Three-Phase Transformer Connections | Electrical Engineering

The service transformer used for interconnection can have a great influence on the impact that Distributed Generation will have on the power quality.

The advantages and disadvantages of the common three-phase transformer connections are:

1. Grounded Wye-Wye Connection:

It is favored because of its reduced susceptibility to ferroresonance on cable-fed loads and fewer operating restrictions when being switched for maintenance. It is also generally well behaved with respect to Distributed Generation interconnection, but there are a couple of issues.

Advantages of Grounded Wye-Wye Connection:

i. No phase-shifting of utility-side voltages. This makes detection of utility faults by DG protection relays more certain.

ii. Less concern for ferroresonance, but it is not immune to ferroresonance.

Disadvantages of Grounded Wye-Wye Connection:

i. Allows DG to feed all types of faults on the utility system.

ii. Does not inhibit the flow of zero-sequence harmonic currents that might be produced from certain kinds of generators.

Because of these two concerns, it may be difficult to parallel some generators using this transformer connection. If the Distributed Generation is a synchronous machine, it may produce a small amount of third-harmonic voltage distortion, depending on the winding pitch of the machine. If a synchronous generator does not have a 2/3 winding pitch, paralleling to the utility system provides a very low impedance path for the third harmonics and the resulting neutral currents may damage generator equipment or simply add unwanted harmonic currents to the utility system.

A neutral reactor may be necessary for some wye-connected machines while they are paralleled to the utility system to:

i. Limit the flow of zero-sequence harmonics (principally, the third)

ii. Limit the contribution of the generator to ground faults

The reactor would be shorted when operating the generator standalone to provide emergency backup power so that a stable neutral is presented to the load.

2. Delta-Wye Connection:

It would probably be favored for serving loads in nearly all cases if it were not for the susceptibility of the connection to ferroresonance in cable-fed systems.

Advantages of Delta-Wye Connection:

i. There is less infeed into utility-side ground faults.

ii. Third harmonics from the DG do not reach the utility system.

iii. Some isolation from voltage sags due to utility-side SLG faults is provided.

Disadvantages of Delta-Wye Connection:

i. It is difficult to detect some SLG faults from the secondary side by voltage relaying alone.

ii. It is susceptible to ferroresonance in cable-fed installations.

iii. Third harmonics in the DG may cause excessive current in the secondary- side neutral.

iv. If islanded on an SLG fault, utility arresters can be subjected to over-voltages.

v. If arresters are islanded on an SLG fault and there is little load, resonant over-voltages can result.

The last two items are common to all transformers with an ungrounded primary connection.

Note that while this connection prevents third harmonics from the generator from reaching the utility system, it does not prevent their flow on the DG side. As with the grounded wye-wye connection, it is generally not advisable to directly connect synchronous alternators that are not 2/3 pitch without inserting an impedance in the neutral to limit the third-harmonic current flow.

While the phase shift can be beneficial to the load in reducing the impact of voltage sags due to SLG faults, it also makes some SLG faults on the utility system more difficult to detect. This increases the chances of islanding at least briefly because it delays fault detection until the utility breaker operates.

Therefore, it is common to add other relaying functions to aid in the early detection of utility-side faults. A negative-sequence relay can make the detection more reliable. While the voltage magnitudes seen on the secondary may not change much during a fault, they will be unbalanced, resulting in detectable negative-sequence voltages and currents.

Another approach is to add relaying on the primary side of the transformer, such as a type 59G relay ground overvoltage that can detect the presence of the SLG fault. This is an overvoltage relay placed in the corner of a broken delta potential transformer that measures zero-sequence voltage.

3. Delta-Delta or Ungrounded Wye-Delta Connection:

While not in the majority, these connections are still common for commercial and industrial loads. Both have similar behavior with respect to serving Distributed Generation. Neither would be the preferred connection for serving most new Distributed Generation installations, but could be encountered in legacy systems where a customer wishes to parallel Distributed Generation.

Some inverter-based systems (fuel cells, photovoltaic, microturbines, etc.) require an ungrounded connection on the Distributed Generation side because the dc side of the inverter is grounded. This is often accomplished by use of a separate isolation transformer rather than the main service transformer. However, either of these connections would also suffice.

The delta secondary is sometimes a four-wire connection with one of the delta legs center-tapped and grounded to serve single-phase 120-V loads. This is common in smaller commercial facilities that have three-phase HVAC equipment along with typical office load. If this is the case, no part of a three-phase Distributed Generation can be grounded while paralleled with the grid.

