Risk Assessment of Damage to Telecommunication Sites due to Lightning in Typical Areas in Vietnam by the Improved Method

Risk Assessment of Damage to Telecommunication Sites due to Lightning in Typical Areas in Vietnam by the Improved Method

1Le Quang Trung, 2Quyen Huy Anh, 3Phan Chi Thach

Faculty of Electrical & Electronics Engineering University of Technical Education

Ho Chi Minh city, Vietnam

AbstractVietnam is in the lightning center of Asia, so dam- age caused by lightning is very large, especially in the telecom- munications field. Telecommunication sites (TSs) are usually built in high positions, and have the antenna tower higher than surrounding structures thus the risk of direct lightning strikes is huge. Currently, the selection of lightning protection solutions for these TSs mainly based on experience and preliminary calcu- lation, not based on the results of risk assessment of damage due to lightning. This paper calculates the risk of loss of service caused by lightning to typical TSs in areas in Vietnam by the improved method with greater detail than the previously sug- gested methods. Then, the useful lightning protection solutions to typical TSs would be suggested for risk protection of the damages due to lightning. The results show that the level of loss of services caused by lightning to TSs in areas in Vietnam much higher than the tolerable value. When the surge protective de- vices (SPDs) are installed on all the incoming service lines, the level of risk caused by lightning can decrease up to 100 times.

KeywordsTelecommunication site; risk of damage due to lightning; lightning protection solution.

1. INTRODUCTION

   Vietnam is in the lightning center of Asia, so the damage caused by lightning is very large, especially in the telecom- munications field.

   Along with national socio-economic growth rate, commu- nication plays an important role, more and more telecommu- nication site have been continuously upgraded and built. To facilitate for the transmitting and receiving, TSs are usually built in higher positions and with adjacent antenna tower, the risk due to lightning is increasingly high. Annually, the dam- ages caused by lightning to the telecommunications field in

   TSs helps engineers designing the lightning protection system give the proper lightning protection solutions to reduce the risk to below the tolerable limit.

   The level of risk of the damage caused by lightning to Telecommunication site (TS) depends on:

   • The dimensions and characteristics of the TSs and the adjacent antenna tower.

   • The dimensions and characteristics of the incoming lines.

   • The environment around the TSs.

   • The density of lightning strikes in the region where the TSs is located.

   The following will analyze and calculate the risk of the damage of the services to typical TSs due to lightning in typical areas in Vietnam by the improved method with greater detail than the previously suggested methods. Then, the reasonable solutions for lightning protection of TSs would be proposed.

2. TYPICAL TELECOMMUNICATION SITE According to [6], a typical TS consists of the telecommu-

   nication building by reinforced concrete and adjacent steel antenna tower.

   Antenna

   22kV

   Vietnam are quite large. According to statistics from the Min- istry of Information and Communications, in 2012 Vietnam Posts and Telecommunication Group had about 457 failures due to lightning, the damage was estimated at 16.7% in total damages caused by natural disasters [4].

   Therefore, application of lightning protection solutions for

   Underground power supply cable

   Telecommunication building

   Distribution board

   BTS

   cabinet

   Earthing system

   Equipotential grid

   Trunking

   telecommunication sites is always necessary. However, cur- rently, designation and selection of lightning protection solu- tions is still based on experience and preliminary calculation, not based on results of risk assessment of the damage caused by lightning. The assessment of risk caused by lightning to

   Fig. 1. Typical telecommunication site.

   A 22kV overhead distribution line feeds a 22kV/380V delta-star primary winding of a transformer. The 380V supply is then fed via underground cable to distribution board. The output of the distribution board is then fed to lighting equip-

   ment, BTS cabinet, warning light, with the power supply for the warning light, telecommunication cables for the an- tenna are put in the trunking. The surrounding of the TS is protected by metal fences.

