Russia is a massive country spannig nine time zones from east to west. Extensive electrification programs conducted in the 1950s, 1960s and 1970s supplied electricity to rural and urban areas. Similar to other European distribution systems, Russia typically uses an ungrounded, three-wire, 6- or 10-kV radial distribution system. Feeders are relatively short and, as a result, the a Russian scheme has more feeders and medium-voltage substations than a comparable North American system.

Aging Infrastructure

Today much of Russia's distribution system is aging and in need of rehabilitation. Many of the substations are lightly loaded because of the present economic conditions in Russia. Cost-effective rehabilitation of the aging, lightly loaded substation equipment was among the primary goals of the Smolenskenergo (Smolensk, Russia) Demonstration Project, named after the participating Russian utility. The project sought to demonstrate that a utility could serve rural loads reliably and with quality voltage regulation. To accomplish this feat, the utility needed to reconfigure existing feeders into longer lines capable of serving more customers from fewer substations and using American-style equipment to offset the problems inherent with longer distribution lines.

With longer lines, however, outages affect more customers. In addition, locating faults along longer lines is difficult, and voltage drops across the length of these lines are more severe. The utility needed a system that could automatically clear and sectionalize faults. The utility also needed to design and implement a regulated voltage scheme.

Safety and equipment damage aspects of single phase-to-ground faults were of concern. Russian regulations limit ground-fault current to no greater than 10 A. With such a low current, industry regulations do not require the circuit to be disconnected immediately but can wait several hours. The fault can present a touch potential hazard to humans and animals during that time. In addition, ground-fault currents also have been found to result in the deterioration of the Russian concrete poles, leading to eventual pole failure. As a result, faulted poles are usually replaced following a ground fault. Replacing these concrete structures costs the utilities time and money.

Ground faults also cause high sustained 50-Hz voltages (greater than 173% of the line-to-ground nominal voltage rating) on the unfaulted phases. The high voltage can lead to cable, potential transformer and other equipment failures.

In addition, single-line ground faults are tough to locate. The low-magnitude current causes minimal damage that is difficult for patrolling linemen to observe. Because current flows everywhere in the system and on all three phases simultaneously during the fault, identifying the faulted feeder proves challenging. Utilities needed to find a better way to sectionalize and isolate ground faults.

A Joint Effort

The U.S. Trade and Development Agency (USTDA) — a government agency that funds feasibility studies throughout the world — and distribution equipment manufacturer Cooper Power Systems (Waukesha, Wisconsin, U.S.) combined forces to finance a US$1.2 million project demonstrating the ability of American-style distribution equipment to meet Russia's immediate goals.

The project demonstrated how an existing three-wire ungrounded system could be reconfigured to reduce substation losses and offset the cost of rehabilitating aging substations. The Smolenskenergo Project impacted two feeders and three substations, totaling 90 km (55 miles) of overhead lines and affecting approximately 1000 customers in the Smolensk region.

In 1998, Russian engineers visited the manufacturer's three-wire ungrounded test-line facility in Franksville, Wisconsin. This test line enabled Russian engineers to study the equipment and proposed solution in a “real-life” application. This allowed Cooper personnel to gauge how American equipment would react on a Russian three-wire ungrounded system. Russian engineers decided that a two-feeder loop scheme with mid-feeder voltage regulation would best serve Smolenskenergo. A loop scheme increases the reliability of two feeders by allowing each to provide an automatically controlled alternate source to the other in the case of a permanent fault on the system. The faulted section is isolated with power restored quickly to the un-faulted section.

To implement the solution, the following equipment was shipped overseas:

• Reclosers geared for loop-scheme application

• Recloser controls

• Voltage regulators, bypass switches and controls

• Sectionalizers for additional fault isolation

• One 400-kVA padmounted transformer to replace existing kiosk-style transformers and low-voltage switch-gear (Fig. 1)

• A pole-mounted capacitor bank for power-factor correction

• Relays for programmable overcurrent protection of the substations

• A supervisory control and data acquisition (SCADA) system using single-frequency radio communication between remote thermal units (RTUs) and master station

• Other assorted equipment to tie it all together.

Let's Get Started

A two-feeder loop scheme was configured (Fig. 2) from three existing 10-kV feeders fed from the Rudnya, Ljubavichi and Mikulino substations. The section of line going to Rudnya was opened, thereby isolating the loop scheme from the Rudnya Substation.

A recloser was placed on the line about halfway between the two remaining substations to act as a normally open tie point between the two feeders. A recloser and voltage regulator were installed at the midpoint of each feeder. Several three-phase laterals were equipped with time-voltage sectionalizers for further fault isolation. One of the sectionalizer locations also included a switched-shunt capacitor bank. These installations were designed similar to the two-pole H-frame commonly used for overhead three-phase distribution transformer sites.

The reclosers used at the feeder's midpoint and normally open tie point are the Cooper solid-dielectric NOVA-style switchgear with Cooper's Form 5 control (Fig. 3). The Form 5 provides directionally sensitive ground fault protection capability and phase overcurrent protection.

