The TPX, TPY and TPZ classes of IEC 61869-2 are defined by the CT error during an asymmetrical fault, that is, with a DC component. In all three the maximum instantaneous error is 10 %. Remanence is what changes: TPX does not limit it, TPY limits it to 10 % and TPZ has almost none, and for TPZ only the AC component of the error is controlled.
Short answer
When a fault starts asymmetrical, the primary current carries a DC component that can saturate a CT core within a few cycles, even if the AC current is small. The TPX, TPY and TPZ classes of IEC 61869-2 are defined for that case: each one sets how much transient error is allowed during a fault duty cycle. In all three, the maximum permitted instantaneous error is 10 %.
Three ways to define a protection CT
The standard groups the protection classes by what they specify (Table 204):
| Designation | Remanent flux limit | How the CT is defined |
|---|---|---|
| P, PR | PR yes; P no | Meets the composite error with a symmetrical steady-state short-circuit current |
| PX, PXR | PXR yes; PX no | Defined by its magnetization characteristic |
| TPX, TPY, TPZ | TPX no; TPY and TPZ yes | Meets the transient error with an asymmetrical short-circuit current |
In practice, each of the three definitions can result in the same physical CT.
The limits of Table 206
With the rated resistive burden connected, the ratio error and the phase displacement at rated current do not exceed these values (5.6.202.5.1):
| Class | Ratio error | Phase displacement | Transient error limit |
|---|---|---|---|
| TPX | ±0.5 % | ±30 min | Maximum instantaneous error of 10 % |
| TPY | ±1.0 % | ±60 min | Maximum instantaneous error of 10 % |
| TPZ | ±1.0 % | 180 ± 18 min | AC component of the error of 10 % |
All limits refer to a secondary winding temperature of 75 °C.
The maximum instantaneous error (3.4.222) is the peak value of the difference between the secondary current multiplied by the rated ratio and the primary current, during the specified duty cycle. It is expressed as a percentage of the peak value of the rated primary short-circuit current. In class TPZ the limit applies only to the AC component of that error (3.4.223), because for a TPZ the accuracy is specified only for the AC component.
Remanence
The remanence factor KR is the ratio, in percent, between remanent flux and saturation flux (3.4.212). The standard sets these limits (5.6.202.5.2):
- TPX: no limit.
- TPY: KR not greater than 10 %.
- TPZ: KR not greater than 10 %. The core design gives a much lower value, so the remanent flux is neglected.
How they are specified
There are two methods, which must not be mixed because the CT would be over-specified (5.6.202.5.3 and Table 207):
- Standard specification. The class, the factor Kssc (the ratio between the rated symmetrical short-circuit primary current and the rated primary current), the duty cycle, the primary time constant Tp and the resistive burden Rb are given. The duty cycle is C-O with the time t′al, or C-O-C-O with the times t′al, t′, tfr and t″al.
- Alternative specification. The class, Kssc, the transient dimensioning factor Ktd, the time constant Ts (only for TPY cores) and Rb are given. This method makes it possible to set requirements that cover several duty cycles. Ktd is usually provided by the protection equipment supplier.
The standard rated resistive burden for these classes is 0.5, 1, 2 or 5 Ω, for a rated secondary current of 1 A. With another secondary current the values change in inverse proportion to the square of the current (5.5.202).
A CT with taps on the secondary meets the accuracy requirements for one ratio only. In a CT with primary reconnection, the requirement can be met at several ratios (notes 1 and 2 of Table 207).
What the plate says
The class marking has a mandatory part, which holds the data needed to know whether the CT meets the requirement, and a supplementary part, mandatory only if the customer specified a duty cycle (6.13.202.6). Examples from the standard, with Kssc = 20 and Ktd = 12.5:
- Rb = 5 Ω, class TPX 20 × 12.5, Rct ≤ 2.8 Ω
- Rb = 5 Ω, class TPY 20 × 12.5, Rct ≤ 2.8 Ω, Ts = 900 ms
- Rb = 5 Ω, class TPZ 20 × 12.5, Rct ≤ 2.8 Ω
Example of the supplementary part: «Cycle 100 ms, Tp = 100 ms» means t′al = 100 ms and Tp = 100 ms.
