Step-up and step-down transformers utilize the same physical principles via electromagnetic induction to alter AC voltage at them but are located on either side of the electrical network and are opposite to one another when it comes to functions they perform in terms of voltage and current. Although they work with the same mechanism, it does not mean that they differ in terms of quality of construction. Rather, the whole difference between them lies in the fact that one transformer has a coil with more turns than the other and this will determine how the transformer is going to affect voltage and current depending on the place of installation and it will also have a tremendous influence on the price of the machine. This article explains all the features of the types of transformers and gives practical advice to their buyers.
The answer is as follows: The step-up transformer is designed with more coils wound on its secondary than on its primary, resulting in a higher voltage at the output than at the input. The same principle is applied to reduce resistive losses by transmitting power at higher voltage using this method. A step-down transformer has fewer turns in the secondary than in the primary; therefore, it produces lower voltage with proportional increase in the output current. Both types of transformers are based on the same equation Vs/Vp = Ns/Np; however, with current being inversely proportional. A transformer will function as either type, if installed the other way around, but step-down transformers may not always produce their rated output when turned into step-up ones due to the winding construction, insulation, and cooling system related to one specific mode of power flow.
What the Two Types Are
Both devices are types of static machines. Each has two or more windings on a magnetic core. The primary winding carries an alternating current and produces a changing flux within the magnetic core. The flux produces an induced voltage in the other, called the secondary winding. The two windings are electrically not connected to each other in a two-winding transformer. In both devices, there are no moving parts.
The only design feature that separates a step-up transformer from a step-down transformer is the turns ratio. In the case of the secondary winding having more turns than the primary winding, the induced voltage in the secondary winding becomes higher, and the transformer is a step-up transformer. If the turns in the secondary winding are less than in the primary winding, it means that its voltage is lower and the transformer is a step-down transformer. All other features of a transformer refer to the way the mentioned turns ratio is realized.
It is important to say that this is the exact point of a rather widespread misunderstanding. The point is that a step-up transformer is not more powerful than a step-down transformer. The fact of a step-up transformer having a power rating of 500 kVA does not mean that it will be a more powerful machine than a step-down transformer of the same power.
How a Step-Up Transformer Works
The usage of a step-up transformer means that the electricity supply is fed into the winding having the smaller number of turns and the utilization is taken from the winding with a larger number. If the turning of the primary is 100 and that of the secondary is 1,000, then the turns ratio gives 1:10 and the voltage at the secondary point of the transformer should be ten times higher than that at the primary point, with some exceptions concerning losses and regulation.
The main point is the effect on current. Since a transformer does not create but only transfers power, if the voltage increases, the current decreases. For instance, if a step-up transformer passes voltage of 10,000 volts from a supply of 1,000 volts, the current will drop to one-tenth of the initial current in the primary winding. This relationship is not a drawback of the transformer, but its main objective as far as it is clear that the higher voltage means the smaller current according to the general physical rule.
Therefore, one may find the bit of the step-up transformer in all power systems from the side of power generation. The generator constructs electricity at a voltage restricted by insulation or more precisely from 11 kV to 25 kV in big generators. After that, step-up transformer increases this voltage to the required transmission value, which may constitute from 275 kV to 400 kV and more. The fundamentals of how these machines are built and where they sit in a network are set out in the basic construction and role of distribution transformers, which share the same principles at a smaller scale.
How a Step-Down Transformer Works
In a step-down transformer the order of windings is reversed, with the supply applied to the winding with the larger number of turns and the load taken from the winding with the fewer turns. In the example of a step-down transformer with the ratio of 10:1, when fed with 10,000 V it will supply 1,000 V, while the output current increases also with the same factor of 10.
One of the reasons for the existence of step-down transformers is that there is a great difference between the voltage that is efficient for transmission from the point of generation to the point of use. While for long-distance transmission the transmission voltage of 275 kV may be employed, no buildings, equipment or apparatus can actually be connected to such high voltage. The voltage must be decreased gradually to the level that is suitable for practical use. For this purpose, it can be reduced in many stages from the transmission to the sub-transmission, then primary distribution and finally down to the level that is acceptable for installations. Several step-down transformations may have to be performed before the supply is established.
Certain features of the working of a step-down transformer should be mentioned. First of all, if voltage decreases, the current carries much higher value at the low voltage winding than at the high voltage winding, and that’s why the low voltage winding must be made heavier than its high voltage counterpart – this is why it is easy to recognize the low voltage winding of the transformer. Secondly, voltage stability is felt to be more important at the output since the installation is tolerable only to a certain range of voltage.The full set of nameplate and specification parameters, and how each one affects behaviour in service, is explained in the interpretation of transformer specifications.
