Manual MRS1 GB
Woodward
DOK-TD-MRS1 Rev.A
9
4.3 Principle of negative sequence protection
An unbalanced-load can be caused due to unequal distribution of current in the grid on account of
unequal loading, unsymmetrical line-to-line short-circuits (one phase and two phase), line interrup-
tion and also switching operations.
Through the unbalanced-load, negative sequence currents occur in the stator, which cause higher
harmonics with odd numbers in the stator winding and higher harmonics with even numbers in the
rotor winding. The rotor is particularly endangered in this because the higher harmonics put extra
load on the rotor winding and induce eddy currents in massive iron content of the rotor which can
even lead to melting of the metal or to the destruction of the metal structure.
An unbalanced-load is, however, permissible in certain limits and with regard to the thermal loading
limit of the generator. In order to avoid a premature outage of the generator in case of unbalanced-
load, the tripping characteristic of the negative sequence protection should be adapted to the ther-
mal characteristic of the generator.
Basically it is established that the better the cooling of the rotor, the lower are generally the per-
missible negative sequence values. This is due to the fact that with better rotor-cooling the maxi-
mum permissible symmetrical load can be chosen higher, however in relation to that, an unba-
lanced-load is permissible to a lesser extent. For turbo-generators the value of the permissible un-
balanced-load is relatively low. Usual values are approx. 10 - 15 % of the load which is permissible
with symmetrical load.
The negative sequence relay MRS1 has a large number of adjustable tripping characteristics.
Protection of almost every type of generator is thereby possible.
In case of unsymmetrical short-circuits in the grid the negative sequence protection relay normally
also picks up. In order to ensure selectivity, to the extent the overload carrying capacity of the ge-
nerator permits it, a tripping time longer than that of the mains protection (e.g. overload protection)
is to be selected.
