Costas Vournas

Relationships between voltage and angle stability of power systems

This paper discusses modelling and theoretical issues associated with voltage and angle stability of power systems. A time-scale decomposition is performed to illustrate how the critical modes can be identified with reduced-order models and the bifurcation phenomena can be explained with these low order models. Examples are given for single and multi-machine systems.

Power System Stabilization via Parameter Optimization - Application to the Hellenic Interconnected System

Dynamic stability problems are usually overcome through the application of Power System Stabilizers. This paper presents an alternative approach for power system stabilization based upon the tuning of the existing generator controllers, both governors and A.V.R.s. The sensitivities of the eigenvalues to the controller parameters are evaluated and an optimization technique is developed to maximize the dynamic stability. Application of the parameter optimization method on a realistic model of the Hellenic Interconnected System has proved an efficient way to stabilize a number of unstable oscillatory modes by relatively small parameter variations. The results are tested with a nonlinear simulation program and the achieved stabilization, as shown by the rotor angle swings, is found to be significant.

Application of interruptible contracts to increase wind-power penetration in congested areas

Wind-power penetration in many cases faces significant barriers due to limited transmission capability. In Greece (and in most countries in Europe), the guaranteed access to the grid offered to wind parks, in conjunction with the existing planning practices, has led to severe limitations in wind power installed in certain congested areas. This paper presents some simple ideas that are currently being applied to the Hellenic Interconnected System, in order to increase the wind-power penetration. The main concept is the introduction of interruptible generator contracts and the control of the power flow through congested corridors by issuing power reduction commands, whenever security limits are violated. The proposed control is implemented using common PLCs of low cost and the existing SCADA system, and is both simple and transparent. The paper presents also a methodology to assess the impact of the proposed techniques on the wind penetration achieved.

Maximizing Power-System Loadability in the Presence of Multiple Binding Complementarity Constraints

In this paper, the problem of maximizing power-system loadability with multiple complementarity constraints representing generator limits is examined. The structure of the loadability surface is investigated, the various type of limits are classified and a general algorithm to optimize settings of control variables, in order to maximize loadability margin is developed. The importance of nonsmooth corner points (CPs) of the loadability surface that are due to the simultaneous occurrence of several loadability conditions is discussed. At such limits, maximization of loadability margin is performed based on techniques provided by nonlinear optimization theory. For this purpose an algorithm to identify multiple binding constraints on a CP is developed. Illustrative examples on small, but realistic systems are included.

Methods for evaluating penetration levels of wind generation in autonomous systems

During the last decades there has been a continuous growth of wind power generation. This trend, especially in Greek islands, where wind potential is high, raises the issue of acceptable penetration levels of wind generation especially in autonomous systems. This work is part of a research program between PPC S.A. –of Greece and NTUA. The method is applied to two islands, Crete and Rhodes, where wind power penetration is expected to be increased within the next years. The power grid of each island is modeled and speed governors, automatic voltage regulators of conventional plants, as well as wind turbines are modeled in detail. Case studies are formed and simulation results are produced, discussed and eventually the penetration levels are determined, together with proposed measures to increase them.

Computational environment to investigate wind integration into small autonomous systems

This paper presents briefly the methodologies and the respective computational tools for the investigation of the wind power exploitation in small autonomous power systems supplying the load demand of small islands. The proposed methodology examines the wind power penetration from both technical and economic point of view. It comprises tools for load forecasting and management, power system analysis and generation system simulation and planning.

Frequency-Based Overload Control of Smart Transformers

A Smart Transformers (ST) is an automated transformer based on the latest power electronics and communication technologies. It aims not only at replacing the traditional transformer, but at providing also ancillary services to the grid, thanks to the greater flexibility offered by power electronics. However, in the case of grid overload caused by high load demand or high production from renewable energy sources, the power electronics have no extra capability. The power semiconductors can be overloaded only for few microseconds, in contrast with the grid components requirement of bearing currents higher than the rated values for several seconds. Thus the ST needs new procedures for dealing with the transients/conditions of the high current requests by the load. This paper presents the Frequency-Based Overload Control (FBOC), an innovative procedure for the overload management that acts in coordination with the droop controller of Distributed Generation (DG) systems, enabling the limitation of the transformer current.

On-Line Load Sensitivity Identification in LV Distribution Grids

This letter proposes a new on-line load sensitivity identification by means of power electronics-based devices. Applying a voltage and frequency perturbation and measuring the consumed power of the loads, the proposed method computes in real time the voltage and frequency dependency of the load active and reactive power. In this work a Smart Transformer application has been proposed, but the method is general for any power electronics converter able to influence dynamically the voltage and frequency in the grid.

Full-converter wind generator modelling for transient stability studies

In this paper the impact of variable speed wind generators on power system transient stability is examined using a generic full converter wind generator model. First the developed model, appropriate for power system stability studies, is described and the grid-side converter control strategy during voltage dips is analyzed. Next, the contribution of a wind farm equipped with full converter wind turbines to the transient stability of a power system is examined. Simulations of a test system show that transient stability margins can be significantly improved, if the wind farm active and reactive power injection is reconfigured in order to satisfy typical present-day grid codes. It is also shown that transient stability can be further improved if the power converters can withstand an additional amount of reactive current during low voltage conditions.

Coordinated frequency and Voltage Overload Control of Smart Transformers

A Smart Transformer (ST) is a power electronics-based transformer that aims not only to substitute the traditional transformer but to upgrade also the LV and MV grid. In order to limit the costs, the ST must be carefully designed, constraining the current carried by the ST. In this paper a Combined Frequency and Voltage Controller is proposed, in order to manage a possible overload without derating the ST. Aiming to reduce the current, this control enhances the ST security against the overload situation interacting with the local Distributed Generation (DG) and the local loads.