FAQ

A: A transformer is a static (stationary) electrical device that, based on the principle of electromagnetic induction, exchanges and transfers electromagnetic energy between two or more winding circuits linked to a common magnetic core. It changes voltage and current levels while keeping the frequency unchanged, which makes it the key equipment for power transmission, distribution and utilisation.

A: A single-phase transformer is a transformer operated from a single-phase AC supply, that is, from one alternating voltage. Its two principal windings are:
• Primary winding - connected directly to the power source.
• Secondary winding - connected to the load or to the device requiring voltage regulation.
Working principle: when an alternating current flows through the primary winding it generates an alternating magnetic field in the core. This field is conducted through the core to the secondary winding and, according to Faraday's law of electromagnetic induction, induces an electromotive force in it. The ratio of the primary and secondary voltages equals the ratio of the number of turns, which determines whether the unit works as a step-up transformer (raises voltage) or a step-down transformer (lowers voltage).

A: • Core - built from laminated silicon steel to minimise energy loss.
• Primary winding - receives the input power.
• Secondary winding - delivers the output power.
• Insulation - prevents short circuits between windings, turns and earth.
• Tank / enclosure - protects the internal components and, in oil-filled designs, holds the cooling oil.

A: Main types:
• Step-up transformer - increases the voltage from the primary side to the secondary side.
• Step-down transformer - reduces the voltage from the primary side to the secondary side.
• Isolation transformer - provides electrical isolation at the same voltage level.
Typical applications:
• Residential and commercial power supply.
• Lighting systems in public and private spaces.
• HVAC (heating, ventilation and air-conditioning) systems.
• Small industrial equipment with lower voltage requirements.
• Electronic devices requiring a stable supply voltage.

A: The choice of 50 Hz results from a combination of historical and technical reasons - essentially a compromise between motor efficiency, transmission losses and the suitability of the frequency for steam-turbine generators:
• Lower core losses - at 50 Hz, transformers and other magnetic equipment show lower core losses than at 60 Hz, giving higher efficiency and less heat.
• Longer equipment life - the lower frequency reduces electrical and mechanical stress on equipment.
• Efficient motors - motors designed for 50 Hz can be more efficient for the same output.
• Long-distance transmission - lower line losses make 50 Hz better suited to long-distance power transmission.
• Lower corona loss - the energy dissipated by the hissing and violet glow around high-voltage conductors is lower at 50 Hz.
Note: 60 Hz is standard in North America and in a few other regions. Tianrun transformers can be designed and manufactured for either 50 Hz or 60 Hz - the required frequency must be stated in the order.

A: Active power (P) is the power actually converted into mechanical, thermal or light energy and used to do useful work. Unit: watt (W) or kilowatt (kW). Active power must be balanced with demand in real time: a deficit causes the system frequency to drop (in severe cases leading to equipment shutdown or grid collapse), while a surplus raises the frequency and equally endangers equipment.
Reactive power (Q) is the power exchanged between the electric and magnetic fields of a circuit. It performs no external work but is indispensable for establishing the magnetic fields that transformers and motors need in order to operate. Unit: var or kvar. Reactive power directly influences the grid voltage: a shortage causes the line voltage to fall (particularly on long lines), while an excess causes it to rise.
Power factor (PF) is the ratio of active power to apparent power: PF = P / S, ranging from 0 to 1. The closer the power factor is to 1, the higher the utilisation of electrical energy and the lower the additional losses in lines and equipment. A low power factor increases line current and line losses and reduces the usable active output of generators and transformers, which is why grids normally require a high power factor, achieved if necessary by capacitive compensation.
Note: In short: active power is the "energy that does work" and governs frequency; reactive power is the "energy that builds the magnetic field" and governs voltage; power factor is the "yardstick of efficiency".

A: The neutral point of a power network is the neutral point of a star-connected transformer or generator. Its earthing methods fall into two main categories:
• Direct earthing of the neutral point.
• Non-earthing of the neutral point, or earthing through an arc-suppression (Petersen) coil.
In a neutral-point earthing complete set of equipment, the two earthing methods are selected by opening or closing the earthing switch. During normal operation the neutral-point voltage is close to 0 V, but asymmetric faults or three-phase unbalance give it a certain value, so a lightning arrester is installed to protect against over-voltage.
Two metal rods placed together form an air gap for lightning protection: when a lightning surge arrives or the neutral-point voltage becomes too high, the air gap discharges, which is equivalent to connecting that point to earth. The single-phase current transformer measures the current flowing from the neutral point to earth.

