17 Common Questions About Power Transformers

1 Why must the transformer core be grounded?
During normal operation of power transformers, the core must have one reliable ground connection. Without grounding, a floating voltage between the core and ground would cause intermittent breakdown discharge. Single-point grounding eliminates the possibility of floating potential in the core. However, when two or more grounding points exist, uneven potentials between core sections create circulating currents between grounding points, causing multi-point grounding heating faults. Core grounding faults can cause localized overheating. In severe cases, core temperature rises significantly, triggering light gas alarms, and potentially causing heavy gas protection to trip. Melted core sections create short circuits between laminations, increasing core losses and seriously affecting transformer performance and operation, sometimes requiring replacement of core silicon steel sheets. Therefore, transformer cores must have exactly one ground point—no more and no less.

2 Why are silicon steel sheets used for transformer cores?
Common transformer cores are made from silicon steel sheets. Silicon steel is steel containing silicon (also called sand) at 0.8-4.8%. Silicon steel is used because it has excellent magnetic properties and can generate high magnetic flux density in energized coils, allowing for smaller transformer size. Transformers always operate under AC conditions, with power losses occurring not only in coil resistance but also in the core under alternating magnetization. Core power losses are called "iron losses," consisting of "hysteresis loss" and "eddy current loss." Hysteresis loss occurs during magnetization due to magnetic hysteresis, with loss proportional to the area enclosed by the material's hysteresis loop. Silicon steel has a narrow hysteresis loop, resulting in lower hysteresis losses and reduced heating.

If silicon steel has these advantages, why not use solid blocks? Because laminated cores reduce another type of iron loss—eddy current loss. During operation, alternating current in coils creates alternating magnetic flux, inducing currents in the core. These induced currents flow in closed loops perpendicular to the flux direction, forming eddy currents that cause heating. To reduce eddy current losses, transformer cores use insulated silicon steel sheets stacked together, forcing eddy currents through narrow paths with smaller cross-sections to increase resistance. Additionally, silicon in the steel increases resistivity, further reducing eddy currents. Transformer cores typically use 0.35mm thick cold-rolled silicon steel sheets, cut to size and stacked in "E-I" or "C" shapes. In theory, thinner sheets and narrower strips would better reduce eddy currents. This would reduce eddy current losses, lower temperature rise, and save material. However, practical core manufacturing considers multiple factors—excessively thin sheets would greatly increase labor costs and reduce the effective cross-sectional area of the core. Therefore, silicon steel sheet dimensions for transformer cores must balance various considerations to achieve optimal design.

3 What is the protection range of Buchholz (gas) protection?

• Internal multi-phase short circuits in the transformer
• Turn-to-turn short circuits, short circuits between windings and core or tank
• Core faults
• Oil level drop or oil leakage
• Poor contact in tap changers or poor welding of conductors

4 What are the differences between main transformer differential protection and Buchholz protection?

• Main transformer differential protection operates on circulating current principles, while Buchholz protection operates based on gas generation during internal transformer faults.
• Differential protection serves as the main protection for transformers, while Buchholz protection is the main protection for internal transformer faults.
• Protection ranges differ:
  A)Differential protection covers:
  • Multi-phase short circuits in main transformer leads and windings
  • Severe single-phase turn-to-turn short circuits
  • Ground faults on windings and leads in high-current grounding systems
• B) Buchholz protection covers:
• • Internal multi-phase short circuits in the transformer
  • Turn-to-turn short circuits, short circuits between turns and core or tank
  • Core faults (overheating damage)
  • Oil level drop or oil leakage
  • Poor contact in tap changers or poor conductor welding

5 How to handle main transformer cooler failures?

• When working power supplies for cooler sections I and II are lost, a "#1, #2 power failure" signal appears, and the main transformer cooler full-stop tripping circuit activates. Immediately report to dispatch and disable this protection set.
• If switching between power supplies I and II fails during operation, the "cooler full stop" indicator illuminates, activating the main transformer cooler full-stop tripping circuit. Immediately report to dispatch to disable this protection set and quickly perform manual switching. If contactors KM1 or KM2 have failed, do not force excitation.
• When any single cooler circuit fails, isolate the faulty cooler circuit.

6 What consequences occur when transformers that don't meet parallel operation conditions are operated in parallel?
When transformers with different transformation ratios operate in parallel, circulating currents develop, affecting transformer output capacity. When transformers with different percentage impedances operate in parallel, loads cannot be distributed according to transformer capacity ratios, also affecting output capacity. When transformers with different connection groups operate in parallel, short circuits will occur in the transformers.

7 What causes abnormal sounds in transformers?

• Overload
• Poor internal contacts causing discharge arcing
• Loose individual components
• Grounding or short circuits in the system
• Large motor starting causing significant load fluctuations

8 When should the tap changer of an on-load tap-changing transformer not be adjusted?

• During transformer overload operation (except in special circumstances)
• When the light gas protection of the on-load tap changer frequently activates
• When the oil gauge of the on-load tap changer shows no oil
• When the number of tap changes exceeds specified limits
• When the tap-changing device shows abnormalities

9 What do the rated values on a transformer nameplate represent?
Transformer rated values are specifications established by manufacturers for normal transformer operation. Operating within these rated values ensures long-term reliable operation with good performance. Rated values include:

• Rated capacity: The guaranteed output capability under rated conditions, expressed in volt-amperes (VA), kilovolt-amperes (kVA), or megavolt-amperes (MVA). Due to high transformer efficiency, primary and secondary winding rated capacities are typically designed to be equal.
• Rated voltage: The guaranteed terminal voltage under no-load conditions, expressed in volts (V) or kilovolts (kV). Unless specified otherwise, rated voltage refers to line voltage.
• Rated current: The line current calculated from rated capacity and rated voltage, expressed in amperes (A).
• No-load current: The excitation current as a percentage of rated current during no-load operation.
• Short-circuit loss: The active power loss when one winding is short-circuited and voltage is applied to the other winding to achieve rated current in both windings, expressed in watts (W) or kilowatts (kW).
• No-load loss: The active power loss during no-load operation, expressed in watts (W) or kilowatts (kW).
• Short-circuit voltage: Also called impedance voltage, the percentage of applied voltage to rated voltage when one winding is short-circuited and the other winding carries rated current.
• Connection group: Indicates the connection methods of primary and secondary windings and the phase difference between line voltages, represented using clock notation.

