By Craig Price and Karen Leix
Power quality is more than just a buzz word in today's competitive utility marketplace. Customers, be they commercial, industrial or residential, demand, and have a right to, expect that the power provided to them will support their endeavors. Utilities are thus faced with an on-going problem of providing an increasing power load to customers for increasingly sensitive equipment. Voltage sags or blinks on electrical distribution systems can cause this sensitive equipment to malfunction, costing customers both time and money.
Georgia Power has a large manufacturing customer with a regional headquarters and pilot assembly line where they test new assembly line processes using computers and other sensitive equipment. They were accustomed to reliable service from Old Alabama Substation. As the area became more developed with the addition of a shopping mall and other commercial establishments, continuous construction was going on, causing dig-ins on underground feeders. Expulsion fuse operations due to the faults on the underground feeders caused breaker operations on feeders parallel to the manufacturing customer. This resulted in voltage dips on the feeder going to the customer.
As part of their underground development, Georgia Power had installed livefront pad-mounted switchgear with expulsion-type power fuses. The industrial customer was becoming critical of the quality of their power, and its possible effect on their susceptible equipment. They were seeing voltage dips from faults not associated with trouble within their own facility.
Expulsion fuses are "zero-awaiting" devices. Before the system is faulted, the expulsion fuse acts as part of the line. When the line is faulted, the element heats to the melting point, then breaks apart at the fusible portion's hottest point. The current continues to flow through the particles of vaporized element and ionized gases. Heat from the arc will burn back the remaining element and release large quantities of gas from the surrounding tube wall. When the alternating waveform of current reaches its zero point, the arc is momentarily extinguished. After passing through zero, the arc may re-establish or re-ignite. This is because the system's recovery voltage rises across the severed ends of the link faster than the dielectric strength growing between the melting fuse ends.
Eventually there is final extinction and removal of the fault from the system through the now electrically open (blown) fuse. All expulsion fuses require at least one-half cycle before they can clear a fault, and in many cases take several cycles to clear. During this clearing process, the voltage at the fuse drops to zero.
On underground systems the impedance of the fault is very low, usually around one ohm. Power at the substation flows to the fault, and this causes the substation bus voltage to collapse. As a result, the other feeders on the same substation bus also experience a dip in voltage.
The duration of voltage dips due to the operation of expulsion fuses is a growing concern for utilities. When a fault occurs at a transformer, an expulsion fuse can isolate the fault and de-energize the transformer. However, because it must take a minimum one-half cycle to clear, a voltage dip at the transformer and on all parallel feeders can last 8-12 ms or longer. This is sufficient time to have a negative effect on motor contactors and electro-mechanical relays, high intensity discharge lamps, adjustable speed motor drives and programmable logic controllers.
Current-limiting fuses typically interrupt current levels up to 50,000 A RMS symmetrical and, in the process, limit peak current magnitude when operating in current-limiting mode. In contrast to expulsion fuses, a current-limiting fuse will reduce the fault duration of these normally high underground faults, and will support system voltage during much of the clearing process.
Typically, a current-limiting fuse has a silver ribbon element, surrounded by fine granular silica sand housed in a fiberglass tube. Before the system is faulted, the fuse acts as part of the line. For low fault currents, the fuse TCC is similar to an expulsion fuse. But when these typically high fault currents flow on underground circuits, the silver ribbon element senses the fault current, and quickly heats and vaporizes along much of its entire length. The high temperature of the resulting arc melts the sand around the arc, forming a glass-like structure called fulgurite. The fulgurite restricts the arc, causing the resistance of the fuse to increase dramatically. This added resistance to the circuit changes the circuit power factor to near unity, shifting the time at which current crosses zero to be at the same time as system voltage zero. And this allows the current-limiting fuse to clear the fault before the natural pre-fault current zero, and typically within a half cycle.
Before the fuse melts, the voltage drops to zero for a short period of time (typically 1-4 ms), not long enough to negatively affect equipment connected to parallel feeders. During the fault clearing process, the current-limiting fuse and the system combine to produce an arc voltage which supports the system voltage on all parallel feeders.
At a recent power quality workshop conducted by Georgia Power -- and attended by the customer experiencing problems -- the topic of current-limiting fuses and how they could improve power quality and help prevent voltage dips was discussed. Meanwhile, Georgia Power was discussing current-limiting fuses during an overcurrent protection workshop conducted by Cooper Power Systems. The manufacturing customer originally proposed that Georgia Power designate a separate substation transformer bank for them, at a cost of over one million dollars. Needless to say, Georgia Power was very anxious to find a current-limiting fuse that could be applied in their existing pad-mounted switchgear, as a much more cost effective solution to accomplish the same goal.
Cooper Power Systems worked with Georgia Power to modify Cooper's X-Limiter fuse to fit the S & C SML-4Z mount. Georgia Power provided a sample of the fuse, as well as a video of the inside of the switching cubicle, showing clearances in the switchgear and removal and installation of the fuse. Specifications were developed including voltage, current and time-current characteristics. When Georgia Power tested mechanical samples, only minor changes were required.
This solution is ideal because no modifications are necessary to install these fuses in the switching cubicle, and the cubicle can be left energized at all times. Because of the interchangeability of these fuses with expulsion fuses, Georgia Power can still use expulsion fuses in the future if they choose.
From a cost standpoint, these fuses are competitive, and require no alterations to the cubicle to install. Georgia Power expanded their direct buried cables instead of encasing them in concrete. Now, if cables are accidentally cut by a dig-in, the chances of it affecting service to critical customers has been diminished. Most importantly, Georgia Power improved the quality of service to their customer and saved at least $900,000 from the original request by the customer for a dedicated substation transformer bank for their service.
With calculated available fault currents at the substation bus of over 5000 A with short lines, it is only a matter of time before the X-Limiter fuses at Georgia Power are called on for high fault current duty. Since the installation of these fuses, there have been low current faults, all of which were cleared by the fuses, with no complaint calls from Georgia Power's critical customers. ET