Download overcurrent protection and voltage sag coordination

OVERCURRENT PROTECTION AND VOLTAGE SAG COORDINATION
IN SYSTEMS WITH DISTRIBUTED GENERATION
Juan Carlos Gomez
Rio Cuarto National University
Rio Cuarto, Cordoba
ARGENTINA
M. M. Morcos
Kansas State University
Manhattan, KS 66506
USA
Abstract- Overcurrent protection and voltage
sag coordination studies are based on the upstream impedance of the point of common
coupling (PCC) or system impedance (Z1).
Voltage sag magnitude is obtained taking into
account the voltage drop on Z1. Sag duration
depends on the operating time of the overcurrent
protective device. The normal coordination
procedure is obtained by comparing the sensitive
equipment (SE) dropout curve with the timevoltage characteristic (TVC) of the protective
device. Under normal operating system
conditions Z1 is approximately constant, but in
the case of distributed generation this impedance
can change from a low value to a high value
when the non-utility generators are the only
supply. The new scenario implies that the
protective device TVC is changed into a zone
which modifies previous methodologies and
increases the SE dropout susceptibility.
Index terms- Distributed generation, power
quality, voltage sags, overcurrent protection.
Each of these power generators has its own
advantages and disadvantages, such as easy
installation based on its small size, fast start,
voltage distortion generator, and uncertain
power availability. In spite of the distribution–
generation promotion carried out by utilities,
they have been expressing concerns related with
potential distribution system problems.
I. INTRODUCTION
One of the important issues that the new
scenario will introduce is overcurrent protection,
where there will be changes in system behavior
and flow of power under short-circuit
conditions. Normally there is no concern with
the DR when its power is less than 10 % of the
minimum load demand in the feeder [1].
It has been predicted that by the year 2010
approximately 20 % of the new generation will
be distributed generation (DG) [1]. There are
several types of power generators that can be
used as DG which, in order of importance, are:





Wind turbines
Fuel cells
Photovoltaic cells
Small and micro turbines
Internal combustion engines.
Currently an extensive task is being carried
out by the Standards Coordination Committee
21 (IEEE SCC 21), where there are numerous
activities by the working group in the new IEEE
Standard P1547 which will provide guidelines
for interconnecting distributed generation with
the power system [2].
In general, distributed resources are defined
as sources of electrical power that are not
directly connected to a bulk power transmission
system. They include both generators and energy
storage technologies, with power rating of 10
MW or less [2].
The most immediate consequence is the need
for verification of the protective device breaking
capacity, which might not be enough due to the
increase of the available short-circuit power.
Short-circuit current value and transient
behavior of generator that provides power
through inverters - such as photovoltaic cells,
fuel cells, wind turbines, and induction
generators - are completely different from the
synchronous generator response. Induction
generators directly connected to the supply will
show a special behavior when a shortcircuit
takes place. Due to the fact that this type of
generator gets its excitation from the mains, it is
not able to maintain the short-circuit currents for
a relatively long time.
The situation when the main supply is
intentionally or unintentionally disconnected
from the power system having at least one DG,
which continues operating with this single
source, is called Islanded Mode Operation. An
aspect that will be studied is the coordination
between overcurrent protection and the voltage
sag ride-through capability of sensitive
equipment (SE).
Voltage sag is almost universally considered
as a non-permanent voltage reduction with
values between 10 % and 90 % of the rated
voltage. The ability of sensitive equipment to
withstand voltage sags without dropout is called
ride-through capability. Many typical ridethrough capability curves have been proposed as
guidelines. The present work is confined to the
lower part only of the voltage sag ride-through
capability curves adopting the Computer
Business Equipment Manufacturing Association
(CBEMA) curve as guideline.
generally separated from the distorted or toohigh current path. In this case it is the shortcircuit main flow path as can be seen in Fig. 1.
Adapted protective device TCC is a curve
transformed into time-voltage characteristic
(TVC), that represents the voltage sag which the
protective device allows to be applied to the SE
under study. Voltage sag applied to SE is
directly the voltage at the PCC or the supply
voltage minus the voltage drop across Z1 due to
short-circuit current [3].
B. Circuit with Distributed Resources
Figure 1 shows the new scheme of the system
with utility and distributed generation where DG
may be representing more than one single
device. When the islanding circuit breaker (ICB)
is closed the source impedance is approximately
the parallel combination of the utility (Area
EPS) and DG impedances.
When ICB is open the source impedance
jumps to a larger value. In case of a fault (Fig.
1), short-circuit fault current will be greatly
attenuated and the operating time of the
protective device will be increased subsequently.
The coordination study needs to be done only
for the two branches on the DG side of the ICB,
with maximum and minimum rated currents of
the protective device.
II. COORDINATION BETWEEN OVERCURRENT
PROTECTION AND SENSITIVE EQUIPMENT
VOLTAGE IMMUNITY
PCC
DG
ZDR
SE
A. Classical Study
The coordination study between the
overcurrent protection device (PD) and the SE
voltage immunity located in parallel branches or
feeders, is done in graphic form, comparing the
adapted time-current characteristic (TCC) of the
protective device with the previously cited
CBEMA curve.
Basically, the PCC will be defined as the
point of the circuit where the SE current is
PD
ICB
EPS
Z1
Fig. 1 System with utility and distributed
resource
C. Transformation of Protecting Device TimeCurrent Characteristic into Time–Voltage
Characteristic
Considering the utility and the DG have
nearly the same Voltage Regulation value, the
voltage sag magnitude at the SE connection
point (or PCC) is given by,
Vs = VEPS – (Z1 // ZDR) x Isc
(1)
Where,
Vs = voltage sag value
VEPS = electric power system voltage
Z1 = utility impedance
ZDR = distributed resource impedance
Isc = short-circuit fault current
The product Z x Isc in (1) can be identified.
During the protective device TVC calculation,
once the rated current has been adopted, each
selected time value will correspond to one value
of the voltage sag magnitude. Isc can be
expressed as function of the PD rated current,
thus the fault current will be a number of times
the rated current.
Normally fuses are built in a
homogeneous way, with the same structure
and with characteristic curves approximately
parallel, thus increasing the number of fuse
elements will increase the rated current.
Hence, the Isc value expressed as multiple of
the fuse rated current has a multiplier that
increases with the decrease in the fuse rated
current and vice versa. A short-circuit
current of 1,000 A will trigger the operation
of fuse of 100 A rated current in nearly 1.4
cycles and the same current will melt the
200 A rated fuse in approximately 20 cycles.
In conclusion, the product Z x Isc can be
considered as function of the source
impedance and PD rated current. Thus, a
reduction in the short-circuit power
(islanding mode) is equivalent to an increase
in the rated current of the protective device
and vice versa.
The previous analysis, although was applied
to fuses, is applicable also to circuit breakers and
to any other type of low-voltage protective
device.
The effect of protective devices rated-current
can be seen in Fig. 2, where the TVCs of the two
fuses mentioned above where plotted. To clarify
the procedure, the methodology for obtaining a
point on each curve is as follows. Considering
an application with a load rated current of 1000
A and a source impedance of 4 % (source
impedance voltage drop - due to rated current is of the same value as voltage regulation), with
two feeders coming out from the bus-bar. They
are protected by fuses with rated currents of 100
A and 200 A, respectively.
An arbitrary time of 10 cycles (200 ms) was
selected for the explanation, for which the fuses
will need currents (to melt) of 600 A and 1200
A, respectively. The fuse rated currents
expressed in pu of the circuit rated current result
0.1 and 0.2, respectively, then the base current
for 10 cycles is 6000 A. Voltage sags which the
fuses allow to be applied to SE connected to the
parallel feeders are:


