Voltage Source Converter Modelling
Introduction
The AC/DC converters in Ipsa represent either voltage source converters (VSC) or line commutated
converters (LCC). A single converter component is used to represent monopole, bipolar and
homopole converters of 6, 12, 24 or 48 pulses.
Line commutated converters rely on the AC voltage waveform to provide the commutation between
the diodes or thyristors which make up the bridge circuits. The thyristors provide the mechanism for
controlling the DC current, diodes do not provide any control. The firing angle for an LCC defines the
point in the AC waveform at which the thyristors are turned on to conduct. They are only turned off
when at the next zero crossing point on the AC waveform.
Voltage source converters utilise insulated gate bipolar transistors (IGBTs) which can be turned both
on and off giving a greater degree of control. These VSCs are also known as pulse width modulated
(PWM) controllers since the IGBTs can be turned on and off many times per cycle.
This tutorial explains how to represent a simple Voltage Source Converter in Ipsa 2.
A simple network is developed which represents two AC systems connected by a DC link. Typical
values are used to represent two back to back voltage source convertors at each end of a 300kV DC
cable. The DC link connects two separate AC systems, one at 400kV and one at 275kV.
The final network is suitable for load flow and fault level studies.
1. Draw the AC Network
Draw the network below using normal AC components:
2. Enter the AC Network Data
Enter the following data for the busbars:
Name
Nominal
Voltage
(kV)
400
150
150
275
Rectifier Grid
Rectifier
Inverter
Inverter Grid
The Grid Infeeds should be set up as follows:
Busbar
Rectifier Grid
Inverter Grid
Voltage
Magnitude
(pu)
1.0
0.95
RMS LLL
(MVA)
X2R LLL
1000
750
20
10
It is important to enter the voltage magnitude for both infeeds. This tells Ipsa that these infeeds
are to be treated as slack sources. Each AC island in the network requires one slack generator.
Now enter some typical transformer impedances, for example:
From Busbar
To Busbar
Rectifier Grid
Inverter Grid
Rectifier
Inverter
Resistance
(pu)
Reactance
(pu)
0.01
0.01
0.1
0.1
Tap
Range
(%)
+/-10
+/-10
Tap Step
(%)
1.0
1.0
Target
Voltage
(pu)
1.0
1.0
3. Test the AC Network
There is now enough data in the network for Ipsa to perform a load flow calculation. When you
run a load flow for the first time Ipsa will ask you to select one slack busbar for each AC island.
Set the ‘Rectifier Grid’ and ‘Inverter Grid’ busbars as slacks:
The slack busbars can also be viewed or edited from the Network > Slacks menu
The load flow results should appear as follows:
4. Draw the DC Network
Draw the two DC busbars and AC/DC Converters as shown below:
The AC/DC Converters should be drawn from the AC busbar to the DC busbar
Use the normal branch component to draw the DC cable. Ipsa knows it is a DC cable due to the
way it’s connected to the AC/DC converters
If you can’t find the AC/DC converter symbol then right click on a blank part of the Ipsa toolbar
and make sure the ‘Draw DC’ toolbar is ticked.
5. Enter the DC Network Data
Enter the following data for the DC busbars. Note that the busbar voltage should be the
maximum DC pole to pole voltage. For example a +/- 150kV link would require busbar voltages
of 300kV.
Name
Rectifier DC
Inverter DC
Nominal
Voltage (kV)
300
300
Now enter a typical DC cable impedance, for example:
From Busbar
Rectifier DC
To Busbar
Inverter DC
Resistance (pu)
0.05
Reactance (pu)
Only resistance data is required for the DC cable. Ratings can also be entered if required.
6. Enter the Rectifier Data
The rectifier converter will be set up to transfer power from the AC network to the DC network.
This will be set up as a voltage source converter (VSC) and configured to control the DC power
transfer to the inverter. The following voltages and power levels need to be set:
Parameter
Type
DC Power (MW)
Value
PWM (VSC)
100MW
AC Reactive
Power (MVAr)
10MVAr
Transformer
Reactance
0.06pu
Description
Select the voltage source converter model
Sets the DC MW power transfer from the rectifier to
the inverter
Sets the MVArs injected into the AC network. A
positive value injects reactive power whilst a negative
value absorbs reactive power.
The per unit value of the rectifier transformer. This is
the reactance of the internal rectifier transformer
from 150kV
The transformer at the Rectifier side will control the AC busbar voltage whilst the rectifier itself
controls the DC power flow.
