Question

What would occur if "matched" (i.e. : X1 to X1 and X4 to X4) lead marking serve connected together and the transformers were of opposite polarity?

Answer 1

Explanation:

Transformer Polarity:

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.

DEFINING POLARITY

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
TERMINAL MARKINGS:

Transformer terminal markings are another key to understanding polarity. They have been standardized for many years 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 rightTHE 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.