Side-by-Side Comparison
| Parameter | Step-up transformer | Step-down transformer |
|---|---|---|
| Turns ratio | Secondary turns > primary turns | Secondary turns < primary turns |
| Voltage | Output higher than input | Output lower than input |
| Current | Output lower than input, in inverse proportion | Output higher than input, in inverse proportion |
| Power | Unchanged apart from losses | Unchanged apart from losses |
| Primary function | Raise voltage to reduce transmission losses | Reduce voltage to a usable level |
| Typical location | At generation, and at points where voltage must be raised for distribution | At substations, at building supply points, and at every point of use |
| Typical voltage pairs | 11-25 kV to 132-400 kV | 132-400 kV down to 11-33 kV, then to 400 V or 230 V |
| Heavier winding | Secondary (high voltage), though insulation dominates | Secondary (low voltage), because current is higher |
| Insulation emphasis | Heavy — the high-voltage winding drives the design and the clearances | Moderate on the high side, minimal on the low side |
| Typical relative cost at equal MVA | Higher, because of insulation and clearance requirements | Lower for the same apparent power rating |
| Efficiency | Very high, typically 98-99% at large ratings | Very high, typically 97-99% depending on size |
| Reversible in principle | Yes, if back-fed within design limits | Yes, if back-fed within design limits |
One row requires some elaboration because it surprises customers. If both types of transformers handle the same power, why is a step-up transformer more expensive than its counterpart? The answer is in the insulation system. A transformer designed with a high voltage secondary must handle the voltage in winding insulation, bushings, clearances to earth and impulse withstand. These requirements are dependent on voltage rather than power. Thus, a 50 MVA 11 kV/132 kV step-up transformer would usually be large and with better insulation and more expensive than a 50 MVA 132 kV/11 kV step-down transformer although both units may seem to be alike on specification.
How They Differ Physically
The external appearance of a step-up and a step-down transformer with the same specifications can be confusing due to the similar construction. And yet, there are certain features the transformer has that help determine its type.
- Bushing height and size. Thanks to the high-voltage winding, the bushing design is different. With that in mind, it is easy to tell which side is used for high-voltage transmission. In a step-up transformer that would be the secondary winding, while in a step-down transformer, it would be the primary one.
- Winding conductor cross-section. If there is an opportunity to make a visual check, it is possible to say whether the winding belongs to a step-up transformer or a step-down transformer by the thickness of the conductors.
- Cooling arrangement. In case of the larger units of both speculations, there are radiators, fans or pumps to provide oil for cooling, but it is not a distinguishing feature for both types, as the cooling class is given in the rating plate.
- Taps used in the winding. The tap changer is usually installed on the high-voltage winding, as this construction allows fine tuning of the transformer for the same number of turns in the winding.
In all cases, the nameplate proves to be of great help in deciding which type of the transformer is available.
Where Each Is Used
The two types occupy opposite ends of a chain that every unit of electricity passes through.
| Stage | Transformer role | Typical rating |
|---|---|---|
| Generation | Generator step-up transformer raises machine voltage to transmission level | 50-1,000 MVA |
| Transmission | Step-up and step-down at interconnection points between voltage levels | 100-1,000 MVA |
| Sub-transmission | Step-down from transmission to sub-transmission voltage | 20-200 MVA |
| Primary distribution | Step-down from sub-transmission to distribution voltage | 5-50 MVA |
| Distribution substation | Step-down to utilisation voltage for an area or a large site | 0.5-5 MVA |
| Point of use | Step-down to the installation’s operating voltage, or local step-up for a specific machine | 0.025-2.5 MVA |
| Specialist local use | Step-up for a specific process or test requirement, such as electrostatic precipitators or test benches | Varies widely |
It can be inferred from the last two rows that both forms of transformations can occur in the same factory facility. A factory can receive electricity by the step-down transformer and then it may have a small step-up transformer to use it for a specific process such as electrostatic precipitators or dielectric test sets, or special equipment which requires higher voltage than what is supplied through the distribution. The direction of the transformation depends on the needs and not on the place where it is located; thus, the sizing problem has to be solved based on the requirements in terms of voltage and power.Getting that arithmetic right is a prerequisite for specifying anything correctly, and the method is set out in the approach to transformer sizing and load calculation.
Types and Configurations
The construction family of the transformers does not tally with the direction of transformation, since both step-up and step-down transformers belong to the same family of construction.
- Single-phase. Made up of two windings on one core, widely used for home and small equipment supply and distribution, commonly used for smaller capacities.
- Three-phase. Consisting of either three single-phase transformers, or one three-phase transformer which has three windings on the same core. The second design favours the majority of transformer constructions due to its inherent capacity for powering transformers of very large rating and in providing installation with other parameters related to transformer operation.
- Autotransformer. A transformer with a single winding on tap that has electromagnetic coupling of circuits. It is the most cost-effective solution when compared to a two-winding transformer for similar purpose.
- Isolation transformer. A transformer with two windings and 1:1 turns ratio designed for specific needs of galvanic isolation.
- Dry and oil-filled types. Their distinction lies in the location where they are used: the dry type can work in closed spaces and the oil-filled ones are mostly employed for transformers that are big enough and have higher capacities, since oil cooling is more effective than air one.
