THE BH CURVE APPLICATION IN POWER TRANSFORMER TUTORIALS

THE BH CURVE APPLICATION IN POWER TRANSFORMER BASIC INFORMATION
What Is BH Curve? How It Is Applied In Transformers?


For actual transformer core materials, the relationship between B and H is much more complicated. For a flux that periodically changes, the B-H curve depends on the magnitude of the flux density and the periodic
frequency.

Figure 1.7 plots the B-H curve for a ferromagnetic core with a 60 Hz sinusoidal flux density having a moderate peak value.

The B-H curve is a closed loop with the path over time moving in a counterclockwise direction over each full cycle. Note that when the magnetizing current is zero (H 0) there is still a considerable positive or negative residual flux in the core.

This residual flux is from crystalline structures in ferromagnetic materials that remain magnetically aligned even after the MMF is removed.

For a given peak amplitude of flux density, the B-H loop becomes narrower at frequencies below 60 Hz, although the width of the loop is not directly proportional to frequency. Even at very low frequencies approaching DC, the B-H curve has a finite area contained in the loop.

As seen in Figure 1.7, magnetic materials are highly nonlinear, so treating m as a constant is clearly an oversimplification. Nevertheless, assuming that materials are linear, at least over some range of flux density, is required in order to do quantitative analysis.

As the peak amplitude of the flux increases, the core goes into saturation; i.e., B increases at a much smaller rate with respect to increasing H. This means that μ gets effectively smaller as B increases. In saturation, the slope dB/dH is approximately equal to μ0. Figure 1.8 plots a typical B-H curve for a ferromagnetic core with a 60 Hz sinusoidal flux density having a large peak value.

This core material saturates at approximately +/- 1.5 Wb/m2 ( +/- 1.5 T), which is a typical saturation value for materials used in power transformers.

The magnitude of H increases greatly when the core goes into saturation, meaning that the peak magnetizing current increases dramatically. Again, the width of the B-H loop becomes narrower at frequencies below 60 Hz for a given peak amplitude of flux.

YΔ (WYE - DELTA) CLOSED / NEUTRAL = PRIM NO-SEC YES TRANSFORMER BANKING BASICS AND TUTORIALS

WYE - DELTA TRANSFORMER BANKING TUTORIALS
Wye - Delta Primary No Secondary Yes Banking Tutorials

WHERE USED
For supplying three-phase, 240 VAC loads with small amounts of 120/240 VAC, single-phase loads. No excessive circulating currents when transformers of unequal impedance and ratio are banked. No problem from third harmonic overvoltage or telephone interference.

WYE-DELTA FOR LIGHT & POWER
This diagram shows the connections for the Y-Delta bank to supply both light and power. This connection is similar to the delta-delta bank with only the primary connections changed. The primary neutral should not be grounded or tied into the system neutral, since a single-phase ground fault may result in extensive blowing of fuses throughout the system. The single-phase load reduces the available three-phase capacity. This connection requires special watt-hour metering.

BANK RATING
The transformer with the midtap carries 2/3 of the 120/240-volt, single-phase load and 1/3 of the 240-volt, three-phase load. The other two units each carry 1/3 of both the 120/240 and 240-volt loads.

CAUTION
The secondary neutral bushing can be grounded only on one of the three transformers.

IMPEDANCE & GROUNDING
The wye-delta connection is one of the most popular connections used today. Transformers are often connected from delta-delta to wye-delta to take advantage of 1.732 times the delta transmission voltage.

In this connection, it is not necessary that the impedance of the three transformers be the same. This connection should not be used with CSP single-phase transformers since when one breaker opens; serious unbalanced secondary voltages may appear.

The wye of this system should not be grounded because then the bank serves as a grounding bank and will supply ground-fault current for a phase-to-ground fault on the primary system. Also for unbalanced three phase loads on the primary system, the secondary acts as a balance coil; therefore, circulating current may result in an overload.

STATIC DISCHARGE
Potentially present on a non-grounded primary wye connection. A high, excessive voltage results on a 3 phase Y-Δ connection on the secondary line to ground when one leg of the primary is open. The voltage present is static with no power and bleeds off when taken to ground.

This static can damage a volt-ohm meter. The static is greater when the secondary feeder is short and lesser when the secondary feeder is long. The static problem is resolved by grounding one phase or the center tap of one transformer on the secondary side, but this usually requires special KWH metering.

This static condition is present only when a primary line is open, not the secondary. This static condition can occur on an open (2-transformers) or closed (3-transformers) bank. This static condition can occur with any primary voltage.