Frequently Asked Questions

Once a dry-type transformer is put into operation it enters the working state and becomes live, so great care must be taken to avoid the danger of electric shock. In actual energizing operation, the following points should be noted:

1. The maintenance work has been finished and the work ticket is closed; maintenance staff have confirmed that the transformer can be put into operation.
2. All related safety measures and warning signs have been removed.
3. The transformer body and its system have been checked and are in good condition, meeting the commissioning requirements.
4. The insulation resistance of the dry-type transformer has been measured and is normal.
5. Switching operations follow the operation ticket order: close the high-voltage side switch first, then the low-voltage side switch.

In short, safety comes first in every energizing operation of a dry-type transformer — any improper operation that could cause an accident must be eliminated.

A distribution transformer is a type of transformer dedicated to supplying power around large transformers. Large transformers usually need more electricity to operate, so distribution transformers are often installed around them. A distribution transformer is different from a transformer's distribution switchgear: the switchgear only transmits electric energy, whereas a distribution transformer transmits both electric energy and voltage.

Fuse arrangements

Fuses should be installed on both the high- and low-voltage sides. The bottom of the high-voltage fuse must be at least 4.5 m above ground, with a horizontal phase spacing of at least 0.5 m; so that the fuse tube can fall smoothly after the fuse melts, the axis of the drop-out fuse should form a 15%–30% angle with the vertical. The bottom of the low-voltage fuse must be at least 3.5 m above ground with a phase spacing of at least 0.2 m.

Drop-out fuse links are selected on the principle that they must melt quickly when a short circuit occurs inside the transformer or on its high/low-voltage outgoing lines; the fusing time must be ≤ 0.1 s. For capacities below 100kVA, the high-voltage fuse rating is selected at 2–3 times the transformer rated current; above 100kVA, at 1.5–2 times. The low-voltage fuse is selected according to the low-voltage rated current.

Lightning protection

Most external threats to distribution transformer operation come from lightning. A lightning protection device should be installed, using gapless metal-oxide surge arresters with synthetic insulation housing, which offer strong power-frequency voltage withstand, good sealing and stable protection characteristics. Note that arresters mounted on the transformer with iron plates connecting them together is an incorrect practice.

Oil-immersed transformers play an active role in improving current and voltage. In production, operation and maintenance, their inspection, use and maintenance levels should be continuously improved. The main components are:

1. Core — made of premium high-permeability cold-rolled grain-oriented silicon steel sheet with fully mitered, three-step step-lap joints, hole-free binding and plate-type clamps.
2. Windings — flat wire made of high-quality oxygen-free copper; LV windings of continuous or helical type; HV windings of continuous or interleaved-continuous type with reinforced insulation at the first and last sections; not lacquered but dried to form an organic whole.
3. Body insulation — a 30 mm integral laminated-wood press ring plus a 30 mm split pressure plate added above the winding inside the core window, and a 70 mm integral laminated-wood support plate below, which strengthen the main insulation and improve the axial dynamic stability of the windings.
4. Leads — with on-load voltage regulation, a high-quality on-load tap changer is used; for off-circuit regulation, an off-circuit tap changer is used for small currents and three single-phase off-circuit tap changers for large currents.
5. Tank — transformers ≤6,300kVA use a barrel-type tank with fixed radiator panels; ≥8,000kVA use a bell-type tank with detachable radiator panels.
6. Fittings — pressure relief valve for capacities ≥315kVA; temperature controller for ≥1,000kVA; gas (Buchholz) relay for ≥800kVA; rapid oil-pressure relay for ≥8,000kVA.

These are the common components of an oil-immersed transformer, provided for your reference.

In terms of price, a dry-type transformer is more expensive than an oil-immersed one. Inside comprehensive buildings (basements, floors, rooftops) and in crowded places, dry-type transformers must be used; oil-immersed transformers are used at independent substation sites. Dry-type units are generally used in box-type (compact) substations; oil-type is generally used for outdoor temporary power supply. Choose according to the available space — where space is large an oil-immersed unit can be used, where space is tight a dry-type unit is preferred. In humid and hot regions, oil-immersed is preferred; if a dry-type unit is used there, forced-air cooling equipment must be fitted.

