PROTECTION AND CONTROL

Relaying for Dual Supply Lines

The importance of a reliable supply of electricity for many manufacturing and heavy industrial process plants cannot be overemphasized. Momentary interruptions and even temporary voltage sags or spikes can have as detrimental an impact as long term interruptions. Such events can wreak havoc with manufacturing processes and the sensitive solid state equipment that is now routinely used to control them.

One solution to the problem is to install dual supply lines. However, depending upon plant location and distribution system configuration, a second supply line could cost millions of dollars. For some heavy power users, even this level of investment can often be justified. But ,what if a company were to make such an investment, only to discover that serious supply problems continue to plague them? Having two supply lines does not always improve the reliability of the power supply for important loads.

Such was the case at an industrial plant in Eastern Ontario.

The plant has a typical two line 115KV supply with two transformers, low side transformer breakers and a normally closed bus tie breaker. The low voltage busses operate at 4200 volts. The low side bus tie potentially offers an uninterrupted supply to the load, but it also introduces the possibility that a fault on one line may affect the second, and the prospect of backfeed through the tie if the transformer breaker fails to clear on a line fault.

Such installations are normally protected by directional over current relays, over/under voltage relays, pilot wire, remote (transfer) trip from the distribution utility, or some combination thereof. Directional over current relays are generally timed and therefore slow. Pilot wire is fast but prone to false operations. Unfortunately, both remote trip and pilot wire require either expensive relaying to enable wireless communication between the terminals, or land lines which can also be expensive. Furthermore, remote tripping is relatively slow, with a time to trip initiation in the range of 40 to 50 ms, and is prone to both false tripping and failure to trip.

The plant in question originally had timed directional over current protection, timed under/over voltage protection, and remote tripping from the local utility, but the remote tripping proved to be so prone to false operations that it was abandoned. Most, but not all, of the other load terminals on the two lines had transfer tripping.

Consequently, every time either of the two lines was affected by lightning or any other fault, the terminals with transfer tripping cleared and left the plant hanging on to the faulted line, until the directional relays finally timed out. During this time the plant would carry the remaining load left on the faulted line, through the low voltage bus tie. The voltage dip at the plant was always so severe that the process control would shut the plant down and / or the large compressor motors would fall out of synchronism and trip off.

In essence, the manufacturer had paid for a Cadillac two supply line system, and as a result had doubled its exposure to line faults because a failure on either supply line would shut down the manufacturing process.

The problem, then, was to find a cost effective local substitute for remote trip or pilot wire protection, one that would clear faults reliably and with sufficient speed to prevent the process control from being affected by the momentary voltage sags that accompanied every electrical fault on either of the supply lines. The solution was surprisingly simple.

Two sets of specialized directional over current relays were installed. Each set was supplied from both sets of transformer breaker current transformers, connected in a differential configuration. This arrangement permits much greater fault discrimination as it results in little or no relay current during normal conditions, but the relay current doubles when fault or load current backfeeds through the low tension bus.

One of the protections utilizes two directional overcurrent relays type KCEG from GEC Alsthom, each directioned for current flow out of one of the 115 KV lines. The setting for the relay is 1000 amps flowing back through the transformer breakers, and it operates on current in any phase. It is supervised by pallet switches from the other breaker and by bus under voltage, set at 3500 volts. Thus configured, the KCEG relays will operate for any line fault on their associated line, or on a backfeed to connected remote loads after a fault clearance at the supply station.

During the past 5 years, the KCEG had one false operation when it tripped both lines. The cause was traced to the delta voltages that were used to direction the relay. The directioning voltages were replaced by four wire star connected potentials to solve the problem.

The KCEG is a reliable, effective protective device, but it does not initiate tripping until about 60 milliseconds or almost 4 cycles after the fault occurs. It was considered advisable to try to reduce this time and thus the plantŐs exposure to the inevitable voltage sag, and to replace the ineffective remote tripping function that had previously been removed.

Accordingly, the author developed a prototype relay called the PCS, which incorporates a number of enhanced features to improve performance and reduce cost, as follows:

• The relay operates in 20 ms, about twice as fast as normal remote tripping and three times as fast as the KCEG.
• The relay sends a trip signal directly to the breaker, eliminating the cost and time delay inherent in auxiliary relays.
• A single relay provides protection for both 115KV lines with separate trip outputs for each transformer breaker.
• The relay is designed to ensure correct directioning for both trip outputs under all voltage collapse conditions.
• The relay is currently configured to trip only when there is reverse current in all three phases. It thus serves the specific purpose of identifying load backfeed conditions, as its function is to replace remote tripping. It should be noted that the relay can also be configured to operate on a single phase of reverse current.
• The pick up setting for the PCS is 500 amps per phase.

The KCEG relays clear slightly more faults than the custom relay, as would be expected because it is designed to operate for all line faults. See fault recorder records of fault cleared by the PCS relay. The fault was a lightning arrester failure right in the switch yard. The voltage dip lasted only 70 milliseconds.

This pilot project has proven conclusively that instantaneous directional over current relays can replace pilot wire relays and/or remote (transfer) tipping with their associated leased telephone circuits in a dual supply line configuration, at lower cost and with increased reliability. This relay configuration can also be used to connect two feeders from different sources to a common load bus for improved security, provided the two sources can be synchronized.