Chapter 2: transformers  (Page 5/6)

• Transformers having a primary and multiple secondaries are frequently found in multiple-output dc power supplies.
• Distribution transformers used to supply power for domestic purposes usually have two 120V secondaries connected in series.
• The three-phase transformer banks used to interconnect two transmission system of different voltages often have a third, or tertiary, set of windings to provide voltage for auxiliary power purposes in substation or to supply a local distribution system.
  • Static capacitors or synchronous condensers may be connected to the tertiary windings for power factor correction or voltage regulation.
  • Sometimes $\Delta$ -connected tertiary windings are put on three-phase banks to provide a low-impedance path for third harmonic components of the exciting current to reduce third-harmonic components of the neutral voltage.

§2.7 Transformers in Three-Phase Circuits

• Three single-phase transformers can be connected to form a three-phase transformer bank in any of the four ways shown in Fig. 2.15. Note that $a=N_1/N_2$ .

Figure 2.15 Common three-phase transformer connections;

the transformer windings are indicated by the heavy lines.

• The $Y-\Delta$ connection is commonly used in stepping down from a high voltage to a medium or low voltage.
• The $\Delta -Y$ connection is commonly used for stepping up to a high voltage.
• The $\Delta -\Delta$ connection has the advantage that one transformer can be removed for repair or maintenance while the remaining two continue to function as a three-phase bank with the rating reduced to 58 percent of that of the original bank.(Open-delta, or V, connection)
• The Y-Y connection is seldom used because of difficulties with exciting-current phenomenon.
  • Because there is no neutral connection to carry harmonics of the exciting current and harmonic voltages are produced which significantly distort the transformer voltages.

• A three-phase bank may consist of one three-phase transformer having all six windings on a common multi-legged core and contained in a single tank.

• They cost less, weigh less, require less floor space, and have somewhat higher efficiency.

• It is usually convenient to carry out circuit computations involving three-phase transformer banks under balanced conditions on a single-phase (per-phase-Y, line-to-neutral) basis.

• $Y-\Delta$ , $\Delta -Y$ , and $\Delta -\Delta$ connections  equivalent Y-Y connections
• A balanced $\Delta$ -connected circuit of $Z_{\Delta }\Omega /\text{phase}$ is equivalent to a balanced Y-connected circuit of $Z_Y\Omega /\text{phase}$ if

$Z_Y=\frac{1}{3}{Z}_{\Delta }$ (2.39)

§2.8 Voltage and Current Transformers

• Transformers are often used in instrumentation applications to match the magnitude of a voltage or current to the range of a meter or other instrumentation.

• Most 60-Hz power-systems’ instrumentation is based upon voltages in the range of 0-120V rms and currents in the range of 0-5 A rms.
• Power system voltages range up to 765-kV line-to-line and currents can be 10’s of kA.

• Some method of supplying an accurate, low-level representation of these signals to the instrumentation is required.

• Potential Transformer (PT) and Current Transformer (CT), also referred to as Instrumentation Transformer, are designed to approximate the ideal transformer as closely as is practically possible.