This document summarizes research on minimizing the impact of resonances in low voltage grids. The research was conducted as part of a larger KTI project involving three parts: studying grid characteristics and interactions; measuring impedances; and developing power electronics. Laboratory experiments showed that virtual techniques using inverters, such as virtual parallel capacitance reduction and virtual resistive harmonic damping, can reduce resonances by virtually reducing grid capacitances and damping oscillations. Large scale computer simulations confirmed these techniques are effective when implemented in many inverters connected throughout a low voltage distribution system. The document concludes these ancillary services provided by inverters have the potential to minimize resonances if implemented as part of the inverters' control systems.

1. Minimizing the Impact of Resonances in
Low Voltage Grids
Peter Heskes January 2011
The EOS LT project KTI is funded in part by SenterNovem
Contents
The KTI project
Grid impedance, interaction and resonances
Impedance measurement
Minimizing the impact of resonances
1

2. The KTI project
Part 1: Research on new boundary conditions, social importance, consequences and
responsibilities.
(TU/e – EES group, Laborelec)
Part 2: Research on characteristics and Part 3: Research on and development
interactions between the grid and of new power electronics to control the
connected appliances and generators quality of the voltage
(ECN) (TU/e – EPE group)
Structure of the KTI project
2

3. Publications
Journal articles
[1] P.J.M. Heskes, J.M.A. Myrzik, W.L. Kling, “Impact of Distribution System’s Non-Linear
Loads with Constant Power on Grid Voltage”, John Wiley ETEP Journal, in-Press.
[2] P.J.M. Heskes, J.M.A. Myrzik, W.L. Kling, “A Harmonic Impedance Measurement System
for Reduction of Harmonics in the Electricity Grid”, International Journal of Distributed
Energy Resources, vol. 5, no. 4, pp. 315-331, Oct./Dec. 2009.
Conference papers
[1] P.J.M. Heskes, J.M.A. Myrzik, W.L. Kling, “Ancillary Services for Minimizing the Impact of
Resonances in Low Voltage Grids by Power Electronics based Distributed Generators”, IEEE
Power Energy Systems General Meeting, Detroit, Michigan, USA, July 24-29, 2011, under review.
[2] P.J.M. Heskes, J.M.A. Myrzik, W.L. Kling, “Harmonic Distortion and Oscillatory Voltages and the
Role of Negative Impedance”, IEEE Power Energy Systems General Meeting, Minneapolis, USA,
July 25-29, 2010.
[3] P.J.M. Heskes, J.M.A. Myrzik, W.L. Kling, “Power Electronic Loads with Negative Differential
Impedance in a Low Voltage Distribution System”, 20th International Conference on Electricity
Distribution, Cired, Prague, 8-11 June 2009.
[5] P.J.M. Heskes, J.M.A. Myrzik, W.L. Kling, “Survey of Harmonic Reduction Techniques
Applicable as Ancillary Service of Dispersed Generators (DG)”, IEEE Young Researchers
Symposium, Technical University of Eindhoven, The Netherlands, February 7-8, 2008.
Grid impedance, interaction and resonances
3

4. Interaction
Grid Operator Point Of Connection Customer
(POC)
Supply voltage Draw current
at from
Interaction
POC POC
via
Grid impedance
Requirements for Requirements on
quality of voltage loads for quality of
(EN 50160) Current
(IEC 61000-3-2)
7 31-1-2011
Interaction
Example of a LV distribution grid, two PCC and one PoC examples are depicted
4

5. Interaction
Simplified grid model with a lumped large number of
resistive loads and parallel capacitances
Interaction
The grid loaded with a non-linear load
5

6. Interaction
Added damping resistance in the distribution grid
Resonances
VPCR = Virtual Parallel Capacitance Reduction
VRHD = Virtual Resistive Harmonic Damping
VPCR VRHD VPCR + VRHD
active active active
6

7. Harmonic impedance measurement
Harmonic Impedance Measurement
The grid voltage is strongly
Injection Current
polluted
signal injection Zgrid
Line
Controlled
current source
12V 230V PoC Zload Grid simulator
Neutral
Injection current for a 50.0Hz grid frequency
I V
Sampled
Data
PC with Matlab A/D
software conversion
Impedance measurement in a laboratory set-up
The injection current stimulus
7

8. Harmonic Impedance Measurement Start
Measure Voltage
Shift Measure Voltage
Magnitude
Measurement (and Current)
Spectrum without
frequencies time series
Stimulus
The system:
• estimates free spots in freq. domain Voltage Transformation to
No
• inject current on free spots Magnitude frequency domain
Spectrum
• collect voltage / current time series below limit?
• does domain transformation Calculate
Impedance
• calculates the impedance spectrum Inject a Current
Spectrum
signal to the grid
Ancillary services for harmonic reduction
8

