Lightning Current Arresters With Low Protection Level

 

LIGHTNING CURRENT ARRESTERS WITH LOW PROTECTION LEVEL – CHANCE AND CHAL CHALLENGE LENGE Dr.-Ing. Peter Hasse, Dehn + Söhne GmbH + Co. KG, Hans-Dehn-Str. 1, 92318 Neumarkt/Opf., Germany e-mail: [email protected]

 Ab st r act —Ther  Abst —There e i s th e r is k of f ir e du e to fl ash ov overs ers bet w een el elect ect r ic al and met al installations if a building will be hitten by lightning. This risk will be avoided by a light ning pr otection system (LP (LPS) S) wit h care carefully fully carried out equipotential bondin g. Lightn ing current arreste arresters rs (LC (LCAs) As) are import ant components to ensure the potential equalisation between the lightning protection systems (LPS) and the electrical installation. Their application is required in IEC62305-3 [1] and their selection and coordination is described in IEC62305-4 [2]. The steadily increasing number of  electronic devices and their growing sensitivity against overvoltages require surge protective devices devices (SP (SPDs) Ds),, with a high li ghtnin g cur rent carrying capability as w ell as a low pr otection leve level. l. The low prot ection level may ca cause use a huge number of poss ible operations of the LCAs wit h possib le follow currents dr iven by the low-voltage mains gene generally. rally. T The he number of  incoming surges, their energy content as well as the number and the amplitude of  power-frequency power-freque ncy foll ow currents dete determine rmine the li fe cycle of the SP SPD. D. Today the assessment of risk becomes more and more important for the design of  electrical and electronic systems also in the field of lightning protection (IEC62305-2 [3]). The assessment of risk demands the specification of an expected life time of the installed devices. Also manufactures of SPDs have to specify the expected life time of  their devices since an increasing number of projects are placed under the aspect of  risk asse assessment. ssment. 1 INTRODUCTION

The increasing electromagnetic sensitivity of electronic devices installed in low-voltage systems requires a reliable mains-voltage supply. The proven concept of a co-ordinated surge protection by SPDs with downstream decreasing protection levels and decoupling elements is sometimes not applicable: Restrictions in the application of such a stepped surge protection concept are given, e.g. in applications with limited space or where high rated currents are required. This led to the design of a new generation of modern spark-gap based LCAs with a low protection level and high surge current carrying capability. The new generation of  combination type SPDs is able to act without downstream installed class II SPDs and

 

therefore without further decoupling elements and offers an adapted protection level. This is the Chance of this special type of arresters as a combination of Class I and Class II SPDs and offers the user an improved protection behaviour in conjunction with a space saving installation. In [4] the design of such a combination type SPD is described. The advantage of SPD, a low other protection level arises the question: Is the coordination between this combination type downstream installed SPDs (e.g. class III SPDs) and customer  devices really ensured? The combination type SPD [4] can be directly installed in parallel to the fine protection elements inside of a customers device to protect. With the low protection level also an other question arises: Does the low protection level cause a huge number of possible operations of the LCAs with follow currents driven by the low-voltage mains? To give a proper answer to this question, it is necessary to investigate the factors of influence on the life cycle of LCAs. Therefore the occurring stresses like surges and overvoltages coming from the mains, have to be compared with the number of surges without impermissible degradation declared by the manufacturer of the LCA. The requirement for a high durability together with a low protection level and the possibility to be coordinated with the customer devices to protect is the Challenge  for the combination type SPD. 2

MAIN REQUIREMENTS ON THE DESIGN OF MODERN LCA LCA´s ´s

During the lightning event and in case of other overvoltages LCAs have to ensure the potential equalization between the LPS and the electrical   installation and the interruption of  possible follow currents. An ideal LCA would have the high surge current carrying capability of a heavy-duty spark gap and the follow current behaviour of a varistor. Reliable surge protection measures require a certain protection level U P, which is not only lower than the withstand capability of the insulation of the electrical installation, but also low enough to protect sensitive electronic equipment. In some cases only a low protection level allows the co-ordination with other downstream installed surge protective devices or SPDs integrated in customer devices. This leads to a new technology of spark-gap based LCA´s with a low protection level activated by an adjustable energy controlled monitoring unit. The introduction of this technology causes 2 major questions: •  Does the low protection level cause a huge number of ope operations rations of the LCAs with possible follow currents driven by the low-voltage mains? consequences for the life cy cycle cle of these LCAs? •  What are the consequences For a proper answer the occurring stresses (like surges and overvoltages coming from the mains), have to be compared with the permitted number of surges, which cause no loss of  functionality of the LCAs?

