Selection and Protection Configuration Optimization of 110kV Transformer Neutral Point Grounding Methods

Introduction

In high-voltage power systems, the transformer neutral point grounding method is a critical factor influencing system safety, reliability, and stability. For 110kV power systems, the choice of neutral point grounding method directly affects equipment insulation levels, overvoltage protection, relay protection configuration, and power supply reliability. In China, 110kV systems typically adopt a partially effective grounding method, where some transformer neutral points are directly grounded while others remain ungrounded, aiming to limit single-phase short-circuit currents while preventing overvoltage threats .

This article analyzes the characteristics, advantages, and limitations of different 110kV transformer neutral point grounding methods, explores optimal protection configuration strategies, and presents future development trends.

1 Key Neutral Point Grounding Methods for 110kV Transformers

1.1 Direct Grounding

Direct grounding refers to the direct connection of the transformer neutral point to the earth. This method effectively fixes the neutral point potential, ensuring that during a single-phase ground fault, the non-fault phase voltage rise does not exceed 1.4 times the phase voltage. This helps lower equipment insulation requirements and reduce costs .

However, a significant drawback is the very high single-phase ground fault current (up to several thousand amperes), which can impact circuit breaker interrupting capacity and system stability. Therefore, direct grounding is generally used in 110kV and higher voltage systems where rapid fault removal is necessary .

1.2 Ungrounded Neutral

In an ungrounded system, the transformer neutral point is insulated from the earth. When a single-phase ground fault occurs, the fault current is very small (mainly the system's capacitive current), allowing the system to continue operating for a short period (typically up to 2 hours). This significantly enhances power supply reliability .

However, in ungrounded systems, single-phase ground faults can cause the non-fault phase voltage to rise to the line voltage level. If the insulation is weak, this may lead to breakdown, escalating into a phase-to-phase fault. Additionally, intermittent arc grounding can generate arc overvoltages, reaching 3–3.5 times the phase voltage, posing a threat to transformer insulation .

1.3 Grounding via Small Impedance

To balance the advantages and disadvantages of direct grounding and ungrounded systems, the impedance grounding method is often used. This includes grounding through a small resistance or a small reactance .

• Small Resistance Grounding: Limits fault current to several hundred amperes, reducing the impact on the system while still enabling rapid protection operation. This method suppresses overvoltages effectively and is suitable for cable-intensive distribution networks with large capacitive currents .
• Small Reactance Grounding: Can offset the system's capacitive current through inductive current, reducing the likelihood of arc reignition. This method is often considered a compensated grounding method .

Grounding via small impedance combines the benefits of both direct and ungrounded systems, offering overvoltage suppression and relatively high power supply reliability. It is widely used in 110kV systems, especially those with significant capacitive currents or requiring high power quality .

2 Protection Configuration for 110kV Transformer Neutral Points

2.1 Overvoltage Threats

The insulation level of a 110kV transformer neutral point is typically semi-insulated, with a withstand voltage rating only one-third of the line end. This makes the neutral point vulnerable to overvoltage damage. Primary overvoltage types include :

• Power Frequency Overvoltage: Arising from line switching, asymmetric short circuits, or sudden load loss.
• Resonance Overvoltage: Caused by oscillations due to interactions between inductive and capacitive elements during system operations or faults.
• Switching Overvoltage: Resulting from the conversion of magnetic and electrostatic energy during the opening or closing of circuit breakers.
• Lightning Overvoltage: Caused by lightning strikes, characterized by high amplitude and short duration.

2.2 Common Protection Devices

To protect the transformer neutral point, the following protection devices are commonly employed :

• Surge Arresters: These limit lightning overvoltage and certain switching overvoltages. However, standard surge arresters are often inadequate for the low insulation level of 110kV transformer neutral points, making selection challenging.
• Isolation Gaps: These protect against power frequency and resonance overvoltages. When overvoltage occurs, the gap breaks down, grounding the neutral point to limit voltage rise. A drawback is the difficulty in precisely adjusting the gap distance, which can lead to protection miscoordination.
• Parallel Connection of Surge Arrester and Gap: This is a widely used protection method. The surge arrester handles lightning overvoltage, while the gap addresses power frequency and resonance overvoltages. The gap also protects the surge arrester from excessive power frequency overvoltages that could cause its failure. This approach offers complementary advantages .

