SPD conectados en paralelo y SPD/filtros conectados en serie

Al diseñar e instalar sistemas de distribución de energía, los ingenieros y contratistas deben seleccionar dispositivos de protección contra sobretensiones (SPD) para proteger los equipos y sistemas eléctricos de sobretensiones transitorias. Para ello, los especificadores deben comprender las diferencias entre los SPD conectados en paralelo y en serie, y dónde aplicar mejor cada tipo de dispositivo. La siguiente descripción resume el diseño y la función de los SPD conectados en paralelo y en serie, y describe algunas aplicaciones comunes para cada uno.

DESCRIPCIÓN GENERAL

La aplicación adecuada incluye saber qué tipo de dispositivo se está instalando y cómo se conecta el SPD al sistema de distribución de energía. En general, un dispositivo conectado en paralelo se instala en un tablero de distribución o panel eléctrico, generalmente a un interruptor de servicio dedicado, como un disyuntor. Alternativamente, los filtros independientes conectados en serie se conectan en línea cerca del equipo de carga protegida. La Figura 1 y la Figura 2 muestran cada tipo.

 

 

DISPOSITIVOS CONECTADOS EN PARALELO

Los SPD conectados en paralelo suelen utilizar varistores de óxido metálico (MOV) para desviar las sobretensiones transitorias de las cargas. Estos componentes sensibles a la tensión conducen solo cuando la sobretensión supera el nivel de sujeción del varistor. Como se muestra en las figuras 3 y 4, estos SPD comienzan a desviar el exceso de tensión cuando la tensión supera el nivel de sujeción. La tensión remanente que se encuentra por debajo de los niveles de sujeción, pero por encima de la onda sinusoidal, se conoce como tensión de paso.

 

En los SPD conectados en paralelo, los MOV se conectan a través de cada modo del sistema eléctrico. Las longitudes de las vías de sobretensión resultantes dependen de las longitudes de los conductores instalados para conectar la unidad a su circuito. La longitud de los cables tiene un efecto directo en el rendimiento del dispositivo, ya que una mayor longitud de los cables equivale a mayores voltajes de paso. En consecuencia, los SPD conectados en paralelo deben montarse lo más cerca posible de los tableros de distribución que protegen. La Figura 5 muestra las vías de sobretensión a través de un dispositivo conectado en paralelo.

 

Los SPD conectados en paralelo son capaces de bloquear transitorios de baja frecuencia y alta energía. Entre sus ventajas se incluyen un menor costo y un tamaño menor que los dispositivos conectados en serie, y procedimientos de instalación que no requieren la interrupción de la alimentación de las cargas. Entre sus desventajas se incluyen las diferencias de rendimiento según la longitud de los cables instalados, las tensiones de paso instantáneas que varían según el ángulo de fase y la falta de atenuación del ruido de alta frecuencia.

DISPOSITIVOS CONECTADOS EN SERIE

Series-connected devices fall into two broad categories – series-connected SPDs and series-connected filters. Typically, series-connected SPD designs are wired in series, but internal suppression components are connected in parallel to the circuit. The benefit of this configuration is that lead length is not added during installation, allowing the device to clamp impulses at tighter levels than a parallel-connect unit. The ampere ratings of series-connected units are limited by their lug or terminal block characteristics.

Series-connected filters are designed to protect sensitive equipment, such as computers and digitally controlled equipment, from high-frequency noise that could disrupt reliable operation. Manufacturers offer these products to provide protection from high-frequency, low-energy transient overvoltages as well as noise filtering. As shown in Figure 6, these devices mitigate voltage whenever transient voltages exceed the instantaneous nominal voltage by a specified amount, regardless of phase angle.

Series-connected filters use a low-pass circuit (consisting of series-connected inductors, capacitors, and resistors) to eliminate high-frequency noise. Because the entire load current passes through the components, the device must be designed to pass all of the current carried by the circuit it serves. As a result, series-connected filters are typically larger and more costly than parallel-connected devices.

Series-connected filters employ many different technologies. Figure 7 below shows a typical hybrid design with MOVs acting as a first line of defense, clamping the initial overvoltage event. The inductors and capacitors then mitigate the remaining let through voltage, providing a filtered sign wave to the connected equipment.

 

Figure 7: Phase-to-Ground, Phase-to-Neutral, and Neutral-to-Ground pathways in a series-connected filter

Benefits of series-connected devices include mitigation of potentially damaging high-frequency noise, a tighter clamping voltage, and performance that is independent of installation practices.

DEVICE SELECTION

During the design process, specifiers must decide where to use parallel-connected SPDs and where to use series-connected filters. The appropriate selection could depend on whether protection is needed against damage to equipment, disruption of operations, or both. It also depends on the location in the power distribution where the device will be installed.

Differing types of transients are likely to occur at different locations within a power distribution system. Studies show that 20-30 percent of transient overvoltages originate externally, from environmental and utility sources, and are more likely to consist of high-energy, low-frequency impulses. That means 70-80 percent originate from equipment and operations inside of a facility, and are more likely to consist of low-energy, high-frequency noise, such as ring waves. If destructive high-energy transients pose the greatest risks, then a parallel-connected device is likely to be the most cost effective solution. Consequently, parallel-connected devices are most often used at or near a building’s service entrance and on panels and equipment that manage large amounts of load. If sensitive equipment could be disrupted by highfrequency noise, then a series-connected filter may be the best choice. Where multiple types of transients may occur or where the characteristics of any power disturbance are unknown, then a staged strategy involving both parallel-connected and series-connected devices should be used. Effective protection is often provided when parallel-connected SPDs are installed at service entrances and distribution panels, and series-connected devices are installed near load equipment.

SUMMARY

Parallel-connected SPDs typically use voltage-sensitive MOVs that conduct current only when line voltage exceeds their maximum continuous operating voltage. Parallel-connected SPDs are capable of clamping high-energy, low-frequency transients.

Series-connected filters provide protection from low-energy transient overvoltages as well as noise filtering. These devices provide consistent clamping of let-through voltages regardless of phase angle, and are usually installed close to critical loads.

Cascading parallel-connected devices at service entrances and power distribution panels, coupled with series-connected devices at both load equipment and internal sources of transients and noise, can provide comprehensive protection from a range of potentially damaging and disruptive surge events.