A: A cable is an essential component of electrical and electronic systems, used to transmit electricity or signals from one place to another. It is usually made up of several thin conductive cores, each insulated and protected by an outer sheath. Note that a single conductor on its own is not a complete cable.
Cables are classified by application as follows:
• Power cables - transmit electrical energy from generation sources to residential, commercial and industrial facilities.
• Communication cables - carry data, audio and video signals for telecommunications, internet and optical-fibre systems.
• Control cables - transmit control signals in automation and instrumentation systems.
• Radio-frequency (RF) cables - carry high-frequency signals for wireless communication, broadcasting and surveillance.
They can also be classified by conductor material (copper or aluminium), by insulation material (rubber, oil-impregnated paper or plastic) and by number of cores (single, double, three or multi-core).

A: The main transformer converts the electrical energy generated at power plants, by stepping the voltage up or down, into low-voltage energy suitable for transmission, distribution and end use. It works on electromagnetic induction: an alternating current in the high-voltage winding creates an alternating magnetic field, which is conducted through the core to the low-voltage winding and induces an electromotive force there.
Main components: core assembly (core, windings, leads and insulation), tank and cover (with oil sampling valves, drain plugs, earthing bolts, etc.), protective devices (conservator, oil level gauge, oil purifier, flow relay, desiccant breather, signal thermometer), cooling system and outlet bushings.
Applications: the three fields of the power system - transmission, distribution and consumption - as well as industry, construction and residential areas, e.g. power machinery, welding and arc furnaces, power supply and distribution, and indoor lighting.

A: Strictly speaking, the term "box-type substation" (often shortened to "box sub") refers to a compact, prefabricated substation. It integrates high-voltage power reception, transformer step-down and low-voltage distribution in one moisture-proof, rust-proof, dust-proof, rodent-proof, fire-proof, theft-proof, heat-insulated, fully enclosed and movable steel enclosure.
Classification:
• By appearance and material: wooden, stone and steel-shell box substations.
• By structure: combined type (American style) and prefabricated type (European style).
• American style is further divided into common-tank and separate-tank designs according to the tank structure; European style is divided into ring-main and terminal types.
Features: compact structure (transformer, HV and LV switchgear and protection devices in one enclosure, small footprint, easy transport and installation); high safety (fully enclosed, protected from animals and moist air, complete internal protection); high flexibility (customised capacity, voltage level and configuration, and mobile supply for temporary or emergency use).
Structure: generally composed of a high-voltage room, a transformer room and a low-voltage room, arranged either in a "T" (pin) layout or a "square" (mu) layout.
Applications: residential communities, industrial parks, shopping malls and similar locations, combining appearance with practicality.

A: See the comparison table below:

A: Dry-type transformer products are safe, flame-retardant, fire-resistant and pollution-free. They can be installed directly at the load centre, are maintenance-free, easy to install and have low overall operating cost. Their main characteristics are:
• Excellent moisture resistance - they operate normally at 100% humidity and can be returned to service without pre-drying after a shutdown.
• Low losses, low partial discharge and low noise.
• Strong heat dissipation - up to 150% of rated load under forced-air cooling (AF).
• A complete temperature protection and control system that reliably safeguards operation.
Because of these advantages, dry-type transformers are increasingly specified for high-density urban areas, commercial buildings, data centres, hospitals, rail transit and other places with stringent fire-safety requirements.

A: See the comparison table below:

A: If the lowest initial purchase price is the only consideration, an oil-immersed transformer may be considered. If safe operation, maintenance cost and environmental protection are taken into account, a dry-type transformer is recommended. In practice the decision should weigh total cost of ownership - installation space, fire protection, maintenance and operating losses - rather than purchase price alone.

