Do you have more information on how to set and verify the value of the commutation reactance of an HVDC-LCC system?
This example can help you understand the following:
- the purpose of the commutation reactance while applying it to the HVDC-LCC element (ElmHvdclcc); nevertheless, the same procedure applies to line commutated converters built using the Rectifier/Inverter element (ElmRec) i.e. the 6 pulse bridge.
- the benefits of correctly setting the commutation reactance
- when to use (and when not to use) the built-in transformer option
- how the overlap angle obtained in load flow / RMS simulation can be validated against a precise EMT simulation
The commutation reactance, Xc, can be defined as the reactance between the terminals of the converter bridge and a fictitious upstream bus whose voltage is perfectly sinusoidal. The commutation reactance is well described in the Technical Reference documentation - please reach to these documents for a more comprehensive description. In this example, the use of a large shunt compensation at the HV terminal of the converter transformer has the effect of creating a commutating voltage at the HV transformer bus itself. Other equipment may have the same effect (e.g. synchronous condensers, STATCOM, etc).
The results of EMT simulation are compared with the results of the load flow model in terms of resulting overlap angles. An RMS simulation has not been executed, since the RMS model is based on the same equations as the load flow model, therefore, identical results are expected as in load flow.
In the case of a built-in HVDC transformer option in the ElmHvdclcc element, the value of the programmed commutation reactance is included always in the simulation. As such, this option, as it will be seen further, generates correct results whenever the source impedance (i.e. the grid reactance) can be neglected i.e. the commutating voltage is found at the HV side of the converter transformer. In all other cases, deviations may occur with this option and therefore, an external representation of the transformer is recommended. The same considerations and conclusions will apply to the Rectifier/Inverter element (ElmRes).
The example contains three study cases, as below. The results are summarised within the attached Excel spreadsheet.
- SC01: both stations have shunt compensation active
- SC02: only the rectifier station has shunt compensation active
- SC03: neither rectifier nor inverter stations have shunt compensation
Within study case SC01, it can be seen that the use of the built-in transformer within both stations generates valid results.
Within study case SC02, the use of the built-in transformer within the inverter station should be avoided, since the commutating reactance will be influenced by the grid reactance as well.
Within study case SC03, the use of the built-in transformer within both stations should be avoided, since each commutating reactance will be influenced by the corresponding grid reactance.
Note 1: the Load flow and RMS models use and are influenced by the commutation reactance.
Note 2: Whenever the commutating voltage is not located at the HV bus of the HVDC transformer, the value of the commutation reactance is dependent on the operation scenario and grid topology (i.e. its value will change every time that the impedance between the HVDC and the commutating voltage bus changes).
Note 3: In almost all cases, the commutation reactance must include the transformer reactance.
Note 4: Whenever an unrealistically low commutation reactance is programmed into the model, the model itself may become highly sensitive: very small variations in the overlap angle will highly impact the active and reactive power of the converter. In such unplausible situations, there is a risk of non-convergence within load flow calculations. Within RMS simulations, there is a risk of non-convergence or various other errors.
Note 5: Whenever the commutating voltage is not located at the HV bus of the HVDC transformer, the commutation reactance will be influenced by the external network. In these cases, special care should be taken whenever performing multiple analyses on the same HVDC system, since each time that the topology of the network changes, the commutation reactance may also change.
Note 6: The load flow and RMS equations used for the overlap angle calculation are assuming that an almost constant DC current exists throughout the commutation period. This is ensured by the existence of sufficiently large DC reactors.