Remote Radio Unit Rru Dc Feed Protectionrev7

7/25/2019 Remote Radio Unit Rru Dc Feed Protectionrev7

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Document technical content created by M J Maytum

2014 Copyright of M J Maytum

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Remote Radio Unit (RRU) DC Feed1protection2

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Abstract: Distributed Base Stations (DBS) split the traditional Radio Base Station1(RBS) antenna tower base equipment into two locations; a Base Band Unit2(BBU) at the tower base and a Remote Radio Unit (RRU) mounted on the top of3the tower. Normally the BBU and the RRU would be connected by a fibre optic4cable to carry the signals and a DC feed to power the RRU. Towers are likely to5be struck with lightning and so some form of protection is necessary to prevent6damage to the DC powering feed and connected equipment. Several example7protection methods and a worked example are given. Feed cable currents and8protection stress levels are calculated for negative and positive lightning flashes.9Clause 1 describes the DBS configuration with term definitions in clause 2.10Clause 3 shows three possible protection configurations and clause 4 determines11the circuit parameters. Clauses 5 through to 7 calculate DC feed cable currents12and the Surge Protective Device (SPD) energy for four variants of lightning13stroke. Clause 8 outlines two other forms of DC feed protection. Finally clause 914summarises and comments on the results.15

Keywords:DBS, RRU, RRH, BBU, SPD, DC powering feed, protection, lightning stroke, positive16lightning, negative lightning17

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Contents1

1. Introduction ........................................................................................................................................................ 121.1 RRH Configuration ..................................................................................................................................... 131.2 History ......................................................................................................................................................... 24

2. Definitions and abbreviations............................................................................................................................ 252.1 Definitions ................................................................................................................................................... 262.2 Abbreviations .............................................................................................................................................. 37

3. Protection configurations ................................................................................................................................... 38

4. Circuit component values .................................................................................................................................. 594.1 Resistances .................................................................................................................................................. 510

4.2 Inductances .................................................................................................................................................. 6

11 4.3 Equivalent experimental tower circuit ....................................................................................................... 612

5. Tower voltage during a negative lightning flash.............................................................................................. 7135.1 First stroke ................................................................................................................................................... 8145.2 Subsequent strokes .................................................................................................................................... 1215

6. Tower voltage during a positive lightning stroke .......................................................................................... 14166.1 Positive stroke ........................................................................................................................................... 1417

7. Tower voltage during an extreme positive lightning stroke .......................................................................... 16187.1 Extreme positive stroke ............................................................................................................................ 1719

8. Alternative protection arrangements ............................................................................................................... 18

20 8.1 Gas Discharge Tube (GDT) reference bonding ...................................................................................... 18218.2 Shielded DC feed ...................................................................................................................................... 1922

9. Lightning stroke results and comments .......................................................................................................... 1923

Annex A (informative) Bibliography................................................................................................................. 2324

25

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1

Remote Radio Unit (RRU) DC Feed1protection2

1. Introduction3

1.1 RRH Configuration4

Traditionally Radio Base Stations (RBS) had the bulk of the equipment at the base of the antenna tower or5mast. Distributed Base Stations (DBS) split the equipment into two sections; a Base Band Unit (BBU) at6the tower or mast base and a Remote Radio Unit (RRU) mounted at the top of the tower or mast. Normally7the BBU and the RRU would be connected by a fibre optic cable to carry the signals and a DC feed to8

power the RRU, see Figure 1. The RRU is also called a Remote Radio Head (RRH). To harmonize with9

[B3] the acronym RRU will be used in this document.10

Where a tower exists, such as power distribution pylons, the RRU may be mounted at an intermediate level11rather than at the tower top.12


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2

Remote RadioUnit RRU

Antennas

Powering: BlueSignal: Yellow

Lightning Rod

Base BandUnit RBU

Tower orMast

13

Figure 1 Distributed Base Station14

1.2 History15

RRU DC feed protection has been under discussion in the ITU-T since the submission of [B1] in 2011. The162012 presentation [B2] gave details the unexpected DC feed protection surge waveshape and the reasons17for it. In 2014 the ITU-T published a Recommendation [B3] giving a more comprehensive explanation of18the DC feed surge protection.19

2. Definitions and abbreviations20

2.1 Definitions21

For the purposes of this document, the following terms and definitions apply.22

Radio Base Station (RBS):Installation intended to provide access to the telecommunication system by23means of radio waves.24[B3]25

