I. Introduction
The general topology of locomotive or suburban unit is depicted in Figure 1. The electrical equipment is composed of input transformer, single-phase voltage-source active rectifier, three-phase traction inverter and traction motor. The active rectifier provides precise and fast control of dc-link voltage, proper current limitation and ensuring of harmonic trolley wire current. To ensure those active rectifier essential characteristics must be used accurate and rapid voltage synchronization method. We are talking about single-phase synchronization method, some advanced synchronization methods are presented in [1]–[10]. The main problems of single-phase voltage synchronization which are possible must be suppressed. We suppose the problem of interference during voltage measurement. Another problematic condition can be rapid changes in voltage magnitude (step changes may occur). There may be a small change in frequency (assumption is ), this is a slow change. Step changes in the position of the voltage vector are not expected.
General topology of classic locomotive or suburban unit