Recent Researches in Circuits and Systems
Graphical Analytical Method to estimate Energy Coordination of
Two Varistors connected in parallel
Hitoshi Kijima, Koreyuki Taketani,
Ryu Tobisawa, Susumu Sakamoto
Electrical Department
Polytechnic University
4-1-1 Hashimotodai Sagamihara 229-1196 Kanagawa
JAPAN
Abstract: Surge Protective Devices (SPDs) are used for protecting apparatus against overvoltage or overcurrent
caused by lightning surge. Typical SPD is a varistor (variable resistor). Varistors can restrict to almost constant
voltage across terminals, and response time is fast, but current capability is small. Therefore two varistors are
normally connected in parallel. It is difficult to analyze the coordination of energy injected into a varistor when two
varistors are installed in parallel on the same line. Because varistor has non linear V-I characteristics, energy
coordination has so far been confirmed only by experimental methods. We propose a new graphic analytical
method to estimate energy coordination in the case of installing two varistors in parallel on the same line.
Key-Words: Lightning, Overvoltage, Surge protective device, Energy Coordination, Varistor
important
requirement
for
designing
and
implementing overvoltage protection of apparatus is
energy coordination between a Class I SPD (current
waveform specifying discharge capacity with
wavehead/wavetail ratio equal to 10/350 μs) installed
on the incursion input (initial stage) of a lightning
surge and a Class II SPD (current waveform
specifying discharge capacity with wavehead/wavetail
ratio equal to 8/20 μs) installed near the protected
apparatus. The Class I SPD (10/350 μs) mainly
discharges most of the impulse current to the earth to
ease the stress caused by the impulse current flowing
into the Class II SPD (8/20 μs). The discharge capacity
of the Class II SPD (8/20 μs) is smaller than that of the
Class I SPD (10/350 μs), but the limiting voltage
(protection level) is low. Therefore, its role is to limit
the incursion overvoltage to an acceptable value. In
this case, it is necessary to set conditions for current
division between the Class I SPD (10/350 μs) and the
Class II SPD (8/20 μs) so that a current exceeding the
discharge capacity does not flow through the Class II
SPD (8/20 μs).
1 Introduction
Both the characteristics of lightning surge and
overvoltage protection technologies have been
introduced in the articles [1]-[17]. Surge Protective
Devices (SPDs) are used for protecting apparatus
against overvoltage or overcurrent caused by lightning
surge.
Typical SPD is a varistor. Varistor is a ceramic
made of Zinc Oxide powder. The V-I characteristic of
a varistor is non-linear. Varistors can restrict to almost
constant voltage across terminals, and response time is
fast, but current capability is small. Therefore two
varistors are normally connected in parallel [18] - [26].
However there is no standard for the coordination
behavior of a multistage protective circuit where two
SPDs are installed in parallel. Therefore, each
manufacturer selects its own SPDs to form protective
circuits. However, some combinations of the
characteristics of these SPDs may cause an incursion
lightning surge current to concentrate only in one SPD
with low operation voltage. .
Therefore it is necessary to establish a systematic
idea for the application of both SPDs. In this study,
when two voltage-limiting SPD’s are connected in
parallel, we propose a graphical analytical method for
the energy coordination of both SPD’s. From the
standpoint of EMC, it is necessary to construct
systems in which multiple SPD’s are installed in
parallel with coordinated energy to design and
implement appropriate overvoltage protection. An
ISBN: 978-1-61804-108-1
２ Problems on the Energy Coordination of
Two Varistors connected in parallel
2.1 Coordination between voltage
limiting SPD’s
72
Recent Researches in Circuits and Systems
decoupling. The following shows why a resistor is
used as a decoupling element.
Normally an inductor is used as decoupling element
in the case of installing SPD in power line due to the
reason that the impedance of an inductor is low for the
50/60Hz current flowing through the line. On the other
hand, normally a resistor is used as decoupling
element in the case of installing SPD in
telecommunication line due to the reason that the
signaling current flowing through the line is very
small so that a voltage drop is negligible.
When an inductor L is used as decoupling element
for power line, lightning current waveform (10/350 μs
or 8/20 μs) should be considered. In particular, in the
case of waveform 10/350 μs, the decoupling effect of
the inductor is limited to the range of the wave front
only. In the long wave tail portion, the voltage drop
Ldi/dt is small, that is, the decoupling cannot be
realized.