 

Main factors of influence on the life cycle of LCAs are: •  technology of the spark gap incoming ming surges or overvoltages •  numbers, amplitudes and energies of inco •  numbers and amplitudes of power-frequency mains follow currents Mains follow by currents and without surges are mainly responsible for numbers of events be handled the LCA a loss of functionality. It the is obviously that LCAswhich with can low protection levels have to operate more frequently. This may lead to a huge number of follow currents and may cause an accelerated degradation. On the other hand it will be shown later  that not in any case of LCA-operation a mains follow current will be initiated. 3

OVERV OVERVOLTAGES OLTAGES IN LOW-VOLTAGE MAINS

A complete description of overvoltages in low-voltage mains is not possible. Overvoltages in low-voltage mains originate from different sources, e.g. direct and nearby lightning strikes or  indirect lightning effects, switching operations of inductive loads or capacitor banks or during fault situations in the power system 3. 3.1 1 Pe Peak ak Amplitude vs. Ra Rate te of Occurrence

To estimate the probability that a certain voltage level will be exceeded, different measurements have been made through the years [5-14] (Figure 1). 10000

C E

1000

A, B, C D E F, G H, I

I

G

100      r      a      e      y        /        1        /      p

F

10

H

[5] 10 [7] [13] [14]

1

A

0.1

D

B

0 1

2

3

4

5 u/kV

10

Fig. 1 - Rate of surge occurrences vs. voltage levels Taking into account that surge amplitudes within buildings may be limited by installed surge protective devices or by flashover of clearances or by the propagation effects in the wiring system, a basic basic dependency for the peak amplitude as a function of number of surges per  year can be found.

 

3. 3.2 2 Dura Duration tion and Energy C Contents ontents of Incoming Surges

The time of interference (duration of a surge or an overvoltage) is significant for the energy contents of the incoming surges. The measurements made with different equipment and with different time resolutions show a broad scattered range of surge durations from a few ns up to some 100 µs. The investigation of [7] shows clearly that the energy content of the incoming overvoltages is constant in a range up to 500 V. This is a clear indication that overvoltages with lower crest values have shorter durations. The energy contents of the measured overvoltages above 500V increase with V2 (as to be expected). The wide range of pulses makes it difficult to determine a typical source impedance. Usually long duration surges (in the range of some µs) can be considered as surges with lower  source impedances as fast and short pulses. Short and fast events as they occur during switching operations of small inductive loads are called burst-pulses of Electrical Fast Transients ( EFTs). For testing purposes a typical waveshape of a single burst pulse is given by the time parameters 5/50ns. These very short burst-pulses can be assumed as electromagnetic waves travelling through the low-voltage system. The behaviour of different types of LCAs during burst stress was tested to check, if the low protection level UP can also be kept during burst pulses. E.g. burst pulses with a crest value of uOC = 4 kV were applied to a spark-gap based LCA with an adjustable energy controlled monitoring (Figure 2).

mains

itotal

load  i prim.

Trigger-Unit

Triggerspark gap

Wtrigger 

isec.

Mainspark gap

ULCA

Fig. 2 - Spark-gap based LCA (combination type t ype SPD) with an adjustable energ energyy monitoring unit unit [4] Figure 3 shows that due to the special trigger unit this energy controlled LCA can limit these

burst pulses to values of UP ≤ 1,5 kV. This LCA was also connected to a power frequency source with a maximum continuous operating voltage Uc. Burst-pulses with a crest value of U oc = 4 kV were applied to determine

 

the number of occurring power power frequency follow currents. It could be shown that (due to the low energy content of the burst pulses) no follow currents occured.  u [kV] 

3.5 Input : Burst-pulse (UOC = 4 kV)  kV) 

3.0 2.5 2.0

LCA output voltage

1.5 1.0 0.5 0.0

0 10 100 0 20 200 0 30 300 0 40 400 0 500 500 600 600 t [ns]  Fig. 3 - Protection behaviour of the combination type LCA (Figure 2) during burst pulse application

These investigations have shown, that short-term pulses, even if they are repetitive, are not able to initiate mains follow currents through an energy-controlled LCA ( Figure 4). u/V

LCA volta e

i/A

200 0

50 0

-200

-50 LCA current

-600 15 ms/ ms/15 150 0 IIm m ulse ulsess 0

5

10

15

20

25

30

35 t/ms

Fig. 4 - Time behaviour of current and voltage during the application of burst pulses to the energy controlled LCA (Figure 2) Therefore burst pulses as an additional stress can be excluded for these kind of energy-

