CT saturation creates a false differential current in the 87T. IEC TR 61869-100 recommends designing the core considering every way the fault can occur, matching the ratio to the rated current of the power transformer and using low-remanence classes (PR, TPY, TPZ or PXR, possibly with Kx).
What the 87T asks of the CTs
Differential protection compares the currents entering and leaving the protected zone. In normal conditions or with an external fault, the vector sum of the currents should be zero. In an internal fault, the sum is almost equal to the short-circuit current at the fault point (IEC TR 61869-100, 2017 edition, clause 11.5.1).
If one CT saturates and another does not, a false differential current appears. In transformer protection, the main problem is not detecting the internal fault, which the relay almost always recognizes, but staying stable for external faults with high through-fault currents, especially if they take time to clear (clauses 11.5.1 and 11.5.3.1).
Transformer differential protection 87T explains the principle and the vector group compensation. This guide deals only with the CT.
The most demanding fault for the CT
The report also considers internal faults at the CT itself, because they can saturate it in the worst case. The most critical situation is a fault on the high-voltage side, between the CT and the transformer (clause 11.5.3.1):
- With low source impedances, the short-circuit current can be very high.
- The rated primary current of the CT is relatively low, because its value is matched to the rated current of the power transformer.
The ratio between the two is large and the core works far above its rated current.
Numerical relays have algorithms that detect saturation and help avoid incorrect trips. Many modern relays need CT accuracy for a short time from the start of the fault; that time is what the relay manufacturer communicates as the time to accuracy limit (IEC 61869-2, 2012 edition, clause 3.4.228).
What the technical report recommends
Clause 11.5.3.2 gathers three recommendations:
| Recommendation | What it means |
|---|---|
| Design the cores considering all aspects of fault occurrence | Internal and external faults, not just the AC current |
| Match the CT ratios to the rated current of the power transformer | The rated primary current of each CT follows the rated current of the side where it is installed |
| Use an anti-remanence type | PR, TPY, TPZ or PXR, the latter possibly with a Kx definition |
Low-remanence classes limit the KR factor to 10 % (IEC 61869-2, clauses 5.6.202.3.5, 5.6.202.4 and 5.6.202.5.2). That keeps the core from reaching the fault with part of its flux range already occupied.
- Are the CTs on both sides new?
- Yes: Low-remanence class (PR, PXR, TPY or TPZ), with ratio matched to the rated current
- No: The relay manufacturer must confirm that the existing cores comply
What happens with the TPZ
The TPZ damps the DC component of the secondary current. The report warns that this damping has consequences when low currents appear with a high primary time constant: small inrush currents, sympathetic inrush of a transformer, or a low fault current contribution from a power plant. CT saturation can occur even at very low current levels.
Those scenarios are hard to stabilize in a differential relay. Unwanted trips for external faults occurred with TPZ when the relay compared unfiltered currents, sample by sample. If the DC components of the CTs are suppressed to a different degree, because of different classes or core sizes, a differential current appears without an internal fault. Relay manufacturers must address that case (clause 11.2.1).
If the relay filters the fundamental component with Fourier, mixing classes in the same zone normally gives no practical problems.
Product standard data that goes with the specification
IEC 61869-2 sets standard values that condition the choice. The rated primary current is taken from the series 10, 12.5, 15, 20, 25, 30, 40, 50, 60 and 75 A, and their decimal multiples or fractions (clause 5.201). The rated continuous thermal current is, by standard, the rated primary current; if a higher one is requested, the preferred values are 120, 150 and 200 % (clause 5.203). The rated short-time thermal current Ith is assigned for 1 s and the dynamic current is 2.5 times Ith (clause 5.204).
With those values, the ratio of each CT is chosen from the standard series closest to the rated current of the transformer side where it is installed. The relay manufacturer confirms that the ratio is compatible with its protection.
What the report’s example includes
The example (clause 11.5.3.3) is a transformer with differential protection between the phase CTs of both sides (87T) and restricted earth fault protection (87N) connected to the phase CTs and the neutral CT of the low-voltage side. The 87N compensates for the loss of sensitivity of the 87T caused by the vector group adaptation, and detects faults very close to the star point with greater sensitivity.
What not to do
- Do not use the same ratio on all CTs without checking that each one is matched to the rated current of the side where it is installed.
- Do not mix cores without checking with the relay manufacturer how it treats the DC component.
- Do not reuse existing CTs with a new relay without checking the relay’s requirements.
- Do not use low-remanence classes in a high-impedance protection that requires high sensitivity: the report considers them unsuitable there.
Related topics
The CT specification for transformer differential protection is compared with that for busbar and line differential protection. Remanence, the class equivalences and the Ktd come into play in the choice. The relay is explained in transformer differential and in restricted earth fault.
When to call us
At Tiza Group we carry out transformer differential protection tests and verify CT saturation with the real burden in Colombia, or elsewhere on request. We also review the specification in protection coordination studies, worldwide, remotely. You can estimate the case with the free CT burden and saturation tool. Write to us and we will quote it.
Related guides
- CT for 87B busbar and 87L line differential
- Equivalences between protection CT classes
- CT remanence and air gap
- Transformer Differential 87T (PDIF): How It Works
- 87T trip without an internal fault: causes and diagnosis
References
- IEC TR 61869-100, application guide for current transformers in protection systems, 2017 edition, clauses 11.2.1 and 11.5.1 to 11.5.3.
- IEC 61869-2, Instrument transformers - Part 2: Additional requirements for current transformers, 2012 edition, clauses 3.4.228, 5.6.202.3.5, 5.6.202.4 and 5.6.202.5.2.
Frequently asked questions
Which fault is the most critical for the CTs of a transformer differential?
The fault on the high-voltage side between the CT and the transformer. With low source impedances the short-circuit current is very high, while the rated primary current of the CT is relatively low because it is matched to the transformer rated current (IEC TR 61869-100, clause 11.5.3.1).
Is it mandatory to use low-remanence classes in an 87T?
The technical report recommends them: PR, TPY, TPZ and PXR, the latter possibly with a Kx factor definition. It is not a requirement of the product standard, which only defines them and limits their remanence (IEC 61869-2, clauses 5.6.202.3.5, 5.6.202.4 and 5.6.202.5.2; IEC TR 61869-100, clause 11.5.3.2).
Can different CT classes be mixed in the same differential zone?
The report states that, with relays that use Fourier filtering, using different classes within the same zone, for example TPZ and a gapless core, normally shows no practical problems. With relays that compare unfiltered currents, sample by sample, unwanted trips have occurred (clause 11.2.1).