CT for Busbar 87B and Line 87L Differential

Short answer

In busbar protection the CT must withstand extreme saturation and different ratios between bays; in line protection, the same CT type at each end. IEC TR 61869-100 recommends type 5PR for busbars and an anti-remanence type for lines, and allows gapless cores in existing installations if tests show they are suitable.

Two applications with a common requirement

Differential protection compares currents and, for an external fault, must remain stable even if the CTs saturate. For both applications in this guide, the technical report IEC TR 61869-100, 2017 edition, asks to consider internal and external faults. In general, the external fault is the most critical (clauses 11.5.1 and 11.5.2).

Each application adds its own problem. In busbars, saturation can be extreme and the CTs are very different from one another. In lines, the two ends are far apart and the relay depends on what each CT measures.

Busbar differential protection (87B)

In most cases, this application has to live with extreme saturation (clause 11.5.4.1). On one hand, short-circuit currents are usually high. On the other, the ratio and type of the CTs can differ widely between bays.

One of the most critical cases is an external fault on the feeder whose CT has the lowest ratio, that is, the lowest rated primary current. The fault current is very high compared with that CT’s rating, and the busbar protection must remain stable.

Modern busbar relays decide between an internal and an external fault with very short times to accuracy limit. If the time is short, the transient factors are low and the demands on the CT are relatively low. Several methods and algorithms, such as saturation detection, keep operation stable for external faults with high through-fault currents and extreme saturation.

Recommendations for the busbar CT

Consider internal and external faults.
Use type 5PR, low-remanence.
In existing plants, gapless cores can be used if tests show they are suitable (clause 11.5.4.2).

Different ratios

Most numerical busbar equipment accepts CTs with different ratios. Each ratio is a flexible parameter that scales the measured secondary current to the current of the busbar object. That is why each CT can be matched to the rated current of its feeder. One limit: the measuring range of the equipment can restrict the maximum secondary current and, with it, the minimum ratio. It is one of the main differences compared with the high-impedance principle.

For a transformer bay, the criterion is the same as in the 87T: the ratio follows the rated current of the bay.

CT data worth gathering for each bay

To review an existing scheme or define a new one, gather for each bay the parameters that IEC 61869-2, 2012 edition, uses to specify the class (clauses 5.6.202.2 to 5.6.202.5):

The ratio and the rated secondary current.
The class and type, for example 5PR, and its accuracy limit factor.
In PX or PXR, the rated knee-point e.m.f., the maximum excitation current and the secondary resistance.
In TPX, TPY or TPZ, the Kssc factor, the duty cycle, the primary time constant and the resistive burden.
The real connected burden: cable plus relay input.
The rated short-time thermal current Ith, assigned for 1 s, and the dynamic current, which the standard sets at 2.5 times Ith (clause 5.204).

Line differential protection (87L)

The report distinguishes two categories (clause 11.5.5.1):

Category	How it decides
Current differential	Sums the currents of the ends
Phase comparison	Compares the angle between the currents of the ends

In phase comparison, with a healthy line or an external fault, the primary currents are in phase in theory. In an internal fault they are in opposition. The line charging currents shift the phase in normal conditions, and CT saturation shifts the current zero crossings, so the phase error can be high. Some special measures in the algorithms give more stability against unwanted operations.

Recommendations for the line CT

The same CT type at each end of the line.
Anti-remanence type.
In existing installations, gapless cores if tests show they are suitable (clause 11.5.5.2).

The report’s example is a two-end line at 50 Hz. Schemes with more ends exist, for example for lines with tapped branches. The 87L line differential protection guide covers the scheme and the communication between ends.

The high-impedance case

The report notes that in high-impedance differential schemes the aim is to detect even low fault currents with high sensitivity. That is why anti-remanence classes, such as 5PR, 10PR, TPY or PXR, are not suitable there. In those schemes the stabilizing resistor controls the saturation caused by remanence (clauses 8.2 and 11.2.1). High-impedance differential protection explains the scheme.

What not to do

Do not use CTs of different types at the two ends of a line.
Do not size the CT of a busbar feeder only with its rated current: the external fault flowing through it can be much larger.
Do not accept an existing CT without a gapped core until you see the tests with the relay.
Do not apply low-impedance recommendations to a high-impedance scheme.

Busbar and line differentials are compared with the CT for transformer differential protection. Remanence, the PX and PXR classes and the Ktd condition the design. The relays are described in busbar differential, differential protection and high impedance. 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 test busbar and line differential protections with secondary injection and review CT saturation with the real burden in Colombia, or elsewhere on request. We also define the specification in protection coordination studies, worldwide, remotely. Write to us and we will quote it.

References

IEC TR 61869-100, application guide for current transformers in protection systems, 2017 edition, clauses 8.2, 11.2.1 and 11.5.1 to 11.5.5.
IEC 61869-2, Instrument transformers - Part 2: Additional requirements for current transformers, 2012 edition, clause 5.6.202.3.5 and Table 204.

Frequently asked questions

Which CT class does the report recommend for busbar differential protection?

Type 5PR, which is low-remanence. In existing installations, gapless cores can be used if tests show they are suitable (IEC TR 61869-100, clause 11.5.4.2).

Can the CTs of a busbar differential have different ratios?

Yes, with modern numerical equipment: each ratio is a parameter that scales the measured secondary current to a bay current. Each ratio can be matched to the rated current of its feeder. The limitation is in the measuring range, which can set the minimum ratio (clause 11.5.4.2).

What type of CT does a line differential use?

The same type at each end of the line, and of an anti-remanence type (clause 11.5.5.2). In existing installations, gapless cores are accepted if tests show they are suitable.