advanced transformers

Open delta open delta banks are used when? Open banks are ____% as efficient as closed banks

Open delta banks are used when the single phase load is predominate over the three phase load 57.7

To complete a circuit , electrical current must return back to the source.in a delta circuit the return flow to the source is in the _______ phase conductors .there is no flow through the earth

Opposing

Wye wye or delta delta banks yield either ____ or ____ degree angular displacement On a wye wye bank with an angular displacement of 180 the x1s are to be tied together and grounded, this connection isn't widely accepted in the industry because the x2 is generally viewed as a neutral only bushing. For this reason most utilities prefer and utilize the zero degree angular displacement wye wye transformer bank

0 to 180

In a wye system the line voltage phase to phase is always ____ time greater than the ____ voltage phase to ground

1.73 Coil

A typical 120/240 volt single phase transformer has 2 coils of 120 volts each that can be connected in series or parallel. When the 2 secondary coils are connected together in series inside the tank then the secondary voltage will be _______ volts when the 2 coils are connected together in parallel the secondary voltage will be __ volts. Either a turns ratio test or a physical check by taking the lid off the transformer is need to check which configuration the secondary coils are in The secondary coils must be parallels for wye secondary connections so the full ____ of the secondary coil is utilized. If they are not paralleled, the proper voltage would still be delivered but the kva would be cut in half

120/240 120 Kva

open wye open delta Open wye open delta only require __ primary phases and a system neutral and can be connected in what 2 ways?

2 Same opposite connection or opposite same connection

Delta Secondary The lighter transformers serves approximately ___ of the single phase load while the power transformers serve approximately ___ each of the single phase load. All together load is 1 1/3 because of imbalance in loading caused by additional single phase load, total transformer kva is greater than the total load delivered. Although this connection is need, it is relatively a ___________ connection.

2/3 1/3 Inefficient

If the fuse is too small it will blow as result of acceptable transformer load, causing an outage to the customers connected to the secondary circuit . If it is too large it a catastrophic failure of the transform could result which may cause the operation of circuit protection devices It is common for companies to fuse transformers based on ___ to ___ % of the rated current at full load

200 to 300 %

Transformer grounding The national electric safety code requires that all systems less than __ volts be grounded The national electric safety code states that the resistance of an electrode shall not exceed ___ ohms

750 volts 25 ohms

In order to supply customers with 3 phase power , transformers coils need to be inter connected onto a configuration that will involve all 3 phases, and be connected so that there is a proper phase rotation where phase a phase b and phase c will be in sequence what are the 2 ways?

A phase to phase connection delta A phase to neutral connection wye

Polarity Make certain the transformers used in the bank are of the same polarity makes connecting secondaries much easier. Using transformers of different polarity will work electrically (if connected correctly) but can cause confusing for line workers in future and create serious safety concerns. Transformers rated 200kva and smaller with a primary rating of 8,660 volts and below will be rated _______ all other will be subtractive

Additive X3 X2 X1 Subtractive is x1 x2 x3

The ends of each coil are stamped or tagged with the letters a b c d the a and c ends connect to one secondary bushing and the b and d ends connect to an other secondary bushing. When paralleling the coils what trick should you remember. It is important to mention that either the ac or bd pair should connect to the x2 and serve as the neutral. It is industry standard for the x2 to serve as a neutral.

Alley cat bad dog

Wye secondary vector diagram was derived from a wye primary vector diagram. Likewise a delta secondary vector diagram was derived from a delta primary diagram, in both cases the secondary vectors were in the same position as the primary vectors it can be said that their _______ __________ was zero degrees

Angular displacement

Transformer loading The higher the kva rating of a transformer the more current it will handle.fuses and bud wires are installed to withstand the maximum currents plus various over currents so it is necessary to calculate the current values of a transformer at full load by dividing the kva rating of the transformer by the ___ voltage rating current at full load can be found remember that a transformer changes voltage and current but not power other words kva in equals kva out

Coil

In delta system phase current is 1.73 times higher than the ?

