Transformer Overvoltage Phenomena and Protection Measures -

Table of Contents

Introduction

With the rapid development of technologies such as UHV and new energy grid connection, transformers-core equipment for power transmission and voltage conversion-are critical to the reliability of power grids and the quality of power supply. However, transformers frequently face the threat of overvoltage during operation: when the operating voltage exceeds the maximum allowable working voltage, an overvoltage phenomenon occurs. This transient voltage anomaly not only accelerates the aging of transformer insulation but can also directly cause insulation breakdown, leading to equipment failure or even grid accidents. A thorough analysis of overvoltage generation mechanisms and the implementation of targeted protection measures are essential to ensuring the safe operation of transformers.

Classification and Hazards of Overvoltage

Overvoltages in power systems are categorized into three main types based on duration and cause: temporary overvoltage, switching overvoltage, and atmospheric overvoltage (lightning overvoltage), each with distinct triggering scenarios and hazard characteristics.

(1) Temporary Overvoltage

Temporary overvoltages are primarily caused by steady-state disturbances such as single-phase ground faults and grid resonance, and include power-frequency overvoltage, resonance overvoltage, and arc grounding overvoltage.

Resonance overvoltage occurs when inductance (e.g., transformer excitation inductance, arc suppression coils) and capacitance (e.g., line-to-ground capacitance, cable capacitance) in the grid form a resonant circuit due to faults or frequent operations, triggering repeated conversion of electrical and magnetic energy under specific conditions. Ferromagnetic resonance, caused by the nonlinear characteristics of transformer excitation inductance, is particularly hazardous, with voltage amplitudes potentially exceeding 2–3 times the rated value.
Arc grounding overvoltage arises from intermittent arc extinction and reignition at the fault point during single-phase ground faults in ungrounded or arc suppression coil-grounded systems. This induces electromagnetic oscillations, with overvoltage amplitudes reaching 2.5–3 times the rated voltage and lasting several seconds to tens of seconds, easily causing insulation fatigue damage.

(2) Switching Overvoltage

Switching overvoltage stems from sudden changes in grid state, such as switching large-capacity equipment, opening/closing no-load lines or transformers, or operational errors that trigger rapid energy release, leading to abrupt electromagnetic energy changes and voltage surges. Typically 3.0–4.5 times the rated voltage, these overvoltages act for only a few power-frequency cycles but accelerate insulation fatigue with repeated exposure in frequently operated distribution networks.

(3) Atmospheric Overvoltage (Lightning Overvoltage)

Atmospheric overvoltage is caused by lightning cloud discharge, divided into direct lightning overvoltage and induced lightning overvoltage:

Direct lightning overvoltage results from direct lightning strikes on equipment or conductors, generating high-amplitude pulse voltages.
Induced lightning overvoltage is induced by rapid changes in electromagnetic fields during lightning discharges.

In lightning-prone southern China, over 15% of annual transformer failures are attributed to lightning overvoltage. Lightning waves feature extremely steep wavefronts (1–4 μs) and high frequencies (above 100 kHz), with voltages reaching 8–12 times the rated value—even 50–200 times the rated value in extreme cases. When invading windings, uneven capacitance distribution creates extreme voltage gradients at the incoming terminals, directly threatening interturn and main insulation.

The severity of overvoltage damage to transformers depends on waveform, amplitude, and duration: lightning waves are short-front pulses, switching overvoltages are long-front pulses, and temporary overvoltages are low-frequency voltage waves. Equipment insulation must be tested against these waveforms. Lightning overvoltage poses the highest risk of insulation breakdown: when lightning waves propagate to transformers, line arresters discharge first, and residual waves pass through the transformer to ground. If there is an electrical distance between the transformer and arrester, residual voltage oscillates and rises along the conductor, potentially exceeding arrester residual voltage. Thus, arresters should be installed as close to transformers as possible. Additionally, resonance and switching overvoltages, though lower in amplitude, have longer durations and can also cause breakdown. Inverse transformation overvoltage is particularly insidious: when the high-voltage side is struck by lightning, arrester residual voltage acts on the high-voltage winding. The impulse voltage drop across ground resistance transfers to the low-voltage winding via the neutral line, and flux from low-voltage winding lightning current induces overvoltages several times to tens of times higher than residual voltage in the high-voltage winding, easily causing insulation breakdown.