A: • Core
• Windings
• Fastening and supporting parts
• Leads
• Insulation
• Cooling device - temperature controller and fan
• Protective device - transformer enclosure

A: • Dry-type power transformers
• Distribution transformers
• Traction rectifier transformers
• Special dry-type transformers such as grounding transformers and arc-suppression coils
• Non-standard (customised) dry-type transformers
• Dry-type reactors

A: See the comparison table below:

Note: Tianrun's mainstream products adopt the SC(L)B process: epoxy resin vacuum casting with a foil-wound low-voltage winding, which offers the best overall balance of safety, short-circuit strength and service life.

A: 1) Coil winding
• SC(L)B: the LV coil is foil-wound - single- or double-layer copper (aluminium) foil, with epoxy resin containing a latent curing agent and F-class composite foil (DMD paper) between layers; the conductor is oxygen-free copper of 99.99% purity for excellent conductivity.
• SC: the LV coil is wire-wound - a cylindrical coil made of several glass-fibre-insulated flat copper wires wound in parallel.
2) Resistance to sudden short-circuit
• SC(L)B: the reactance height equals the winding height, the same as for the HV coil, so the axial short-circuit force generated under a sudden short circuit is relatively small. Because the whole winding is a single wide, thin foil, it strongly resists axial tensile rupture.
• SC: multiple parallel conductors give a very large end helix angle and a strong end leakage field, producing a very large short-circuit force; unless the structure is adequately reinforced, axial damage is likely.
3) Low-voltage temperature rise - the LV winding is close to the core and dissipates heat with difficulty. The SC(L)B foil structure allows ventilation surfaces to be arranged conveniently and air flow to be increased, so the LV temperature rise is on average 5 - 7 K lower than for a wire-wound structure, improving overload capability and service life.
4) Load loss - for the same voltage class and rating, the SC(L)B foil structure reduces skin-effect additional losses by an average of 5% compared with SC.
5) Insulation performance - in the SC(L)B foil structure the voltage gradient between layers of the LV winding is very low, giving higher dielectric strength; the larger turn-to-turn capacitance also improves withstand of over-voltages and lightning impulses.

A: • Thin insulation structure with epoxy resin casting (thin-insulation type);
• Epoxy resin wound structure (wound type);
• Thick insulation structure cast with filled epoxy resin (filled type);
• Vacuum impregnation structure (open type).

A: Model designation: SC (B) (Z) 10 - XXXX / *

SCZB9-1000/10 therefore means: a three-phase dry-type on-load tap-changing transformer with a rated capacity of 1000 kVA; the HV winding is wire-wound at a voltage class of 10 kV and the LV winding is foil-wound at 0.4 kV.

A: • Type 9: no-load and load losses comply with GB/T 10228-2008.
• Type 10: no-load loss is on average 10% lower than Type 9, and load loss on average 5% lower than Type 9.

A: • Coil winding process;
• Core production process;
• Assembly process.

A: See the comparison table below:

A: • 30 - the silicon steel sheet thickness is 0.30 mm;
• Q - cold-rolled grain-oriented silicon steel;
• 130 - the maximum specific iron loss, P17/50 = 1.30 W/kg.

A: • Copper foil: common width 600 - 1000 mm, maximum thickness 2 mm.
• Aluminium foil: common width 600 - 1200 mm, maximum thickness 2 mm.

A: Tianrun dry-type transformers are generally Class F, with an insulation system temperature of 155 °C. The maximum temperature rise at Class F is 100 K, which leaves a comfortable margin for overload operation and long service life.

A: Casting from bottom to top ensures that the resin fully penetrates the interior of the insulating materials and completely covers the coils. Heating and vacuum evacuation fully degas the casting resin, preventing bubbles and guaranteeing casting quality. The casting tank can reach a vacuum of 0.2 mbar.

A: • Epoxy resin;
• Curing agent;
• Plasticiser;
• Accelerator;
• Colour paste.
For a filled resin system, a filler such as SiO2 is also required.

A: • A wound-core transformer is one whose core is made from a core roll produced on a special winding machine.
• An amorphous-alloy transformer is one whose core is made of amorphous (non-crystalline) alloy material, giving a very low no-load loss.

A: Tianrun uses a German JORG core cross-cutting line to produce the core. It offers high shearing accuracy, small burrs, multi-head feeding, automatic stacking and step-lap stacking. This reduces the number of times the sheets are vibrated and preserves the intact grain structure of the silicon steel, which lowers no-load loss and noise.