The midpoint installation included two single-phase voltage regulators connected in open delta with CL5C control for midfeeder voltage control and an RTU with radio for communication with the master SCADA unit (Fig. 4). The sectionalizers were time-voltage controlled because current control was not sensitive enough for the small ground fault currents of the system.

Microprocessor-based relays with directionally sensitive earth fault (dsef), phase overcurrent and negative-sequence-tripping capabilities were installed at the two substations. The zero-sequence polarizing voltage is obtained from the broken delta secondary of a three-phase, substation-bus-connected potential transformer. In a typical Russian overcurrent-protection scheme, current transformers (CTs) monitor only two phases because protection does not include ground faults. A third CT was added to each of the two feeder breakers to allow for residual current determination. All three CTs must have closely matched turn ratios to avoid an error residual current in the CT secondaries. Although the error current would not cause erroneous ground-fault tripping when there was not a fault, it could lead to misoperation during a ground fault.

The installation of a SCADA system allowed monitoring and supervisory remote control at four points in the system: the three-recloser sites and the capacitor/sectionalizer site. Single-frequency data radios were the selected means of communications.

There was no clear line of sight from master station to all remote stations, therefore, the radio at the highest elevation was converted to a repeater mode. All other remote and master directional antennas were pointed toward it. The SCADA firmware and software allow downloading of the entire Form 5 database and recording voltage, current, power, power factor and tap position from the voltage regulator and capacitor bank.

Smolenskenergo and Cooper installed more than 300 pieces of equipment at nine sites within a 20-km (12-mile) radius of Rudnya in December 2000 (Fig. 5). Commissioning occurred in February 2001. The SCADA system (master station, RTUs and radios) went operational in June 2001.

Commissioning checks included instantaneous voltage, current, power and power factor recorded at each recloser and regulator. Staged single line-to-ground faults proved the effectiveness of the dsef of the relays at the substations. Transfer switch operation was tested at each recloser. Naturally occurring weather-related faults provided a test of the loop-scheme operation.

Phase-overcurrent protection of relays and reclosers uses inverse time-current characteristics. Overcurrent-protection coordination is performed in the usual manner.

Definite time-protective elements in the relays and the reclosers provided ground-fault protection. Tripping for ground faults occurs after 10 to 30 seconds, allowing enough time to alert the system dispatcher of the ground fault and to let temporary faults self extinguish. It also enables transient conditions such as transformer inrush current to dampen.

Ground-fault sensing in the substation relays and midpoint reclosers must be directional because capacitive current flows in each phase of all feeders during the fault. The distinguishing feature is that the residual current always flows toward the fault. If the relay senses a residual flow into a feeder, the fault is on the feeder. If the flow is out of the feeder, the fault is elsewhere.

Likewise, the midpoint recloser senses residual current flowing toward the source if the fault is on another feeder or between it and the source. It will not trip in this condition. If the current flow is toward the load side of the recloser, the fault is a down line fault and the recloser trips. Ground-fault pickup must be as low as one ampere.

Ground-fault sensing in the tie-point recloser is nondirectional because it must be able to trip for ground faults in either direction when in the closed state, depending upon the source. Tripping on a ground fault helps to locate the fault and to save the pole.

Commissioning tests also involved operation of both feeders supplied from only one end. In this case, the tie recloser is closed and the Mikulino Substation breaker is open. The midpoint regulator operates in reverse power-flow mode, and the midpoint recloser operates on the Alternate 1 set of time-current characteristic curves.

Lessons Learned

The Smolenskenergo Demonstration Project has achieved its goals. To recap the experience and benefits of the Smolenskenergo loop scheme:

• Phase-to-phase faults and phase-to-ground faults have been experienced on the line. Both have been properly detected and cleared. The loop scheme has operated properly to isolate the faulted section and restore power to the unfaulted sections. Voltage regulation has been improved.

• Longer line operation has been satisfactorily demonstrated on the Russian three-wire ungrounded system.

• The ability to operate longer lines allows reconfiguration of the system in such a way as to reduce the number of substations required to supply the load. Resulting cost savings can be significant by reducing substation losses and rehabilitation dollars (fewer substations need to be rehabilitated).

Acknowledgments

Moscow's Power Engineering Institute provided technical evaluation and equipment importation support. The consulting firm ROSEP (Moscow, Russia) performed design work. Project sponsors were the RAO Unified Electric System (Moscow) and the USTDA. The author also would like to acknowledge Antone Bonner with Cooper Power Systems for his invaluable support and contribution to the writing of this article.

Aleksander Nikolaevich Prudnikov has been the director of Affiliate Western Electrical Networks for Smolenskenergo since 1995. Prior to that, he was the chief engineer of Affiliate Western Electrical Networks. He graduated from Moscow Power Engineering Institute in 1976. His concentrations include electrosupply of cities, industrial firms and agriculture.