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CC II RR EE DD 17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003

DOS_Ametani_A1 Session 2 Paper No 4 - 1 -

Fig. 2 Measured results of transient voltage andcurrent waveforms along a counterpoise

(a) Transient voltage

EXPERIMENTAL INVESTIGATION OF A TRANSIENT INDUCED VOLTAGETO AN OVERHEAD CONTROL CABLE FROM A GROUNDING CIRCUIT

Akihiro AMETANI, Tomomi OKUMURA, Naoto NAGAOKA, Nobutaka, MORIDoshisha University - Japan

[email protected]

1. Introduction

Electromagnetic disturbance of low voltage controlcircuits in generator stations and substations and ofconsumer electronic/digital circuits are becoming a significant problem as a number of electronic/digitalcircuits composed of sensitive semiconductors have beenadopted. In fact, a few disturbances have been experiencedin Japan [1-3]. During last 10 years more than 300disturbances in the control circuits in the power stations and substations are informed, and about 70% is estimated due tolightning. Among the 220 cases, 104 cases are of the voltage

class 66-77kV, 45 cases/110-154kV, 14 cases above 154kV,and 57 cases the voltage class unknown. The incoming pathof a lightning surge causing the disturbances has beeninvestigated in the hydraulic generator stations and substations, and the result indicates two dominant paths : (1)via VTs and CTs, (2) via grounding systems due to arresteroperation in a main circuit and etc. It is statistically foundthat more than 20% of the lightning surges are comingthrough the grounding systems, and another 20% throughVTs and CTs.

The present paper carries out an experimentalinvestigation of a transient voltage and current of anoverhead control cable induced from a single groundingconductor, i.e. a counterpoise, when a lightning current

flows into the grounding conductor probably by an arresteroperation. The transient voltages and currents on the cablecore and sheath are measured under various termination of the cable.

2. Experimental setup

Fig. 1 illustrates an experimental setup. A naked Cuconductor (counterpoise) with the radius of 1cm is buriedunderneath the earth surface at the depth of 30cm with thetotal length of 4m. A coaxial single core cable (3D2V) withthe length of 2m is suspended above the earth surface at theheight of 10cm. A pulse generator (PG) is connected to oneend of the counterpoise through a resistance 500 via a leadwire of 10m. When measuring a sheath voltage, oneterminal of an oscilloscope is connected to a voltagereference line with the length of 60m of which the other endis grounded independently from the counterpoise.

Terminating conditions of the both ends of theoverhead cable core and sheath are changed as summarizedin Table 1. For examples, the sheath ends are eitherconnected (ON) / disconnected (OFF) to the counterpoise orindependently grounded (IG) from the counterpoise. Thecore ends are either open-circuited or connected through amatching resistance to the sheath. Measured results of core

and sheath voltages and currents are summarized in Table 1.

PG

4

0.1m

2m1 1

10

3D-2V500

node 0 1 2

0.3m 1

Fig. 1 Experimental setup

(b) Transient current

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3. Measured Results and Discussions

3.1 Voltage and Current along Counterpoise

Fig. 2 shows measured results of transient voltages andcurrents along a counterpoise. Wave propagationcharacteristics along a counterpoise have been discussed indetail in reference [4].

3.2 Sheath Ends Open-Circuited

Fig.3 shows measured results of transient voltage andcurrent waveforms on the cable core ( core to sheath ) and

sheath ( to voltage reference wire ) in the case of the sheath both ends being open-circuited (OFF, OFF). (a) is the coreto sheath voltage at the receiving end, (b) the sheathreceiving-end voltage and (c) the sheath current in case11.It is clear in Fig. 3(a) that the core voltage is nearly the sameas the sheath voltage at the receiving ends. The maximumdifference is less than 2V as observed in Table 1independently from the core termination. The phenomenonis readily explained by a traveling wave theory .

Define the surge impedance matrices between nodes 0to 1, 1 to 2 and 2 to right in Fig.1 as [Z 1] and [Z 2]respectively. Then, the refraction coefficients 1f from theleft to the right at the node 1 is given in the followingequation[5].

