Transformer Polarity Transformer

refers to

hand side -

Polarity

phase transformer phase transformer - Copyright 2002 Kilowatt Classroom, LLC. Classroom, 2002 Kilowatt Copyright Primary H2 Terminal phase unit, H3 will be to the right of phase unit, H3 will - Secondary X2 Terminal Primary Bushing H2 bushing secondary. Secondary Bushing X1 -

Secondary Bushing X2 (Neutral)

from X1 to X2. In practice, from X1 to X2. In

hand side of a distribution transformer. This hand side of a distribution transformer.

- .

will be , the Primary Bushing H1 is always on the left , the Primary Bushing tapped 120 / 240 volt three

- Additive Polarity

Subtractive Polarity cycle when the applied voltage (or current in the case of a current trans- voltage (or current in the case of a cycle when the applied - Transformer Polarity Transformer

phase transformers with existing electrical systems. A knowledge of A knowledge of electrical systems. with existing phase transformers - hand side (if the transformer is a three hand side (if the transformer - 24 kV Potential Transformer

A Typical Distribution Transformer

on the left, X1 will be on the right

An understanding of polarity is essential to correctly construct three is essential to correctly of polarity An understanding are Additive Polarity and the H1 and X1 bushings are physically placed diagonally oppo- are Additive Polarity and the H1 and X1 always (PT’s and CT’s) and large substation transformers are Subtractive Polarity, so the H1 and (PT’s and CT’s) and large substation transformers when facing the low voltage bushings when facing the low The position of the High Voltage Bushings is standardized on all power and instrument trans- High Voltage Bushings is standardized The position of the bushing primary and center refers to the relative direction of the induced voltages between the high voltage terminals and direction of the induced voltages between refers to the relative - Two Secondary Bushing X3 Primary Bushing H1 Primary H1Terminal Secondary X1 Terminal from H1 to H2 the secondary induced voltage direction from H1 to H2 the is

banks and to properly parallel single or three to properly parallel banks and relays. devices and protective to power metering current transformers potential and also required to connect polarity is where step voltage regulators principle used in is the underlying subtractive polarity theory of additive and The basic buck or boost the applied line voltage. an autotransformer is connected to either the series winding of Transformer Polarity During the AC half the low voltage terminals. former) brought out of the transformer. the way the leads are Bushing Arrangement this: formers. The rule is H2 is on the right and the Primary Bushing H2). Distribution Transformers site each other. Since H1 is development of electrical distribution systems and has been adhered to in or- standard was developed very early in the transformers need to be replaced or paralleled with existing equipment. der to prevent confusion in the field when Instrument Transformers the transformer. This standard was later adopted to make it easier to read elec- X1 Bushings will be on the same side of diagrams. trical schematics and construct phasor The Importance of Polarity of Polarity The Importance H1 and X1 on same side of transformer. H1 and X1 bushings are located diagonally opposite. H1 and X1 bushings are located diagonally

Transformers Sheet 2

Ind = 150

hand bushings. If -

30 VAC 120 VAC

30 = 90). If the voltmeter reads 150 volts, 30 = 90). If the voltmeter reads 150 volts, -

Jumper 120 volts, the transformer ratio is 4:1 (480 / 120 = 4). 120 volts, the transformer - Temporary

hand secondary bushing are temporarily jumpered together and a test are temporarily jumpered together hand secondary bushing -

V Transformer Polarity Test Polarity Transformer Ind = 90

30 VAC 120 VAC hand bushings is less than the applied primary voltage, the transformer is Subtractive Polarity. than the applied primary voltage, the hand bushings is less - In situations where the secondary bushing identification is not available or when a transformer has or when a transformer is not available bushing identification where the secondary In situations hand) primary bushing and the left hand) primary bushing -

been rewound, it may be necessary to determine the transformer polarity by test. The following procedure can be procedure by test. The following the transformer polarity to determine it may be necessary been rewound, used. The H1 (left voltage is measured between the right the transformer primary. The resultant voltage is applied to the polarity is transformer is Additive Polarity because is greater than the applied voltage, the the measured voltage voltage primary voltage. If, however, the measured voltage is being added to the applied such that the secondary across the right applied to the secondary insulation, the test voltage to avoid the possibility of damaging the Note: For safety and the rated secondary voltage. a reduced voltage and should not exceed primary should be at if the transformer is actually rated 480 In the example below, (120 / 4 = 30). If in a secondary voltage of 30 volts of 120 volts to the primary will result Applying a test voltage voltmeter will read 90 volts (120 transformer is subtractive polarity, the + 30 = 150). the transformer is additive polarity (120 and direction of the primary and secondary voltages. The red arrows indicate the relative magnitude Polarity Test Polarity Test Jumper Temporary

Transformers Sheet 3

circuited - bushing PT’s -

ground voltage,

out type meter and relay - scale current rating of - type units with low primary current -

bushing PT’s would normally be connected bushing PT’s would normally be connected - Primary (H1) Polarity Mark