Distributed Base Station (DBS):One kind of Radio Base Station, where the Remote Radio Unit (RRU)26and Base Band Unit (BBU) can be installed separated.27[B3]28


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Remote Radio Unit (RRU):29(Syn. Remote Radio Head (RRH))30The radio frequency module of Radio Base Station which can be installed separately. Optical fibre is31

commonly used to connected radio frequency module and base band unit of Radio Base Station.32 [B3]33

Base Band Unit (BBU):The base band module of Radio Base Station which can be installed separately.34Optical fibre is commonly used to connected base band module and radio frequency module.35[B3]36

surge reference equalizer:A surge protective device used for connecting equipment to external systems37whereby all conductors connected to the protected load are routed, physically and electrically, through a38single enclosure with a shared reference point between the input and output ports of each system.39[B4]40

2.2 Abbreviations41

BBU Base Band Unit42

DBS Distributed Base Station43

RBS Radio Base Station44

RRU Remote Radio Unit45

RRH Remote Radio Head46

RTN Return47

SPC Surge Protective Component48

SPD Surge Protective Device49

3. Protection configurations50

Figure 2 shows the common arrangement where the floating DC feed has protection applied at the RRU51and the BBU. During lightning stroke events the protection, the red block in Figure 2, becomes a surge52reference equalizer, lightning event bonding the DC feed to the local tower structure via the black53connection line from the protection block to the tower.54


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Remote RadioUnit RRU

Protection

Powering: BlueSignal: Yellow

Lightning Rod

Base BandUnit RBU

Tower orMast

Protection

55

Figure 2 Protection (red block) applied to DC powering feed at tower base and top56

Figure 3 through to Figure 5 shows some protection configurations for a -48 V DC feed, assuming the57lightning stroke current flows from tower top to bottom. The lightning stroke current, IS, divides between58the tower, IT, and DC feed, IRand IF. Because of the values of tower and DC feed impedances, most of the59

stroke current will flow in the tower. In the Figures, the tower impedances are RTand LTand the DC feed60 impedances are RRand LRfor the return and RFand LFfor the -48 V. The mutual inductance between the61tower and DC feed is represented by MTRF.62

-48 V

RTN

SPD4SPD3

SPD2SPD1

RBURRU

TowerStroke

IS IT

RR

RT

LR

LT MTRF

IR

RF LFIF

MTRF

MTRF

63

Figure 3 2 SPD Protection of DC powering feed64

In Figure 3, SPD1 and SPD2 at the tower top and SPD3 and SPD4 at the base reference equalize the DC65feed voltage to the local tower voltage during a lightning stroke.66


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5

SPD1

SPD2

SPD3

-48 V

RTN

SPD4

SPD5

SPD6

RBURRU

TowerStroke

IS IT

RR

RT

LR

LTMTRF

IR

RF LFIF

MTRF

MTRF

67

Figure 4 3 SPD Protection of DC powering feed68

Figure 4 is similar to Figure 3, but has the addition of an extra SPD, SPD 2 and SPD5, across the DC feed69at the tower top and base to limit the differential feed surge voltage.70

SPD1

SPD2

-48 V

RTN

SPD3

SPD4 RBURRU

TowerStroke

IS IT

RR

RT

LR

LTMTRF

IR

RF LFIF

MTRF

MTRF

71

Figure 5 Single SPD for reference equalization72

Figure 5 uses a single SPD for reference equalization, SPD1 and SPD3, and another SPD, SPD2 and SPD4,73across the DC feed at the tower top and base to limit the differential feed surge voltage.74

4. Circuit component values75

Reference [B1] measured a three leg, 5 cm square tube, folded 24 m tower in a laboratory environment76fitted with a 6 mm2DC feed cable.77

4.1 Resistances78

The three mild steel tower legs had 2 mm walls, making the total cross-sectional area, A, 3 x (50 2-462) =791152 mm2. The calculated tower resistance will be:80


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6

m

A

lR

T1.3

101152

24105.1 6

7 81

Where:82

l= 24 m83A= 1152x10-6m284 = 1.5x10-7m (mild steel)85

The actual measured tower resistance was 2.2 mequivalent to 6.6 mper leg.86

Similarly the 6 mm2DC feed cable resistance calculates to 68 musing = 0.17x10-7m (copper).87