 

controlled LCAs with low protection level. This allows the following assumption: Due to the low energy contents of burst pulses (which are not able to initiate mains follow currents) in further considerations only overvoltages with a duration above 1 µs have to be taken into account. The basis for the the further are the results of investigations [10,11]overvoltages. which were performed with goalconsiderations to detect low-voltage insulations endangering Therefore the measuring equipment was adjusted to detect only overvoltage events with a time duration tmin > 1µs. 3. 3.3 3 Assessme Assessment nt of Thre Threat at to SP SPDs Ds with L ow-protection L eve evels ls

Modern spark-gap based LCAs with low protection levels are generally SPDs with a trigger  circuit. Several conditions for the ignition of a power frequency mains follow current can be defined depending on the trigger circuit design as well as on the spark gap characteristics themselves. To assess the threat by overvoltages to a SPD the boundaries of follow current initiation have to be defined. Independend from the LCA-technology two boundaries can be found (Figure 5). •  Duration of the surges:   If the time of interference is smaller than a given limit no follow current will occur (boundary b1 or b2). •  Energy contents of the incomming surges: If the surge current remains smaller  than a defined value, the LCA is not activated independent from the duration of the surge (boundary a1 or a2).

 

isurge /kA LCA with energy-controlling trigger circuit[2]

50 20 10 5

LCA without energycontrolling trigger circuit

2  b1) 1 0.5

 b2) a2)

0.2 0.1

a1)

1

2

5

10 20 50 100 200 t / µs

Fig. 5 - Boundaries of a follow current initiation in LCAs From a very conservative point of view, all measured surges above a certain peak value and longer than a certain duration can be considered as surges generated from a combination wave generator with a fictive impedance of Zf  =  = 2 Ω. They are able to initiate follow currents. These considerations allow a correlation between the measured overvoltages and the resulting surge currents. 3. 3.4 4 Application to an E Energy nergy C Control ontrol led LC LCA A

The minimum specific energy to initiate a mains follow current in a LCA with energy 2

controlling trigger determined W/Rof= I16 A s. This value corresponds to a 8/20µs current i8/20circuit  = 1 kA[4] or was a 10/350µs pulsewith current peak = 240 A (Figure 6).

 

u/kV 1.2

i/A 1200

total curren curr entt

a)

0.8

LCA-voltage

0.4

800 400

0

main spark gap current

0

10

20

30

40

u/kV 1.2

0 t/µ s

i/A

b)

main spark gap current

180 LCA-voltage

0.8

120 total current

0.4

60

0.0

0

0.0

0.2

0. 4

0.6

0.8

t/ms

Fig. 6 - Behaviour of the LCA with energy controlling trigger circuit at different surge currents a) LCA operates in the 8/20µs-pulse tail (imax = 1 kA) only b) LCA operates already in the 10/350µs pulse-front (ipeak = 240 A) Acc. to the previous assumption of a representative source impedance of Z f  =  = 2 Ω  follow currents may occur if the overvoltages exceed a value of û = 2 kV. The rate of occurrence of  this overvoltage level is (acc. to [10]) p = 1..3 events per year. This rate of occurrence describes only the number of overvoltages which are able to initiate mains follow currents and is not the total number of overvoltage per year. The total number of  overvoltages may activate the energy-controlled LCA, but only a few of them have sufficient

 

energy contents to initiate follow currents. The number of overvoltages in connection with mains follow currents increases rapidly with a lower crest value boundary (Figure 5 a 1) or a lower duration boundary (Figure 5 b 1): This is the case at LCAs without an energy controlling trigger unit. 4

LIFE CYCLE TEST

The durability of a LCA installed in the low-voltage system depends on the number of  overvoltages with high energy content, on the number of follow currents and their peak values. In [15] prospective short- circuits at different points of installations were investigated (household and smaller industrial installations). It could be shown that approximately 95% of  all measured values of the prospective short-circuit currents were smaller than i p ≤ 5 kAeff . Modern LCAs with their high follow current interrupt rating i fi  are able to limit mains follow currents to low values and to extinguish them (independent from the prospective short-circuit current at the point of installation). It is also reasonable, that overvoltages with an energy content high enough to cause follow currents, are shared equally equally:: This means, that in a life cycle test all synchronization angles with respect to the mains voltage should be considered. 4. 4.1 1 Life cycle tes testt procedure

