Medium-Voltage Current Transformer (CT): Classes

What it is and where it sits

The current transformer (CT) is an instrument transformer whose secondary delivers a current proportional to the primary current. In medium voltage it is installed inside switchgear, in the cable compartment or next to the switching device, and it feeds the protective relay, the energy meter and, in some ring main units (RMU), a self-powered relay that takes its energy from it.

Its standard is IEC 61869-2, applied together with the general requirements of IEC 61869-1. The IEEE counterpart is IEEE C57.13. The general guide to the equipment is the one on the current transformer.

Main parts

• Primary: a bar that passes through the core, or a winding of a few turns.
• One or more ring cores, each with its own secondary winding.
• Epoxy resin insulation cast under vacuum or by pressure gelation, which forms a rigid, oil-free body.
• Secondary terminals with taps when the CT is multi-ratio, and an earthing terminal.

How it works

The primary current creates a flux in the core that induces the secondary current. A small part is used to magnetize the core, and that excitation current is the source of the error. The CT forces the current; the voltage at its terminals is whatever the connected load (relay, meter and leads) requires, called the burden.

While that voltage stays below the knee point of the excitation curve, the error is small. When the fault is large or the burden is high, the core saturates and the secondary current is distorted. That is why metering cores and protection cores are designed differently:

• The metering core is accurate near rated current and saturates early, which protects the meters. Its instrument security factor (FS 5 or FS 10) is the multiple of rated current at which its composite error reaches 10 % with the rated burden; the lower the value, the better the meter is protected.
• The protection core keeps its error bounded up to many times the rated current, so that the relay sees the real fault.

Variants and differences with other voltage levels

• Block or support type: a CT encapsulated in resin with its primary, mounted in the switchgear.
• Window or ring-core type: the cable or bar passes through the core; it has no primary insulation of its own.
• Zero-sequence ring-core (core balance): it encircles the three phases and measures their sum, that is, the residual current. It detects earth faults of a few amperes in isolated-neutral or impedance-earthed systems. The earth conductor of the cable screen must pass back through the ring core, and the cables must be centered.
• Multi-core: one metering core and one or two protection cores in the same body.

At high and extra-high voltage the CT is a stand-alone oil-paper or SF6 device, with capacitance and dissipation factor requirements that are not applied in medium voltage; see the high-voltage current transformer. The low-power sensors that replace the CT in modern switchgear are covered in non-conventional instrument transformers.

Rated values and nameplate

According to IEC 61869-2:

• Secondary current: 1 A or 5 A.
• Rated burden of metering cores and class P protection cores: 2.5, 5, 10, 15 and 30 VA; higher values are allowed.
• Metering classes: 0.1, 0.2, 0.2S, 0.5, 0.5S, 1, 3 and 5. The S classes keep their accuracy at low currents, useful for revenue metering.
• Protection classes P: 5P and 10P, with an accuracy limit factor (ALF) of 5, 10, 15, 20 or 30. A 5P20 has a maximum composite error of 5 % at 20 times rated current with its rated burden.
• Class PX: defined by the knee-point voltage (Ek), the maximum excitation current at that voltage and the secondary winding resistance (Rct). Some differential and high-impedance protections require it.
• Rated short-time thermal current (Ith), normally for 1 s, and rated dynamic current (Idyn), which by default is 2.5 times Ith.

The nameplate also shows the ratio, the taps, the highest voltage for equipment and the polarity marking of each core.

Integration with protection and control

The protection core feeds overcurrent 50/51 and 50N/51N, directional 67 and, on motors and transformers, differential. The zero-sequence ring-core CT feeds sensitive earth fault. In a digital substation, the CT is modeled in IEC 61850 with the logical node TCTR.

Two rules are not negotiable: the secondary is earthed at a single point and is never left open with current in the primary. The CT wiring guide explains the usual connections.

Commissioning and maintenance tests

The current and voltage transformer tests include:

• Ratio and polarity of each core and tap.
• Excitation curve and knee point; see the CT excitation test.
• Secondary winding resistance and actual burden of the circuit, including the leads.
• Insulation resistance of the primary and secondaries.
• Primary injection on zero-sequence ring-core CTs and on self-powered relays, which tests the complete chain.
• Demagnetization after tests with direct current.

Common failures and what to check first

• Sensitive earth fault trip with no fault: cable screen incorrectly connected to the ring core.
• Differential that operates at the first load step: reversed polarity or wrong tap.
• Low energy metering: a protection core used for metering, or the ratio set wrongly in the meter.
• Self-powered relay that does not trip: supply CT with the wrong ratio or connection.
• Cracks or discharges in the resin: moisture, contamination or overvoltages.
• Open secondary at a test block or a loose terminal.

Testing and commissioning with Tiza Group

At Tiza Group we test medium-, high- and extra-high-voltage CTs in substations and industry, and we verify their circuits all the way to the relay, in Colombia or elsewhere on request. Write to us and we will propose the scope of the tests and commissioning.

References

• IEC 61869-1, Instrument transformers: general requirements.
• IEC 61869-2, Additional requirements for current transformers.
• IEEE C57.13, Requirements for instrument transformers.
• IEEE C37.110, Guide for the application of current transformers used for protective relaying purposes.
• Blackburn and Domin, Protective Relaying: Principles and Applications.

Frequently asked questions