Page 501 - Industrial Power Engineering and Applications Handbook

Accuracy 2 % Current (ratio) erroP at % of rated primary current + Phase displacement angle 6 (Figure 15.18)
class in minutes at % of rated primary current
~
% rated. I, 5 20 50 100 120 5 20 100 120
0.1 0.4 0.2 NA 0.1 0.1 15 8 5 5
0.2 0.75 0.35 NA 0.2 0.2 30 15 10 10
0.5 1.5 0.75 NA 0.5 0.5 90 45 30 30
1 .o 3.0 1.5 NA 1 .o 1 .o 180 90 60 60
3.0 - - 3 - 3 Not specified
5.0 - - 5 - 5 Not specified

Table 15.9 Limits of error for protection CTs
loo
Composite error = - il/T joT (K; i2 - i,)* dt %
I1 Accuracy Current error at Phase displacement Composite error
class rated primary angle 6 (Figure at rated
where current 15.18 at rated accuracy limit
K,, = rated transformation ratio % primary current primary current
minutes
%
I, = actual primary current (r.m.s.) ~
I2 = actual secondary current (r.m.s.) 5P +1 + 60 5
i, = instantaneous value of the primary current 10P *3 - 10
i2 = instantaneous value of the secondary current 15P +5 - 15
T = duration of one cycle
= 1/50 s or 20 ms for a 50 Hz system. As in IEC 60044-1
(viii) Knee point voltage
are required to commence their saturation beyond 120%
This is the point on the magnetic curve of the laminated of the primary rated current and saturate fully by 500%,
core of the CT at which the saturation of the core will as a system is not warranted to operate on an overload or
start. It is defined as the point where an increase of 10% short-circuit, and will be interrupted through its protective
in the secondary voltage will increase the excitation current devices. Thus a low knee-point voltage or a low saturation
I,,, by 50% (Figure 15.19). Beyond this point, a very level is needed to protect the connected instruments from
large amount of primary current would be required to fault currents (overcurrent factor) on the primary side.
further magnetize the core, thus limiting the secondary For example, in a measuring CT of 1000/5 A, the
output to a required level. secondary current will be in direct proportion to the
primary current from about 50 to 1200 A and the core
(ix) Instrument security factor (SF) will start saturating beyond 1200 A.

This is the ratio of instrument limit primary current to Overcurrent factor for instruments
the rated primary current. Consequently a high SF will
mean a high transformation of the primary current and As in IEC 6005 1, the measuring instruments are required
can damage instruments connected to its secondary. For to have an overcurrent factor of not more than 120% for
measuring instruments therefore it is kept low, as it is two hours for instruments of all accuracy classes, 200%
required to measure only the normal current and not the for 0.5 second for class 0.5 or less, and 1000% for 0.5
fault current. second for class 1 accuracy and above. Overcurrents or
durations longer than this may damage the instruments.
15.6.2 Measuring current transformers
Accuracy class
These are employed for the measurement of power circuit
currents through an ammeter, kW, kWh or KVAr and This defines the maximum permissible current error at
power factor meter, or similar instruments requiring a the rated current for a particular accuracy class. The
current measurement. They must have a specified accuracy standard accuracy classes for the measuring CTs may be
class as in IEC 60044-1 and the secondary current one of 0.1, 0.2, 0.5, 1, 3 and 5. The limits of error in
substantially proportional to the primary within a working magnitude of the secondary current and the phase error,
range of about 5-120% of its primary rated current. They as discussed in Section 15.6.1 and shown in Figure 15.18,