Advantages of Delta-Delta or Ungrounded Wye-Delta Connection:

i. More economical transformer installation for smaller three-phase service with some single-phase loads is possible.

ii. The load is isolated from ground faults on the utility side.

iii. DG would not typically feed utility-side ground faults except when resonance occurs.

iv. Ungrounded interconnection can be provided for inverter-based systems requiring it.

Disadvantages of Delta-Delta or Ungrounded Wye-Delta Connection:

i. Utility-side SLG faults are difficult to detect.

ii. Utility arresters are subjected to high steady-state over-voltages if islanded on an SLG fault. This is true for delta-wye connections as well.

iii. These connections are highly susceptible to ferroresonance in cable-fed installations.

iv. There are more restrictions on switching for utility maintenance.

Three-phase switchgear may be required on the primary because there are several problems that can occur if one attempts to perform single phase switching. This will increase the cost of the interconnection.

The prompt detection of SLG utility faults using voltage relaying is a problem with these connections. This will delay fault detection until after the utility breaker has opened, resulting in at least a brief island.

This can result in over-voltages and a resonant condition common to all ungrounded primary connections. Supplementing voltage relaying with negative- sequence relaying on the DG side can make the detection more certain. Also, it is common to add a ground overvoltage relay (59G) on the primary side to detect the continuing presence of a ground fault.

Grounded Wye-Delta Connection:

This is an interesting connection because of the conflicting application considerations. Many utility engineers believe this is the best winding connection for interconnecting generation to the utility system. This is the connection used for nearly all central station generation.

There are many advantages, including:

i. Utility-side faults are easily detected partly because the transformer itself actively participates in ground faults.

ii. Triplen harmonic voltages produced by the generator do not cause any current to flow because it is blocked by the delta winding. Therefore, nearly any generator can be paralleled with this connection.

iii. Protection schemes are well understood based on many years of experience with utility generation.

Despite these benefits, one may be surprised to learn that this connection is not permitted on distribution systems without a great deal of study and special considerations that may result in costly modifications to the system. In fact, it may not be possible to accommodate the connection on some distribution systems because of the inconvenience to other customers.

The connection is often referred to as a “ground source” because it contributes to ground faults and will generally disrupt the ground fault relaying coordination on the feeder. Other feeders connected to the same substation bus may be disrupted also. Figure 6.23 shows how the connection contributes to an SLG fault on a four-wire, multi-grounded neutral distribution system, the most common in the United States. The thicker arrows show the normal contribution expected from the main utility source.

Only one phase is involved on the distribution side, and the fault appears to be a line- to-line fault from the transmission side. The thinner arrows show the paths of the current from the grounded wye-delta interconnection transformer. The currents flow back through the substation and contribute additional current to the fault. The amount contributed would depend on the size and impedance of the transformer. This contribution will be dependent on the capability of the DG to feed a short circuit. In some cases, the contribution due to the transformer alone will be larger.

This characteristic has a number of possible adverse side effects when present on the distribution system:

i. Increased fault current means increased damage at the fault site, which will eventually lead to more sustained interruptions and reduced reliability.

ii. The connection is likely to cause sympathetic tripping of the feeder breaker for faults on other feeders. The transformer supplies ground current to other feeders connected to the same substation bus. Many customers who would normally see only a sag would be subjected to interruptions.

iii. Ground trip pickup levels must be increased, and more delay must be used to maintain coordination, which results in less sensitive fault protection. (An alternative is to use directional overcurrent relaying.)

iv. Sags for ground faults will generally be somewhat deeper (the transformer makes the system appear more solidly grounded).

v. If fuse saving is being attempted, the fault infeed, which is likely to be larger than from the DG itself, makes this much more difficult to achieve.

vi. The transformer itself is subject to short-circuit failure when a ground fault occurs. This is particularly true for smaller transformer banks with impedances less than 4 percent. A special transformer must generally be ordered.

vii. The transformer is also subject to failure thermally because the feeder load is rarely balanced. Thus, the transformer will act as a sink for zero-sequence load currents.

The distribution system can almost always be engineered to work with grounded wye-delta connections. This makes the Distributed Generation interconnection protection more certain and straightforward. However, this may require costly modifications that could become an insurmountable barrier for small- and medium- sized Distributed Generation. The utility must also be willing to accept special transformers and operating procedures that are different from the rest of the system.