Methods for reducing transformer losses

Power transformers are one of the most important pieces of equipment in power systems and are fundamental to ensuring reliable power supply. With the rapid development of the national economy, the demand for transformers will continue to increase. However, with the increase in the installed capacity of power transformers, the energy they consume is also increasing, which is inconsistent with China's advocacy for building an energy-saving society. It is necessary to adopt corresponding technical measures to reduce the losses of transformers themselves, so it has become very necessary to study how to reduce transformer losses.

No-load losses of power transformers

　　The no-load losses of a transformer mainly include the hysteresis loss, eddy current loss, and additional loss of the core material. Because the no-load loss of the transformer is excitation loss, it is independent of the load.

1) Hysteresis loss is the loss generated due to the hysteresis phenomenon during the repeated magnetization of ferromagnetic materials. The magnitude of hysteresis loss is proportional to the area of the hysteresis loop.

2) Eddy current loss. Because the core itself is a metal conductor, the electromotive force generated due to the electromagnetic induction phenomenon will generate a circulating current in the core, which is the eddy current. Because there is eddy current flowing in the core, and the core itself has resistance, eddy current loss is caused. 3) Additional iron loss. Additional iron loss is not completely determined by the transformer material itself, but mainly related to the structure and production process of the transformer. The main reasons for additional iron loss are: high-order harmonic components in the magnetic flux waveform, which will cause additional eddy current loss; loss increase due to deterioration of magnetic properties caused by mechanical processing; increase in local losses at the core joints and T-shaped areas of the core column and yoke, etc.

Main methods to reduce no-load losses

　　Since no-load loss is an important parameter of a transformer, accounting for only 20%~30% of the total transformer loss, to reduce no-load loss, it is necessary to reduce the total core amount, unit loss, and process coefficient. The main methods to reduce no-load loss are as follows:

　　(1) Using high-permeability silicon steel sheets and amorphous alloy sheets. The thickness of ordinary silicon steel sheets is 0.3~0.35 mm, with low loss, and 0.15~0.27 mm can be used. At the same time, if stepped stacking is used, iron loss can be further reduced by about 8%. Laser irradiation, mechanical indentation, and plasma treatment can further reduce the loss of high-permeability silicon steel sheets. Amorphous alloy sheets and silicon steel sheets with a silicon content of 6.5% made according to the rapid cooling principle have smaller eddy current losses than general high-permeability silicon steel sheets.

　　(2) Reduce the process coefficient. The process loss coefficient is related to many factors, such as the silicon steel sheet material, whether the punching and shearing equipment is annealed, and the clamping degree. The accuracy of the punching and shearing equipment's tools, reasonable tool installation, and adjustment are also important.

　　(3) Improve the core structure. The core is not punched, not bound with glass adhesive tape, the end face is coated with solidified paint, and the interphase yoke is bound with high-strength steel strip. The pull plates connecting the upper and lower clamps on both sides of the core column are made of non-magnetic steel plates. For large-capacity core sheets, no paint treatment is applied to improve the filling factor and cooling performance. Strong pressure tooling and adhesive are used to make the two yokes of the core into a solid, flat, and high-precision vertical whole. Reducing the core lap width can reduce losses, and for every 1% reduction in the lap area, the no-load loss will be reduced by 0.3%. Mixing different grades of silicon steel sheets in the core will consume energy, so less or no mixing should be done.

　　(4) Reduce the core window size. Changing the constant-turn insulation (thickness) of the winding to variable-turn insulation, for example, for a 120,000/110 transformer, according to the impact voltage distribution, the turn insulation thickness of the high-voltage winding head and tap section is 1.35 mm, and other sections are 0.95 mm. As a result, after reducing the window size, the iron weight is reduced by 1.67%. Under safe conditions, reasonably reducing the main air gap distance between high and low voltage, reducing the inter-coil oil channel, reducing the interphase distance, strengthening insulation treatment (adding corner rings, partitions, etc.), and using a half-oil channel structure for the winding will shorten the core column center distance, reduce the core weight, and reduce iron loss.

　　(5) Design a non-resonant core. By designing the resonant frequency of the core in a suitable frequency range, it is impossible to produce strong resonance, which has a significant effect on reducing noise and can save energy used for noise reduction.

　　(6) Using wound core transformers and three-dimensional core transformers. Wound cores have 4 fewer corners than traditional laminated cores, continuous winding makes full use of the orientation of silicon steel sheets, and annealing process reduces additional losses. For R-type wound cores, the section duty cycle is close to 100%. The yoke of the three-dimensional core is a triangular three-dimensional arrangement, which is 25% lighter than the planar wound core yoke. These factors indicate that wound cores and three-dimensional cores are more energy-efficient.

Load losses of power transformers

　　When a power transformer is in operation, current flows through the windings, generating load losses. Load losses, also known as copper losses, include basic winding DC losses and additional losses.

1) Basic copper loss. For small-capacity transformers, load loss mainly refers to basic copper loss, and the proportion of additional loss caused by leakage magnetic field is very small.

2) Additional losses. Additional losses mainly include three types of losses: winding eddy current loss, circulating current loss, and stray loss:

　　(a) Winding eddy current loss. When a large-capacity transformer is operating, the ampere-turns of the winding will generate a large leakage magnetic field. The so-called leakage magnetic field refers to the fact that part of the magnetic flux passes through the air, and part of the magnetic circuit is the core. Because the conductors of the winding are in the leakage magnetic field, the leakage magnetic flux will cause eddy current loss in the conductors.

