Grounding transformers is a critical safety procedure that ensures the proper functioning of electrical systems and prevents dangerous fault currents. Failure to properly ground a transformer can lead to catastrophic equipment damage, service disruptions, and even loss of life.

In this comprehensive guide, we will delve into the intricacies of transformer grounding, covering best practices, industry standards, and practical implementation techniques.

What Is a Grounding Transformer

A grounding transformer is a specialized type of transformer designed to provide a low-impedance path to ground for electrical systems. Its primary purpose is to limit the magnitude of transient overvoltages and provide a reliable ground fault current path.

Unlike ordinary transformers, which are used to step up or step down voltage levels, grounding transformers have a unique winding configuration that allows for the creation of a neutral point. This intentional connection to ground enables the transformer to effectively manage fault currents and maintain a stable ground potential. By providing a conductive path for zero-sequence currents, grounding transformers help to minimize voltage fluctuations and limit the duration of ground faults.

Types of Grounding Transformer

There are two main types of grounding transformers: grounded wye-delta and zig-zag transformers.

Grounded Wye-Delta

Grounded wye-delta transformers, also known as wye-delta grounding transformers, consist of a primary winding connected in a wye configuration and a secondary winding connected in a delta configuration. The neutral point of the primary winding is solidly grounded, providing a direct connection to ground. This arrangement allows for the effective grounding of the electrical system and the creation of a return path for ground fault currents.

One of the main advantages of grounded wye-delta transformers is their ability to handle high fault currents while maintaining a low impedance path to ground. The delta-connected secondary winding helps to circulate and suppress zero-sequence currents, reducing the impact of ground faults on the primary circuit. Additionally, the grounded neutral point of the primary winding serves as a reference point for protective relaying equipment, enabling accurate fault detection and isolation.

Zig-Zag

Zig-zag transformers, also known as Z-type or interconnected star transformers, feature a unique winding configuration that provides a low-impedance path for zero-sequence currents. The windings are arranged in a zig-zag pattern, with each phase consisting of two half-windings connected in series. This arrangement creates a virtual neutral point, which is then grounded, allowing for the effective grounding of the electrical system.

Compared to grounded wye-delta transformers, zig-zag transformers offer several advantages:

• Lower zero-sequence impedance, resulting in better ground fault current management
• Reduced voltage drop during ground fault conditions
• Improved voltage balance in ungrounded systems
• Smaller physical size and lower cost compared to equivalent grounded wye-delta transformers

Applications of Grounding Transformers

• Converting Ungrounded Delta or Wye Systems to Grounded: In ungrounded systems, such as delta-connected windings, a single-phase grounding fault can lead to transient overvoltages and insulation damage. By employing a grounding transformer, like a Zig-Zag connection, the system can be effectively grounded, providing a low-impedance path for ground-fault currents and minimizing the risk of Equipment Damage.
• Providing Grounding for Renewable Sources like Wind and Solar: As the integration of renewable energy sources continues to grow, proper grounding becomes crucial. Grounding transformers offer a reliable solution for wind and solar power systems, ensuring a stable ground reference and protecting against ground faults. By creating an effective ground-fault current path, these transformers contribute to the safe operation of renewable energy infrastructure.
• Mitigating Telephone Interference from Neutral Currents: In some cases, neutral currents flowing through the grounding system can cause interference with nearby telephone lines. Grounding transformers, such as the Zig-Zag transformer, help alleviate this issue by providing a low-impedance path for the neutral current. By diverting the current away from the telephone lines, the transformer minimizes electromagnetic interference and ensures clear communication.
• Providing a Ground Source During Abnormal Conditions like Generator Islanding: When a generator becomes islanded from the main power grid, maintaining a stable ground reference is essential for the protection schemes and safe operation of the electrical equipment. Grounding transformers step in to provide a reliable ground source during these abnormal conditions. By establishing a solid grounding point, the transformer enables the proper functioning of protective devices and prevents dangerous potential differences.

Specifying and Sizing Grounding Transformers

When selecting a grounding transformer for your electrical power systems, there are several critical parameters to consider to ensure safe operation, reliable protection schemes, and compliance with relevant standards.

