A Study on Intermittent Ground Faults on Floating Wye Delta Connected Transformer Banks
Abstract – Intermittent ground faults. In the past, floating wye-delta transformer banks are most common for supplying ungrounded systems. For some industrial establishments, the use of an ungrounded system is very much desirable in order to ensure continuity of service and thus eliminating operational downtime. However, this practice is becoming obsolete due to concerns on establishing a reference to ground, stabilizing the voltage to ground, and detection of ground faults. This study assessed the possibility of high transient peak voltages resulting from intermittent ground faults on floating wye-delta transformer banks.
Introduction
Distribution transformers are banked or connected depending on the service configuration requirements. One of the most popular connections used in the past is the floating wye-delta connection where the delta is left ungrounded, in order to ensure continuity of service. Due to the disadvantages presented by ungrounded systems, grounding one of the phases of delta connected secondaries provided a means of obtaining a grounded system. In this way, a grounded system could be obtained at a minimum expense where existing delta connections did not provide access to the system neutral.
Current industry practices still make use of the ungrounded systems for the sake of ensuring continuity of service. However, leaving delta connected secondary ungrounded exposes the transformer and the loads connected to the effects of intermittent ground faults. Intermittent ground faults on ungrounded systems may cause high transient peak voltages which could lead to multiple, simultaneous failures of electrical and electronic devices.
Floating Wye Delta Connection
Floating wye-delta connection utilizes three single-phase transformers whose primaries are wye connected with the star point left ungrounded; and a delta connected secondary, see figure 1.
The increased reliability offered by this type of connection is taken from the fact that the first phase-to-ground fault on the secondary has little or no effect on the continuity of operations.
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Intermittent Ground Faults
Despite the fact that an ungrounded system has no intentional connection to ground, in reality, an ungrounded system is coupled to ground through distributed phase capacitances. This makes the system unable to effectively control the transient and steady-state voltages to ground. When one phase is grounded, a low magnitude current flows to ground through the distributed phase capacitances. If fault resistance is low, the predominant impedance is the capacitance in which arc is extinguished at zero current, maximum voltage(current leads the voltage by 90° in capacitive circuits). It becomes possible for the high voltage to re-ionize the arc path and for the arc to restrike, possibly every half cycle.
Figure 4a shows the normal voltage vectors rotating counter-clockwise with a zero neutral-to-ground potential. Considering a phase-A to ground fault as shown in fig. 3 and 4; at the instant phase-A is grounded, phase-A to ground potential decreases to zero. During this time, the neutral point rises to a value equal to VL-N while current flows through the distributed phase capacitance as shown in fig. 4b.
As the current through the capacitor zeroes, If(t) = 0, the arc extinguishes, leaving the capacitance at the neutral-to-ground potential, VL-N. This establishes a new reference point for the rotating phasor, where the neutral-to-ground potential is now equal to VL-N. During the next half-cycle, as the vectors rotate, the phase A voltage changes from zero to twice VL-N (dotted lines). If this voltage is sufficient enough to break down the gap in the ground fault, an arc may restrike causing another swing to the neutral-to-ground potential. This swing may either be in the positive or negative direction depending on the fault conditions and may cause another restrike.
The phenomena of arcing grounds and the escalation of the normal rated voltage to 5 to 6 times may cause high transient peak voltages which could lead to multiple, simultaneous failures of electrical and electronic devices, especially those with lower Basic Impulse Level (BIL).
Simulation
Figure 5 shows the simulation model for an intermittent ground fault on the floating wye-delta transformer bank. The bank involves a 50kVA and two (2) 15 kVA transformers whose secondaries are connected in an ungrounded delta system.
The simulation of an intermittent ground fault was created using MATLAB SimPowerSystem Software. Distributed capacitance was modeled by connecting 3-19µF in a grounded wye configuration. The sizes of the phase capacitance were computed by assuming a 1-ampere charging current (1 to 2-ampere charging current is typical for 480V systems). The ground fault was modeled by using a switch that is programmed to close at a definite time and opens at current zero in order to simulate an arcing ground.
Results and Discussions
The following figures show the voltage and current waveforms captured during the entire simulation. Figure 6 shows the voltage across the fault gap. As shown in the figure, the line-to-neutral voltage shifts between positive and negative directions every time the fault is established. The maximum voltage recorded was twice the line-to-neutral voltage prior to the fault.
Figure 7 shows the fault current waveform at the instant the fault is established. The maximum fault current recorded exceeded 1kA which lasts within a fraction of a millisecond.
Figure 8 shows the voltage output of the floating wye-delta transformer bank. The high transient peak voltages of magnitudes ranging from 400V-480V which recur every time the arc restrikes. These high transient peak voltages may cause multiple, simultaneous failures of electrical and electronic devices connected to the load center, as well as a possible failure of the transformers serving these loads.
Figure 9 shows the line currents at the primary side of the floating wye-delta transformer bank. A peak current of 2.3A was recorded which lasted for about a fraction of a millisecond. This current magnitude doesn’t warrant the protective fuses to trip thus no fault isolation was initiated, allowing high transient peak voltages to damage the equipment and connected loads. The intermittent ground fault can last until the insulation of any of the utility equipment fails, permanently shorting the phase to ground.
Conclusion
Floating wye-delta connection is one example of an ungrounded system. Ungrounded systems, while offering higher reliability by ensuring continuous operation in spite of a single, secondary phase-to-ground fault, allow high transient peak voltages to form from intermittent ground faults. These overvoltages are very much dangerous, considering that these may lead to multiple, simultaneous failures of the connected loads. Without establishing a proper reference to the ground for distribution transformers, the risks of transient overvoltages due to intermittent ground faults cannot be eliminated.
References
Short, T.A.(2004). Electric Power Distribution Handbook. Florida: CRC Press LLC.
Johnston, M.(2014). Installations and Instructions of Corner-Grounded Systems. IAEI.
Matsch, L.W (1964). Capacitors, Magnetic Circuits, and Transformers. New Jersey: Prentice-Hall, Inc.
Das, J.C (2010). Transients in Electrical System: Analysis, Recognition, and Mitigation. USA: McGraw-Hill.
Transient Overvoltages on Ungrounded Systems from Intermittent Ground Faults. Retrieved January 20, 2015 from the World Wide Web: http://www.eaton.com
Hi sir gud am, is there any specific explanation during energization of ungrounded wye- delta 3 phase bank transformer, that when closing the cut-outs simultaneously, it was during the 3rd phase closing ,the result was the first cut-out fuse will open ? , that was my experience during that time , I was assigned in Emergency crew.
our remedy during the time was we synchronized the 2 cut-outs during energization. thanks