To test the durability of a LCA to be installed at the mains the test procedure taken from the preconditioning test procedure described in [16] was applied to the energy-controlled LCA. With a power-frequency source of Uc and prospective short-circuit currents from ip = 5 kA to ip = 25 kA 8/20µs-surges (I max = 5kA) starting from a synchronisation angle of 0° in steps of  30 electrical degrees were applied to inititate possible follow currents. The test was repeated as long as the pass criteria of [16] could be fulfilled by the LCA. The pass-through energy provided from the mains was measured. Furthermore a 35 A fuse was included into the test circuit. The test was finished if the fuse operates. This additional pass criteria is not equivalent to a general end-of-life of the LCA. It indicates only, that the LCA is not longer selective with an 35 A fuse. These tests were ® performed with different types of LCAs of the DEHNventil -family [2]. The results are summarized in Figure 7. Also in case of high prospective short circuit currents (i p > 5 kA) where the follow current frequency increases, the mains follow current can be limited to negligible values (comparable to those ones of MOVs). )

Figure 8 shows the behaviour of the LCA (DEHNventil®  during the application of the mains

voltage UC  = 230 V with a prospective short circuit current i P = 25 kA. The mains follow current if  was  was limited to 500 A  peak or 1.5 % of iP. The let-through integral of this follow current doesn’t exceed the value of the melting integral of a 35 A NH-fuse.

 

This remarkable limitation of follow currents is the result of the immediate increase of the arcvoltage within the spark gap. As faster this arc-voltage reaches the present value of the mains voltage as lower the peak values of follow currents will become. The property of this kind of spark gaps to reach high arc voltages within a short time leads furthermore to the behaviour that only for some few synchronisation angles a high energy surge causes a noticeable follow current (Figure 8). If the arc-voltage within the spark gap is in the order of the mains voltage (Figure 8) the influence on other connected devices is small. This is comparable with the limiting behaviour  a MOV based SPD.

32 - 36

4,0 3,0

8 - 11 8 - 12

2,0 1,0

+

34 - 36

20 - 23 20 - 24

–

+

–

40

–

+

+

i P  = 25 kA *s] 0 iP  = 5 kA kA 0° 60 60°° 12 120 0°18 °180° 0°24 240 0°30 °300° 0° 60° 120° 0°18 180 0°24 °240° 0°3 300° 0° 60 60°° 12 120°1 0°18 80°2 °24 40° 0°30 300° 0° 0° 60 60°° 12 120 0°180° °180° ²W/ 

R[ 4,0

44 - 48 44 - 48

3,0

52 52 - 53

56 - 59 56 - 60 64 63 - 65

2,0 1,0

68 - 71 68 - 72

–

+

–

+

80 - 81 80 - 84

76

–

+

–

0

i P  = 25 kA iP  = 5 kA 18 180° 0°24 240° 0°30 300° 0° 0° 60 60°° 12 120° 0°18 180° 0°24 240°3 0°300 00°° 0° 60 60°° 12 120° 0°18 180°2 0°240 40°30 °300° 0° 0° 60 60°° 12 120° 0°18 180° 0°24 240° 0°30 300° 0°36 360° 0°

Su Surg rgee at   ? electr.

Fig. 7 - Pass through-energy of the t he LCA DEHNventil® during a life cycle test at ip = 5 kA and ip = 25 kA

 

u/V 400 0

mains voltage

-400

spark gap voltage

i / kA 40

 prospective short circuit current

20 0 -10

follow current of  the LCA

0

5

10

15

20 t / ms

i / kA

1

current flow for ∆t ≈ 7 ms

0 10

15 t / ms 20

i p = 25 kA

Fig. 8 - Limitation of the mains follow current of the LCA (DEHNventil ®) at 10 kA (8/20µs) and a prospective short circuit current i P = 25 kA r.m.s. Figure 7 shows clearly, that only at a few synchronisation angles a mains follow current is initiated. The given numbers correspond to synchronisation angles in the positive and negative of the mains increased voltage where currents During theshorttest procedurehalf-wave the pass through-energy evenlyfollow and not rapidly.occur. At a prospective circuit current of ip = 25 kA the pass through-energy reaches the defined pass criteria at 85 surges and at a prospective short-circuit current of ip = 5 kA even at 117 surges. The claimed protection level of up = 1,5 kV and a sufficient insulation have always been kept during these test. With these test sequences the number of stresses was determined where no inadmissible change of functionality of the LCA occurs. With the above mentioned rate of occurrence of overvoltages (causing follow currents) of  p = 1..3 per year a life time of the investigated LCA (DEHNventil ®) of tLCA ≥ 33 years can be extrapolated.

