IEEE Power & Energy Magazine - March/April 2021 - 71

OCCTO's goals and objectives are to maintain a stable electricity supply and enhance the efficiency of supply systems as much
as possible. To achieve these objectives, OCCTO undertakes the
role of conducting studies by its advisory committee, from an
expert perspective, which is formulating regulations and codes,
developing long-term policies and cross-regional network development plans, and studying network access application.

The Japanese Version of Connect and Manage
In the Japanese version of connect and manage, as illustrated
in Figure 1, studies are being implemented to introduce the
probabilistic evaluation of power flow and nonfirm access in
addition to the N−1 intertrip scheme. The probabilistic evaluation of power flow is an effort to increase the available transmission capacity (ATC) through the evaluation of a power
system based on the assumption of actual power flow, which
can be made by an evaluation of variable renewable energy
output and generation probability according to demand conditions. Nonfirm access is the concept of generator connection without a network facility upgrade, in which new generators under normal conditions agree to curtail their output if
there is congestion.

Network Access Rule
In Japan, a new generator wanting to connect to a power
system is provided with a transmission capacity in the
order of their application as a general rule (i.e., a first-come,
first-served queue). Transmission and distribution utility
companies are obliged to provide network access under the
Electricity Business Act in Japan. Accordingly, the capacity
of the power system is secured in the order of application,

No Contingency

100

N-1 Contingency

and when the system runs short of ATC, generation interconnection is granted only after the corresponding network
upgrades are completed.
Also, the present cost allocation rule requires network
upgrade expenses to be borne based on the beneficiariespay principle that states that specific generators near a direct
connection attachment facility need to pay these upgrade
costs. In contrast, the network upgrade costs associated with
a bulk power system could be allocated to all network users
and not just the specific generators near the connection.
Under the access rule in Japan, the N−1 intertrip scheme
is expected to address new interconnection service requests
and minimize any increases in electricity costs.

The Basic Concept of the
N−1 Intertrip Scheme
Power System Development for
Generation Network Access
The N−1 criterion for all N−1 credible contingencies is the
basic concept for power system development of transmission
and transformation systems (7 kV or higher) with generation
network access. This concept, which is one of the typical
technical standards related to facility security and power
system stabilization, remains the same even after the application of the N−1 intertrip scheme.
The N−1 intertrip is a mechanism to expand operational
capacity by instantaneously intertripping (or curtailing;
hereafter, intertripping includes curtailing) a generator using
a relay system in the event of a single contingency. Figure 2
illustrates the concept of the N−1 intertrip scheme.

No Contingency

100

Line A
100 MW/Circuit
× One Circuit

Line A
100 MW/Circuit
× Two Circuits
ATC 200 MW

Line A
100 MW/Circuit
× One Circuit

Line A
100 MW/Circuit
× Two Circuits
ATC 100 MW

N-1 Contingency

200

100

Trip Signal
(Curtailment)
Relay

100

100

100
Existing New
G
G

Existing
G
Cannot Connect to the Grid
(Reinforcement Is Needed)
(a)

100

100

0
New
G

100
Existing
G

New
Existing
G
G

Trip
(Curtailed)

Possible to Connect to
the Grid
(b)

figure 2. An overview of the N−1 intertrip scheme. (a) Present operation and (b) applying N−1 intertrip scheme.
G: generation plant.
march/april 2021

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IEEE Power & Energy Magazine - March/April 2021

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