LPIT Fault Detection, Reliability and Temperature

Short answer

IEC 61869-6 requires an LPIT with a detected fault to give a zero analog output or set the invalid data flag of the digital output, and to recover on its own only once it has stabilized after a loss of power. The manufacturer must declare MTTF, MTBF and a failure mode analysis, and may declare as replaceable without recalibration only the parts that are marked and proven. Accuracy is also tested over a temperature cycle.

Short answer

A low-power instrument transformer (LPIT) has its own electronics and power supply. It can fail, lose power or run hotter than expected. The protection that uses it needs to know when its measurement stops being reliable.

IEC 61869-6, 2016 edition, asks for three things. First, that a detected fault shows at the output: zero on the analog one, invalid data on the digital one. Second, that the manufacturer declares its reliability and which parts can be replaced without recalibration. Third, that accuracy is also checked over temperature and over frequency.

What must happen when the LPIT detects a fault

Clause 6.604 covers fault detection and the maintenance alert. Its rules are these:

A fault detected automatically must give a zero analog output or set the invalid data flag on the digital output.
At a minimum, a failure of the transmission system must be detected automatically, or that system must allow the relay to supervise it.
If the output stops because the power supply is interrupted, the output must be zero in analog voltage and inactive in digital.
When power returns, and until the LPIT stabilizes, the analog output stays at zero and every digital output is marked invalid.
When the LPIT stabilizes, it must return to normal operation automatically.
A detected maintenance need must be announced. On a digital output, that sets the corresponding quality bits in the data stream.

What the output of an LPIT shows on a loss of power

Normal operation. Output within its accuracy class
Power is interrupted. Analog output at zero and digital output inactive
Power returns. The LPIT starts up and stabilizes
While it stabilizes. Analog at zero and digital marked invalid
Automatic recovery. Returns to a valid output without intervention

For the relay that receives the signal, the difference is large. A zero with the right flag lets a function be blocked. A zero without a flag is mistaken for zero current, and a protection can go blind without warning. How that mark travels in the digital output is explained in the guide on sample quality in IEC 61850.

Reliability the manufacturer must declare

Clause 6.606 asks the manufacturer for reliability information according to standards such as IEC 60812 and IEC 61025. That includes:

the mean time to failure (MTTF);
the mean time between failures (MTBF);
a failure mode and effects analysis (FMEA) of the main parts that receive maintenance;
a block diagram showing how the parts relate and how redundancy is handled, if there is any;
the identification of the parts that receive maintenance and of their procedures.

The standard notes that one way to improve reliability is adequate redundancy. It also asks the manufacturer to provide the controls needed to avoid spurious operation due to loss or insufficiency of supply, loss of an internal component or malfunction of a component.

A basic idea: the reliability of an LPIT must be treated like that of the electrical components of the substation. It is not assessed like that of an accessory.

Clause 6.605 adds an operating rule. Parts that must be accessed, such as switches, taps, fuses, inputs and outputs, must be in accessible positions.

Parts replaceable without recalibration

The standard restricts what can be changed in the field. Components that can be replaced without calibration must carry an appropriate marking, and the manufacturer must demonstrate it by test or documentation. No other component can be changed without recalibrating the whole LPIT.

The test of clause 7.2.6.605 exists for this. Accuracy is tested at ambient temperature, rated frequency, rated input signal and rated burden, if applicable, with the components already replaced. The LPIT must still meet its class.

In practice, this affects your maintenance plan. If a spare part is not marked as replaceable, changing it means recalibrating the whole assembly, with the outage that implies.

Accuracy tests

The accuracy tests of clause 7.2.6 apply to measuring and protection LPITs. There are four:

Basic accuracy. For measuring, it is tested at each input value in the product standard, at rated frequency, with the rated burden if applicable and at ambient temperature. For protection, it is tested with the rated signal. A multipurpose LPIT passes both.
Temperature cycle. Accuracy is repeated while the equipment goes through its temperature range.
Accuracy versus frequency. It is tested at both ends of the rated frequency range.
Component replacement. The test already described.

For protection, the product standards add their own tests, such as transients.

The temperature cycle

Clause 7.2.6.603 describes the cycle. The rated primary signal is applied continuously at maximum temperature and at ambient temperature, and is not applied at minimum temperature. The LPIT works with its rated burden, if applicable, and its indoor and outdoor parts are exposed to their maximum and minimum ambient temperatures.

The temperature variation is at least 5 K per hour. It can be higher only if the manufacturer allows it. The manufacturer declares the thermal time constant, written with the Greek letter tau. Each step lasts at least three times that constant, and accuracy is measured at the instants marked in figure 608 of the standard.

When the LPIT is partly indoor and partly outdoor, both parts are tested at both extremes, with these rules:

both parts experience the ambient air temperature;
the indoor part is at its maximum when the outdoor part is at its maximum;
the indoor part is at its minimum when the outdoor part is at its minimum.

Under normal service conditions, the error measured at each point must stay within the class limits.

What not to do

Do not accept an LPIT without the MTTF and MTBF figures or the failure mode analysis.
Do not program your protection to take a zero as real current if the equipment has an output with an invalid data flag.
Do not replace a part that is not marked as replaceable and expect to keep the accuracy class.
Do not compare the reliability of two devices without knowing which parts each figure covers.

Faults and temperature affect sample quality and relate to insulation and auxiliary power, to electromagnetic immunity and to synchronization. The overview is in merging unit and non-conventional instrument transformers.

When to call us

At Tiza Group we review how your protection reacts when an LPIT fails or restarts, and we check the digital output against the real configuration of your system. We offer IEC 61850 engineering and current and voltage transformer testing, in Colombia or elsewhere on request. If you have already had a strange trip or block, we quote the relay trip analysis, which we can do remotely, worldwide.

References

IEC 61869-6, Instrument transformers - Part 6: Additional general requirements for low-power instrument transformers, edition 1.0 of 2016, clauses 6.604 to 6.606 and 7.2.6.601 to 7.2.6.606 (figure 608).
IEC 60812 and IEC 61025, to which clause 6.606 refers.

Frequently asked questions

What must an LPIT do when it detects an internal fault?

Under IEC 61869-6, the analog output must go to zero or, if the output is digital, the invalid data flag must be set. At least a failure of the transmission system must be detected automatically, or the system must allow the relay to supervise it.

Can a component of an LPIT be replaced in the field without recalibrating it?

Only if the component is identified with an appropriate marking and the manufacturer has shown, by test or documentation, that the LPIT keeps its accuracy class after the change. No other component can be replaced without recalibrating the whole LPIT.

What temperatures does the temperature cycle test of an LPIT cover?

The test takes the equipment between the minimum and the maximum ambient temperature of its range, with a variation of at least 5 K per hour and at least three thermal time constants at each step. If the LPIT has an indoor part and an outdoor part, each part is tested at the extremes of its own range.