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Overcurrent Protection and Voltage Sag Coordination in Systems with Distributed Generation

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.

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Overcurrent Protection and Voltage Sag Coordination in Systems with Distributed Generation

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  1. Overcurrent Protection and Voltage Sag Coordination in Systems with Distributed Generation J. Carlos Gomez1 M. M. Morcos2 1Rio Cuarto National University, Rio Cuarto, Cordoba, ARGENTINA 2Kansas State University, Manhattan, KS, USA

  2. Introduction • It has been predicted that by the year 2010 approximately 20 % of the new generation will be distributed generation (DG) • 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.

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

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

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

  6. Coordination between Overcurrent Protection and SE Voltage Sag Ridethrough Capability • 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 • The effect of this situation on the coordination between overcurrent protection and the voltage sag ride-through capability of SE needs to be studied.

  7. Classical Study • The coordination study is done in a graphic form, comparing the adapted TCC of the protective device with the CBEMA curve • 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 • PCC is defined as the point of the circuit where the SE current is separated from the distorted (or too-high) current path.

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

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

  10. Coordination Graph • Vs = VEPS – (Z1 // ZDR) x Isc where, Vs = voltage sag value VEPS = electric power system voltage Z1 = utility impedance ZDR = distributed resource impedance Isc = short-circuit fault current

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

  12. Conclusions • Sensitive equipment protection against voltage sags can be provided to overcurrent protective devices • Protective device TVC moves into a zone which will be up and to the left of the SE immunity curve • 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.

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