4.2 Inductances88

To enable short connection leads to the surge generator, the tower was bent back on itself to form a U or89hairpin shaped structure. Had the tower been straight, the equation for a straight conductor inductance,90L=0.2l(ln(2l/r)-0.75) H, based on [B5] could have been used. This equation results in an inductance value91of 32 H per tower leg or 11 H with three in parallel. A tower inductance relationship is given in [B6],92which gives a rule of thumb of 0.84 H for every metre of tower height, giving an inductance value of9320 H for a 24 m tower.94

The inductance of a hairpin shaped tower can be treated as rectangle with sides of length L and width d95formed by a conductor of radius r. The inductance equation [B9] for a rectangle is:96

Hr

ll

r

dd

d

dl

d

ll

l

dl

l

dddldlL

2ln

2lnlnln224.0

22222297

This equation results in an inductance value of 17 H per tower leg or 5.8 H with three in parallel.98

Using the equation for the 6 mm2 DC feed cable gives a value of 32 H. Bending the tower round in a99hairpin to apply the surge generator reduces the tower and cable inductance.100

The mutual inductance between the tower and cable is neglected as the recorded current waveforms did not101show any major mutual inductive effects.102

4.3 Equivalent experimental tower circuit103

Figure 6 shows the simplified circuit for [B1] using the values from previous clauses showing currents and104component voltages.105


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SPD1

+SPD2

RBU

RRUTop

Stroke

LC

IT

32 H

Cable

LT

5.8 H

IC

RT RC

68 m2.2 m

VLC

Base

VLT

VRCVRT

VSPD

Tower

IS

Ground106

Figure 6 Folded 24 m tower simplified circuit107

The tower path is lower in impedance than the cable path so most of the current flows in the tower path. As108most of the current is in the tower path the tower acts like a voltage generator to drive current through the109cable path. The voltage across the parallel arms must have the same value, giving:110

RCSPDLCRTLT VVVVV 111

Rearranging in terms of VLC112

RCSPDRTLTLC VVVVV 113

Integrating this equation with time will result in the voltseconds applied to the cable inductance, LC, which,114when divided by the cable inductance will give the cable current, IC. This approach is used to determine the115cable current in the following clauses.116

5. Tower voltage during a negative lightning flash117

The median stroke values from [B7] will be used to emulate a negative lightning flash. The parameter118values are:119

First stroke: 30 kA, 5.5/75 and 5.2 C120


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Subsequent strokes: 12 kA, 1.1/32 and 1.4 C121

Inter-stroke interval: 60 ms122

Subsequent stroke number: 3 to 5123

5.1 First stroke124

Figure 7 shows the simulated first stroke tower current. The slow start to current rise is to model more125accurately the naturally occurring stroke initial current rise. A double exponential lightning equation gives126a rapid initial rate of rise and overestimates the inductive voltage levels. The alternative equation given in127[B8] was used to emulate the current waveform slow initial rise.128

Time s

0 20 40 60 80 100 120

TowerCurrent

A

0

5000

10000

15000

20000

25000

30000

129

Figure 7 First stroke tower waveform130


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Figure 8 shows the tower inductive (VLT) and resistive (VRT) voltage components as a result of the first131stroke current.132

time vs i

Time s

0 20 40 60 80 100 120

ComponentVoltageV

-5000

0

5000

10000

15000

20000

25000

30000

35000

40000

Inductive Voltage

Resistive Voltage

133

Figure 8 First stroke tower voltages134

The stroke current rate of rise develops a substantial tower inductive voltage (40 kV red line) and this135determines the voltage applied to the cable and the consequent current in the cable. The tower resistive136voltage developed in this case is negligible (green line). Until the combined voltage thresholds (VSPD) of the137two SPDs, SPD1 and SPD2, is exceeded, substantive current cannot flow in the cable. If, for example the138combine threshold voltage was 200 V only the portion of the tower voltage exceeding 200 V will build up a139current in the cable.140

Figure 9 shows the portion of the inductive voltage that exceeds 200 V in red and that below 200 V in141 black. The fill represents the volt-seconds (172 mVs) applied to the cable inductance, LC.142