Secondary (X1) Polarity Mark X1

scale value of switchboard indicating instruments, scale value of switchboard bushing PT’s, rated for the phase - -

high secondary voltage. Draw - Type CT - down the magnitude of the measured quantity to a value that of the measured quantity to a value that down the magnitude - circuit the CT secondary when the instrument is removed from the - (PT’s) are voltage transformers which are used to supply a proportional which are used to supply a (PT’s) are voltage transformers Instrument Transformers Instrument

ground, or with 3 two - 300:5 Window phase Wye systems would normally be metered with 3 single phase Wye systems would normally be - to -

phase Delta systems are metered, 2 two X2 -

type CT’s, such a pictured below, the conductor, bus bar, or bushing which passes type CT’s, such a pictured below, the conductor, Current Transformers and Potential (voltage) Transformers are used to supply a reduced to supply a reduced are used (voltage) Transformers and Potential Current Transformers - delta. Three (CT’s) are used to supply a proportional current to the current input of metering and relay- (CT’s) are used to supply a proportional -

Potential Transformers Potential Transformers The standard PT secondary volt- input of metering and relaying equipment. voltage to the voltage the standard full age is 120 VAC to match relays. The transformer high power metering equipment, and protective PT ratios are expressed as the ratio of the high volt- be the same as the nominal system voltage. The Potential Transformer pictured at the left has a 24000 age divided by the secondary voltage. volt secondary, so the ratio is 200:1 (24000 / 120 = 200). volt (24 kV) primary and a standard 120 Where three open connected phase connected to ground. having one primary bushing on each transformer

All instrument transformers are subtractive polarity. All instrument transformers The load on an instrument transformer is referred to as a “burden”. transformer is referred to as a The load on an instrument

Bushing PT -

Current Transformers current is 5 amps to match the standard full ing equipment. The standard CT secondary switchboard indicating devices, power metering equipment, and protective relays. CT ratios are expressed as a ratio metering equipment, and protective relays. CT ratios are expressed as a ratio switchboard indicating devices, power secondary. of the rated primary current to the 5 amp 5 amps of secondary current when 300 amps flows through the primary. The 300:5 CT pictured below will produce current will vary accordingly. For example, with 150 amps through As the primary current changes the secondary current will be 2.5 amps ( 150 : 300 = 2.5 : 5 ). the 300 amp rated primary, the secondary On the Window or Donut one primary turn. On Window through the center of the transformer constitutes can be changed by taking multiple ratings, where the primary conductor size is small, the ratio of the transformer CT has a ratio of 100:5, placing wraps of the primary conductor through the window. If, for example, a window the ratio to 50:5. Some types of two primary conductor wraps (two primary turns) through the window will change ratio CT to be utilized for several equipment employ this method to calibrate the equipment or to permit a single different sizes (ampacities) of equipment. connected; an open Caution: the secondary of a Current Transformer must always have a burden (load) secondary can result in the development of a dangerously cases incorporate shorting contacts which short circuit for calibration. Polarity marks identify (are adjacent to) the H1 and X1 terminals. value of current or voltage to instrument circuits. They provided isolation from the high voltage system, permit voltage system, permit from the high They provided isolation instrument circuits. or voltage to value of current circuit for safety, and step grounding of the secondary by the instruments. can be safely handled Burden Polarity Instrument Transformers Instrument 200:1 Two

Transformers Sheet 4

Transformer Secondary Circuit Grounds

CT Primary Polarity Mark ) will be used to identify the polarity terminals on electrical the polarity terminals on electrical ) will be used to identify + Polarity Marking Polarity polarity marks are shown on Instrument Connection Diagrams, Instrument Connection are shown on polarity marks

element devices such as an ammeter or voltmeter, but is essential -

Line Power Diagrams. The polarity mark is usually shown as a round dot, on shown as a round mark is usually Diagrams. The polarity Line Power

Typical Current and Potential Transformer Connection Diagram Typical Current and Potential Transformer To insure correct wiring, To insure Instrument Current Coil

PT Primary Polarity Mark PT Secondary Polarity Mark CT Secondary Polarity Mark drawing. (Red dots were The PT, CT, and instrument polarity marks are shown by the red dots on the above used in this example only for clarity.) connected in parallel. Current elements of the instruments are connected in series, voltage inputs are Polarity is not a consideration on single protective relays. for proper operation of power measuring devices, and for directional or differential drawings. illustration with polarity marks as shown in the may also have the terminals identified Instrument transformers understood not have polarity marks on them, it is Sheet 3. If instrument transformers do of the 300:5 CT on polarity. and the X1 (secondary) terminals are that the H1 (primary) marks, polarity connections may also have polarity other equipment which require proper Meters, relays, and Connection Diagram. must be obtained from the Instrument but usually this information Polarity Marks Polarity Marks and Three Control Schematics, form of a Sometimes alternate marking, in the and X1 terminals of PT’s and CT’s. or adjacent to, the H1 ( / ), or plus/minus sign ( square dot, slash mark