Alternative approaches and dynamic analysis considerations for detecting open phase conductors in three phase power systems

Highlights

• Open phase conditions can disrupt plant operations, damage critical assets, and cause cascading system failures.

• Typical existing plant protective relay schemes do not adequately detect open phase conditions under all system conditions.

• Different approaches for detecting open phase conditions have been deployed.

• There are pros and cons with each detection method.

• This paper discusses key considerations for analyzing open phase conditions and selecting a detection system.

Abstract

Open phase conductors in three-phase power systems can be difficult to detect with conventional protection relay schemes. Such events can have adverse consequences to power system equipment reliability and performance. The resultant voltage unbalance associated with open phase events can cause excessive heating in transformer core and coil assemblies and tanks, reduce the available starting and running torque of motors, increase motor acceleration time, cause inadvertent tripping of critical loads, and thermally damage plant equipment.

Power system response to an open phase condition is highly dependent on a number of factors, including the type of open phase condition (events involving one or two phase conductors, coupled with or without a ground), the location of the open phase, the topology of the power system, transformer core design and winding connections, and type and magnitude of system loading.

This paper briefly describes industry operating experience with open phase events. It summarizes the various alternative approaches for detecting open phase conductors on large station service transformers. Dynamic modeling considerations and techniques are described and a summary of analytical results which convey the challenges, advantages, and disadvantages associated with different detection strategies are presented. The role symmetrical components and sequence components can play in understanding the impact of open phase conditions on power system equipment also is discussed.

Introduction

In January 2012, a mechanical failure of an underhung isolator caused an open circuit in a single phase conductor of a three-phase, 345 kV overhead power line feeding the two system auxiliary transformers (SATs) at the Byron Nuclear Station, Unit 2. The open phase caused unbalanced voltages on the plant buses, the automatic trip of certain plant equipment, and a significant plant transient. The event revealed a previously unanalyzed design vulnerability in the station’s offsite to onsite power system. Subsequent reviews of industry operating experience indicate that open phase events occur in industrial power systems more frequently than desired [1]. For example, IAEA Safety Report No. 91 [2] summarizes fourteen open phase events at nuclear power plants in various countries worldwide. These events involved systems operating at 115 kV–400 kV and were due to a variety of causes including broken or fatigued conductors, misoperated circuit breaker poles, failed insulators, and loose connections.

Open phase conditions (OPCs) can be difficult to detect, cause inadvertent trips of critical plant loads, and damage equipment. When a motor is supplied from a wye-delta or delta-wye transformer, an open phase on the primary (or line) side of the transformer results in increased current that may go undetected by the motor’s overload protection because the positive sequence current is not excessive. However, the voltage unbalance at the terminals of the machine may cause excessive heating due to negative sequence currents. Significant voltage unbalance can cause running motors to stall and trip off. Negative sequence voltages produce torques during motor starting conditions which retard motor acceleration causing longer acceleration times and create the potential for mission critical loads already running to stall or trip on overload due to abnormal starting currents.