Time s

0 10 20 30 40 50

InductiveVoltageV

-5000

0

5000

10000

15000

20000

25000

30000

35000

40000

< 200 V

> 200 V

143

Figure 9 First stroke waveform above 200 V144

Figure 10 integrates the red portion (>200 V) volt-seconds of Figure 9 and shows the cumulative millivolt-145seconds applied to the cable inductance LC. As the volt-seconds divided by the cable inductance is the value146of current, the cumulative waveshape is also the cable current rise waveshape. The total volt-seconds147


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applied is 172 mVs and that would give a peak inductive current of 172m/32 = 5380 A. The inductive148cable energy stored is 0.5*32*53802= 460 J.149

Time s

0 5 10 15 20 25 30

CumulativemVs

0

20

40

60

80

100

120

140

160

180

200

150

Figure 10 Cumulative millivolt-seconds applied to the cable inductance LC151

Figure 11 shows the simplified circuit elements just after the cable current peak.152

SPD1

+SPD2

RBU

RRUTop

Stroke

LC

IT

32 H

Cable

LT

5.8 H

IC

RT RC

68 m2.2 m

VLC

Base

VLT

VRCVRT

VSPD

Tower

IS

Ground

5380 A30000 A

153

Figure 11 Simplified circuit elements for cable current decay154

After the cable current has peaked, the cable stored inductive energy discharges. The voltage polarity155across the cable inductance (VLC) and tower (VLT) reverses after the current peak as the stroke di/dt156


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becomes negative. The voltage across the cable inductance will essentially be the combined SPD clamping157voltages of SPD1 and SPD2 (200 V) and the tower voltage. The previous voltage equation becomes158

RCSPDRTLTLC VVVVV 159

The modified equation shows that the combined SPD voltage now adds to the tower voltage instead of160subtracting from it, so reducing the time for the cable inductive energy to discharge.161

Time s

25 50 75 100 125 150 175 200

Tower+SPDvoltageaftercurrentpeak

V

-2500

-2000

-1500

-1000

-500

0

162

Figure 12 Tower voltage after current peak163

The tower voltage plus the voltages of the SPD1 and SPD2 is applied to the cable inductance. Figure 12164shows the combined tower and SPD voltage after the stroke current peak. By integrating the total voltage165with time, the time at which the volt-second product reaches 172 mVs can be found. The 172 mVs value is166reached at 168 s. At 168 s the induced current in the cable will be zero.167

Figure 13 shows the tower and cable currents. The cable current drops to zero at 168 s or 151 s after the168current peak.169

The SPD energy deposited can be found from integrating the cable current and multiplying by the SPD170voltage (200 V). The total SPD energy deposited was found to be 68 J comprised of 5 J in the current rise171and 63 J in the current decay. The total energy deposited in each SPD is 34 J. This is less than 10 % of the172energy (460 J) that was stored in the cable inductance.173


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Time s

0 50 100 150 200 250

FirststroketowercurrentA

0

5000

10000

15000

20000

25000

30000

CableCurrentA

0

1000

2000

3000

4000

5000

6000

First stroke tower current

Cable current

174

Figure 13 First stroke tower and cable currents175In summary, the significant first stroke parameters were:176

Peak tower current 30 kA (stroke current 35 A)177

Peak cable current 5.38 kA178

Cable volt-second integral during current rise 172 mVs179

Cable stored energy 460 J180

Cable current time from peak to zero 161 s181

SPD energy 68 J (about 34 J each SPD)182

5.2 Subsequent strokes183

Subsequent strokes are typically lower in amplitude, faster rate of current rise and shorter decay times, but184they occur in multiples of 3 to 5. Figure 14 shows the simulated subsequent stroke tower current.185


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Time s

0 10 20 30 40 50 60 70 80 90 100

Subsequentstroketowercurrent

A

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

186

Figure 14 Subsequent stroke tower current187Figure 15 shows the tower inductive and resistive voltage components as a result of the subsequent stroke188current.189

Time s

0 10 20 30 40 50

Towercomp

onentvoltages

V

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

Inductive voltage

Resistive voltage

190

Figure 15 Subsequent stroke tower voltages191

The fast rate of current rise causes a peak tower voltage of over 80 kV. The treatment of these values192followed the same procedure as the first stroke clause. Significant parameters were:193

Peak tower current 12 kA (stroke current 14 kA)194





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Figure 16 shows the tower and cable currents. If there were 5 subsequent strokes the total energy deposited200in a single SPD (SPD 1 or SPD2) would be 5*7 + 34 = 69 J.201

