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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 19, NO. 4, OCTOBER 2004
Validation of ZnO Surge Arresters
Model for Overvoltage Studies
Micaela Caserza Magro, Student Member, IEEE, Marco Giannettoni, and Paolo Pinceti
Abstract—This paper presents an improvement of the procedure to determine the parameters of the simplified model for
metal-oxide surge arresters, derived from the IEEE standard
model. The main innovation introduced by the paper lays in the
possibility to define an approximate dynamic model even if data
about residual voltages for steep current pulse are not defined in
the manufacturer’s data sheets. This model has a wide rangeability
and its effectiveness is good for both medium- and high-voltage
arresters. The effectiveness of the model was tested for several arresters of different manufacturers. The residual voltages reported
in the datasheets and obtained by the manufacturers through a
discharge test are compared with the simulations performed with
Matlab. The possibility of defining a dynamic model for surge
arresters even with missing data makes the proposed model a
useful tool for insulation coordination studies involving steep front
transients.
Index Terms—Arresters, overvoltage protection, surge protection.
I. INTRODUCTION
M
ETAL-OXIDE surge arresters (MOA) are used as protective devices against lightning and switching overvoltages in medium- and high-voltage power systems. Simulation
tools are necessary to perform studies of insulation coordination in electrical power systems. Until now, several models have
been proposed to describe the arresters’ behavior under different
voltage/current stresses. The hardest task is the definition of a
dynamic model for steep surges. For waveshapes with a time
to crest shorter than 8 s, the dynamic effects play an essential
part. In fact, the residual voltage increases as the time to crest
of the current pulse decreases and the residual voltage peak anticipates the current pulse crest as well.
It is very often difficult to identify the dynamic model parameters from the available data. The model proposed by the IEEE
W.G. 3.4.11 [1] represents the arrester as two nonlinear resistors separated by an R-L filter. The filter elements are calculated
by an iterative procedure. The starting values can be calculated
starting from the residual voltages and the physical data of the
arrester. The trouble with this model is the procedure to approximate the parameters; in fact, iterative corrections on different
elements are required until a satisfactory behavior is obtained.
Starting from the IEEE model, a simplified model was proposed in [2]. In the simplified model, parameters are identified
Manuscript received July 22, 2003; revised September 29, 2003. Paper no.
TPWRD-00389-2003.
The authors are with the Department of Electrical Engineering, University
of Genova, Genova 16145, Italy (e-mail: caserza@die.unige.it; giannettoni@die.unige.it; pinceti@die.unige.it.).
Digital Object Identifier 10.1109/TPWRD.2004.832354
using a formula that does not require any iterative correction
and that makes use only of the data reported on manufacturer’s
datasheets. The comparison of the residual voltage calculated
with this model and the residual voltage reported in the constructor’s datasheets for a given current pulse show fair accuracy.
Experiences using the simplified model in overvoltage studies
have shown two major problems.
1 ) The model may loose accuracy for low-range mediumvoltage (MV) surge arresters: error may raise up to 10%
for MOA in the range between 3 kV and 30 kV.
2 ) In the datasheets of several manufacturers, not all of the
data required to calculate the dynamic parameters are
available. Very often, the manufacturer does not declare
the residual voltage for steep pulse (with a risetime
between 0.5 and 1 s).
The goal of this paper is to present an enhanced procedure to
define the simplified model parameters respecting the following
issues.
• The model is accurate also for medium-voltage
(MV) arresters in the whole range of rated voltages.
• The model can be defined also in case of missing
data in the datasheet (even if with poorer accuracy).
II. MODEL PORTING
The model was implemented in the Matlab environment [3],
[4] instead of in the alternative transient program (ATP). The
use of Matlab gave us the following advantages.
• The integration algorithm has been modified, and
this allows avoiding additional resistors that both the
IEEE model and the simplified model had to introduce
to avoid integration errors (Fig. 1). Also, resistor R in
Fig. 1(b) is not necessary any more.
• The simulation environment is flexible; results can
be presented using customized graphics, and specific
logic can be introduced to automate voltage coordination studies.
The result obtained by the simulation is the voltage at the
arrester terminals after applying a current pulse to the arrester
itself. The residual voltage is the maximum value of this voltage.
The nonlinear resistors A0 and A1 are based on the curves
proposed by the IEEE W.G. 3.4.11 which are shown in Fig. 2.
Voltages are expressed in per unit, referring to the residual
voltage measured during a discharge test with a 10-kA light.
ning current pulse
0885-8977/04$20.00 © 2004 IEEE
MAGRO et al.: VALIDATION OF ZnO SURGE ARRESTERS MODEL FOR OVERVOLTAGE STUDIES
Fig. 1.
