- Aug 2, 2016 9:30 pm GMT
- 17261 views
Transformer polarity is fundamental to grasping how transformers function and how they’re utilized. Understanding polarity is essential to properly paralleling single-phase transformers and connecting instrument transformers (current and potential) to metering devices and protective relays. It has always been a challenge to explain transformer polarity in a manner easily understood by students. Occasionally, trainees will ask why there are two transformer polarities rather than one. This is a logical question for which the answer has been shrouded in mystery. This paper is intended to clarify various technical elements of transformer polarity in a manner useful for training specialists teaching transformer classes.
Let’s start with the definition of polarity: The electrical property of a body that either develops magnetic poles or has terminal points between which exists a difference in potential. The word “polarity” itself refers to these poles, meaning positive and negative (or north and south, as with Earth’s magnetic poles). Poles are given terminals that designate direction of current flow. Poles of electrical polarity (positive and negative) are present in every electrical circuit. In theory, electrons flow from the negative pole to the positive pole. In a direct current (DC) circuit, one pole is always negative, the other pole is always positive, and electrons flow in one direction only. In an alternating current (AC) circuit, the two poles alternate between negative and positive, and electron flow reverses back and forth. In every situation, there are two poles: positive and negative—or, in transformers, additive and subtractive.
UNDERSTANDING TRANSFORMER POLARITY
Transformer terminal markings are another key to understanding polarity. They have been standardized for many years (IEEE Std C57.12.70-2000) in the following manner: Terminals shall be distinguished from one another by marking each terminal with a capital letter (H for the primary winding and X for the secondary) followed by a subscript. Terminal designation for primary terminals is easy to remember, since it never changes. When facing the transformer from the front side, the H1 terminal is always on the left and the H2 terminal is always on the right. Secondary terminal markings are different depending on the polarity of the transformer, as confirmed on the nameplate. If the polarity of the transformer is subtractive, the X1 terminal is on the left and the X2 terminal is on the right. Conversely, if the transformer polarity is additive, the X2 terminal is on the left and the X1 terminal is on the right.
When only one lead of the high-voltage winding is brought out (the other being connected to the tank internally), it shall be designated as H1. For polarity marking and testing, the H1 terminal shall always be located on the left when facing the front side of the transformer. The terminals of any winding whose leads are brought out of the case shall be numbered 1,2,3,4, etc., the lowest and highest numbers marking the full winding and the intermediate numbers marking portions or taps. Thus, If the transformer has a center tap used as a neutral, it shall be designated as X2. Internally, secondary winding leads are marked A,B,C,D from left to right.
TRANSFORMER COIL WINDINGS
Transformer polarity depends on which direction the coils are wound around the core (clockwise or counterclockwise) and how the leads are brought out from the winding ends to the terminals. The two coil windings have distinct orientation with respect to one another—each coil can be wound around the core clockwise or counterclockwise. If the primary and secondary coils are wound in opposite directions, the polarity is additive; if wound in the same direction, it’s subtractive.
AC POWER FLOW
Another element in determining transformer polarity is noting how power flows through the windings. To understand this, voltage and current that flows across the transformer windings must be observed when peak positive voltage is being produced. In a 60 Hz AC circuit, voltage changes polarity 120 times per second. Any time the current is flowing into one of the primary terminals, it will be flowing out of one of the secondary terminals.
When the potential of primary terminal H1 “goes positive” and the secondary terminal on the right also goes positive, current flow is in on the H1 terminal and out on the secondary terminal on the right.
When the potential of primary terminal H1 “goes positive” (i.e. during the first half-cycle of AC) and the secondary terminal on the left goes positive at the same time, the current flow is in on the H1 terminal, and out on the secondary terminal to the left.
Note that the secondary terminal designation depends on polarity; with subtractive polarity X1 is on the left, and with additive polarity it is on the right.
TRANSFORMER POLARITY TEST
It is unusual for a line crew to need to conduct a polarity test, since polarity is confirmed on nameplates; however, there could be a situation where a nameplate is missing and polarity needs to be verified. Verifying the polarity of a transformer can be accomplished with a simple voltage test using the following steps:
- Make a temporary connection between the primary and secondary terminals of the left side of the transformer (when facing the front of the transformer).