Coil current Coil current =phase current /1.732 Phase current = coil current x 1.732

Delta secondary vector The line voltage and ___ _____ are equal in a delta system.when a neutral is tapped from the center point of one of the coils, a power leg is created __ coils from the neutral. The voltage between the neutral and the power leg is 1.73 times greater Than the neutral to hot leg voltage

Coil voltage

Kva ratings As a a general rule, transformer banks with wye connected secondary will be composed of transformers of ______ size because all three pots share the single phase load equally. Pots with delta connected secondaries serving combination single and three phase loads will usually have 1 transformer of greater size because one transformer in the bank serves ______ of the single phase load

Equal 2/3

It is important to understand that the neutral conductor is part of the circuit and will current all the time. The grounding conductor connects the neutral or the transformer case to earth and should only carry current during a _____ In a pad mounted transformer the grounding conductor is connected to a siren ground rod that is typically located in the secondary section of the transformer pad. The concentric and secondary neutral connect to the grounding scheme

Fault

Phase to phase or phase to ground connections There's transformers will have primary voltage rating that depots 2 numbers separated by a slash (/). The first number is the primary coil rating which should be the smaller of the 2 numbers second number is the system voltage. Transformers with this designation can be connected phase to phase or phase to ground and will provide the rated voltage. They are the most versatile transformers because they can be used for both types of connections. If the letter y is printed after the system voltage, this is an indication to the line workers that if the transformer is installed on a 20,800 volt system it should be connected phase to ________

Ground.

Wye primary vectors A vector is drawn over the primary coil of each transformer below with an arrow tip representing the ____ and a trial representing the ______ When applying vectors to transformer banking the ________ is voltage

H1 representing arrow tip H2 tail Magnitude

Open wye open delta The same opposite connection is so called because the same primary bushings are connected together __ to ___ and grounded while the opposite secondary bushings are connected together __ to __

H2 to H2 X1 to X3

Wye delta bank The floating wye point can be a ____ to line workers because it is not ________ A power leg exist only when ? The secondary coils are connected in _________ the voltage between the neutral and power leg is 1.73 times greater than the neutral to hot leg voltage. Power leg has no use electrically and is simply a by product of a delta secondary. Three phase motors only see the h to h voltage and do not notice the power leg

Hazard Delta and open delta secondaries when a ground is attached to the winding midpoint of one of the transformers

Turns ratio testing A standard turns ratio test will give a good indication that the transformer is operational and that there are not shorts between the turns in the windings. It will not show a weakness in the insulation. Test is carried out by energizing the high voltage coil with a low voltage, such as 120 volts. The device can then compare the voltage output with he known input voltage. The ratio on the nameplate and measured ratio should be equal. Turns ratio is calculated by dividing the input voltage by the output voltage. When doing this test remember that transformers work both ways. For this test the low input voltage must be connected to the _________ voltage winding. If the input voltage is applied to the low voltage winding , a high voltage will be present on the primary bushings

High

When paralleling transformer banks the primary and secondary angular displacements must be ______

Identical

Angular displacement is an indication of the voltage rise relationship between the primary and secondary. When considering a wye wye. Transformer bank that has zero degree angular displacement, it can be said that a rise in voltage in the h1 causes an instantaneous rise in voltage on the x1. The h1 and x1 are in _____. Angular displacement becomes important when paralleling transformer banks. Only banks with similar angular displacement can be paralleled

In Phase

Continuity testing This can be done with a multi meter. It gives off a sound when the 2 probes contact one another either directly or through a conductive object. Feature is especially useful when proving the _______ of transformers coils. Audible sound should be heard between the h1 and h2 bushings and same with secondary bushings .where should an audible sound not be heard? Hastings makes a transformer tester called tlc which stands for transformer last check used to verify distribution transformers do not have shorted or open windings prior to energizing

Integrity Between primary and secondary windings

Impedance Every effort should be made to select transformers with the same impedance. Mismatched impedance between parables units or banks requires de rating because the load does not then divide in proportion to the ____ ratings; the lowest impedance transform will supply a greater share of the load and possibly end up being _______.