　　(b) Lead loss. Lead loss is the sum of the resistance losses of the various leads of the transformer.

　　(c) Stray loss. Stray loss is the loss generated when the leakage magnetic flux passes through steel structural parts (such as plate clamps, steel pressure plates, pressure nails, bolts, and oil tank walls, etc.).

Main methods to reduce load losses

　　Load losses account for 70%~80% of the total losses, including winding DC resistance losses (basic losses), eddy current losses in conductors, circulating current losses between parallel conductors, lead losses, and stray losses in structural parts (such as clamps, steel pressure plates, tank walls, bolts, core pull plates, etc.). The main methods to reduce load losses are as follows:

　　(1) Limit additional losses caused by stray magnetic flux. Conduct ampere-turn balance calculations and adjust ampere-turns according to the results; adopt a "low-high-low" or "high-low-high" arrangement for the windings; limit the width and thickness of the flat conductors; select the most suitable transposition method based on magnetic field calculations; use transposed conductors or composite conductors.

　　(2) Reduce the size of the main and longitudinal insulation structures. Using the "equal impact voltage gradient" distribution technique on the high-voltage windings can reduce the size of the longitudinal insulation; use thin paper tubes and small oil gaps between windings; use corrugated paper for main insulation; use molded parts with exactly the same shape and equipotential lines, with corner rings shaped to match equipotential lines, using segmented molded corner rings as structural components; wind the inner diameter of the winding on insulating paper, but set axial oil channels in the middle of the line segments; use more acetal resin-coated wires, replacing 0.45 mm thick paper-wrapped flat wires with QQ-2 or QQB type acetal wires, as the former has a turn insulation of 2 × (0.056～0.079) mm, a high winding fill factor, and meets the turn insulation requirements; use more cylindrical windings, as there are no inter-disk oil channels, cooling mainly relies on axial vertical oil channels, with good heat dissipation, good fill factor and impact characteristics, uniform ampere-turns, and small short-circuit forces; appropriately reduce the main insulation (radial, end) distance.

　　(3) Use relevant processes according to calculations. Determine the longitudinal insulation structure according to impact calculations, maintaining good shape of shims, struts, and chamfered metal parts; calculate stray magnetic fields and eddy current distribution to guide transposition methods; axially distribute windings evenly, using non-magnetic materials for core banding; set special shielding for the core and yoke to mitigate the electric field; use one tap per layer for the tap winding; use an assembly method in the process, with the inner winding directly wound on the insulating cylinder, strictly controlling height and diameter tolerances, with small assembly gaps, using a new hot-fitting process, using integral support plates and pressure plates, and using Dinelson paper at the winding transposition points, with pressure drying, and placing the windings in an insulated drying room to prevent moisture.

　　(4) Use low-loss, low-resistance conductors. Use oxygen-free copper wires drawn using the up-drawing method, or made using a continuous copper extrusion machine. If it can be used in transformers, it can save energy and reduce volume, and has certain application prospects.

　　(5) Utilize insulation structure characteristics to design for smaller size. Utilize the liquid dielectric properties of transformer oil, appropriately setting cover layers, barriers, shields, and insulation layers; utilize the "distance effect" of oil to add partitions to create small oil gaps; utilize the "volume effect" of oil to use corrugated paper; utilize the "thickness effect" of the insulation layer in oil to add insulation to increase the breakdown voltage, but it should not be too thick; utilize the characteristics of the distance between the partition and the maximum electric field strength in the oil to set the partitions.

　　(6) Adopt advanced insulation structures. Adopt suitable windings to improve the fill factor, and adopt a new spiral (or continuous) winding with axial oil channels to effectively reduce the winding volume. Use a non-metallic or non-magnetic material clamping structure at the stray magnetic flux concentration points, and use electromagnetic shielding to channel the stray magnetic flux, which can reduce load losses by 3%～8%.

　　(7) Optimize internal winding protection. Internal winding protection measures include capacitive rings, electrostatic windings, series compensation (additional inter-disk capacitance), equipotential screens, and twisted windings or internally shielded windings. They all reduce the overvoltage acting on the main and longitudinal insulation under impact, thereby reducing the size and energy consumption of the transformer.

　　(8) Adopt oblong windings and Yyn0 connection and reduce height for energy saving. Using oblong cores, windings, or elliptical windings or rectangular windings with rounded corners has proven to be more energy-efficient than traditional circular cross-sections. Using Yyn0 connection has lower tap voltages than Dyn11 connection, and three phases can share one tap switch, with a simple structure and small size. For a 500 kVA transformer, the former reduces conductor weight by 2%, iron weight by 6%, and oil weight by 11%, thus saving materials and energy. For dry-type transformers, the higher the winding, the more obvious the temperature difference between the top and bottom, so appropriate height reduction is beneficial for heat dissipation and energy saving.

　　Summary: In summary, this article mainly analyzes the causes of no-load and load losses in power transformers, and proposes detailed solutions for reducing no-load and load losses. These methods can effectively reduce the large losses in power transformers. Due to the complex problems encountered in practical engineering applications, there are still many problems. Therefore, further in-depth research is still needed on how to reduce power transformer losses.