Angle stability improvement by switching shunt reactors in mixed overhead-cable lines

The growing sensibility to environment issues contributed in recent years to the erection of barriers in the construction of new overhead transmission lines. One of the solutions to overcome these problems is the use of cable lines for high (HV) and extra-high (EHV) voltage. However, the coexistence of overhead and cable lines in transmission systems, in the so called “mixed” lines, raises a number of issues regarding the behaviour of such systems both in transient and steady state. In particular, the definition of angle or transient stability of a system plays a strategic role and provides the actions of design and planning of the system. The purpose of this study is to investigate the angle stability of a system containing a mixed line: a control strategy of shunt reactors, located to compensate reactive power request of cable line, will be proposed as countermeasure to severe disturbance and then as improvement of power system stability. The analysis, based on detailed time simulations, will show that the timing of control actions of shunt reactors is critical for the success of emergency control: in particular, the size and the timing of reactive power involved in switching operation determine whether these control actions play positively or not in avoiding angular separation and system instability.