Grid implementation of a 400MVA 220/150kV phase shifting transformer for power flow control

Load flow studies concerning the power exchange in the border region of Belgium and France, have shown that the old but important 220kV line between the RTE substation of Chooz (F) and the Elia substation of Monceau (B) could go into a serious overload condition in case of failure or maintenance on other transnational power lines or in case of outages on neighbouring interconnection transformers. The importance of this power line to the industrial Charleroi (B) region obliges special measures to be taken to protect the line against overloading while staying in service at all possible grid conditions. The chosen solution consists in implementing a phase shifting autotransformer (PST) in the 150kV substation of Monceau. The main characteristics were deduced from the results of the load flow calculations (the simulations were done with and without a phase shifting unit): a 220/150kV 400MVA autotransformer (with OLTC for voltage regulation) able to provoke a phase displacement between the 220kV and 155kV sides in the range of -15° +3° at full load; also by means of an OLTC. From these characteristics, it is clear that shifting away the overload current is the main purpose of the unit. The implementation of the phase shifting transformer has also several operational consequences. Not only the protection, control and command of the unit, but also various (external) grid events (lightning strokes, single and multiple phase line faults, autoreclose of the circuit breakers) affect the design of the PST. The transformer itself will have an influence on the power flow behaviour of the grid, the protection schemes and the automatic systems in the substation. It is obvious that a close cooperation and exchange of mathematical simulation models (of both grid and transformer) between the grid operator and the manufacturer is necessary to cover all possible operational issues before the putting into service of the transformer in march 2006. With these experiences, the authors develop the expertise for the grid behaviour studies and transformer performance for new PST implementations such as 400/400kV 1400MVA phase shifting transformers to be placed at the electrical border between Belgium and the Netherlands.