20 kV Cast Resin Dry Type Transformer: Insulation, PD and Loading Limits

Cast resin dry type transformers are the default choice for 20 kV indoor distribution where insulating oil is not acceptable: inside buildings, cable tunnels, underground substations, hospitals, data halls and process plants with a fire or contamination risk. Because the windings are encapsulated in a filled epoxy system under vacuum, the unit has no liquid to leak, no bunded oil pit and a much lower fire load than an oil-immersed design of the same rating.

Where a 20 kV Cast Resin Unit Fits

A 20 kV nominal system is served by equipment designed for a highest system voltage of 24 kV. Insulation coordination for that class follows IEC 60076-3, which sets a rated lightning impulse withstand voltage of 125 kV peak and a power-frequency withstand of 50 kV rms for the 24 kV class. The primary winding therefore needs a heavier insulation build than a 10 kV unit, and the cast body is dimensioned around that requirement rather than around the thermal duty alone.

Vacuum-Cast Winding Construction

The low voltage winding is a foil or wire coil reinforced with glass fibre; the high voltage winding is a layer winding with glass-fibre reinforcement between layers. The assembled coil is dried, placed in a mould and cast under vacuum with a filler-loaded epoxy resin, so voids and gas pockets are removed before the resin gels. A film-type vacuum deaerator and controlled mixing keep the filler evenly distributed in the mixture, which is what allows the finished body to resist cracking and internal bubbles. The result is a winding that does not absorb moisture, so it can be energised after long storage without a drying process, and whose glass-fibre and resin constituents are self-extinguishing and do not release toxic gas at high temperature.

Partial Discharge Performance and Insulation Evaluation

Partial discharge is the decisive acceptance test for cast resin insulation because it detects voids, delamination and sharp electrode edges that a power-frequency withstand test alone would pass. IEC 60076-11 requires a partial discharge measurement on every unit and applies an acceptance level of 10 pC for class F windings, measured at 1.3 times the rated phase-to-earth voltage divided by the square root of three. Routine production results in the range of 5 pC are achievable when the mould, the deaeration cycle and the electrode screening are under control, and that margin is what protects the insulation against long-term erosion.

Temperature Rise Classes and Loading

Dry type transformers are classified by the thermal class of their insulation system: class F permits an average winding temperature rise of 100 K and class H permits 125 K above the reference ambient. IEC 60076-12, the loading guide for dry type power transformers, is used to check short-time overload and cyclic duty, because the absence of oil means there is no large thermal reservoir to absorb a sudden load step. A cast body heats and cools relatively quickly, so the winding hot-spot, not the average, governs emergency loading limits.

Noise, Installation and Environment

Sound level is determined to IEC 60076-10, and modern cast resin designs run about 10 to 15 dB(A) below first-generation units of the same rating because of improved core clamping and lower core flux density. Since air is the only cooling medium, the enclosure must allow convection: IP00 or IP20 arrangements are usual indoors, with clearances that satisfy the impulse withstand distance for the 24 kV class. Units are commonly supplied with a temperature monitoring system using PT100 sensors in each winding, which trips at alarm and trip thresholds set from the class F or class H limit.

Cast Resin Compared with Oil Immersed

Frequently Asked Questions

Q: Why is the design voltage class 24 kV for a 20 kV system?
A: Equipment voltage class follows the highest system voltage rather than the nominal value, and the 24 kV class carries a 125 kV peak impulse rating with 50 kV rms power-frequency withstand under IEC 60076-3.

Q: What partial discharge figure should be specified?
A: Specify the IEC 60076-11 acceptance level of 10 pC for class F windings, and ask for routine measured values so that the margin is visible in the test report.

Q: Can the unit be overloaded like an oil-immersed transformer?
A: Not to the same extent. Without an oil reservoir the thermal time constant is short, so emergency loading must be checked against the winding hot-spot using the dry type loading guide.

Q: Does the cast body need maintenance?
A: No oil sampling or oil filtration is required. Maintenance is limited to cleaning dust from the cooling ducts, checking connections and verifying the temperature sensors and tripping circuit.

Q: Is a 20 kV cast resin unit suitable for outdoor use?
A: It can be installed outdoors in a weatherproof enclosure with adequate ventilation, but the standard arrangement is an indoor room because the resin surface and the cooling ducts must stay free of water and conductive dust.