Analysis of Lightning Surge Protection

Analysis of Lightning Surge Protection

Created by: Glen Zhu | Updated Date: May 24th, 2024

1. Test standard for lightning surge immunity of electronic devices

The international standard IEC 61000-4-5 for the test of lightning surge immunity of electronic devices. The standard mainly simulates various situations caused by indirect lightning strikes:

(1) When lightning strikes external lines, a large amount of current flows into the external lines or grounding resistance, resulting in interference voltage.

(2) Indirect lightning strikes (such as those between cloud layers or within cloud layers) induce voltage and current on external lines.

(3) When lightning strikes objects near the line, a strong electromagnetic field established around it induces a voltage on external lines.

(4) Interference is introduced by ground currents when lightning strikes near the ground and passes through the common grounding system.

In addition to simulating lightning strikes, the standard also simulates disturbances introduced by switch operations in occasions such as substations, such as voltage transients caused by switch actions, including:

(1) Interference generated during main power system switching (such as capacitor bank switching).

(2) Interference caused by minor switch toggling near equipment in the same power grid.

(3) Thyristor devices with resonant circuit switching.

(4) Various systemic faults, such as short circuits and arcing faults between equipment grounding networks or grounding systems.

The standard describes two different waveform generators, one is a waveform induced on the power line by lightning strikes;

Both of these lines belong to overhead lines, but their impedances are different: the surge waveforms induced on the power line are narrower (50µs), with a steeper leading edge (1.2µs); while the surge waveforms induced on communication lines are wider, but with a gentler leading edge. In our analysis of circuits, we mainly focus on waveforms induced on power lines by lightning strikes and briefly introduce lightning protection techniques for communication lines.

2. The working principle of simulated lightning surge pulse generation circuit

The figure above shows the pulse generation circuit when simulating the surge voltage induced in the transmission line by lightning striking the distribution equipment, or the high-voltage backlash generated by the lightning current passing through the common ground resistance after lightning strikes. The single pulse energy at 4kV is 100 joules.

In the diagram, Cs is the energy storage capacitor (approximately 10µF, equivalent to a thundercloud capacitor);

Us is the high-voltage power supply;

Rc is the charging resistor;

Rs is the resistance-forming resistor for pulse duration (forming resistance of discharge curve);

Rm is an impedance matching resistor Ls as inductance forming current rise.

Lightning surge immunity tests have different parameter requirements for different products. The parameters in this diagram can be slightly modified according to product standard requirements.

Basic parameter requirements:

(1) Open-circuit output voltage: 0.5-6kV, divided into 5 levels of output, with the final level determined by user and manufacturer negotiation;

(2) Short-circuit output current: 0.25-2kA, for different level tests;

(3) Internal resistance: 2 ohms, additional resistances of 10, 12, 40, and 42 ohms available for other level tests;

(4) Surge output polarity: positive/negative; when surge output synchronizes with a power source, phase shifts from 0-360 degrees;

(5) Repetition frequency: at least once per minute.

The severe level of the lightning surge immunity test is divided into 5 levels:

  • Level 1: Environment with relatively good protection;
  • Level 2: Environment with certain protection;
  • Level 3: Ordinary electromagnetic interference environment, no special installation requirements specified for equipment, such as industrial workplaces;
  • Level 4: Environments under serious disturbance, such as civilian overhead lines and unprotected high-voltage substations;
  • Level X: To be determined through consultation by the user and the manufacturer.

In the circuit, the 18µF capacitor can have different values selected based on different severity levels, but after reaching a certain value, it basically does not make much sense.

For the 10 ohm resistor and 9µF capacitor, different values can be chosen based on different severity levels. The minimum value for the resistor can be 0 ohms (this is how it is in American standards), and the 9µF capacitor can also be chosen to be very large. However, after reaching a certain value, it basically does not make much sense.

