Transformers
1.Introduction
As the scale of power systems continues to expand and automation levels steadily increase, substations play an increasingly vital role in transmission and distribution networks. The 110kV substation, as a key node in medium- to high-voltage power grids, is responsible not only for collecting and distributing electricity but also for ensuring the stability and safety of grid operation. In this context, ensuring reliable grounding and fault protection is a central challenge in power system design and operation.
The grounding system is often the “invisible safeguard” of grid safety. Among its components, the grounding transformer plays a pivotal role in establishing a neutral point in systems where it is otherwise unavailable. It works in conjunction with resistors or reactors to control grounding current and voltage, contributing to protection coordination, transient overvoltage suppression, and system stability.
This article explores the structure, operating principles, key functions, typical applications, and engineering considerations of grounding transformers in 110kV substations, aiming to present a comprehensive understanding of this essential yet often overlooked equipment.
2. Importance of Grounding in Power Systems
In three-phase AC power systems, the method of neutral grounding significantly impacts system safety, reliability, and protection behavior. When a single-phase-to-ground fault occurs, different grounding methods lead to varied fault current magnitudes, voltage distributions, and insulation requirements.
For 110kV and higher voltage systems, it is common to use ungrounded, high-resistance grounded, or reactance-grounded schemes to limit fault current, reduce arc re-ignition, and control overvoltages. However, many transmission systems, such as delta-connected transformers or three-wire systems, naturally lack a neutral point. To enable effective grounding, a neutral must be artificially created.
This is the context in which grounding transformers are applied. They provide an artificial neutral point and, together with resistors or reactors, establish a safe and stable grounding path—making them indispensable components of a modern substation’s grounding system.
3. Structure and Working Principle of Grounding Transformers
3.1 Definition
A grounding transformer is an electrical device used to provide a neutral point for systems that lack one. Its primary function is to enable system grounding in order to implement protective schemes such as resistance grounding, reactance grounding, or solid grounding.
3.2 Common Types
Grounding transformers come in several structural forms:
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Zig-Zag Type
The most common design, composed of three single-phase windings connected in a special zig-zag configuration. It offers a high zero-sequence impedance and compact structure, ideal for grounding applications. -
Delta-Wye (Δ-Y) Type
Typically used when a neutral point and additional low-voltage load support are required. -
Wye-Wye (Y-Y) Type
Rarely used in grounding scenarios due to lower performance in handling unbalanced currents.
3.3 Operating Principle
Under normal conditions, a grounding transformer remains idle, serving only as a standby neutral point. It becomes active during ground faults, especially single-line-to-ground faults, by providing a return path for zero-sequence current. This enables the protective relays to detect and isolate the fault effectively while also limiting overvoltages and stabilizing the system.
4. Grounding Methods in 110kV Systems
The 110kV level lies in the transition zone between medium and high voltage. Grounding strategies at this level must balance system scale, protection sensitivity, fault current control, and equipment insulation requirements. Common methods include:
4.1 Ungrounded Systems
Used in small or isolated grids. They allow continued operation during a single-line-to-ground fault but suffer from issues like overvoltage, resonance, and detection difficulty.
4.2 Solid Grounding
Provides immediate and effective fault detection but results in high fault currents, which are unsuitable for 110kV systems.
4.3 Resistance Grounding
Widely used in 110kV systems. It uses grounding resistors (typically 200–1000A) to limit fault current and protect equipment while maintaining protection sensitivity.
4.4 Reactance Grounding
Uses a reactor in the neutral path to control fault current and reduce overvoltage. It is suitable for special applications.
4.5 Petersen Coil (Arc Suppression Coil) Grounding
Primarily used where the system is highly capacitive and prone to arc faults. It cancels the zero-sequence current through tuning.
Typically, 110kV substations use a “grounding transformer + resistor” configuration to achieve high-resistance grounding that balances protection and reliability.
5. Core Functions of Grounding Transformers in 110kV Substations
Although grounding transformers do not transmit power under normal conditions, they are essential for maintaining system stability. Their main functions include:
5.1 Providing a Neutral Point
In systems lacking a natural neutral, the grounding transformer creates an artificial one, enabling grounding and voltage stabilization.
