This document discusses controlling parallel DC-DC converters in a DC microgrid using a virtual output impedance method. It begins with introductions to power converters and DC converters. It then discusses types of DC converters, their performance parameters, output impedance, and how output impedance mismatch can cause circulating currents. It proposes using a virtual output impedance control loop to compensate for differences in converter output voltages and reduce circulating currents. Simulation results show this method can decrease circulating currents to near zero while maintaining voltage regulation within allowed deviations.

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Power Electronic Systems & Chips Lab., NCTU, Taiwan
Control of Parallel DC-DC Converters in a DC Microgrid
Using Virtual Output Impedance method
電力電子系統與晶片實驗室
Power Electronic Systems & Chips Lab.
交通大學 • 電機控制工程研究所
台灣新竹‧交通大學‧電機控制工程研究所‧電力電子實驗室~鄒應嶼 教授
Sushil Kumar

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Outline
1. Introduction
2. Types of DC Converter
3. Performance parameter of DC converter
4. What is output impedance ?
5. Circulating Current and output Impedance
6. Microgrid
7. Control of DC-Dc converters by using virtual impedance
8. Droop control Startagy
9. How Virtual impedance work?
10. Simulation Study
11. Advantages and disadvantages of this scheme
12. Conclusion
13. Question Arises

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Introduction
Power Converter:-
For the control of electric power, power conditioning the
conversion of one form to another from is necessary that is
done by static power converters
DC Converter
 A DC converter is an electrical circuit which accepts a DC input
and generates a DC output of a different voltage.
 Usually achieved by high frequency switching action employing
inductive and capacitive filter elements
 DC converter is used for unregulated DC to regulated DC.
:

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Types of DC Converter
1. Buck Converter :- Voltage step down and current step up
2. Boost Converter:- Inverse of Buck, Voltage up ,current down
3. Buck Boost converter:- Output Voltage may be less then or
higher than input voltage, as per user requirement, hybrid of
Buck and boost both hence buck-boost.

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Performance parameters of DC-DC converters
There are many performance parameter whose take attention
while we design the converter. These are:-
1. Operating Frequency
2. Inductor Selection
3. Capacitor Selection
4. Switch component

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What is Output Impedance ?
• The output impedance is defined as the output voltage
response of a converter for the excitation of current (load
current, VL) at constant input voltage (Vin) and a specific
value of duty ratio D.
• Output impedance is mismatch due to unequal output voltage
or variation in current.
• It is very necessary that output impedance of output
converters should be same.

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Circulating Current and Output impedance
• The mismatch in the output voltages will increase the current
sharing difference and flow of circulating current.
• We can also say that output impedance mismatching is
responsible for circulating current or vice versa.
• The circulating current phenomena we can understand by this
table 1.
Where, Vdc1, Vdc2 are the output currents R1, R2 are cable resistances, I1 and I2 are
o/p currents and Ic12 and Ic21 are the Circulating currents of two converters
respectively.

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Microgrid
Microgrid :-
 An active power distribution network
 Capability of autonomous distribution
 Renewable energy sources connected with common bus
 Can say Local power station
 Work as a autonomous as well as tie with Grid
Types:-
 AC Microgrid
 DC Microgrid

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Control of DC-DC Converters by using Virtual Impedance
Method
Importance of Virtual Impedance :-
 Virtual impedances can be used to match the grid-side
impedance in AC microgrids
The Virtual Impedance can be programmed at the certain
frequency to form the expected harmonic impedance, which
enables the function of harmonic sharing and damping.
Virtual Impedances can be employed to realize the resonance
damping in the LCL filters. By using the additional current
sensors and multiple control loops, the resonance peaks
imposed by LCL filters can be attenuated.

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Continue…
The virtual impedance loop can either be embedded as an
additional degree of freedom for active stabilization and
disturbance rejection, or be employed as a command reference
generator for the converters to provide ancillary services.
connect a virtual impedance in parallel or series with the input
impedance of the load converter , the magnitude or phase of the
load converter’s input impedance is modified in a small range of
frequency, to solve the instability problem of a cascaded system

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Droop control strategy using virtual impedance
Three control levels:-
 Primary control:- Voltage and Freq. stabilization after islanding
Secondary Control:- check for any deviations caused by primary
Tertiary control:- Govern the power flow between Microgrid & main
Grid
Droop Control Method:-
It is method to share load between parallel connected sources
 In order to obtain proper load sharing we use decentralized droop
control method i.e. No communication between the sources hence
name decentralized.

