Evaluating power transformers: Don’t just stare at the kVA on the nameplate

The project load list is 900 kW and the alternative transformer nameplate is 1000 kVA. Looking at the numbers, it seems that there is still room-is this equipment directly usable? Don’t rush to compare the size first. kW and kVA are not the same thing at all, and the rating on the nameplate has a set of prerequisites. As long as the parameters are not correct, the “margin” on the paper may not exist at all.

When evaluating power transformers, the easiest pit to step on is also here: people often only remember a capacity number, but forget to check the corresponding voltage, current, cooling method and load conditions.

Unify the Load to the Same Caliber

The transformer is usually calibrated with kVA or MVA because its rated capacity is limited by both voltage and current. The insulation system has to withstand the voltage, and the winding heating is mainly linked to the current. If the effective power of the load is only expressed in kW, the power factor is missed. It is obviously unscientific to use it directly to compare with the kVA on the nameplate.

Therefore, when we see 900kW alone, we still cannot conclude that 1000kVA is absolutely enough. You must first find out: is this 900kW the maximum continuous load or short-term peak? Is the power factor using the design estimate or the measured value? Is there any inrush current when the motor starts in the load, or harmonics brought by rectifier, frequency converter and UPS equipment? Find out all these details, and then convert the active power into the apparent power of the load, so that the two figures are really comparable.

In addition, it is necessary to distinguish between “power” and “power consumption. KVA is not a unit used to calculate electricity bills. Even if it is the same transformer, as long as the load rate and power factor are different, the actual transmission of active power is not the same, the loss is naturally very different.

The Capacity Is Right, and the Current on Both Sides Must Be Checked

For three-phase transformers, the relationship between rated capacity S, line voltage U and line current I can be checked by applying the formula S = √ 3UI. In the case of the same capacity, the lower the voltage, the larger the corresponding current. Whether the cables, bus bars, connection terminals and protection devices on the low-voltage side can withstand such a large current is often exposed earlier than “whether the capacity is enough.

After getting the nameplate, it is best to honestly write down the rated voltage on the secondary side of the 1., the rated current on each side, the frequency, the number of phases, the winding connection group and the tap changer position, and then take it to the single-line diagram to check one by one. Don’t just write a “high pressure side” or “low pressure side. If parallel operation is required, the variable ratio, phase sequence, connection group and impedance voltage must all be matched, otherwise there will be circulation or uneven load distribution.

The 1 column of frequency is not a decoration either. It must not be passed at a glance. If the voltage does not change and the frequency drops, the ratio of V/Hz will rise, which can easily lead to over-excitation of the iron core. Don’t think that the equipment can turn at this frequency, change the frequency can directly apply the original rated voltage and capacity.

The Nameplate Rating Is Valid Under Specific Operating Conditions

The so-called rated capacity refers to the apparent power that the equipment is allowed to continuously transmit under the specified cooling method, temperature rise limit, ambient temperature and frequency conditions. To put it bluntly, it is not a rigid commitment out of context.

Take dry-type transformer, natural air cooling and fan forced air cooling, can allow the capacity is two different things. If the fan is broken, the original capacity supported by strong cold will not be able to be retained. Changes in the ventilation path of the enclosure, poor exhaust in the data center, elevated ambient temperature, and even changes in installation altitude will directly affect the heat dissipation of the equipment.

“There is no universal percentage in the world that can specify the maximum allowable load rate for all equipment.” Continuous load, periodic load and short-term overload have completely different meanings. Exceeding the nameplate rating in a short period of time does not mean that the equipment can be operated for a long time. When making this kind of evaluation, you have to take into account the initial temperature, overload duration, hot spot effect, insulation system grade, and even the bearing limit of leads and accessories.

Don’t Simply Add All the Loads Together

If there are both ordinary lighting loads, frequency converters and rectifiers on a load meter, if the power of their nameplates is increased by one light, the key factors will definitely be missed. The non-sinusoidal current will increase the eddy current loss and other additional losses of the winding, and the temperature of the local hot spot may be much higher than the expected value calculated only by the fundamental current. Motor starting, frequent shock loads, or periodic operation can also cause the difference between the peak and average values to be a thousand miles.

To take over this kind of project, we must know the load waveform, harmonic spectrum, working system (temporary load rate), peak value and simultaneous coefficient, and we must not only stare at a total power figure. In a three-phase system, it has to be checked phase by phase. The total kVA does not exceed the upper limit, which cannot prove that the current and temperature rise of each phase are within the safety line. What’s more, load imbalance will also involve neutral current and overall voltage quality.

Even if You Change the Equipment with the Same Capacity, Things Will Not End

The old equipment is 1000 kVA and the new equipment is 1000 kVA, but this does not mean that they can be directly replaced on site. The impedance percentage not only determines the short-circuit current and the voltage drop after loading, but also relates to the load distribution during parallel connection. Insulation level, tap range, cooling method, environmental level and terminal outlet direction must also be carefully aligned with the existing system. This is especially true for multi-winding products. Don’t take a total capacity to summarize all operating conditions; the capacity, voltage, current and impedance between windings of each winding must be clearly listed separately.

Before asking for an inquiry from the manufacturer, please be sure to integrate the nameplate data, single line diagram, actual load curve and site environment into a technical input document. First, the normal operation, peak and emergency state are clearly defined, and then the voltage, frequency, power factor, harmonics, impedance, cooling requirements and installation boundaries are clearly defined. If the supplier only gets the 1 dry sentence “need a 1000kVA transformer”, then most of the deviations that come out later cannot be remedied by slightly increasing the capacity.