Tests
- Ratio error and phase displacement. They are measured at rated current and corrected to 75 °C, by direct measurement with a reference CT. For low leakage reactance CTs, Annex 2E gives an indirect method useful on site and for monitoring (7.3.5.204).
- Error at the limit. This is the type test that demonstrates compliance with the asymmetrical fault. If the CT is of low leakage reactance, it can be done indirectly; if not, it must be done directly (7.2.6.204 and Annex 2B). In the routine test, low leakage reactance CTs are tested indirectly (7.3.5.205).
- Time constant Ts and resistance Rct. They are measured as routine tests. The measured time constant must not differ by more than ±30 % from the specified one (7.3.202).
A TPX core must be demagnetized before the direct test because of its high remanence. For a TPY it may be necessary if the remanence is not negligible (2B.3.2).
What not to do
- Do not ask for TPY with a time constant Ts without knowing whether your relay uses it: in the alternative specification, Ts is the only CT parameter that enters the calculation of Ktd.
- Do not mix the standard specification with the alternative one.
- Do not compare the errors of a 5P CT with those of a TPY: one is composite error in steady state, the other is instantaneous error with asymmetry.
Related topics
The TP classes are defined over a duty cycle, which is explained in C-O and C-O-C-O cycles. The calculation of the Ktd factor and the secondary time constant enter the core sizing. To compare with other classes, see PX and composite error. These CTs are used in high voltage and with automatic reclosing. Our free CT burden and saturation tool covers classes 5P, 10P and PX; the TP classes are calculated with the duty cycle.
When to call us
At Tiza Group we verify that the protection CTs in your substation match what your relay needs, and we check their plate data with measurements on site. See our instrument transformer testing, in Colombia or elsewhere on request. If you need to define the CT for a new protection, engineering design and studies, remote and worldwide, includes it. Write to us for a quote.
Related guides
- CT Duty Cycles: C-O and C-O-C-O
- CT Rct and Ts: Winding Resistance and Time Constant
- Who Sizes the Protection CT: IEC TR 61869-100
- High-Voltage Current Transformer (CT): Types
- Current Transformer (CT): Parts, Functions and Tests
References
- IEC 61869-2, Instrument transformers - Part 2: Additional requirements for current transformers, edition 1.0 (2012), clauses 3.4.212, 3.4.221 to 3.4.234, 5.5.202, 5.6.202.1, 5.6.202.5, Tables 204, 206 and 207, 6.13.202.6, 7.2.6.204, 7.3.5.204, 7.3.5.205, 7.3.202 and Annexes 2B and 2E.
Frequently asked questions
When is a TPY CT used instead of a 5P?
The standard gives the difference in definition, not an application rule: P CTs are defined by composite error under symmetrical steady-state current, and TP CTs by the error with an asymmetrical fault. Which class fits is set by the relay manufacturer or the protection study. TP CTs are used where the error with a DC component must be bounded.
What does Rb = 5 Ω, class TPY 20x12.5, Rct ≤ 2.8 Ω, Ts = 900 ms mean?
It is a TPY CT with a rated resistive burden of 5 Ω, a rated symmetrical short-circuit current factor Kssc of 20 and a transient dimensioning factor Ktd of 12.5. The winding resistance does not exceed 2.8 Ω and the secondary time constant is 900 ms. It is the plate example from IEC 61869-2.
Why does class TPZ specify the phase displacement and not remanence?
The core of a TPZ CT is designed so that remanence is well below 10 %, so the standard treats it as negligible. In exchange, it requires a phase displacement of 180 ± 18 minutes at rated current, which is verified as a routine test and from which the time constant Ts is obtained.