Causes and Detection Methods of Partial Discharge in Oil-Immersed Transformers

The main and longitudinal insulation of oil-immersed transformers is primarily composed of oil-impregnated paper with a high dielectric constant and insulating oil with a lower dielectric constant. In this type of insulation, the electric field in the oil section is relatively soft, but when it reaches a certain level, partial discharge occurs, and sometimes even breakdown. Therefore, the basic approach is to eliminate oil gaps in high electric field areas or subdivide them to increase the breakdown strength of unit oil gaps. This paper studies and analyzes several typical structures and factors that lead to partial discharge in transformers, summarizing the causes and characteristics of partial discharge in oil-immersed transformers.

Typical Structures and Factors Causing Partial Discharge in Transformers

Due to the different properties of insulating materials, along with design or manufacturing factors and the presence of bubbles and impurities inside the insulation, an uneven electric field distribution is created within the insulation structure. This often leads to an excessively concentrated electric field in certain areas, making partial discharge more likely. The following are some typical structures prone to partial discharge due to electric field concentration.

1. Lead Wires

In transformer insulation structures, the arrangement of lead wires is abundant, and the electric field between lead wires is highly uneven. When two lead wires with the same purpose are parallel or perpendicular, the maximum electric field strength appears on the surfaces of the lead wires at the closest points between them. Under the same conditions (ignoring the outer insulation layer), the maximum electric field strength is approximately 10% higher when the two lead wires are perpendicular rather than parallel. The area where the lead wires are drawn out from the high-voltage winding and the regulating winding outside the tank wall is also prone to partial discharge due to electric field concentration.

2. End Insulation Structure

In ultra-high voltage power transformers, an electrostatic ring is usually installed at the winding ends to improve the impulse voltage distribution and to shield and even out the electric field at the ends. At this point, the wedge-shaped oil gap (also known as "oil wedge") formed between the electrostatic ring and the end ring becomes an area of electric field concentration. The maximum electric field strength on the electrostatic ring surface can generally be calculated using the empirical formula:

where U is the test voltage (kV). It can be seen that the maximum electric field strength is related to the main insulation distance of the winding (m), end insulation distance (h), curvature radius of the electrostatic ring (r), and the thickness of its insulation layer (d), as shown in Figure 1. Generally,  H=(1.5~2.4)m, R/m=0.1~0.5

3. Protruding Metal Electrodes

The surfaces of protruding metal electrodes in transformers, such as welds on the inner wall of the oil tank, welding slag, burrs left during lead wire welding, nuts on tap changers, and burrs formed during cutting of iron cores, all contribute to electric field concentration, significantly increasing the electric field, whether it is grounded or charged. Therefore, in transformer design and manufacturing, efforts should be made to avoid and eliminate protruding metal electrodes.

4. Impurities

In the transformer insulation structure, oil has the lowest dielectric constant compared to paper and cardboard, making the electric field strength borne by oil in the composite insulation structure relatively high. Among the three insulating materials, oil also has the lowest breakdown field strength, which determines that the weakest part of transformer insulation is the oil gap. The breakdown field strength of oil is related to impurities in the oil, such as metallic and non-metallic particles, moisture content, and gas content. Impurities distort the electric field in the oil, as shown in Figure 2, where a spherical impurity with radius RRR distorts the electric field in a uniform electric field.

Let ϵ1 and E1 represent the dielectric constant and electric field strength of the oil, and ϵ2 and E2 represent the dielectric constant and electric field strength of the spherical impurity. Then,

When ϵ2<ϵ1, E2>E1, meaning the field strength inside the impurity is greater than that in the oil, equivalent to the impurity being a spherical air gap. For transformer oil, with ϵ1=2.2 and air ϵ2=1.0, E2=1.22E1. Thus, the presence of air gaps in the oil significantly reduces the partial discharge inception voltage.

When ϵ2>ϵ1, E2<E1, meaning the field strength inside the impurity is less than that in the oil.

