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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
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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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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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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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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
Peak cable current 2.17 kA195
Cable volt-second integral during current rise 69 mVs196
Cable stored energy 75 J197
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Cable current time from peak to zero 70 s198
SPD energy 13 J (about 7 J each SPD)199
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
Peak tower current 35 kA (stroke current 41 kA)219
Peak cable current 6.1 kA220
Cable volt-second integral during current rise 196 mVs221
Cable stored energy 595 J222
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Cable current time from peak to zero 343 s223
SPD energy 210 J (about 105 J each SPD)224
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
Peak tower current 350 kA (stroke current 406 kA)252
Peak cable current 55.9 kA253
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Cable volt-second integral during current rise 1.79 Vs254
Cable stored energy 50 kJ255
Cable current time from peak to zero 112 s256
SPD energy 464 J (about 232 J each SPD)257
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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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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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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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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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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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