AQ-200 Current transformer selection

AQ-200 Current transformer selection

Introduction

Current transformer selection is important part of complete protection system design. Current transformer must be able to measure currents according to the application needs in all situations. Specifying current transformers is sometimes challenging because it requires a lot of detailed data from: wiring impedance, protection functions and setting thresholds, protected equipment and back-ground network. It also requires knowledge about different CT characteristics available. If current transformer selection is done incorrectly it may lead to unselective protection or even damage to the protection devices and to the protected equipment.
  1. Overestimation of the short-circuit current can lead to feasibility problems, overrating and high CT costs.
  2. Underestimation of the short-circuit currents can lead to failure to detect the fault, thus destroying the equipment, placing the operator in danger and causing operating downtime.
  3. An output power or accuracy error can result in a malfunction or in failure to trip of the protection devices, thus destroying the equipment, placing the operator in danger and causing operating downtime.
  4. An error in defining the accuracy class of a metering winding will lead to incorrect energy billing and thus a loss of income for the electrical utility or the customer.

Definitions

General definitions

According to IEC 61869-2:2012 when applicable.

Current transformer

Instrument transformer in which the secondary current, under normal conditions of use, is substantially proportional to the primary current and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections. Current transformers are defined by their ratio, power and accuracy class. Their class is chosen according to the application.

Measuring current transformer

Current transformer intended to transmit an information signal to measuring instruments and meters. 

From this type of current transformer good accuracy is required around the nominal current value. Measuring instruments do not need to withstand as high currents as protection relays, for this reason measuring current transformers have lowest possible Safety Factor (SF) to protect the measuring instruments through earlier saturation.

Protective current transformer

A current transformer intended to transmit an information signal to protective and control devices. Protective current transformer classes include: P, PR, PX, PXR, TPX, TPY and TPZ.

Protection current transformer must saturate sufficiently high to allow a relatively accurate measurement of the fault current by protection relay whose operation threshold can be set to very high value. Protection current transformers are thus expected to have an Accuracy Limit Factor (ALF) that is usually high. Keep in mind that the protection relay must be able to withstand these high secondary current values.
Standard symbols are according to IEC 61869-2:2012.


General

Accuracy limit factor

Accuracy limit factor is the value how many times the nominal current value can be exceeded to stay within the specified accuracy class. When accuracy limit factor is exceeded the secondary output of the current transformer may be distorted and this may affect to protection relay operation for example by causing selectivity issues. When selecting current transformer following must be considered: the rated accuracy limit factor ALF is rarely the same as the actual accuracy limit factor ALF’.

This following equation is used for dimensioning protection relay current transformer:

Following equation is used to calculate actual accuracy limit factor ALF’:

Following equation can be used to calculate minimum required rated accuracy limit factor:

Following theoretical equation can be used to dimension the CT in a way that it will not
saturate during transients:

Current transformer rated power

The current transformer needs to have enough power to output the needed secondary current in all cases to the connected burden. If output power is not high enough compared to the connected burden it leads to saturation of the CT.

Following equation can be used to calculate minimum required rated power of the CT:


Wire lead burden

Following equation can be used to calculate secondary wire burden:

CT requirements according to protection functions

Definite time overcurrent protection

The threshold Iset is usually between 2 to 10 times the CT rated current. To ensure that the
CT saturation does not affect the protection, it is necessary to have no saturation up to the
setting point. Usually there is additional safety margin (KAQ=2).

Calculate current transformer rated burden:

Calculate actual accuracy limit factor:

Check if accuracy limit factor is acceptable:

Accuracy limit factor is within the acceptable range as the actual accuracy limit factor is
greater than the requirement 16.7 > 16.

Calculate DC transient effect:

Calculate needed rated power of current transformer:

This kind of CT is too large and expensive, however there is two possibilities: change the CT secondary to 1A or ignore the transient DC component. Ignoring transient DC component may cause additional delay to tripping time.

When transient DC component is ignored the 10VA current transformer is suitable.

Differential overcurrent protection

AQtivate setting tool contains calculations for current transformer selection for differential
overcurrent protection in Transformer Wizard module.