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ELEKTROTEHNIKI VESTNIK 75(3):159-164,2010ENGLISH EDITION

Universal SPD coordination towards an effective surge

protection of power supply networks

Murko Vladimir1, Nermin Suljanovi2 , Aljo Muji2 , Jurij F. Tasi31 Iskra Zaite Ljubljana2

Fakultet Elektrotehnike Univerziteta u Tuzli3 Fakulteta za elektrotehniko Univerze v LjubljaniE-pota: nermin.suljanovic@untz.ba

Abstract. The paper describes the principles of effective surge-protection devices (SPDs) coordination for bothstandard test impulses. Since the SPD coordination is strongly affected by the overvoltage protection componentsand SPD topology, laboratory measurements for different SPD types and topologies were conducted. The foundresults are given in the paper. The impact of the test surge impulse shape is recognized as important.

Keywords: surge protection, surge protection devices (SPDs), SPD coordination, metal-oxide varistors

1 INTRODUCTION

The equipment installed in households, businessbuildings or factories is vulnerable to surges that mayoriginate due to atmospheric discharges or energyrelease present in a power supply system (e.g. switchingactions or fuse clearing a fault) [1]. Due to vulnerabilityof electronic systems to lightning discharges, surgeprotection must be completed with an exceptional care.If surge protection is not completed in a proper manner,protected equipment may go out of operation. Protectionof appliances from overvoltages coming from powersupply installation should be completed in two or morestages since energy-withstand capability of surgeprotective devices (SPD) also means higher protectionlevels.

To allow for effective overvoltage protection, SPDsin two or more stages should be coordinated thusenabling each stage to successfully fulfill its tasks; first,to divert a large amount of the transient energy to theground and second, to clamp the overvoltage and reduceit to the level at which the protected device canwithstand it.

According to the classification defined by IEC61643-1, SPDs installed at a service entrance to protectagainst direct lightning currents belong to Class I andwhich must be tested with the surge impulse 10/350 swith the peak of up to 100 kA [2]. SPDs intended forprotection against indirect lightning effects and whichmust be tested with an impulse 8/20 s and anamplitude of 20 kA belong to Class II.

The ANSI/IEEE standard requires tests with an 8/20s impulse with a peak of 100 kA [3]. These standardstake different positions as to which simulation/testimpulses should be considered for the SPD tests for abuilding or a house struck by a lightning stroke.

Problem to be solved is that the protection level (orresidual voltage) of SPDs at a power-line entrance is

due to the need of a large energy withstand capabilitytoo high for the protected equipment. The solution is toapply a secondary surge arrester not withstanding aslarge transient energy as the first but reducing theresidual voltage remaining at the first surge-protectionstage to an appropriate level.

The SPD coordination requires some decouplingimpedance that will take over the voltage drop betweenthe first and the second stage and ensure that the secondSPD doesnt take over the major stroke.

2 SPD COMPONENTS

Gas discharge tubes (GDT) and metal-oxide varistors(MOV) are the most common components of surge-protective devices. In this section we will brieflydescribe the major advantages and disadvantages ofGDTs and MOVs important for effective surgeprotection.

2.1 Gas discharge tubesGDTs usually consist of two or three electrodes in aglass or ceramic, inert gas (neon or argon) filledpackage. The electrodes are aligned with a small gapbetween themselves.

A typical V-I curve for GDT is shown in Figure 1.GDT has a symmetrical V-I characteristic [1].When a voltage applied to the GDT electrodes is belowits striking voltage (DC firing voltage of the gap), thecurrent through the arrester is close to zero. At point A,GDT switches from an insulating to a conducting state.Once a potential reaches the striking voltage, thevoltage across GDT suddenly collapses causingnegative incremental resistance dV/dI.

The segment of the curve between points B and D isknown as a glow region. This region can be divided intotwo subregions. In the first subregion (BC), the voltageacross GDT is approximately independent of the current

and is called a normal glow region. The secondReceived 29 September 2010

Accepted 16 October 2010

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160

subregion is characterized by poresistance dV/dI.

Figure 1. Typical V-I relationship for a g

When the current increases (DE),decreases to the level termed the ar

stays until the surge passes away. Tconductive until its current falls bevalue.

After the surge disappears, thereduced to the arc extinguishing thvalue (point F), the arc is stoppedglow discharge [1].

2.2Metal-oxide varistorsMOVs are made of a ceramic mamixing ZnO with a small amount of agrains are characterized by their lowbeing surrounded by granular layershigh resistance. This structure beconnected in series/parallel ensuperformance of the varistor. The varicharacteristic can be divided into thre1. Low electric-field region

A low voltage is applied at the vardiodes dont conduct and the varisinsulator.2. Medium electric-field region

In this region, the current suddeelectric field reaches the value abovecurrent in this region varies from 1 m3. High electric-field region

The voltage drop at MOV isdetermined by the voltage drop oresistance while the voltage drop atneglected due to the tunnel effect.linear zone of the V-I characteristic.

The published measurement resresidual voltage show the dynamic nasince the MOV residual voltage depesurge wavefront shape [5]. In otherresidual voltage increases when thedecreases. The second dynamic prMOV residual voltage reaches its mcurrent maximum.

MURKO, S

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e arrestor remainslow its withstand

current will bee current. At thisand replaced by a

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ly rises when the100 kV/mm. Theto 1 kA.

linear since it isn the ZnO grainthe barrier can beherefore, this is a

lts of the MOVture of the varistords on the current-words, the MOV

current front timeperty is that theximum before the

2.3 GDT vs. MOVTo meet requirements impostandards, different surge pproposed.

Some manufacturers favourthe first stage due to theiramount of the surge eneUnfortunately, there are soGDTs, such as requirementpresence of a follow-up currenetc. Application of GDTs in thmandatory installation of a coignition, which may be alsoinductance between the two stcommonly based on MOVs.

MOVs are resistors wcharacteristic. Their operation

circuits in the power supply nwith GDTs. Application of Minternal surge protection offrequently avoided due to thewithstand a direct lightning str

3 SURGE TEST IMPULSES

BE MET

SPDs coordination is stronglyof the type of the surge impulseare decoupled by inductancconsiderable difference in the f

8/20 s and of 10/350 s, therethe tail and the energy carrieimpulse shape will stronglsharing between SPDs in thevoltage drop on the decoupling

Until 1995, both IEC andsuggested SPD to be tested witpulse. However, IEC inintroduces SPD classes and re1 should be tested with an imthis is the impulse which correof the direct lighting stroke [2,define SPD satisfying require

and IEEE.The problem to be copped

SPD behavior during tests withof 10/350 s, respectively. Thesignificantly longer, thus carryiof the transient energy. Duripulses behave similarly. Acoordinated SPDs, the currentimpulse of 10/350 s and tdecoupling inductance is smaSPD (with a lower protectisignificant amount of the enerfor such scenario. When

corresponds to a 8/20 s shap

LJANOVI , MUJI, TASI

sed by the IEC/ANSIrotection solutions are

ise the use of GDTs inbility to divert a largergy into the ground.

e disadvantageous offor arc extinguishing,

t, unwanted fuse burningfirst stage also requires

centrated inductance forused as a decoupling

ges. The second stage is

ith a nonlinear V-Icauses no short short-

twork unlike is the caseVs in the first stage ofa certain structure ispinion that MOVs cantke.

: REQUIREMENTS TO

BY SPD

affected by the selectionfor the reason that SPDs

e. While there is noront between impulses of

is a notable difference ind by the impulses. Theaffect current/energy

two stages (due to theinductance).ANSI/IEEE documents

h a 8/20 s surge-currentits document 61643-1uires that SPDs of Classulse of 10/350 s since

sponds to the parameters6, 7]. The main aim is toents laid down by IEC

with is the changeableimpulses of 8/20 s andimpulse of 10/350 s is

ng a much larger amountng the rise time, these

the impulse tail ofgradient is low for thee voltage drop on thell. As the second-stagen level) takes over ay, it should be designeda surge-test impulse

e, the fall time is much

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UNIVERSAL SPD COORDINATION TOWARDS AN EFFECTIVE SURGE PROTECTION OF POWER SUPPLY NETWORKS 161

shorter and the first stage SPD takes over the mainstroke. Furthermore, the lightning surge-current sharingbetween the neighboring structures (e.g. houses) alsovaries depending on the current shape due toinductances of the connecting cables.

As seen from the above, the design of the surgeprotection system is affected by the used surge-impulsewaveform.By the time the test-impulse shape is standardized, SPDshall have to meet both requirements to allow for areliable operation.

Also, the above standards dont deal with thesubsequent lightning strokes in the sense foreseen forSPDs. According to IEC 61312-1, a lightning strokeconsists of the first stroke (90% of the negative polarityand 10% of the positive polarity), subsequent strokes ofa negative polarity and a long duration stroke that mightbe either of a positive or negative polarity. Figure 2

shows the first three strokes and a direct lightning strokeinto a tower [7].

Figure 2. First three strokes of a negative five-stroke multiplelightning flash and a direct lightning discharge to an tower([7])

The shape of the lightning strokes following the firstone has a steep front and is of a short duration.According to IEC 61312-1, the subsequent strokes aremodeled with an impulse of 0.25/100 s and with a peakof 50 kA. When SPDs are coordinated in two stages, thecoordination principles for the impulse of 10/350 s donot apply to subsequent strokes (the voltage drop on thedecoupling inductor is proportional to the currentgradient).