2.3 Relay Protection Configuration

Relay protection for a 110kV transformer neutral point mainly includes the following aspects :

• Zero-Sequence Current Protection: For directly grounded transformers, zero-sequence current protection is configured to quickly remove ground faults. The protection is usually divided into sections, with short time delays for fault localization and longer time delays for tripping all sides of the transformer.
• Zero-Sequence Voltage Protection and Gap Current Protection: For ungrounded transformers, zero-sequence voltage protection and gap current protection are set up. When a ground fault causes the system to lose its ground point, leading to neutral point voltage rise, the gap breaks down. Gap current protection or zero-sequence voltage protection acts with a time delay (0.3–0.5s) to trip the transformer on all sides.
• Backup Protection Coordination: To ensure selectivity, zero-sequence protection time delays must be coordinated. For example, the time delay for a backup protection on a transformer should be longer than that of the line protection it backs up .

3 Optimization Recommendations and Case Analysis

3.1 Limitations of Traditional Methods

While the use of surge arresters parallel with gaps is common, this approach has several shortcomings :

• Difficulty in Surge Arrester Selection: It is challenging to find standard surge arresters that meet the requirements of both high continuous operating voltage and low lightning impulse residual voltage for 110kV transformer neutral points.
• Challenges in Gap Setting: Air gap breakdown voltage is subject to dispersion, making it difficult to accurately coordinate the gap operation for "loss of ground" and "with ground" fault conditions.
• Complexity of Relay Protection: Protection against "loss of ground" (such as zero-sequence overvoltage and gap overcurrent protection) may malfunction, necessitating additional blocking criteria, which increases complexity and reduces reliability.

3.2 Advantages of Grounding via Small Reactance

Research and practice indicate that grounding the neutral point via a small reactance offers significant advantages over traditional partial grounding methods :

• Reduced Insulation Level Requirements: After adopting small reactance grounding, the insulation level of the transformer neutral point can be lowered from 35kV to 20kV, obviating the need for surge arresters and gaps and simplifying the protection configuration.
• Unified Grounding Mode: This method eliminates the occurrence of an isolated ungrounded system, allowing the simplification or omission of related protection, thereby enhancing reliability.
• Retention of Advantages: It maintains the benefits of partial grounding, such as simple and reliable zero-sequence protection, while limiting single-phase short-circuit currents.

3.3 Case Study Analysis

An example is a 110kV terminal substation transformation. The original design used a surge arrester parallel with a gap for neutral point protection. However, after adopting small reactance grounding, the insulation level requirement of the transformer neutral point was reduced, protection devices were simplified, and operational reliability was improved. Calculations showed that the grounding resistance could limit the fault current to a few hundred amperes, and the zero-sequence protection could be easily coordinated .

Another case involved a fault in a 110kV substation where a transient single-phase ground fault on the incoming line led to neutral point gap breakdown and transformer tripping. Analysis revealed that although the line fault was transient, the feedback from a large number of asynchronous motors on the load side provided energy for the arc, sustaining the fault. This highlights that for transformers with significant motor loads (equivalent sources), complete neutral point protection, including zero-sequence overcurrent, gap current, and zero-sequence voltage protection, is essential during the design phase .

4 Conclusion and Outlook

The selection of 110kV transformer neutral point grounding method and its protection configuration is a multifaceted task that requires consideration of system structure, load characteristics, and reliability requirements. While the traditional partial grounding method combined with surge arresters and gaps is common, it faces challenges in device selection and setting coordination. The small reactance grounding method offers a promising alternative, potentially lowering insulation requirements, simplifying protection, and improving reliability .

Future development trends will focus on the following areas:

• Application of New Devices: Such as composite gaps or controllable gaps used in parallel with surge arresters, enhancing protection reliability and accuracy .
• Digital Protection Technology: Utilizing microcomputer-based protection with advanced algorithms (e.g., waveform identification, harmonic analysis) to improve the sensitivity and reliability of ground fault protection .
• Standardization and Modularization: Developing standardized and modular neutral point protection equipment to simplify design and maintenance .

In summary, optimizing the 110kV transformer neutral point grounding method and protection configuration is crucial for enhancing the safety, reliability, and economic operation of the power system. With technological advancements, more intelligent and efficient solutions are expected to emerge and gain widespread application.