   The parameters and characteristics of the typical TS locat- ed at Ho Chi Minh City and of the service lines connected to the station are shown in TABLE I, II and III as follows:

   TABLE I. ENVIRONMENT AND TELECOMMUNICATION SITE CHARACTERISTICS

   TABLE III. CHARACTERISTICS OF THE TELECOM LINE

   Parameters	Comment	Symbola	Value
   Length (m)		LT	1000
   Line installation factor		Cl/T	0.5
   HV/LV transformer		CT/T	1
   Line shield	UW/T=1.5	PLD/T	1
   		PLI/T	0.5
   		CLD/T	1
   		CLI/T	1
   		KS4/T	0.6
   Reduction factor for surge protec- tive device on input of equipment	no SPD	k3	1
   Reduction factor for surge protec- tive device on input of service line	no SPD	k5	1
   Coordinated SPDs	no SPD	PSPD/T	1

   c.For more information about symbols and values adopted refer to standard IEC 62305-2, AS/ANZ 1768

3. RISK ASSESSMENT OF DAMAGE DUE TO LIGHTNING BY THE IMPROVED METHOD

   The improved method of risk assessment of damage caused by lightning is built on calculation method recom- mended by IEC 62305-2 standard [1], but the level of detail of the parameters such as: the probability of dangerous dis- charge based on structure construction materials; the proba- bility of dangerous discharge based on internal wiring type; the number of service lines and shielding factor along the distribution line is considered added based on references from

   [2] and [3].

   This method has been presented in detail in [5], the proce- dure of risk assessment has been shown in Fig. 2. According to [1], the components of the risk R2 are expressed by:

   rf

   Parameters	Comment	Symbola	Value
   Ground flash density (flash- es/km2/year)		Ng	12
   Structure dimension (m)		L×W×H	5×4×3
   Antenna tower height (m)		Hanten	45
   Location factor	isolated	CD	1
   Probability of a dangerous dis- charge based on structure type	reinforced concrete	Ps	0.2
   Type of floor	ceramic	rt	10-2
   Risk of fire	low	10-3
   Fire protection	extinguishers	rp	0.5
   Shield at external structure bounda- ry	none	KS1	1
   Shield at internal structure boundary	none	KS2	1
   Protection measures	no LPS	PB	1
   Coordinated SPDs	none	PEB	1
   Protection measures	equipotential grid	PTA	10-2
   Protection against touch voltages	none	PTU	1
   Hazard	no special hazard	HZ	1
   Loss	by electric shock	LF	10-2
   	by physical damage	LO	10-3
   Probability of a dangerous dis- charge based on structure type	unscreened	pi	1

   Where:

   R2 = RB

   + RC

   + RM

   + RV

   + RW

   + RZ

   (1)

   a.For more information about symbols and values adopted refer to standard IEC 62305-2, AS/ANZ 1768

   Parameters	Comment	Symbolb	Value
   Length (m)		LP	600
   Line installation factor		Cl/P	1
   HV/LV transformer		CT/P	1
   Line shield	UW/P=2.5	PLD/P	1
   		PLI/P	0.3
   		CLD/P	1
   		CLI/P	1
   		KS4/P	0.4
   Reduction factor for surge protective device on input of equipment	no SPD	k3	1
   Reduction factor for surge protective device on input of service line	no SPD	k5	1
   Coordinated SPDs	no SPD	PSPD/P	1

   TABLE II. CHARACTERISTICS OF THE POWER LINE

   b.For more information about symbols and values adopted refer to standard IEC 62305-2, AS/ANZ 1768

   RB = ND×PB×rp×rf×LF×nz/nt (2)

   RC = ND×PC×LO×nz/nt (3)

   RM = NM×PM×LO×nz/nt (4)

   RV = NL×PV×rp×rf×LF×nz/nt (5)

   RW = NL×PW×LO×nz/nt (6)

   RZ= Nl×PZ×LO×nz/nt (7)

   (Refer to [1] for the meaning of symbols)

   1. Assessment of dangerous events due to lightning in the risk components

      • The number of dangerous events due to flashes to structure is determined by the equation:

        ND = NG× CD× AD× 10-6 (8)

      • While NL (9), Nl (10) are the number of dangerous events due to flashes to and near the service lines, are determined by the equations:

        NL= NG× Cl× CE× CT× AL× 10-6 (9)

        Nl= NG× Cl× CE× CT× Al× 10-6 (10)

        In (9) and (10), the coefficients such as: installation factor Cl, environmental factor CE and line type factor CT are men- tioned.

