What is a Surge Generator?
Surge is a transient overvoltage in a power system and is produced in electrical/electronic circuits by the events such as load switching, capacitor bank switching, device faults, and lightning discharges. Usually, the duration of the surge is short, typically from several µs to ms range. The nature of surge waveforms can be oscillatory or, impulsive with the rising wavefront usually on the order of 0.5 μs to 10 μs.
The highest surges are produced by direct lightning strikes to overhead power lines. Surges that enter a facility typically range from 10 kV to 50 kV. Since this Impulse voltage can severely damage electrical/ electronic devices, it is therefore necessary to test devices to work against surges.
Figure 1: Standard surge voltage and current waveform as per IEC 60
Surge/impulse voltages are used to test the surge voltage immunity of the electrical/electronic devices. Also, surge/ impulse currents are required to test the equipment such as lightning arresters, fuses, and many other technical applications such as thermonuclear fusion and plasma devices. A surge generator is a device that is used to generate standard impulse voltage/current pulses to test the surge immunity of electrical/electronic products.
A surge generator is a combined surge current/voltage generator that generates the idle mode a standard surge voltage waveform with the pulse shape 1.2 / 50 µs and a surge current with the standard pulse shape 8 / 20 µs to test the surge immunity of a product in a controlled testing set-up. The EMC standard IEC/EN61000-4-5 is the surge immunity standard. The surge generators are designed to meet the IEC/EN61000-4-5 standard.
Figure 2: Surge Generator for EMC Testing
Usually, the surge immunity test is done with the help of CDN (Coupling decoupling networks). The CDNs couple a surge generator to the equipment under test (EUT) and prevent dangerous voltages from being sent back into the ac power system.
How the impulse/surge voltage waveform is generated?
The surge generators may use any impulse voltage waveform circuits (for example MARX circuit).
Figure 3: Basic impulse voltage generating circuit.
Figure 4: Impulse waveform
A basic impulse voltage waveform generator circuit is shown in figure 3. Here, C1 is the charging capacitance that has the voltage rating in KV; C2 is the discharging capacitance or load capacitance. The symbol S represents the spark gap. Initially, the C1 capacitor is charged by a DC voltage source until the charged voltage is enough to break down the spark gap. When a breakdown occurs in the spark gap, the spark gap acts as a closed switch; hence, C1 discharges through load capacitance (Figure4). It results in an impulse voltage waveform with desired nature as the output. The resistors R1 & R2 and capacitor C2 decide the waveform shape.
How is the surge current waveform is generated?
Figure 5: Surge current waveform circuit
The basic circuit of an impulse current generator is shown in the figure 5. The capacitor C consists of one or more capacitors that are connected in parallel (capacitor bank). The inductor L is an air-cored high current inductor; usually, a spiral tube of a few turns, and R is the dynamic resistance of the equipment under test (EUT). Initially, a DC source charges the capacitor bank to a specified value results in a breakdown that occurs across the spark gap. Now, the capacitor bank discharges and delivers the impulse current through the R-L elements.
Specifications details of surge generators:
Surge voltage pulse shape: Represents the standard surge voltage waveform 1.2 μs ± 30 % / 50 μs ± 20 %. Here, the time 1.2 μs represents the time to reach peak amplitude, and the time of 50 μs represents the time to reach 50% peak amplitude from peak amplitude.
Pulse voltage (Open circuit): Represents the peak amplitude of the voltage surge in KV that can be provided by the surge generator.
Surge current pulse shape: Represents the standard surge current waveform 8 μs ±20 % / 20 μs ± 20 % Here, the time 8 μs represents the time to reach peak amplitude, and time 20 μs represents the time to reach 50% peak amplitude from peak amplitude.
Pulse current (Short circuit): Represents the peak amplitude of the current surge in KA that can be provided by the surge generator.
Polarity of output voltage/output current: Represents the Polarity of output voltage/output current from the surge generator. The surge generator has the capability to generate voltage/current impulse in Positive/negative/alternating polarity.
Maximum stored energy: Represents the maximum/total energy storage of the surge generator’s capacitors. It is represented in joules.
Output impedance: Represents the output impedance of the surge generator in Ω.
Pulse repetition: Represents the number of pulses per second that can be provided by the surge generator. For example, a surge generator can provide 1- 999pulses/sec.
Time between the surges: Represents the time duration between the two impulses of a surge generator. It is represented in Seconds.
Charging time for maximum charging voltage: Represents the minimum time taken by the surge generator to attain its maximum charging voltage level for its operation. It is represented in Seconds.
Energy storage capacitor: Represents the capacitance value of energy storage capacitors of the surge generator. It is represented in µF.
Trigger mode: Represents the triggering mode of a surge generator. The surge generators are available with Auto/Manual mode triggering options.
Mains power: Represents the power supply rating. For example, 230V, 50Hz/60Hz.
Fuse Type: Represents the current rating of the fuse in A.
Dimensions: Represents the dimension of the surge generator in L X W X H mm3.
Weight: Represents the weight of the surge generator in Kg.
Operating temperature: Represents the safe operating temperature limit of the surge generator in °F (°C).
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