Lightning Surge Test System Components and Operation

1. Introduction

To realistically simulate voltage or current surges caused by lightning strikes or switching transients that equipment may encounter in operation and to evaluate the immunity level of electrical and electronic equipment to lightning surges, standards require the equipment under test (EUT) to be energized (DC or AC supply) and subjected to surge immunity tests using a combined wave generator that produces a 1.2/50us open-circuit voltage and an 8/20us short-circuit current. This verifies the interference immunity of the equipment in practical operation. This article describes the main components and operating principles of lightning surge test equipment based on laboratory practice and presents a test platform and analysis of different coupling methods.

2. Surge Waveform Definition

Standard GB 17626.5-2019 provides a more practicable waveform and parameter definition for testing. Examples are shown in Figures 1 and 2.

Figure 1 Open-circuit voltage waveform at the generator output without CDN (1.2/50 us)

Figure 2 Short-circuit current waveform at the generator output without CDN (8/20 us)

The short-circuit current waveform parameters are also defined by the front time and the time to half-value, consistent with the open-circuit voltage definition. The standard defines:

• Front time: Tf = 1.25 × Tr = 8 × (1 ± 20%) us. The factor 1.25 is the reciprocal of the difference between the 0.9 and 0.1 thresholds.
• Duration: Td = 1.18 × Tw = 20 × (1 ± 20%) us. The factor 1.18 is empirical.

3. Surge (Impulse) Test System

The surge test system mainly consists of a surge signal generator and coupling/decoupling networks. To generate combined waveforms that meet the standard definition, GB/T 17626.5-2019 specifies the circuit topology and performance requirements for the combined wave generator.

3.1 Surge Signal Generator

The combined wave generator is the key component of the surge test system. Its primary function is to generate the surge waveform. GB/T 17626.5-2019 provides the circuit diagrams for combined wave generators. Figure 5 shows the schematic for a 1.2/50us-8/20us waveform generator.

Figure 5 Circuit diagram of the combined wave generator (1.2/50 us - 8/20 us)

As shown in Figure 5, the 1.2/50us-8/20us generator circuit is composed of two parts: a charging circuit and a pulse-shaping circuit. The charging circuit consists of the high-voltage source U, charging resistor Rc, and storage capacitor Cc. The pulse-shaping circuit comprises pulse-duration adjustment resistors RS1 and RS2, the impedance-matching resistor Rm, and the inductance Lr that shapes the rise time.

Working principle: with the switch open, the charging circuit charges the storage capacitor Cc via Rc and Cc. After Cc is charged to the required voltage, the main switch closes to form the discharge loop. At the initial moment after closing, Cc discharges into Rm, Lr, and RS2. The measured surge voltage across RS2 is in its rising phase. During this interval, Lr stores energy in the pulse circuit. As the energy in Cc decreases, both Lr and Cc discharge through RS1, Rm, and RS2, causing the measured surge voltage across RS2 to fall from its peak. Rm ensures the generator internal impedance is 2 ohm, producing the required 1.2/50us-8/20us combined surge waveform.

Figure 6 shows the schematic for a 10/700us-5/320us waveform generator.

Figure 6 Circuit diagram of the combined wave generator (10/700 us - 5/320 us)

The 10/700us-5/320us generator similarly separates into charging and pulse-shaping circuits. The charging circuit uses the high-voltage source U, charging resistor Rc, and storage capacitor Cc. The pulse-shaping circuit uses pulse-duration adjustment resistor RS, Rm1, Rm2, and the capacitance Cs that adjusts the rise time. When switch S1 is closed, external matching impedance is used.

The charging circuit operation is the same as for the 1.2/50us-8/20us generator. The pulse-shaping differs because the 10/700us waveform has much longer rise and duration times, so a capacitor Cs replaces the inductance Lr. The built-in impedance Rm1 is set to 15 ohm and Rm2 is 25 ohm as required by the standard, giving an internal impedance of 40 ohm. When switch S1 is closed, Rm2 is shorted out so an external matching resistor can be used. This yields the required 10/700us-5/320us surge waveform.

