Detailed Explanation of Dry-Type Transformer Principles

Dry-type transformers are critical electrical equipment playing vital roles in operation. Primarily used in high-voltage transmission, distribution, and industrial power applications, their functioning is based on the principle of electromagnetic induction.

Introduction to Dry-Type Transformers

I. Common transformer classifications can be summarized as follows: 1. By phase: ① Single-phase transformers: Used for single-phase loads. ② Three-phase transformers: Used for voltage step-up/step-down in three-phase systems. 2. By cooling method: ① Dry-type transformers: Rely on natural air convection cooling or enhanced fan cooling. Primarily used in high-rise buildings, highway toll stations, local lighting, electronic circuits, and other small-capacity applications. ② Oil-immersed transformers: Utilize oil as the cooling medium, such as oil-immersed self-cooled, oil-immersed air-cooled, oil-immersed water-cooled, or forced oil circulation types. 3. By application: ① Power transformers: Used for voltage step-up or step-down in transmission and distribution systems. ② Instrument transformers: Such as voltage transformers and current transformers, used for measuring instruments and relay protection devices. ③ Test transformers: Capable of generating high voltage for testing electrical equipment. ④ Special-purpose transformers: Includes electric furnace transformers, rectifier transformers, regulating transformers, capacitive transformers, phase-shifting transformers, etc.

II. Definition of Dry-Type Power Transformers

1. Simply put, dry-type transformers refer to power transformers where the core and windings are not immersed in insulating liquid (insulating oil). 2. Primarily composed of a core made of silicon steel sheets and windings cast with epoxy resin. Insulating cylinders are placed between high and low voltage windings to enhance electrical insulation, with spacers supporting and constraining the windings.

Epoxy-cast dry-type transformers are critical power equipment in distribution systems. As epoxy resin is a flame-retardant, self-extinguishing solid insulating material, it offers both safety and cleanliness. Thus, epoxy resin cast dry-type transformers feature oil-free operation, flame resistance, low operational losses, and outstanding disaster prevention capabilities, making them widely adopted. Compared to oil-filled transformers, dry-type units eliminate risks of fire, explosion, and pollution due to the absence of oil. Losses and noise are reduced to new levels, enabling co-location of transformers and low-voltage switchgear within the same distribution room.

Transformer Structure and Working Principle

I. Structure (1) Core Functional component: Forms the magnetic circuit of the transformer Core material: Thick silicon steel laminations (0.35–0.5 mm) Core configuration: Power transformers primarily employ core-type structures. (2) Windings: The electrical circuit component of the transformer. Made by winding copper or aluminum conductors around insulated paper. The primary and secondary windings are concentrically mounted on the core columns. For insulation convenience, the low-voltage winding is typically placed inside the high-voltage winding. However, in large-capacity, low-voltage, high-current transformers, the low-voltage winding is often placed outside the high-voltage winding due to the difficulty of lead-out wire processing.

II. Principle

The primary components of a transformer are an iron core and two windings mounted around it. These windings are magnetically coupled but electrically isolated. Applying an alternating voltage to the primary winding generates an alternating magnetic flux linking both windings, inducing electromotive forces e₁ and e₂ respectively. The magnitude of these induced voltages is determined by Faraday's law of electromagnetic induction:

Electromagnetic induction refers to the phenomenon where a changing magnetic flux induces an electromotive force. As long as: (1) the magnetic flux changes, and (2) the number of turns in the primary and secondary windings differs, the voltage can be transformed.

Dry-Type Transformer Nameplate\

Nameplate

Explanation 1. Insulation Class of Transformers

Insulation class refers to the thermal resistance rating of the insulation material used in the windings of an electric motor (or transformer).                           A     E      B     F     HMaximum Allowable Temperature (°C): 105  120   130  155  180Winding Temperature Rise Limit (K):     60    75    80   100  125 Performance Reference Temperature (°C):    80    95   100  120  145

2. Insulation Level of Transformers The insulation level of transformers, also known as insulation strength, corresponds to the protection level and other insulation components. It refers to the withstand voltage value: LI75 AC35/AC3LI -- Lightning impulse withstand voltage (75kV) AC -- Power frequency withstand voltage (High voltage 35kV, Low voltage 3kV) 3. Temperature Rise and Cooling Methods ① For air-cooled transformers, temperature rise refers to the difference between the measured component temperature and the ambient cooling air temperature. For water-cooled transformers, it refers to the difference between the measured component temperature and the water temperature at the cooler inlet. ② Transformer cooling methods are designated by the type of cooling medium and circulation method. They are categorized as: Dry self-cooled (AN), Dry air-cooled (AF), Oil-immersed self-cooled (ONAN), Oil-immersed air-cooled (ONAF), Forced oil-air-cooled (OFAF), Forced oil-water-cooled (OFWF), Forced oil-guided air-cooled and water-cooled (ODAF and ODWF). 4. No-Load Current and No-Load Losses ① When the secondary winding is open-circuited and the primary winding is energized with rated voltage at rated frequency, the current flowing through the primary winding is the no-load current. ② The active component of the no-load current constitutes the loss current, and the absorbed active power represents the no-load loss.

5. Impedance Voltage and Load Losses ① For a two-winding transformer, the voltage applied to the primary winding when the secondary winding is short-circuited and the primary carries rated current is called the impedance voltage. ② The active power absorbed when the secondary winding is short-circuited and the primary carries rated current is called the load loss.

Daily Inspection of Transformers

Regular inspections are essential to ensure normal transformer operation. During inspections, pay attention to the following two aspects:

Temperature

The operating temperature is a critical indicator for normal transformer operation. The normal operating temperature range is generally 50°C to 100°C. Dry-type transformers can operate safely within 140°C because the F-class insulation system has a temperature rating of 155°C. If abnormal temperatures are detected, promptly analyze and identify the cause. Sound

During normal operation, transformers emit a uniform humming sound. If abnormal sounds are detected—such as significantly increased noise or partial discharge noises—immediate investigation and corrective action are required.

Transformer Power-Off Maintenance\05

(1) Disconnect the low-voltage circuit breaker of the transformer to be maintained, open the isolating switch, and hang the corresponding identification tag on the handle. (2) Disconnect the high-voltage load switch of the transformer. After confirming it is in the open position, close the grounding switch, secure the fuse, and hang the relevant identification tag. (3) Enter the oil transformer room. First, use a high-voltage tester to confirm the transformer is de-energized. Then open the high-voltage isolating switch. Inspect the casing, porcelain insulators, and leads for deformation. Replace any damaged components. Check the oil level for normality. If oil leakage is detected, replace the rubber gasket. Inspect the silica gel for effectiveness. Replace immediately if discoloration or severe degradation is observed.

Post-Inspection\06

(1) Verify no tools remain in the transformer room or on the transformer. Cross-reference the pre- and post-transformer tool/material lists to confirm completeness. (2) Remove all installed grounding wires and corresponding protective signage. (3) Perform corresponding energization switching operations.