Impacts of Distributed Generation on Power Quality

1. Course : POWER QUALITY AND MANAGEMENT
Prepared by :
Shivam
Impacts of Distributed Generation on Power Quality

2. Introduction
• This paper This paper studies the impacts on power quality due to the
interconnection of multiple distributed generators on a distribution utility feeder.
• “Impacts of Distributed Generation on Power Quality“ : e main purpose of this
paper is to discuss the basic understanding of power quality in relation to the
distributed generation.
• Different scenarios were implemented in which solar and wind plants were
modeled with high variability of load and generation to observe their impacts on
system’s power quality.
• All the modeling and simulations were carried out using a high fidelity
electromagnetic real-time transient simulation tool.

3. About the Research Paper -
• An EMTP simulation platform is chosen to do this work.
• All simulations were carried out in the real-time transient simulation
tool RSCAD/RTDS. This project started with the modeling of a Florida-
based distribution feeder on which circuit reduction methods have
been applied , to reduce the model from thousands of nodes to a
seven bus model suitable for simulation in RSCAD/RTDS.
• Then solar and wind energy models that are provided in RSCAD have
been modified to fit the ratings and requirements pursued. Multiple
cases and scenarios were modeled and simulated using real data and
results were monitored at different nodes.

4. Distributed generation
• Distributed generation in simple term can be defined as a small-
scale generation. It is active power generating unit that is connected
at distribution level.
• Electric Power Research Institute (EPRI) defines distributed
generation as generation from a few kilowatts up to 50 MW.
• International Energy Agency (IEA) defines DG as “Power generation
equipment and system used generally at distribution levels and
where the power is mainly used locally on site”

5. Power Quality
• Power quality refers to a wide variety of electromagnetic phenomena
that characterize the voltage and current at a given location on the
power system .
• The power quality study in general deals with harmonic injections,
voltage fluctuations, voltage sag/swell, flicker, impact of low
frequency anti islanding signal injections and many other phenomena
that accompany the integration of DG in a distribution system.

6. Power Quality Issue & Significance of this work
• A major issue related to interconnection of distributed resources onto the power
grid is the potential impacts on the quality of power provided to other customers
connected to the grid.
• The significance of this work relates to the increase in DG penetration level on
power grids in the form of localized generation in the distribution side.
• Since most of the DG units that are connected to the grid are inverter connected
(even wind generations are now a days connected through inverters), there is a
potential threat to the grid from high frequency harmonics coming out of these
power electronic equipment.
• Although those harmonics are mostly localized and get filtered out by the
inductances from the transformers and lines, still their impacts on the network,
especially nearby magnetic circuits.

7. • Over-voltages due to reverse power flow: If the downstream DG output exceeds the
downstream feeder load, there is an increase in feeder voltage with increasing distance. I
• The figure illustrates one voltage regulation problem that can arise when the total DG
capacity on a feeder becomes significant. This problem is a consequence of the
requirement to disconnect all DG when a fault occurs.
• Figure shows the voltage profile along the feeder prior to the fault occurring. The intent of
the voltage regulation scheme is to keep the voltage magnitude between the two limits
shown.
• This is why it is crucial to understand the potential impacts on power quality and
investigate whether there are substantial evidence of power quality problems that may
require new solutions and additional infrastructure.

8. Modeling And Simulation
• The modeling and simulations of the distribution network and the
DGs was done in RSCAD/RTDS which is capable of solving system
equations in real-time.
• The time step size used for the different scenarios was 2μs which is
suitable for capturing the higher harmonics injected by the fast
switching power electronic converters of the wind and solar plants.
• The solar and wind energy models that are provided in RSCAD have
been modified to fit the ratings and requirements pursued.

9. Distribution System
• Parameters of the distributes system :
• Feeder – 12.47 kV ---- connected to 138kV substation ---- via 22 MVA,
138/12.47 kV Transformer
• Feeder SVR = +-10% regulation capability.
• 4 capacitor banks (Total = 3.3 MVAR)
• Average Loading of Feeder : 5MW
• Figure – 1 shows the detailed feeder diagram as provided by the utility
and Figure - 2 shows the reduced model diagram that was
implemented in RSCAD.

12. PV & Wind Plants Setup
• Two solar and one wind plants were used to implement different case studies and
scenarios.
• The first solar plant is a 2.25 MW plant and the second is a 0.35 MW. The wind
plant is rated at 2 MW.
• The grid VSC maintains constant capacitor voltage. It is current regulated, with
real component used to regulate the capacitor voltage, and the quadrature
component used to adjust terminal voltage.
• Wind Turbine and Multimass controls are to feather the turbine blades if the
turbine speed rises above 1 pu (1.2 pu for DFIG), the controller does nothing for
speeds less than 1 pu.

