MICROGRID COMPATIBILITY OF PHOTOVOLTAIC AND WIND POWER SYSTEMS

Proceedings of the 5th Annual ISC Research Symposium
ISCRS 2011
April 7, 2011, Rolla, Missouri
MICROGRID COMPATIBILITY OF PHOTOVOLTAIC AND WIND POWER SYSTEMS
Anshuman Vaidya
[email protected]
Badrul H. Chowdhury
[email protected]
Electrical & Computer Engineering
Missouri S & T
ABSTRACT
Renewable energy sources, like wind and photovoltaic (PV)
power plants can be used to provide grid-friendly services in
the form of additional active and reactive control for frequency
and voltage regulation respectively. Current wind and PV
power systems are operated in simply an energy supply mode
and are not required to participate in ancillary power services.
These services will become critical when operating in an
islanded micro-grid. The maximum active and reactive power
capability of the wind and photovoltaic power plants along with
their limitations are formulated and discussed. A direct power
control scheme is simulated with both types of resources in a
test microgrid. Simulation results display the potential of wind
energy when utilized either in parallel with or islanded from the
main grid.
1. INTRODUCTION
The need for high service availability, high environmental
quality and high power quality has dramatically increased the
desirability of renewable energy resources in the power grid.
The advantage of the renewable class of distributed generation
is that their fuel source (water, sunlight or wind) is abundantly
available, and, because of their modular nature, they can be
easily deployed near customer sites, thereby decreasing the
over-dependence on typical radial distribution lines to bring
power from central generating locations.
However, the application of renewable as well as other
DERs, especially in a microgrid environment is fraught with
technological challenges [1]. They are significantly different
from the conventional forms of generation in that solar
insolation and wind are intermittent in nature, and therefore, at
any given point in time, a desired or commanded amount of
electric power output from these renewable resources cannot be
guaranteed. As such, when applied in a microgrid environment,
the customer’s kW capacity demand may not be met at all times
leading to frequency excursions that may become
uncontrollably low, eventually leading to a blackout. Another
serious disadvantage of present-day inverter-fed renewable
energy plants is that the inverter is forced to operate at unity
power factor, which means that these plants cannot supply
reactive power to the microgrid. This form of operation may
lead to low voltages during peak load conditions, and could, in
fact, lead to a voltage collapse because of the lack of dynamic
reactive reserves.
The renewable energy resources are incapable of
responding to frequency and voltage changes on the
distribution system. For example, the present preferred method
of operation of a PVP is to track the maximum power point at
unity power factor [2-3]. At high penetrations, this type of
control can be detrimental to overall system operations. In
order to have greater control over distributed resources in a
microgrid, a communication-enhanced inverter-based control
scheme will be developed for both active (P) and reactive (Q)
power control. Active and reactive power modulation are only
now beginning to be discussed in the literature [4-6]. Such
inverter-based controls will become critical as the penetration
of renewable resources continues to grow.
Some solutions for correcting these two serious
drawbacks of renewable energy plants – namely the inability to
respond to the dynamic nature of the microgrid’s frequency as
well as the inability to regulate microgrid voltage are presented
in this paper.
2. MICROGRIDS
Microgrids are autonomous small-scale power grids at the
lower distribution voltage levels consisting of interconnected
loads and DER [7-9]. As an integrated system, a microgrid is
capable of operating in parallel with the grid or in an islanded
mode, thus providing higher reliability and flexibility of
operation. Although some features of the microgrid exist in the
present state-of-the-art, the overall concept of the microgrid is
still far from being widely adopted. Nevertheless, it is serving
as a catalyst for the development and deployment of many
DERs [10-15]. To provide uniformity, the interconnection and
operation of distributed generation, including PV and wind
plants on the grid is governed by IEEE 1547 standards [12-13].
A microgrid can provide immunity from system-wide
disturbances as long as certain operating conditions are
satisfied. In the past, there has been a general reluctance to
using renewable energy-based generation in an energy-limited
grid, such as a microgrid. Most devices within RENDER, such
as solar photovoltaic plants, or wind power plants, or fuel cell
1
plants, have to be connected to the grid through inverters; since
inverter-fed energy sources lack adequate short circuit capacity,
they cannot supply fault currents during system disturbances.
Consequently, these devices are forced to trip at the first sign of
a system problem, unless low- or high-voltage ride through
capabilities are provided for these inverter. More and more
inverters are now appearing in the market equipped with this
capability [4-5, 16-17].
is the tip speed ratio (TSR)
is the pitch angle of the wind turbine.
c1-c6 constants are provided by the manufacturer.
Also, Cp is called the power transmission coefficient.
Some of the more complex technical challenges facing
the operation of a microgrid in the presence of renewable
resources may be summarized as follows:
The different variations of power level can be seen in Fig.
1 with varying pitch angle and rotor speed.
1
1
0.035
=
− 3
λi λ + 0.08* β β + 1
(3)
λ
β
• Surviving fault-induced transients.
• Providing adequate dynamic damping to ensure
oscillatory stability.
• Providing adequate voltage and frequency regulation.
• Ensuring transient/voltage stability of the microgrid.
• Providing communication among entities within the
microgrid.
• Assessing the dynamic state of the microgrid in real
time.
3. WIND POWER PLANT (WPP)
There are two major classifications amongst wind generation
units: fixed speed generation and variable speed generation.
The fixed speed generators have a design speed for which they
have maximum efficiency whereas for other speeds their
efficiency is lower. Variable speed generators have the
maximum power tracking capability that extracts maximum
available power out of the wind at different speeds thereby
resulting in more efficient operation. Also the variable speed
generators reduce mechanical stresses on the turbine thus
increasing the lifetime of the turbine. Thus variable speed
generators are more commonly installed.
When designing a wind turbine the amount of power
generated by the turbine can be associated with the torque
generated by the wind. The relationship between the torque and
the wind can be formulated as below.
3
1 ρ * A*v *Cp
Twind = *
ωr
2
(1)
and,
C p = c1 *(
λi
Amongst the variable speed generators, there are two
major kinds, synchronous generators with direct power
electronic converters and doubly fed induction generators with
rotor side power electronic converters. Both have the above
mentioned advantages of variable speed generators but the
power electronic ratings of the two machines are different. A
doubly fed induction generator (DFIG), as shown in Fig. 2,
allows us to use converters of partial rating instead of
conventionally used full scale converters. The power
electronics used in this topology are rated around 30-40% of
the total rating of the system. Usually, the slip varies between
40% at sub-synchronous speed and -30% at super-synchronous
speed.
2.1.P-Q Capability
Where,
ρ is the density of air.
A is the rotor area.
V is the wind velocity.
ωr is the rotor speed.
c2
Fig.1. Variation of power with respect to the pitch angle and
wind turbine speed. (wind speeds 10m/s and 12 m/s).
− c5
− c3 * β − c4 ) * e λi + c6 * λ
(2)
The capability of a DFIG presents similarities to the
conventional synchronous generator capability. Reactive power
capability depends on three factors, active-power generated, the
slip and the limitations due to stator and rotor maximumcurrents as well as the maximum rotor voltage. In order to
understand the power capability curve, the following circuital
relationships are obtained from the equivalent circuit of DFIG
as discussed in [18]. The active power varies with the wind
speed and the slip is assumed to be constant here. Other three
limitations can be formulated as discussed below.
2
Fig.2. Schematic for DFIG used in WPP [19]
(a) Stator current limitation
This limit takes into account the stator heating due to the
stator winding’s Joule losses. The PQ curve depicting stator
current limitation is straight forward. The curve will form a
circle with a center at 0 and a radius of the product of the stator
current and the stator voltage as described in equations below:
Center = 0
(4)
Radius = I s * Vs
(5)
(b) Rotor current limitation
This limit takes into account the rotor heating due to the
rotor winding’s Joule losses. The rotor current limitation in PQ
diagram of a DFIG is derived by assuming a rotor current with
the rated magnitude and a variable angle relative to the stator
voltage. The formulated power equation with a given rotor
current and a given stator voltage is free of slip. The angle is
varied with a fixed rotor current magnitude which forms a
circle in the complex plane with the following center and radius
equation.
Center = − Vs
2
 1 
*