Generation Mechanisms of Overvoltage

(1) Switching Overvoltage: Transient Oscillation of Electromagnetic Energy

(2) Atmospheric Overvoltage: Steep Invasion of Lightning Waves

(3) Temporary Overvoltage: Energy Accumulation and Oscillation Under Steady-State Disturbances

(1) Switching Overvoltage: Transient Oscillation of Electromagnetic Energy

Taking the no-load switching of a step-down transformer—the most common scenario in power grids—as an example, the secondary-side capacitance has minimal impact on the primary side when converted, so it can be neglected. The primary winding’s line-to-ground capacitance ( $C_{Fe}$ ) and interturn capacitance ( $C_{t}$ ) become key parameters. Since $C_{Fe} \gg C_{t}$ in power transformers, the effect of interturn capacitance can be further simplified for qualitative analysis.

When a no-load transformer is switched off, if the no-load current is non-zero, the magnetic field energy stored in the primary inductance $L_{1}$ ( $\frac{1}{2} L_{1} I_{a}^{2}$ ) and electric field energy stored in $C_{Fe}$ ( $\frac{1}{2} C_{Fe} U_{a}^{2}$ ) oscillate in the parallel circuit formed by $L_{1}$ and $C_{Fe}$ . This oscillation, with a frequency of several hundred to several thousand Hz and lasting only a few power-frequency cycles, converts magnetic field energy entirely to electric field energy when current reaches zero, raising capacitor voltage to a maximum and triggering switching overvoltage. Higher winding inductance and lower line-to-ground capacitance increase overvoltage amplitude, explaining why switching overvoltages typically do not exceed 4.5 times the rated voltage.

(2) Atmospheric Overvoltage: Steep Invasion of Lightning Waves

Lightning waves have steep wavefronts (1–4 μs) and high frequencies (above 100 kHz), with wavefront steepness up to 500 kV/μs. This rapidly changing voltage generates significant displacement current in windings. When lightning waves propagate to transformer terminals, they act as high-frequency, high-voltage signals. At high frequencies, winding inductive reactance $\omega L$ increases significantly, while capacitive reactance $\frac{1}{\omega C}$ decreases sharply, so current flows primarily through interturn and line-to-ground capacitances.

Since the low-voltage winding is close to the core, its large line-to-ground capacitance approximates a short circuit, making it effectively grounded. The voltage distribution of lightning waves along the winding is determined by the ratio of $C_{t}$ to $C_{Fe}$ : interturn capacitances near the incoming terminal carry more current due to shunting, creating extreme voltage gradients where the first few turns may experience voltages hundreds of times the rated value. This uneven voltage distribution is the primary cause of interturn insulation breakdown.

(3) Temporary Overvoltage: Energy Accumulation and Oscillation Under Steady-State Disturbances

Temporary overvoltages arise from steady-state grid disturbances, with three typical scenarios:

Power-frequency overvoltage: Caused by single-phase ground faults, load rejection, or line charging. In effectively grounded systems, healthy phase voltages rise to line voltage levels during single-phase faults; sudden load rejection increases generator speed and unadjusted excitation current, raising bus voltages to 1.5–1.8 times the rated value for several seconds.
Resonance overvoltage: Occurs when inductance and capacitance form resonant circuits due to faults or operations, triggering electrical-magnetic energy conversion. Ferromagnetic resonance, driven by nonlinear transformer inductance, causes frequency drift and voltage amplitudes up to 2–3 times the rated value.
Arc grounding overvoltage: Induced by intermittent arc extinction/reignition during single-phase faults in ungrounded systems, causing electromagnetic oscillations. Arc extinction releases capacitive energy to inductance, while reignition amplifies energy accumulation, leading to overvoltages of 2.5–3 times the rated voltage.

Overvoltage Protection Measures

A mature protection system has been developed to address overvoltage hazards and mechanisms, combining external direct lightning protection, internal winding protection, and specialized supplementary measures to ensure transformer safety.

