Zero threshold surge suppressor

Details Zero threshold surge suppressor Zero threshold surge suppressor

2000-12-29

2004-08-10

Jackson, Stephen W. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Load shunting by fault responsive means

C361S058000, C361S091100, C361S113000, C361S118000

Reexamination Certificate

active

06775117

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the suppression of transient energy to protect sensitive loads and more particularly to a zero threshold surge suppressor which provides a low impedance path for transient energy to flow and be dissipated.
2. Description of the Related Art
Protecting electrical and electronic equipment from ac power line disturbances is a growing concern. The industry trend has been a transformation of electrical systems from electromechanical to a sophisticated, electronic rich environment. Sensitive electronic equipment, such as programmable logic controllers, solid state motor controllers, variable frequency drives, robotics and microprocessor-based equipment have been added to boost productivity, save energy and carry out tasks more efficiently. It is clearly important to protect this proliferation of sensitive electronic equipment from the harmful effects of transients.
Voltage surge and transient suppressors are commonly employed between power sources and sensitive electrical circuitry to protect such circuitry from surges and transient spikes which can occur as a result of inductive load switching, capacitive load switching, lightening strikes or other transient events.
Utilities use capacitor banks to regulate system voltage levels as load profiles vary in an effort to minimize on-line generator capacity. As peak loads increase, additional capacitor banks become necessary for voltage support. Utilities are adding more capacitors to sub-transmission and distribution circuits to support voltage during high load periods and, in some cases, to provide power factor correction for the utility grid. Typically, utility capacitors are switched on in the morning as system load builds up and off in the evening as the load drops off.
Capacitor voltage cannot change instantaneously when system voltage is applied. As such, energizing a capacitor causes a collapse in system voltage followed by a rapid recovery and an oscillating transient. The actual magnitude of the capacitor switching at various points in the distribution system depends on several factors: 1) method of capacitor switching (i.e., oil switch, vacuum contactor, vacuum breaker, SF6 breaker), 2) presence of any transient limiting devices (i.e., inrush reactors, tuning reactors, pre-insertion resistors or inductors), 3) point in the voltage waveform at which the capacitor is first energized, 4) stiffness of the utility network (i.e., available short circuit current) and 5) presence of other capacitors on the network. With multiple capacitor banks in the system, switch-on spikes may exceed 200 percent and switch-off spikes are appreciable.
The magnitude of the transient measured at the point in the network where the capacitor is connected may be vastly different than the magnitude measured at a customer's site several miles away. Typically, the further away from the switched capacitor, the lower the magnitude of the transient as a result of the added system impedance. However, the presence of other capacitors on the network, either at low or medium voltage, may have a significant impact on the transient magnitude. Capacitors that do not employ de-tuning reactors will often magnify an otherwise benign capacitor switching transient to unacceptable levels. Utility capacitor switching transients are typically 1.3 to 1.4 per unit overvoltage range, but have been observed near the theoretical maximum of 2.0 per unit. However, if low voltage capacitors are present, transient overvoltages on the low voltage bus under some conditions may reach as high as 3.0 to 4.0 per unit with severe consequences for many types of equipment.
Sensitive loads, such as variable speed drives (i.e., 20 HP and less), commonly trip when the utility switches capacitor banks for power factor correction and/or voltage regulation. These transients can cause sensitive equipment to trip upon momentary overvoltage, resulting in loss of productivity and, in many cases, substantial losses due to scrap. The most common source of transients is utility switching of medium and high voltage capacitors for voltage regulation and power factor correction. Capacitor switching creates low frequency transients which cause sensitive loads to shut down on overvoltage fault. Prior art surge protection devices will typically limit a transient to 1.8 to 2.0 per unit overvoltage, but small drives will trip at 1.3 to 1.4 per unit overvoltage.
The traditional method of protecting variable speed drives or other sensitive loads from utility side transients has been the installation of line reactors in series with the sensitive load to increase line impedance and limit the transient at the drive terminals. Further information on series reactors may be found in U.S. Pat. No. 4,158,123, titled “Series Reactor”. However, in many cases, the line reactors will only lessen the transient problem and not eliminate it. Line reactors are intended to limit inrush current and attenuate harmonics, but may not always be sufficient to eliminate transient related drive problems. Each variable frequency drive must be equipped with an input reactor. Additional disadvantages with the use of a series reactor are: 1) sufficient space is required for the reactor to be located adjacent the electrical load, 2) substantial production downtime occurs when installing the reactor, 3) the reactor dissipates extra energy and 4) the reactor does not provide for protection of other sensitive loads on the same bus.
Transient voltage surge suppression systems help to reduce or eliminate harmful transients, surges and electrical line noise, thus preventing damage to sensitive electrical equipment. Many transient voltage surge suppression systems utilize multiple parallel metal oxide varistors (MOV's). As the voltage reaches the MOV's rated voltage level, the impedance of the MOV changes state, providing a low impedance path for the transient to follow. This allows the excess energy to be diverted away from the protected load.
MOV's are voltage clamping devices usually connected directly across a power line. An MOV does not clamp until the occurrence of a voltage transient exceeds the line voltage by a sufficient amount. As the voltage transient rises, the MOV's nonlinear impedance results in a current spike through the MOV that rises faster than the voltage across it. This produces the desired voltage clamping action. However, the clamping characteristic of a MOV is too high to protect sensitive loads from the 200 percent and higher voltage spikes generated by most utility switched capacitor banks.
Another disadvantage with the use of MOV's is that when subjected to a sustained overvoltage or a large transient exceeding its capacity, the MOV can go into a “thermal avalanche” or “thermal runaway” condition where the zinc oxide material of the MOV will break down and can initiate a short circuit condition.
Because MOV characteristics are unsuitable for protecting small drives, a suppressor with a lower voltage characteristic is necessary. The zero threshold surge suppressor of the present invention is designed to reduce the voltage spike below the overvoltage trip level of the adjustable-speed motor drives. The zero threshold surge suppressor is a capacitor based, phase to phase surge suppressor wherein the suppressed spike amplitude is dependent on the time constant of the zero threshold surge suppressor resistor-capacitor circuit.
The present invention provides for suppression of low frequency transients to acceptable levels as well as transients generated by transfer switch operations with the use of a passive diode bridge and a electrolytic capacitor bank to shunt transient energy away from sensitive equipment.
The advantages of the zero threshold surge suppressor over a series reactor are: 1) the zero threshold surge suppressor can be installed without production downtime, 2) the zero threshold surge suppressor can be located out of the production area, 3) the zero threshold surge suppressor is more efficient, 4

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