The northern Italian 380-kV transmission system is interconnected to neighbouring countries by eight 380-kV circuits, eight 220-kV circuits and one 132-kV circuit. These interconnections form three power corridors: Italy to France, Switzerland and Austria/Slovenia. In 2008, the net yearly import to Italy through these interconnections totaled some 42 TWh.

The theoretical import transfer capacity through the northern interconnectors to Italy is around 12,000 MW, but in practice, the net transfer capacity (NTC) at times of peak demand is 7190 MW in the winter and 6090 MW in the summer. However, the commissioning of two merchant lines — a 380-kV circuit from Cagno, Italy, to Mendrisio, Switzerland, and a 150-kV circuit from Tirano, Italy, to Campocologno, Switzerland, which were commissioned in 2008 and 2009, respectively — has provided an additional NTC on the northern Italian border of 350 MW in the winter and 300 MW in the summer.

These NTC values are determined by different scenarios that take into consideration circuit constraints, the N-1 security operation of the interconnected system, transient overloads and the transmission reliability margin. During the NTC evaluation process, the availability of circuits to resolve grid congestion arising during normal system operation as a result of grid system faults is checked. Some five operational strategies are considered, including the automatic or manual connection of power flow devices such as phase-shifting transformers (PSTs).

To increase the existing import capacity by improving the use of the existing interconnectors, the Italian transmission system operator (TSO), Terna, installed a PST on the 380-kV double-circuit transmission line from Rondissone, Italy, to Albertville, France.

Functional Specification

The load flow, contingency and feasibility studies formed the basis for the PST specification, but the unit design had to take into account the limited space at Rondissone substation, which resulted in the need for one set of three-phase PSTs for each of the two transmission circuits. Also, the units had to comply with Italian railway limits, as transportation by road was not possible.

The regulation angle was specified in a functional manner as X 12 degrees, with a line current of 70% of the circuit rating. This allowed potential suppliers the opportunity to produce a design comprise between the minimum short-circuit impedance required for the short-circuit withstand of the windings and tap changer and a minimum no-load regulation angle for the most compact unit. The N-1 criteria was applied to the operation of the coolers, and an emergency rating of 120% rated load — with N-1 coolers in addition to operation at rated load without coolers — was specified.

The asymmetric quad-booster concept for the PSTs was selected because it allows maximum use of the on-load tap changer capacity. Also, as the asymmetric quad-booster does not require a 400-kV centre tap in the series winding, the connections between the series and shunt transformers were simplified. The PSTs had to be designed with the ability to move from standby mode to maximum advance phase shift in a minimal amount of time to relieve transmission line overloading.

Design Specification

The electrical design of the PSTs from ABB consist of a three-phase series transformer with six 400-kV oil/air bushings and a three-phase shunt transformer with three oil/air bushings. The connections between the regulation windings of the shunt transformer and the excitation windings of the series transformer are made by oil/oil bushings in an oil-filled duct. This design feature allows separate handling and factory testing of the transformers and minimum exposure of the windings to atmosphere during transportation and on-site assembly.

The on-load tap changer, with 33 tap positions, consists of three single-phase units with a common motor drive giving course/fine regulation. This design satisfies the functional specification in terms of speed of operation, short-circuit withstand and overload capability. Type and special tests in accordance with IEC 60076 were completed at the factory, together with load loss and temperature rise tests for up to 120% of rated current.

Substation Installation

The PSTs installed on the 380-kV double-circuit Rondissone-Albertville transmission line scheme is a type of configuration with a control system offering the following advantages:

• The circuits to Albertville remain in service with the PSTs disconnected.

• Normal operation of the 380-kV circuits occurs with the PSTs in standby mode.

• It is possible to insert a PST in service, return it to standby mode and disconnect it without ever interrupting the operation of the transmission lines.

The PSTs have a short-circuit impedance at rated voltage between 11% and 13.5%.impedance. When a PST is in service, the internal impedance tends to reduce the transfer capacity of the transmission line, an effect contrary to that required in a post-contingency situation in which the capacity increases to reduce overloading on parallel circuits. However, this can be avoided by bringing the PST to an equivalent zero position before opening the bypass. The equivalent zero position means the voltage drop due to the load current is approximately equal to the voltage injected by the series windings of the PST.

The PST switching sequence required to achieve this operational mode can be performed manually or as an automated command sequence. The equivalent zero tap positions as a function of load current are placed in the supervisory control and data acquisition system; this parking position of the tap changer can be set to a value that ensures the equivalent zero position is reached in a minimal amount of time.

Maintenance of the PSTs is similar to that required by 400-kV transformers in a strategic location; the PSTs are subject to Terna's standard maintenance disciplines. To support maintenance planning and provide early indicators of incipient faults on the two PSTs and two 400-kV transformers installed at Rondissone substation, each unit is equipped with a transformer electronic control (TEC) system and a bushing monitoring system. Data from the TEC is transmitted through fibre optics to a common server PC located in the substation. The data from the TEC and bushing monitoring systems can be accessed from remote locations through the intranet using standard Internet browsers.