DISADVANTAGES OF Y-Y CONNECTION IN THREE PHASE SYSTEM

What are the disadvantages in using y-y connection?

The Y-Y transformer connection was poorly understood in the early days of power engineering and it received a very bad reputation when it was first used; in fact, this connection was avoided for a long time until its limitations were overcome by good engineering practice. Some of the inherent disadvantages of the Y-Y connection are discussed below:

1. The presence of third (and other zero-sequence) harmonics at an ungrounded neutral can cause overvoltage conditions at light load. When constructing a Y-Y transformer using single-phase transformers connected in a bank, the measured line-to-neutral voltages are not 57.7% of the system phase-to-phase voltage at no load but are about 68% and diminish very rapidly as the bank is loaded. The effective values of voltages at different frequencies squared. With sinusoidal phase-to-phase voltage, the thirdharmonic component of the phase-to-neutral voltage is about 60%, so the effective voltage across the winding is calculated as follows:

2. There can be a large voltage drop for unbalanced phase-to-neutral loads. This is caused by the fact that phase-to-phase loads cause a voltage drop through the leakage reactance of the transformer whereas phase-to-neutral loads cause a voltage drop through the magnetizing reactance, which is 100 to 1000 times larger than the leakage reactance.

3. Under certain circumstances, a Y-Y connected three-phase transformer can produce severe tank overheating that can quickly destroy the transformer. This usually occurs with an open phase on the primary circuit and load on the secondary.

4. Series resonance between the third harmonic magnetizing reactance of the transformer and line-to-ground capacitance can result in severe overvoltages.

5. If a phase-to-ground fault occurs on the primary circuit with the primary neutral grounded, then the phase-to-neutral voltage on the unfaulted phases increases to 173% of the normal voltage. This would almost certainly result in overexcitation of the core, with greatly increased magnetizing currents and core losses. This is illustrated in Figure 9.4. A bold X marks the location of a B phase-to - ground fault with the neutral of the voltage source either ungrounded or connected to ground through a large impedance. The voltage across the B phase winding collapses and the applied voltages across the A phase and C phase windings are now equal in magnitude to the phase-to-phase voltages.

6. If the neutrals of the primary and secondary are both brought out, then a phase-to-ground fault on the secondary circuit causes neutral fault current to flow in the primary circuit. Ground protection re laying in the neutral of the primary circuit may then operate for faults on the secondary circuit. This is illustrated in the Figure 9.5. The two magnetically coupled windings are connected by a dotted line. Any fault current in the secondary neutral is transformed into neutral current in the primary circuit through the second transformer law.

The obvious remedy for some of the disadvantages of the Y-Y transformer connection would be to simply solidly ground both the primary and secondary neutrals. In fact, this is standard practice for virtually all Y-Y transformers in systems designed by utility companies. Unfortunately, solidly grounding the neutrals alone does not solve the problem of tank overheating, ferroresonance, and operating primary ground protection during secondary faults.

 ADVANTAGES OF Y-Y CONNECTION IN THREE PHASE SYSTEM