3.2 Coupling/Decoupling Networks

Coupling/decoupling networks can be divided into coupling network and decoupling network by function. The coupling network transfers the combined wave generator output to the EUT, limits return currents from the EUT into the generator to protect the generator and to reduce waveform distortion. The decoupling network provides sufficient decoupling impedance to prevent surge energy from entering the mains and affecting other equipment supplied from the same power source. Without a decoupling network, surge protective devices in other connected equipment can prevent the application of surge to the EUT and affect test results.

Coupling/decoupling networks are categorized by application: power-line coupling/decoupling networks and interconnection-line coupling/decoupling networks. Power-line networks include single-phase AC or DC and three-phase AC networks. Interconnection-line networks include unshielded asymmetric and unshielded symmetric networks.

Surge coupling to the EUT is typically via capacitive coupling or gas discharge tube coupling. Gas discharge coupling noticeably affects the generator output waveform, so capacitive coupling is more commonly used. Smaller coupling capacitance results in lower residual voltage on the supply side but lower efficiency in producing the impulse current; larger capacitance increases coupling efficiency but also increases residual voltage. To balance output efficiency and residual voltage, the standard specifies 18uF for line-to-line coupling (differential mode) and 9uF for line-to-ground coupling (common mode). Power-line coupling/decoupling network design must meet the waveform parameter requirements at the network ports. The decoupling network provides a relatively high impedance to the surge while not affecting the normal supply to the EUT. Coupling components are high-voltage capacitors whose role is opposite to the decoupling network: they must allow the surge waveform to pass intact. Interconnection-line coupling/decoupling networks must also meet the waveform parameter requirements; their coupling elements may be capacitors, clamps, or surge arresters.

Select the appropriate coupling/decoupling network according to the product under test. See Figure 7 for selection guidance.

Figure 7 Selection of coupling/decoupling network

GB/T 17626.5-2019 provides topology diagrams for coupling/decoupling networks. Figures 8 through 11 show single-phase/DC line-to-line coupling, single-phase/DC line-to-ground coupling, three-phase line-to-line coupling, and three-phase line-to-ground coupling, respectively.

Figure 8 Line-to-line coupling for AC/DC lines

Figure 9 Line-to-ground coupling for AC/DC lines

Figure 10 Line-to-line coupling for three-phase AC

Figure 11 Line-to-ground coupling for three-phase AC

From Figures 8 to 11, the decoupling network topology is consistent for single- and three-phase power: an LC low-pass filter formed by decoupling capacitors C between lines and decoupling inductances L on each line.

Decoupling inductance L should not be excessively large, otherwise it will cause a significant voltage drop across the coupling/decoupling network and result in large physical size, complicating manufacturing and installation. To meet the standard requirement that the voltage drop across the coupling/decoupling network under rated current does not exceed 10% of the network input voltage, for EUTs with per-phase rated current ≤ 200 A, L should be ≤ 1.5 mH. For rated current > 200 A, the L value should follow the curve shown in Figure 12.

Figure 12 Reactance value of decoupling line in coupling/decoupling network for rated current > 200 A

For coupling networks, because the simulated real-world conditions differ for line-to-line and line-to-ground coupling, the design differs. Given that the low-voltage power network source impedance to earth is 12 ohm, for a combined wave generator with a virtual impedance (defined as the ratio of the open-circuit voltage peak to the short-circuit current peak) of 2 ohm, an additional series resistor of 10 ohm is required for line-to-ground coupling to increase the effective source impedance.

All power-line coupling/decoupling networks apply only to the 1.2/50us-8/20us surge waveform.

Figures 13 to 15 show coupling/decoupling networks for unshielded asymmetric interconnects, unshielded symmetric interconnects, and unshielded outdoor symmetric communication lines, respectively.