If there are suspended water droplets in the transformer oil, with a relative dielectric constant of water ϵ2=80, then E2=0.078E1; if it is a metal sphere, ϵ2→∞, then E2=0.

When the dielectric constant of the impurity is greater than that of the oil, ϵ2>ϵ1, the field distortion further increases the field strength in the oil. The maximum electric field strength in oil can be represented by:

For oil containing suspended water droplets, E′max=2.84E1; if the oil contains metal particles, ϵ2→∞, then E2=3E. At this point, corona discharge or local micro-discharge may occur on the impurity surface, often accompanied by bubble generation, further intensifying partial discharge. Therefore, impurities in oil have a significant impact on the electric field strength in the oil.

To enhance the partial discharge strength of transformer oil, it is necessary to filter out impurities, degas and dehydrate the oil under high vacuum and at a specific temperature, thereby reducing impurities, gas, and moisture content in the oil. Transformers should be filled with oil in a vacuum, and allowed to stand for a period before testing or operation to allow the oil to absorb any remaining gas.

All metal parts in transformers must have a fixed potential. During lead welding, steps should be taken to prevent welding slag and metal particles from entering the insulation and winding, forming metal impurities with floating potential.

Mechanism and Characteristics of Partial Discharge in Transformers

1. Mechanism and Influencing Factors of Partial Discharge in Transformers

For most stable discharges within transformers, the essential mechanism is bubble discharge. The difference lies in the formation mechanism of bubbles and the surrounding medium, which could be oil, solid insulation, or even conductors. During transformer operation, it is unavoidable to have various air gaps with differences in size and shape, each subjected to different overvoltages. As a result, multiple forms of discharge may coexist. Townsend-type discharges may contribute to long-term insulation degradation without causing immediate faults; however, stream-type discharges, such as brush discharge or main insulation discharge, can quickly damage insulation, making them the primary targets in partial discharge (PD) detection. In specific weak points of insulation, Townsend-type discharge may evolve into stream-type discharge, increasing discharge magnitude.

Air gap discharge, typically resulting from material defects such as air gaps or varnish leaks within insulation media like paper or cardboard, has a high electric field intensity and large discharge magnitude, reaching up to 103 to 107 pC, with a discharge waveform duration that can exceed 10 μs. Gases produced from the decomposition of oil and paper accumulate, forming bubbles that lead to local discharge with magnitudes up to 10 pC.

Each partial discharge releases positive and negative charges, accompanied by a sharp current pulse, emitting electromagnetic waves in all directions. Additionally, PD may generate ultrasonic waves, light, and local overheating, producing new chemical compounds like H₂ and C₂H₂. The frequency of discharge current ranges from several kHz to hundreds of MHz, with the main frequency components around 10 MHz. Due to the high insulation strength of the transformer oil-barrier structure, transformers can emit high-frequency electromagnetic waves during PD, with peak frequencies up to GHz. The wavelength of light generated during discharge is typically between 500–700 nm. The molecular collisions within the discharge region produce a pressure, accompanied by sound waves from PD, with sound wave frequencies ranging from 10 Hz to the 107 range, with higher intensity bands in the tens to hundreds of kHz.

Many factors influence PD in transformer oil and oil-paper insulation, such as temperature, pressure, humidity, impurities, and impulse voltage. The main effects of these factors include:

a. Water molecules in oil, with high dielectric constants, align along the electric field direction, causing field distortion that triggers PD. Thus, increased humidity or impurities increase the discharge rate but reduce pulse duration and discharge magnitude.

b. Temperature significantly impacts PD in oil. Higher temperatures increase convection and bubble pressure, reducing the likelihood of space charge accumulation in concentrated electric field areas, hence decreasing PD. However, in oil-paper insulation air gaps, temperature increases charge mobility and solid conductivity, reducing breakdown strength and enhancing PD.

c. Increased oil pressure affects gas solubility and release, raising bubble pressure, reducing discharge rates, and increasing PD inception voltage.

d. Forced oil circulation in transformers increases oil flow rate, enhancing the electrostatic field concentration at the winding ends.

e. Large conductive particles in oil, when subjected to an external electric field, generate a surface field strength significantly higher than the applied field. When the field exceeds the oil breakdown threshold, corona discharge occurs on the particle surface, or micro-discharges may occur locally, generating bubbles and further intensifying PD.