        However, environmental factor CE in (9) and (10) without reference to the terrain where the service lines go through as: the pole height h, the horizontal distance between the outer wires b and shielding factor Sf of the object height H, the distance to the service line x (Fig. 2), and in this case, the number of lightning strikes directly to line service follow [3] is determined by equation:

        NL = NG×Cf×10-6 (11)

        Where:

        Cf = (b + 28×h0.6) ×10-1(1 – Sf) (12)

        To improve accuracy when calculating the number of lightning strikes directly and indirectly on the overhead ser- vice lines, coefficient CE in (9) and (10) should be replaced by the coefficient Cf defined in (12). The number of lightning strikes to and near overhead service lines is defined by the following equations:

        NL = NG×AL×Cl×Cf ×CT ×10-6 (13)

        Nl = NG×Al ×Cl ×Cf ×CT×10-6 (14)

      • Number of dangerous events due to flashes near a structure, is equal to:

        NM = NG×AM ×10-6 (15)

        Identify the characteristics of the telecomunication site to be protected

        Identify the types of loss relevant to the telecommunication site

        Determine the tolerable risk RT

        Caculation of risk areas: AD, AM, AL, Al

        Identification of probability factors Px (x= B, C, M, V, W, Z)

        Caculate the level of risk R

        Telecommunication site or service protected for this type of loss

        Yes

        R < RT

        No

        Intall adequate protection measures suitable to redure R

        (Refer to [1] for the meaning of symbols)

        Fig. 2. Procedure for risk assessment for telecommunication site.

        x

        Sf H

        h h

   2. Assessment of the Px in the risk components

   1. Probability of physical damage to a structure PB :

      According to [1], when calculating risk component RB related to physical damages due to flashes to the structure, the value of the probability PB is determined only depending on the lightning protection level designed [1].

      To increase the accuracy of the probability PB for risk component RB in IEC 62305-2, should be added the probabil- ity that external wiring carries a surge from structure that causes physical damage (Pewd) and the probability of danger- ous discharge based on structure construction materials (ps) according to standard [2], as follows:

      PB =k1×ps + Pewd (16)

   2. Probability of failure of internal systems PC:

      When calculating the probability PC for risk components RC, the coefficients are taken into account such as: the coor- dinated SPD system is installed PSPD and the depending on shielding, grounding and isolation conditions CLD.

      While in [2], when calculating this probability, the factors are considered such as: the probability of dangerous dis- charge based on structure construction materials (ps); the probability of dangerous discharge based on internal wiring type (pi); reduction factor for surge protective device on input of equipment (k3); correction factor for impulse level of equipment (kw) and probability that external wiring carries a surge from structure that causes a damaging overvoltage to internal equipment (Pwedo).

      When calculating the probability PC, the coefficients as calculated probability Pw in [2] should be added as follows:

      PC = 1 (1 k1×ps×pi×k2×k3×kw)(1 Pwedo) (17)

   3. Additional the number of service line number when calculating the probability of lightning flashes to and near the service line:

   If there are many service lines connected to the structure in the separate rout, the probability of flashes to the service line will increase along with the number of service lines. Therefore, to increase the accuracy when calculating proba- bility PU, PV, PW and PZ, the number of service lines should be added. The equations for calculating PU, PV, PW and PZ for overhead lines are defined as follows:

   PV/oh = PEB×PLD×CLD×noh (18)

   PW/oh = PSPD×PLD×CLD×noh (19)

   Pz/oh = PSPD×PLI×CLI×noh (20)

   And the equations for calculating PU, PV, PW and PZ for underground lines are defined as follows:

   PV/ug = PEB×PLD×CLD×nug (21)

   PW/ug = PSPD×PLD×CLD×nug (22)

   Pz/ug= PSPD×PLI×CLI×nug (23)

   (Refer to [1] for the meaning of symbols)

   2h b

   2h 2H

   Fig. 3. Shielding power line by nearby object.

4. CALCULATION OF THE RISK FOR TYPICAL TELECOMMUNICATION SITE

   Applying the improved method as in Sction 3, calculate the value of risk of service due to lightning to typical TS using the values of TABLE I, II and III. (The rate between the number of people in zone (nz) and the total number of people in the structure (nt) has been chosen as equal to 1). It is possible to obtain the value for risk R2=0.01636. This result is greater than the tolerable risk for loss of service is equal to 0.001 [1].

   Similar calculation for the case of typical telecommunica- tion site is located in different areas in Vietnam with ground flash density change from 1÷17 (flashes/km2/year) and the height of antenna tower change from 20÷80 (m). The results of risk calculation are shown in TABLE IV and the results are compared with tolerable risk values in Fig. 4.

   The level of risk of loss of services caused by lightning to telecommunication sites in typical areas in Vietnam is much greater than the tolerable value for the loss of services. The level of risk depends mainly on the ground flash density at the region where the telecommunication site is located; the

   characteristics of the incoming cable and existing protection measures against lightning for the telecommunication site; the higher antenna tower is the more level of the risk increas- es.