9. VPCR and VHRD
VPCR = Virtual Parallel Capacitance Reduction
VRHD = Virtual Resistive Harmonic Damping
The inverter’s basic block diagram with focus on the grid interfacing part
Time domain lab experiments
Experimental result of vgrid polluted with
Laboratory set-up 10% of the 11th harmonic
9

10. Inverter model
Time domain lab experiments + Output filter Grid model
model
iinv Lout igrid Lgrid Rgrid
DC
Power H-bridge voltage
supply Cout vgrid source
model
model
fundamental with 10%
- of 11th harmonic
2
1 H-Bridge
driver model
iinv (t)
0
Controller
model
-1
-2
1 1.02 1.04 1.06 1.08 1.1
2
inverter currents
1
Igrid (t)
0
VPCR = off
VRHD = off
-1
-2
1 1.02 1.04 1.06 1.08 1.1
time (seconds)
Experimental result of iinv and igrid without activated ancillary services
Inverter model
Time domain lab experiments + Output filter Grid model
model
iinv Lout igrid Lgrid Rgrid
DC
Power H-bridge voltage
supply Cout vgrid source
model
model
fundamental with 10%
- of 11th harmonic
2
1 H-Bridge
driver model
0
iinv (t)
Controller
model
-1
-2
1 1.02 1.04 1.06 1.08 1.1
2
inverter currents
1
Igrid (t)
0
VPCR = on
VRHD = off
-1
-2
1 1.02 1.04 1.06 1.08 1.1
time (seconds)
Experimental result of iinv and igrid with activated VPCR
10

11. Inverter model
Time domain lab experiments + Output filter Grid model
model
iinv Lout igrid Lgrid Rgrid
DC
Power H-bridge voltage
supply Cout vgrid source
model
model
fundamental with 10%
- of 11th harmonic
2
1 H-Bridge
driver model
iinv (t)
0
Controller
model
-1
-2
1 1.02 1.04 1.06 1.08 1.1
2
inverter currents
1
Igrid (t)
0
VPCR = on
VRHD = on
-1
-2
1 1.02 1.04 1.06 1.08 1.1
time (seconds)
Experimental result of iinv and igrid with activated VPCR and VRHD
Large scale model
11

12. Large scale computer simulations with the validated inverter model
Goal: estimate the total grid impedance at the LV Busbar
5 streets with 10 inverters each on line 1
A customer 1 customer 2 customer 9 customer 10
10m 10m 10m
Cable 50 Al
customer 11 customer 12 customer 19 customer 20
10m 10m 10m
Cable 50 Al
customer 21 customer 22 customer 29 customer 30
A
10m 10m 10m
Cable 50 Al
customer 31 customer 32 customer 39 customer 40
10m 10m 10m
Cable 50 Al
customer 41 customer 42 customer 49 customer 50
10m 10m 10m
Cable 50 Al
Cap. load
Zhome
Single Home connection
Inverter
12

13. The ancillary service inverter controls the (50Hz) and the 3 th, 5th,
7th, 9th and 11th harmonic.
3 5 7 9 11
Inverter’s controller block with resonators on the fundamental
(50Hz) and the 3th, 5th, 7th, 9th and 11th harmonic.
Large scale computer simulations
9
7 11
5
3
100 inverters + 100 capacitive
loads connected
no ancillary services active
Harmonic impedance measured at the substation busbar with
100 inverters as well as 100 capacitive loads connected.
13

14. Large scale computer simulations
11
9
7
3 5
100 inverters + 100 capacitive
loads connected
limited VPCR active
Harmonic impedance measured at the substation busbar with
100 inverters as well as 100 capacitive loads connected.
Large scale computer simulations
100 inverters + 100 capacitive
loads connected
full VPCR active
Harmonic impedance measured at the substation busbar with
100 inverters as well as 100 capacitive loads connected.
14

15. Large scale computer simulations
100 inverters + 100 capacitive
loads connected
both VPCR and VRHD active
Harmonic impedance measured at the substation busbar with
100 inverters as well as 100 capacitive loads connected.
Conclusions
Based on study, simulations and laboratory measurements:
• VPCR virtually reduces capacitances that can bring resonances,
• VPCR + VRHD virtually reduces capacitances and damp resonances.
These two ancillary services can be implemented in power electronics
based inverters for DG. The actual working range depends on the
performance of the control system.
Author Name-Country-SessionX- 30
P.J.M. Heskes Netherlands Session 2 Paper ID 0549 ID
BlockY-Paper
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16. Thank you for your kind
attention
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