7. Enter the Inverter Data
The inverter will be set up to transfer power from the DC network to the AC network. This will
be set up as a voltage source converter (VSC) and configured to control the DC power transfer
through the inverter to the AC side. The following voltages and power levels need to be set:
Parameter
Type
DC Power (MW)
DC Voltage (pu)
AC Reactive
Power (MVAr)
Transformer
Reactance
Value
PWM (VSC)
Not set
0.99pu
-20MVAr
0.06pu
Description
Select the voltage source converter model
Set by Ipsa as a results from the load flow
Sets the DC voltage level that the inverter will
maintain. The inverter will set the power extracted
from the DC link in order to maintain this DC voltage
Sets the MVArs injected into the AC network. A
positive value injects reactive power whilst a negative
value absorbs reactive power.
The per unit value of the rectifier transformer. This is
the reactance of the internal rectifier transformer
from 150kV
The transformer at the Inverter Grid will control the Inverter AC busbar voltage whilst the
Inverter itself controls reactive power imported or exported to the AC network. The real power
from the Inverter to the AC network is calculated by the load flow analysis such that the DC
voltage is maintained at the target set in the Inverter.
8. Test the Combined AC and DC Network
There is now enough data in the network for Ipsa to perform a load flow calculation on the
complete network. Ipsa will again ask you to select a busbar for the DC network. Select the
‘Inverter DC’ busbars as the DC area slack since the Inverter is the device that controls the DC
system voltage:
The slack busbars can also be viewed or edited from the Network > Slacks menu
The load flow results should appear as follows:
9. Reversing the Power Flow Direction
The following steps should be taken to reverse the DC power flow.



10.
At the Rectifier Converter:
o Delete the DC Voltage
o Ipsa will calculate the DC Power Flow
At the Inverter Converter
o Enter a DC Voltage
o Enter the required DC Power Flow (MW)
Change the slack busbars
o Go to Network > Slacks
o Move the DC Area Slack busbar from the Rectifier to the Inverter
Adding Fault Analysis Data
Grid infeed components are required in order to represent a fault contribution from the rectifier
and inverter. The fault contribution is dependant on the specific protection and control circuits
used by the converters and it therefore specific to each manufacturer. It is typically in the range
of 100% to 150% of the full load current rating of the converter. Therefore two grid infeeds are
added to the model to provide a 1.0 per unit fault level contribution. The grid infeed data is as
shown below;
Parameter
RMS LLL (MVA)
X2R LLL
Value
100
1
Description
RMS 3 phase fault contribution in MVA
3 phase X/R ratio
The fault level results without an additional grid infeed for a 3 phase fault at 100ms are shown
below;
Symmetrical 3 Phase Fault Level at 100ms
The fault level results with an additional grid infeed for a 3 phase fault at 100ms are shown
below;
Symmetrical 3 Phase Fault Level at 100ms
Converter Data Summary
The following table describes the data fields that are used for a Voltage Source Converter;
Parameter
DC Power (MW)
DC Voltage (pu)
AC Reactive Power (MVAr)
Transformer Reactance
Commutation Reactance
Voltage Ratio
(Advanced Option)
DC Current Trip Limit (pu)
(Advanced Option)
Description
Defines the DC side power output of the rectifier in MW. Ipsa
will calculate this value for the Inverter end converter
Defines the DC side target voltage at the Inverter end. This
value is ignored for the Rectifier. This is in per unit and must be
between 0.5 and 2.5 pu.
Sets the MVArs injected into the AC network. A positive value
injects reactive power whilst a negative value absorbs reactive
power.
The per unit value of the rectifier transformer. This is the
reactance of the internal rectifier transformer in per unit on the
system MVA base.
The converter commutation reactance in per unit
The ratio of RMS line to line voltage on the AC side to the DC
voltage. This value is used to calculate the DC voltage and DC
current. The voltage ratio is used in conjunction with the DC
Current Trip limit. This value defaults to 1.275 and must be less
than 1.625.
This sets the maximum per unit DC current that can be
absorbed or supplied by the DC side of the converter.
Parameter
Pulse number
AC Current Trip Limit (pu)
DC Equivalent Capacitance (pu)
Description
Defines the number of pulses in the converter. Valid values are
6, 12, 24 and 48 only.
This value is used to calculate the harmonics generated by an
unfiltered converter automatically. It is also possible to
manually configure the harmonics by entering harmonic source
data.
This sets the maximum per unit AC current that can be
absorbed or supplied by the AC side of the converter.
The DC Equivalent capacitance in per unit.