Combined Current and Voltage Transformer (IEC 61869-4)

Short answer

A combined transformer puts a current transformer and a voltage transformer in the same enclosure. IEC 61869-4 keeps for each part the error limits it would have on its own and adds two requirements of its own: the current must not push the voltage transformer out of its class, and the voltage must not push the current transformer out of its class.

Short answer

A combined current and voltage transformer is a single field device with two instrument transformers inside. IEC 61869-4 defines it as an instrument consisting of a current transformer and a voltage transformer in the same enclosure. Each part keeps the requirements of its own standard. What is new is that they share space and therefore influence each other. The standard limits that influence and checks it with tests of its own.

What IEC 61869-4 covers

Part 4 is an additional product standard. It does not repeat what parts 1, 2 and 3 already say. It refers to them and only changes or adds what a combined unit needs: error limits, temperature rise, marking and tests.

With a single device in the bay, the support structure and the space taken up are reduced. That makes it attractive in high-voltage and extra-high-voltage substations. When you buy one, you receive two instruments in one enclosure, with the rating plate and the tests of each part.

Error limits of each part

Clause 5.401.1 sets the base rule. The error limits of a combined metering unit are those of a metering current transformer (IEC 61869-2, clause 5.6.201) and those of a metering voltage transformer (IEC 61869-3, clause 5.6.301). For a combined protection unit, they are those of a protection CT (5.6.202) and those of a protection voltage transformer (5.6.302).

In other words, class 0.2 on a combined unit means the same as class 0.2 on a stand-alone CT or voltage transformer. There is no “relaxed” class for being combined.

Mutual influence

Clause 5.401.2 defines the requirement specific to the combined unit. It works in two directions.

What varies	What must still be met
The CT operates between 5 % of rated current and the rated continuous thermal current	The voltage transformer does not exceed the error and phase displacement limits of its class, within the specified burden range and between 80 % and 120 % of rated voltage
The voltage transformer operates between 80 % of rated voltage and rated voltage multiplied by the voltage factor	The CT does not exceed the error and phase displacement limits of its class, within its current range and its burden range

The two physical mechanisms are different.

From current to voltage. The current in the CT primary creates a stray magnetic field. That field induces a small voltage in the voltage transformer winding. The standard calls it the voltage induced by the rated continuous thermal current. It sets the maximum variation of the voltage error.
From voltage to current. The whole assembly sits at line voltage. That voltage produces a capacitive current in the CT, which adds to the measured current. The standard measures it as a voltage drop across a resistor connected to the CT secondary.

How the influence is checked

Clause 7.2.6.401 deals with the effect of the CT on the voltage transformer. The laboratory first measures the voltage error with no current in the CT. It then passes the rated continuous thermal current through a conductor loop of defined geometry. In that loop, the return conductor sits at the distance at which another phase would be on the real line. It measures the voltage induced in the secondary and calculates the largest possible variation of the error. That variation is added to the error measured without current. The result cannot exceed the class limit.

For a metering voltage transformer, the variation is referred to 80 % of rated secondary voltage. For a protection one, to 2 %.

Clause 7.2.6.402 deals with the opposite effect. With the voltage transformer not energized, the CT error is determined. Then 120 % of rated voltage is applied to the voltage transformer, and also rated voltage multiplied by the voltage factor. The capacitive current that appears in the CT is measured with a resistor in its secondary. The error variation is calculated for 5 % of rated current, the most sensitive point, and added to the measured error. Again, the total must stay within the limit.

The full procedures, with the loop geometry and the treatment of results, are in those clauses of the standard. This guide describes what they demonstrate, not how they are carried out.

Temperature rise

Clause 6.4.1 requires the temperature rise not to exceed the values of IEC 61869-1, clause 6.4, with the test voltage applied and the CT carrying its rated continuous thermal current. The CT burden is the rated burden at unity power factor. The voltage transformer carries its rated burden, or the highest one if it has several, at a power factor between 0.8 lagging and 1.

