Selection of power transmission transformers: First, clearly define their role within the power system

Project documentation often only specifies voltage and capacity. Looking at these two parameters alone, it seems enough to make inquiries, but once you enter the technical communication link, you will immediately get stuck: is this equipment connected between the generator and the transmission grid? Or is it used to connect two transmission voltage levels? Or is it hung in the terminal distribution link close to the load? You should know that the location of the equipment is different. Once a system failure occurs, the consequences, including voltage regulation, grounding, transportation and even power outages, will all change greatly. Instead of arguing at the beginning about whether a certain capacity is “large” or not, it is better to simply go back to the single-line chart and have a look.

Therefore, when a request for quotation simply states “power transformer,” the most reliable approach is to first attach a single-line diagram. The drawings clearly and explicitly indicate the power supply, busbars, power flow direction, as well as upstream and downstream equipment. After all, a name can only represent a broad category and cannot possibly substitute for the specific project boundary conditions.

clarify its specific role in the system

The generator step-up transformer is responsible for pulling up the generator terminal voltage to the transmission voltage level. The transmission or contact transformer connects different transmission voltage levels, and the trend direction in the middle will change back and forth with the operating conditions of the system. Distribution transformers are even more closely tied to end users; their primary function is to step down medium voltage to the low voltage required by commercial, residential, or industrial loads. In fact, there is considerable overlap among the capacity ranges of these three, so you should never draw conclusions based solely on the rated capacity.

Also, don’t let those similarly sounding names fool you. The phase-shifting transformer for transmission is to change the phase angle at both ends of the line by injecting a controllable orthogonal voltage, in order to regulate the AC active power flow, while the fixed phase-shifting transformer, which supplies power to the front end of the rectifier or inverter, is based on multiple sets of secondary windings with different phases to cooperate with multi-pulse rectification. Although their names both include the term “phase shift,” their original design purposes and the input conditions they require are vastly different.

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The so-called “primary winding” is the side that is connected to the power supply during operation, while the “secondary winding” is the side that supplies power to the load. Note that these designations are not rigidly tied to “high‑pressure side” and “low‑pressure side.” When dealing with communication devices or systems that feature reverse-flow protocols, misunderstandings can easily arise—like talking at cross-purposes. The safest way is to mark the input, output, high pressure side, low pressure side, and the power flow direction under normal and special operation modes in black and white on the drawing.

The turns ratio also cannot be simply and roughly calculated by dividing the two nominal voltages. The actual supply voltage range, target load voltage, tap changer strategy, winding resistance, leakage inductance, load current, and power factor all affect the final terminal voltage. In addition, capacity is based on apparent power; since the active power of the load is known, the power factor, efficiency, harmonics, start-up or impact load, and even future expansion plans are all counted under the same caliber.

Translate the inquiry criteria into verifiable data

1 a reliable technical input, at least the rated voltage of the primary and secondary sides, the maximum voltage, frequency, phase number, grounding mode and acceptable voltage deviation of the equipment must be specified. Next, we will finalize the connection group, tap range, impedance percentage, insulation level, and whether parallel operation is required. 1 more words here, the impedance percentage is not the bigger the better, nor the smaller the better; it will also involve the size of short-circuit current, load voltage drop, and current distribution between parallel loads.

Also, never delay addressing short-circuit conditions until after you’ve submitted your quote and then proceed with the calculations. The expected short-circuit current at the installation point, how long the fault can last, how long the protection device can be removed, and whether multiple through faults will occur, all of which should be included in the verification of short-circuit tolerance and protection coordination. If the project requires parallel operation, it is even more necessary to check the voltage ratio, phase sequence, connection group and phase shift angle, frequency, impedance and capacity difference item by item. For less than one item, there may be trouble with circulation or uneven load distribution.

The structure and cooling method must be determined by working backward from the actual on-site conditions

Whether to choose a dry-type or a liquid-immersed (oil-filled) transformer is definitely not something that can be dismissed with a simple “indoor or outdoor.” You need to evaluate, based on the specific installation site, all factors including fire and leak boundaries, energy density, maintenance requirements, noise levels, footprint, and the total life-cycle cost. If a dry-type transformer solution is proposed, the site conditions regarding humidity, condensation, dust, salt spray, corrosive gases, and ventilation must also be clearly specified. If the enclosure’s protection rating is later upgraded, whether the airflow resistance and temperature rise can still accommodate the rated capacity will need to be re-verified.

Forced cooling does indeed enhance heat dissipation, but it also means the system becomes more reliant on fans, pumps, power supplies, control systems, and routine maintenance. If the equipment is divided into several cooling stages, the allowable capacity, standby power supply, alarm mechanism, fault capacity reduction and on-site heat exchange conditions corresponding to each stage must be stated one by one. If even the cooling conditions aren’t specified and only the total capacity is thrown out, people discussing the technology won’t even be on the same page.

Compare all the proposals under the same project scope

During the vendor comparison phase, all candidate solutions must be evaluated under the same set of electrical, environmental, and operational conditions. In addition to the most intuitive purchase price, keep an eye on these hard indicators: guaranteed loss, efficiency, temperature rise, impedance tolerance, noise, size, weight, test range, document delivery, and deviation table. Transformer this year-round live operation of the equipment, its no-load loss, load curve, maintenance cycle, spare parts inventory, installation ease, even the risk of downtime, all in the whole life cycle of the total cost.

Finally, go back and 1 through the single line diagram: which two systems is this equipment connected? How does the trend go under normal and abnormal conditions? What level of protection is the fault removed? If the cooling auxiliary machine is disconnected, how much load can it carry? Until these four core issues are thoroughly clarified, no matter how fancy the model name is, it is not enough to become the 1 reliable transformer specification that can be used for design and quotation.