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Single line to ground fault (SLGF) is the most frequent fault probable to occur in an electric power system. The effect of ground fault is determined by voltage transformer and transformer configurations. In this paper, the simulation showed the performance generator within the SLGF at various transformer configurations. Simulation was conducted in DIgSILENT PowerFactory 14.0. and the results were analyzed, presenting comparison of the fault impact at various transformer connections. The model of transformer connection for each side (primary and secondary) used were Y, Yn, Z, Zn and ∆ . The Y, Z and ∆ secondary side of transformer configuration were utilized to block the single line to ground fault at the generator bus. It was clearly shown that the SLGF at the generator bus was highly dependent upon the type of the transformer configurations used during the ground fault at the secondary side of transformer. INTRODUCTION In general, a step up transformer in electric power station can be categorized as unit generator-transformer configuration, unit generator-transformer configuration with generator breaker, cross-compound generator and generator involving a unit transformer [1]. Ground fault at the transmission line or busbar can affect the system configuration of the generator. Knowledge of ground fault at transformer winding connections is essential to choose an appropriate transformer for service requirement. Research and applications on transformers have been carried out for decades. IEEE std.C57.12.702000 [2] provides guides and recommended practices for terminal marking and connections for distribution and power transformers. IEEE std. C57.116-1989 [3] provides guides for direct connection of transformers to generators, while IEEE std. 519-1992 [4] and IEEE std. 142-2007 [5] address the harmonics and system grounding related to transformers, respectively. The transformer configurations with the propagation of voltage sags [6] can influence the performance of voltage sags inside the industry facility, depending on the function of transformer configurations used in the service transformer. [7] describes the effect of the voltage transformers on the operating conditions of a ground-fault protection system for unit-connected generators. The magnitude of ground fault current, especially at the generator and transformer are determined by the generator and transformer winding impedance [1],[8]. The protection for generators are influenced by the arrangement and selection of how the generators are united into the system and by the overall generating station arrangement. This paper presents the effect of transformer connection, which were denoted as wye (Y), wyegrounded (Yn), zigzag (Z), zigzag grounded (Zn) and delta (∆) in each side primary and secondary for unit generator-transformer configuration. TRANSFORMER CONFIGURATION The primary and secondary winding of the transformer can be connected in combinations of Y, Yn, Z, Zn or ∆ configurations, which can result in twenty five possible connection combinations [9][10] Y-Y, Y-Yn, Y-Z, Y-Zn, Y-∆, Yn-Yn, Yn-Y, Yn-Z, Yn-Zn, Yn-∆, Z-Y, Z-Yn, Z-Z, Z-Zn, Z-∆, Zn-Y, Zn-Yn, Zn-Z, Zn-Zn, Zn-∆, ∆-Y, ∆-Yn, ∆-Z, ∆-Zn and ∆-∆. The simple schematics for Yn-Yn and ∆-Yn configurations are shown in Figure 1. The zero-sequence impedance into a transformer depends on the configuration of the winding. The zero-sequence impedance of a ∆ winding is infinite, whereas the zero-sequence impedance of a Y-connected winding is a composite series of the zero-sequence impedance of the transformer and the impedance of any neutral grounding devices that might be present. Thus, an ungrounded Y-winding would present infinite zero-sequence impedance because the absence of a neutral grounding connection appears as an open circuit in series with the zero-sequence impedance of the transformer winding itself [11]. The impedance of the transformer itself depends on several factors in the construction of the transformer. Three-phase transformers, which are constructed so that a closed, low-impedance path exists for the flow of zero-sequence flux within the transformer, have a lower zero sequence impedance than the transformers without such a path. One such path is the transformer core. Transformers with core-form construction have lower zero-sequence impedances than units which have shell-form cores. Three-phase transformers with ∆ windings have the lowest zero-sequence impedance, and in the absence of actual test data, it is often assumed that the zerosequence impedance of core-form transformers with ∆ winding is about 0.85 times having positive-sequence leakage reactance of such transformers [11]. The zero-sequence impedance of shell-form transformers has about the same magnitude as the positive-sequence leakage reactance of such transformers. Conversely, a three-phase transformer-bank, consisting of three, single-phase transformers connected in Y-Y configuration, has a very high zero-sequence impedance [9]. a)Yn-Yn connections b)∆-Yn connections Figure 1. Connection’s diagram of Yn-Yn and D-Yn transformer [9] Zero-sequence components of current can flow through a Yn-Yn connected transformer if a neutral path exists on both sides of the transformer. An example is shown in Figure 2, where a ∆-Yn connected transformer (T1), supplies power to a Yn-Yn connected transformer (T2). A fault on the load side of T2 produces a zero-sequence current, which flows in the primary and secondary windings of that transformer. Zero-

Ground Fault Currents in Unit Generator Transformer at Various Voltage and Transformer Configurations