Transformers

Describe the conservator tank

A large cylinders above the bank of the radiators, contains reserve of oil at higher level than main tank, there is a pre set limit which triggers an alarm if the oil falls under this amount. Ensures main transformer fukk Absorbs thermal expansion and contraction of the insulating oil Excludes air from main tank When fitted with a silica gel breather, any air drawn into the conservator tank is kept dry.

when a single phase transformer is double wound what are the effect

An increase in the secondary current when load increases, greater demagnetising force. Reduction in primary flux, reduced back emf (voltage) induced in the primary, increasing the supply voltage causing more current to flow in the primary.

What transformer has a 1:1 turns ratio?

An isolating transformer

how simple single phase transformer works

As polarity of AC changes, magnetic field of primary builds up and magnetic field collapses, the moving magnetic field cuts across windings of secondary coil and induces an EMF, the effect of emf is known as mutual induction. If the insulated coils were wound in one direction, the effect of mutual inductance would be much stronger. This is the principle on which the transformer is based.

How are transformers identified

By their cooling method

How can overheating be monitored in a transformer

By thermometer in the top of the oil By the resistance of the transformer windings

What is the use and description of a current transformer

Can transform large current down to a smaller current and isolate meters and equipment from high voltage being measured. Assuming no losses current can be calculated with the turns ratio. Ip/Is=Np/Ns. Current transformers usually have either 5 or 1 ampere secondary currents.

Name types of transformer protective devices

Conservator tanks/ Buchholz relay/ Pressure relief devices

What are four effects of overvoltage in a CT transformer

Electric shock/ Damage to the insulation of secondary winding/ Damage to insulation of leads, damage to relays, instruments/ possible permanent change to the magnetic characteristics of the CT core, affecting future accuracy.

Why is a smaller burden preferred

Greater accuracy if a current transformer

What is the use of tapping points on a high voltage side of a transformer

If can increase and decrease the output voltage. If turns are added to winding, output voltage decreases. If turns subtracted from winding, output voltage increases.

What are the hazards of a Current transformer

If the secondary terminals of CT are left open circuited in service, no secondary current can flow, primary current become excitation current. Magnetisation of the core becomes much greater than normal and the core tends to saturate, flux in core increase rapidly which induces enormous spikes of voltage twice in every cycle, these peaks can measure several thousand volts.

Explain instantaneous sum of the fluxes in the cores of three phase transformer

If the three primary windings are connected to a three phase supply, their fluxes will differ in phase by 120 degrees. As a result of this, the instantaneous sum of the three fluxes in the cores A, B and C will be zero.

What are some types of instrument transformers

Impedance matching transformers/ Voltage transformers/ Current transformers

What are Eddy currents?

In a solid iron core, a current will flow in a conducting path, as a result this induces EMF in the centre and outer limbs of a solid iron core.

What are the four functions of a temperature indication gauge

Indicates the oil temperature Switches on the cooling system, fan/ pumps, when temp reaches a preset level. Operates an alarm when temp reaches preset level

How are Eddy currents reduced?

Instead of a solid iron core, the core is instead made up of many thin laminated slices of iron insulated from one another by a thin layer of varnish.

How are the windings referred to in three-phase transformers?

Instead of primary and secondary they are referred to as either High voltage or Low voltage, Low voltage are always placed inside next to the core and the high voltage windings on the outside, if phase to earth appears the voltage to earth is much lower.

What is the kVA

It is the maximum that the transformer can supply when fully loaded.

Choice of transformer is decided by

Its current rating/ Physical size/ Input and output voltage/ Environment

Iron loss is reduced by

Laminations silicone between steel. Iron loss is the result of Eddy currents and hysterisis. Iron loss is when power is absorbed by the magnetic core of a transformer.

What is hysteresis?

Polarity changes between P-N and N-P. Energy is consumed in this process and heat is produced in the core. Silicon steel has a low hysterisis loss, it is used for transformer laminations.

What are two types of three-phase power transformer

Power transformers- designed to run at almost constant load (full-load rating) Distribution transformers- designed for variable load (less than full-load rating) They are similarly constructed.

Describe pressure relief devices

Pressure relief or explosion vents, fitted ontop of transformers and act as safety valves by reducing internal pressure if major fault occurs within transformer. Either spring loaded or have thin non-metallic membrane that fractures under pressure. If the build up pressure is relatively slow, the explosion vent will operate satisfactorily a prevent tank bursting.

