In a distance relay the effect of CT saturation is complex and depends on the measuring principle: it can cause both failure to operate and unwanted trips. IEC TR 61869-100 states that the relay manufacturer defines the CT requirements from the worst fault cases relative to the zone limits, with additional consideration if the voltage comes from a CVT.
How a distance relay measures
A distance relay measures voltage and current and calculates the impedance between its location and the fault. Since the impedance per kilometer of a line is almost constant, the measured impedance is proportional to the distance to the fault. The relay compares that impedance with the set reach and, if it is lower, trips (IEC TR 61869-100, 2017 edition, clause 11.4.1).
Because of errors in the current and voltage transformers, in the line data and in the settings, zone 1 must not cover the entire line. Covering all of it risks an overreach that is not accepted. That is why three zones are used:
| Zone | Function |
|---|---|
| Zone 1 | Protects only part of the line |
| Zone 2 | Time-delayed, covers the section between the end of zone 1 and the remote busbar |
| Zone 3 | Remote backup |
Distance protection 21 explains the complete scheme. What matters here is what it demands of the CT.
What saturation does to the measurement
The technical report warns that the influence of CT saturation on distance protection is very complex. It depends on the technology, the manufacturer and the application. Two factors weigh in:
- The operating characteristic. An electromechanical relay naturally tends to reduce its reach, because it measures a lower current when the CT saturates. A static relay behaves similarly if it uses circular characteristics.
- The measuring algorithm. In numerical relays the impact varies widely and depends on the impedance algorithm. There is a risk of failure to operate and also of unwanted trips under severe saturation.
Modern relays include functions that compensate for CT saturation and reduce the risk of incorrect operations. Even so, saturation is the reason why the CT must meet specific requirements.
Who defines the CT requirements
Clause 11.4.2 of the technical report says it directly: the relay manufacturer defines and supplies the CT design requirements. They are based on the worst fault cases of the network, which depend on the fault position relative to the protected line and the zone limits. These requirements can be critical for the stability of the relay against saturation.
If the voltage comes from a capacitor voltage transformer (CVT), additional consideration is needed.
The report refers to IEC 60255-121 for a more detailed approach, with several significant fault scenarios and examples. Its own example is limited to two fault positions and three-phase faults, and it can be extended with that approach.
Which CT parameters come into play
The relay manufacturer’s requirements are expressed with the IEC 61869-2, 2012 edition, parameters for the transient error classes:
- Kssc factor: ratio between the rated symmetrical short-circuit primary current and the rated primary current (clause 3.4.232).
- Ktd factor: dimensioning for the DC component. In the alternative specification, it is normally given by the protection supplier (Table 207, note 3).
- Duty cycle: one fault (C-O) or two with auto-reclosing (C-O-C-O). On a line with auto-reclosing, the second fault matters because the core may be left with residual flux (clauses 3.4.224 and 3.4.230).
- Time to accuracy limit: the time during which accuracy must be maintained. It is set by the protection’s critical measuring time (clause 3.4.228).
- Rated resistive burden.
In its design example, the report describes the time to accuracy limit as the first assumption that comes out of the relay algorithm. From there the work moves on to relay tests. The example shows the path, which is of more use to those who write the relay documentation than to the designer. The designer must apply the simplified results from the relay documentation: Ktd factor, Kssc factor and the guidance on remanence.
What to ask the relay manufacturer
Before specifying the CT, ask in writing for:
- The Kssc factor and the Ktd factor that their protection needs.
- The duty cycle assumed: one fault or two with auto-reclosing, and the times of each.
- The resistive burden to be used in the calculation.
- The guidance on how to treat remanence, which the report lists among the results the manufacturer supplies (clause 11.4.3.1.1).
- If the voltage comes from a CVT, what additional consideration applies.
With those answers the designer can specify the CT parameters without repeating the manufacturer’s analysis.
What to do in a project
- Ask the relay manufacturer for the CT requirements in the standard form: Kssc, Ktd, duty cycle and burden.
- Calculate the limiting voltage with those factors and with the real burden of the wiring and the relay.
- Review the existing CT if the relay is new. The voltage the new relay requires may not be available in the old core.
- If there is a CVT, include its transient behavior in the requirements.
What not to do
- Do not size the CT of a line with the same criterion as an overcurrent protection.
- Do not use a single fault scenario to define the CT. Consider faults near the zone limits.
- Do not ignore auto-reclosing: the second fault can start with remanence.
- Do not assume the relay compensates for any saturation. The limits are given by the relay manufacturer.
Related topics
The distance CT is calculated with the Ktd factor and compared in the equivalences. The TPY class and the duty cycle come into play when there is auto-reclosing. The relay that uses it is distance protection, on high-voltage lines.
When to call us
At Tiza Group we define with you the CT requirements for your distance relays as part of protection coordination studies, worldwide, remotely, and we test CTs and protection with secondary injection in Colombia, or elsewhere on request. You can estimate saturation with the free CT burden and saturation tool. Write to us and we will quote it.
Related guides
- CT transient dimensioning factor Ktd
- Equivalences between protection CT classes
- TPX, TPY and TPZ CT classes: transient error
- Distance Protection 21 (PDIS): What It Is and How It Works
- Distance 21 trip on load or power swing
References
- IEC TR 61869-100, application guide for current transformers in protection systems, 2017 edition, clause 11.4. The report cites IEC 60255-121 for the fault scenario method.
- IEC 61869-2, Instrument transformers - Part 2: Additional requirements for current transformers, 2012 edition, clauses 3.4.224, 3.4.228, 3.4.230, 3.4.232 and Table 207.
Frequently asked questions
Who defines the class and the dimensioning factor of the CT for a distance relay?
The relay manufacturer, based on the worst fault cases of the network: the fault positions relative to the zone limits. The designer applies those general requirements and specifies the CT parameters for the project (IEC TR 61869-100, clause 11.4.2).
What happens to a distance relay if the CT saturates?
It depends on the technology and the measuring algorithm. An electromechanical relay tends to reduce its reach because it measures a lower current. A numerical relay can fail to operate or trip unwantedly under severe saturation, although modern relays include functions that compensate part of the effect (clause 11.4.1).
Does the type of voltage transformer matter?
Yes. If the voltage is measured with a capacitor voltage transformer (CVT), the report asks for additional consideration when defining the CT requirements. The report gives no further detail (clause 11.4.2).