 

The measured pass through-energies of the tested LCAs are always lower then the melting energy of a 35 A fuse, which means that the LCAs of the DEHNventil ®-family keep their  claimed selectivity to a 35 A fuse during their whole determined life time. This is remarkable with respect to the fact, that the SPDs of the DEHNventil ®-family are encapsulated LCAs, without blowing any hot gases or particle emission during operation. 5

CONCLUSIONS

The design of a modern spark-gap based LCA with a low protection level and high surge current carrying capability has to consider a possible increase of operations and a possible increase of mains follow currents. The number of overvoltages, their energy contents as well as the number and r.m.s.-values of prospective short-circuit currents at the points of  installation are influencing the life time of spark-gap based LCA.s. Only surges with sufficient energy contents are able to initiate mains follow currents. Due to their short time of interference burst-pulses or EFT are not able to initiate follow currents in the investigated LCA. With the measured behaviour, that only surges with a sufficient energy content and a certain duration can initiate follow currents, a rate of occurrence of these overvoltages (p = 1..3 per year) could be found. The investigations were performed with LCAs with a high follow current interrupt rating and have been done an addition to the existing operating test [16]. The measured number of  surges up to n > 80 without a loss of functionality delivers for the investigated LCAs (with their  high follow current limiting behaviour) a prospected life time of tLCA ≥ 33 years even at very high prospective short circuit currents (e.g. i p = 25 kA).. 6 REFERENCES

[1] IEC 62305-3, Ed. 1: Protection against lightning - Part 3: Physical damage to structures and life hazard, IEC 81/214/CD [2] IEC 62305-4, Ed. 1: Protection against lightning - Part 4: Electrical and electronic systems within structures IEC 81/212/CD [3] IEC 62305-2, Ed. 1: Protection against lightning - Part 2: Risk management IEC 81/213/CD [4] R. Brocke, P. Hasse, F. Noack, P. Zahlmann, “Spark gap based lig lightning htning current arresters th without mains follow currents ”, Proceeding of the 24   International Conference on Lightning Protection, pp. 654-659, Rhodes-Greece 2000. [5]  ANSI/IEEE C62.41-1991, “IEEE Recommended Practice on Surge Voltage in LowVoltage AC Power Circuits.” [6] Goedbloed, J. J. “Transients in Low-Voltage Supply Networks” IEEE Transactions, EMC29, No. 2, May 1987, pp. 104-115.

 

[7] Meissen, W. “Überspannungen in Niederspannungsnetzen.” (Overvoltages in low-voltage networks) Elektrotechnische Zeitschrift(etz), vol. 104, 1983, pp 343-345 [8] Meissen, W. “Transiente Netzüberspannungen (Transient overvoltages in Networks)”: Elektrotechnische Zeitschrift(etz), vol. 107, 1986, pp 50-55 [9] Standler, R. B. “Transients on the Mains in a Residential Area”, IEEE Transactions, EMC31, May 1998, pp 170-176 [10]Ackermann, G.; Hudasch, M.; Schwetz, S.; Stimper, K.: “Überspannungen in Niederspannungsanlagen (Overvoltages in low-voltage installations)” Elektrotechnische Zeitschrift (etz), Vol. 114 (1993), pp 218-223 [11]Ackermann, G.; Scheibe, K.; Stimper, K.: “Isolationsgefährdende Überspannungen im Niederspannsbereich” (Overvoltages hazardous to insulation in low-voltage systems)” Elektrotechnische Zeitschrift (etz), Vol. 116 (1997), pp 36-40 [12]Martzloff, F. D. and Gruzs, T. S. “Power Quality Site Surveys: Facts, Fiction, and Fallacies“, IEEE Transaction, IA-24, No. 6, Nov./Dec. 1988, pp 100-1018 [13]Harich, H.; Enders, W.: “Transiente Überspannungen in Niederspannungsanlagen (Transient overvoltages in low-voltage installations)”, Bericht der Bundesversuchs- und Forschungsanstalt Arsenal, Vienna 1982 [14]IEC-Contribution 28A (France) 25 juin 1986:  Contribution du Comité national francais pour la révision du rapport 664 de la CEI [15]F. Noack, J. Pospiech, "Kurzschluß-Kenngrößen von Nieder-spannungsnetz Nieder-spannungsnetzen en (Characteristics of short-circuit currents in low-voltage mains)", Elektrotechnische Zeitschrift (etz),Vol. 116(1995), No. 5, pp 218-223 [16]IEC 61643-1:1998: “Low-voltage surge protective devices - Part 1: Surge protective devices connected to low-voltage power distribution systems - Performance requirements and testing methods.”