1 Coordination of relay protection system of a biomass power plant Spin Valis *Matej Žnidarec, *Srete Nikolovski, *Damir Šljivac, *Danijel Topić *Faculty of Electrical Engineering, Computer Science and Information Technology Osijek/Department of Power Engineering/Osijek/Croatia mznidarec@etfos.hr, srete@etfos.hr, sljivac@etfos.hr, dtopic@etfos.hr Abstract — In this paper coordination of overcurrent relay protection of biomass power plant Spin Valis International among with surrounding distribution network was performed. Paper described working process of power plant, all electrical parameters of the facilities of which the simulation model was made of and the way of achieving coordination and selectivity by using results of short-circuit simulations. System check of overcurrent protection system coordination is conducted by simulations of three phase and single phase short-circuit faults for different places in grid. Main and backup relay trip check was based on short- circuit current values and current-time characteristics of relays near short-circuit fault location. I. INTRODUCTION As humankind’s energy needs grew through 20 th century, electric grid developed into the most complex system in the world. Because electrical engineers couldn’t manage system like grid without help of automation, they developed protection system which consist of relays. This paper’s goal is to achieve coordination of overcurrent relay protection of biomass combined heat and power (CHP) power plant Spin Valis International. Power plant International shown in Fig 1. is owned by wood industry Spin Valis Ltd. located in Požega, Republic of Croatia. Overcurrent protection system protects elements of network from harmful effects like thermal overheating, mechanical stresses and humans from high touch voltages. Requirements that every protection system must fulfill are fastest time of reaction, selectivity, sensitivity and reliability. In this paper, coordination of overcurrent protection system was realized with modern computer software tool for analysis of electric power systems in short-circuit states and relay system coordination. First chapter described technical specification of the power plant and surrounding distribution network. In second chapter, modelling process of power plant and surrounding distribution process is described. Last two chapter dealt with coordination of overcurrent system using short-circuit analysis and time-current curves of relays. Coordination check of overcurrent protection was also done by observing individual relay trip in a case of short-circuits for various of places in simulation model. In a case that main relay doesn’t trip, backup relay should clear the fault if it exists. Figure 1. Biomass CHP biomass power plant Spin Valis International II. TECHNICAL SPECIFICATION OF POWER PLANT AND SURROUNDING DISTRIBUTION NETWORK Biomass CHP power plant Spin Valis International is integrated into existing 10 kV distribution network. Power plant uses Organic Rankine Cycle for electricity and heat production. Three phase squirrel cage induction generator produces 1525 kW of power. Generator data is given in Table 1. Facility produces 4 MW of excess heat for district and industrial use [1]. TABLE I. INDUCTION GENERATOR Type Squirrel cage three phase induction generator Rated power 1818 kVA Vector group Delta Rated voltage 660 V 5%  Frequency 50 Hz Rated rotor speed 3028 min -1 Rated stator current 1583 A Power factor 0.88 ind. Nearby power plant a 110/35/10 kV supply substation Požega 1 which is operated by national transmission system operator HOPS is placed. According to Ref. [2] there are two operating states of surrounding distribution network and power plant, normal and auxiliary. Normal operating state is accomplished with connection of distribution network and power plant to the 110/35/10 kV substation Požega 2 over 10 kV line bay Spin Valis. Substation consists of four transformers. First pair consists of two parallel connected 110/35 kV transformers. Second pair consists of two parallel connected 35/10 kV transformers. 110/35 kV