Response Time of Surge Protective Devices With Spark Gap Technology

Response Time of Surge Protective Devices With Spark Gap Technology

I. INTRODUCTION

Effective surge protection is vital to ensure reliable and uninterrupted operation of modern low-voltage power grids that incorporate sensitive electrical and electronic equipment . Surge protective devices (SPDs) are installed in AC power systems that integrate components of different materials and technologies, such as metal-oxide varistors, spark gaps, and gas discharge tubes. Actually, spark gap technology is commonly used in SPDs and systems in Europe, thanks to the capability of spark gaps to (i) handle surge currents of high energy content, (ii) exhibit zero leakage current, and (iii) effectively interrupt the power frequency follow current from the power grid after successful operation.

Although extensive work has been conducted on the transient behavior of spark gap-based SPDs with a clear focus on the interruption of the power frequency follow current , the response time of spark gaps is rarely investigated and discussed in the literature. Furthermore, UL1449  and forthcoming IEC61643-01 lack a definition of the response time of SPDs; an interesting analysis on the response time of SPDs has been provided in, covering however only varistor technology.

This study focuses on the transient response of SPDs that integrate spark gap technology. The response time of commercially available spark gap-based SPDs commonly installed in main switchboards of AC power systems (nominal voltage: 230/400V) is investigated by employing lightning and switching impulse voltages. Thus, the impulse behavior of SPDs is studied for a wide range of surge events that may be exposed to in the field in the case of fast-front and switching transients. Experimental results are analyzed, and the effect of the applied impulse voltage waveform is discussed.

II. EXPERIMENTAL ARRANGEMENT

The device under test (DUT) is a voltage switching surge protective device (SPD) employing triggered spark gaps between power lines and ground. It should be mentioned that the SPD adopts a connection type 1 configuration per IEC 61643-12 , which is applicable to three-phase (230/400 V) TN-C systems. Fig. 1 depicts the schematic diagram of the configuration of the surge protective components integrated into the commercially available DUT with a declared response time < 100 ns and protection level of 1500 V; these ratings are commonly claimed by manufacturers of 230V SPDs.

The response time of the SPD was determined by employing lightning and switching impulse voltages of 1.2/50μs and 250/2500μs waveforms, respectively; 5 hits of positive and 5 hits of negative voltage polarity, were applied to the DUT. Experiments were conducted at the High Voltage Laboratory of the Aristotle University of Thessaloniki, Greece. This experimental investigation is important when considering that the sparkover voltage-time curve of spark gap is statistical in nature (spread δU, δt in Fig. 2) and highly dependent on the steepness of the applied voltage (Fig. 2); this sparkover behavior is inherent in spark gap technology.

  

Fig. 3 shows the single-stage impulse voltage generator (140kV/ 245J), with interchangeable components, that was used to stress the line to earth protection mode of the SPD (L-PEN). The voltage at SPD terminals was monitored by a high voltage probe Tektronix P6015A and the discharge current was measured by using a Pearson 310 current  transformer both connected to  a Tektronix ΤDS 3064B digital oscilloscope (600 MHz, 10 GS/s). Both voltage and current data were acquired via connecting the digital oscilloscope to a personal computer  through a KUSB-488B adapter enabling live data monitoring.

III. RESULTS AND DISCUSSION

For the purpose of this work, the response time of spark gap-based surge protective devices (SPDs) is defined as the time duration that the overvoltage at SPD terminals attains values that threaten the safe operation of protected equipment. A schematic representation of the proposed definition is given in Fig. 4. It is suggested that the “clock” starts counting the response time at the time instant that the voltage exceeds a threshold value, that is, the temporary overvoltage peak of 630 V (√3 ∙ 255 ∙√2), that may originate due to LV-system faults per IEC 61643-11, up to the time instant that the overvoltage is mitigated to values lower than the half of the sparkover voltage peak.