Time s

0 10 20 30 40 50 60 70 80

Subsequentstr

oketowercurrentA

0

2000

4000

6000

8000

10000

12000

Cable

currentA

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

Subsequent stroke tower current

Cable current

202

Figure 16 Tower and cable currents203

6. Tower voltage during a positive lightning stroke204

The median stroke values from [B7] will be used to emulate a positive lightning stroke. The parameter205values are:206

35 kA, 22/230 and 16 C207

6.1 Positive stroke208

Figure 17 shows the simulated positive stroke tower current. The slow start to current rise is to model more209accurately the naturally occurring stroke initial current rise.210


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Time s

0 50 100 150 200 250 300 350 400 450 500

Positivestroketowercurrent

A

0

5000

10000

15000

20000

25000

30000

35000

40000

211

Figure 17 Positive stroke tower current212Figure 15 shows the tower inductive and resistive voltage components as a result of the subsequent stroke213current.214

Time s

0 50 100 150 200 250 300 350 400 450 500

Towercomp

onentvoltages

V

-1000

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

Inductive voltage

Resistive voltage

215

Figure 18 Positive stroke tower voltages216

The rate of current rise causes a peak tower voltage of 12 kV. The treatment of these values follows the217same procedure as the first stroke clause. Significant parameters were:218






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Figure 16 shows the tower and cable currents. This figure shows that, with the 35 kA stroke current, when225the stroke current time to half value is 250 s or longer the cable current decay waveshape tends to a linear226ramp. This was noticed on the oscilloscope shots of [B1] where the surge generator used had a time to half227value of 400 s to 440 s.228

Time s

0 50 100 150 200 250 300 350 400 450 500

PositivestroketowercurrentA

0

5000

10000

15000

20000

25000

30000

35000

40000

Cablecurrent

A

0

1000

2000

3000

4000

5000

6000

7000

8000

Positive stroke tower current

Cable current

229

Figure 19 Positive stroke tower and cable currents230

7. Tower voltage during an extreme positive lightning stroke231

The recommended extreme (1 % population) stroke values from [B11] were used to emulate an extreme232positive lightning stroke. The positive stroke parameter values given are:233

350 kA peak current, 11 s time to current peak, 40 s time to half value and 500 kA/s di/dt234maximum235

For reference the extreme negative stroke parameter values given are:236

160 kA peak current, 6 s time to current peak, 80 s time to half value and 500 kA/s di/dt237 maximum238

Both waveforms have a 500 kA/s maximum rate of rise and will generate the same value of peak239inductive voltage.240


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7.1 Extreme positive stroke241

Figure 20 shows the simulated extreme positive stroke tower current. This simulation only achieves a242maximum di/dt of 100 kA/s and not the recommended 500 kA/s.243

Time s

0 20 40 60 80 100 120 140

Extremepositivestroketowercurrent

A

0

50000

100000

150000

200000

250000

300000

350000

400000

244

Figure 20 Extreme positive stroke tower current245

Figure 21 shows the tower inductive and resistive voltage components as a result of the extreme positive246stroke current.247

Time s

0 20 40 60 80 100 120 140

TowercomponentvoltagesV

0

100000

200000

300000

400000

500000

600000

700000

Inductive voltage

Resistive voltage

248

Figure 21 Extreme positive stroke tower voltages249

The rate of current rise (105 kA/s) causes a peak tower voltage of 620 kV. The treatment of these values250follows the same procedure as the first stroke clause. Significant parameters were:251




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Cable volt-second integral during current rise 1.79 Vs254

Cable stored energy 50 kJ255



Figure 22 shows the tower and cable currents. The tower inductive voltage dominates making the cable258current waveshape similar to the stroke current waveshape.259

Time s

0 20 40 60 80 100 120 140

TowercurrentA

0

50000

100000

150000

200000

250000

300000

350000

400000

CablecurrentA

0

10000

20000

30000

40000

50000

60000

70000

80000

Tower current

Cable current

260

Figure 22 Extreme positive stroke tower and cable currents261

8. Alternative protection arrangements262

8.1 Gas Discharge Tube (GDT) reference bonding263

Clause B.5 Series connected GDTs for DC power applications in [B10] shows how a series combination264of GDTs can develop a combined arc voltage higher than the protected DC supply voltage. This series265arrangement avoids the possibility of the GDTs continuing to conduct after a lightning surge.266

