Abstract：Phase shifting transformers are power flow control devices that regulate power flow distribution by adjusting the phase angle between voltage and current on either side of their insertion point in a line. Phase shifting transformers can be classified as either two core or single core based on the number of cores, and as symmetrical or asymmetrical based on their operating principle. This paper focuses on the commonly used symmetrical two core Phase shifting transformer as a case study. The symmetrical two core Phase shifting transformer (referred to as “phase shifting transformer”) consists of a series transformer and an excitation transformer. Conventional main protection schemes for these transformers employ multiple differential relays, requiring numerous current transformers (CTs) and complex interconnections. To address this challenge, this paper proposes a novel Phase shifting transformer main protection scheme based on a streamlined CT configuration, building upon an analysis of the electromagnetic coupling within the Phase shifting transformer's various components. The proposed scheme involves placing CTs at the three-phase input and output terminals of the Phase shifting transformer to obtain the necessary current information. A CT is also placed on the secondary side of one phase of the series transformer to obtain its secondary current. Furthermore, the secondary currents of the other two phases of the series transformer can be calculated based on the delta connection of its secondary windings. Using these measured and calculated currents, the proposed streamlined protection scheme is formed, consisting of one magnetic balance differential protection element (KCD) and three comprehensive differential protection elements (KZD1~KZD3). The magnetic balance differential protection (KCD) utilizes the CT on the secondary side of one series transformer phase, along with the corresponding phase's input and output CTs. This protection element, based on the ampere-turn balance principle between the transformer's primary and secondary windings, detects internal faults within that series transformer; KZD1 and KZD2 use the series transformer secondary CT along with the input and output CTs of the other two phases, while KZD3 uses the input and output CTs of all three phases. The underlying principle of the comprehensive differential protections (KZD1~KZD3) is the integration of electrical balance differential protection and magnetic balance differential protection. This is achieved by combining the ampere-turn balance equations for the transformer windings with the Kirchhoff's current law (KCL) equations for the “T” connection. This allows a single differential relay to detect multiple fault types. Furthermore, In the proposed novel main protection scheme, the four differential relays have different and overlapping protection zones. By analyzing the operational combinations of these relays, the fault discrimination range can be narrowed. Moreover, the mal operation of the comprehensive differential protection during the no-load switching-in of the Phase shifting transformer can be prevented by second harmonic block of magnetizing inrush current. Finally, a simulation model was developed on the PSCAD/EMTDC platform, and the operation of each differential relay was simulated and analyzed under various fault locations, fault phases, and fault severities. The feasibility and effectiveness of the proposed scheme are verified through extensive simulation experiments.
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2026, Vol. 41 