[1f ] = 2 [Z 2] ( [Z 1] + [Z 2] ) -1 (1)

where [Z 1] =

s

c

g

R

R

Z

1

1

00

00

00

,

[Z2] =

smn

mcn

nn g

Z Z Z

Z Z Z

Z Z Z

(2)

and Z g : self surge impedance of a counterpoiseZc : self surge impedance of a cable coreZs : self surge impedance of a cable sheathZm : mutual impedance between a core and a sheathZn : mutual impedance between a counterpoise and a

cableR 1c, R 1s : terminating impedance of a core and a sheath

at node 1

When the both ends of the cable core and sheath areopen-circuited, the terminating impedances are difined by :

R 1c = R 1s = infinite ( ) (3)Substituting the above condition into eq. (1), the

following refraction coefficient matrix is obtained [5].

[1f ] =00/

00/001

g n

g n

Z Z

Z Z (4)

The above equation indicates that the sending-endvoltages on the core and the sheath are the same. When thecore sending end is short-circuited to the sheath, therefraction coefficient matrix [ 1s] is defined in the followingequation [6].

[1s] = ( )1121 '2 + t T Z T Z U (5)where

=0

0]'[ 1

g Z Z ,

=110001][T : rotation matrix (6)

and subscript t for a transposed matrix.Then, the refraction coefficient is given in the following

2 2 matrix.

[1s] = 0/01

g n Z Z (7)

Max. current [A]sheath tocounterpoise

core - to - sheathresistance[ ]

max. core to sheathvoltage [V]

max. sheathvoltage to

reference wire [V]core sheathcase

No.send receive R 1c R 2c send receive send receive send receive send receive

11 OFF OFF 0.82 0.89 35.99 34.12 0.0077 0.0129 0.0231 0.017714 OFF OFF 60 60 1.32 0.47 35.20 34.38 0.0088 0.0060

21 ON OFF 1.85 1.03 52.42 43.50 0.0088 0.0127 0.0481 0.023622 ON OFF 60 0 1.09 1.40 52.17 43.73 0.0091 0.007623 ON OFF 0 60 1.19 1.50 51.90 43.28 0.0094 0.009424 ON OFF 60 60 1.33 1.45 51.75 43.82 0.0108 0.009431 OFF ON 1.71 0.72 45.53 38.00 0.0092 0.0129 0.0132 0.026232 OFF ON 60 0 1.57 0.83 47.75 37.22 0.0106 0.009433 OFF ON 0 60 1.66 0.90 47.05 37.77 0.0117 0.007934 OFF ON 60 60 1.30 0.97 46.67 38.38 0.0100 0.006341 ON ON 1.71 0.79 44.97 36.56 0.0104 0.0094 0.2106 0.209542 ON ON 60 0 1.85 1.03 46.98 37.31 0.0104 0.006243 ON ON 0 60 1.61 1.20 44.78 37.53 0.0113 0.005044 ON ON 60 60 1.61 1.05 45.64 37.72 0.0099 0.005151 1.45 0.80 14.89 15.49 0.0221 0.026752 60 0 1.64 1.53 15.00 14.99 0.0073 0.0083 0.0205 0.027253 0 60 1.53 1.38 14.80 14.84 0.0069 0.0052 0.0208 0.027854

independentgrounding

60 60 1.60 1.32 15.47 15.03 0.0065 0.0044 0.0209 0.0271

Table 1 Experimental conditions and measured results

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It is observed from eq. (4) and (7) that the core voltageis the same as the sheath voltage at the sending endindependently from the open and the short circuit of the core

and the sheath, and is given as 0)/( E Z Z g n , where 0 E isa traveling-wave voltage incoming to node 1 along thecounterpoise. The sending end voltage g V of the

counterpoise is given by 0 E as in eq.(7), if no circuit isconnected to the cable sending end. The characteristicimpedances calculated by the Cable Parameters of theATP-EMTP[7] are given in eq.(8) at 10MHz.