10 Why do current-source inverters require larger transformer capacity?
Transformer design typically considers rated capacity rather than rated power since current relates only to rated capacity. For voltage-source inverters, the input power factor is close to 1, so rated capacity and rated power are nearly equal. Current-source inverters differ—their input-side transformer power factor equals at most the power factor of the load induction motor. Therefore, for the same load motor, the rated capacity must be larger than for transformers used with voltage-source inverters.

11 What factors affect transformer capacity?
Core selection relates to voltage, while conductor selection relates to current—conductor thickness directly affects heat generation. In other words, transformer capacity relates only to heat generation. For a well-designed transformer operating in poor heat dissipation conditions, a 1000kVA unit might operate at 1250kVA with enhanced cooling. Additionally, rated capacity relates to allowable temperature rise. For example, a 1000kVA transformer with an allowable temperature rise of 100K might exceed 1000kVA capacity if allowed to operate at 120K in special circumstances. This shows that improving transformer cooling conditions can increase its rated capacity. Conversely, for the same capacity inverter, transformer cabinet size can be reduced.

12 How to improve transformer efficiency?

• Select low-loss, high-efficiency energy-saving transformers whenever possible
• Choose transformer capacity reasonably based on load conditions
• Maintain transformer average load factor above 70%
• Consider replacing with smaller capacity transformers when average load factor is consistently below 30%
• Improve load power factor to enhance the transformer's ability to deliver active power
• Reasonably configure loads to minimize the number of operating transformers

13 Why accelerate technical retrofitting of high-energy-consumption distribution transformers?
High-energy-consumption distribution transformers mainly refer to SJ, SJL, SL7, S7 series transformers, whose iron and copper losses are much higher than currently widespread S9 series transformers. For example, compared to S9, S7 has 11% higher iron losses and 28% higher copper losses. Newer transformers like S10 and S11 are even more energy-efficient than S9, while amorphous alloy transformers have iron losses equivalent to only 20% of S7 transformers. Transformers typically have service lives of several decades. Replacing high-energy-consumption transformers with high-efficiency models not only improves energy conversion efficiency but also achieves considerable electricity savings over their lifetime.

14 What is eddy current? What harm does eddy current cause?
When alternating current flows through a conductor, it creates an alternating magnetic field around the conductor. This alternating field induces currents within solid conductors. Since these induced currents form closed loops within the conductor similar to water vortices, they are called eddy currents. Eddy currents not only waste electrical energy, reducing equipment efficiency, but also cause heating in electrical devices (such as transformer cores), potentially affecting normal equipment operation when severe.

15 Why must transformer instantaneous protection avoid low-voltage short-circuit current?
This primarily considers selectivity in relay protection operation. High-voltage side instantaneous protection mainly protects against severe external transformer faults. During setting, if the protection doesn't avoid maximum short-circuit current on the transformer low-voltage side, the protection range would extend to low-voltage outgoing lines since short-circuit current values don't change significantly in a short range near the low-voltage outlet. This would compromise selectivity. While non-selective protection is more reliable, it creates operational inconvenience. For example, many industrial parks have 10kV main distribution rooms (10kV bus + outgoing circuit breakers), with each workshop having low-voltage distribution rings (ring main units + transformers). If circuit breakers don't avoid maximum short-circuit current on the transformer low-voltage side, low-voltage main switches (ring main unit load switch fuses) and high-voltage circuit breakers would both operate, causing operational difficulties.

16 Why aren't two paralleled transformers allowed to have neutral points grounded simultaneously?
In high-current systems, to satisfy sensitivity coordination requirements for relay protection, some main transformers must be grounded while others remain ungrounded. At a station with two main transformers, not grounding both neutral points simultaneously primarily addresses coordination of zero-sequence current and zero-sequence voltage protection. In substations with multiple paralleled transformers, typically some transformer neutral points are grounded while others remain ungrounded. This limits ground fault current to reasonable levels and minimizes the impact of operational mode changes on the magnitude and distribution of zero-sequence currents throughout the grid, improving sensitivity of zero-sequence current protection systems.

17 Why perform impulse closing tests before putting newly installed or overhauled transformers into operation?
Disconnecting an unloaded transformer from the grid creates switching overvoltages. In small-current grounding systems, these overvoltages can reach 3-4 times the rated phase voltage; in high-grounding current systems, they can reach 3 times the rated phase voltage. Therefore, to verify whether transformer insulation can withstand rated voltage and operational switching overvoltages, multiple impulse closing tests must be performed before commissioning. Additionally, energizing unloaded transformers produces magnetizing inrush current, which can reach 6-8 times the rated current. Since magnetizing inrush creates significant electromagnetic forces, impulse closing tests also effectively verify transformer mechanical strength and whether relay protection might maloperate.