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World's First Microprocessor-Based Numerical Relay

Through active research on a numerical protection relay incorporating a microcomputer both in Japan overseas, Toshiba commercialized the world’s first microprocessor-based numerical current differential protection relay.

World's First Microprocessor-Based Numerical Relay

The first scientific paper describing a numerical relay is considered to have been a report by G. D. Rockefeller of the U.S., which was published in 1968. This was before the arrival of microcomputers, and research was being conducted throughout the world on the operation of protection relays using large-scale process computers, including by our company. These efforts were not yet at the level of practical realization, however, because the scale of the devices was too large and the speed of operation was insufficient. The situation changed with the appearance of microcomputers, when the feasibility of the numerical relay rapidly increased. In the latter half of the 1970s, current differential protection systems became a target of research. The current differential protection system is a highly sensitive and highly reliable system for detecting and protecting against faults in power systems. When the sum of the current flowing in from each terminal of a power transmission line is zero, the system judges that there is no fault, whereas in the event that the value is not zero, a fault is judged to have occurred. In addition to analog frequency modulation (FM), digital pulse-code modulation (PCM) was also researched as a method of data communication for this system. A microprocessor-based numerical current differential protection system using PCM communication performs digital conversion by sampling after synchronizing the instantaneous value of the current of each terminal, requiring highly advanced technology. Basic research on a microprocessor-based numerical current differential protection relay of this type, also referred to as a PCM relay, had been carried since the early 1970s. After several field tests, the design progressed to the point where the protection function could be maintained for a long period with a high degree of reliability even under the high-voltage and high-current conditions of an electrical substation and in various temperature environments. In 1980, the world’s first microprocessor-based current differential relay, developed by our company, was applied to the 275 kV Azusagawa transmission line of The Tokyo Electric Power Company (TEPCO).

One of the key features of the PCM current differential relay is its sophisticated built-in function for out-of-step detection. In the event that any of the generators steps out due to an inability to maintain synchronization with the system, the voltage becomes zero at some point in the electric power system at the time when the generator phases are reversed by 180°. This point is called the out-of-step center. When an out-of-step has occurred, stable operation of the generator can be continued by isolating the system at the out-of-step center. A PCM current differential relay uses the transmitted bus voltage phase information of the other terminals to directly compare the voltage phase at both ends of the line and, when it judges that an out-of-step center has entered the line from the 180° reversal of both phases, reliably isolates the system at the out-of-step center.

These functions of the PCM current differential relay such as communication error detection capability, out-of-step detection by voltage phase comparison, and so on cannot be easily realized by the FM current differential relay. As a result, the PCM current differential relay has now established its position as the most reliable transmission line protection system, and is used throughout Japan in power systems ranging from 66 kV systems to extra-highvoltage trunk power systems.

The evolution of microprocessor-based numerical type protection relays, which began with the PCM current differential relay, has been gradually expanding to other areas including circuit selecting protection relay for the 66 kV Yokaichiba transmission line, which was also introduced in 1980. However, the first-generation DI type numerical relay suffered from the problem of heat generation due to its use of a bipolar type device. This problem was solved by the use of a special heat pipe.

Remarkable progress was seen in arithmetic devices and, a few years later, a metal-oxide semiconductor (MOS) type high-speed arithmetic device appeared that generated less heat. This triggered a shift to the era of the secondgeneration DII series MOS type numerical relays beginning in 1985. Since that time, their application has rapidly expanded throughout Japan and they have been playing the leading role in the protection relay field.

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