IEEE Power & Energy Magazine - March/April 2021 - 74

The Generators Subject to the
N−1 Intertrip Scheme
All generators newly connected to the transmission
power system (7 kV or higher) will be permitted
network access under the condition of implementing the N−1 inter trip scheme, except in cases where
a network facility upgrade is necessar y, including
cases in which
✔ the generator, for example, a must-run generator necessary to maintain system reliability, cannot participate in the N−1 intertrip scheme
✔ the power flow exceeds facility capacity as a result of
the connection of a new generator
✔ a network facility upgrade is economically more
advantageous than installing N−1 intertrip scheme
equipment (excluding the opportunity cost of loss
caused by the N−1 intertrip scheme).
The N−1 intertrip scheme requires the construction
of a highly reliable dedicated system because the relay
system is used instead of a network facility upgrade, and

an instantaneous response is required in the case of a
transmission contingency. In particular, if the intertripping amount, which is necessary to respond to a single
contingency in the transmission power system (7 kV or
higher), is to be provided by the relatively small-scale
generators connected to the distribution power system
(6.6 kV) (hereinafter called distribution generators), it
will be necessary to intertrip an enormous number of
distribution generators, and the N−1 intertrip system will
become more complex.
From the viewpoint of system reliability and certainty,
it is undesirable to intertrip distribution generators. But
considering that many renewable energy generators
in Japan are small scale and connected to distribution
systems, it is necessary to utilize the effect of the N−1
intertrip scheme in the connection of distribution generators and to pursue control of these assets. Thus, we
need to establish a mechanism that a generator subject
to the N−1 intertrip is not simply selected from newly
connected generators but from the most appropriate

Immediately After N -1 Failure
Ex-Ante Power
Flow Is Large
Curtailment of
All Generator
Outputs Subject
to the N -1
Intertrip
Scheme Is
Required

Operational Capacity
(With N -1 Intertrip Scheme)
D

Facility
Capacity
Per
Single Line

C
B
A

Before
Adjustment
Ex-Ante Power
Flow Is Small

Planned Outage Period for Maintenance
(Including Continuation of N -1 Failure Period)

Curtail All Generator Outputs
Subject to N -1
Intertrip Scheme
Same as on the Left

After
Adjustment

Operational Capacity (With N -1 Intertrip Scheme)

Facility
D
Capacity C
B
Per
A
Single Line

Before
Adjustment

Curtail on Last-In, First-Out
Basis Among Generator
Outputs Subject to the N -1
Intertrip Scheme

Curtail on Pro Rata Basis
Among Generator Outputs
Subject to the
N -1 Intertrip Scheme

B
A

D
C
B
A

After
Adjustment

After
Adjustment

Generator Connected Within Operational Capacity (Not Subject to N -1 Intertrip Scheme)
Generator Subject to N -1 Intertrip Scheme (Prior Application)

figure 4. The operation during first-step application.
74

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march/april 2021
IEEE Power & Energy Magazine - March/April 2021

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