Figure 13 Coupling/decoupling network for unshielded asymmetric interconnects

Figure 14 Coupling/decoupling network for unshielded symmetric interconnects

Figure 15 Coupling/decoupling network for unshielded outdoor symmetric communication lines

GB/T 17626.5-2019 specifies the following:

1. For unshielded asymmetric interconnect coupling/decoupling networks (Figure 13), R = 40 ohm, and CD is a 0.5 uF capacitor with a gas discharge tube.
2. When using a 1.2/50us combined wave generator, for unshielded symmetric interconnect coupling/decoupling networks (Figure 14), Rc = n × 40 ohm, where n is the number of conductors in the interconnect. CD may be a capacitor, gas discharge tube, clamp, avalanche device, or any element that allows the EUT to transmit data normally while meeting the specified surge waveform parameters.
3. When using a 10/700us combined wave generator, for unshielded outdoor symmetric communication line coupling/decoupling networks (Figure 15), Rc = 25 ohm and CD is a 0.5 uF capacitor with a gas discharge tube.

4. Test Configuration and Test Procedure Requirements

4.1 Test Configuration

The standard gives explicit requirements for items to note, test arrangements, and procedures for surge testing. The surge test system configuration typically includes the following equipment:

• Equipment under test (EUT)
• Supporting equipment, if required
• Cables of specified types and lengths
• Coupling/decoupling network
• Combined wave generator
• Reference ground plane

Figures 16 to 18 show typical surge test configurations for power ports, unshielded interconnect ports, and shielded interconnects, respectively.

Figure 16 Surge test configuration for power ports

Figure 17 Surge test configuration for unshielded interconnect ports

Figure 18 Surge test configuration for shielded interconnects

4.2 Test Procedure Requirements

After arranging the test according to the applicable product standard, product family standard, or the generic standard, perform the test setup and configuration in accordance with the following requirements.

4.2.1 Confirm the test level according to the applicable standard, as shown in Figure 19.

Figure 19 Test levels

4.2.2 Number of pulses

Unless otherwise specified in the relevant product standard, for each coupling path apply five positive and five negative polarity surge pulses to DC power ports and interconnect lines. For AC power ports, apply five positive and five negative polarity surge pulses at each phase angle of 0°, 90°, 180°, and 270°.

4.2.3 Pulse interval

The interval between consecutive surge pulses must not exceed 1 minute. Intervals shorter than 1 minute increase test severity because rapid successive charging may cause cumulative voltages that exceed the intended test level.

4.2.4 Typical operating condition

The EUT shall be tested in a typical operating state to simulate real-world conditions.

4.2.5 Ports to which surges are applied

All input, output, and signal ports of the EUT that are within the scope of the applicable product standard shall be tested. In some cases with multiple identical lines, a representative subset of lines may be selected. Surge testing of output ports is recommended only for output ports through which a surge could enter the EUT (for example, switching of a high-power load).

For common-mode tests, each line shall be tested in turn unless the product family standard specifies otherwise.

Because the EUT may contain surge protective devices such as varistors or gas discharge tubes, or because the EUT itself exhibits nonlinear voltage-current characteristics, the test levels should be applied incrementally from low to high until the required level is reached. The EUT shall meet the pass criteria specified in the test plan or standard at all applied levels.

5. Surge Calibration Parameters and Calibration Configuration

Surge test equipment is typically calibrated periodically (commonly annually) by a third-party calibration laboratory, which issues a calibration report. The equipment may be used normally as long as the parameters in the calibration report comply with the standard requirements.

5.1 Combined Wave Generator Calibration Parameters

GB/T 17626.5-2019 specifies requirements for the generator output parameters. See Table 1 and Table 2.

Table 1 Tolerance ranges for open-circuit voltage and short-circuit current amplitudes at the combined wave generator output

Table 2 Tolerance ranges for the time parameters of the surge waveform at the combined wave generator output