   Recalculating for typical TS above with installed SPD on all telecommunication and the low-voltage power line, the risk of loss of service caused by lightning will decrease up to

   100 times. The results of risk calculation are shown in TABLE V and the results are compared with tolerable risk values in Fig.5.

   Results in TABLE V and Fig.5 indicate that installation of protective measures against lightning for the telecommunica- tion site is needed. Surge reduction filter, SPDs on power lines, telecommunication cables and equipment inside should be installed. Besides, determining the location for installation of lightning protection devices and coordinating protection measures in order to achieve maximum efficiency is also important tasks should be taken into account.

   TABLE IV. THE VALUE OF RISK OF LOSS OF SERVICE R2

   Ng (flashes/km2/year) Hanten (m)	The value of risk of loss of service R2
   	1	3	5	7	9	11	13	15	17
   20	0.00135	0.00404	0.00674	0.00944	0.01213	0.01483	0.01753	0.02022	0.02292
   40	0.00136	0.00408	0.00679	0.00951	0.01224	0.01495	0.01767	0.02040	0.02311
   60	0.00138	0.00414	0.00689	0.00965	0.01241	0.01516	0.01792	0.02068	0.02343
   80	0.00140	0.00421	0.00702	0.00983	0.01264	0.01545	0.01826	0.02107	0.02388

   Fig. 4. The values of risk R2 compare with tolerable risk values RT.

   TABLE V.THE VALUE OF RISK OF LOSS OF SERVICE R2, AFTER INSTALLING PROTECTIVE DEVICES.

   Ng (flashes/km2/year) Hanten (m)	The value of risk of loss of service R2×10-5
   	1	3	5	7	9	11	13	15	17
   20	1.3444	4.0332	6.7220	9.4108	12.0996	14.7884	17.4772	20.1660	22.8549
   40	1.3446	4.0339	6.7232	9.4125	12.1018	14.7910	17.4803	20.1696	22.8589
   60	1.3450	4.0351	6.7252	9.4153	12.1053	14.7954	17.4855	20.1755	22.8656
   80	1.3456	4.0368	6.7280	9.4191	12.1103	14.8015	17.4927	20.1839	22.8750

   1. Hanten =20 m b) Hanten =40 m

   c) Hanten =60 m d) Hanten =80 m

   Fig. 5. The value of risk R2 compare with tolerable risk values RT, after installing protective devices.

5. CONCLUSION

This paper analyzed and calculated the risk of loss of service to typical TSs due to lightning in typical areas in Viet Nam by the improved method. The level of risk of loss of service caused by lightning for typical TS in areas in Viet Nam is shown with ground flash density change from 1÷17(flashes/km2/year) and the height of antenna tower changes from 20÷80 (m). The results of calculation indicat- ed that the level of risk greater than the tolerable value. The level of risk increases and depends mainly on density of lightning strikes.

Therefore, designation and installation of lightning pro- tection measures to minimize the risk of the damages of the services caused by lightning for TS in areas in Vietnam is necessary. When the SPDs are installed on all the power lines and telecommunication cables, the level of risk caused by lightning can decrease up to 100 times.

Risk assessment of the damages due to lightning at TSs helps forecast the damages of the services, which supports the engineers designing the lightning protection system give proper lightning protection solutions to reduce the risk to below the tolerable limit.

ACKNOWLEDGMENTS

This research was supported by Ho Chi Minh City Uni- versity of Technology and Education under a research at the Electrical Power System and Renewable Lab.

REFERENCES

1. IEC 62305-2, Protection against lightning – Part 2: Risk manage- ment, 2011.

2. Australian/New Zealand Standard, Lightning protection, 2007.

3. IEEE 1410, Guide for Improving the Lightning Performance of Electric Power Overhead Distribution Lines, 2004.

4. ITCnews, In 2012, VNPT damaged more than 58 billion caused by natural disasters, itcnews.vn, May 2013.

5. Le Quang Trung, Quyen Huy Anh, Phan Chi Thach, Compare Different Recent Methods and Propose Improved Method for Risk Assessment of Damages Due To Lightning, Journal of technical education science (JTE), No.32, 7.2015.

6. Quyen Huy Anh, Bui Kim Cuong, Overall lightning protection solution for telecom site, Journal of information technology and communications of Viet Nam, No.1, 9.2013.

7. National technical regulation on lightning protection for telecom- munication stations and outside cable network, Ministry of infor- mation and communications of the Socialist Republic of Viet Nam, 2011.