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Additional resistors. (a) IEEE dynamic model. (b) Simplified model.
Fig. 2. Characteristic of the two nonlinear elements A0 and A1 proposed by
the IEEE W.G. 3.4.11.
The two inductances (mH), in the simplified model, are calculated by the following equations:
(1)
(2)
is the
where is the arrester rated voltage (in kilovolts);
residual voltage for 10-kA lightning current pulse with 8/20-ms
is the residual voltage for 10-kA steep curshape (kV);
rent pulse (
s) [in kilovolts]. The decrease time T2 is not
specified here since it can vary between 2 and 20 s and each
manufacturer can choose the preferred value.
Data required to calculate the above equations are not alawys
reported in the datasheets. A procedure to overcome this
problem is presented in the next section.
III. MODEL TESTING WITH EXTENDED DATABASE
The simplified model in [2] was tested only with arresters
having maximum continuous operating voltage (MCOV) between 4 and 37 kV. To evaluate the application range of this
model, it has now been considered a larger set of arresters with
a MCOV ranging between 3 and 360 kV, called in the following,
the extended database. More than 200 arresters from six manufacturers from all around the world have been considered. The
list of the considered arresters is reported in Appendix A.
V
Fig. 3.
Errors on residual voltages for 10-kA lightning pulse (
Fig. 4.
K versus rated voltage.
).
Simulation results presented herein after refer to one family
of arresters from Appendix A, with complete datasheets. The
simulated arresters belong to discharge classes 2, 3, and 4.
As Fig. 3 shows, the simplified model is not accurate for
low-range medium-voltage surge arresters; error may raise up
to 10% in the range between 3 and 30 kV for a 10-kA lightning
pulse. Fig. 3 also shows that the relative error for values higher
than 30 kV is fairly small, around 1%. The relative error is
computed, comparing the calculated voltage with the residual
voltage reported in the manufacturer’s datasheets
(3)
where
is the residual voltage obtained by running the
is the residual voltage reported in the
simulation, and
manufacturer’s data sheet.
The inaccuracy in the medium-voltage (MV) range of the
simplified model can be ascribed to the dynamic part. In fact,
since the error is positive, the computed residual voltage is
higher than the corresponding residual voltage reported in the
datasheets. This fact may come from a too high value of the
and ).
dynamic parameters (i.e.,
Voltages indicated in the datasheets are obtained by means of
laboratory tests so they include both static and dynamic effects.
To verify this hypothesis, a new parameter based on the
residual voltages to the lightning and steep front pulses is
introduced. This parameter is defined as
where the residual voltages are available in the datasheets, and
it is plotted as a function of MCOV in Fig. 4.
has the same behavior
It is apparent that the parameter
of the relative error. For high values of , the error increases,
while when is below about 1.18, the model is quite accurate.
According to this, new values for the dynamic elements are defined.
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 19, NO. 4, OCTOBER 2004
Fig. 5. Errors on residual voltages for 10-kA lightning pulse (V
the model for an extended database.
) using
Fig. 7. Errors on residual voltages for 10-kA lightning pulse (V
) using
the enhanced model without data on residual voltages to a steep front pulse.
Fig. 6. Errors on residual voltages for 10-kA steep pulse (V r
enhanced model.
) using the
Fig. 8. Errors on residual voltages for 10-kA steep pulse (V ) using the
enhanced model without data on residual voltages to a steep front pulse.
When is below 1.18, inductances are defined as in the simplified model, using (1) and (2).
For higher values of , we found out that inductances are
better defined using (4) and (5)
(4)
(5)
where
is the MOV-rated voltage.
. Tests
These values have been obtained assuming
show that this value leads to the best match of calculation with
the real discharger behavior.
The model calculated with the enhanced procedure has been
tested with the extended database of arresters with a 10-kA
- s shape). As Fig. 5 shows, the model
lightning pulse (
is quite accurate for the whole range of rated voltages since the
error is always below 1%.
To determine if the behavior remains accurate also for transients due to steep front surges, the test was extended to 10-kA
- s shape.
steep pulse with
As Fig. 6 shows, the model’s behavior is quite good in the
range of the MV since the error is around 2.5%.
Results obtained with arresters from different manufacturers
confirm the enhanced model validity for transients due to steep
current pulses. Figs. 5 and 6 confirm that the accuracy of the
model is quite constant even with varying discharge classes.
