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Impacts of Distributed Generation on Power Quality

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Impacts of Distributed Generation on Power Quality

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Impacts of Distributed Generation on Power Quality

  1. 1. Course : POWER QUALITY AND MANAGEMENT Prepared by : Shivam Impacts of Distributed Generation on Power Quality
  2. 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. 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. 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. 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. 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. 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. 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. 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.
  10. 10. 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.
  11. 11. 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
  12. 12. 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.
  13. 13. 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.
  14. 14. • 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
  15. 15. • 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.
  16. 16. 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.
  17. 17. Voltage profiles (case 2 vs case 1)
  18. 18. Case 3 : One Solar & one Wind Connected Substation current (case 3) Frequency components of substation current
  19. 19. Current injected at bus Bu204 Frequency components of current at bus Bu204
  20. 20. Voltage profiles (case 3 vs case 1)
  21. 21. 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
  22. 22. 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
  23. 23. 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
  24. 24. Acknowledgment Research Topic : Impacts of Distributed Generation on Power Quality Research Paper :

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