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Droop control in DC Microgrid
• In the parallel DC-DC Converter, the output voltage of the
converter given a droop with increase in load power.
Fig.1. DC Microgrid Architecture
Fig.2. P-V Droop Characteristics
As P1max < P2max, the sloop or droop of voltage converter 1 is
greater than that of 2. At any particular voltage V*, the power shared
by Converter 1 is P1* and that of converter 2 is P2*. The total load is
sum of P1* and P2*

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Effect of droop control
• Voltage regulation is adverse effected by droop control so the
slope of droop characteristics is designed keeping in mind the
max. value of voltage deviation.
• Cable resistance that is connected
to source and point and common
coupling is also mentioned
because the higher value of
resistance has poor voltage
regulation and due to this we
can not give more sloop to the
droop characteristics.
Load sharing will be better if
cable resistance is less.
Fig.3. Effect of cable resistance on P-V
droop Characteristics

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Effect of droop control
• When droop gain used is higher, current sharing error of the
converters reduces
• Voltage regulation is increased
• High value of droop gains provide stabilizing effect on DC
Microgrid but also increase losses.
A trade off is required is selecting the droop gain as a small value and a
higher value can adversely affect the system

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Droop Control using Virtual impedance
 Two DC-DC converters connected
in parallel
The equivalent ckt. DC power supplies
connected to a common load through
resistive output impedance.
Nominal output voltage should be same.
In case of minor difference of output voltage , a circulating
current flows between the sources
Virtual Impedance is programmed to reduce the
circulating current

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How virtual impedance work ?
The Control Input to the voltage loop expressed
as:-
Vo* = Vref – RdIo ……………….(1)
Where
Io is the measured output current
RD is the virtual impedance
Vref is the no load output reference voltage
If Ev is the maximum output deviation, RD and
Vref is
Vref = Vn – Ev/2 …………(2)
RD = Ev/ Imax ………….(3)
Where Vn is the nominal output voltage
Imax is the max output current
The virtual impedance control loop
compensate the difference in voltage
reference Vo* = V*o1-V*o2
The current sharing between the two converters are
Δ Io = Io1-Io2= Δ Vo* /RD

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Simulation Study
• Simulation study done by connecting two buck converter back to
back. The parameter used are given in table 1.
• The design parameter given in
Table 2.
The design equation of the buck
converter
L= 1.25 (1-D)RL / 2f
C= (1-D) / 8Lf2 (ΔVo/Vo)
Where,
D is the duty ratio
ΔVo is the peak to peak ripple output
Voltage, Vo is the reference voltage

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Design Parameter
 Nominal output voltage = 380 V
 Nominal Output voltage = 120 V
 Virtual Output impedance = 8 Ω
 Proposed control strategy output time = 0.5 s
 Sudden increase in load at 0.8 s
 Load resistance change from 100 Ω to 80 Ω
 Maximum Voltage deviation= 5 %
Current before applied virtual impedance
different and after 0.5 second the difference
goes almost zero even after 0.8 second sudden
increase in load but voltage drop due to this
action

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Advantages and disadvantages
Advantages:-
1. Circulating current decrease aprrox. to zero by using this method.
Disadvantages:-
1.Voltage deviations caused due to load variation is the major disadvantages of
this method.
2. Secondary control and territory control also required for further control the
grid system network

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Conclusion
• Output impedance matching is one of the most important concept
in converter
• If impedance don’t match the circulating current will be flow in
between the converter that are connected parallel
• If the output resistance
difference between the two
converter is more, then the
current sharing error and
voltage regulation is
deteriorated. (Fig.7)

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Conclusion
1.With higher value of virtual output impedance the load sharing
has improved
2. Current sharing error between the converter will be reduced
3.Voltage deviation or droop will be more
4. For a smaller value of virtual impedance voltage deviation is
less but current sharing error is higher.
5. Additional control i.e. secondary control for synchronization
and territory control is also required to control the bidirectional
power flow.

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Questions Arises
1. if there are more then two converter connected in parallel than
this technique will be effective or not?
i.) The value of virtual impedance changed every time or
predefined.
ii) If the value is predefine or fixed then how can we give
the proportional feedback as we required to compensate
the current sharing error.
2. What about the Cable resistance, It will be same or variable
because all converter not at same distance. It could effect
stability.
3. What about the output impedance- I think output impedance also
a control variable in this case.

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4. Line impedance is hard to calculate because its may very with
different load and in this approach conductor resistance also
mentioned , so this is the point for confusion.
5. If circulating current is more and so after applied virtual
impedance, voltage should not be more or less than maximum
voltage deviation for example if output is 100 V and voltage
deviation is +/- 5% so maximum voltage deviation will be 5 V so
product of Virtual impedance and circulating current is less than
5 V, if more current not be removed.

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