At one time Greenville Electric Utility System, Greenville, Texas, U.S., had both a primary and backup transmission relaying scheme installed on all its 69-kV lines. This set up required a pair of copper conductors to be installed between the substations to ensure instantaneous tripping for all phase and ground faults. Since the operating experience with this scheme was not satisfactory, the relays were disabled, forcing us to rely solely on electromechanical directional overcurrent relays to clear all faults. To coordinate the relays, a short time delay was necessary, resulting in a voltage sag until the backup relays cleared the fault. This voltage sag caused all digital devices plugged into the system to reset every time a fault occurred. In 1992, I began a study to find a cost-effective relay scheme that would eliminate such inconveniences to our customers.

The first option investigated was installing phase and ground stepped distance relays at all substations. The Zone 1 elements would be set to reach 90% of the protected line length, which would ensure instantaneous tripping on the majority of faults, but not all. Another idea was to use a Zone 1 extension scheme, which included a Zone 1 element that overreached the next substation on the initial fault but pulled back the Zone 1 reach to 90% coverage on the subsequent reclose. Under these circumstances a substation could be de-energized while the breakers reclosed. Both ideas lacked the coverage we needed. The only way to ensure instantaneous tripping for all faults was to install a communications channel between the substations. Looking for the most cost-effective means to establish this channel, I compared several possibilities and their cost estimates:

If fiber optics were used, we would have to install about 193,000 ft (58,826 m) of cable, costing about US$0.50 per ft, and 16 fiber optic relays, costing about US$12,000 each. Total cost was estimated at US$288,500.

If leased phone lines were used, we would need eight lines, which would cost US$1400 for installation and US$15,500 per year.

If we used conventional microwave in the gigahertz frequency range, the cost would be US$280,000 for eight lines at US$35,000 per line. If we used copper conductors, the cost would be US$34,700 for the materials plus installation. This option was the least expensive for hardware, but based on our previous experience with the pilot wire relays, I thought we probably would not get the high quality service we wanted.

If we used 960 MHz radio, we would spend US$10,000 per line section for a total cost of US$80,000. This option looked like a good possibility.

In conjunction with the study for the best communications choice, I also wanted to find the best relays for the project. In a comparison of nine relays, Schweitzer Engineering Laboratories, Inc., Pullman, Washington, U.S., offered the best price on its directional overcurrent relay and its single zone distance relay. We decided to use the Schweitzer directional overcurrent relay with the 960 MHz radios, with the assumption that tone equipment would be required to interface between the radios and the relays.

The Idea While conducting the protective relaying study, we were also installing a new SCADA system, which would use a 960 MHz Point-to-Multi-point Multiple Address System (MAS) for remote terminal unit (RTU) communications. During one of our discussions with the supplier of the MAS, Microwave Data Systems (MDS) of Rochester, New York, U.S., I mentioned the relaying project. I asked whether a contact closure at one end of a radio link would allow the radio to transmit that contact closure to the other end and close a contact on the radio at the remote end. The MDS representative said the radios have an option called the "ear and mouth" (E&M) card, which generates a tone of 3825 Hz on the baseband of the radio. The MDS rep did not know the speed of the E&M cards. I reasoned that if the cards could be used in lieu of traditional tone equipment, I could save at least US$6000 per line section and greatly simplify the relaying circuits. To get an answer, I asked the factory to set up two radios and conduct a test. The factory reported that all of the tests demonstrated a speed of between 17 and 19 mseconds. They guaranteed a speed of at least 20 mseconds, which was a little slower than the 14 mseconds speed of traditional tone equipment. I asked myself if an increase in speed of 6 mseconds would be worth an additional US$48,000 for tone equipment. Later, MDS modified the card to increase its speed for my application to 10 mseconds, which put it in the range of the tone-equipment speed for which I was originally shooting.

Before deciding whether to use the cards at the slower speed, I needed to know how to interface the cards into the protective relays. Our substation batteries are rated at 48 and 125 V, but the cards, which have a maximum rating of 30 V, could not operate at these voltages. I could have installed auxiliary relays, which would have slowed the operating time of the relay scheme, but I decided that there might be another solution. I called Schweitzer Labs and was told that it could make the relays for 12 V inputs. Where could I get the 12 V dc supply? I considered a dc to dc converter to use the batteries in the substations, or an extra station battery, but these solutions seemed to complicate the problem. I called MDS and explained the problem. MDS said the radios I was considering an output of 12 V on their back ends. The design was starting to materialize.

Trip and Guard Two aspects of reliability, relating to dependability and security, must be considered when designing a protective relaying scheme. Dependability is defined in terms of the ability of the scheme to operate when a fault occurs. Security is defined in terms of the scheme not operating when there is no fault. Dependability and security operate in a mutually exclusive way, since a decrease in dependability will increase security.

Traditional tone equipment has a double stage for processing a trip signal. Before tone equipment is allowed to send a trip signal, a loss of guard signal must immediately be followed by the reception of the tripping signal. Could the E&M cards be used in this application for what is basically an on/off tone generation, which is considered to be less secure than the trip and guard signals required for tone equipment? The E&M cards might misoperate as a result of spurious radio signals from outside sources. Was there a way to protect against this possibility? There was. By using the Permissive Overreaching Transfer Trip (POTT) scheme, I could take advantage of its highly secure method of transmission relaying to offset the possible insecurity problem posed by the on/off tone of the E&M card.

