IEEE Power & Energy Magazine - March/April 2021 - 72

The operational capacity is the upper limit of power that
can flow under normal conditions, and it ensures stable power
transmission even in the event of a single contingency. If the
thermal capacity per line is 100 MW, the operational capacity
of a parallel two-line transmission system used to be set at
100 MW considering a single contingency (short-term overload may be allowed in some cases). The N−1 intertrip scheme
is designed to allow a 200 MW power flow (i.e., operational
capacity is set at 200 MW) under normal conditions on the
premise that power flow is limited to the thermal capacity per
one line (100 MW) by intertripping either an existing or new
generator instantaneously in the event of a single contingency.
Figure 2 depicts an example where the new generator is subject to intertripping.
Effective power system development can be achieved
through an increase in operational capacity by the N−1 intertrip scheme and the resulting decrease in network facility
upgrades. On the other hand, a generator needs to be tripped
in the event of a single contingency, requiring consideration
to be paid to the impact on securing supply-demand balance.
For this reason, if the impact of N−1 intertripping on reliability is small, power system development should be implemented based on the application of the N−1 intertrip scheme.
However, a network facility upgrade should be implemented
when intertripping has a major impact on reliability under
generator scenarios such as
table 1. The concept for applying the N−1 intertrip
scheme to a power system.
Transmission Power System
(7 kV or Higher)
Bulk power system
(top two voltages)

Concept of Application

Loop

Carefully determine for
each system based on
the features of the loop
system1

Radial

Apply as a rule2

Power systems excluding bulk
power system

Apply as a rule2,3

1

Apply the N−1 intertrip scheme to loop systems that are
considered capable of being controlled simply.
2
Exclude the cases where the intertripping amount exceeds
the allowable amount per failure of the facility subject to
the N−1 intertrip scheme.
3
Exclude the cases where simple control is difficult because
the system constitutes the loop system with a bulk power
system.

✔ a generator that has a capacity exceeding the allow-

able intertripping amount, which is set to ensure supply reliability
✔ a generator that has difficulty in restarting generation,
with such a difficulty requiring a considerable period
for reinterconnection and output increase to its previous dispatch level
✔ a must-run generator necessary to maintain system
reliability.

Applicable Power Systems
Concerning the power system to which the N−1 intertrip
scheme is applied, there are concerns that are associated with
the application of the scheme to a bulk power system, including
the following:
✔ A large intertripping amount is required, resulting in
a significant reduction in supply capacity and a large
decline in frequency.
✔ The number of generators under control by intertripping becomes extensive, resulting in a great number of
targets and an overly complex system.
✔ A significant impact is expected if intertripping fails
to operate as intended.
These concerns are prominently visible in a bulk power
system constituting a loop system. The concept of the application of the N−1 intertrip scheme to power the system is
detailed in Table 1.

Intertripping Amount Allowable From
a Supply Reliability Standpoint
Since a radial bulk power system may have a large intertripping amount under the N−1 intertrip scheme and its reliability is affected significantly in terms of frequency and
reserve margin, it is necessary to set the allowable intertripping amount in advance.
table 3. An example of the allowable intertripping
amount for each facility subject to the N−1 intertrip
scheme (indicated in red).

Area
table 2. The normal frequency variation range.

72

Area

Frequency

Hokkaido

50 ± 0.3 Hz

Eastern Honsyu

50 ± 0.2 Hz

Western Honsyu

60 ± 0.2 Hz

Okinawa

60 ± 0.3 Hz

ieee power & energy magazine

Area with large
system scale

System
Scale
(MW)1

Intertripping
Amount Limit
Necessary
to Keep the
Frequency
Within Normal
Variation
(MW)2

Intertripping
Amount
Limit Taking
Into Account
the Reserve
Margin of
Each Area
(MW)3

56,530

400

2,550

500

250

Area with small 5,210
system scale
1

Maximum power demand in 2018
Normal frequency variation (low side): -0.2 Hz (-0.3 Hz
for Hokkaido and Okinawa)
3
Calculated based on the demand (2017 supply plan) of
each area during the light load period
2

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

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