Ice NP900 Series, Инструкция по применению

Страница: 316 / 504

Application Guide - NP900 Series 316 (504
)
A996A
On the other hand differential protections negative properties are that it is not the easiest
to set up to operate correctly and second set of current transformers are required thus
increasing the installation cost. In bigger scale power transformers this still is marginal
cost.
In following chapter the principles of the transformer and how to set the differential
protection correctly for an example application are presented.
1.1.1.4 T
RANSFORMER PROPERTIES AND BASIC CONCEPTS FOR DIFFERENTIAL
PROTECTION
To set correct differential protection parameters at least transformer nominal data needs
to be known. Minimum requirement is to know the transformer nominal power and the
HV/LV side nominal voltages as well as transformer special properties like tap changer
and auxiliary windings. Also transformer vector group is important to know for matching
the transformer per unitized vectors as well as HV/LV side CT ratios and properties. In this
chapter the setting and principle of transformer differential protection are shown step by
step.
Figure 1.1.1.4-151 Transformer and its components forming the “differential zone”.
First let’s define the area we are working in is the area in between the CTs. This is called
the “differential zone” which means that the currents going inside from another side must
come out from the other side. It doesn’t matter if the signal is scaled either higher or lower
or its phase angle is shifted, both side currents have to match. Otherwise there is problem
within the protected zone which either blocks or keeps the current inside the zone.
Following example shows a typical minimum information from the transformer name plate
data and what to do with it.