=

32832816

32838116

161665

0 Z at MHz f 10= (8)

Thus, the transient induced voltage to the cable is estimatedto be about 30% of the counterpoise voltage. The maximumcounterpoise voltage being about 120V in Fig. 2, the cable

voltage is evaluated roughly to be 30V which is smaller thanthe measured result in Fig. 3(b). The discrepancy isestimated due to a direct induction from the current leadwire.

The sheath current in Fig. 3(c) is quite oscillatory,similarly to the core-to-sheath voltage in Fig. 3(a). The highfrequency component is estimated due to multiple reflectionof the coaxial and the earth-return modes of propagation(coaxial ,/1951 smv = earth sm /236 ) at the cable ends.On the contrary, the sheath voltage shows a rather smooth

waveshape, similarly to the counterpoise voltage. The propagation velocity on the counterpoise is measured to beabout sm /100 , which gives a dominant transientfrequency g g f 4/1= 6.3MHz, or one cycle T = 160ns.

3.3 Sheath Sending-End Connected to Counterpoise

Fig.4 shows transient sheath voltages when the sendingend of the cable sheath is short-circuited to the counterpoiseat the node 1 in Fig. 1, i.e. Case 2i in Table 1. The receivingend of the sheath is open-circuited. It is observed that thesheath voltage at the sending end is similar to that of thecounterpoise voltage at the node 1 in Fig. 2(a). Thereceiving-end voltage is attenuated at the wavefront and theoscillatory waveform becomes distinct. The sheath currentin Fig. 4(c) is less oscillatory than that in Case 11, Fig. 3(c)and the peak current at the sending end is nearly twice of

Case 11.The node 1 voltage is analytically determined in thesame manner as that in Sec.3.2. The sheath voltage is thesame as the counterpoise voltage because those are shortcircuited. The voltage is roughly given by :

s g s s g Z Z Z E V V 2/20 +== V c (9)

where 0 E : traveling-wave voltage incoming to node 1 alongthe counterpoise

Applying the characteristic impedances in eq.(8),

s g V V = cV 0)11/10( E

With 0 E 50V at the node 1 in Fig. 2(a), the aboveequation gives the following result.

s g V V = cV 45.5V

The value agrees well with the measured results in Table1. It should be noted that the sheath voltage in Case 2i isgreater than that in Case 1i.

(a) Core to sheath voltage at the receiving end

(b) Sheath voltage at the receiving end

(c) Sheath current in Case 11

Fig.3 Transient voltage waveform on a cable in Case 1i

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(a) Sending end

(b) Receiving end

(a) Sending end

(b) Receiving end

(c) Voltage difference between thecounterpoise and the sheath

Fig.5 Transient sheath voltages in Case3i

(c) Sheath current in Case 21

Fig.4 Transient sheath voltages in Case 2i

3.4 Sheath Receiving-End Connected to Counterpoise

Fig.5 shows transient voltages in the case of the sheathreceiving end being short-circuited to the counterpoise atnode 2 (Case 3i). The maximum voltage is smaller in Case3i than in Case 2i as observed in Table 1.

Fig. 5(c) shows the voltage difference between thecounterpoise and the sheath at the sending end. Thecounterpoise voltage is composed of Fig. 5(a) and (c), and becomes similar to that in Fig. 2(a).

3.5 Both Ends of Sheath Connected to Counterpoise

Fig.6 shows transient voltages and current waveformson the cable sheath in Case 4i. It is clear that thesending-end voltage waveform in Fig. 6(a) is nearly the

same, except the

maximum value, as that in Fig. 4(a) in the case of the sheathsending end short-circuited to the counterpoise. Thereceiving-end voltage in Fig. 6(b) is similar to Fig. 5(b)except the initial part. As a result, the maximum sheathvoltage is the smaller in Case 4i than that in Cases 2i and 3ias observed in Table 1, and this agrees with the conventional practice of the both ends of a cable sheath to beshort-circuited to a counterpoise.

Currents waveforms in Fig. 6(c) are quite different fromthose in Cases 1i to 3i which are highly oscillatory. The peak value of the current is far greater in Fig. 6(c). Thereason is simply due a closed circuit composed of the sheathand the counterpoise.

