8 Key Measures to Reduce Partial Discharge in Power Transformers

Growing Requirements for Power Transformer Cooling Systems and the Function of Coolers

With the rapid development of power grids and the increase in transmission voltage, power grids and electricity users are demanding increasingly higher insulation reliability for large power transformers. Since partial discharge testing is non-destructive to insulation yet highly sensitive, effectively detecting inherent defects in transformer insulation or safety-threatening defects generated during transportation and installation, on-site partial discharge testing has gained widespread application. It has been listed as a mandatory commissioning test item for transformers with voltage ratings of 72.5 kV and above.

1.Partial Discharge and Its Principles

Partial discharge, also known as electrostatic ionization, refers to the flow of electrostatic charges. Under a certain applied voltage, electrostatic charges first undergo ionization at positions with weaker insulation in areas of stronger electric field, without causing complete insulation breakdown. This phenomenon of electrostatic charge flow is called partial discharge. Partial discharge occurring near conductors surrounded by gas is referred to as corona.

Partial discharge is an electrical discharge occurring at localized positions within the internal insulation of transformers. Since the discharge is localized and has low energy, it does not directly cause complete breakdown of the internal insulation.

For partial discharge testing of transformers, China initially implemented requirements only for transformers rated at 220kV and above. Later, the new IEC standard stipulated that partial discharge measurement should be performed when the equipment's maximum operating voltage Um ≥ 126kV. The national standard similarly specifies that for transformers with maximum operating voltage Um ≥ 72.5kV and rated capacity P ≥ 10,000kVA, partial discharge measurement should be conducted unless otherwise agreed.

The partial discharge test method follows the provisions in GB1094.3-2003, with the standard limit set at not exceeding 500pC. However, in actual contracts, customers often require limits of ≤300pC or ≤100pC. Such technical agreements require transformer manufacturers to maintain higher product technical standards.

2.Hazards of Partial Discharge

The severity of partial discharge hazards relates to its causes, location, and the levels of inception and extinction voltages. Higher inception and extinction voltages mean less hazard, and vice versa. In terms of discharge characteristics, discharges affecting solid insulation pose the greatest hazard to transformers, reducing insulation strength or even causing damage.

3.Causes of Partial Discharge

Factors causing partial discharge include inadequate design considerations, but most commonly originate from the manufacturing process:

• Sharp edges and burrs on components that distort the electric field and lower the discharge inception voltage;

• Foreign objects and dust that cause electric field concentration, leading to corona discharge or breakdown discharge under external electric fields;

• Moisture or gas bubbles. Due to the lower dielectric constant of water and gas, discharge occurs first under electric field influence;

• Poor contact of suspended metal structural components forming field concentration or causing spark discharge.

4.Measures to Reduce Partial Discharge

4.1 Dust Control

Among factors causing partial discharge, foreign objects and dust are extremely important triggers. Test results show that metal particles larger than 1.5μm can produce discharge quantities far exceeding 500pC under electric field influence. Both metallic and non-metallic dust create concentrated electric fields, lowering the inception discharge voltage and breakdown voltage of insulation.

Therefore, maintaining a clean environment and core body during transformer manufacturing is crucial, and strict dust control must be implemented. Sealed dust-proof workshops should be established based on the degree to which products may be affected by dust during manufacturing. For example, during wire straightening, wire paper wrapping, winding fabrication, winding assembly, core stacking, insulation component manufacturing, core assembly, and core finishing, absolutely no foreign objects or dust must be allowed to remain or enter.

4.2 Centralized Processing of Insulation Components

Insulation components are particularly vulnerable to metal dust contamination, as once metal dust adheres to insulation components, it is extremely difficult to remove completely. Therefore, centralized processing in an insulation workshop is necessary, with a dedicated mechanical processing area isolated from other dust-producing areas.

4.3 Strict Control of Silicon Steel Sheet Burrs

Transformer core laminations are formed by longitudinal and transverse shearing processes, which inevitably create burrs of varying degrees. These burrs not only cause inter-lamination short circuits, forming internal circulating currents that increase no-load losses, but also effectively increase core thickness while reducing the actual number of laminations. More importantly, during core assembly or operation under vibration, burrs may fall onto the core body, causing discharge. Even burrs that fall to the tank bottom may align under electric field influence, causing ground potential discharge. Therefore, core lamination burrs should be minimized as much as possible. For 110kV products, core lamination burrs should not exceed 0.03mm; for 220kV products, they should not exceed 0.02mm.

4.4 Cold-Pressed Terminals for Lead Wires

Using cold-pressed terminals for lead wires is an effective measure to reduce partial discharge quantities. Phosphor bronze welding produces numerous spatter particles that easily scatter onto the core body and insulation components. Additionally, the welding boundary area requires isolation with water-soaked asbestos rope, introducing moisture into the insulation. If moisture isn't thoroughly removed after insulation wrapping, it will increase the transformer's partial discharge quantity.

4.5 Rounding of Component Edges

Rounding component edges serves two purposes: 1) Improving electric field distribution and increasing the discharge inception voltage. Therefore, metal structural components in the core such as clamps, pull plates, foot pads, brackets, press plates, outlet edges, bushing riser walls, and magnetic shielding plates on the inner tank walls should all undergo edge rounding. 2) Preventing friction that produces iron filings. For example, contact parts between clamp lifting holes and ropes or hooks require rounding.

4.6 Product Environment and Core Finishing During Final Assembly

After vacuum drying of the core, core finishing must be performed before tank installation. Larger products with more complex structures require longer finishing times. Since core pressing and fastener tightening are performed with the core exposed to air, moisture absorption and dust contamination may occur during this period. Therefore, core finishing must be conducted in a dust-proof area. If finishing time (or exposure time in air) exceeds 8 hours, re-drying treatment is required. 

After core finishing, the upper tank section is installed followed by vacuum pumping and oil filling. Since the core insulation absorbs moisture during the finishing stage, dehumidification treatment is necessary, achieved by vacuum pumping the product. This is an important measure to ensure the insulation strength of high-voltage products. The vacuum level is determined based on core and environmental humidity and moisture content standards, while vacuum duration is determined based on furnace exit time, environmental temperature, and humidity.

4.7 Vacuum Oil Filling

The purpose of vacuum oil filling is to eliminate dead spots in the transformer's insulation structure through vacuum pumping, completely expel air, and then fill with transformer oil under vacuum conditions to ensure complete impregnation of the core. After oil filling, transformers must stand for at least 72 hours before testing, as the degree of insulation material impregnation depends on insulation material thickness, oil temperature, and immersion time. Better impregnation reduces the possibility of discharge, making sufficient standing time essential.

4.8 Tank and Component Sealing

The quality of sealing structures directly affects transformer leakage. If leakage points exist, moisture will inevitably enter the transformer interior, causing the transformer oil and other insulation components to absorb moisture—this is one factor causing partial discharge. Therefore, reasonable sealing performance must be guaranteed.