In AC circuits, electrical machines, called transformers, are often used. All of them are designed to convert the current value, but the tasks can be completely different. Therefore, in electrical engineering, there are such concepts as a current transformer (CT), voltage (VT) and a power transformer (TS). Any of them will work only if the transformer windings are connected correctly.
What is a current transformer?
Current transformers are called electrical appliances that are used in high current circuits in order to conduct safe current measurements, as well as to connect protective devices with low internal resistance.
Structurally, such devices are low-power transformers, sequentially included in the circuit of electrical equipment, where there is medium and high voltage. In the secondary circuit of the device take readings.
Standards for current transformers normalize the following technical indicators of devices:
- Transformation ratio.
- Phase shift.
- The strength of the insulating material.
- The value of the load capacity in the secondary.
- Terminal marking.
The main rule that you need 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 TT:
- Connection of 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 devices (IP), relay protection circuits and galvanic isolation of equipment from high-voltage lines for the safety of staff.
The design of the VT does not fundamentally differ from the TS. They lower the voltage to 100 V, which is already supplied to the IP. The instrument scales are calibrated, taking into account the transformation coefficient of the measured voltage on the primary winding.
What is a power transformer
The main electrical machines used in substations and in everyday life are power transformers. They play the role of voltage converters of one magnitude to another, while maintaining the shape of the electrical signal. There are lowering and raising electric cars.
TCs are three-phase and single-phase for two or three windings. Three-phase are usually used to redistribute energy in powerful electrical networks, single-phase can be found in any household equipment, for example, 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:
- Full star outline. 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 terminals of the CT of the same value should 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 earth bus, then a relay will trip in the star (in the zero wire) .
- Connection diagram of transformer windings into an incomplete star. This option involves installing TTs not in all phases, only in two. The secondary windings are also connected to the relay by a star. Such a scheme is effective only when shorting between phases. With a phase short to zero (where the CT was not installed), the protection system will not work.
- The triangle circuit is on transformers, the star is on the relay. Here, the CTs with their opposite terminals of the secondary windings are connected in series with a triangle. The vertices of this triangle pass to the rays of the star, where the relays are installed. It is used for such types of protection circuit as remote and differential.
- CT connection scheme according to the principle of two difference phases. The circuit only reacts to phase-to-phase faults with sensitivity necessary.
- Scheme of zero sequence filtering currents.
Connection diagrams of voltage transformer windings
With regards to the VT, when they feed the relay protection and measuring equipment, they use both phase-to-phase voltage and linear (between phase and ground). 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 interphase voltages, a star when connecting three VTs, if phase and linear voltages are simultaneously used for measurements and protection.
For electrical devices with two additional secondary windings, an switching circuit is used where the primary and secondary main windings are connected by a star. Using an open triangle, additional windings are assembled. Such a circuit can be used to obtain the voltage of the 0th sequence for the relay system to respond to a short circuit in a circuit with a grounded wire.
Connection diagrams of power transformer windings
For three-phase networks, there are three main schemes for connecting the windings of power transformers. Each of the methods of such a connection has its own effect on the operation mode of the transformer.
A star connection is when there is a common point of union of the beginnings or ends of all windings (zero point). Here is the following pattern:
- Phase and linear currents have the same magnitude.
- The phase voltage (between phase and neutral) is less than the linear voltage (between phases) by a root of 3.
With regards to the windings of higher (VN), medium (MV) and lower (LV) voltage, the following schemes are more often used:
- They are connected by a star of the VN winding, removing the wire from point zero for increasing and decreasing T of any power.
- CH windings are connected in the same way.
- HV windings are rarely connected by a star at step-down transformers, but when this happens, the neutral wire is output.
The triangle connection involves the series connection of the transformer in the 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 last and the beginning of the last with the end of the first. From the tops of the triangle come outlets of electricity. In such a connection scheme of the windings of a three-phase transformer, there is a pattern:
- Phase and line voltages have the same magnitude.
- Phase currents are less than linear to the root of 3.
In a triangle, as a rule, the LV windings of any step-down and step-up three-phase T are connected by two, three windings, as well as powerful single-phase assembled in groups. For BH and CH, a triangle joint is usually not used.
A zigzag-star connection is characterized by alignment of the magnetic flux over the phases of the transformer, if the load on them in the secondary windings is unevenly distributed.
Schemes and connection groups of transformer windings
In addition to the connection schemes, there are groups by which they mean 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 differences can vary within 360 degrees. The factors determining the group are:
- Direction of winding turns.
- The location method on the core of the coil.
For convenience, the designations of the groups adopted an hourly angular count divided by 30 degrees. Therefore, it turned out 12 groups (from 0 to 11). With all the basic schemes for connecting the transformer windings, all displacements 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 shape of such a current is not sinusoidal, since higher harmonic components are present here. The third harmonic is responsible for transmitting the phase voltage curve without distortion (a distorted shape is undesirable for the equipment to operate).
To obtain the third harmonic, a prerequisite is to connect at least one winding into a triangle. If the star-star transformer windings connection scheme, for example, in transformers with two windings, is adopted as the basic one, it is impossible to obtain the third harmonic without additional technical intervention. Then a third winding is rolled onto the transformer, which is connected by a triangle, sometimes without leads.