Grid implementation and operational use of large phase shifting transformers
Warichet, Jacques Leonard, Jean-Louis Rimez, Johan Bronckart, Olivier Van Hecke, Jozef #
CIGRE Session edition:2010 location:Paris
In the Central West Europe (CWE) grid, Belgium is rather strongly interconnected with about
5500 MVA border crossing capacity on both its northern and southern borders. The short
average distance between those borders (roughly 150 km) causes the Belgian grid to
participate substantially in almost every north-south exchange in the CWE grid. Effective
CWE markets, together with a huge wind generation capacity in the Baltic area, give rise to
increased north-south exchanges. This results in mostly unforeseeable transits up to 2500 MW
through the Belgian grid in both north-south and south-north directions. Historically, the
Belgian grid has not been designed to cope with this kind of transits. Consequently, high
transits might endanger the grid safety.
To cope with this situation, the Belgian Transmission System Operator (TSO) decided to
install Phase Shifting Transformers (PST’s) such that all international transiting power would
pass through them without decreasing the actual border capacity. The northern border was
chosen, and three PST’s were installed in series with the 400 kV border crossing lines. The
combined action of the PST’s aims at reducing the power transits to safe levels. In the actual
CWE grid, maximum phase shifting allows to reduce north-south transits by about 1250 MW
in each direction. In addition to reducing massive north-south transits, these PST’s will
further enhance overall grid safety, by redistributing a given transit along different paths in
the Belgian grid.
For this application, the PST is a combination of two three-phase transformers (a series and a
regulation transformer) connected together by oil ducts. With a rating of 1400 MVA, their
maximum angle displacement is ± 25° (in no-load conditions). The PST is of the symmetrical
indirect design and has no voltage regulation.
The main concern regarding the operational use is the volatility of the loop flows. The
increase of the electricity trade and the high penetration of wind generation in Europe led to
very important and frequent power flow changes both in direction and in amplitude. The
Belgian TSO operates the three PST’s in order to stabilize these loop flows within a predetermined range. It will thus be possible to schedule a more stable outage plan and to
better assess the power exchange capacities at the border for the different time horizons.
Additionally, a phase shifting realized on two PST’s in opposite direction allows a better N-1
congestion management within the Belgian system in real time with limited impact on the
neighboring TSO’s. Specific coordination tests and procedures were set up with the TSO’s of
the CWE area.
With the availability of all three units since October 2008, System Engineers are unanimous
concerning the key-role the PST’s have in the grid security and efficiency. Until now, the
units are playing a major role for limiting the power flow through Belgium, to counter
congestion problems in foreign grids and to resolve local congestions in the 400 kV and the
220 kV power grids by redirecting east-west power flows.