13. The Photovoltaic Model Setup
• The PV model consists of a
• PV array
• DC-DC converter
• three phase bridge inverter
• AC filter and a transformer
• The PV model as built in RSCAD. The DC-DC converter controls the DC link voltage
by controlling its duty cycle through a Maximum Power Point Tracking (MPPT)
algorithm. In this case, incremental conductance method is used to keep the
operating point of the PV at its maximum power.
• Then the three phase inverter transforms the DC voltage into AC voltage to connect
to the transformer and eventually to the grid

15. The Wind Turbine Model Setup
• The wind turbine system model is a doubly fed induction generator (DFIG) based
wind energy system with a back-to back converter connected to the grid from
one side and to the rotor from the other side.
• This is referred to as a type 3 wind energy system that is characterized by
bidirectional flow of energy between the grid and the rotor of the induction
machine.
• The wind turbine model involves much more sophisticated controls than the PV
model.

17. Case Studies: Results
Case 1 : No DG
Case 2 : Two Solar Plant Connected to feeder
Case 3 : One Solar & one Wind Connected
• For each case, different scenarios were chosen that were expected to have
some impacts on the system. For instance, in some cases, focus was on a very
fast increase in the solar irradiance and in another case it had a sharp
decrease.

18. • The currents at the substation have no harmonic components.
• The currents in the three phases are unbalanced since the loading of the three phases
is unbalanced.
Case 1 : No DG

19. • Table shows the THD as produced by the
simulation model for the currents in phase “a”
of each bus. In all buses, the THD is below the
limit set by standards.
• The Figure shows the voltage profiles
at buses Bu200 and Bu205 to show
how the voltage is being affected by
the change of load.

20. Case 2 : Two Solar Plant Connected to feeder
• Two solar plants (2.25 MW and 0.35 MW) were connected at buses Bu204
and Bu205, respectively. Fig. 11 and 12 show the waveform and frequency
spectrum of the substation currents, respectively.

22. Voltage profiles (case 2 vs case 1)

23. Case 3 : One Solar & one Wind Connected
Substation current (case 3) Frequency components of substation current

24. Current injected at bus Bu204
Frequency components of current at bus Bu204

25. Voltage profiles (case 3 vs case 1)

26. 1. Harmonics
• For case1 showed that there is only one fundamental frequency in the
current meaning that there are no harmonics in the system.
• In case 2, when the two PV plants are introduced, the currents injected
by the PV plants had some distortions and showed some harmonic
components especially of the 33th and 37th orders.
• In case 3, harmonics appear at the output current of the PV plant having
a slightly distorted current waveform.
• IEEE standard 1547 suggests that the current THD must not exceed 5 %, all
the THDs in these cases are well within the limit. The other fact is dominant
harmonics are of higher order which may not exceed the imposed THD limit
but their frequency ranges may be of concern for EMI related impacts.
Smart grid communication devices may get affected due to that.
Power Quality Concerns

27. 2. Voltage Fluctuations.
• In case 1 showed that the bus voltages changed as the loading changed. Even
though the feeder is equipped with voltage regulation devices (four capacitor
banks and one SVR), still at some loading conditions the voltage went either
above or below the +- 5 %
• In case 2, the voltage at bus Bu200 near the substation was not affected much
and the voltage change was insignificant. The system load was chosen to
exceed the limit which is a peak of up to 8 MW. So the capacitor banks were
fully working but still were not able to boost the voltage up.
• In case 3 shows that the voltage at the bus near the substation is stiff and
only some minor fluctuations appear. But the voltage at bus Bu205 that is
close to both the PV and the WT had voltage fluctuations of mean value of
0.05 p.u

28. 3. Flicker
• Flicker is an impression of unsteadiness of visual sensation caused by the
voltage fluctuation [11]. Flicker evaluation is done based on [12] that
describes the probability of flicker perceptibility based on instantaneous
flicker levels gathered over a period of 10 minutes.
• The model block used computes a cumulative probability function (CPF) and
then reduces it to a single value, short term probability.
• For MV systems the voltage fluctuation should be less than 3%. The values
found in the tested case were all below the 3% limit, therefore no flicker was
reported in the studied cases

29. Acknowledgment
Research Topic : Impacts of Distributed Generation on Power Quality
Research Paper :