SCB & SGB Dry-Type Transformers Explained

1. Introduction

A transformer operates based on the principle of electromagnetic induction. The main components of a transformer are the windings and the core. During operation, the windings serve as the path for electric current, while the core serves as the path for magnetic flux. When electrical energy is input to the primary winding, the alternating current creates an alternating magnetic field in the core (i.e., electrical energy is converted into magnetic field energy). Due to magnetic linkage (flux linkage), the magnetic flux passing through the secondary winding continuously changes, thereby inducing an electromotive force (EMF) in the secondary winding. When an external circuit is connected, electrical energy is delivered to the load (i.e., magnetic field energy is converted back into electrical energy). This "electricity–magnetism–electricity" conversion process is realized based on the principle of electromagnetic induction, and this energy conversion process constitutes the working principle of a transformer.

U1N2 = U2N1

U1: Primary Voltage；N1: Number of Primary Winding Turns；U2: Secondary Voltage；N2: Number of Secondary Winding Turns

According to the Chinese national standard GB 1094.16, a dry-type transformer is clearly defined as a transformer whose core and windings are not immersed in insulating liquid. Its insulating and cooling medium is air. Broadly speaking, dry-type transformers can be divided into two main types: encapsulated and open-wound.

• The "SC(B)" type refers to an epoxy-resin-cast dry-type transformer (the "B" in the model designation indicates that the windings are made of copper foil; the "B" in "SG(B)" has the same meaning). The high-voltage winding is fully encapsulated with epoxy resin, while the low-voltage winding is generally not fully cast with epoxy resin—only the end turns are sealed with epoxy resin (this is also because the low-voltage side carries higher current, and full casting would adversely affect heat dissipation). Currently, SC(B)-type dry-type transformers are the mainstream products on the market, and this article uses them as an example for analysis. Most SC(B)-type transformers have Class F insulation, with a few rated at Class H.

• The "SG(B)" type is an open-wound dry-type transformer that uses NOMEX insulation paper from DuPont (USA) for turn-to-turn insulation. The low-voltage winding is made of copper foil, and both high- and low-voltage windings undergo VPI (Vacuum Pressure Impregnation) insulation treatment. The surface is coated with a layer of epoxy insulating varnish. Most SG(B)-type dry-type transformers have Class H insulation, with a few rated at Class C.

• There is another type of dry-type transformer, designated as "SCR(B)", which is an encapsulated type but not cast with epoxy resin. It is fully encapsulated using NOMEX paper and silicone gel, based on French technology. This product has very limited market demand. All SCR(B)-type dry-type transformers have Class H insulation.

2 Advantages of Dry-Type Transformers

• Safe, flame-retardant, fireproof, explosion-proof, pollution-free, and can be installed directly in the load center;

• Maintenance-free, with low overall operating costs;

• Excellent moisture resistance—can operate normally under 100% humidity and can be re-energized without pre-drying after being shut down;

• Low losses, low partial discharge, low noise, strong heat dissipation, and capable of operating at 150% of rated load under forced-air cooling conditions;

• Equipped with a comprehensive temperature protection and control system, providing reliable assurance for safe operation;

• Compact size, light weight, small footprint, and low installation cost.

3.Disadvantages of Dry-Type Transformers

• Under the same capacity and voltage rating, dry-type transformers are more expensive than oil-immersed transformers;

• Voltage rating is limited—typically up to 35 kV, with only a few models reaching 110 kV;

• Generally used indoors; when used outdoors, a protective enclosure with a high ingress protection (IP) rating is required;

• For cast-resin windings, if damaged, they often need to be scrapped entirely, as repair is usually difficult.

4. Structure of Dry-Type Transformers

4.1 Windings
(1) Layer-type winding: Made by stacking flat or round conductors and winding them in a helical pattern to form multiple layers. Insulation or ventilation ducts are placed between layers. The winding is cast and cured under vacuum using a mold and specialized casting equipment. Process: stacked helical winding → placed into mold → vacuum casting.

(2) Foil-type winding: Made by winding thin, wide conductors, with one turn per layer. Interlayer insulation also serves as turn-to-turn insulation. Foil-type windings generally use axial cooling ducts: during winding, spacer strips are inserted at designated turn positions and later removed to form axial air channels. After winding on a foil winding machine, the coil only needs to be heated and cured—no mold or casting is required.

Why is the high-voltage winding placed on the outer layer and the low-voltage winding on the inner layer?
Because the low-voltage side operates at a lower voltage and requires smaller insulation clearance, placing it closer to the core reduces the distance between the winding and the core, thereby reducing the overall transformer size and cost. Additionally, the high-voltage winding usually has tap connections; placing it on the outside makes operation more convenient and safer.

4.2 Core

• Constructed by stacking multiple laminations of silicon steel coated with insulating varnish;

• The core is clamped primarily by clamping frames and clamping bolts;

• Upper and lower clamping frames compress the core and windings via tie rods or tie plates;

• Core insulation components include frame insulation, bolt insulation, or tie-plate insulation.

