History Protection - Relaying in Japan

Authors: Phil Beaumont, Toshiba, UK, Tadashi Nakamura and Noriyoshi Suga, Toshiba, Japan

Towards enhanced reliability

Towards enhanced reliability
With rapid economic growth, power demand increased and large power stations were constructed in remote locations. This resulted in the requirement to develop a 500kV network. One of these was the 500kV Fukushima trunk line in the North East of Japan. With greater interconnection, the power system also became more complex and as a consequence of large-scale blackouts, both at home and overseas, in 1965 the decision was made to enhance the reliability of protection relays.

Automatic supervision and continuous monitoring features were introduced as countermeasures to improve the reliability of protection. These features were provided for both primary and back-up protections. Supervision covered current and voltage transformer circuits as well as binary input and trip output signals. Fault detection relays were another addition. In this scheme, the trip outputs for each of the two main protections are individually connected in series with the trip outputs from each of the two fault detection relays in order to avoid false tripping. This arrangement also presented the opportunity to individually test trip output contacts without tripping the circuit breaker. This new approach reduced the time required to detect and clear failures, reduced the need for manual testing and maintenance, and significantly reduced the occurrence of human errors.
The static type relay system shown in Figure 11 featured an automatic supervision function and the introduction of multiphase auto-reclosing. This type of auto-reclosing is widely used in Japan for reclosing on double-circuit lines. In this form of auto-reclosing, only the faulted phases are tripped and reclosed when the terminals of double-circuit lines are interconnected during the dead time through at least two or three different phases. Multi-phase auto-reclosing enables high-speed auto-reclosing for multi-phase faults without synchronism and voltage check, and minimizes the possibility of outages in the case of double faults on double-circuit lines. Figure 13 shows all of the fault types that could occur on a parallel line, the possibility of auto-reclosing for these faults using multi-phase auto-reclosing, and the conditions necessary to permit auto-reclosing for the case where two different healthy phases are available.
Because this type of auto-reclosing can be relied upon to avoid a complete power failure, even in the event of multiple faults which extend over two lines, it was adopted for Extra-High-Voltage critical main lines. Note that the protection devices that are used in conjunction with multi-phase auto-reclosing must be able to ensure identification of the faulted phase.
In 1969, a static phase comparison scheme was developed for the 275kV system in Fukushima, offering improved selectivity to that of directional comparison schemes. This protection uses the principle that the phase currents at both ends of the protected line tend towards the in-phase condition for internal faults, and are in anti-phase for load or through fault currents (Figure 16). This was a phase-segregated scheme enabling multi-phase auto-reclose, and included self-supervision features. A successor to this relay was the ‘sliced-level’ type phase comparison relay, which was developed to overcome the difficulties of its application to multi-terminal lines and was presented to the IEEE PES Summer Meeting held in Mexico City in 1977.
As the power system evolved it became increasingly complex, and the use of multi-terminal lines became more widespread. Although multi-terminal lines introduced difficulties with regard to maintenance outages, stability issues and fault clearance times their use was principally driven by the availability and cost of land in addition to the usual economies relating to primary equipment. These problems pressed the development of PCM current differential relaying which is the main protection scheme in use today. Before we focus on the PCM relay it is worth mentioning its predecessor - the Frequency Modulation (FM) current differential protection,  which was introduced in 1977 and is shown in Figure 7. The phase comparison scheme could not be expected to operate properly to clear internal faults on three terminal lines when current flowed out of the protected section, and therefore the FM current differential scheme was introduced to achieve a much improved selectivity overcoming the ‘outflow’ effect for internal faults on multi-terminal lines. The new scheme also included a countermeasure to improve the speed of the relay for faults on transmission lines adjacent to underground power cables for delays caused by transient harmonic oscillation. Frequency modulation technology was used to transmit the current data between terminals with the vector summation of currents at each terminal being calculated to determine whether or not the fault was internal, or external to the protected circuit.   It is also worthy of note that this protection was also applied in the UK at 400kV as a teed feeder protection after first undergoing switching and fault throwing tests in 1985 as part of a system trial.

BeijingSifang June 2016