Файл: Metodichka_Kuzmenko_dlya_elektrikov.doc

Current transformers (CTs) are commonly used in metering and protective relaying in the electrical power industry where they facilitate the safe measurement of large currents, often in the presence of high voltages. The current transformer safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured.

Current transformers are often constructed by passing a single primary turn (either an insulated cable or an uninsulated bus bar) through a well-insulated toroidal core wrapped with many turns of wire. Current transformers are used extensively for measuring current and monitoring the operation of the power grid. The CT is typically described by its current ratio from primary to secondary. Common secondaries are 1 or 5 amperes. For example, a 4000:5 CT would provide an output current of 5 amperes when the primary was passing 4000 amperes. The secondary winding can be single ratio or multi ratio, with five taps being common for multi ratio CTs. Typically, the secondary connection points are labelled as 1s1, 1s2, 2s1, 2s2 and so on. The multi ratio CTs are typically used for current matching in current differential protective relaying applications. Often, multiple CTs will be installed as a "stack" for various uses (for example, protection devices and revenue metering may use separate CTs). For a three-stacked CT application, the secondary winding connection points are typically labelled Xn, Yn, Zn. Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary, as this will produce a dangerously high voltage across the open secondary and may permanently affect the accuracy of the transformer.

Specially constructed wideband current transformers are also used (usually with an oscilloscope) to measure waveforms of high frequency or pulsed currents within pulsed power systems. One type of specially constructed wideband transformer provides a voltage output that is proportional to the measured current. Another type (called a Rogowski coil) requires an external integrator in order to provide a voltage output that is proportional to the measured current. Unlike CTs used for power circuitry, wideband CTs are rated in output volts per ampere of primary current.

Voltage transformers

Voltage transformers (VTs) or potential transformers (PTs) are another type of instrument transformer, used for metering and protection in high-voltage circuits. They are designed to present negligible load to the supply being measured and to have a precise voltage ratio to accurately step down high voltages so that metering and protective relay equipment can be operated at a lower potential. Typically the secondary of a voltage transformer is rated for 69 or 120 Volts at rated primary voltage, to match the input ratings of protection relays.

The transformer winding high-voltage connection points are typically labelled as H1, H2 (sometimes H0 if it is internally grounded) and X1, X2, and sometimes an X3 tap may be present. Sometimes a second isolated winding (Y1, Y2, Y3) may also be available on the same voltage transformer. The high side (primary) may be connected phase to ground or phase to phase. The low side (secondary) is usually phase to ground.

The terminal identifications (H1, X1, Y1, etc.) are often referred to as polarity. This applies to current transformers as well. At any instant terminals with the same suffix numeral have the same polarity and phase. Correct identification of terminals and wiring is essential for proper operation of metering and protection relays.

While VTs were formerly used for all voltages greater than 240V primary, modern meters eliminate the need VTs for most secondary service voltages. For new, or rework, meter packages, VTs are typically only installed in primary voltage (typically 12.5kV) or generation voltage (13.2kV) meter packages.

Pulse transformers

A pulse transformer is a transformer that is optimised for transmitting rectangular electrical pulses (that is, pulses with fast rise and fall times and a constant amplitude). Small versions called signal types are used in digital logic and telecommunications circuits, often for matching logic drivers to transmission lines. Medium-sized power versions are used in power-control circuits such as camera flash controllers. Larger power versions are used in the electrical power distribution industry to interface low-voltage control circuitry to the high-voltage gates of power semiconductors. Special high voltage pulse transformers are also used to generate high power pulses for radar, particle accelerators, or other high energy pulsed power applications.

To minimise distortion of the pulse shape, a pulse transformer needs to have low values of leakage inductance and distributed capacitance, and a high open-circuit inductance. In power-type pulse transformers, a low coupling capacitance (between the primary and secondary) is important to protect the circuitry on the primary side from high-powered transients created by the load. For the same reason, high insulation resistance and high breakdown voltage are required. A good transient response is necessary to maintain the rectangular pulse shape at the secondary, because a pulse with slow edges would create switching losses in the power semiconductors.

The product of the peak pulse voltage and the duration of the pulse (or more accurately, the voltage-time integral) is often used to characterise pulse transformers. Generally speaking, the larger this product, the larger and more expensive the transformer.

3 - Phase Transformers

3 - Phase Electrical Power Transformer In a 3 phase transformer, there is a three-legged iron core as shown below. Each leg has a respective primary and secondary winding.

Most power is distributed in the form of three-phase AC. Therefore, before proceeding any further you should understand what is meant by 3 phase power. Basically, the power company generators produce electricity by rotating (3) coils or windings through a magnetic field within the generator . These coils or windings are spaced 120 degrees apart. As they rotate through the magnetic field they generate power which is then sent out on three (3) lines as in three-phase power. 3 phase transformers must have (3) coils or windings connected in the proper sequence in order to match the incoming power and therefore transform the power company voltage to the level of voltage we need and maintain the proper phasing or polarity.

3 - Phase Power Is More Efficient Than Single Phase Three phase electricity powers large industrial loads more efficiently than single-phase electricity. When single-phase electricity is needed, It is available between any two phases of a three-phase system, or in some systems , between one of the phases and ground. By the use of three conductors a 3 phase system can provide 173% more power than the two conductors of a single-phase system. Three-phase power allows heavy duty industrial equipment to operate more smoothly and efficiently. 3 phase power can be transmitted over long distances with smaller conductor size.

