Evaluation of the Distributed Generation Effect on the Power Quality of the Grid N. KASMAS, S. PAPATHANASSIOU, A. KLADAS School of Electrical and Computer Engineering National Technical University of Athens Iroon Polytechniou st., 15780 Zografou, Athens GREECE Abstract: Technological advancements and institutional changes in the electric power industry constantly increase the penetration level of Distributes Generation (DG) sources in the grids. The connection of new installations is subject to utility defined technical requirements for the interconnection, usually resolved at the expense of the investor. As the interest for installing new generation facilities escalates, the adoption of transparent and easily applicable technical evaluation procedures becomes more imperative. In this paper, a methodology and relevant limits are presented, which address fundamental power quality considerations and are applied by electric utility engineers. Issues addressed are the steady state and fast voltage variations, flicker and harmonic emissions. Simplified evaluation procedures are presented, largely based on the relevant IEC publications, which are suitable for application by utility engineers in practical situations. Key-Words: Distributed Generation, Power Quality, Distribution Networks, Voltage Variations, Flicker, Harmonics. 1 Introduction The penetration of Distributed Generation (DG) resources (wind turbines, photovoltaics, fuel-cells, biomass, micro-turbines, small hydroelectric plants etc., ranging from sub-kW to multi-MW sizes) in distribution grids is increasing world-wide. The incorporation of DG units in the grids alters their traditional operating principle and poses new problems, regarding power quality, supply reliability and safety of operation. To speed-up the evaluation and connection process, without compromising the operating and safety requirements of the grid, proper technical evaluation procedures are required, which must be transparent, objective, widely accepted and, most important, easily applicable by utility engineers. This is now recognized by utilities and international organizations, working for the adoption of uniform technical procedures (e.g. [2-6]). In this paper a framework of technical criteria and requirements is presented, which permits the efficient and reliable evaluation of new DG installations, regarding their connection to the grid. The methodology presented mainly concerns installations intended for connection to the MV level (typically sized above a few hundred kW). Issues addressed here are the slow and fast voltage variations, flicker and harmonic emissions. Other important considerations (protection requirements, network capacity, fault level contribution etc.) are only briefly commented, due to space limitation reasons. 2 Slow Voltage Variations Traditionally, utilities have imposed limiting values to the acceptable steady state voltage deviations from the nominal value, both at the MV and LV levels, which should not be exceeded in normal operation of the system. During the last decade, the statistical nature of the voltage variations has been recognized and relevant norms have been issued, such as the European Norm EN 50160, [7], which imposes statistical limits, in the sense that a small probability of exceeding them is acceptable. The evaluation procedure presented in the following ([6]) utilizes 10-min average values of the voltage and can be applied in two stages. At a first stage, the maximum steady-state voltage change ε(%) at the PCC is evaluated, using the following relation ( ) ( ) 100 (%) 100 cos cos 3% n k k k S S R ε ψ φ ψ φ ≅ + = + ≤ (1) where S n is the maximum continuous output power of the DG installation, S k the network short circuit capacity at the PCC, ψ k the phase angle of the network impedance and φ the phase angle of the DG output current (using generator convention). R=S k /S n is the short circuit ratio at the PCC.