 Zs + Zm 
*
Fig.3. Three limitation of a DFIG model in transferring active
and reactive power.
Fig. 3 displays the effects of these limitations on the PQ
capability of the system. The blue line in the figure constitutes
the stator current limitations on the DFIG. The red line displays
the rotor current limitations and the green lines represents the
limitations due to rotor voltage. Because of these limitations
DFIG can operate only in the overlapping region of all the tree
limitations. This region lies between red upper curve and blue
lower curve.
Fig. 4 displays the variation of capability according to the
wind speed. In this figure, the DFIG machine operating region
after taking care of the three limitations is displayed for wind
speeds ranging from 9m/s to 14 m/s. As observed, the DFIG
capability of providing active and reactive power decreases
with decrease in the wind speed. Also, DFIG machine
operating region max outs at 12 m/s and the pitch angle of the
wind turbine is to be controlled above this wind speed to avoid
any damage to the system.
Wind Turbine Capability Curves for different wind speeds.
1.5
(6)
9 m/s
10 m/s
1
11 m/s
12 m/s
(7)
(c) Rotor voltage limitation.
The rotor voltage limitation is essential for the rotor speed
interval, because the required rotor voltage to provide a certain
field is directly proportional to the slip. Thus, the possible rotor
speed is limited by the possible rotor voltage.

