Overcurrent Protection and Voltage Sag Coordination in Systems with Distributed Generation

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.