In the case of two voltage limiting SPD’s connected
in parallel, when a discharge current starts to flow
through the SPD’s, the amount of divided impulse
current that flows through each SPD depends on its
voltage and current characteristics, that is, the current
is inversely proportional to the SPD resistance value
(which is variable). In this case, coordination must be
improved by the impedances (long wires, inductors,
resistors) connected between them.
Considering low-voltage power system or
telecommunication system of buildings, apparatus
located near the entrance is generally characterized by
a high impulse withstand voltage. Deeper into the
building, the impulse withstand voltage of the
apparatus tends to become lower and more sensitive to
overvoltages. Therefore, an SPD installed at
downstream of a low-voltage power system or
telecommunication system usually has lower
operation voltage than an SPD installed at
upperstream of a low-voltage power system or
telecommunication system, and has a smaller current
discharge capacity.
To coordinate both, the upstream SPD must operate
as early as possible and bear most of the current, and
the downstream SPD must not be overloaded.
Therefore, in order to achieve coordination between
the SPD's so that the voltage applied to the protected
apparatus does not exceed the tolerance and the SPD's
do not suffer overload, an SPD installed near the
entrance of a building must have a large discharge
capacity, whereas an SPD installed deeper inside must
have a smaller discharge capacity. However, the latter
must have a low operating voltage so that impulse
currents are distributed according to the characteristics
of each SPD. Thus, it becomes necessary to connect a
decoupling impedance.
lightning current
SPDa
SPDb
Fig. 1 Multistage SPD consisting
of SPDa, SPDb and Rd
Therefore, the following numerical expression is
used on the assumption that resistor is used as a
decoupling element.
The meanings of the symbols of the numerical
expression are as follows:
V1: Voltage across SPDa terminals
i1: Current flowing through SPDa
R1: Instantaneous V1 / i1 ratio in
SPDa
V2: Voltage across terminals of
decoupling resistance Rd
i2 : Current flowing through Rd
V3: Voltage across terminals of
SPDb
i3: Current flowing through SPDb
R3: Instantaneous of V3 / i3 ratio in
SPDb
2.2Numerical expression of energy
coordination when two voltage limiting
(voltage clamping type) SPD’s are used
Figure 1 shows the basic combination of two SPD's
(hereinafter referred to as “multistage SPD”) using
two varistors as voltage limiting SPD's. It is basically
shown below that the energy coordination of SPDa
and SPDb can be analytically determined based on
their voltage-current characteristics.
In Figure 1, the discharge capacity and limitation
voltage Ures of SPDa are high, and in SPDb, the
discharge capacity is smaller and limitation voltage
Ures is lower than SPDa. Rd is a resistor for
ISBN: 978-1-61804-108-1
Rd
73
Recent Researches in Circuits and Systems
V1 = R1・i1
V2 = Rd・i2
V3 = R3・i3
where
i2＝i3
V1 = V2 +V3
When all currents are i0
i0＝i1+i2 = i1+i3
From (1) to (6)
i1 = {(R3+Rd) / (R1 +
R3+Rd )}i0
i2 = {R1 / (R1 + R3+Rd )}i0
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
Therefore, it is possible to find how the lightning
waveform current i0 is split between SPDa and SPDb.
In this case, however, R1 and R3 are constants. The
voltage-current characteristics of these elements are in
general non-linear. That is, since R1 is a function of i1
and R3 is a function of i3, it is difficult to carry out an
analytical solution.
Therefore, in this paper, we propose a graphical
analytical method and show an example of calculation
as follows.
Fig.2 V-I characteristics of SPDa ,
SPDb and Rd
By obtaining Ia and Ib for various Vc’s in Figure 2,
the V-I characteristics of the multistage SPD formed
by the combination of SPDa and SPDb can be found as
shown in Figure 3.
3 Evaluation by the graphical
analytical
method
for
energy
coordination using voltage-limiting SPD
In Figure 1, the limiting voltage Ures must be large
at the initial stage SPDa and small at the latter stage
SPDb, and must be kept lower than the impulse
withstand voltage of the protected apparatus. In
addition, the surge current waveform and energy
distribute between both SPD’s must be lower than the
value tolerated by each SPD.
