CT remanence is left after interrupting a current that saturated the core, or after DC tests on the secondary. A gapless core can keep almost all of its flux; classes PR, PXR, TPY and TPZ limit it to 10 %, and the standard names the air gap as the means to achieve that.
What remanence is and why it matters
A current transformer (CT) keeps flux in the core after saturating. That remanent flux persists until the core is demagnetized. Demagnetization only happens when high primary currents of the right polarity flow and produce a high voltage across the burden (IEC TR 61869-100, clause 8.3).
Remanence reduces the flux margin left before saturation in the next fault. If the flux starts near the maximum, the CT saturates sooner and delivers a distorted current to the relay. The remanence factor KR measures how much flux is left, as a percentage of the saturation flux.
What effect it has on protection
IEC TR 61869-100, clause 8.2, distinguishes two cases:
- Internal fault (dependability). Remanence is generally not considered. If the CT does not saturate with the AC current, remanence never causes a failure to operate. Its possible consequence is an additional delay, normally of no importance.
- External fault (security). An unwanted trip for a fault outside the protected zone is never accepted. There, remanence is taken into account.
When a gapless core is dimensioned considering remanence, the high initial flux reduces the operating range of the magnetization curve. Relay manufacturers compensate with an additional over-dimensioning factor. That factor depends on the maximum possible remanent flux of the core material, which in most practical cases is not known.
Why a gapless core keeps so much flux
The technical report notes that the maximum remanence of gapless cores depends on the magnetic material and its alloy. Several references classify gapless cores as high-remanence and assume a maximum. The report’s measurements found more recent materials with a higher remanence than that of older cores. High remanence is an unwanted result of optimizing those materials for other criteria.
For that reason, the worst case is taken in the calculation: the maximum possible remanence of the material.
What the air gap does
Inserting one or more air gaps in the core is the means that IEC 61869-2 mentions for limiting the remanence factor (clause 5.6.202.3.5, note). With the air gap, remanence is limited by design and stops being an unpredictable value.
The low-remanence classes, which limit KR to 10 %, are PR, PXR, TPY and TPZ (clauses 5.6.202.3.5, 5.6.202.4 and 5.6.202.5.2). The standard observes that a PXR with a large core and few ampere-turns can have difficulty meeting the limit; in that case a higher value can be agreed (clause 5.6.202.4, note 202).
The technical report proposes using gapped cores as the rule in any new low-impedance protection application. Two reasons it gives:
- They do not have the drawback of an unpredictable remanence level.
- They can normally be dimensioned at a size smaller than or equal to that of an equivalent gapless core.
Low-remanence classes are not a disadvantage in transient behavior compared with a gapless core of the same saturation voltage. The TPZ, and the TPY with a specified low secondary time constant, require individual adjustment according to the relay.
How it is verified
IEC 61869-2 defines remanent flux as the flux that stays in the core 3 minutes after interrupting a magnetizing current large enough to drive it into saturation (clause 3.4.211). Annex 2B.2 describes three measurement methods: AC, DC and capacitor discharge. If the core was not demagnetized beforehand, the measurement must be corrected with a second test with the secondary terminals reversed.
For a TPX core the standard states that it must be demagnetized before each measurement, because of its high remanence. In a TPY the remanent flux is usually so low that it can be neglected (Annex 2B.2.1). These data are useful for asking the manufacturer for the measured KR factor of its core, with the method and the saturation definition used.
Causes of remanence
IEC TR 61869-100, clause 8.3, lists the causes.
| Side | Cause |
|---|---|
| Primary | Interruption of fault currents, especially with a DC component |
| Primary | Quasi-DC currents (GIC) induced in overhead lines by solar storms |
| Primary | Loads with a variable DC component, such as arc furnaces |
| Secondary | Polarity tests with DC current pulses |
| Secondary | Wiring continuity test with a buzzer |
| Secondary | Secondary winding resistance measurement |
| Auto-reclosing cycle | The first fault of a C-O-C-O cycle leaves the core with flux for the second |
| Wiring | Single-phase fault with a common return wire shared by several phases |
The last cause deserves an explanation. When the three CTs share a neutral conductor up to the relay, the voltage drop in that conductor can magnetize the cores of the healthy phases from the secondary. If one of those cores has high remanence, it saturates and current flows through the relay with no primary current. The report recommends cables with one return conductor per phase over long runs.
When an air gap makes sense
- Is the protection a high-impedance differential?
- Yes: Not needed. The stabilizing resistor controls remanence
- No: Is the CT new?
- Yes: Gapped core (PR, PXR or TPY)
- No: Verify with relay tests before reusing it
For existing installations, the report states that gapless cores can be used in busbar and line differential protection if tests show they are suitable. The final decision for each relay is made with relay tests. Differential functions are the most exposed to this problem.
What not to do
- Do not assume that a gapless protection core has low remanence. Ask the manufacturer for the value.
- Do not run polarity, continuity or resistance tests with DC current without planning the demagnetization afterward. The standard does not describe it here, and neither does this guide.
- Do not reuse an old CT with a new relay without checking that it meets that relay’s requirements.
Related topics
Remanence uses the saturation flux and the Ktd factor. It affects transformer differential protection and busbar and line differential protection. The overview is in current transformer. For a first check with classes 5P, 10P or PX, use our free CT burden and saturation tool.
When to call us
At Tiza Group we carry out current and voltage transformer tests in Colombia, or elsewhere on request, and verify that existing CTs are suitable for new relays. If you are going to replace electromechanical relays with digital ones, we also review the CTs as part of secondary engineering design, worldwide, remotely. Write to us and we will quote it.
Related guides
- CT saturation flux and remanence factor
- CT transient dimensioning factor Ktd
- CT for 87T transformer differential protection: specification
- Current Transformer (CT): Parts, Functions and Tests
- High-Impedance Differential Protection: What It Is and How It Works
References
- IEC 61869-2, Instrument transformers - Part 2: Additional requirements for current transformers, 2012 edition, clauses 5.6.202.3.5, 5.6.202.4 and 5.6.202.5.2, Table 204 and Annex 2B.2.1.
- IEC TR 61869-100, application guide for current transformers in protection systems, 2017 edition, clauses 8.2 and 8.3.
Frequently asked questions
What causes remanence in a current transformer?
On the primary side: interruption of fault currents with a DC component, quasi-DC currents induced by solar storms, and loads with a DC component, such as arc furnaces. On the secondary side: DC tests such as polarity checks, buzzer continuity checks and winding resistance measurement (IEC TR 61869-100, clause 8.3).
Can remanence prevent the protection from tripping for an internal fault?
As long as the CT does not saturate with the AC current of the fault, remanence does not cause a failure to operate; at most, a delay. The risk is in security: an unwanted trip for an external fault is not accepted, which is why remanence is considered in those cases (IEC TR 61869-100, clause 8.2).
Does a high-impedance differential protection need a gapped CT?
No. In that scheme the stabilizing resistor controls the saturation caused by remanence, and no remanence over-dimensioning factor needs to be applied either (IEC TR 61869-100, clause 8.2). Low-remanence classes can even be unsuitable when high sensitivity is sought.