5.2 Limiting Fault Currents
In combination with grounding resistors or reactors, they limit single-line-to-ground fault currents to safe levels (hundreds of amps).
5.3 Suppressing Transient Overvoltages
They prevent damage from arc re-ignition or switching surges by stabilizing system voltages during faults.
5.4 Creating a Zero-Sequence Path
They offer a return path for zero-sequence current during unbalanced conditions, enabling protective relays to function properly.
5.5 Enhancing Protection Sensitivity
They improve relay performance by ensuring proper current flow during faults.
5.6 Ensuring Personnel and Equipment Safety
Grounding transformers help prevent abnormal voltages and equipment enclosures from becoming live, protecting both personnel and infrastructure.
6. Typical Grounding Transformer Configurations
The configuration depends on the system and grounding method:
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Single Zig-Zag Grounding Transformer
The most common and compact design, suitable for single- or double-busbar systems. -
Zig-Zag with Series Resistor
Widely used in 110kV systems to limit ground fault current effectively. -
Shared Grounding Transformer for Two Transformers
Applies when multiple transformers share a common neutral grounding point. -
Grounding Transformer with Capacitor and CT
Used in systems requiring arc suppression, resonance monitoring, or enhanced protection.
7. Design and Selection Principles
When designing grounding systems, several factors must be considered for proper transformer selection:
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Rated Voltage Level
The insulation rating must match the 110kV system. -
Short-Time Current Withstand
Typically designed for 500A–1000A for 10 seconds. -
Core Structure
Zig-Zag types are preferred for most 110kV applications. -
Grounding Method Compatibility
Must support the planned use of resistors or reactors. -
Installation Environment
Outdoor/indoor, oil-immersed or dry-type, and mounting style. -
Reliability and Maintainability
Favor designs with minimal maintenance requirements and proven durability.
8. Practical Engineering Case
In a 110kV wind power substation in a northern province of China, the main transformer is connected Y/Y/Δ and has no available neutral point. A Zig-Zag grounding transformer (500kVA, 800A) with a 20Ω grounding resistor is installed to construct a stable ground path.
After commissioning, the system has experienced several ground faults that were correctly identified by protection relays. The grounding transformer functioned as expected, limiting fault current and ensuring no equipment damage occurred. It played a vital role in maintaining system integrity.
9. Potential Issues and Maintenance Considerations
Although not a high-load device, grounding transformers require proper monitoring. Common issues include:
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Overheating
Repeated or prolonged fault currents can cause thermal damage. -
Insulation Degradation
Especially in oil-filled transformers, periodic oil testing is necessary. -
Corrosion or Loose Connections
Regular inspection is essential, particularly in outdoor environments. -
Protection Malfunction
Incorrect resistor/reactor configuration may impair protective functions.
It’s recommended to establish a routine inspection schedule, including infrared scanning, insulation resistance tests, and current logging to ensure reliable performance.
10. Future Trends and Innovations
With the rise of digital substations and smart grids, grounding transformers are evolving in the following directions:
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Intelligent Monitoring
Real-time data on current, temperature, and transformer health. -
Dry-Type and Maintenance-Free Designs
More environmentally friendly and easier to maintain. -
Modular Structures
Simplify installation and integration in compact substations. -
System Integration
Deeper coordination with digital protection systems and asset management platforms.
In the future, grounding transformers may serve not only as passive protection devices but as integral parts of intelligent grid infrastructure.
11. Conclusion
Though often overlooked, grounding transformers are indispensable in ensuring the safe and stable operation of 110kV substations. They provide a reliable neutral point, limit ground fault currents, suppress overvoltages, enable protective relay operation, and safeguard both personnel and equipment.
As power grids continue to evolve toward greater complexity and intelligence, grounding transformers must also keep pace through smarter, more resilient designs. Proper design, selection, and maintenance of these transformers are critical to supporting the future of safe and reliable electric power systems.