Zr + Zm
2 
Center = − Vs * 

 (Zr + Z s ) * Zm + Z s * Zr 
Radius =


Zm
Vr
* Vs * 

s
 (Zr + Z s ) * Zm + Z s * Zr 
Reactive Power (p.u.)
 Zm 
Radius = I r * Vs * 

 Zm + Zs 
0.5
13 m/s
14 m/s
0
DFIG
-0.5
-1
(8)
-1.5
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Active Power (p.u.)
(9)
Fig.4. Variation of DFIG capability according to different wind
speeds. (Bold red line shows the maximum DFIG capability).
3
1
Fig. 5. (a) PV-Inverter integrated system capability (b) Inverter Efficiencies [26].
3. PHOTOVOLTAIC (PV) POWER PLANTS (PVP)
Photovoltaic (PV) power plants are only capable of providing
active power because of its’ DC type system build. So, PV
systems generally rely on inverters to provide reactive power
support.
Inverters can be used to define the phase angle of the
current going to the mains grid, hence can be regulated as
required. One fundamental limitation of inverters is the
maximum current carrying capacity. If an inverter is to be
designed for providing additional reactive power support, it
needs to be oversized to accommodate the increased amount of
current in the system. Because of the introduction of reactive
power, we then consider apparent power with zero power factor
as the input (instead of active power) to fulfill the law of
conservation of energy and a combination of active and reactive
power at the output [24]. Also, PV faces one more limitation
due to intermittent availability of raw power and the use of an
efficient controller.
Within these limitations, the PV-inverter integrated system can
be controlled with a response time in the order of milliseconds.
Figure 5 shows the capability of a PV system and general
efficiency of different inverters [26]. This figure displays the
operating region of a PV-inverter integrated (PV) system. PV
system can operate efficiently in the yellow region with very
low response time. When the input power to the inverter drops
below 5%-15% of the rated power (Figure 5, part b), the
inverter shuts down because of unavailability of sufficient
power to drive itself. Also, according to the manufacturer’s
ratings, the inverter can be overloaded for a short period of
time.
4. SIMULATION
4.1. Circuit Description
A 1.5 MW wind turbine connected to a 13.2 kV, 6-bus test
distribution system exports power to the grid through a 2 km,
13.2 kV feeder line, as shown in Fig. 6.
Wind turbines using a DFIG, consist of a wound rotor
induction generator and an AC/DC/AC IGBT-based PWM
converter modeled by voltage sources. The stator winding is
connected directly to the 60 Hz grid while the rotor is fed at
variable frequency through the AC/DC/AC converter. The
DFIG technology allows extracting maximum energy from the
wind for low wind speeds by optimizing the turbine speed,
while minimizing mechanical stresses on the turbine during
gusts of wind. In this type of model, the IGBT voltage-sourced
converters (VSC) are represented by equivalent voltage sources
generating the AC voltage averaged over one cycle of the
switching frequency. This model does not represent harmonics,
but the dynamics resulting from control system and power
system interaction is preserved.
Figure 6. Simulation model of Microgrid.
4.2. Direct Power Control
By controlling the state variables like rotor current, rotor speed
to achieve active and reactive power control, direct power
control scheme is implemented in this system. The simulation
results display that the wind power system under observation,
follows the commanded active/reactive power, as shown in Fig.
7. This simulation shows the time taken by the WPP to respond.
First sub plot is displaying the commanded active power in p.u.
from the WPP, whereas the second subplot shows commanded
reactive power in p.u. . Third and fourth subplot shows the
active and reactive power response respectively according to
the commanded value.
4
Fig.7. WPP response on commanded requirement of active and
reactive power.
4.3. Wind power system in Microgrid dynamics
The wind power plant is now connected to the microgrid
network to simulate it under load. This system is tested first
with the wind power plant connected as a source, not
participating in frequency or voltage regulation. The system is
tested for three-phase fault conditions near bus 6 and bus 3. The
voltage – frequency responses on each bus can be seen in
Figure 8. We see that, if a fault is away from the slack bus (Bus
1), system attains stability after some time. But if the fault is
near the slack bus, the system becomes unstable. This unstable
operation can be avoided if the WPP is allowed to participate in
frequency regulation by varying its’ active/reactive power
according to the changes in frequency and voltage profile.
(a) Voltages and Frequency response for fault near Bus 6.
5. FUTURE WORK AND CONCLUSIONS
Capability and limitations of wind and photovoltaic power
plants were discussed and a microgrid model was simulated to
satisfy the claims.
In the next piece of work, a controller for the wind power
plant would be designed which would autonomously control
the active and reactive power output coming from the WPP
according to the frequency of the system and the voltage profile
of the bus connecting the WPP to the grid. This would show
that a WPP, supporting a microgrid increases the stability. Also,
a DPC enabled photovoltaic (PV) source would be incorporated
in the system to observe the combined dynamics.
6. ACKNOWLEDGMENTS
The authors gratefully acknowledge the support of Intelligent
Systems Center for production of this work.
(b) Voltages and Frequency response for fault near Bus 3.
Fig.8 Fault on the microgrid with a clearing time of 0.27 Sec
7. APPENDIX
Vs
Is
Vr
Ir
Zm
Zr
Zs
5
DFIG Stator Voltage.
DFIG Stator Current.
DFIG Rotor Voltage.
DFIG Rotor Current.
Mutual Impedance between stator and rotor.
Rotor Impedance.
Stator Impedance.
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