A. Lightning Impulse Voltage Tests

Standard lightning impulse voltages up to 17kV have been used to study the transient response of SPDs against lightning-related overvoltages.  

Figs. 5a and 5b depict the open circuit impulse voltage of 6kV, 1.2/50 μs, and  the corresponding typical voltage record at the SPD terminals (L-PEN), respectively. The voltage at the SPD terminals increases up to the sparkover of the spark gap (~1450 V) and then, due to the sudden change of the SPD impedance, a discharge current flows resulting in a residual voltage of ~1000V associated with the relatively low current flow from the impulse voltage generator; the response time of the SPD is estimated as ~70ns, in line with the declared response time (< 100ns) by the manufacturer.

    

Fig. 6 shows the  experimentally  derived  variation  of the mean response time of the SPD as a function of the impulse voltage peak. It is evident that the response time of the SPD decreases with increasing impulse voltage peak. It must be noted that for impulse voltages with a peak lower than ~4000 V, the response time is longer than the value of 100ns, which is the upper limit declared by the SPD manufacturer. Thus, lightning impulse voltage tests with a peak lower than the standard value of 6000V, 1.2/50μs, as specified by IEC 61643-11 [15] and UL 1449, shall be integrated into updated standards to evaluate the response of SPDs realistically. Longer response times, although associated with lower sparkover voltages (Fig. 7), may impose a high risk of failure of sensitive equipment. The reason behind this is that the destructive effect on equipment can be attributed not only to the peak overvoltage but also to the duration of the overvoltage that affects the specific energy stressing the protected equipment . It is noteworthy that lightning impulse voltage tests with peak values higher than 6 kV, as suggested by IEC and UL , may result in sparkover voltages exceeding the declared protection level of the SPD.

B. Switching Impulse Voltage Tests

Standard switching impulse voltages up to 17kV have been used to study the transient response of SPDs against switching transients. Figs. 8a and 8b depict the open circuit impulse voltage of 6kV, 250/2500 μs, and the corresponding typical voltage and current record at the SPD (L-PEN), respectively. The voltage at the SPD terminals increases up to the sparkover of the spark gap followed by a sudden drop of the SPD impedance, which is associated with a response time of ~3000ns! The significantly longer response times than the commonly declared value of 100 ns (i) call for further investigations on the performance of the trigger circuit (component T in Fig. 1) of spark gaps under overvoltages with a low rate of rise and (ii) question the technical value of the declared response time by manufacturers in case of switching transients.

   

Fig.  9 shows  the  experimentally  derived  variation  of the mean response time of the SPD as a function of the switching impulse voltage peak. Obviously, there is a sharp decrease in the response time of  the SPD with increasing switching impulse voltage peak, but the response time is always at least an order higher than the declared response time of 100 ns. It must be noted  that  these  long  response  times  are  associated  with relatively low sparkover voltages that attain values in the range of ~850-1000 V (Fig. 10), which even though they are certainly below the declared protection level of 1500 V they may still be harmful to sensitive equipment in case of prolonged duration. These experimental findings on long response times, which are well beyond the response time declared by manufacturers, stress the need for a new category of the response time of SPDs against  switching  transients;  the  latter  may  occur  in  power systems and the response time of the SPDs against slowly rising overvoltages can be a critical protection parameter.

     

It should be mentioned that results of this work regarding the response time are not only related to the spark gap technology but can be also directly associated with other switching and combination type SPDs employing air gaps and gas discharge tubes.

IV. CONCLUSIONS

The response time of surge protective devices employing triggered spark gap technology has been experimentally investigated by employing lightning and switching impulse voltage tests. Experimental results have shown that the response time of spark gap-based surge protective devices declared by manufacturers, that is typically 100 ns, is dubious in the case of fast-front  transients  and is certainly exceeded in the case of switching transients. These findings stress the need for an update of forthcoming standards to include a strict definition of the response time of surge protective devices; this amendment will resolve  issues  associated  with manufacturers' subjective declaration of response time in the surge protection industry.