Kva Overloaded

Delta secondaries do not allow all three transformers to share the single phase load equally so one transformer of the bank must provide the majority of the lighting load. It will always be the transformer with the x2 grounded serving as a neutral because this transformer must provide the majority of the single phase load it must sized ______ than the other 2 transformers in the bank and is referred to as the ______ transformer because it handles the lighting load the other transformers share somewhat equal portions of single phase and three phase load and are referred to as power transformers

Larger Lighting

Phase to ground connection only These transformers also have a primary rating that depicts 2 numbers separated by the slash . First number is the system voltage and second number is the primary coil rating smaller number. Transformer with this designation have one primary bushing and can only be connected between phase and ground that will provide the rated voltage. The insulation of the primary coil of these transformers is reduced at the neutral end. If the letters grd are next to they y this is an indication to the line work that the ________ end of the primary coil must be grounded for the transformer to operate properly

Neutral

dual rated voltages This transformer has 2 primary coil ratings, and can be installed on systems of differing voltages. This is achieved by the installation of an external __ _____ switch on the side of transformer case. The switch changes the primary ____ turns ratio. The dual voltage switch should only be operated when?

No load Coil Only when it is de energized

If one transformer of a three transformer bank with a delta secondary goes bad, the bank can be temporarily re wired to an _____ bank in emergency situations

Open

Wye delta Each transformer is connected to the system and the wye point is floated not grounded. Because the wye point is floated transformers with 2 primary bushings must be used. Grounding the wye point would create a ground source for the primary system this could subject the transformer to severe _____ during a primary system disturbance or load imbalance. Since the wye is floated this connection can be used on both 3 and 4 wire systems.with one unit out of service a bank of the single phase units can be reconnected as an _______ __________ which may temporarily supply power to customer until permanent repairs are made

Overloading open wye open delta

Secondary connections on wye If the phase to phase voltage of the service is need is 1.73 times the phase to neutral voltage of the service needed connect wye ________ the secondary coils when making wye secondary connections

Parallel

Wye wye banks Wye wye bank each transformer is connected to the system between a phase and grounded neutral. Each of the three transformers provide one hot leg on the secondary side to deliver three phase power. Secondary coils are ________ leaving x3 idle. always use the x1 and x2. What are the 2 main advantages of wye wye bank? What is the disadvantage?

Paralleled All three transformers can share load equally They can be built using single or dual primary bushing transformers If one transformer fails the customer is down to one phase until the defective transformer is fixed or replaced

Wye / delta primary connection rule If the primary coil voltage ratings of the transformers match the phase to neutral voltage connect wye If the primary coil voltage ratings of the transformer match the phase to ______ voltage of the system connect delta Transformers in delta connection Must have 2 primary bushings delta connections cannot derive from single bushing transformers

Phase

Engineers can choose from three different primary voltage designations: Phase-to-phase, Phase-to-ground, OR __________

Phase-to-phase or Phase-to-ground

Wye delta and delta and delta wye connected transformer banks yield a __- degree or a 210- degree angular displacement. Each bank wye delta or delta wye can yield either the 30- degree or 210- degree angular displace at depending open the ________ of the transformers used when building the bank. Most utilities utilize the 30- degree angular displacement wye delta with subtractive transformers, and the 210 degree angular displacement with additive transformers

Polarity

Secondary vectors Since a rise in voltage on the h1 bushing causes a rise in voltage on the x1 bushing (regardless of polarity) the h1 and x1 will have the same instantaneous ______. Therefor the secondary vectors are drawn over the secondary coils with the tips facing the ____ bushings Secondary vector diagrams are derived from the ____ and _____ primary vectors. So the secondary vectors must always remain _______ to the primary vectors

Polarity X1 Wye and delta Parallel

Before selecting any transformer, testing it is a good idea, especially for used transformers. When selecting single phase transformers that are to be banked or parallel together you should check for circuit voltage ,system voltage, transformer voltage rating, kva ratings of the transformers and what other 2 things ?

