Overcurrent Protection and Voltage Sag Coordination in Systems with Distributed Generation J. Carlos Gomez 1 M. M. Morcos 2 1 Rio Cuarto National University, Rio Cuarto, Cordoba, ARGENTINA 2 Kansas State University, Manhattan, KS, USA

Introduction 4 It has been predicted that by the year 2010 approximately 20 % of the new generation will be distributed generation (DG) 4 Currently an extensive task is being carried out by the IEEE SCC 21 – in the new IEEE Standard P1547 – which will provide guidelines for interconnecting distributed generation with the power system.

Distributed Resources 4 Defined as sources of electrical power that are not directly connected to a bulk-power transmission system, including both generators and energy storage technologies 4 Main power generators used as DG:  Wind turbines  Fuel cells  Photovoltaic arrays  Small and micro turbines  Internal combustion engines.

Overcurrent Protection Issues 4 The new scenario will introduce changes in system behavior and flow of power under short-circuit conditions 4 Need for verification of the protective device breaking-capacity 4 Induction generators will show a special behavior when a short circuit takes place 4 Short-circuit current value and transient behavior of generator that provides power through inverters are different from synchronous generator response.

Voltage Sag Ridethrough Capability of Sensitive Equipment 4 Voltage sag is considered as a non-permanent voltage reduction with values between 10 % and 90 % of the rated voltage 4 The ability of sensitive equipment (SE) to withstand voltage sags without dropout is called ride-through capability 4 Computer Business Equipment Manufacturing Association (CBEMA) curve was adopted as ridethrough capability guideline.

Coordination between Overcurrent Protection and SE Voltage Sag Ridethrough Capability 4 Islanded Mode Operation is the situation when the main supply is disconnected from the power system having at least one DG, and continues to operate with this single source 4 The effect of this situation on the coordination between overcurrent protection and the voltage sag ride-through capability of SE needs to be studied.

Classical Study 4 The coordination study is done in a graphic form, comparing the adapted TCC of the protective device with the CBEMA curve 4 Adapted protective device TCC is a curve transformed into TVC, that represents the voltage sag which the protective device allows to be applied to the SE under study 4 PCC is defined as the point of the circuit where the SE current is separated from the distorted (or too- high) current path.

Circuit with Distributed Resources 4 When the islanding circuit breaker (ICB) is closed the source impedance is approximately the parallel combination of the utility and DG impedances 4 When ICB is open the source impedance jumps to a larger value.

Protective Device (100A and 200 A fuses) 4 Homogeneous fuses have parallel TCC curves 4 For 200 ms, will need melting currents of 600 A and 1200 A 4 Fuse rated currents in pu of the circuit rated current result 0.1 and 0.2, and base current is 6000 A. 4 For 100 A fuse, V s (%) = 100 – (0.04 * 0.1 * 6000) = 97.6 % 4 For 200 A fuse, V s (%) = 95.2 %

Coordination Graph 4 V s = V EPS – (Z 1 // Z DR ) x I sc where, V s = voltage sag value V EPS = electric power system voltage Z 1 = utility impedance Z DR = distributed resource impedance I sc = short-circuit fault current

New Coordination Scenario 4 If the ICB opens during parallel operation the source impedance increases 4 For example changing the source impedance from 0.04 to 0.06 pu and maintaining similar rated currents 4 Protection given by the 100 A fuse is still satisfactory, but the 200 A fuse curve intersects with the immunity curve.

Conclusions 4 Sensitive equipment protection against voltage sags can be provided to overcurrent protective devices 4 Protective device TVC moves into a zone which will be up and to the left of the SE immunity curve 4 The area is bordered by the two TVCs of the maximum protective device, and will be wider as the difference between the utility and DR impedances increases.