The Necessity of Grounding Transformers

In power systems, proper grounding of various components is of great significance. It can significantly improve the availability, basic insulation level, overvoltage withstand capability and short-circuit withstand capability of components. However, some system parts may lack the neutral connection required for grounding. In such cases, auxiliary components like grounding transformers are needed to achieve the grounding function.

Grounding transformers create grounded neutral connections for ungrounded three-phase systems, such as delta-connected and ungrounded star-connected systems. When a single-phase ground fault occurs in such isolated or ungrounded three-phase systems, since there is no grounding path, the zero-sequence fault current will not be able to return. At this time, although the system can still continue to operate, the fault will cause the voltage of the unfaulted lines to increase by a factor of √3 (1.73), which will subject the insulation of transformers and other components of the system to 173% overstress. Moreover, although solid-state devices such as metal oxide varistors (MOVs) are often used to suppress voltage spikes, under fault conditions, even without sufficient flashover voltage, the thermal effect generated by the leakage current may damage these devices.

Obviously, if the system lacks sufficient fault tolerance, it is extremely vulnerable in such situations. Therefore, grounding transformers effectively avoid such situations by providing a grounding path for the fault current. In general, grounding transformers are mainly used for the following aspects:

1.Facilitate the connection of phase-neutral loads to the system.

2.Provide a flow path for the zero-sequence current of ground faults when line ground faults occur.

3.Provide a circulation path for the third harmonic caused by the exciting current when ungrounded transformers are energized.

4.Achieve a short circuit through a relatively low impedance path between the neutral and the ground to maintain the system neutral at a potential close to the ground potential.

5.Protect equipment from overvoltage transients caused by reignition ground faults.

Comparison of Grounding Transformer Connection Types

Grounding transformers mainly have two connection methods: direct grounding and resistance grounding, and there are also two main types of winding configurations: zigzag and star-delta connected windings.

1. Direct Grounding System and Resistance Grounding System

The direct grounding system grounds the power system components directly through the grounding transformer. This grounding method is simple to install and can bring many safety improvements to the originally ungrounded system. However, because the grounding transformer itself has a small resistance, it does not have current limiting capability.

To build a safer system, a neutral grounding resistor is usually added to the grounding transformer to limit the size of the ground fault current. This is resistance grounding. When implementing resistance grounding, the choice of the resistance ohm value is quite critical. The resistance must be high enough to prevent large fault currents from flowing freely in the system, and at the same time it must be low enough to limit thermal damage. The layouts of these two grounding systems are shown in Figure 1.

Figure 1: Schematic Diagrams of the Direct Grounding System and the Resistance Grounding System

2. Zigzag and Star-Delta Connections

The geometric structure of the zigzag connection can limit the circulation of the third harmonic and can operate without a secondary delta-connected winding, which reduces the cost and size of the transformer. In contrast, the star-delta connected grounding transformer requires a secondary delta-connected winding or adopts a structural design of a 4-leg or 5-leg iron core arrangement. Under both of these connection methods, the system can provide a return magnetic flux path for the load imbalance on the primary winding. The two connection configurations are shown in Figure 2.

Figure 2. Wye-Delta and Zig-zag connected configurations of Grounding Transformers.

Most modern power systems tend to choose the double-winding star-delta configuration for grounding transformers. This is because compared with the single-winding zigzag configuration, it has many advantages:

The design difficulty of the zigzag transformer is higher.

The double-winding transformer is easier to replace and upgrade.

The double-winding star-delta transformer can be used for secondary load and metering, and requires less protection devices and insulation.

The star-delta transformer configuration technology is more mature, and most manufacturers produce it. On the contrary, there are fewer manufacturers focusing on zigzag technology.

The double-winding transformer can provide auxiliary power from the secondary winding.

III. Correct Selection of Grounding Transformers

When selecting a grounding transformer, the system requirements must be fully considered. The following key parameters should always be paid attention to throughout the selection process:

1. Primary System Voltage

This is one of the most crucial considerations when selecting a grounding transformer, that is, the system voltage to which the transformer needs to be connected. In addition, the ability of the transformer to withstand lightning impulses is also an important factor in terms of voltage, which is usually measured by the basic impulse level of the transformer.

2. Rated Kilovolt-Ampere (kVA)

Grounding transformers only operate for a short time during ground faults. Therefore, compared with continuous-operation transformers with the same kVA rating, they are smaller in size and lower in cost.

3. Fault Current and Duration

This refers to the total fault current expected to flow through the system when a ground fault occurs, which is also the current that must be grounded through the transformer. Therefore, the transformer must be able to withstand currents in the range of 100 - 1000 amperes within a specific period of time without being damaged by the current flow or thermal effects.

4. Continuous Neutral Current

The selection basis of the grounding transformer is not the kVA rating but the current rating. The continuous neutral current refers to the amount of current flowing through the neutral circuit without triggering any protection devices or equipment trips.

5. Impedance

The impedance value can be defined as the ohmic resistance per phase or as a percentage. When selecting the impedance value, it should be ensured that when a single-phase ground fault occurs, the voltage of the unfaulted phases does not exceed the temporary overvoltage withstand capability of the transformer and other equipment in the system.

6. Primary Winding Connection

It is crucial to clarify the primary connection type of the transformer, that is, to determine whether it is based on a star or zigzag design configuration.

7. Auxiliary Load

For transformers with star and zigzag primary winding configurations, attention should be paid to the size factor when considering auxiliary loads. This cannot be ignored when selecting a suitable transformer and correctly operating the selected transformer.

8. Secondary Connection

If the transformer has a secondary winding, it is necessary to specify the connection type (delta or star) of this winding and its voltage rating.

IV. Design Key Points of Grounding Transformers

The above characteristics help to select a suitable grounding transformer for the system. However, when designing the transformer, some other key factors also need to be considered:

Apply special paint to the transformer body according to actual needs.

Consider the installation and operation environment of the transformer, such as indoors or outdoors. Extra protective measures need to be taken when used outdoors.

When selecting transformer oil (silicone oil, mineral oil and natural ester oil), it is necessary to combine the application scenarios and oil characteristics and make corresponding design adjustments.

Determine the connection type according to the site situation, such as whether a live front-end design is required, and consider the terminal position (on the side wall or under the cover) and the closed or exposed state.

The temperature rise is usually 65 °C, but it will change due to location, ventilation and application scenarios. Adjustments should be made in the design when necessary.

Consider environmental factors, such as the altitude of the site, and install corresponding protection mechanisms to ensure the stable operation of the transformer under various conditions.

For resistance grounding, it is necessary to ensure that the rated voltage of the neutral grounding resistor is consistent with the line-to-ground voltage of the grounding transformer, and the current rating and duration are matched with the grounding transformer.