Introduction

Testing of distributed busbar protection systems requires a good understanding of the architecture of the system and the functionality of each of its components. The article describes two basic distributed protection systems:

• Remote units that provide the interface with the process and a central unit that performs the bus bar differential protection function
• Remote units determining the fault's direction and a central unit performing the bus bar directional comparison protection function

The remote units in both cases can be multifunctional IEDs, which will require additional testing to cover the complete functionality of such a system.
The testing of the remote units is the first step in the process. It is required in order to ensure that all components of the bus protection system are functioning as specified by the manufacturer.  If a directional comparison bus protection system is used, it is important to properly simulate the transition between pre-fault to fault condition. While the testing of conventional bus protection systems requires the simulation of currents only, the testing of the directional functions in the remote units requires simulation of both currents and voltages.  Each of the above described schemes imposes different requirements for the tools used for testing of the protection system. At the same time, the requirements for testing are affected by the type of test being performed - type testing, acceptance testing, commissioning, routine testing, or troubleshooting.
The behavior of bus protection systems is also dependent on their response to CT saturation in case of high current external faults. Simulation of CT saturation thus becomes another requirement discussed in the article together with methods for performing such tests when necessary.
IEC 61850 has a significant impact on the way distributed bus protection systems can be designed and implemented. The article analyzes such systems and discusses the methods and tools for their testing. Comparisons between conventional and IEC 61850 based cases are presented. The last part of the article addresses the commissioning of distributed bus protection systems and the importance of accurate time synchronization of multiple test devices operating as a distributed test system.