Polarity of the transformers Impedance of the transformers

Each coil in a transformer has positive and negative end. On a single phase transformer, the h1 and x1 are always ______, and the h2 and x3 (sometimes x2) are __________. A reversed polarity on one transformer of a three phase bank will result in extremely high _______ flow when energized. When making a wye connection , the negative polarity terminals are all connected together, and the positive terminals are connected to each individual phase. When making a delta connection, a positive terminal is always connected to a negative terminal and connected to the phase. Regardless of polarity, delta banks present the same appearance

Positive Negative Current

When interrupting the primary coil rating of a transformer remember that the smaller number (whether it is printed first or last) is the voltage that needs to be impressed on the _________ coil for proper operation. The large number refers to the voltage of the system that the transformer should be installed ( assuming that system voltage is always read phase to phase)

Primary

insulation testing Refers to at times as megger and is sued in troubleshooting and preventive maintenance programs to test by insulation in electrical apparatus, cables, wire or networks , can be used to check for transformer coil insulation damage (and coil continuity) all connections including the ground on the x2 terminal must be ________ for this test. When testing the insulation the readings between any primary and secondary bushings should read _____, and reading between any bushing and transformer case should read _____ as well. This would indicate insulation is not damaged.

Removed Infinity

Delta When the 3 coils are each connected phase to phase then 3 phase are connected in _______. This connection is know as a delta connection that being said the h1 connects to h2 all the way around as well on the secondary side x1 to x3 connects all the way around

Series

Phase to phase connections only These transformers will have a primary voltage rating that depicts only 1 number. This number serves as north the primary voltage rating and ________ voltage. Transformers with this designation have 2 primary bushings and should be connected between 2 primary phases that will provide the rated voltage

System

In a delta system the coil voltage equals ?

System voltage

Fusing the primary Fusing protects the transformer from overload,short circuits and any follow through current form a lighting or switching surge. Primary fusing on a three phase transformer bank has to be small enough to clear a secondary short, but large enough to prevent nuisance fuse blowing because of motor starting or a brief overload fusing of transformers has what 2 main objectives ?

To prevent damage to the transformer from overloads or short circuits on secondary side of transformer Isolate a transformer that has an internal fault from the circuit so that circuit protection devices do not have to clear the fault

Tap changers Some distribution transformers are supplied with taps in order to provide the same secondary voltage at different location on the system where the primary voltage may no be the same, and the utility doesn't want to add high cost voltage regulation. Never forget that when the tap setting is raised , the output voltage is lowered. The tap changer is a no load Manual switch that is connected the the primary coil and increases or decrease the _____ ______ of the transformer with each tap setting depending on the manufacturer each tap lower or raises the secondary voltage by __ or ___%. This handle is located either inside or outside the tank. The tap changer is connected to the primary coil. If a tap is raised the number of turns of the primary coil is _______ which maintains the output voltage as the primary voltage drops

Turns ratio 2.5 or4.5 % Reduced

Whenever transformers are banked together , a graphical representation of the electrical quantities and connections can be made. The graphical representation is made with _________. They are lines that have magnitude (some numeric quantity) and direction they can be associated with the coils of a transformer to graphically depict connections and voltages

Vectors or voltage phasors

when is a current limiting fuse used?

When a transformer is installed on a circuit that has a very high fault current capability, a current fuse is installed in series with a fuse cutout

The term open bank is used because when 2 transformers are banked together , they do not complete the entire delta or wye configuration. One leg of the delta or wye is gone or open. as a general rule if the customer load is a combination of three phase and single phase, with the single phase load being larger than the three phase load, an open bank may be used. three phase power cannot be derived from an open bank if the secondary is connected open ______?so if the 3 phase power is to be delivered when open banks are used the secondary connection will be open ________. primary side can be either open wye or open delta depending upon the system voltage and primary coil voltage rating of the transformers

Wye Delta

120/208 , 277/480, 240/416, 347/600 are what typed of secondary connections ?

Wye secondary