Transcript Overcurrent Protection and Voltage Sag Coordination in Systems

Overcurrent Protection and Voltage
Sag Coordination in Systems with
Distributed Generation
J. Carlos Gomez1
1Rio
M. M. Morcos2
Cuarto National University, Rio Cuarto,
Cordoba, ARGENTINA
2Kansas State University, Manhattan, KS, USA
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.
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:
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


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Wind turbines
Fuel cells
Photovoltaic arrays
Small and micro turbines
Internal combustion engines.
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.
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.
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.
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 toohigh) current path.
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.
Protective Device
(100A and 200 A fuses)
 Homogeneous fuses have
10000
1000
Time (seconds)
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.
100
10
1
0.1
 For 100 A fuse, Vs (%) =
100 – (0.04 * 0.1 * 6000) = 97.6 %
 For 200 A fuse, Vs (%) = 95.2 %
0.01
10
100
1000
Prospective current (A)
10000
Coordination Graph
 Vs = VEPS – (Z1 // ZDR) x Isc
100
80
Voltage (% )
where,
Vs = voltage sag value
VEPS = electric power
system voltage
Z1 = utility impedance
ZDR = distributed resource
impedance
Isc = short-circuit fault
current
60
40
20
0
0.01
0.1
1
10
100
1000
Time (cycles)
CBEMA
Fuse 100 A
Fuse 200 A
10000
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.
100
Voltage (%)
80
60
40
20
0
0.01
0.1
1
10
100
1000
10000
Time (cycles)
CBEMA
Fuse 100 A
Fuse 200 A
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.