Introduction

Bus Protection Principles
The behavior of bus protection systems during different fault conditions is critical to the stability and security of the electric power system, due to the tripping of the multiple lines and transformers connected to the bus. Fast fault clearing for faults on the bus is very important, while at the same time operation of the bus protection for external faults must be avoided in order to reduce the probability of a system disturbance.

For many years, transmission bus protection systems were based on a centralized principle using physical summation of all incoming currents into the bus differential relay. The differential current is defined as:

Idiff(t) = i1 + i2 + … + iN            (1)

High or low impedance relays with fast operating times protect busses at different voltage levels. This principle is quite simple, but requires all current transformers to be more or less identical and is susceptible to the impact of CT saturation for high current external faults (Figure 1).
 Low impedance bus differential protection relays with multiple restrained inputs significantly reduced the impact of CT saturation and allowed using current transformers with different CT ratios. They typically work based on single or multi-slope characteristic defined by an operating Iop and restrained Ibias current, where:

Idiff(t) = |idiff(t)| = |∑i|                      (2)              Ibias(t) = |i1| + |i2| + … + |iN| (3)  (See Figure 2).

However, the high cost limited their application to a relatively small number of critical substations.
The advancements in communication technology have resulted in significant changes in the design and implementation of bus protection for transmission substations. The conventional centralized bus protection relays are being replaced by communications based distributed bus protection systems that are the subject of this article. Two main types of bus protection are discussed:

• Based on remote (peripheral) units that provide the interface with the process and a central unit that performs the bus bar differential protection function
• Based on remote (peripheral) units that provide the interface with the process and determine the direction of the fault and a central unit that performs the bus bar directional comparison protection function

The remote units in both cases can be multifunctional IEDs by themselves.  In the case of IEC 61850 based systems a new type of device – the merging unit – is introduced.

IEC 61850 has a significant impact on the way distributed bus protection systems can be designed and implemented. This article analyzes such systems.  Comparisons between the conventional and IEC 61850 based cases are presented.