For the 100 A fuse; Vs (%) = 100 – (0.04 *
0.1 * 6000) = 97.6 %
For the 200 A fuse, Vs (%) = 100 – (0.04 *
0.2 * 6000) = 95.2 %.
The same procedure allows the calculation of
the complete fuse TVC curves.
D. Effect of Protective Device Rated Current on
Coordination
The protecting device rated current is crucial
for the coordination between protecting device
and SE immunity curve. The reason is due to the
fact that, as the rating current increases the
curves go nearer, and when they intersect there
are points of mis-coordination.
E. Transformation of the Protective Device TVC
in a Time–Voltage Zone
If during parallel operation of utility and DG,
due to any cause, the IB opens, the circuit
suffers a source impedance increase, modifying
the situation. Fig. 2 was drawn for a similar
situation as before, but changing the source
impedance from 0.04 to 0.06 pu and maintaining
similar fuse rated currents. It can be seen that the
protection given by the 100 A fuse is still
satisfactory, but the curve representing the 200
A fuse now intersects with the immunity curve
of the sensitive equipment, which might signify
SE dropout.
100
Voltage (%)
80
by the curve locations when the supply is the
utility plus DG and the new curve location
having DG as only supply. In the present case, it
is the zone enclosed by the positions of the 200
A-fuse curves for both source impedances.
III. CONCLUSIONS
Sensitive equipment protection against
voltage sags can be given for overcurrent
protective devices, provided that careful
coordination has been carried out. The readily
available protective device TVC moves now into
a zone which shall be up and left to the SE
immunity curve if effective coordination is
desired. The area is bordered by the two
locations of the maximum protective device
TVCs, and will be wider as the difference
between utility and DR impedances increases.
60
IV. REFERENCES
40
20
0
0.01
0.1
CBEMA
1
10
100
Time (cycles)
Fuse 100 A
1000
10000
Fuse 200 A
Fig. 2 Fuse time-voltage characteristics and
CBEMA curve for DG alone
It may be deduced that the protective device
TVC has been transformed into a zone bordered
[1] A.M. Borbely and J.F. Kreide, Distributed
Generation, The Power Paradigm for the
New Millennium, CRC Press, Boca Raton,
FL, 2001.
[2] IEEE Draft Standard for Interconnecting
Distributed Resources with Electric Power
Systems, IEEE Standard P1547, 2001.
[3] J.C. Gómez and M.M. Morcos, “Voltage sag
mitigation using overcurrent protection
devices,” Electric Power Components and
Systems, Vol. 29, pp. 71-81, 2001.