(1) External Direct Lightning Protection

At the substation level, direct lightning protection is the first line of defense:

Lightning rods: Divert lightning currents to ground, with protection coverage determined by height and number of rods. Multiple rods expand coverage. To prevent counterattacks, grounding grids must meet resistance requirements and maintain safe distances from transformers and circuit breakers.
Lightning wires: Installed above incoming lines near substations, they reduce lightning wave intrusion probability. When lightning strikes outside the protected line, line impedance limits arrester current, and impulse corona on conductors reduces wave steepness and amplitude, lowering arrester residual voltage for better insulation coordination.

(2) Arrester Protection: Core Protection Means

Grid arresters are primarily gapless metal oxide arresters (MOA) and gapped valve-type arresters, with a shared selection principle: arrester rated voltage must not be lower than temporary overvoltage at the installation site, and neutral-point arrester rated voltage must not be lower than the transformer’s maximum phase voltage.

If rated voltage is too low: Gapped arresters may fail to extinguish arcs during single-phase faults, causing explosions; MOAs degrade from excessive energy absorption.
If rated voltage is too high: Impulse discharge voltage and residual voltage increase, reducing protection effectiveness.

Gapped valve-type arresters use nonlinear valve discs that exhibit high conductivity under large lightning currents (maintaining low residual voltage) and low conductivity at normal voltages (limiting power-frequency follow current). Gapless MOAs, leveraging zinc oxide valve discs, suppress follow current without gaps, offering fast response, low residual voltage, and high current capacity—making them the mainstream choice for transformer protection.

According to GB311.1-1997 Insulation Coordination for High-Voltage Transmission and Distribution Equipment, 6kV transformers have a lightning impulse withstand voltage of 60kV, and 10kV transformers 75kV. For 6kV systems, the YH5WS-10/30 MOA has a residual voltage ≤30kV, giving an insulation coordination factor $K_{c} = \frac{60\,\text{kV}}{30\,\text{kV}} = 2 \geq 1.4$ . For 10kV systems, the YH5WS-17/50 MOA yields $K_{c} = \frac{75\,\text{kV}}{50\,\text{kV}} = 1.5 \geq 1.4$ , meeting safety redundancy requirements. $K_{c}$ is typically ≥1.4 for lightning impulses and ≥1.15 for switching impulses, considering insulation type, overvoltage distribution, and environmental aging.

(3) Enhanced Insulation: Supplementary Protection

For medium and small transformers (≤35kV), enhanced insulation is a key supplementary measure: thickening high-voltage winding insulation and reinforcing interturn insulation at incoming/terminal turns to withstand voltage gradients. However, thicker insulation impairs heat dissipation and reduces interturn capacitance, potentially increasing voltage gradients—making this suitable only for low-voltage, cost-sensitive scenarios.

(4) Increased Interturn Capacitance: Optimized Voltage Distribution

Higher interturn capacitance $C_{t}$ improves voltage distribution uniformity: a larger $C_{t}$ relative to $C_{Fe}$ reduces voltage gradients and overvoltage risk. Techniques like interleaved windings and capacitor screens are used, particularly in lightning-prone areas such as mountain substations or wind farm step-up stations.

(5) Neutral-Point Overvoltage Protection

For substations in lightning-prone areas with single power sources, three-phase lightning wave intrusion is common, requiring focused neutral-point protection. The primary solution is neutral-point MOAs, which protect against both lightning and switching overvoltages with fast response, low residual voltage, and high current capacity. In lightning-prone provinces like Guangdong and Guangxi, neutral-point MOA installation rates exceed 90%. Designed with high rated voltages, they remain reliable during minor neutral-point shifts and withstand 1x power-frequency phase voltage during single-phase faults.

For extreme neutral-point shifts (overvoltage ≥2x power-frequency phase voltage), a parallel “arrester + horizontal rod gap” scheme is used: arresters handle lightning/switching overvoltages, while gaps limit high-frequency power-frequency overvoltages on arresters, creating a mutually protective mechanism.

(6) Specialized Protection for Multi-Winding Transformers

Three-winding transformers (high, medium, low voltage) face electrostatic induction overvoltage risks, especially when low-voltage windings are open. Lightning waves on the high-voltage side transfer to low-voltage windings via electrostatic coupling and electromagnetic induction, and low capacitance to ground raises electrostatic voltage, threatening insulation. Arresters at low-voltage winding outlets directly suppress intruding overvoltages.

