IEEE Power & Energy Magazine - January/February 2015 - 72

5 p.u.
4
3

Stable

5 p.u.

IG

4

Unstable

IG

3

δ

PG

2

2

PG

1
0

δ

Short
Circuit
0.5

0

-1

Short UG
Circuit

1

UG

0
1
t (s)

1.5

(a)

0

0.5

1
t (s)

-1
-2

1.5

(b)

figure 18. Transient stability issues.

begins to replace conventional generation, however, the ability
of the system to resist these deviations decreases. modern generating plants such as wind and solar power plants are made up
of generating units that connect to the power system through
frequency converters. these converters, however, are most
often controlled in such a way that operation of the generating plants is independent of the system frequency, resulting in
reduced system inertia. the same is true of motor loads that utilize power electronics. Import and export of electricity through
hVdc connections will also mean a shift in the location of inertia in the power system.

The Nordic Problem
the nordic power system is a synchronous system made up of
the power systems in finland, norway, Sweden, and Zealand
(eastern Denmark). It is connected to the continental european
power system exclusively through hVdc connections and therefore has a frequency isolated from the frequency of the much

Lowest Frequency (Hz)

49.7
49.65
49.6
49.55
49.5
49.45
49.4
49.35

ay

M

Ja
nu
ar

y

20
04
O
20
ct
05
ob
Fe er 2
00
br
ua
6
ry
20
Ju 08
N
ov ly 2
00
em
9
be
r2
01
Ap
0
ril
20
Au
12
gu
D
ec st 2
01
em
3
be
r2
01
4

49.3

figure 19. Recent frequency disturbances in the Nordic
system with frequencies under 49.7 Hz due to an N-1
contingency.
72

ieee power & energy magazine

larger continental system. Because of the relatively small size of
the nordic system-maximum load is only about 70,000 mw-
power imbalances lead to relatively large frequency excursions.
the nordic countries aim to keep the instantaneous system frequency always above 49.0 hz after the loss of a large production
unit and above 49.5 hz 30 s afterward.
as time goes on, it is becoming more apparent that the system frequency is reaching lower levels than before, as can be
seen in figure 19. It is believed that this is caused by a reduction in the number of synchronous generators connected to the
system at any given time, which affects the amount of inertia
in the system and thus power regulation.
the frequency of the system begins to decrease the
instant a production unit is lost. the frequency reaches its
lowest point (begins moving up again) when the frequency
containment reserve (fCr) provides enough power to compensate for the loss in generation.
the nordic countries are working together in the nordic
analysis group, within entSo-e's nordic regional group, to
develop tools and methods with which to mitigate this issue.

Implementation of Real-Time Inertia Estimation
to ensure that the system frequency remains above 49.0  hz,
more real-time information regarding the amount of inertia in
the system is required. the balance between production and
load is readily available by observing the frequency, but the current state of inertia in the system is more difficult to determine.
the position of circuit breakers connecting synchronous
generators to the power system has been investigated as a
method for helping to determine the online inertia. these
data are then connected to information regarding the inertia constant and nominal apparent power for every machine,
after which the inertia of the system can be calculated. there
are, however, problems with this method.
first, the data for the inertia constants in the tSo databases
of the generators seem to be unreliable. Large numbers of
machines appear to have the same integer value for their inertia
constants. historically, this parameter has not been regarded as
january/february 2015



Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - January/February 2015

IEEE Power & Energy Magazine - January/February 2015 - Cover1
IEEE Power & Energy Magazine - January/February 2015 - Cover2
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IEEE Power & Energy Magazine - January/February 2015 - Cover3
IEEE Power & Energy Magazine - January/February 2015 - Cover4
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