Instrument Transformers in Power System Protection

Protection or measuring devices require
data on the electrical rating of the equipment
to be protected. To obtain this data, following intermediary devices are
needed:

• voltage transformers (VT)
• current transformer (CT)

These devices fulfill the following functions: 

• reduction of the value to be measured
  (e.g.1500/5 A)
• galvanic isolation
• providing the power required for data
  processing and for protection operation
  itself. 

Current Transformers

The CTs are characterized by the following
values. 

CT voltage 

This is the operating voltage applied to the
CT primary. Note that the primary is at the
HV potential level and that one of the
secondary terminals is generally earthed.
As for other equipment, the following is also
defined :
maximum1 min. withstand voltage at
standard frequency
maximum impulse withstand voltage.
e.g. for 24 kV rated voltage, the CT must
withstand 50 kV voltage for 1 min at 50 Hz
and 125 kV impulse voltage. 

Rated transformation ratio 

It is usually given as the transformation ratio
between primary and secondary current
I1/I2. Secondary current is generally 5 A
or 1 A. 

Accuracy level 

It is defined by the composite error for the
accuracy limit current.
e.g. 5P10 means 5% error for 10 In
10P15 means 10% error for 15 In
5P and 10P are the standard accuracies
classes.
5 In, 10 In, 15 In, 20 In are the standard
accuracy limit currents. 

The accuracy limit factor 

is the ratio
between the accuracy limit current and the
rated current.
Class X is another way of specifying CT
characteristics based on “knee-point
voltage” (fig.1, CT response in the saturated
state).

Accuracy level power

Secondary power at rated current for which
the accuracy level is guaranteed. Expressed
in VA, it indicates the power that the
secondary can deliver for its rated current,
while respecting the rated accuracy class.
It represents the total consumption of the
secondary circuit, i.e. the power consumed
by all the connected devices as well as the
connecting wires.
If a CT is loaded at a power rating lower
than its accuracy level power, it’s actual
accuracy level is higher than the rated
accuracy level. Likewise, a CT that is loaded
too much loses accuracy. 

Admissible short time current 

Expressed in RMS kA, the maximum current
admissible for 1 second (Ith) (the secondary
being short-circuited) represents CT thermal
overcurrent withstand. The CT must have
the capacity to withstand short-circuit current
for the time required to clear it. If the clearing
time t is other than 1 sec., the current the CT
can withstand is Ith / Vt.
Electrodynamic withstand expressed in peak
kA is at least equal to 2.5 x Ith.

Normal values of rated currents

At the primary (in A)
10 – 12.5 – 15 – 20 – 25 – 30 – 40 –
50 – 60 – 75 and multiples or decimal
submultiples. 

When subjected to very strong current, the
CT becomes saturated, i.e. the secondary
current is no longer proportional to the
primary current.
The current error which corresponds to the
magnetization current becomes very high.

CT Saturation Point or Knee Point

For direct time overcurrent protection, if
twice the setting current does cause
saturation, the operation is ensured no matter
how strong the fault. 

However, there are special types of CT that do not saturate. This are specific “wide band” current sensors. These sensors, most often without magnetic
circuits and therefore not subject to
saturation. Linked to an electronic device,
their response is linear.
These CTs are used and supplied with the
digital technology protection units. They only
require knowledge of the primary rated
current.

Voltage Transformers

Voltage transformers reduced the high voltage across the power system into a value that can be connected to small protective relays. Normally, the secondary side of the VT used for protective relaying has a maximum voltage of 120 Volts. 

The voltage transformer have the following
characteristics (IEC186). 

A few additional key concepts for understanding voltage transformers:

Reference: