IEEE Power & Energy Magazine - September/October 2015 - 68

responses measured using PMUs
can derive both the fault type and
its position, so the dispatcher can
quickly determine what kind of
fault occurred and on which part
of the power grid. This information
is important as dispatchers decide
whether further measures should be
taken to maintain safe operations.

Parameters

Assessing Generator
Ancillary Services

Output Changes

Ensuring safe and reliable generator
operation, requires ancillary services
such as speed control and excitation
control. When the system frequency
deviates from the nominal frequency
over a certain threshold, the unit
power output and frequency curve
Frequency Changes
can be analyzed to assess their contribution to the primary frequency
regulation. Based on the active power
and the frequency measurement
provided by the generator's PMU
figure 9. The performance assessment of a generator's primary frequency regulation.
during a frequency disturbance, the
performance parameters of the generator's primary frequency regulation can be calculated. These
parameters include delay time, response time, frequency dead
1.2
2
1
zone, speed variation ratio, and contribution energy. Figure 9
8
0.8
6
0.6
shows the result of such an assessment of a generator's primary
4
0.4
0
10
20
30
40
50
60
frequency regulation during a frequency disturbance. Monitort (s)
ing of a generator's excitation control can be carried out in a
(a)
similar way.
8
0.8
A generator's primary frequency regulation function is
6
0.6
Threshold
Thr
h
eshold
extremely
important in maintaining the frequency of a power
4
0.4
2
0.2
grid within a secure range when it suffers an active power
0
deficit, such as the tripping of a large generator. In a number of
0
10
20
30
40
50
60
t (s)
tests on some generators' primary frequency regulation function, even though it seemed that this function had switched on, it
(b)
was actually switched off. The system frequency response may
be deteriorated in this condition. For this reason, every generafigure 10. The detection of voltage dips during faults.
tor with the primary frequency regulation function switched on
should be monitored and assessed. Unlike in previous years
when the SGCC had no such assessment system in place,
0.7
0.6
today all large generators are assessed, and system frequency
0.5
characteristics are thus improved.
Active Power (p.u.)

ddU (p.u.)

U (p.u.)

Evaluations

0.4
0.3
0.2
0.1
0
-0.1

Detected Power Ramp

30
Time (s)

Intelligent Alarm System for Cascading Tripping
of Wind Turbines and Wind Farms
60

figure 11. The identification of power ramps during cascading failures.
68

ieee power & energy magazine

Wind turbines in China suffer from cascading tripping
induced by internal or external wind farm faults. Such
cascading tripping may occur when groups of wind farms are
located close to one another electrically, and events like these
put considerable pressure on utility companies. However,
because of commercial secrecy, utility companies have no
september/october 2015

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - September/October 2015