1. Analysis and Mitigation of Harmonic Currents due to Clustered Distributed Generation on the Low Voltage Network
Authors: Ehab Shoubaki, Somasundaram Essakiappan, Johan Enslin
Presenter: Dr. Ehab Shoubaki, Ph.D
Postdoctoral Fellow,
Energy Production and Infrastructure Center (EPIC)
UNC Charlotte, NC, USA
EPIC.UNCC.EDU
The authors wish to acknowledge and thank Duke Energy for their financial support to this project.

2. Limitations of Grid Connected Inverter certification:
Industry Reference standards:
“IEEE Standard for Conformance Test Procedures for Equipment Interconnecting Distributed Resources with Electric Power Systems”, IEEE
“Inverters, Converters, Controllers and Interconnection System Equipment for Use with Distributed Energy Resources”, UL 1741
Partially concerned with limiting Harmonic currents injected into the grid. Tested with a stiff, pure sinewave AC source.
Analogy: An inverted pendulum control apparatus need not be tested on a movable (non-stiff) table. Common sense ?
In reality: Myriad resonance modalities will manifest between DG PV inverters and the grid which is neither stiff or a pure sinewave. But cost pressures push industry to design “for the tests”. Therefor problems tend to become more apparent as clustering in close proximity occurs.

3. Dynamic Model of standard two stage inverter with reservoir DC capacitance:
Voltage at point of common coupling
Output Filter Inductance
DC-Link Voltage
Output current Sensing
Controller
Circuit model
High Fidelity Reference current

4. Essential Dynamics of VAR capable two stage grid tied PV inverter.
Full-bridge on one side of output filter inductance. Modulates DC-Link voltage into an AC waveform.
Grid (with variable stiffness) on other side.
Controller actuates SPWM duty cycle to balance required output current in output inductor.
Output inductor tends to “run-away”. So fast high bandwidth OCR loop required to steer current.
But BW cannot be infinite , and dwindles at higher harmonics. i.e. Controller cannot catch up with preexisting high frequency distortion on the Grid voltage.
Grid Voltage feedforward is straightforward solution, but cannot be perfect due to cost and/or technological limitations at higher harmonics.

5. Lumped Circuit Model of a Single Inverter (Including Feedforward)
Point of common coupling feed forwad
Output Filter Capacitance
Inverter current controller impedance

6. Output impedance of single and clustered DG’s
With Feed Forward
Limited BW of voltage sensing
Single DG
Clustered DG’s
(up to 5000 on a single LV network)
Possibility of Hick-up Scenario

7. What about Hick-ups due to small signal instability ?
Stability very much depends on Magnitude of Zline/Ztotal
Nyquist Diagram for 800 DGs , unstable

8. Voltage and Current divider effect
Steady state oscillations due to stiff distortion from the primary circuit and/or current reference
From Grid center to PCC
Voltage and Current divider effect
Total Admittance

9. Mitigation ~ What can be done ?
Provide extra damping (Stability):
Reduce Steady State oscillations (Harmonics):
Virtual RC damping branch.
Virtual negative capacitance (naturally only manifests within bandwidth of OCR loop). NOTE : Too much can cause instability.
General guidelines:
Applied negative capacitance should only compensate for the output filter capacitance.
Implement virtual RC branch with corner frequency above line but within the OCR bandwidth.
Goal is to load harmonic currents AND diminish oscillations.

10. Validation (Simulation: MATLAB)
24 DG’s, No mitigation
Negative capacitance allows for 177 DG’s
Negative Capacitance and damping : 708 DG’s

11. Conclusions
Certification with a Stiff Sinewave source not adequate for high penetration scenarios.
Disturbance and resonance from/with grid cannot be cost effectively canceled at high frequencies through feedforward voltage.
Analysis suggests some mitigation techniques to increase stability and reduce harmonic distortion with increased clustering.
Microgeneration on the Low voltage network is becoming ever so popular and the need for detailed dynamic analysis is ever present.