Additionally, zero-sequence voltage from asymmetric faults or breaker malfunctions transfers to low-voltage windings via capacitive coupling. To mitigate this, high-synchronism breakers are used, and 3 Y-connected capacitors are installed on low-voltage bus bridges to increase capacitance to ground, absorbing overvoltage energy and reducing amplitude/steepness.

(7) Low-Voltage System Coordinated Protection

Long low-voltage feeder lines are vulnerable to lightning strikes. When struck, lightning currents induce inverse transformation overvoltage in high-voltage windings (2–3 times rated voltage) via electromagnetic induction, threatening insulation. Low-voltage side MOAs suppress impulse flux in low-voltage windings, blocking overvoltage transfer. Transformers with Y/Zn11 connection groups inherently suppress inverse transformation overvoltage, providing enhanced protection when paired with low-voltage arresters.

(8) Other Specialized Protection Measures

For specific overvoltage scenarios, supplementary measures include:

Arc Suppression Coils: Suppressing Arc Grounding Overvoltage

In ungrounded or arc suppression coil-grounded distribution systems, arc suppression coils compensate for capacitive current during single-phase faults, minimizing fault current and preventing intermittent arc reignition. Auto-tuning coils track real-time capacitive current changes, limiting arc grounding overvoltage to ≤2x rated voltage.

Reactors: Limiting Switching and Power-Frequency Overvoltage

Series reactors: Restrain transformer inrush current and switching overvoltage by increasing circuit inductance to reduce current change rates. For example, series reactors on low-voltage sides of large transformers limit inrush current to 2–3x rated current.
Shunt reactors: Absorb capacitive reactive power in long high-voltage transmission lines, reducing power-frequency overvoltage from line charging effects. In 500kV lines, shunt reactors limit overvoltage to ≤1.3x rated voltage, indirectly protecting transformers.

Grounding System Optimization: Reducing Ground Potential Rise

Optimized grounding grids (e.g., copper-clad steel electrodes, additional grounding poles) reduce substation ground resistance to ≤0.5Ω, minimizing ground potential rise during lightning strikes and preventing counterattacks. In lightning-prone areas, “independent grounding + equipotential bonding” further reduces ground potential differences.

Varistors and Surge Protectors (SPD): Low-Voltage End Protection

Varistors (MOV) or SPDs at low-voltage feeder outlets suppress induced lightning and switching overvoltages, preventing inverse transformation overvoltage. These devices offer nanosecond response and low residual voltage, ideal for low-voltage distribution system end protection.

SCOTECH Transformer Overvoltage Protection: Full-Scenario Solution List

This list addresses overvoltage protection needs across voltage levels and scenarios, following the core logic of direct lightning protection + core voltage limiting + specialized suppression + system coordination. It aligns with SCOTECH transformer design features, matching protection measures and customized solutions for direct use in project design, customer technical engagement, and product selection.

