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) Rectifier Grid 400 Rectifier 150 Inverter 150 Inverter Grid 275 The Grid Infeeds should be set up as follows: Busbar Voltage Magnitude (pu) RMS LLL (MVA) X2R LLL Rectifier Grid 1.0 1000 20 Inverter Grid 0.95 750 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 Resistance (pu) Reactance (pu) Tap Range (%) Tap Step (%) Target Voltage (pu) Rectifier Grid Rectifier 0.01 0.1 +/-10 1.0 1.0 Inverter Grid Inverter 0.01 0.1 +/-10 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 Nominal Voltage (kv) Rectifier DC 300 Inverter DC 300 Now enter a typical DC cable impedance, for example: From Busbar To Busbar Resistance (pu) Reactance (pu) Rectifier DC Inverter DC 0.05 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 Value Description Type PWM (VSC) Select the voltage source converter model DC Power (MW) 100MW Sets the DC MW power transfer from the rectifier to the inverter AC Reactive Power (MVAr) Transformer Reactance 10MVAr 0.06pu 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 Value Description Type PWM (VSC) Select the voltage source converter model DC Power (MW) Not set Set by Ipsa as a results from the load flow DC Voltage (pu) 0.99pu 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 AC Reactive Power (MVAr) Transformer Reactance -20MVAr 0.06pu 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. 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 10. 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 Value Description RMS LLL (MVA) 100 RMS 3 phase fault contribution in MVA X2R LLL 1 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.