3. Common Mode Surge Suppression Circuit

When designing surge protection circuits, assume that the common mode and differential mode are independent of each other. However, these two parts are not truly independent because the common-mode choke coil can provide a considerable amount of differential mode inductance. This part of differential mode inductance can be simulated by discrete differential mode inductors.

To utilize the differential mode inductance during the design process, common mode, and differential modes should not be processed simultaneously but rather done in a specific order. First measure common-mode noise and filter it out. By using a Differential Mode Rejection Network, the differential mode component can be eliminated so that common-mode noise measurement becomes direct.

If designed common-mode filters need to ensure that differential mode noise does not exceed allowable limits as well then mixed noise from both modes should be measured together. Since it’s known that common-mode components are below noise tolerance level; the only exceeding part would be from the differential model which could attenuate using leakage from the common-mode filter’s impedance for reducing this excess part.

For low-power supply systems; choke coils’ impedance due to their difference between modes is sufficient enough to solve radiated problems caused by differences since source impedance for radiation is small hence only a few effective inductions will work effectively against them.

To suppress surge voltages below 4000Vp, it is generally only necessary to use an LC circuit for current limiting and smoothing filtering, reducing the pulse signal as much as possible to a level of 2-3 times the average value of the pulse signal. Since L1 and L2 have a current flow of 50 Hz power grid passing through them, inductance is easily saturated. Therefore, L1 and L2 generally use a common mode inductor with high leakage inductance.

It can be used for both AC and DC applications. It is commonly seen in EMI filters for power supplies and switch-mode power supplies but less frequently on the DC side. It can be found on the DC side in automotive electronics.

The purpose of adding a common mode inductor is to eliminate common mode interference on parallel lines (both two-wire and multi-wire). Due to impedance imbalances between two lines on circuits, common mode interference ultimately manifests itself differentially. Using differential filtering methods makes it difficult to filter out this type of interference.

When should you use a common mode inductor? Common mode interference usually comes from electromagnetic radiation coupled over space; therefore, whether it’s AC or DC transmission over long distances requires adding a common-mode choke if there are long line transmissions involved. For example: many USB cables have magnetic rings added online. At the input of switch-mode power supplies where AC voltage is transmitted over long distances requires adding one too. Usually not needed on the DC side unless remote transmission is required because if there isn’t any common-mode interference then adding one would just be wasteful without any gain for your circuit.

Power filter design typically considers both differential and common modes when designing filters. The most important part of a common-mode filter is its choke coil which has significant advantages compared with differential chokes due to its extremely high induction value despite being small-sized components; however, when designing these coils their leakage must also be considered – that’s their differential induction value which usually accounts for around 0.5%~4% compared with their nominal values calculated assuming they’re at least 1% effective as CM chokes instead.

How does leakage occur? A tightly wound circular coil that fills up an entire week will concentrate all magnetic flux within its “core” even without using magnetic cores themselves! However, if such coils aren’t fully wound or are loosely wound then some flux will leak out from within its core area instead – this effect increases proportionally based upon relative distance between wire turns & permeability characteristics associated with spiral-core bodies like those used by CM chokes here.

Common-mode chokes have two windings designed so that currents flowing through them conduct along opposite directions across their core areas thereby canceling each other out completely resulting zero net magnetic field inside them overall! If safety considerations require single-line winding configurations though then gaps between these windings become quite large leading naturally towards more “leakage” effects occurring outside our desired region(s) thus creating non-zero fields at points we care about after all… This means that CM choke leakage becomes equivalent to differential-inductance values since related fluxes must leave core regions somewhere else before returning back into closed loops again!

Generally speaking CX capacitors can withstand surge voltage impacts up to 4000Vp while CY capacitors can handle shared-voltage surges up until around 5000Vp max limit-wise depending upon how well matched they are together alongside proper sizing choices made regarding parameters like L1/L2/CX2/CY etc., but if total capacitances exceed 5000P installed throughout machine wiring systems then higher-rated capacitors need selecting plus additional surge suppression circuits featuring clamping functions may also prove necessary beyond certain limits.