- Connect a portable voltmeter across the primary and secondary terminals on the right side of the transformer.
- Apply a low voltage (around 120 volts) to the primary terminals; this will result in a voltage of about 12 volts across the secondary winding (provided the turns ratio is 10:1).
- If the voltmeter indicates the sum of the voltages (120 + 12 =132) the polarity is additive.
- If the voltmeter indicates the difference in the voltages (120-12 = 108) the polarity is subtractive.
Voltage readings may vary somewhat depending upon the turns ratio of the transformer. If the indicated voltage is more than the applied voltage, the polarity is additive; if it’s less than the applied voltage, the polarity is subtractive. Care should be taken to avoid connecting the 120 volt source across the secondary terminals, as high voltage will be present at the primary terminals.
Another way of understanding polarity is to view the windings on the same horizontal plane (similar to a single-winding autotransformer) along with the direction of current flow in each winding.
THE ORIGIN OF POLARITY
Transformer polarity became an inherent electrical consideration when the first AC transformers were developed back in the late 1800s. At that time, pioneers learned what polarity meant when they attempted to parallel transformers for more capacity. They quickly discovered that the transformers had to be the same voltage, and would only operate properly in parallel when the terminals were connected a certain way. There were no standard markings for the transformer terminals, and nameplates did not include any indication of polarity. Frequently, connecting of these early transformers was done by trial-and-error, and electrical workers were exposed to hazards created by short-circuits and damaged transformers. Eventually the industry recognized the need for clarification and standardization of various aspects of transformer manufacturing, including polarity.
In 1918, the American Institute of Electrical Engineers and other organizations established standards for external transformer lead markings. These markings served as the basis for establishing polarity as we know it today. The basic standard was as follows: The leads of any winding (high voltage or low voltage) brought out of the case shall be numbered 1,2,3,4 etc. The lowest and highest numbers represent the full winding and the intermediate numbers represent fractions of the winding or taps.
The first transformers were simply wound with no consideration of polarity. The origin of the polarity concept is obscure, but apparently, early transformers having lower primary voltages and smaller kVA sizes were first built with additive polarity. In the early 1900s, almost all transformers were manufactured with additive polarity. When the kVA and voltage values were extended, a decision was made to switch to subtractive polarity.
WHY TWO POLARITIES?
As the industry became more familiar with transformers, it was determined that with a two-winding transformer, voltage stress occurs between the two windings as a result of the difference in potential (voltage) of the two windings. The magnitude of the voltage stress is affected by the transformer polarity or direction of current flow in the two windings. Engineers discovered that as voltage stress increases, the life of the transformer is shortened. Winding insulation failures were the main result of the increased voltage stress. It was discovered that subtractive transformers produce less voltage stress than additive transformers.
As an example, assume we have a two-winding transformer with a primary voltage of 25,000 volts and a secondary voltage of 7,200 volts. A comparison of the voltage stress between the windings for additive and subtractive polarities can be determined as follows:
As you can see by the voltage, stress between the windings is considerably higher with an additive transformer. This was a factor in establishing a standard that transformers over 8,660 volts would have subtractive polarity. The reduction in voltage stress would result in longer life for transformers with higher voltages. The obvious question that comes to mind is, “Why not make all transformers subtractive polarity?”
Since there were large numbers of additive transformers in service, it was decided to continue manufacturing additive polarity transformers for voltages below 8,660 volts. Manufacturing transformers with different polarities is strictly a U.S. standard. Canadian standards are additive and Mexican standards are subtractive (not the clearest answer to the lineworker’s question, but hopefully somewhat understandable). Lineworkers should be informed that polarity differences are not a significant problem in the field, as transformer nameplates confirm polarity, and it is rare lineworkers are confronted with paralleling or banking transformers of different polarities.
The voltage stress between windings is considerably higher with an additive transformer.
Today’s standard has evolved over time from ANSI (American National Standards Institution) to the IEEE (Institute of Electrical and Electronic Engineers).
By Alan Drew, Vice President of Research & Development
Northwest Lineman College all rights reserved