1. Microgrid Unbalance Compensator –
Mitigating the negative effects of
unbalanced microgrid operation
Sung Pil Oe, Prof. Mark Sumner,
Prof. Mark C. Johnson
University of Nottingham, UK
3. Background
• Connection of large number of micro-generators and new loads
(heat pumps, EV/PHEVs) on the distribution network presents a
number of technical challenges to DNOs.
• A new approach to the way the traditionally passive distribution
system is designed, managed and operated is required.
Microgrid:
• Small-scale semi-autonomous LV distribution network consisting of
loads, micro-generation and energy storage units designed to meet the
electrical and heat demands of the energy cell it serves.
• Connected to the LV distribution network at the PCC downstream of the
distribution substation transformer.
• Introduced to facilitate the integration of large numbers of micro-
generation, distributed storage and active loads in the LV network.
• Allows intelligent coordination of loads, generation and storage to make
it appear to the utility as a single controllable entity minimising negative
impacts to the wider network.
4. Motivation
Motivation
• Mitigate the negative effects of unbalanced operation propagating to the
wider electricity network, increase utilization of existing network assets
and reduce losses.
• Facilitate the connection of a large number of synchronous generator-
based micro-generation units by limiting the technical problems
associated with its unbalanced operation
Unbalanced Operation
VUF exceeds
1.3%
5. Microgrid Unbalance Compensator
What is a Microgrid Unbalance Compensator (MUC)?
Shunt connected three-phase four-leg voltage source converter (VSC)
designed to detect the unbalanced three-phase load current and
perform unbalanced compensation.
Main Components:
• Four-leg PWM IGBT-
based VSC with LCL filter
• Voltage/Current sensors
• Advanced control system
6. Microgrid Unbalance Compensator
Advanced Control System
Main Components:
• DSOGI-PLL (positive
sequence detector)
• DC Link Control
(PI controller)
• Proportional Resonant (PR)
current controller +
Harmonic Compensator (HC)
• Compensation reference
current generator
(p-q Theory)
• Sine-PWM signal generator
8. Unbalance Compensation Strategy
• Two main compensation strategies based on the p-q Theory have
been identified :
1. “Constant Source Power (CSP)” Strategy
• Ensure constant power under unbalanced operating conditions
• Aims to provide optimal power flow to the synchronous
generator-based micro-generator
2. “Sinusoidal Source Current (SSC)” Strategy
• Ensure sinusoidal and balanced source currents even under
unbalanced load and unbalanced and distorted source voltage
conditions
9. Prototype MUC
• 15 kW Triphase PM15F42C power
module
• All controllers implemented in
Matlab/Simulink
• All data logged through Simulink
10. Experimental Setup: CSP Strategy
Balanced
(Ω)
Unbalanced
(Ω)
Phase A 31.6 15.7
Phase B 31.4 31.4
Phase C 31.2 31.2
Unbalanced microgrid load
• Percentage Voltage Unbalance Factor
(%VUF) is used as a measure of the 3-
phase terminal voltage unbalance
%𝑉𝑈𝐹 =
𝑛𝑒𝑔𝑎𝑡𝑖𝑣𝑒 𝑠𝑒𝑞𝑢𝑒𝑛𝑐𝑒 𝑉 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡
𝑝𝑜𝑠𝑖𝑡𝑖𝑣𝑒 𝑠𝑒𝑞𝑢𝑒𝑛𝑐𝑒 𝑉 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡
× 100
11. Experimental Results
CSP Strategy:
• Oscillating power component has been significantly reduced (≈ 87%)
reduction).
910𝑊𝑝−𝑝
3900𝑊𝑝−𝑝
1300𝑊𝑝−𝑝
• Neutral current is
effectively compensated
• %VUF decreases
(2.7% 0.2%)
Key Result: MUC is able
to reduce the oscillating
active power component
thus reducing the wear
and tear of the SG.