What do we mean by suppression? Essentially what happens here involves lowering peak amplitudes somewhat while transforming original spike signals into new waveforms having relatively wider widths yet flatter amplitudes overall; however energy levels remain largely unchanged during this process regardless.

Although capacities associated with dual CY caps tend to be rather small meaning limited stored energies available making little impact toward suppressing shared modes effectively enough alone hence why the primary focus falls onto larger-scale components like L1/L2 instead whose sizes/costs often impose practical limitations preventing optimal performance against lightning-induced shared-voltages unfortunately.

In figure (a), L1 suppresses the common-mode surge voltage with CY1, and L2 suppresses it with CY2. When calculating, only one of them needs to be calculated. To accurately calculate L1, a set of second-order differential equations needs to be solved. The results show that the capacitor charges follow a sine curve and discharges follow a cosine curve. However, this calculation method is relatively complex, so here we use a simpler method.

Assuming that the common-mode signal is a square wave with an amplitude of Up and width of τ, and the voltage across the CY capacitor is Uc, the current flowing through the inductor is a sawtooth wave with a width equal to 2τ:

The current flowing through the inductor is:

The average current flowing through the inductor during 2τ period is:

From this, we can obtain the voltage change of CY capacitor during 2τ period:

The above formula is the calculation formula for the parameters of inductance L and capacitance CY in the common mode surge suppression circuit. In the formula, Uc is the voltage across CY capacitor, which is also the output voltage of the surge suppression circuit.

∆Uc is the voltage change across CY capacitor. However, due to the long period and small duty cycle of lightning pulses, it can be considered that Uc = ∆Uc. Up is the peak value of the common mode surge pulse, q is the charge stored by CY capacitor, τ is the width of the common mode surge pulse, L represents inductance, and C represents capacitance.

According to the above formula, assuming that Up=4000Vp for surge peak voltage and C=2500p for capacitance, with an output voltage Uc=2000Vp from the surge suppression circuit; then a value of 1H is required for inductance L. Obviously this value is very large and difficult to achieve practically. Therefore, this circuit has limited capability for lightning-induced common-mode suppression and needs further improvement.

Differential mode surge voltage suppression mainly relies on filtering inductors L1 and L2 shown in the diagram as well as filtering capacitor CX. The selection of parameters such as L1, and L2 filtering inductor values along with the CX filtering capacitor can also be calculated using the formulas below.

But in the above equation, L should be equal to the sum of L1 and L2, C = CX, and Uc is the differential mode suppression output voltage. Generally, the differential mode suppression output voltage should not exceed 600Vp because many semiconductor devices and capacitors have a maximum withstand voltage near this voltage. After passing through the two filtering inductors L1 and L2 as well as the CX capacitor filter, although the amplitude of lightning surge differential mode voltage decreases, the energy basically does not decrease because after filtering, the pulse width increases. Once a device breaks down, most cannot recover to its original state.

According to the above formula, assuming that surge peak voltage Up = 4000Vp and pulse width is 50µS, if the output voltage Uc of differential mode surge suppression circuit is 600Vp, then LC needs to have a value of 14mH × µF. Obviously, this value is relatively large for surge suppression circuits in general electronic products. In comparison, increasing inductance is more advantageous than increasing capacitance. Therefore it is best to use an inductor with three windows made of silicon steel sheets as its core material and has a relatively large inductance (greater than 20mH) as a surge inductor. This type of inductor has both common-mode and differential-mode high values of inductance and is not easily saturated.

By the way,the electrolytic filter capacitor behind the rectifier circuit also has functions for suppressing surging pulses. If we include this function too, then we cannot choose an output voltage Uc of 600Vp but can only select Ur (400Vp), which represents the highest rated withstand voltage for capacitors.

