01 Importance of Transformer Core Grounding
During normal operation, the core of a power transformer must be reliably grounded at one point. If the core is not grounded, it may form a floating potential, leading to intermittent breakdown discharges to ground. A single-point grounding effectively eliminates this floating potential. However, when two or more grounding points occur, circulating currents form between the grounding points, causing overheating faults in the core. In severe cases, this can damage the transformer's performance, requiring replacement of the silicon steel sheets. Therefore, the transformer core must have only one grounding point.
02 Scope of Gas Protection
Gas protection is primarily used to protect transformers from the following faults: multiphase short circuits inside the transformer; inter-turn short circuits, winding-to-core or winding-to-tank short circuits; core faults; oil level drop or oil leakage; poor contact or insecure welding in tap changers.
03 Handling Faults in Main Transformer Coolers
When a main transformer cooler fails, the following measures should be taken: If the working power supply for sections I and II is lost, immediately report to dispatch and disable relevant protections. If the power source switching fails, report to dispatch and manually switch. If any circuit in the cooler loop fails, isolate the faulty circuit.
04 Conditions for Parallel Operation of Transformers
For parallel operation of transformers, the following conditions must be met to avoid circulating currents or short circuits: identical voltage ratios; matching percentage impedances; identical connection groups.
05 Causes of Abnormal Transformer Sounds
Abnormal sounds from a transformer may result from: overload; poor internal contact or discharge sparking; loose components; system grounding or short circuits; significant load changes caused by large motor startups.
06 Restrictions on Tap Changer Adjustments in On-Load Tap Changers
Under the following conditions, adjustments to the tap changer of an on-load tap-changing transformer are not allowed: transformer operating under overload (except in special cases); frequent gas protection signals; no oil in the oil gauge; exceeding the specified number of tap changes; abnormal operation of the tap-changing device.
07 Meaning of Transformer Nameplate Ratings
The nameplate ratings of a transformer include: rated capacity (output capability guarantee under rated conditions); rated voltage (guaranteed terminal voltage under no-load conditions); rated current (calculated line current based on rated capacity and voltage); no-load current (percentage of magnetizing current relative to rated current during no-load operation); short-circuit loss (active power loss when one winding is shorted and the other reaches rated current); no-load loss (active power loss during no-load operation); short-circuit voltage (percentage of applied voltage relative to rated voltage when one winding is shorted and the other reaches rated current); connection group (connection method of primary and secondary windings and phase difference between line voltages).
08 Relationship Between Current Source Inverters and Transformer Capacity
Current source inverters require larger transformer capacities because the input-side transformer's power factor is at most equal to the load induction motor's power factor. Compared to voltage source inverters, their rated capacity is larger.
09 Factors Affecting Transformer Capacity
Transformer capacity is mainly related to heat generation. Core selection depends on voltage, while conductor selection depends on current. Additionally, the nominal capacity is also related to allowable temperature rise.
10 Methods to Improve Transformer Efficiency
To improve transformer efficiency, the following methods can be adopted: selecting low-loss, high-efficiency energy-saving transformers; choosing transformers with appropriate capacity based on load conditions; maintaining an average load factor above 70%; replacing transformers with smaller capacity when the average load factor is below 30%; improving load power factor; rationally distributing loads to reduce the number of operating transformers.
11 Technical Upgrades for High-Energy-Consumption Distribution Transformers
High-energy-consumption distribution transformers (e.g., SJ, SJL, SL7, S7 series) have higher iron and copper losses compared to the S9 series. Replacing them with newer models (e.g., S10, S11 series or amorphous alloy transformers) can significantly improve energy conversion efficiency and save electricity.
12 Generation and Hazards of Eddy Currents
Eddy currents are induced currents generated within a solid conductor due to alternating magnetic fields created by AC current in nearby conductors. Eddy currents not only waste electrical energy and reduce equipment efficiency but also cause heating, which can severely affect normal operation.
13 Selectivity of Transformer Instantaneous Protection
Transformer instantaneous protection must avoid low-voltage short-circuit currents to ensure selectivity of relay protection actions. Failing to avoid the maximum short-circuit current on the low-voltage side will expand the protection range to the low-voltage outgoing lines, losing selectivity.
14 Considerations for Neutral Grounding in Parallel Transformers
When two transformers operate in parallel, their neutral points cannot be grounded simultaneously. This is mainly due to coordination issues with zero-sequence current and zero-sequence voltage protection. Operating some transformers with neutral points grounded limits ground fault current levels and improves the sensitivity of zero-sequence current protection.
15 Purpose of Impulse Closing Tests for New or Overhauled Transformers
Newly installed or overhauled transformers must undergo impulse closing tests before being put into operation. These tests verify whether the insulation can withstand rated voltage and operational overvoltages. Additionally, they consider the impact of magnetizing inrush current-generated electromagnetic forces on the transformer’s mechanical strength and relay protection.