Describe the Buchholz Relay

Safety device that is normally fitted between the conservator tank and the main transformer tank, operates in event of an internal fault, protecting the transformer from further and more serious damage. In normal conditions the oil chamber is full and in open position In minor fault conditions, gas produced in the oil gathers at top of relays oil chamber, forcing oil level down causing upper mercury switch to float down, as switch lower, alarm circuit contact made. In major fault the lower mercury switch bridges the contacts that form part of an electrical trip circuit.

Copper loss is reduced by

Shorter copper length/tighter windings/neat. Copper loss is power loss in transformer windings. It occurs as a result of current flowing through the resistance of the windings.

Why is the high voltage winding on the outside when low voltage and high voltage wound together

So tappings can be added and due to insulation.

For the burden to be give accurate readings what is needed

The burden of the instrument and connecting cables must not exceed the burden rating of the CT. Common burden ratings of CTs are 15 VA for 5 ampere secondary current and 5 VA for 1 ampere secondary current.

Efficiency for transformers

The ratio between its useful power output and the power input in kilowatts. Efficiency depends on on the power factor aswel as the size of the load.

What are transformers rated in and why

They are rated in VA or kVA. It is not given in Watts because, for any wattage, the current in the windings must increase when a load lowers the power factor below unity.

describe simple single phase transformer

Two coils of inducted conductor are wound on a core, they are electrically seperate but linked magnetically by magnetic flux in core. Transformers wound this way are called DOUBLE WOUND. Coil connected to A.C is primary. Coil connected to load in secondary. When transformer switched on mutual inductance occurs.

Explain voltage regulation

When transformer loaded, secondary terminal voltage falls because the flow of the current causes a voltage drop in the resistance and reactance of the windings. This decreases in voltage is called the regulation of the transformer. Is it expressed in a percentage. Voltage regulation= Voltage no load- Voltage full load/ Voltage no load.

What is the common winding connection used in New Zealand

delta -star DV11, the a phase is at the top of the delta and the a phase is at 11o clock in the star.

What are the four basic winding connections of a three phase transformer

delta-star delta-delta star-star star -delta

What is the use of an impedance matching transformer

designed to offer an impedance match to a particular circuit to offer either a high impedance (ac resistance) interface with an ac supply. Used in hot watch switching.

What is the use of voltage transformers also describe

double-wound, electrically isolated, step down transformers, used to step down the voltage being measure to an acceptable value, usually not more than 110 volts, for use in measuring equipment such as a voltmeter. They also electrically insulated from high-voltage lines. Used in power distribution in SWER. Primary and secondary currents are small. So the ratio of secondary volts to primary volts can be kept as constant as possible.

What are some means of cooling a transformer

for small transformers natural air for up to 50kVA plain tanks may be cooled in mineral oil for 50kVA-MVA trange, tubed tanks with cooling tubes for oil to circulate and cool down back into tank. Higher rating transformers use fans fitted to their radiators or cooling tubes also oil pumps may be used to circulate cold oil into tanks.

A voltmeter or the voltage coil of a wattmeter must be connected how

in parallel to the secondary of the voltage transformer

An ammeter or the current coil of a wattmeter must be connected how

in series to the secondary of a Current transformer. The CT is also connected in series with the load.

What are applications of voltage transformers

indicating instruments, recorders and protection systems

What are applications of instrument transformers

may supply the trip coil of a circuit breaker or provide input current and voltage for protection systems. They can step down large currents and extra-high voltages to a level where they can be safely applied to a standard meter. They electrically isolate the instruments from supply and provide a means of measuring electrical values at a remote point.

What is the lead on the secondary side of the current transformer called

the burden, expressed in volt amperes. Burden is calculated by Burden= I squared Z. I is the CT secondary current and Z is the total impedance of the meter and any connected leads.

What are two types of three phase transformers and which is the most common

three single-phase transformers each providing one third out the output required OR combining the three in a single three phase unit., the second option is most common as it saves on initial cost and space and the energy losses are not as great.

Why do transformers need to be cooled

when a transformer is in use, its temperature rises until heat is dissipated through its case to the air surrounding it at the same rate that the heat in generated. So there must be a means of cooling or else the transformer will overheat and cause damage.