In AC circuits, electrical machines called transformers are often used. All of them are designed to convert the value of the current, but the tasks at the same time can be completely different. Therefore, in electrical engineering, there are such concepts as a current transformer (CT), voltage (VT) and power transformer (TC). Any of them will work only with the correct connection of the transformer windings.
What is a current transformer
Current transformers are electrical devices that are used in high-current circuits in order to carry out safe current measurements, as well as to connect protective devices with low internal resistance.
Structurally, such devices are low-power transformers connected in series in the circuit of electrical equipment, where there is a medium and high voltage level. Readings are taken in the secondary circuit of the instrument.
Standards for current transformers standardize such technical indicators of devices:
- Transformation ratio.
- Phaseshift.
- Strength of insulating material.
- The value of the load capacity in the secondary.
- Terminal markings.
The main rule to remember when assembling the connection diagram of the current transformer windings is the inadmissibility of idling in the secondary circuit. Based on this, you can select the following operating modes for the TT:
- Connecting load resistance.
- Short circuit operation (short circuit).
What is a voltage transformer
A separate group of transformers used in AC networks with voltages above 380 V. The main task of the devices is to supply power to measuring instruments (IP), relay protection circuits and galvanic isolation of equipment from high-voltage lines for the safety of maintenance personnel.
The design of the HP does not fundamentally differ from the TS. They lower the voltage to 100 V, which is already supplied to the IP. Instrument scales are calibrated taking into account the transformation ratio of the measured voltage on the primary winding.
What is a power transformer
The main electrical machines used in substations and at home are power transformers. They act as voltage converters of one value to another, while maintaining the shape of the electrical signal. There are step-down and step-up electric machines.
TS are three-phase and single-phase for two or three windings. Three-phase are usually used to redistribute energy in powerful electricalnetworks, single-phase can be found in any household equipment, such as power supplies.
CT winding connection diagrams
There are such basic schemes for connecting the secondary windings of a current transformer when powering protective relay devices:
- Scheme of a full star. In this case, current transformers are switched in all power phase lines. Their secondary windings are connected by a star circuit with relay windings. All CT terminals of the same value must converge to the zero point. According to this scheme, a relay will respond to a short circuit (short circuit) of any phase. If a short circuit occurs on the ground bus, then the relay will operate in the star (in the zero wire).
- Scheme for connecting the transformer windings into an incomplete star. This option involves the installation of a CT not on all phases, only on two. The secondary windings are also connected to the star relay. Such a scheme is effective only when shorting between phases. If the phase is short-circuited to zero (where the CT was not installed), the protection system will not work.
- Diagram on transformers, star on relays. Here, the CTs are connected in series with a triangle with their opposite terminals of the secondary windings. The vertices of this triangle go to the rays of the star, where the relay is installed. It is used for such types of protection scheme as remote and differential.
- SchemeCT connections according to the principle of two phase difference. The circuit only reacts to phase-to-phase short circuits with the sensitivity required.
- Zero-sequence current filtering circuit.
Wiring diagrams for voltage transformer windings
With regards to VTs, when they feed relay protection and measuring equipment, they use both phase-to-phase voltage and line voltage (between phase and earth). The most commonly used schemes are based on the principle of an open triangle and an incomplete star.
A triangle is used when there is a need for two or three phase-to-phase voltages, a star when connecting three VTs, if phase and linear voltages are used simultaneously for measurements and protection.
For electrical devices with two additional secondary windings, a switching circuit is used, where the main windings of the primary and secondary purposes are connected by a star. With the help of an open triangle, additional windings are assembled. With this circuit, you can get the voltage of the 0-th sequence for the response of the relay system to a short circuit in a circuit with a grounded wire.
Wiring diagrams for windings of power transformers
For three-phase networks, there are three main schemes for connecting the windings of power transformers. Each of the ways of such connection has its own influence on the mode of operation of the transformer.
Star connection is when there is a common point of union of the beginnings or ends of all windings (zero point). Here is the followingpattern:
- Phase and line currents have the same value.
- Phase voltage (between phase and neutral) is less than linear voltage (between phases) by the square root of 3.
With regards to the windings of high (HV), medium (SN) and low (LV) voltage, schemes are more often used:
- Connect the HV windings with a star, leading the wire from the zero point for raising and lowering T of any power.
- CH windings are connected in the same way.
- HV windings are rarely star-connected for step-down transformers, but when they do, the neutral wire is brought out.
Triangle connection involves connecting the transformer in series in a circuit where the beginning of one winding has contact with the end of the other, the beginning of the other with the end of the latter and the beginning of the latter with the end of the first. From the vertices of the triangle there are outlets of electricity. In such a connection scheme for the windings of a three-phase transformer, there is a pattern:
- Phase and line voltages are the same value.
- Phase currents are less than linear currents by the square root of 3.
In a triangle, as a rule, the LV windings of any step-down and step-up three-phase T are connected to two, three windings, as well as powerful single-phase assembled in groups. For HV and MV, delta connection is not normally used.
Zigzag-star connection is characterized by the alignment of the magnetic flux in the phases of the transformer, if the load on them in the secondary windings is unevenly distributed.
Schemes and groups for connecting transformer windings
In addition to connection schemes, there are groups, which are understood as nothing more than a displacement of the vector directions of the linear EMF of the primary windings relative to the electromotive force in the secondary windings. These angular discrepancies can vary within 360 degrees. The factors that determine the group are:
- The direction of the turns of the winding.
- The method of location on the core of the coil.
For the convenience of designating groups, we adopted an hourly angular count divided by 30 degrees. Therefore, there were 12 groups (from 0 to 11). With all basic connection schemes of transformer windings, all shifts by an angle multiple of 30 degrees are possible.
What is the third harmonic for
In electrical engineering there is the concept of magnetizing current. It is he who forms the electromotive force (EMF). The form of such a current is not sinusoidal, since higher harmonic components are present here. The third harmonic is responsible for the transmission of the phase voltage curve without distortion (a distorted form is undesirable for equipment operation).
To obtain the third harmonic, a prerequisite is a delta connection of at least one winding. If the star-star transformer winding connection scheme is taken as the basic one, for example, in transformers with two windings, it is impossible to obtain the third harmonic without additional technical intervention. Then the third winding is wound onto the transformer, which is connected in a triangle, sometimes without leads.