The previous Figure 5 circuit can be used with single GDTs for SPD1 and SPD3.267


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19

SPD1

SPD2

-48 V

RTN

SPD3

SPD4 RBURRU

TowerStroke

IS IT

RR

RT

LR

LTMTRF

IR

RF LFIF

MTRF

MTRF

268

Figure 23 Single GDT for reference equalization269

When the tower stroke voltage causes the sparkover of the GDTs, the return, RTN, cable is bonded to the270

tower top and base. As a result a portion of the stroke current will flow in the cable RTN for the stroke271 duration. SPDs SPD2 and SPD4 should be clamping type voltage limiters to avoid shorting the DC supply.272The feed (-48 V) cable current will be similar to that of the clause 4.3 circuit.273

8.2 Shielded DC feed274

The Appendix I, Isolated protection solution: example of [B3] shows how a cable shield and a power feed275isolation barrier in the RRU could be used to remove the need of SPD elements. The withstand voltage of276the isolation barrier needs careful consideration to comprehend the maximum stroke voltages. In this277document peak values of 80 kV have been calculated just for nominal conditions on the 24 m tower.278

9. Lightning stroke results and comments279

Table 1 Lightning stroke values and results280

Stroke

(clause #)

Peak

towercurrent

kA

Cable

peakcurrent

kA

Stroke

waveshape

Cable

currenttime to 0

s

Vsintegral

peak

mVs

Tower

energyJ

Cable

energyJ

Total

SPDenergy

J

Negativefirst (5.1)

30(100 %)

5.4(18 %)

5.5/75 160 170 2610(100 %)

460(18 %)

68(2.6 %)

Negativesubsequent(5.2)

12(100 %) 2.2(18 %) 1.1/32 70 69 418(100 %) 75(18 %) 13(3.1 %)

Positive(6.1)

35(100 %)

6.1(17 %)

22/230 340 200 3550(100 %)

600(17 %)

210(5.9 %)

Extremepositive(7.1)

350(100 %)

56(16 %)

4.5/40 110 1800 355000(100 %)

50000(14 %)

460(0.13 %)

Percentage values shown are relative the tower current peak or tower energy


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20

Table 1 results were obtained by the following procedure:281

1) Estimate the inductive and resistive components of the tower and DC feed cable.2822) Define the lightning stroke waveform applied to the model.283

3) Assume most of the stroke current flows in the tower284

4) Calculate the tower voltage from the stroke current and the tower inductance and resistance.285

(in this example the resistive voltage was negligible)286

5) Subtract the nominal SPD voltage from the result of 4).287

6) Integrate the result of 5) with time to give a cumulative Vs waveform.288

7) Divide the result of 6) b y the cable inductance to give the SPD current versus time waveform.289

8) Multiply the result of 7) by the nominal SPD voltage to give the SPD power versus time waveform.290

9) Integrate the result of 8) for the cable current duration time to give the SPD cumulative energy.291

Only a nominal value of SPD clamping voltage was used. A further refinement of this procedure would be292to incorporate a model for the SPD clamping voltage versus current.293

The lightning parameters used for analysis of the first three table data rows were median stroke values from294[B7]. The forth extreme positive stroke row used recommendations from [B11], which represents the more295stressful values occurring in the field. For a given amplitude of stroke current, the energy deposited into the296SPD increases with increasing time to half value of the stroke. Longer times, over about 250 s, to half297value cause the SPD current decay waveshape to approach a linear ramp.298

The transition from a truncated stroke waveshape to a linear ramp warrants further investigation. The299 following (simplistic) approach provides some indication of the transition region. The actual tower current,300iT, can be approximated to:301

TC

CST LL

Lii 302

Where:303

iS= Peak stroke current304LT= Tower inductance305LC= Cable inductance306

In Figure 24 the tower current is approximated to a triangular waveform. The waveform rises in time TFto307a peak of current of iTMAXand then linearly decays to 50 % of iTMAX, in time TDT, measured from the current308

peak. The cable current reaches zero in time 2TDT.309


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21

Voltag

e

TowerCurre

nt

CableCurrent

0

0

0

iCMAX

iTMAX

vTMAX-vSPD

-vSPD

0.5iTMAX

2TDTTF0

TF0

iTactual

iTlinear approximationTDT

2TDC

310

Figure 24 Simple analysis of cable current decay time to zero311

In the time to peak current, the tower current rate of rise generates an inductive voltage, VTMAX, of312LT*iTMAX/TF. The voltage applied to the cable inductance, LC, will be reduced by the SPD clamping voltage313VSPD. The peak cable current, iCMAX, will be:314