Figure 2 shows an example of the voltage-current
characteristics curve A of voltage limiting SPDa at the
initial stage and an example of the voltage-current
characteristics curve B of voltage limiting SPDb at the
latter stage.
In addition, the figure shows curve D describing the
voltage-current characteristics when a decoupling
resistor Rd of 0.5 Ω - which is suitable for their
coordination - is used, and also curve C, which is the
sum of B and D. Since the terminal voltage of SPDa
and the terminal voltage of SPDb+Rd are always equal,
drawing a constant voltage line Vc parallel to the
horizontal axis in Figure 2, we can obtain currents Ia
and Ib flowing through the terminal voltages of SPDa
and SPDb from the intersections with curve A and
curve B.
ISBN: 978-1-61804-108-1
Fig.3 V-I characteristics in the
case of multistage SPD
From Figure 3, for example, it is possible to obtain
the current waveform flowing through SPDa and
SPDb when the lightning peak current of 15 kA
(10/350 μs) shown in curve I of Figure 4 strikes
directly and flows into this multistage SPD.
74
Recent Researches in Circuits and Systems
4 An example of graphical
analysis of tolerance limit
energy
Using the above procedure, we analyzed the
tolerance limit energy for a lightning current with peak
value of 700 to 5000 A.
Table 1 shows specifications of each SPD. The
tolerance limit energy of SPDa and SPDb are 50J and
27J respectively.
Table 1 Specifications of each SPD
Current(kA）
Fig. 4 Current waveform which
flows to each SPD when a direct
lightning current (15kA:10/350)
flows into multistage SPD
Energy (J)
V1mA(V)
SPDa
40
50
250
SPDb
40
27
135
For the decoupling resistance Rd, we examined the
cases Rd=0.1 Ω and Rd=0.5 Ω in addition to Rd=2Ω.
That is, by obtaining Ia and Ib corresponding to
values of current from curve I at each time and
re-plotting them at their respective times on the time
axis of Figure 4, it is possible to obtain the current
waveform curve II flowing through SPDa and the
current waveform III flowing through SPDb.
After obtaining the currents Ia and Ib flowing
through SPDa and SPDb, the voltage V across
terminals of SPDa and SPDb corresponding to each
current value can be obtained from Figure 2. Using the
voltage V across terminals and current i (Ia,Ib) above
and plotting the instantaneous power (W) = V×I at
each time instant, the injected power × time waveform
can be obtained for each SPD as shown in Figure 5.
The area under the curve in Figure 5 (that is, the value
obtained by integrating the instantaneous power in
Figure 5 with respect to time) shows the energy (in
Joule) injected into the SPD.
(1) For Rd=2Ω
Figure 6 shows the result of tolerance limit energy
analysis when the decoupling resistance value is set
to 2 Ω.
Fig.6 Tolerance limit energy in the
case of Rd= 2 Ω
In this case, the energy injected into SPDa exceeds
the tolerance limit energy at 600 A. On the other hand,
SPDb has still sufficient tolerance limit energy.
Therefore, it can be found that the discharge capacity
of both SPD’s is not used well from the standpoint of
energy coordination. In this case, since the
decoupling resistance is too large, the load on SPDa
becomes too large.
Fig. 5 Power–time characteristics
in the case of multistage SPD
ISBN: 978-1-61804-108-1
75
Recent Researches in Circuits and Systems
In this case, the energy injected into SPDa exceeds
the tolerance limit energy at 950 A, while the
corresponding tolerance limit energy for SPDb
occurs at 800 A. Consequently, it can be said that
proper energy coordination has been achieved.
(2) For Rd=0.1Ω
Figure 7 shows the result of analyzing tolerance
limit energy when decoupling resistance value is set
to 0.1Ω.
5 Conclusions
In this paper, we considered energy coordination
when two SPD’s are connected in parallel. Major
results are shown below.
(1) We proposed that instantaneous power and
energy can be obtained graphically from the value of
the current flowing through each voltage limiting
SPD.
(2) For the energy coordination of two SPD’s, it is
important to specify a decoupling resistance value so
as to allow surge currents up to their respective
tolerance limits.