4 SPD COORDINATION PRINCIPLES

As mentioned above, SPDs installed at a power-supplyservice entrance should withstand a direct lightningstroke. To cope with the stress, such SPD is exposed to,requires a high protection level and coordination of thetwo used SPDs. The role of the second SPD is besidesreducing the residual voltage also to avoid flowing ofthe surge current through an electrical installation of astructure that would cause EMC problems [8-10]. Asthe energy, coordination is assumed successful if thedissipated energy through SPDs in both stages is less or

equal to its maximal energy withstand capacity value.

Efficient coordination also means that the second-stageSPD is not over-dimensioned.

The two-stage SPDs are decoupled by inductor L(Figure 3). The lack of impedance would cause anoverstress of the second SPD due to its lower protection

level. The voltage drop on the first SPD equals the sumof the voltage drops on the inductor and on the secondSPD, respectively:

VSPD1=L di/dt+VSPD2 (1)Inductance L can be found in the wiring inductance.

If the wiring provides satisfactory impedance, aconcentrated inductance can be avoided. A concentratedinductance is preferred when the surge current should beprevented from flowing through a structure installation.

Figure 3. Two-stage SPD coordination

If the first SPD is GDT, inductance L is also requiredfor the first SPD to reach the sparkover voltage.

Lets consider coordination with a surge impulse of10/350 s applied at the input of the circuit presented inFigure 3. If GDT is used as the first SPD and if anappropriate inductance guarantees ignition of a gappedSPD, GDT ensures a low-impedance connection to theground and a low voltage drop (but also a short-circuitin the power network). The second SPD is commonlybased on the MOV technology. Low impedance isimportant in the tail of the impulse, with the currentgradient small and the inductance representing smallimpedance. Since the energy stored in the 10/350 simpulse is in its tail, application of GDT ensures that thesecond SPD will not be overstressed. Despite the aboveshown GDT coordination advantage, no air-gap can beused in the power supply networks. After GDT isinitiated, the electric arc cant be extinguished until the

power-frequency voltage has crossed zero. The current,known as the follow-up current, continues flowingthrough GDT even when the surge current hasdisappeared. A short-circuit in the power-supplynetwork is present until an electric arc is extinguishedwhich causes tripping of a current-limiting fuse even atsmall lightning strokes. This tripping doesnt occurwhen using MOVs as the first SPD. Let us demonstratethe use of surge protection of a base station in a cellulartelephone network. The overcurrent protection in suchfacilities is achieved by using a 35 A fuse withstandingup to 6 kA surge impulse of 10/350 s without tripping[11]. If GDT is used as the first SPD and a lightning

stroke of 5 kA occurs, however, GDT causes a short-

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UNIVERSAL SPD COORDINATION TOWARDS AN EFFECTIVE SURGE PROTECTION OF POWER SUPPLY NETWORKS 163

Our laboratory measurement proved that the secondstage doesnt take over the major stroke. Figures 4-band 4-c show the surge current and the current flowingthrough MOVs in the second stage. The measurementsconfirmed a successful coordination of MOVs in

protection against direct lightning strokes.Figures 5 and 6 demonstrate coordination and current

sharing between a serial connection of MOVs andGDTs with MOVs in the second stage. Ourmeasurements proved a successful universalcoordination for the test impulses of 10/350 s and 8/20s, with the advantage of the arc extinguishing due toapplication of MOV in the same branch with the GDT.Measurement depicted in Figure 6 shows that the MOVin the second stage are not overstressed even at a lowinductance of 1 H for the impulse of 8/20 s.

a) Measurement setup

b) Measured surge current in the second stage (670 A)

c) Measured residual voltage (740 V)

Figure 5. Current sharing in the case of coordination of MOVand GDT in the first stage with the MOV in the second stage

a) Measurement setup

b) Surge current 8/20 s, 40 kA

c) Current in the second stage, 4.4 kA

Figure 6. Current sharing in the case of coordination of MOVand GDT in the first stage with the MOV in the second stagefor the test impulse of 8/20 s

Coordination of MOVs in the first stage with a cascadeconnection of MOVs and GDTs in the second stage wastested in accordance with the measurement setup givenin Figure 7-a. The protection levels of MOVs and GDTswere selected so as to keep the residual voltage at thesecond stage bellow 800 V at the 10/350 s impulse

with the amplitude of 50 kA (Figure 7-b). Themeasurement showed that such SPD topology ensures avery low protection level, even at high surge currents.

6 CONCLUSION

The SPD coordination should be effective for both testsurge impulses, i.e. 10/350 s and 8/20 s. The SPDcoordination is strongly affected also by the componentsbuilt-in SPDs and by the applied topology.

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164 MURKO, SULJANOVI, MUJI, TASI

a) Mesurement setup

b) Measured surge current (50 kA) and residual voltage afterthe second stage (