Why must the core be grounded?
During normal operation, the transformer core must have one and only one reliable ground point. Without grounding, a floating voltage would develop between the core and ground, leading to intermittent breakdown discharges from the core to ground. Grounding the core at a single point eliminates the possibility of a floating potential. 

However, if the core is grounded at two or more points, uneven potentials between core sections will cause circulating currents between grounding points, resulting in multi-point grounding faults and localized overheating. Such core grounding faults can cause severe local temperature rise, potentially triggering protective tripping. In extreme cases, melted spots on the core create short circuits between laminations, significantly increasing core losses and severely affecting transformer performance and operation—sometimes requiring replacement of the silicon steel laminations for repair. Therefore, transformers must not have multiple grounding points; only one and exactly one grounding point is allowed.

5.Temperature Control System

The safe operation and service life of a dry-type transformer largely depend on the safety and reliability of the winding insulation. If the winding temperature exceeds the insulation’s thermal withstand limit, the insulation will be damaged—this is one of the main reasons for transformer malfunction. Therefore, monitoring the operating temperature and implementing alarm and trip controls are critically important.

(1) Automatic fan control: Temperature signals are measured by Pt100 resistance temperature detectors embedded in the hottest part of the low-voltage winding. As the transformer load increases and operating temperature rises, the system automatically starts the cooling fans when the winding temperature reaches 110°C, and stops them when the temperature drops to 90°C.

(2) High-temperature alarm and over-temperature trip: Temperature signals from the windings or core are collected by PTC nonlinear thermistors embedded in the low-voltage winding. If the winding temperature continues to rise and reaches 155°C, the system outputs an over-temperature alarm signal. If the temperature further increases to 170°C, the transformer can no longer operate safely, and an over-temperature trip signal must be sent to the secondary protection circuit.

(3) Temperature display system: Temperature values are measured by Pt100 thermistors embedded in the low-voltage winding and directly display the temperature of each phase winding (with three-phase monitoring, maximum value display, and historical peak temperature recording). The system provides a 4–20 mA analog output for the highest temperature. If remote transmission to a computer is required (up to 1200 meters away), it can be equipped with a computer interface and one transmitter, enabling simultaneous monitoring of up to 31 transformers. The Pt100 thermistor signal can also trigger over-temperature alarms and trips, further enhancing the reliability of the temperature protection system.

6. Enclosure of Dry-Type Transformers

Depending on the characteristics of the operating environment and protection requirements, dry-type transformers can be equipped with different types of enclosures. Typically, an IP20-rated enclosure is selected, which prevents solid foreign objects larger than 12 mm in diameter and small animals such as rats, snakes, cats, and birds from entering the transformer, thereby avoiding severe faults like short circuits and power outages, and providing a safety barrier for live parts.

If the transformer must be installed outdoors, an IP23-rated enclosure can be used. In addition to the protection offered by IP20, it also prevents water droplets falling at angles up to 60° from the vertical direction. However, the IP23 enclosure reduces the transformer’s cooling capacity, so attention must be paid to derating its operating capacity accordingly when selecting this type of enclosure.

7. Cooling Methods of Dry-Type Transformers

Dry-type transformers employ two cooling methods: natural air cooling (AN) and forced air cooling (AF).

Under natural air cooling, the transformer can operate continuously at its rated capacity for an extended period.

Under forced air cooling, the transformer’s output capacity can be increased by 50%, making it suitable for intermittent overload operation or emergency overload conditions. However, during overload operation, load losses and impedance voltage increase significantly, resulting in uneconomical operation; therefore, prolonged continuous overload operation should be avoided.

8.Test Items for Dry-Type Transformers

• Measurement of DC resistance of windings:
  Checks the welding quality of internal conductors, the contact condition between tap changers and leads, and whether phase resistances are unbalanced. Generally, line-to-line resistance imbalance should not exceed 2%, and phase-to-phase imbalance should not exceed 4%. Excessive DC resistance imbalance can cause circulating currents among the three phases, increasing circulating current losses and leading to undesirable effects such as transformer overheating.

• Check voltage ratio at all tap positions:
  Verifies whether the number of turns is correct and whether all tap connections are properly wired. When applying 1000 V to the high-voltage side (and its various taps), check whether the transformer outputs approximately 400 V on the low-voltage side.

• Check three-phase winding connection group and polarity.

• Measure insulation resistance of core-insulated fasteners and the core itself.

• Measure insulation resistance of windings:
  Evaluates the insulation level between high-voltage, low-voltage windings, and ground. Typically, a 2500 V megohmmeter is used, and the measured insulation resistance values (HV–LV, HV–ground, LV–ground) must exceed the specified standard values.