A: • Design - adjust the core proportion factor to avoid resonance frequencies;
• Materials - use high-quality silicon steel sheet to reduce magnetostriction;
• Processing - advanced core production lines keep burrs below 0.03 mm;
• Assembly - advanced five-step lamination technology minimises core joint air gaps.
At site level, two further measures are effective: erecting sound-insulation walls around the transformer, which significantly lowers the sound level; and process measures such as stepped core joints, bonding the sheared edges with resin varnish after stacking, binding and coating the core limbs with fixing varnish, and using silicone rubber pads at component connections for vibration damping.

A: Through optimised design, selection of high-quality materials and continuous improvement of processing and manufacturing techniques.

A: The 8 factory (routine) tests are:
• Ratio measurement;
• DC resistance measurement;
• Insulation resistance measurement;
• Power-frequency withstand voltage test;
• Induced withstand voltage test;
• No-load current and no-load loss measurement;
• Load loss measurement;
• Partial discharge measurement.
The 2 type tests are: lightning impulse test and temperature-rise test.

A: • DC resistance measurement;
• Ratio measurement;
• Power-frequency withstand voltage test.

A: Partial discharge is discharge that occurs only at a local position of the insulation and does not immediately form a complete breakdown or flashover through the whole insulation. Its hazards are twofold:
• Direct bombardment of the insulation at the discharge point causes local damage that gradually extends until the insulation breaks down;
• The heat, ozone, nitrogen oxides and other active gases produced by the discharge corrode the local insulation, increase its conductivity and eventually cause thermal breakdown.

A: A sudden short-circuit test is a simulation of the most severe fault a transformer may experience in service. The very large short-circuit current causes high losses, rapid heating of the windings and a sharp temperature rise; over a longer time this inflicts severe thermal damage. At the same time the electromagnetic forces acting on the transformer are very large and destroy the dynamic stability of the active part, cracking the coils.

A: It is the length of time for which the transformer can operate continuously under conditions exceeding its rated load. With forced-air cooling (AF) a dry-type transformer can run at up to 150% of rated load, and the foil-wound LV design (lower temperature rise) further improves overload capability.

A: Temperature. It is controlled by forced-air cooling and by reducing the load. Keeping the winding hot-spot temperature within the limits of the insulation class (Class F, 155 °C) is the single most effective way to ensure a long service life.

A: • Temperature controller - performs three-phase temperature scanning, controls the start and stop of the cooling fans (80 °C / 100 °C), issues an over-temperature alarm (130 °C) and an over-temperature trip (150 °C), and can transmit signals remotely to a computer terminal.
• Enclosure - prevents people and foreign objects from approaching the transformer while it is energised, avoiding danger.
• Fan - increases air flow to reduce the temperature rise caused by operating losses, ensuring continuous normal operation.

A: • The connection (vector) groups are identical;
• The rated voltage ratios are identical;
• The rated impedance voltages are identical (a deviation of ±10% is permitted).

A: Yyn0 has a star-connected HV winding and a star-connected LV winding with the neutral point brought out. Dyn11 has a delta-connected HV winding and a star-connected LV winding with the neutral point brought out.
• Dyn11 eliminates the third-harmonic voltage; because the primary side is delta-connected, the secondary neutral point is stable and can be earthed. It is the preferred connection for distribution transformers.
• Yyn0 is usually used for small-capacity transformers. For large capacities it is not recommended because of the high eddy-current losses in metal structural parts.

A: No. Their rated voltages and rated transformation ratios are different, so they cannot replace one another. The correct rated voltage must always be confirmed before ordering.

A: By means of off-circuit (or on-load) tap links on the HV winding. Taking a rated HV of 10 000 ± 2 x 2.5% V as an example, the tap positions correspond to 10 500 V, 10 000 V and 9 500 V. For models with on-load tap-changing (code Z), the tap can be changed under load.

A: Before energising, thoroughly clean the active part of the transformer and carry out insulation measurement. Dust, moisture or foreign objects on the insulation surface are the most common causes of arcing and discharge at commissioning.