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(a) Sending end

(b) Receiving end

( c) Current in Case 41

Fig.6 Transient sheath voltage and currentwaveforms in Case 4i

(a) Core-to-sheath voltage at the receiving end

(b) Sheath voltage at the sending end

(c) Sheath voltage at the receiving end

(d) Sheath current in Case 51

Fig.7 Transient voltage and current waveforms inCase 5i (independent grounding)

3.6 Sheath Grounded Independently from Counterpoise

A transient voltage and current induced to a controlcable from a counterpoise is desired as small as possiblefrom the viewpoint of the insulation and the electromagneticimmunity of the control circuit in a power station and asubstation. To achieve this, the paper investigates anindependent grounding (Case 5i in Table 1) of the cablesheath from the counterpoise. The cable sheath is groundedat the both ends to a buried vertical rod apart by 2m from thecounterpoise.

Fig.7 shows measured results of transient voltage andcurrent waveforms. (a) is the core-to-sheath voltage at thereceiving end. A maximum voltage difference between thecore and the sheath is observed to be less than 1.5V, which

is

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similar to those in the Cases 1i to 4i in Table 1.Fig.7(b) and (c) are the sheath voltages at the sending

and the receiving ends respectively. A comparison with thesheath voltage waveforms in Case 1i to 4i (Figs. 3 to 6)indicates that there is no fast rise of the voltage at thewavefront, and the maximum sheath voltage in Case 5i is

less than 15.5V, while it reaches about 36V in Case 1i,52.5V in Case 2i, 48V in Case 3i and 47V in Case 4i. Thus,it is clear that the maximum sheath voltage is reduced to 1/3of that in the cases of the cable sheath being connected to thecounterpoise.

Fig. 7(d) shows the sheath current in Case 51. The sheathcurrent is observed to be similar to that in Fig. 3(c), Case11in the beginning, and to be far smaller than that in Fig. 6(c),Case 41.

4. Conclusions

The paper has investigated experimentally transientvoltages and currents on an overhead cable induced from a

counterpoise, representing an induced voltage and current ona control cable in a power station and a substation. Based onthe investigation, the following remarks have been obtained.(1) No significant voltage difference appears between the

cable core and the sheath.(2) The sheath voltage is smaller in the case of the sheath

being open-circuited, i.e. not connected to acounterpoise, than that in the case of the sheathconnected to the counterpoise.

(3) When a cable sheath is connected to a counterpoise, thesheath voltage becomes the smallest and the sheathcurrent is the largest in the case of the both endsconnected to the counterpoise. The sheath voltage is thelargest in the case of the sending end connected to the

counterpoise.(4) The sheath voltage is far smaller in the case of thesheath being grounded independently from thecounterpoise than that in the case of the sheathconnected to the counterpoise. Thus, the independentgrounding of a control cable could be an effectivemeans to reduce the transient overvoltage on a controlcable in a power station and a substation, unless avoltage difference between the independent groundingand the counterpoise (or mesh) dose not cause a problem.

References

[1] T. Hasegawa, et.al., A fact-finding analysis oflightning disturbances and substations, IEE ofJapan, Annual Meeting Records, Paper 1218,1992

[2] Technologies of Countermeasures against Surgeson Protection Relay and Control Systems, Reportof Japanese ETRA, vol.57, No.3, Jan.2002

[3] T. Sonoda, et.al., An experimental study on surgesinduced from grounding grid to low-voltagecontrol circuits, IEE of Japan, Research Meeting,Paper HV-01-129, 2001

[4] A. Ametani, et.al. : Basic investigation of wave propagation characteristics on an undergroundnaked conductor, Proceedings of ICEE 02, Jeju(Korea), pp.2141-2146, 2002

[5] A. Ametani : Distributed-Parameter Circuit Theory,Corona Pub.Co. (Tokyo), 1990.2

[6] N. Nagaoka and A. Ametani, Transient calculations

on crossbonded cables, IEEE Trans., vol.PAS-102, pp.779-786, 1983[7] A. Ametani : Cable Parameters Rule Book, B.P.A.

1996. 4

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