Grid implementation and operational use of large phase shifting transformers

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.status: publishe