References:
[1] H. Kijima, K. Murakawa, Lightning surge
response improvement by combinations of
varistors and GDTs, WSEAS Transactions on
power systems, Issue 2, vol. 7, pp60-69, 2012
[2] H. Kijima, K.Takato, K. Murakawa, Lightning
protection for gas-pipelines installed under the
ground, International Journal of systems, Issue 1,
vol. 5, pp117-126, 2011
[3] H. Kijima, T. Hasegawa, Electrical force analyzed
results on switchgear, WSEAS Transactions on
power systems, Issue 1, vol. 5, pp32-41, 2010
[4] H. Kijima, M. Shibayama, Circuit breaker type
disconnector for SPD, WSEAS Transactions on
power systems, Issue 5, vol. 4, pp167-176, 2009
[5] H. Kijima, A development of earthing resistance
estimation instrument, International Journal of
geology, Issue 4, vol. 3, pp112-116, 2009
[6] H. Kijima, Lightning surge response improvement
by combinations of varistors and GDTs, 15th
WSEAS Int. Conf. on circuits, pp132-137, 2011
[7] H. Kijima, K. Ochi, Lightning protection for
gas-pipelines, 10th WSEAS Int. Conf. on power
systems, pp171-176, 2010
[8] H. Kijima, M. Shibayama, Disconnector for SPD,
13th WSEAS Int. Conf. on systems, pp434-44,2009
[9] H. Kijima, Experimental results on earth potential
rise when arrestor operating, 12th WSEAS Int. Conf.
on communications, pp311-315, 2008
[10] Vemon Cooray, The Institution of Engineering
and Technology, p1036, ISBN 978-0-86341-744-3,
2010
Fig.７ Tolerance limit energy in the
case of Rd= 0.1 Ω
In this case, the energy injected into SPDb exceeds
the tolerance limit energy at 500 A, but SPDa has
sufficient tolerance limit energy.
(3) For Rd=0.5Ω
Figure 8 shows the result of analyzing tolerance
limit energy when decoupling resistance value is set
to 0.5Ω.
Fig.8 Tolerance limit energy in the
case of Rd= 0.5Ω
ISBN: 978-1-61804-108-1
76
Recent Researches in Circuits and Systems
[11] H. Kijima “Overvoltage protective device and
method of overvoltage protection, Japanese patent
No.3854305, 2006
[12] H. Kijima, Overvoltage protective device and
method of overvoltage protection, Korean patent
No. 10-0845224, 2008
[13] H. Kijima, Overvoltage protective device and
method of overvoltage protection, Australian
patent No.2006246468, 2008
[14] H. Kijima, Overvoltage protective device and
method of overvoltage protection, Chinese patent
No.ZL20068000361.X, 2010
[15] H. Kijima, Overvoltage protective device and
method of overvoltage protection, USA patent
No.7764481, 2010
[16] H. Kijima, H. Sumiya, Power distribution board,
Japanese patent No.3141491, 2008
[17] H. Kijima, Disconnector and overvoltage
protection device, USA patent No.7983014, 2011
[18] Application notes, Combining GDTs and MOVs
for Surge Protection of AC Power Lines, Circuit
protection specialists, Littelfuse Inc.
[19] Application notes TAN1001, Lightning surge
protection for electronic equipment - a practical
guide, MTL Surge Technologies
[20] Application notes TAN1009, Surge protection
for electrical power installations, MTL Surge
Technologies
[21] M. Loboda, Overvoltage hazard for international
installations and equipment, 28th Int. Conf. on
lightning protection, pp1081-1085, 2006
[22] J. Schimanski, Discharge capability and residual
voltage of class I and class Ⅱ SPDs, 28th Int. Conf.
on lightning protection, pp1160-1164, 2006
[23] V. Murko, Coordinating SPDs with MOV
varistors at direct lightning stroke, 28th Int. Conf.
on lightning protection, pp1174-1179, 2006
[24] M. Shiozaki, Experimental study for the
application of zinc oxide type class I SPD, 28th Int.
Conf. on lightning protection, pp1155-1159, 2006
[25] C. Avendano, Design of SPDs class I for low
voltage systems using combination of varistoros,
28th Int. Conf. on lightning protection,
pp1165-1168, 2006
[26] IEC 61643-1, Surge protective devices connected
to low voltage power distribution systems,
Requirements and test methods, 2005
ISBN: 978-1-61804-108-1
77