The test (7.2.2) applies current and voltage at the same time. That requires the winding that supplies the current to be insulated from the primary for the full system voltage. If there is no such transformer, the standard allows two other arrangements and states that the three methods give the same result. The choice is left to the manufacturer. Winding temperature rise is measured by the increase in resistance.

Insulation and routine tests

In the impulse test (7.2.3.1), the waves are applied to the short-circuited CT primary, connected to the terminal of the voltage transformer that is at high voltage in service. The same connection applies to the chopped impulse and the multiple chopped impulse.

In routine tests, the induced voltage used to test the voltage transformer is also the power-frequency test of the CT primary (7.3.1.301). The accuracy of each part is checked according to its own standard. The error variation obtained in the type test is taken into account (7.3.5.401).

Marking

The terminals of each part are marked as on a stand-alone transformer (6.13.401). That is, P1, P2, S1 and S2 for the CT and the IEC 61869-3 designation for the voltage transformer. This avoids confusion when connecting and testing. The guide on current transformer terminals and secondaries shows how they are read.

The rating plate carries the characteristics of each part separately (6.13.402.1). In addition, the voltage transformer carries the voltage Ue induced by the short-time thermal current. It is measured with the voltage transformer primary short-circuited and with a burden of 15 VA or the rated burden. The standard allows it to be calculated from the voltage measured with the continuous thermal current, multiplied by the ratio of the short-time thermal current to the continuous one.

What to ask for in the specification

The class of each part, metering or protection, and their rated burdens.
The continuous and short-time thermal current of the CT, with the short-circuit current data for the site.
The voltage factor and the system earthing type.
The mutual influence test report and the Ue value on the rating plate.
The edition of the standard to which the unit was tested.

What not to do

Do not accept a combined unit with a single accuracy report. There must be results for each part and for the mutual influence test.
Do not assume the CT class holds by itself at high voltage: the standard requires it to be checked.
Do not connect to either part a burden outside the specified range: the class limits of both parts are guaranteed only within that range.

The combined transformer brings together the functions of current and voltage transformers. It relates to terminal marking, the metering classes, the VT classes, the inductive voltage transformer and the high-voltage CT.

When to call us

At Tiza Group we test current and voltage transformers, combined units included, before they are energized. We verify ratio, polarity, excitation, insulation and secondary wiring, and deliver the report with full traceability. Learn about our service of testing and commissioning of current and voltage transformers, available in Colombia, or elsewhere on request. Write to us and we will send you a quote.

References

IEC 61869-4, Instrument transformers - Part 4: Additional requirements for combined transformers, 2013 edition, clauses 3.1.401 to 3.1.408, 5.401, 6.4.1, 6.13.401, 6.13.402, 7.2.2, 7.2.3.1, 7.2.6.401, 7.2.6.402, 7.3.1.301 and 7.3.5.401.
IEC 61869-2, Instrument transformers - Part 2: Additional requirements for current transformers, 2012 edition, clauses 5.6.201 and 5.6.202.
IEC 61869-3, Instrument transformers - Part 3: Additional requirements for inductive voltage transformers, 2011 edition, clauses 5.6.301 and 5.6.302.

Frequently asked questions

Does a combined transformer have the same error limits as two separate transformers?

Yes. IEC 61869-4 (clause 5.401.1) requires the current part to meet the limits of IEC 61869-2 and the voltage part those of IEC 61869-3, for both metering and protection. The difference is that both sets of limits must be met at the same time, with the influence of each part on the other included.

What is mutual influence in a combined transformer?

It is the effect one part has on the other because they share an enclosure. The current flowing in the CT primary induces a small voltage in the voltage transformer. In turn, the applied voltage drives a capacitive current in the CT. The standard requires that neither one pushes the other out of its class.

What extra data does the rating plate of a combined transformer carry?

Besides its own data, the voltage part carries the voltage Ue that the short-time thermal current of the CT induces in its secondary when the voltage transformer primary is short-circuited (clause 6.13.402.2). The plate may give instead the ratio between that induced voltage and the current, in millivolts per kiloampere.