IV. MODEL RANGEABILITY
To calculate the two inductances
and
with (1) and (2),
it is necessary to know the residual voltage for a steep current
pulse. If these data are not available, we assumed that the two
inductances can be calculated using (4) and (5). This is equivais equal to 1.12
.
lent to assuming that
To validate this hypothesis, the arresters with a complete
datasheet have been selected. Residual voltages for 10–kA
s) have been calculated using the
lightning pulses (
model obtained with (4) and (5), and compared with the
residual voltages reported in the datasheets.
Results are shown in Fig. 7. The model seems to be quite
accurate, with an error of about 1%, not very different from the
and
are calculated from (1) and
error of the model where
(2).
s) gave good
Also, the tests with a 10-kA steep pulse (
results as shown in Fig. 8. The average error is about 3%.
Of course, the accuracy of the approximate model for steep
pulses is lower than the accuracy of the model calculated using
(see Fig. 6).
the real
Nevertheless, the approximate model makes it possible to run
reasonable simulations with steep pulses also with missing data.
is supported by a logic
Assume a conventional value for
consideration. In fact, there is no reason not to believe that arresters with poor datasheets do not perform like arresters with
more detailed datasheets, since datasheets do not affect currents
and voltages.
V. CONCLUSION
A procedure for defining an approximate model when data
on residual voltages to steep current pulses are not available
is proposed in the paper. The model is based on the simplified
model [2], with R removed, but its rangeability is extended to
the whole HV and MV level. To calculate the model parameters
for low range MV arresters, two new formulas are introduced.
The accuracy of the model calculated with the new procedure
was tested with a large set of arresters with satisfactory results
in the whole range.
MAGRO et al.: VALIDATION OF ZnO SURGE ARRESTERS MODEL FOR OVERVOLTAGE STUDIES
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VariSTAR AZG4 (rated voltages 3–400 kV).
GE:
Tranquell (rated voltages 3–144 kV).
Joslyn:
ZIP (rated voltages 3–144 kV);
ZSP (rated voltages 3–192 kV).
Passoni-Villa:
SCA (rated voltages 60–420 kV).
Ohio Brass:
DynaVAR VH2 (rated voltages 3–150 kV);
DynaVAR VH3 (rated voltages 3–192 kV);
DynaVAR VH4 (rated voltages 84–396 kV).
REFERENCES
[1] Working Group, “Modeling of metal oxide surge arresters,” IEEE Trans.
Power Delivery, vol. 7, pp. 301–309, Jan. 1992.
[2] M. Giannettoni and P. Pinceti, “A simplified model for zinc oxide surge
arresters,” IEEE Trans. Power Delivery, vol. 14, pp. 393–398, Apr. 1999.
[3] Using MATLAB, Version 6.1.
[4] MATLAB Function Reference (Volume 1: Language), Version 6.1.
Fig. 9. Flowchart to calculate elements L and L .
The flowchart in Fig. 9 shows how to calculate the values of
the dynamic parameters in all of the possible conditions. First,
the availability of the residual voltages to the steep front pulses
is checked. If
is not reported on the datasheets,
the dynamic parameters are computed using the formulas depending only on the rated voltage . Otherwise, the parameter
is computed. If the value of is lower than
1.18, dynamic parameters are calculated using
; otherwise, the parameters are calculated using the rated voltage .
APPENDIX A
List of tested surge arresters.
—
ABB:
—
PEXLIM R (rated voltages 42–192 kV);
—
PEXLIM Q (rated voltages 42–360 kV);
—
PEXLIM P (rated voltages 42–360 kV);
—
EXLIM R (rated voltages 42–168 kV);
—
EXLIM Q-E (rated voltages 42–228 kV);
—
EXLIM Q-D (rated voltages 132–420 kV);
—
EXLIM P (rated voltages 42–444 kV);
—
EXLIM T (rated voltages 180–624 kV).
—
Cooper Power Systems:
—
VariSTAR AZG2 (rated voltages 3–240 kV);
—
VariSTAR AZG3 (rated voltages 3–312 kV);
Micaela Caserza Magro (S’02) was born in Genova,
Italy, in 1976. She received the electrical engineering
degree from the University of Genova, Genova, Italy,
in 2001, where she is currently pursuing the Ph.D.
degree.
Marco Giannettoni was born in Genova, Italy, in
1970. He received the Ph.D. degree in electrical engineering from the University of Genova, Genova,
Italy, in 2001.
His research interests include communication systems (field bus) for industrial automation.
Paolo Pinceti was born in Genova, Italy, in 1957. He
received the Ph.D. degree in electrical engineering
from the University of Genova, Genova, Italy, in
1987.
Currently, he teaches “Measurement for Industrial
Applications” at the University of Genova. His research interests include power system measurement,
protection, and automation.