4. Commonly used devices for suppressing lightning strike surges

Surge protective devices mainly include ceramic gas discharge tubes, zinc oxide varistors, semiconductor thyristor surge protectors (TVS), surge suppression inductors, X-class surge suppression capacitors, etc., and various devices that need to be used in combination.

There are many types of gas discharge tubes with generally large discharge currents reaching tens of kA. The discharge voltage is relatively high. It takes a certain amount of time for the tube to ignite and discharge, and there is residual voltage present making its performance somewhat unstable. Zinc oxide varistors have good volt-ampere characteristics but are limited by power.

The current is relatively smaller compared to gas discharge tubes. After multiple lightning overcurrent breakdowns occur, the breakdown voltage value will decrease or even fail completely. Semiconductor TVS has the best volt-ampere characteristics but generally has low power and higher cost. Surge suppression coils are the most basic lightning protection devices; a three-window core must be selected to prevent AC saturation from flowing through the power grid; X capacitors are also necessary and should use capacitors with larger allowable ripple currents.

Gas Discharge Tube (GDT)

A gas discharge tube refers to a type of surge arrester or antenna switch tube used for overvoltage protection, with two or more electrodes inside the tube filled with a certain amount of inert gas. Gas discharge tubes are gap-type lightning protection components used in communication system lightning protection.

The working principle of the discharge tube is that when a certain voltage is applied between the two poles of the discharge tube, an uneven electric field is generated between the poles: under this electric field, the gas inside the tube begins to ionize. When the external voltage increases to exceed the insulation strength of the gas, causing a breakdown between the poles, transforming from an insulating state to a conducting state. After conduction, the voltage between the two poles of the discharge tube remains at a residual pressure level determined by the arc path, which is generally very low, thereby protecting electronic devices connected in parallel from damage due to overvoltage.

Some gas discharge tubes are encapsulated in glass as outer shells while others use ceramics as outer shells. Inert gases with stable electrical properties (such as argon and neon) are filled inside these tubes. The common discharge electrodes in gas discharge tubes are usually two or three separated by inert gases. Based on electrode numbers, gas discharge tubes can be divided into bipolar and tripolar types.

Ceramic bipolar gas discharge tubes consist mainly of pure iron electrodes, nickel-chromium-cobalt alloy caps, silver-copper welding caps, and ceramic bodies. Radioactive oxides are coated on internal electrodes for improved performance along with radioactive elements on inner walls for enhanced characteristics. Electrodes come in rod-shaped and cup-shaped structures; rod-shaped ones require an additional cylindrical heat shield between the electrode and body wall for uniform heating distribution preventing local overheating and leading to fracture risk. Heat shields also have radioactive oxide coatings reducing dispersion further. In cup-shaped electrode models, molybdenum mesh at the mouth reduces dispersion while the cesium element within minimizes it too.

Tripolar gas-discharge tubes also consist mainly of pure iron electrodes,nickel-chromium-cobalt alloy caps,silver-copper welding caps, and ceramic bodies. In contrast to bipolar ones, a nickel-chromium-cobalt alloy cylinder has been added acting as a third pole i.e., ground electrode.

(1) DC breakdown voltage. This value is determined by applying a voltage with a low rise rate (dv/dt=100V/s).

(2) Impulse (or surge) breakdown voltage. It represents the dynamic characteristics of discharge tubes, often determined by a voltage with an ascent rate of dv/dt=1kV/µs.

(3) Rated impulse discharge current. The rated discharge current for an 8/20µs waveform (rise time 8µs, half-peak duration 20µs), is typically discharged 10 times.

(4) Standard discharge current. Defined by the rated effective value of a 50Hz AC current, specifying each discharge time as 1s and discharging 10 times.

(5) Maximum single impulse discharge current. The maximum single discharge current for an 8/20µs current waveform.

(6) Withstand frequency withstand current value. The maximum effective value of continuous currents from nine cycles at a frequency of 50Hz for the single maximum impulse discharge current in an 8/20µs waveform.

