IEEE Power & Energy Magazine - March/April 2021 - 70

Such a scheme would be particularly relevant in systems with-
or that expect-high renewable penetration not only in Japan but
also in many other countries/regions.
To this end, the Organization for Cross-Regional Coordination of Transmission Operators (OCCTO) in Japan has defined
the desired future evolution of the power system network as 1)
ensuring the appropriate reliability, 2) providing the smooth
use of the power system with a reasonable cost, and 3) ensuring the soundness of power system facilities. OCCTO thus
commenced studies toward the Japanese version of connect
and manage, specifically N−1 intertrip and nonfirm access to
improve the efficiency of power system facilities. The phrase
" connect and manage " is also used, for example, in the United
Kingdom, where the Office of Gas and Electricity Markets
introduced a scheme in 2010 to provide earlier grid interconnection to new and existing generation projects.
In this article, we focus on the N−1 intertrip scheme,
one measure in the Japanese connect and manage scheme, to
increase operational capacity by intertripping/curtailing generators instantaneously using a relay system in the event of a
single contingency. The N−1 intertrip approach can mitigate
transmission congestion and improve energy security.
In general, transmission congestion may decrease the
operational security level. This is because flexible resources
might be isolated from areas that need flexibility due to the
upper limit of operational capacity. If operational capacity is
expanded by the implementation of the N−1 intertrip scheme
and transmission bottlenecks are eliminated, the supply

from flexible resources to areas that need flexibility may
increase under normal conditions, improving the normal
condition operational security level. Such a scheme would be
particularly relevant in systems with-or that expect-high
renewable penetration not only in Japan but also in many
other countries/regions.
On the other hand, the N−1 intertrip scheme requires
advanced control systems that can instantaneously locate
a failure and automatically curtail generator output to a
specific, effective level, and such instantaneous control
must be evaluated appropriately for its influence on the
reliability of the entire network. Thus, its application
requires careful design and highly reliable control system implementation.
This article outlines the expected effects of the N−1 intertrip
scheme as well as the evaluation of the reliability required for
its application and highlights relevant points to consider when
determining the location and scale of application.

An Outline of Japanese Connect
and Manage
Overview of OCCTO
OCCTO was established in April 2015 as a government-authorized organization that all electricity companies must join. The

(MW)
Operational Capacity Increased
by the Application of
the N -1 Intertrip Scheme
Operational
Capacity

ATC

Maximum Power
Flow (Assumed)

Assumed Power Flow

Nonfirm Access
Facilitates the Use of ATC in
the System

Probabilistic Evaluation of Power Flow
Contributes to Improving the Accuracy of the Assumed Maximum Power Flow

figure 1. An overview of the Japanese version of connect and manage. ATC: available transmission capacity.
70

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