CSPDFTMAXTFCMAX

LVTiLTi // 315

Simplifying to316

CSPDFCTMAXTCMAX LVTLiLi // 317

Generally the TFVSPD/LCfactor will be small making318


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22

CTMAXTCMAX LiLi / 319

During the tower current decay time the cable inductive voltage, vCD, will be320

SPDDTTMAXTCD

VTiLv 2/ 321

or322

DTSPDDTTMAXTCD

TVTiLv 2/2 323

The time, 2TDC, for the cable current to decay to zero is given by324

CDCMAXCDC viLT /2 325

Simplifying326

SPDDTTMAXTTMAXTDTDC

VTiLiLTT 2/22 327

TMAXTSPDDTDTDC

iLVTTT /21/22 328

The equation above indicates that the cable current decay linearizes when the 2TDTVSPD/(LTiTMAX) factor329becomes significant compared to unity. Inserting the example values gives 400TDT/(5.8iCMAX) =33069TDT/iTMAX, where TDTis in microseconds. In clause 6.1 with iTMAX= 35 kA and TDT=200 s a linear331ramp is noticeable. The factor for this condition is 69*200/35000 = 0.39, making 2TDC= 2*200/1.39 = 290332s. The actual calculated value using exponential decays was 340 s.333

The above shows the transition from a cable current truncated exponential decay waveshape to a linear334decay ramp is determined by the 2TDTVSPD/(LTiTMAX) factor becoming significant compared to unity. For a335given setup VSPDand LTwill be fixed and the variables are TDTand iTMAX. In the example given, a336linearized ramp occurred with a factor of TDT/iTMAX= 200 s/35 kA = 5.7 nAs.337

Further the approximate SPD energy, ESPDcan be calculated with338

CTDCTMAXSPDSPD LLTiVE / 339

Using previous values ESPD= 200*35000*100 s*5.8/32 = 20*35*5.8/32 = 126 J. The actual calculated340value using exponential decays was 210 J. Thus the simplistic linear approach can be used to establish341orders of magnitude but not to any level of accuracy.342

The analyzed tower was 24 m and folded. Higher towers will result in proportionally larger values of stress343as the inductances will be proportional to the tower height.344


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23

Annex A345346

(informative)347348Bibliography349

These bibliographical references provide background information, but do not need to be consulted to350understand this document.351

[B1] Mr Politis Zafiris, Lightning current through MOV based SPDs in RRH applications, Raycap SA352

[B2] Mick Maytum, Towering Powering protection problem on Remote Radio Head (RRH) cellular353systems, PEG 2012 Conference354

[B3] ITU-T Recommendation K.97 (02/2014): Lightning protection of distributed base stations355

[B4] IEEE Std 1100-1999 - IEEE Recommended Practice for Powering and Grounding Electronic356Equipment357

[B5] IEEE Std 518-1982 - IEEE Guide for the Installation of Electrical Equipment to Minimize Electrical358Noise Inputs to Controllers from External Sources359

[B6] E. A. Williams, G. A. Jones, D. H. Layer, T. G. Osenkowsky, National Association of Broadcasters360Engineering Handbook, Focal Press361

[B7] CIGR (Council on Large Electric Systems) Technical Bulletin (TB) 549 (2013), Lightning362Parameters for Engineering Applications363

[B8] F. Heidler, Z. Flisowski, W. Zischank, C. Mazzetti, Parameters of lightning current given in IEC36462305 Background, Experience and Outlook, 29th International Conference on Lightning Protection,36523rd 26th June 2008 Uppsala, Sweden366

[B9] Rectangular loop inductance,367http://www.cvel.clemson.edu/emc/calculators/Inductance_Calculator/rectgl.html, Clemson University,368retrieved 2014-10-28369

[B10] IEEE PC62.42.1 - Draft Guide for the Application of Surge-Protective Components in Surge370Protective Devices and Equipment Ports - Gas Discharge Tubes (GDTs)371

[B11]W. R. Gamerota, J. O. Elisme, M. A. Uman and V. A. Rakov, Current Waveforms for Lightning372

Simulation, IEEE Transactions on Electromagnetic Compatibility, Vol. 54, No. 4, August 2012, pp 880-888373

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