(7) Insulation resistance. The maximum effective value of continuous currents from nine cycles at a frequency of 50Hz for the single maximum impulse discharge current in an 8/20µs waveform.

(8 )The capacitance between electrodes in the discharge tube, generally ranging from 2 to 10pF, is the smallest among all transient interference absorber devices.

Gas discharge tube characteristic parameter table (APC)

model

Nominal DC breakdown voltage (V)

DC error (±%)

Breakdown voltage (V)

Nominal withstand impulse current 8/20µs wave (kA)

Rated withstand current at power frequency 50Hz/1s (kA)

Single impulse current withstand capacity 8/20µs wave (kA)

Rated current for industrial frequency 50Hz 9 cycles (A)

Insulation resistance (GΩ)

Capacitance (PF)

Bipolar discharge tube

R098XA

90

20

≤700

5

5

10

20

≥1

≤1.5

R158XA

150

20

≤700

5

5

10

20

≥1

≤1.5

R238XA

230

20

≤800

5

5

10

20

≥1

≤1.5

R358XA

350

20

≤800

5

5

10

20

≥1

≤1.5

R478XA

470

20

≤900

5

5

10

20

≥1

≤1.5

R608XA

600

20

≤1200

5

5

10

20

≥1

≤1.5

R808XA

800

20

≤1400

5

5

10

20

≥10

≤1.5

R1008XA

1000

20

≤1600

5

5

10

20

≥10

≤1.5

Triode discharge tube

3R077CXA

75

20

≤600

5

5

10

>35

≥1

≤2

3R097CXA

90

20

≤700

5

5

10

>35

≥1

≤2

3R157CXA

150

20

≤700

5

5

10

>35

≥1

≤2

3R237CXA

230

20

≤800

5

5

10

>35

≥1

≤2

3R357CXA

350

20

≤800

5

5

10

>35

≥1

≤2

3R477CXA

470

20

≤900

5

5

10

>35

≥1

≤2

3R607CXA

600

20

≤1200

5

5

10

>35

≥1

≤2

Metal Oxide Varistor (MOV)

Varistors are generally composed mainly of zinc oxide, with small amounts of other metal oxides (particles) added, such as cobalt, manganese, bismuth, etc., and pressed into shape. Due to the combination of two different types of materials together, it is equivalent to a PN junction (diode). Therefore, varistors are essentially made up of numerous PN junctions connected in series and parallel.

MOV Resistor Characteristic Parameter Table (Xi’an Radio Factory)

model

MOV voltage

Maximum continuous voltage

Maximum continuous voltage current

Peak-to-Peak Current

Pulse current life value 8/20µs/10 times

Interdigital capacitor 1kHz

8/20µs/2 times

2ms square wave

Vd (V)

AC (V)

DC (V)

Vc (V)

Ip (A)

(A)

(J)

(A)

(pF)

