Measuring and Instrument Transformers – What are they used for?

Current Transformer

The current transformer is a suitably designed measuring transformer in which, during normal operation, the secondary current is almost proportional to the primary current, and its phase differs from the primary current phase by an angle that is close to zero in the case of proper connection of terminals. Current transformers are used to measure currents with high values, which can not be measured by directly switching on the meters due to exceeding their measuring ranges. Another advantage of measuring transformers is the galvanic isolation of measuring instruments from the measuring circuit, which is under high voltage.

Construction of the Current Transformer

The windings are wound on a typical ferromagnetic core (usually toroidal), wound from a single transformer plate. The primary current “Ip” flows through the primary winding, which is transformed from the primary side to the secondary side of the transformer. In the secondary winding there flows the secondary current “Is”, which powers the electronic circuits of measuring instruments, meters or relays. The transformer windings are carefully insulated from each other, which protects against breaking the high voltage from the primary side to the secondary circuit. The sum of all currents in the magnetic circuit is zero, because the current transformer’s secondary circuit is shorted by low impedance and current flows in it, which almost completely compensates for the primary flow.

IpNp ≈ IsNs

From the above dependence, one can calculate the value of primary current ”Ip” based on the measurement of the secondary current “Is” and the number of primary windings “Np” and secondary “Ns”.

Ip ≈ Is(Ns/Np)

The ratio of the number of coils is called a transformer turns ratio or uncorrected turns ratio. The transformer turns ratio has a value similar to the actual current transmission equal to the ratio of the effective value of currents actually flowing through the transformer.

Ki = Ip/Is

The actual transmission “Ki” is variable, because the ratio of current values depends on the transformer load and the value of the primary current. In operation of the transformer, therefore, the rated gear “Kn” is used equal to the ratio of rated currents.

Kn = Ipn/Isn

The rated values of the primary and secondary currents are the values of those currents to which the transformer’s operation is referred. In practice, after measuring the value of the secondary current, the value of the primary current is calculated from the following formula:

Ip = KnIs

The measuring transformers are used to power measuring instruments and are more accurate than the protection transformers that are used to power the protection relays. The protection relays are less accurate, but they meet the requirements in the field of total error in a much wider range of currents, which exceed the rated values ​​even several dozen times. Thanks to this, they ensure correct operation of protections in conditions of overloads and shortages in the power network. However, measuring transformers are more accurate, but only in a narrow range of currents and even at currents slightly larger than rated, they show large negative measurement error value, which effectively protects the measuring instruments attached to them from damage during an overload or short circuit in the power grid. Therefore, measuring and protection transformers can not be used interchangeably, even if they have the same rated transmission and similar limit error values.

Voltage Transformer

Voltage transformers are used to transform high voltage into normalized low, voltage supply circuits for measuring instruments, meters, energy meters, etc.

Construction of the Voltage Transformer

Inductive voltage transformers are usually built as single-phase. In three-phase systems, such transformers are combined into a suitable system or three-phase transformers are used. Depending on the number of secondary windings, voltage inductive transformers may have one secondary winding or many secondary windings. Depending on the purpose, the voltage transformers are divided into:

• voltage transformers for measurements,
• designed to power instruments,
• voltage transformers for security,
• voltage transformers for measurements and protections.

In single-phase voltage transformers as a protection there may be a residual voltage winding, intended to connect in a set of three single-phase transformers into an open triangle in order to create a zero voltage component in case of ground faults, suppressing ferroresonance vibrations. Basic parameters of voltage transformers are as follows:

• rated primary voltage,
• rated secondary voltage,
• rated transmission “Kn” (Kn = V1n/V2n),
• rated insulation level,
• rated power,
• accuracy class,
• rated voltage coefficient,
• thermal limit power,
• voltage error ((KnVs – Vp/Vp) * 100),
• angular error.

To power measuring instruments, measuring voltage transformers are used, characterized by high accuracy of transformations at primary voltages close to the rated ones. In the selection of voltage measuring transformers, in addition to determining the type and type of the transformer, the following aspects should be determined and adjusted:

• transformer connection system,
• rated primary voltage,
• rated secondary voltage,
• rated power and accuracy class.

The choice of the transformer’s rated power and its accuracy class is of significant importance when choosing transformers for measuring systems. The rated power of the transformer depends mainly on the sum of the rated powers of devices and devices connected to the secondary winding. In order for the transformer to work within the required accuracy class, the secondary load of the transformer must not be lower than 25% of the rated load and must not exceed the rated load. Voltage transformers used to supply protection systems should be characterized by appropriate accuracy of voltage transformations in fault states where distorted waveforms occur.