Primary Voltage

Grounding transformers are available in a wide range of primary voltages to accommodate various electrical systems. Common primary voltages include 480V, 600V, 2400V, 4160V, 12.47kV, and higher, depending on the specific requirements of the installation.

kVA Rating

The kVA capacity should be sufficient to handle the maximum expected ground-fault current without exceeding the transformer’s thermal limits. Factors such as the system voltage, impedance, and required fault clearing time should be considered when determining the appropriate kVA rating. Undersizing the transformer can lead to overheating and potential damage, while oversizing can result in increased costs and space requirements.

Impedance

Grounding transformer impedance plays a vital role in limiting the ground-fault current to a level that can be safely handled by protective devices and minimize equipment damage.

The impedance value is typically expressed as a percentage of the transformer’s rated kVA and voltage. Lower impedance values result in higher fault currents, which may require more robust protective relaying equipment. Conversely, higher impedance values limit fault currents but may impact the sensitivity and coordination of protection schemes.

Winding Connections

The most common connection for grounding transformers is the Zig-Zag (Z-type) configuration, which provides a low zero-sequence impedance path for ground-fault currents while maintaining high positive and negative sequence impedances. This connection effectively limits the fault current and minimizes the impact on the healthy phases. Other winding connections, such as wye-delta or grounded wye-delta, can also be used depending on the specific application requirements and grounding objectives.

How to Ground a Transformer Secondary

The most common method for grounding the transformer secondary is through a solid connection to the neutral point. This intentional connection provides an effective path for ground-fault currents, limiting the potential difference between the transformer winding and ground during abnormal conditions.

When grounding the secondary of a transformer, the following steps should be taken:

1. Connect the neutral point of the secondary winding to the grounding electrode conductor using a suitable conductor size.
2. Ensure the grounding connection is secure and capable of withstanding the maximum ground-fault current.
3. Install a grounding conductor from the neutral point to the grounding electrode, following the shortest possible path.
4. Use an appropriately sized grounding electrode, such as a ground rod or building frame structural metal, to establish a low-resistance connection to earth.

How Should Transformers Be Grounded

In general, transformers should be grounded in accordance with the following guidelines:

• Wye-connected transformers: Ground the neutral point of the wye-connected winding.
• Delta-connected transformers: Use a grounding transformer, such as a zig-zag grounding transformer, to provide a neutral point for grounding.
• Ungrounded systems: Consider using a grounding transformer to create an artificial neutral point for grounding purposes.

What Is the NEC Code for Grounding Transformers

The National Electrical Code (NEC) provides requirements for grounding transformers in Article 250. Some key requirements include:

• NEC 250.30(A): Grounding of transformer enclosures and equipment grounding conductors.
• NEC 250.30(A)(1): Grounding of separately derived systems, such as transformers.
• NEC 250.30(A)(2): Bonding of equipment grounding conductors to the grounded conductor (neutral) at the transformer.
• NEC 250.30(A)(3): Sizing of the grounded conductor (neutral) for transformers.

Additionally, NEC Article 450 covers the installation and protection of transformers, including grounding requirements for specific transformer types and configurations.

Do You Ground Both Sides of a Transformer

In most cases, only one side of a transformer is grounded, typically the secondary side. Grounding both the primary and secondary windings can lead to undesirable circulating currents and potential safety hazards. However, there are specific situations where grounding both sides may be necessary, such as:

• Grounding the primary and secondary windings to separate grounding electrodes to prevent the transfer of ground-fault currents between systems.
• Grounding both sides of a transformer when it is used to supply power to a separately derived system, such as a backup generator.

What Happens if You Don’t Ground a Transformer

Failing to properly ground a transformer can result in various negative consequences, including:

1. Increased risk of electrical shock hazards to personnel due to the lack of a low-resistance path to ground.
2. Potential for transient overvoltages, which can damage electrical equipment and insulation.
3. Inability of protective devices, such as overcurrent relays and fuses, to detect and clear ground faults effectively.
4. Uncontrolled ground-fault currents, leading to thermal damage and potential fire hazards.
5. Reduced power quality and reliability due to the presence of zero-sequence currents and voltages.