2
Features ..........................................................................................................1
Application .......................................................................................................2
Description of function .....................................................................................2
Connection diagram ........................................................................................5
Connections ....................................................................................................6
Intermodular control signals ............................................................................7
Signal abbreviations ........................................................................................7
Output relay module ........................................................................................8
Auxiliary power module ...................................................................................9
Applications ...................................................................................................10
Technical data ...............................................................................................15
Spare parts ....................................................................................................16
Information required with order .....................................................................17
The complete User’s Manual for the differential relay SPAD 330 C is composed of the
following partial documents:
General differential relay description
34 SPAD 4 EN 1
General characteristics of C-type relay modules
34 SPC 2 EN 1
Differential relay module
34 SPCD 1 EN 1
Instructions for mounting and maintenance of relays
of the SPA-300 series
34 SPA 11 EN 1
Contents
The differential relay SPAD 330 C is designed
to be used as a fast interwinding short-circuit
and interturn fault protection for two-winding
power transformers and power plant generator-
Application
transformer units. The relay is also adopted for
three-winding power transformers provided that
the power transformer is characterized by a
unidirectional power flow.
The differential relay compares the phase
currents on either side of the protected object.
If the amplitude or phase angle difference of
the currents, or both, exceed the threshold
value setting, the relay operates. The current
transformer secondary currents measured on
either side of the protected object are desig-
nated I
1
and I
2
. Under normal service conditions,
that is when there is no fault in the protected
area, the I
1
current is equal to the I
2
current and
the differential current I
d
= 0, expression 1.
I
d
= I
1
- I
2
(1)
In practice the differential current I
d
deviates
from zero also under normal service conditions.
When protecting power transformers, differ-
ential currents are caused by inaccuracies of
the current transformers, changes in the tap-
changer position, no-load current of the power
Description of
function
transformer and temporarily by transformer
connection inrush currents. As the transformer
load current increases, the differential current
caused by the current transformer inaccuracies
and the tap-changer grows in the same
proportion as the load current. At short-circuits
occurring outside the protected area, and when
motors are started, the load current may grow
high enough to cause partial saturation of the
current transformers. In such a case the
differential current may grow to many tens per
cent of the load current. Because of these facts
the operation of the differential relay has to be
stabilized to the load current. The higher the
load current in a biased differential relay, the
higher the differential current is required to
cause a relay operation. The bias current I
b
of
the relay is determined by expression 2:
l
b
= ( I
1
+ I
2
) /2
(2)