IEEE Electrification Magazine - March 2015 - 30

E (MJ)

∆f (Hz)

0.6
0.4
0.2
0
0.2
0.4
0.6
0.8

0

20

40

60

80 100 120 140 160 180 200
t (s)

16
14
12
10
8
6
4
2
0

0

20

40

60

Without FESS
FESS (Pn = 1 MW)
FESS (Pn = 1.5 MW)

80 100 120 140 160 180 200
t (s)
FESS (Pn = 1 MW)
FESS (Pn = 1.5 MW)
(b)

(a)

Figure 5. The dynamic simulation results for frequent variations in renewable power, with the FESS sized for the major power loss (Pn = 1 MW,
En = 15 MJ), and with FDB = 0.1 Hz and R = 1 Hz/Pn: (a) the time evolution of system frequency deviation without an FESS, or with an FESS of
1 MW or 1.5 MW, and (b) the time evolution of the FESS stored energy for different Pn values of the FESS.

interfaced with the grid by power electronic converters. To
evaluate the security improvement that can be provided
by this technology, time-domain simulations were performed for major foreseen wind/PV disturbances in operating conditions that may provide the largest transient
frequency deviations in this power system. This includes
cases of minimum consumption, with maximum penetration of wind and PV and also with a high share of generation and spinning reserve provided by
hydro power plants.
For time-domain simulations of the
variable-speed pumped hydro (VSPH)
units, a dynamic model was developed
based on the works presented in Suul,
Fraile-Ardanuy et al., Lung et al.,
Kuwabara et al., and Rodríguez-Bobada
et al. This model includes fast frequency control provided by a frequency
droop characteristic, also including a
speed pulling-back control, to prevent
the speed of the machine from
violating a lower and upper limit. To
improve the machine speed transient
response, a feed-forward wicked gate
signal was also included.

Security Assumptions
In this study, the expected major renewable power loss
disturbance, caused by a sudden bus-bar or line section
disconnection, is 27 MW of wind power loss. For the frequent renewable power variations disturbance, a 5%
change in 5 s was considered for wind power production
and a 50% change, also in 5 s, was considered for PV
power production. Besides renewable power variations,
three-phase short-circuits in any bus
of the transmission system were also
considered for the dynamic security
assessment of this power system.
During frequent power variations,
the system frequency deviations must
not systematically violate the range of
±0.5 Hz. For major disturbances, the
power system was considered to lose
security if, after a power loss, the
frequency transient behavior provokes
automatic load-shedding. In dynamic
simulations, automatic load shedding
was assumed to be activated for a
minimum frequency drop of 0.8 Hz
combined with a minimum rate of
change of 2.5 Hz/s, or for a minimum
frequency drop of 1.8 Hz. For the most
severe disturbances, usually provoked
by a short-circuit, load shedding cannot be avoided. Therefore, for these disturbances, the system was considered to lose security for frequency behaviors that can provoke power stations automatic
disconnection (assumed to occur for frequency deviations
that violate the range of ±3 Hz).

For major
disturbances, the
power system was
considered to lose
security if, after a
power loss, the
frequency transient
behavior provokes
automatic load
shedding.

Analyzed Scenario
In the analyzed scenario, an installed
power of 140 and 15 MW was considered for wind and PV
power production and a 74-MW capacity was assumed for
the cluster of new planned pumped hydro power plants.
At off-peak hours, the wind power penetration is assumed
to be around 76%, totaling, with the hydro production, a
renewable power penetration of 78%. At noon, a 69% penetration was assumed for the renewable power, with 66%
provided by the wind plus PV power generation and the
remaining by hydro generation.

30

I E E E E l e c t r i f i c ati o n M agaz ine / March 2015

Obtained Results
The security improvement that can be provided by the frequency control action of VSPH units was evaluated, by



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