2. Characteristics of Partial Discharge Signals in Transformers

The main causes of internal PD in transformers are electric field concentration, resulting in oil breakdown, with common discharge types including bubble discharge, suspended particle discharge, tip discharge, surface discharge, and air gap discharge. Over recent years, to understand PD development, extensive research on pulse waveform and frequency characteristics has been conducted by universities such as Tsinghua University. These studies commonly employ PD models, like those in Figure , placed in specific containers of transformer oil. By measuring discharge pulse waveforms, UHF, ultrasonic, and even light signals, researchers have analyzed the PD pulse waveforms and progression.

Corresponding to the four discharge models mentioned, pulse durations generally last several μs for oil gap discharge, whereas other discharges persist for tens to hundreds of ns, with pulse rise times of a few ns. Discharge current, inception voltage, etc., vary widely depending on model type and size. Research has shown that apart from oil gap and internal dielectric discharge having distinctive waveforms, other discharge processes correlate well with air gap test results. Studies on transformer oil discharge reveal significant variability in PD repetition rate and pulse amplitude distribution, often displaying sudden bursts in frequency clusters. The waveform envelope exhibits a dual-exponential decay, with successive small pulses gradually increasing in amplitude until a sudden interruption, with the last small pulse exhibiting the highest amplitude. This phenomenon is due to charge deposition on the oil wall, causing field strength to decrease and extinguishing the discharge. The energy from the discharge promotes oil decomposition and electrostatic force, generating larger bubbles. The enhanced field near the sharp electrode allows the next discharge to occur in the larger bubble, further increasing discharge magnitude until the bubble reaches a critical size, after which the field strength is insufficient to sustain PD, leading to an abrupt cessation.

PD generates sound waves with a principal frequency fff (peak frequency) related to the discharge energy ωωω as follows:

where c is the speed of sound (m/s), p is pressure (Pa), and ω is the energy per unit length (J/m). For instance, a 4-meter gap discharge has f=1.5 kHz, while a weak discharge has f=150 kHz.

Researchers such as E. Howells identified that transformer PD ultrasonic frequencies primarily center around 150 kHz. Domestic institutions like the Wuhan High Voltage Research Institute and Tsinghua University observed peak frequencies in the ranges of 30–160 kHz and 70–150 kHz through testing and modeling. Field detection and model tests of electromagnetic waves emitted by transformer PD indicate that neutralization of positive and negative electrons in oil excites UHF electromagnetic waves exceeding 1.5 GHz, with a broad spectrum propagating outward as TEM waves. The spectral characteristics of these electromagnetic waves correlate with the geometry of the discharge source and insulation strength of the gap. Figure shows measured UHF signals from transformer PD.

Oil-immersed transformers, due to the properties of insulation materials, design, manufacturing factors, and internal bubbles and impurities, create uneven electric field distributions, leading to localized electric field concentrations and potential PD. Typical cases include lead wires, end insulation structures, protruding metal electrodes, and impurities.

Experimental studies indicate that most stable discharges within transformers are bubble discharges, differing based on bubble formation mechanisms and the surrounding medium, whether oil, solid insulation, or conductors. During each partial discharge, positive and negative charges neutralize, producing a steep current pulse that radiates electromagnetic waves in all directions. Additionally, ultrasonic waves, light, and localized overheating generate new chemical substances, such as H₂ and C₂H₂. This research on the causes and characteristics of PD in oil-immersed transformers provides essential theoretical and technical support for future analysis of transformer operational conditions and condition-based maintenance.