3BHB005243R0105 KUC755AE105 ABB Excitation control of medium and high voltage frequency conversion board

Description

The overtemperature protection serves to protect a winding
against inadmissible temperature rise due to an overcurrent. It is
suitable for machines, transformers and lines. The temperature
rise of the winding is derived from the current using a thermal
image of the protected unit. In contrast to an overload protection
function, the overtemperature function can be used for machines
of any rating and thermal capacity. It monitors temperature rise
and not the absolute temperature of the winding. Thus neither
the ambient temperature nor the efficiency of the cooling system
bear any influence on protection.
The operation of the overtemperature function is based on a
thermal image, which models the temperature rise of the protected winding. When the current changes, the temperature rise
changes from an initial value to a final value according to one or
more exponential functions. For a transformer, these represent,
for example, the thermal behaviour of the cooling water, the oil,
the copper of the windings etc. In the case of a motor or a generator, they concern the iron, end windings, windings in the slots
etc. One of these functions is always more pronounced than the
others, e.g. the transformer oil or motor iron. The overtemperature function utilises two models for constructing the transient
temperature rise. The first employs an exponential function and
the second “universal” model a universally applicable function
(see Fig. 3.5.19.1). The first model is suitable for the case where
one exponential function is so predominant, that the others may
be either neglected, or approximately taken into account by ad-
RE. 216 1MDU02005-EN / 3 / Rev. I ABB Switzerland Ltd
3.5.19-7
justing the thermal time constant. The second model is to be
preferred for more complex transient temperature rise functions.
This protection function does not monitor instantaneous values
as do most of the others. The temperature rise is determined by
integration over a given period, e.g. 1/40 of the thermal time
constant.
The overtemperature function determines the temperature rise
curve from the following:

the final temperature rise given by the current

the rate of temperature rise by virtue of a transient function.