• AC withstand voltage test of windings:
  Assesses the main insulation strength between HV, LV, and ground via dielectric strength testing. This test is decisive in detecting localized defects introduced during manufacturing. For dry-type transformers, the typical test voltages are: 35 kV for the 10 kV winding and 3 kV for the 0.4 kV winding, each applied for 1 minute without breakdown to be considered acceptable.

• Switching and interlock tests for circuit breakers on all sides of the transformer:
  Verifies the reliability of protective relay operations and confirms that switching equipment is intact and defect-free.

9. Impulse Switching (Inrush) Test

(1) When disconnecting an unloaded transformer, switching overvoltage may occur. In power systems with an ungrounded neutral or neutral grounded through an arc-suppression coil, the overvoltage magnitude can reach 4–4.5 times the phase voltage; in systems with directly grounded neutral, it can reach up to 3 times the phase voltage. To verify whether the transformer insulation can withstand full voltage or switching overvoltage, an impulse test is required.

(2) Energizing an unloaded transformer produces magnetizing inrush current, which can reach 6–8 times the rated current. The inrush current decays rapidly initially—typically reducing to 0.25–0.5 times rated current within 0.5–1 second—but complete decay may take much longer, up to tens of seconds for large-capacity transformers. Due to the large electromagnetic forces generated by inrush current, the impulse test is performed to evaluate the mechanical strength of the transformer and to assess whether protective relays might maloperate during the early decay phase of inrush current.
Generally, newly installed transformers undergo 5 impulse tests, while overhauled transformers undergo 3 impulse tests.

10. No-Load Test

The purpose of the no-load test is:

• To measure the transformer’s no-load loss and no-load current;

• To verify whether the design and manufacturing of the core meet technical specifications and standards;

• To detect core defects such as local overheating or poor local insulation.

During the test, the high-voltage side is open-circuited, and rated voltage is applied to the low-voltage side. No-load loss is primarily core (iron) loss.

Defects detectable via no-load test include:

• Poor insulation between silicon steel laminations;

• Local short circuits or burn damage between core laminations;

• Insulation failure in core-through bolts, steel binding straps, clamping plates, upper yokes, etc., causing short circuits;

• Loose, misaligned silicon steel sheets or excessive air gaps in the magnetic circuit;

• Multi-point grounding of the core;

• Inter-turn or inter-layer short circuits in windings, or unequal turns in parallel branches causing ampere-turn imbalance;

• Use of high-loss, low-quality silicon steel sheets or errors in design calculations.

11.Short-Circuit Test

The short-circuit test primarily measures short-circuit loss and impedance. It is conducted at commissioning to verify the correctness of winding structure, and after winding replacement to check for significant deviations from previous test results.

The test power supply may be three-phase or single-phase, applied to the high-voltage side while the low-voltage side is short-circuited. During the test, the high-voltage side current is raised to its rated value, and the low-voltage side current is controlled to remain at rated current.

12.Handling Abnormal Conditions of Dry-Type Transformers

12.1 Abnormal Transformer Noise

• Mechanical noise caused by:

• Loose core clamping bolts;

• Deformation of core corners due to mishandling during transport or installation;

• Foreign objects bridging parts of the core;

• Loose fan mounting screws or foreign debris inside the fan;

• Loose enclosure mounting screws causing panel vibration and noise;

• Loose low-voltage busbar fixing screws or lack of flexible connections, leading to vibration and noise.

• Excessively high input supply voltage causing over-excitation and louder humming noise.

• Noise from high-order harmonics: irregular in pattern—varying in volume and intermittently present. Mainly caused by harmonic-generating equipment (e.g., electric furnaces, thyristor rectifiers) on the supply or load side feeding harmonics back into the transformer.

• Environmental factors: small transformer room with smooth walls creates a resonant "speaker box" effect, amplifying perceived noise.

12.2 Abnormal Temperature Display

• Sensor not inserted into the socket on the back of the temperature display unit—fault indicator light illuminates;

• Loose connection at sensor plug increases resistance, causing falsely high temperature readings;

• Infinite temperature reading on one phase indicates an open circuit in the platinum resistance wire of the sensor;

• Abnormally high reading on one phase suggests the platinum resistor is in a partially broken (intermittent) state.

A transformer operates based on the principle of electromagnetic induction. The main components of a transformer are the windings and the core. During operation, the windings serve as the path for electric current, while the core serves as the path for magnetic flux. When electrical energy is input to the primary winding, the alternating current creates an alternating magnetic field in the core (i.e., electrical energy is converted into magnetic field energy). Due to magnetic linkage (flux linkage), the magnetic flux passing through the secondary winding continuously changes, thereby inducing an electromotive force (EMF) in the secondary winding. When an external circuit is connected, electrical energy is delivered to the load (i.e., magnetic field energy is converted back into electrical energy). This "electricity–magnetism–electricity" conversion process is realized based on the principle of electromagnetic induction, and this energy conversion process constitutes the working principle of a transformer.