A: • Rated capacity (kVA);
• Voltage ratio (kV);
• Short-circuit impedance (Uk %);
• Load loss and no-load loss (or the performance level code);
• Connection (vector) group;
• No-load current;
• Insulation level;
• Protection grade (IP code);
• Frequency (50 Hz / 60 Hz);
• Service conditions (altitude, ambient temperature);
• Cooling method (AN / AF);
• Cable entry and exit arrangement, and accessories.

A: Example: a customer orders a standard SCB10-2000/10/0.4, Dyn11, Uk = 6%.

A: • Bottom-in bottom-out
• Bottom-in top-out
• Bottom-in side-out
• Top-in top-out
• Top-in bottom-out
• Top-in side-out
• Side-in side-out
• Side-in bottom-out
• Side-in top-out

A: • Standard horizontal side outlet - the outgoing busbar is arranged parallel to the transformer.
• Standard vertical side outlet - the outgoing busbar is arranged perpendicular to the transformer.

A: • A non-standard product requires redesign of the transformer body and of the moulds, adding a design cycle and a mould production cycle as well as tooling cost and new-product development cost.
• Most non-standard products also need higher-cost materials - for example, reinforced insulation for high-altitude applications.

A: • Clean the active part thoroughly and carry out insulation measurement (see D12);
• Carry out the on-site tests listed in D2: DC resistance, ratio and power-frequency withstand voltage;
• Verify the temperature controller settings (fan 80 °C / 100 °C, alarm 130 °C, trip 150 °C);
• Confirm the enclosure protection grade, earthing and cable connections.

A: IEC (International Electrotechnical Commission) currently has 89 technical committees (TC) and 107 sub-committees (SC) and has developed thousands of international standards covering electrical, electronic and related technologies. By the end of 2000 IEC had already published 4 885 international standards, and the number is considerably larger today.
Major technical committees and their core standards:

A: The familiar "IP protection grade" is defined by IEC 60529. It specifies the degree of protection provided by an enclosure against the ingress of solid foreign objects (including dust) and liquids (water). It is used very widely:
• Consumer electronics - e.g. IP68 for mobile phones and watches;
• Industrial equipment - e.g. mining machinery that must exclude dust, water or corrosive liquids;
• Outdoor facilities - outdoor luminaires, typically IP65, exposed to wind, rain and sand;
• Medical equipment - devices that may be cleaned and disinfected must exclude liquids.
Note: For dry-type transformers, common enclosure grades are IP00 (no enclosure), IP20 (standard indoor enclosure) and IP30; higher grades are available on request.

A: • Direct adoption - some countries or regions adopt IEC standards directly as national standards.
• Modified adoption - more commonly, countries slightly modify IEC standards or adopt them as equivalent, converting them into their own national standards (for example China's GB standards).
• Conformity assessment - IEC itself does not certify, but it has established three conformity assessment systems: IECEE (CB Scheme, mutual recognition of safety testing and certification), IECEx (certification for explosive atmospheres) and IECQ (quality assessment of electronic components).
Companies normally contact the national standards body or an authorised certification body to obtain standard texts and to arrange product testing and certification.

A: • Basic standards - terminology, units and symbols, graphical symbols, terminal markings, standard voltages, current ratings and frequencies, insulation coordination, insulation systems;
• Raw material standards - working fluids for electrical instruments, insulating materials, measurement methods for electrical properties of metals;
• General safety, installation and operation standards - electrical installations in buildings and in severe outdoor conditions, equipment for explosive gas atmospheres, electrical equipment in industrial machines, enclosure protection, live-working tools;
• Measurement, control and general testing standards - energy measurement and load control, measurement of basic electrical quantities, industrial process measurement and control, nuclear instrumentation, instrument transformers, high-voltage test techniques;
• Power generation and utilisation standards - rotating machines, steam turbines, transformers, power capacitors, relays, photovoltaic systems, electric traction, welding, electro-heating, electric vehicles;
• Power transmission and distribution standards - switchgear and controlgear, wires, low- and high-voltage fuses, surge arresters, telecontrol, protection and communication equipment;
• Electronic component and assembly standards - semiconductor devices and integrated circuits, capacitors and resistors, miniature fuses, cables, electromechanical components, magnetic components;
• Telecommunication, electronic system, IT standards - radio communication, information technology equipment, data processing equipment, audio/video systems, medical electrical equipment, measurement and limitation of radio interference, alarm systems, navigation instruments.