its payments and maximize its comfort level, while balancing this with the objectives of an aggregator that aims to minimize the cost of electricity purchased in a pooled wholesale market. This paper presents a fast distributed algorithm for aggregating a large number of households with a mixture of discrete and continuous energy levels. A distinctive feature of the method in this paper is that the nonconvex demand response (DR) problem is decomposed in terms of households as opposed to devices, which allows incorporating more intricate couplings between energy storage devices, appliances, and distributed energy resources. The proposed method is a fast distributed algorithm applied to the double smoothed dual function of the adopted DR model. The method is tested on systems with up to 2560 households, each with 10 devices on average. The proposed algorithm is designed to terminate in 60 iterations irrespective of system size, which can be ideal for an on-line version of this problem. Moreover, numerical results show that with minimal parameter tuning, the algorithm exhibits a very similar convergence behavior throughout the studied systems and converges to near-optimal solutions, which corroborates its scalability.