For the breaker to trip at a substation, the protective relay must first detect a fault on the line and also receive a transfer trip from the remote end. Therefore, two actions must take place before a trip signal is sent to the breaker. If spurious signals were to key the E&M card in the absence of a fault, no breaker would be tripped. Because the POTT scheme is highly secure, it is less dependable, since the loss of the communications channel will prevent it from operating. However, the Schweitzer relay has directional overcurrent phase and ground features should the POTT scheme fail to operate, which overcomes this potential problem.

The wiring diagram (Fig. 1) shows the simplicity of the design; only three wires are required to interface to the radio. The 12 V power supply on the back of the radio is wired into the "E" contact of the E&M card. The wiring continues over to the Permissive Trip and Direct Trip inputs of the relay and then returns to the negative terminal on the radio. The "E" contact closes only when a transfer trip is received from the remote end. The "M" lead is wired to the A1 auxiliary contact on the relay. The A1 contact is masked as Zone 2 and will close if the relay senses a phase or ground fault in the forward direction or if the breaker is open. The direct trip input, which does not trip the breaker, was wired into the scheme to output to the A3 contact when a transfer trip is received from the remote end. The A3 contact is wired into our SCADA system and is used for testing of the communication path by transmitting a tripping signal, which can be verified as having been reported on the SCADA by the Operations Center. Although the Schweitzer relay is capable of reclosing, we left the electromechanical reclosing relays in service with the intention of replacing them with solid state relays.

Additional features of the installation included the masking of the A2 contact to close when a tripping signal is being sent to the breaker. The relays were not mounted in the breaker control panel because we were hesitant to drill into an energized panel. Instead, the relays were installed in the old pilot wire cabinets, which were large and had ample space. We intend to install a latching relay with a light emitting diode on the breaker control panel to indicate when the relay has tripped the substation breaker. An on/off switch was wired into the tripping circuit to interrupt the circuit during testing, which will avoid accidental breaker tripping during the procedure. In addition to the Zone 2 overcurrents being used for the POTT scheme, Zone 1 overcurrent settings trip the breaker instantaneously and the Zone 3 overcurrent settings trip the breaker after a 60 cycle time delay.

Equipment Layout The equipment has a compact design (Fig. 2). The solid state relay, the 960 MHz radio and the radio duplexer are mounted in a stacked configuration. The relay, radio and duplexer occupy 21 inches (53.34 cm) of vertical space in the 19-inch (48.26-cm) rack. Full relay protection for three transmission lines can be installed in one 6 ft (1.8 m) tall rack. The radio antennas are mounted on 55-ft (16.7 m), class 3, wood poles, which are installed in 10-ft (3 m) holes and backfilled with foam. The top of the antennas are mounted 40 ft (12 m) from the ground, which allows 5 ft (1.5 m) for lightning protection (Fig. 3).

FCC Licensing Before we could proceed with the relaying scheme, we had to apply for FCC licenses for the eight paths. Microwave Planning, Dallas, Texas, performed the required frequency coordination and provided all of the information I needed to fill out the FCC forms. I sent the forms on March 2, 1993. The FCC contacted me in June and expressed some reservations about the applications. Their first concern was addressed by sending them a description of the antennas to be used with the radios. Their second concern was related to the antenna pole for the LTV/E-Systems substation, which they felt would interfere with the glide slope of Majors Field on the south side of town. Since we had already had deadend structures inside the substation that were higher than the proposed antenna poles, I sent photos of the structures with a note stating that the new pole would not exceed the height of the existing deadends. We received the licenses on July 23, 1993.

This relaying scheme was first installed on the line between our Steam Plant Substation and Industrial Park Substation on May 23, 1994. A thunderstorm moved into the territory on June 15, 1994 and lightning struck the line twice that evening with the relay scheme operating as planned (Fig. 4). The lightning strike caused Phase C to ground fault with about 5200A, as seen by the Steam Plant relay. The Phase C voltage dropped to 18.1 kV, phase to ground.

About 1/2 cycle after the initiation of the fault, the Steam Plant relay sensed the fault and transmitted a transfer trip to the remote end by closing the A1 contact on the back of the relay. About 1-3/4 cycles after the initiation of the transfer trip, a permissive trip signal arrived from the remote end. Once the permissive trip was received, a tripping signal was sent to the two ring bus breakers. It took about 2 1/4 cycles for the relay to detect the fault and to receive a trip from the remote end. Once the tripping signal was sent, the remaining time was a function of the mechanical operation of the breakers and the time to extinguish the arcing on their contacts, an elapsed time of about 3-3/4 cycles for a total fault duration of 6.5 cycles, as shown by the event record on the relay. The record also showed the fault to be 2.65 miles (4.3 km) from the substation.

Our 42 MW Steam Plant Unit No. 3, which remained on line during both faults, would have been knocked off line using the old backup scheme. Our crews patrolled the line the next day and did not find any physical damage to the shield wire or the insulators. Typically, a slow clearing time on the line almost always required repairs. In addition, no customers called in to report that any digital devices reset during the fault.

Cost The 16 relays were purchased for US$45,888. The 16 radios, the antennas and associated cabling were purchased for US$75,291. The cost per protected line was US$15,148. The conclusion of the project demonstrated that service improvements for our customers can be accomplished at a low cost when innovation and creativity are used in solving problems. In this case, a new relay design provides high speed instantaneous tripping for all faults in addition to backup relaying for greater reliability and customer satisfaction. TDW

J. Kraig Kahler received the BSEE degree with emphasis on power systems from Kansas State University in 1985 and the MBA in finance from the University of Colorado in 1990. He has worked for Missouri Public Service Co. and for Colorado Springs Utilities. He is currently engineering manager for Greenville Electric Utility System and is a member of IEEE.