Application Scenario	Core Operating Conditions	Core Protection Measures	SCOTECH Customized Adaptation
Outdoor Substations/Wind Farm Step-Up Stations<br>(≥35kV, lightning-prone/mountainous areas)	High direct lightning risk; combined lightning/switching overvoltage; neutral-point potential shift risk	1. External direct lightning protection<br>2. High-voltage side gapless MOAs<br>3. Neutral-point MOA + rod gap parallel protection<br>4. Optimized winding capacitance distribution	1. Custom interleaved windings/capacitor screens at transformer incoming terminals to optimize voltage distribution;<br>2. SCOTECH-customized neutral-point MOAs matching transformer maximum phase voltage;<br>3. Zero-distance arrester installation interfaces to reduce residual voltage rise from electrical distance;<br>4. Grounding grid optimization recommendations, paired with lightning rods/wires for full direct lightning coverage
Urban Distribution Substations<br>(10/6kV, urban core areas, frequent load switching)	Frequent switching operations; repeated switching overvoltage; no direct lightning but induced lightning risk; low resonance risk	1. High-voltage side gapped valve-type/MOA arresters<br>2. Series reactors for inrush current limitation<br>3. Low-voltage side surge protectors (SPD)<br>4. Enhanced incoming terminal insulation	1. Thickened interturn/ground insulation at high-voltage winding incoming terminals to resist switching overvoltage fatigue;<br>2. Pre-integrated interfaces for low-voltage reactors/SPDs, supporting customized low-voltage protection components;<br>3. Optimized transformer excitation inductance to reduce resonance probability under frequent switching
Rural Distribution Transformers<br>(10/6kV, remote lightning-prone areas, frequent single-phase faults)	High direct/induced lightning risk; significant arc grounding overvoltage; long rural lines, high inverse transformation overvoltage risk	1. Pole-mounted arresters + low-voltage side MOAs<br>2. Neutral-point grounding via arc suppression coils<br>3. Low-voltage side SPD surge protection<br>4. Inverse transformation overvoltage suppression	1. Promote Y/Zn11 connection group transformers to inherently suppress inverse transformation overvoltage;<br>2. Standard low-voltage MOA interfaces, directly compatible with low-voltage SPDs;<br>3. Optimized neutral-point insulation to support auto-tuning arc suppression coils;<br>4. Zero-distance arrester mounting brackets for pole-mounted transformers
Industrial Plant Dedicated Transformers<br>(10/35kV, multi-winding, high load fluctuation)	Multi-winding electrostatic induction overvoltage; switching overvoltage from load switching; zero-sequence voltage coupling risk; low-voltage inverse transformation overvoltage	1. Independent arresters for each winding<br>2. Low-voltage side shunt capacitors for overvoltage absorption<br>3. Series reactors for inrush/overvoltage limitation<br>4. Layered insulation reinforcement	1. Independent reinforced insulation for each winding: optimized capacitance distribution on high-voltage sides, thickened ground insulation on low-voltage sides;<br>2. Pre-installed interfaces for 3 Y-connected capacitors on low-voltage bus bridges to absorb zero-sequence overvoltage;<br>3. Custom series reactors tailored to load fluctuation-induced inrush current;<br>4. Arrester selection matching tables for multi-winding voltage levels
Long-Distance Transmission Transformers<br>(≥110kV, transmission lines ≥50km)	Power-frequency overvoltage from line charging; significant switching overvoltage from line switching; steep lightning wave propagation	1. High-capacity MOAs on high-voltage sides<br>2. Line-side shunt reactors for capacitive reactive power absorption<br>3. Lightning wave steepness suppression<br>4. Enhanced transformer insulation class	1. Custom high lightning impulse withstand transformers adapted to long-distance lightning wave steepness;<br>2. Optimized transformer excitation characteristics to reduce resonance with line reactors;<br>3. High-current-capacity MOAs on high-voltage sides, addressing power-frequency/switching overvoltage;<br>4. Lightning wave buffer components at incoming terminals to reduce wave steepness
Compact Substations/Prefabricated Transformer Stations<br>(10/6kV, urban/park areas, space-constrained)	No independent direct lightning protection; high equipment integration, small electrical distances; induced lightning + internal switching overvoltage	1. Integrated MOAs inside the cabinet<br>2. Varistor (MOV) supplementary protection<br>3. Incoming line surge suppression<br>4. Equipotential bonding protection	1. Integrated arrester mounting positions for zero-distance transformer-arrester integration;<br>2. Built-in low-voltage side varistors to suppress internal switching overvoltage;<br>3. Equipotential bonding design inside cabinets to reduce ground potential counterattack risk;<br>4. Optimized winding outlet structure for compact spaces, minimizing voltage gradient unevenness

Supplementary Notes

All solutions support integrated matching of SCOTECH transformers and protection components, reducing customer secondary selection and installation costs.
For extreme conditions (e.g., severe thunderstorm zones, high altitude, heavy pollution), enhanced options like anti-pollution insulation coatings, high-current arresters, and independent grounding terminals are available.
All adapted solutions comply with GB311.1-1997, with insulation coordination factors ≥1.4 (lightning impulse) and ≥1.15 (switching impulse), ensuring safety redundancy.

Conclusion

Transformer overvoltage protection is a systematic engineering task that balances technical feasibility and economic efficiency, considering grid structure, operating environment, and equipment characteristics. From external direct lightning protection to internal arrester and insulation reinforcement, supplemented by arc suppression coils and reactors, a comprehensive protection network is formed. Future advancements in digital twin and online monitoring technologies will drive the evolution of transformer overvoltage protection toward intelligent, proactive systems, laying a solid foundation for more reliable power grids.

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