Open-Phase Studies included in ETAP 12 Unbalanced Load Flow Module

Preview of ETAP 12 hints at a major enhancement to the Unbalanced Load Flow Module.

Irvine, Calif. – November 18, 2012 — ETAP®, the leading provider of software solutions for the design, optimization, and online operation of mission critical electrical power infrastructure, has given a preview to an enhancement of a module in the upcoming ETAP 12 release.

“One of the major enhancements will be to the Unbalanced Load Flow (UBLF) module to accurately simulate steady-state open-phase operating conditions of power systems,” said Shervin Shokooh, Chief Operating Officer at ETAP. “This enhancement was added specifically to provide solutions in response to an open-phase event that occurred at a nuclear generation plant earlier this year. In this event, the protective devices at the utilization buses could not detect an open-phase fault condition on the primary side of the source transformer.”

In ETAP 12, an open-phase condition can be simulated and studied by placing a graphical icon on the one-line diagram at the terminals of branch elements. The UBLF program would open a phase (A, B, C) and run the unbalance load flow study for a steady-state solution without the initial transients. For transformers, the effects of an embedded winding, no-load current and losses, transformer type (core and shell), and inter-phase A, B, C mutual coupling, including zero-sequence are included.

New enhancements added to the Unbalanced Load Flow module for open-phase conditions include:

  • Graphical Placement of the Open-Phase on the One-Line Diagram: Using a new icon from the study toolbar, one can easily place open-phase faults (phase A, B, or C) at any terminal of the three-phase branches, including two-winding and three-winding transformers, cables, transmission lines, impedances, and reactors.
  • Induction Motor Modeling: The modeling of induction motors has been enhanced to handle severe unbalanced system conditions caused by an open-phase fault, including the effects of negative-sequence current.
  • Transformer Type: The capability to model various types of transformers has been added, including Shell and Core with 3, 4, and 5 limbs, for both 2-winding and 3-winding transformers.
  • Transformer Magnetization Coupling: Based on the no-load data, the magnetizing impedance for positive, negative, and zero sequence couplings are now calculated and taken into account.
  • Transformer Embedded Winding: The effects of an embedded (buried) winding, for two and three winding transformers are included.
  • Report Current Flows Inside Transformer Embedded Winding: The zero-sequence current circulating inside the embedded delta–connected winding is reported. Leading to the development of the UBLF module and, as part of the verification and validation (V&V) process, we have conducted numerous studies and have created benchmarks to simulate open-phase conditions in various networks, including the off-site power supply system of a nuclear power plant. V&V test cases have been created to simulate electrical network behavior with different models and under various operating conditions.

The following is a summary of our findings:

  • Under low loading, certain transformer configurations, and specific operating conditions, detection of an open-phase condition on the primary side of a transformer can be difficult on the secondary side.
  • In some cases, detection of the fault is not possible by monitoring the phase currents and voltages.
  • Depending on the location of the relays/monitoring devices, the effects of motor back-emf and voltage drop across cables can be significant for detection.
  • Better accuracy of simulation results is obtained with an actual detailed system model rather than a simplified model.
  • The ETAP enhancements mentioned above for motor and transformer modeling are essential to obtain accurate results.


About ETAP
Founded in 1986 and headquartered in Irvine, Calif., ETAP is the global market and technology leader in electrical power system modeling, design, analysis, optimization, and predictive real-time solutions. The Company’s software technologies ensure that power systems are designed for optimal reliability, safety, and energy efficiency; when deployed in real-time mode, they enable organizations to manage energy as a strategic asset, maximize system utilization, lower costs, and achieve higher levels of financial stability. To date, more than 50,000 licenses of the Company’s ETAP and ETAP Real-Time products have been used in demanding generation, transmission, distribution, and industrial power system projects around the world. Visit etap.com for more information

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