Wind_Farm_Transient_Simulation_on_ATP Wind Farm Transient Simulation on ATP

Carlos M. Torres Ortolaza
carlos.torres@ece.uprm.edu

Camille Guzmán Torres
camille.guzman@ece.uprm.edu



Abstract In this paper the analysis of the transient
stage of a wind turbine when it is initially connected to
power distribution system IEEE 13 Node Test
Feeders. The type of induction generator used to run
the simulation is a self-excited induction generator.


I
.

I
NTRODUCTION


ind energy has become an increasing
source of electrical energy production in
recent years. In this project we examine the
effect caused by a wind farm in a transmission
system when it is initially connected to the grid.
For propose of this paper only one wind turbine
of 2 MW was used in the analysis. The
simulation has been made using only one wing
turbine connected to a power distribution system
IEEE 13 Node Test Feeders. The type of
induction generator (IG) used to run the
simulation is a self-excited induction generator.

II.

T
HEORY


Because of growing interest of using wind
energy as a source of electrical energy
production, it is necessary to study the possible
impact a wind turbine will produce on the power
network where it is connected. In order to
investigate the effects, suitable wind energy
models must be used.

The technology used in most of the land based
wind farms is stall regulated wind turbines with
conventional fixed induction generators which
are connected directly to the to the power
system. Low and medium self-excited induction
generators
are
ideally
suited
for
non-
conventional energy systems such as wing-
electric generators, micro hydel power stations,
etc. In rural areas where large wind resources are
available, stand alone wind turbine driven
induction
generator
can
meet
the
local
requirements. Low cost, simple and reliable
operation, minimal maintenance, and inherent
short circuit protection capability, are only some
of the advantages of this system.
The effects of wind turbines on power systems
can be classified in two groups: steady-state
security and power quality. The goal of steady-
state security is to seek network power stability
conditions when the wind power is initially
injected into a system. Power quality analysis
studies the effects of wind power fluctuations in
the form and level of electrical waves
(harmonics, fluctuations, etc.).

Besides the induction generator a wind energy
conversion system also consists of a blade, hub,
and a coupler. So the wind energy conversion
system follows four aspects [2]:

1.

Stochastic wind energy
Because the speed of the wind is affected by the
temperature, pressure, humidity, and died degree,
actives of the sun and the moon, topography of
the wind farm, etc., the speed of the wind is
stochastic and uncontrolled, hence the wind
energy.
2.

Large inertial wind turbine
The wind turbine blade diameter is very large, so
the turbine inertia is very large, and the wind
energy density is very low.
3.

Flexible coupling between the wind turbine
and the induction generator
Owing to the low rotational speed of the wind,
the turbine can to be connected directly to the
generator, so the torque of the turbine must be
transferred by a gear box and a coupler. As a
result the rigidity between the turbine and the
induction generator is very low.
4.

Induction generator
The induction generator is used because of its
low cost, reduced maintenance, rugged, and
brushless rotor. The use of some capacitors
provides reactive power to the excitation.

The connection of the wind farm to the nearest
primary substation may not be cost effective. So,
it is more attractive to use existing rural
W distribution systems for the connection of
individual or small wind farms.

III.

D
ISTRIBUTION
S
YSTEM


The distribution system used in the analysis is
the IEEE 13 Node Test Feeder (Fig. 1). It a small
one, yet displays interesting characteristics. It is
a 4.16 kV short and highly loaded feeder. This
system serves a total load of 5 MVA.

This feeder is characterized by:

1.

Short and relatively highly loaded for a 4.16
kV feeder
2.

One substation voltage regulator consisting
of three single-phase units connected in wye
3.

Overhead and underground lines with
variety of phasing
4.

Shunt capacitor banks
5.

In-line transformer
6.

Unbalanced spot and distributed loads



Figure 1

IV.

I
NDUCTION
G
ENERATOR
M
ODEL


The structure of squirrel cage induction
generator is same as induction motor have
aluminum bar winding laid into the slots of the
rotor core and short-circuited at both ends.
Single-phase equivalent circuit of three-phase
cage generator is similar to threephase
transformer equivalent circuit with one winding
is short-circuited, and the same circuit models
apply as shown in Fig. {2}. The circuit shown in
Fig. {2} can be used in steady state operation. In
this circuit, the machine core losses have been
ignored. The successfully build up in self-excited
induction generator occurs when X
m
have the
right value in saturation. The simulation of the
induction generator is based upon the following
assumptions:

a.

Saturation effects and core losses are
neglected
b.

Space harmonics on air-gap flux are ignored
c.

The stator transients are neglected

A 2 MW induction generator (IG) model for
wind farms was used in our simulations. The
parameters shown below are based on a 6O
MVA base. It uses a capacitor for power factor
correction (X
PFC
), but it was removed in the
simulations in order to simplify it.


Figure 2

V.

S
IMULATION AND
R
ESULTS


ATP/ETMP program was used to analyze the
transient in a power system when a wind farm is
connected to help supply the increasing load.
Figure 3 is the schematic of our IG model
(without X
PFC
) in ATP. The parameters were
changed from p.u. (60 MVA and 0.69 kV bases)
to its equivalent in Ohms using a Z
base
=
0.007935
. A three-phase source is connected
in series to the R
s
to model the voltage created in
the stator. It has a 690 V rating, but a 4.16 kV
source was used in the model to eliminate the
need for a transformer. The elimination of the
Xmer reduces the reality of the simulation,
however this action minimizes simulation
problems
(multiple
frequency,
inaccurate
waveforms, etc.) that we faced before. The slip
used in the simulation is 2% because at this
value the torque-slip curve presented in Ref [2] is close to its peak. The output in a real wind
farm varies time-to-time because of the
stochastic nature of wind, so a fixed slip is not
ideal. However, simulating this behavior is quite
complex using ATP, so behind of our scope. The
use of power electronics are becoming widely
used to stabilize the output for a certain range of
wind energy, so our assumption of a fixed output
can be acceptable.

The Wind farm was connected in the node 680,
far away from the other sources, including a DG
(Distributed Generation) at node 634, to balance
the power injection in the distribution system.
The simulation consisted in two parts; in the first
part the wind farm is disconnected from the main
distribution system, but it feed a load connected
at the same node, and the second part has this
load feed originally from the main distribution
system and later the wind farm is connected to
inject power in the main system. We want to see
the transient produced in the loads when the