1. Appearance — the encapsulation differs: with a dry-type transformer the core and windings are directly visible, while an oil-type transformer only shows its outer enclosure.
2. Lead bushings — dry-type transformers mostly use silicone-rubber bushings, while oil-type transformers mostly use porcelain bushings.
3. Capacity and voltage — dry-type transformers are generally for distribution, mostly below 1,600kVA and 10kV, with some up to 35kV; oil-type transformers can be made in all capacities and all voltage classes, including the 1,000kV UHV test line currently under construction in China.
4. Insulation and cooling — dry-type transformers generally use resin insulation with natural air cooling (fans for large capacities), while oil-type transformers use insulating oil for insulation, with oil circulation carrying winding heat to the radiator panels.
5. Applications — dry-type transformers are mostly used where "fire-proof and explosion-proof" performance is needed, such as large buildings and high-rise buildings; because oil-type transformers may spray or leak oil and cause fire in an accident, they are mostly installed outdoors at sites where an "accident oil pit" can be dug.
6. Load capacity — a dry-type transformer should generally operate at its rated capacity, while an oil-type transformer has better overload capability.
7. Cost — for the same capacity, the purchase price of a dry-type transformer is much higher than that of an oil-type one. Dry-type model designations usually begin with SC (epoxy-resin cast encapsulated), SCR (non-epoxy resin cast solid-insulation encapsulated) or SG (open type).

A high-frequency transformer is a power transformer working at frequencies above medium frequency (10kHz). It is mainly used as the transformer in high-frequency switching power supplies, and also in high-frequency inverter power supplies and high-frequency inverter welding machines. Its main uses are reflected in the following points:

1. Higher conversion efficiency — in power supplies using high-frequency transformers, the switching devices work in a state of instant conduction and cut-off (continuously switching on and off), which is why it is called a "switching power supply". Losses are about 30% lower than with a traditional iron-core transformer.
2. High-frequency pulse transformation — the core of an ordinary transformer cannot do this, because a ferrite core has a much higher magnetic permeability.
3. Compact size — for the same power, the high operating frequency allows a much smaller core and overall size.

High-frequency transformers are also applied in everyday appliances such as radios, televisions and microwave ovens — especially in switching power supplies.

1. The most common is a short circuit between turns or between layers of a single-phase winding. Once the short-circuit current becomes very large, the winding deforms severely under electromagnetic force and the fault expands further. This fault can only be protected by differential protection and the transformer's own heavy gas (relay) operation, but if it persists beyond a certain time the transformer will be damaged.
2. Long-term overload, insulation ageing and transformer oil deterioration can cause a short circuit from live parts to earth. If heavy gas protection fails to act, the powerful electromagnetic force deforms the winding until it burns out.
3. An external short circuit can upset the transformer's dynamic stability and also cause an internal short circuit.
4. Internal insulation defects can cause local discharge to grow progressively, thermal instability or local electrical breakdown, resulting in a live-part-to-earth short circuit.
5. There are many other causes, e.g. when the load changes sharply the turn insulation paper may wear through by friction under long-term electromagnetic force (especially if the winding is not tightly wound), or frequent external short circuits and repeated reclosing cause turn-to-turn short circuits, particularly at conductor transposition points (the so-called "scissors mouth") where wrapping is insufficient.

Oil leakage arises partly from defects hidden in the transformer design and manufacturing process, and partly from improper installation and maintenance. Leakage most often occurs at radiator interfaces, flat butterfly-valve caps, bushings, porcelain insulators, weld seams, sand holes and flanges.

1. Air leakage — an invisible form of leakage. Examples include the bushing head, the conservator diaphragm, the glass of the safety airway, weld sand holes and steel with sand inclusions. Over the years, most serious accidents in power systems have been winding burn-outs and severe damage to the active part caused by short circuits at the transformer LV outlet.
2. Classification of oil leakage — transformer oil leakage can be divided into internal and external leakage; external leakage is further divided into weld-seam leakage and sealing-surface leakage.
   Internal leakage: most commonly, oil from oil-filled bushings or from the on-load tap changer's switch oil chamber leaking into the transformer body.
   External leakage: weld-seam leakage is caused by sand holes in welded steel plate areas; sealing-surface leakage is more complex and must be analyzed case by case — preventing it should be treated as an important task during transformer overhaul or installation.

Yes. In addition to the standard series shown on our Products page, Suzhou Boyuan designs and manufactures all kinds of special transformers according to customer requirements. Send us your specifications — furnace data, voltage, capacity, impedance, connection group and site conditions — through the contact form, and our engineering team will reply with a tailored proposal.