Also read about 3 phase isolation transformers here.  For an excellent source for these all transformer types check out TEMCo 3 phase transformers.  Or check with Isolation Transformer Sales for 3 phase isolation transformers.  These two companies manufacture some of the most recognized high quality 3 phase transformers available today.

In a three-phase transformer, there is a three-legged iron core as shown below. Each leg has a respective primary and secondary winding.

3 phase electrical power Transformer

The three primary windings (P1, P2, P3) will be connected at the factory to provide the proper sequence (or correct polarity) required and will be in a configuration known as Delta. The three secondary windings (S1, S2, S3) will also be connected at the factory to provide the proper sequence (or correct polarity) required. However, the secondary windings, depending on our voltage requirements, will be in either ?Delta? or a ?Wye? configuration.  

3 - Phase Transformer Delta and Wye Connections In a 3 phase transformer, there is a three-legged iron core as shown below. Each leg has a respective primary and secondary winding.  

3 Phase Transformer Delta and Wye Connections

3 - Phase Transformer Winding Combination   As can be seen, the three-phase transformer actually has 6 windings (or coils) 3 primary and 3 secondary. These 6 windings will be pre-connected at the factory in one of two configurations:  

Three primary Windings in Delta and Three Secondary Windings in Wye

These are the designations which are marked on the leads or terminal boards provided for customer connections and they will be located in the transformer wiring compartment. In both single and 3 phase transformers, the high voltage terminals are designated with an “h” and the low voltage with an “X”  

Special Three Phase Delta Connected Transformers There are certain situations where only a very small portion of a building loads require 120V single-phase . A special transformer is available and you should be familiar with it.  

Special Three Phase Delta Connected Transformers

The 240 Volt 30 Delta Connected Secondary With 120 Volt 10 Lighting Tap. As you can see there is no point in a Delta at which an equal potential to all three lines and the grounded neutral can be made. This is a disadvantage of a Delta compared to a Wye secondary connection
This Delta secondary connection has only one winding (S3) with a neutral conductor. The mid-point of winding S3 is tapped which gives the XI and X3 to neutral a voltage reading of 120 volts. In a 3-phase system, winding S3 is the workhorse; it has to carry all the 120V lighting and appliance loads plus one-third of all the 3 phase loads. (The 120V loads must not exceed 5% of the nameplate KVA, and the total of the nameplate KVA must be derated by 30%). Winding S1 and S2 cannot carry any 120 volt loads as there is no neutral connection to these windings. Windings S1 and S2 can only carry one-third of the three-phase loads each, and the 240 volt single-phase loads.

Caution: A240 volt Delta connected transformer with a 120 volt neutral tap creates a condition called “high leg” As indicated in the above diagram, the voltage between Phase B (X2) and the neutral tap will be 208 volts; therefore, no 120 volt single-phase loads can be connected between X2 and the neutral tap.  

Single Phase Transformers Connected to Form Three Phase Bank   Normally , when 3 phase is required, a single enclosure with three primary and three secondary windings wound on a common core is all that is required. However three single-phase transformers with the same rating can be connected to form a three-phase bank. Since each single-phase transformer has a primary and a secondary winding, then 3 single-phase transformers will have the required 3 primary and 3 secondary windings and can be connected in the field either Delta-Delta or Delta-Wye to achieve the required 3 phased transformer bank, as shown below.  

Single Phase Transformers Connected to Form Three Phase Bank  

3 - Phase Transformer: Delta-Delta Utilizing 3 single-phase transformers is normally not done because it is more expensive than utilizing 1 three-phase transformer. However, there is an advantage which is called the open Delta or V-Connection and it functions as follows: A defective single-phase transformer in a Delta-Delta 3 phase bank can be disconnected and removed for repair. Partial service can be restored using the remaining single-phase transformer open-Delta until a replacement transformer is obtained. With two transformers three-phase is still obtained, but at reduced power. 57.7 of original power. This makes it a very practical transformer application for temporary emergency conditions

Open Delta 57.7%

3 - Phase Transformer: Delta-Delta

Перелік скорочень

MMF (magnetomotive force) – магніторушійна сила;

EMF (electromotive force) – електрорушійна сила;

CEMF (counter electromotive force) – проти електрорушійна сила;

VHF (very hith frequency) – дуже висока частота;

PLC (programmable logic controller) – програмований логічний контролер;

PC (programmable controller) – програмований контролер;

TTL (transistor-transistor logic) – транзисторно-транзисторна логіка;

SSR (solid state relay) – напівпровідникове реле;

AND (logic function) – логічна функція I;

NO (logic function) – логічна функція НЕ;

OR (logic function) – логічна функція АБО;

XOR (logic function) – логічна функція, що виключає АБО;

NAND (logic function) – логічна функція І-НЕ;

NOR (logic function) – логічна функція АБО-НЕ;

ANSI (American National Standard Institute) – Американський інститут національних стандартів

IEC (International Electrotechnical Commission) – Міжнародна електротехнічна комісія

MCB (Miniature Circuit Breaker) – малогабаритний автоматичний вимикач;

MCCB (Moulded Case Circuit Breaker) – автоматичний вимикач у відлитому корпусі;

IEEE (Institute of Electrical and Electronic Engineer) – інститут інженерів з електротехніки та електроніки.