Automatic System for the Control of Stability of an HVDC link - Implementation in Red Eléctrica de España (REE)

Authors: J Martin, Red Electrica de España (REE), C. de Arriba, J. Rodriguez, GE Grid Automation, and E. Leon, Siemens, Spain

The introduction of an on-line short circuit estimation process that automatically enable the operation mode (bipolar or monopolar of the HVDC link, or total disconnection, as the one developed in ASPAS), allows to reduce the operating margins maintaining the same reliability level and so optimize the operation, allowing substantial increase of the energy transferred through the link, with the corresponding economic benefits to REE.
ASPAS also facilitates companies in migrating to a more digital model based fundamentally on three pillars: Communications, Big Data and Artificial Intelligence. The right combination of these three pillars will optimize the operation of the electrical system, reducing costs and polluting emissions, accelerating the adoption of non-polluting energies and renewable energies, while maximizing economic performance and a closer way of interacting with users.

The Problem
The Spanish transmission power system owned by Red Eléctrica de España (REE) includes a submarine high voltage direct current (HVDC) link laid in the Mediterranean Sea that connects the Mallorca island (Santa Ponsa substation) with the Spanish mainland (Morverdre Substation).
This link is a Line Commutated Converter (LCC) in bipolar configuration with return conductor with a capacity of 2x200MW equipped with optical signal activated thyristors at ±250kV and six filters 2x100 + 4x41MVAr.
Figure 2 shows where HVDC connects to AC island grid. Normally, the energy flows from Spanish mainland to Mallorca island, being fundamental the total amount of short circuit power in Santa Ponsa substation for a correct HVDC operation.
Values of short circuit power in Santa Ponsa from 1300 to 500 MVA require changing the operational topology and the operation mode of the HVDC from a bipolar configuration to a monopolar one, which also implies a reduction in the flow of Active Power. Values lower than 500 MVA require a complete disconnection of the HVDC link, to avoid commutation failures or dangerous oscillatory behavior that could seriously affect the operation of the entire grid.

In Figure 3 we observe a current oscillation due to a trip on Santa Ponsa – Valldurgent circuit (DC1 on Figure 2) due to this trip the HVDC link is working only with 220/66kV transformers in Santa Ponsa, the ones in service condition after trip, this grid situation produces a current oscillation (power) unacceptable for a safe grid operation.
Figure 3 shows the oscillation from one phase of one HVDC pole, event that provoke the trip of the link by subsynchronous resonance protection. This oscillation is harmful for electrical system, not only for the affection to generation and loads on island due to grid voltages oscillation, also, for overvoltage produced in close trip instants of HVDC link due to filters operation to low short circuit power currently in grid.

Relion advanced protection & control.
BeijingSifang June 2016