Tiempo de respuesta de los dispositivos de protección contra sobretensiones con tecnología Spark Gap

I. INTRODUCCIÓN

Una protección eficaz contra sobretensiones es vital para garantizar un funcionamiento fiable e ininterrumpido de las redes eléctricas de baja tensión modernas que incorporan equipos eléctricos y electrónicos sensibles. Los dispositivos de protección contra sobretensiones (SPD) se instalan en sistemas de alimentación de CA que integran componentes de diferentes materiales y tecnologías, como varistores de óxido metálico, descargadores de chispas y tubos de descarga de gas. De hecho, la tecnología de descargadores de chispas se utiliza habitualmente en los SPD y sistemas en Europa, gracias a la capacidad de los descargadores de chispas de (i) gestionar sobrecorrientes de alto contenido energético, (ii) presentar corriente de fuga cero e (iii) interrumpir eficazmente la corriente de seguimiento de la frecuencia de la red eléctrica después de un funcionamiento correcto.

Aunque se ha realizado un trabajo extenso sobre el comportamiento transitorio de los SPD basados ​​en descargadores de chispas con un enfoque claro en la interrupción de la corriente de seguimiento de la frecuencia de la red eléctrica, el tiempo de respuesta de los descargadores de chispas rara vez se investiga y analiza en la literatura. Además, UL1449 y la futura IEC61643-01 carecen de una definición del tiempo de respuesta de los SPD; se ha proporcionado un análisis interesante sobre el tiempo de respuesta de los SPD, que, sin embargo, solo cubre la tecnología de varistores.

Este estudio se centra en la respuesta transitoria de los SPD que integran tecnología de descarga de chispas. Se investiga el tiempo de respuesta de los SPD basados ​​en descarga de chispas disponibles comercialmente que se instalan comúnmente en los cuadros de distribución principales de los sistemas de energía de CA (voltaje nominal: 230/400 V) empleando voltajes de impulso de conmutación y de rayo. Por lo tanto, se estudia el comportamiento de impulso de los SPD para una amplia gama de eventos de sobretensión a los que pueden estar expuestos en el campo en el caso de transitorios de conmutación y de frente rápido. Se analizan los resultados experimentales y se discute el efecto de la forma de onda de voltaje de impulso aplicada.

II. DISPOSICIÓN EXPERIMENTAL

El dispositivo bajo prueba (DUT) es un dispositivo de protección contra sobretensiones transitorias (SPD) que emplea descargadores de chispas activados entre líneas eléctricas y tierra. Cabe mencionar que el SPD adopta una configuración de tipo de conexión 1 según IEC 61643-12, que es aplicable a sistemas TN-C trifásicos (230/400 V). La figura 1 muestra el diagrama esquemático de la configuración de los componentes de protección contra sobretensiones transitorias integrados en el DUT disponible comercialmente con un tiempo de respuesta declarado < 100 ns y un nivel de protección de 1500 V; estos valores nominales son comúnmente declarados por los fabricantes de SPD de 230 V.

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.