Transformer design & system grounding (wye vs. grounded wye)

There are two basic types of electrical systems: grounded and ungrounded. Today, the most common and widely used systems are grounded systems. Grounded systems can be divided further into two basic categories: solidly grounded and impedance grounded. So, technically speaking we can divide electrical systems into one of three categories:

1. Ungrounded
2. Solidly Grounded
3. Impedance Grounded

When selecting a transformer for any particular application, it is important to know which of the above categories your system falls under as it will directly impact the transformer’s design. Choosing the right transformer for your system will ensure your unit has the correct insulation rating and design. We’ll start with a brief overview of the three systems listed above.

Types of Electrical Systems

Ungrounded Systems

As its name implies, an ungrounded system is one where none of the circuit conductors in the system have any intentional connection to ground or earth. This type of system was once the prevailing system in use, but it is now less common due to the challenges it presents during arcing faults. A fault on an ungrounded system can raise the voltage level on the unfaulted phases to two or three times the normal voltage. This increased voltage level can cause insulation failure at the transformer if it is not designed for these scenarios. 

A transformer with a delta connected winding or ungrounded wye connected winding will always be designed with additional insulation to protect against these overvoltage spikes should a fault occur. Only an ungrounded type wye or delta winding should be used for ungrounded systems.

Solidly Grounded Systems 

(Why Wyes Are Wise)

A solidly grounded system is one with a neutral point directly connected to the earth, usually at the transformer neutral.

(Solidly grounded connections often bond the neutral directly to ground at the transformer’s winding supply power to an electrical system.)

Due to the direct connection to ground via the system neutral, a large amount of fault current will flow during an arcing event. The system is designed with very low resistance to facilitate this large current flow, and the result is lower transient overvoltages. For such a system, you will not see the voltage rise significantly (higher than the line to neutral system rating) on any of the unfaulted phases during an arcing event. This allows for the insulation rating of the system to be configured based on the line to neutral value of the grid voltage rather than the line to line rating. The transformer's winding remains at the level of the normal system voltage (since there will be no overvoltages two to three times the system’s standard rating). For example, in a 12470 GY 7200 system, the insulation rating can follow the line-to-neutral, 7200 volt value.

Impedance Grounded Systems

Impedance grounded systems are grounded through some sort of impedance like a resistor or reactor. Instead of having the neutral point directly connected to earth at the transformer, the system’s source transformer neutral is connected to earth at the impedance. 

There are several different types of impedance grounded systems:

1. High resistance grounded 
2. Low resistance grounded
3. Reactance grounded

Each system behaves a little differently when an arcing fault occurs, and depending on which system you employ, it may require a special design change for the transformer’s winding insulation.

Note that the GY and Y refer to the insulation of the windings and should correspond to the electrical system where the transformer is installed. 

Grounded Systems in relation to transformer windings

In the same way that an electrical system will be insulated according to whether it is grounded or ungrounded, the insulation for transformers will follow the same rules.

The type of grounding system you use for your project will also dictate which bushing and winding configuration you will need on your transformer. Let's look at the most common configurations for:

1. Grounded Wye Windings
2. Wye Ungrounded Windings
3. Delta Ungrounded Windings

Grounded Wye Windings

Systems that utilize the neutral conductor will generally have a transformer that has insulation which is 57.7% of the full phase-to-phase rating. The insulation is reduced at the neutral and on through to the windings. What this means is that such a transformer winding cannot be used in an ungrounded circuit. As we mentioned above, an ungrounded circuit will produce overvoltages two to three times the phase voltage at the transformer. Using a transformer designed for an effectively grounded system in an ungrounded system could result in transformer insulation failure. Let's look at the differences between grounded wye systems with common H0/X0 and separate H0 & X0

Grounded Wye with Common H0/X0

A common H0/X0 configuration does not provide a separate H0 bushing at the primary terminals. Instead, the H0 lead inside the tank is linked to the X0 lead and brought out as a single bushing in the low voltage cabinet as a common H0/X0 terminal. In this design, the H0 neutral is allowed to be selected in accordance with the low voltage winding BIL rating. In other words, if the low voltage winding BIL is 30kV, then the H0 terminal will have a BIL rating that is 30kV (IEEE C57.12.00-2015, Table 3, see note “d”). In this setup, the neutral of the primary winding is limited by the secondary windings.

Grounded Wye with Separate H0 & X0

A separate H0 and X0 configuration provides a separate H0 bushing with a ground strap with the primary terminals. This configuration also allows for a common solidly grounded neutral as well, via the bond strap connection at the tank wall between the H0 and X0 terminals. This also results in the H0 terminal having its BIL limited by the low voltage winding BIL.

Wye Ungrounded Windings

For systems that are ungrounded or may experience overvoltages that would exceed the phase (coil) voltage of the transformer, a fully-insulated wye winding is needed which can withstand the higher voltages that arise on the unfaulted phases during a line-to-ground fault.

Since a typical ungrounded wye winding is fully-insulated, it is suitable for both ungrounded and grounded systems. 

As a general rule, the secondary/low voltage connection of the transformer is always fully-insulated when configured as a wye, and includes a ground strap that can be bonded to the tank for solidly grounded applications or removed for ungrounded applications. This option gives the customer the most versatility, and since there is virtually no cost savings for requesting a GY rated winding on the LV, a fully-insulated Y connection with a ground strap is customarily supplied.

Delta (ungrounded) Windings

Unlike a wye-connected winding, delta ungrounded windings have no common neutral point between all three phases. Delta connections typically indicate an ungrounded circuit. There are methods of solidly grounding a delta winding to earth, but these are not often employed due to the challenges posed by such a configuration. A delta-connected winding will always be fully insulated. 

Winding Insulation and Its Effects on Transformer Components

Arresters

In a solidly grounded system, the line-to-ground fault current is higher (typically between 25% and 110% of the three phase fault current). Consequently, since the fault current is higher, the transient overvoltages are lower. For this reason, ground-to-neutral type lightning arresters may be applied.

Fusing

In addition to arresters, the transformer fuses which are connected in the same pathway will vary based on whether the primary winding is solidly grounded or ungrounded. For an ungrounded primary connection, the fuses can see higher overvoltages under a fault and should be sized accordingly.

We can illustrate this point by showing the different fuse coordinations for backup partial range current limiting fuses connected in series with expulsion style bayonet fusing.

Transformers with grounded wye-connected windings will often utilize a L-N rated backup current limiting fuse with a voltage rating that is lower than the L-L nominal system rating. For a standard 12470 GY/ 7200 primary, a CLF with an 8.3kV rating may be selected. For a 12470 Y/ 7200 winding, a CLF rated for 100% of the nominal line-to-line voltage will be selected–a 17.2kV rated CLF in this case. 

It is important to select the primary winding type that best suits the system where the transformer is being installed. This ensures the overcurrent and surge protective devices will be configured correctly to provide optimum protection for your system.

Conclusion

In conclusion, the type of grounding system used in an electrical system has a significant impact on the design of the transformer that is used. It is important to choose a transformer with the correct winding insulation and configuration to ensure that it can withstand the overvoltages and fault currents that are likely to occur in the system.

If you are unsure about the type of transformer configuration your project requires, our team of technical sales representatives is here to help.