MYD-05K330

33

20

26

73

1

50

0.6

5

900

MYD-05K390

39

25

31

86

1

50

0.8

5

500

MYD-05K470

47

30

38

104

1

50

1.0

5

450

MYD-05K560

56

35

45

123

1

50

1.0

5

400

MYD-05K680

68

40

56

150

1

50

1.2

5

350

MYD-05K820

82

50

65

145

5

200

1.7

20

250

MYD-05K101

100

60

85

175

5

200

2.0

20

200

MYD-05K121

120

75

100

210

5

200

2.5

20

170

MYD-05K151

150

95

125

260

5

200

3.0

20

140

MYD-05K201

200

130

170

355

5

200

4.0

20

80

MYD-05K221

220

140

180

380

5

200

4.5

20

70

MYD-05K241

240

150

200

415

5

200

5.0

20

70

MYD-05K271

270

175

225

395

5

200

6.0

20

65

MYD-05K361

360

230

300

620

5

200

7.5

20

50

MYD-05K391

390

250

320

675

5

200

8.0

20

50

MYD-05K431

430

275

350

745

5

200

9.0

20

45

MYD-05K471

470

300

385

810

5

200

10.0

20

40

MYD-05K621

620

385

505

1025

25

1250

45.0

100

130

5. Ultra-high surge voltage suppression circuit

Example 1

The diagram above is a schematic diagram of an electrical circuit that can withstand strong lightning surge pulse voltages. In the diagram: G1 and G2 are gas discharge tubes, mainly used to suppress high-voltage common-mode surges, and they also have the ability to suppress high-voltage differential-mode surges; VR is a varistor, mainly used to suppress high-voltage differential-mode surges. After being suppressed by G1, G2, and VR, both the amplitude and energy of common-mode and differential-mode surge pulses are significantly reduced.

The breakdown voltage of G1 and G2 can be selected from 1000Vp to 3000Vp, while the varistor’s clamping voltage is generally taken as 1.7 times the maximum value of the power frequency voltage.

After breakdown occurs in G1 and G2, subsequent currents will be generated. It is necessary to add a fuse to prevent excessive subsequent current from causing a short circuit in the circuit.

Example 2

Added two varistors VR1, VR2, and a discharge tube G3, the main purpose is to enhance the suppression of common mode surge voltage. Since varistors have leakage current, and general electronic products have strict requirements for leakage current (less than 0.7mAp), a discharge tube G3 is added in the diagram to make the leakage current of the circuit to ground equal to zero under normal circumstances. The breakdown voltage of G3 should be much lower than that of G1 and G2. By using G3 for leakage isolation, the breakdown voltage of varistor VR1 or VR2 can be selected relatively low accordingly. VR1 and VR2 also have a strong inhibitory effect on differential mode surge voltage.

Example 3

G1 is a three-terminal discharge tube, which is equivalent to installing two two-terminal discharge tubes in one housing. It can replace the G1 and G2 discharge tubes in the above two examples. In addition to two-terminal and three-terminal discharge tubes, there are also four-terminal and five-terminal discharge tubes, each with different uses.

Varistors are generally composed mainly of zinc oxide, with small amounts of other metal oxides (particles) added, such as cobalt, manganese, bismuth, etc., and pressed into shape. Due to the combination of two different types of materials together, it is equivalent to a PN junction (diode). Therefore, varistors are essentially made up of numerous PN junctions connected in series and parallel.

Example 4

Added two varistors (VR1, VR2), the main purpose is to isolate the subsequent current generated after G1 breakdown, to prevent excessive subsequent current from short-circuiting the input circuit. However, since the maximum peak current of VR1 and VR2 is generally only a fraction of G1’s, so in this example, the suppression capability against ultra-high surge voltage is much worse than that of Example 3.

Example 5 – directly manufactures surge protection devices on PCB boards

Directly making a surge protection device on the PCB board can replace the gas discharge tube, which can suppress tens of thousands of volts of common-mode or differential-mode surge voltage shocks. The distance between the electrodes of the surge protection device is generally required to be strict. When the input voltage is AC110V, the distance between the electrodes can be selected as 4.5mm; when the input voltage is AC220V, it can be selected as 6mm. The middle electrode of the surge protection device must be connected to one end of the power line and to a port on the PCB board.

Example 6 – PCB Board Air Gap Discharge Device Replacing Discharge Tube

Making an air gap discharge device directly on a PCB board, with a normal discharge voltage range per millimeter being 1000-1500V, approximately 4500-6800Vp for a climbing distance of 4.5mm, and approximately 6000-9000Vp for a climbing distance of 6mm.

6. Connection of various lightning protection devices

The installation sequence of lightning arrester devices must not be mistaken. The discharge tube must be at the forefront, followed by surge suppression inductors and varistors (or discharge tubes), and then semiconductor TVS thyristors or X-class capacitors and Y-class capacitors.

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