12. Experimental Setup SSC Strategy
Test 1
A programmable AC source is
used to generate a three-phase
unbalanced and distorted voltage
Resistance (Ω) Inductance (mH)
Phase A 227.2 15
Phase B 15.7 30
Phase C 11.8 15
<Unbalanced Load>
13. Experimental Results Test 1
• After activating the MUC, three-phase currents are balanced and in-
phase with the voltage and neutral current is effectively compensated.
Before
Compensation
After
Compensation
isa 2.32 A
14.07 A
isb 19.41 A
13.77 A
isc 21.65 A
13.49 A
<Compensation Result>
Key Result: MUC is able
to perform unbalance
compensation even
under highly unbalanced
and distorted operating
conditions
15. Experimental Results Test 2
Phase
Before
Compensation
After
Compensation
A -8.04 A 4.52 A
B 10.16 A 4.13 A
C 9.69 A 4.26 A
2.1 kW3.3 kW
<Compensation Result>
• The total three-phase power imported from the grid is reduced which
effectively means that the excess power generated by the single-phase
micro-generator is recirculated to the other two phases
Test 2 Scenario:
• Unbalance caused by a single-phase micro-generator connected to phase A .
• The loads on the other two phases are made equal (36Ω + 15mH).
• The excess power generated by the micro-generator on phase A (1.6kW) is less
than the combined power (≈3.3 kW) required by the loads on phase B & C.
16. Experimental Results
SSC Strategy Test 3
Test 3 Scenario: The excess power generated by the micro-generator on phase A
(3 kW) is now greater than the combined d load demand (≈ 400 W) of the other two
phases. Simulates the case of low demand and excess PV generation.
Before
Compensation
After
Compensation
Phase A -17.87 -4.34
Phase B 2.00 -4.78
Phase C 1.98 -4.73
<Compensation Result>
Key Result: MUC allows
the surplus power
generated within the
microgrid to be exported
in a balanced manner
17. Experimental Setup SSC Strategy
Test 4 With Community Energy Storage
Prior to activating the battery charger :
• Phase A is exporting approximately 3 kW (single-phase generation)
• Phase B is importing approximately 340 W (resistive load)
• Phase C is importing approximately 340 W (resistive load)
18. Experimental Results
SSC Strategy Test 4
Test 4 Result :
After activating the battery charger to charge 2 kW (without MUC controller enabled):
• Phase A is exporting approximately 2.3 kW (3kW → 2.3kW)
• Phase B is importing approximately 1 kW (340W → 1 kW)
• Phase C is importing approximately 1 kW (340W → 1 kW)
19. Experimental Results
SSC Strategy Test 4 Continued
Test 4 :
After activating the MUC controller:
• The three-phase power imported from the grid is nearly 0 kW
• Power generated from phase A micro-generator is redistributed to the other two phases
to meet the demand and the excess power is charged to the battery in a balanced way.
20. Power Converter Control Design
Consideration
• Which feedback signal is used for control?
• Cheaper inverters try to cut costs by not having an extra current
sensor at the grid-side.
• Need to consider the phase shift introduced by the LCL filter capcitor
to ensure unity power factor. (Typically sized 5% of rated power)
• Possible cause of increasing reactive power at DNO level?
21. Conclusion
• MUC is able to ensure the microgrid behaves as a “good citizen” from the
view point of the distribution network operator (DNO) even under
unbalanced operating conditions.
• It has been demonstrated that the MUC allows the recirculation of power,
thus allowing other customers to benefit from excess generation
originating from another phase within the microgrid. Also able to export
balanced power back to the grid or charge a battery storage.
• MUC will benefit the DNO by reducing losses and increasing utilization of
the existing distribution network assets thus deferring unnecessary
network reinforcements, especially beneficial for urban areas (smart
buildings, smart offices, etc.)
• MUC is able to reduce the %VUF at the PCC thus allowing increased
connection of micro-generation units.
• The MUC will benefit the microgrid owners by maximizing self
consumption, protecting synchronous-generator based micro-generators
and compensation of reactive power.