- Pad-mounted and pole-mounted transformers, or transformers in substations. They are rather concerned with the installation methods than with the principle of transformers’ work.
For work done over three phases, the way the windings are connected — whether through delta or wye — significantly affects how phases of primary and secondary work together, along with whether a neutral is there and how unbalanced loads and triplen harmonics are handled by the transformer. An example would be a delta-wye connection that offers a neutral at the secondary and blocks the zero-sequence current from passing through it.The practical implications of each combination are significant enough to drive the specification, and a representative example of how a three-phase unit is presented for procurement is the three-phase transformer range, where voltage ratio, connection group, impedance and cooling class appear together because they must be selected together.
Prices and Cost Drivers
Transformer pricing is conditioned by similar factors in the opposite direction, where direction of the installed unit does not matter as much as voltage class or voltage rating.
| Cost driver | Effect on price | Why |
|---|---|---|
| Power rating (kVA or MVA) | Dominant driver; price rises faster than rating | More core steel, more copper, more oil, larger tank |
| Voltage class | Major driver, especially on the high-voltage side | Insulation, clearances and bushing costs scale steeply with voltage |
| Cooling class (ONAN, ONAF, OFAF) | Moderate | Radiators, fans and pumps add cost but raise usable rating |
| Impedance specification | Moderate | Low impedance costs more in material; affects fault level and regulation |
| Tap changer (off-circuit or on-load) | Significant for on-load | On-load tap changers are complex mechanisms with their own controls |
| Loss specification | Significant | Low-loss designs use more active material to reduce operating cost |
| Connection group and neutral arrangement | Minor to moderate | Affects winding configuration and any neutral bushing |
For the buyer, the key factors are that when comparing price offers from multiple suppliers, a product specification should be taken into account, since, after all, cheap price often translates into cheaper voltage class, small cooling margin or higher losses than in a good deal; and that the price of the product is only one part of the total cost, since losses are paid throughout the entire service life of the product. Reconciling capital cost against lifetime energy cost is the central exercise in transformer procurement, and the realistic price bands for each rating and class are set out in the cost structure for electrical transformers.
Can a Step-Down Be Used as a Step-Up?
In theory, it is possible. A transformer is a reversible device and turning the supply towards what is being called its secondary will mean that the voltage on what is being called primary will be higher than expected. On some occasions, this is done on purpose in case of emergency.
However, in reality the following four points define the reason why this is a bad idea unless made as part of a rational engineering choice. They are usually installed on the high-voltage winding which implies that reverse feeding of the transformer will situate them in unsuitable places for voltage adjustment. The insulation and distances were made with the assumption that the high voltage will appear on a certain winding, but back-feeding can expose this voltage to a wiring arrangement not convenient for it. The parameters for cooling and current ratings were set with a certain current distribution through the windings assumed, which will now turn when the power flow gets reversed. The protection and earthing system that would normally be designed for a particular direction of power flow will not be suitable for reversed flow either.
To summarize, reversal is theoretically possible to do, may be appropriate in certain situations, but is better clarified with the manufacturer rather than assumed. If one does require both directions at site, using a transformer properly selected for the job would be a much smarter approach than compromising by using whatever transformer one happens to have.
How to Tell Which You Have
Four tests in an order of reliability.
- Check the rating plate. It states the rated voltage of each winding as identified by terminal markings as well as the rated power, type of connection group, impedance, and the class of cooling. This is the most accurate test and can be carried out within seconds. If it is not possible to identify rating plate because of its illegibility or absence then other tests will become just approximations.
- The next test is to make a comparison of bushings and the clearance. The high-voltage side is identified by the side where bushings are larger and spacing and clearance to the earth are wider.
- Make a check of the size of the conductors whenever it is possible to see it. The heavier conductor belongs to the winding with the higher current and lower voltage.
- Trace the connection. Follow the supply line to find out which winding is the primary one and compare it with the rated voltage of the winding being investigated.
There are two remarks about field identification. If a transformer has a ratio of 1:1, for example, that is the isolation transformer, it does not step-up or step-down voltage and an attempt to identify the transformer by the direction will yield only a wrong result. Moreover, a transformer having many taps can be connected in such a way as to change its nominal ratio. Thus what is shown on the plate may not reflect the actual situation.
FAQ
What is the difference between step-up transformers and step-down transformers?
The differentiation resides in the number of coils in winding. A step-up transformer has a greater number of coils in the secondary coil because of which the voltage of the output is greater than that of the input as well as the current is less proportionately. The purpose of the step-up transformer is to boost the voltage level to ensure the efficiency of power transmission. Meanwhile, a step-down transformer decreases the number of coils in its secondary coil leading to the decrease of voltage and increase of current in the proportion. The step-down transformer is used to lower the voltage to an appropriate level. Though both transformers operate at the same efficiency level, the following equation is true for both types of transformers Vs/Vp = Ns/Np.
What is the disadvantage of a step-up transformer?
There are three drawbacks that should be taken into account. To begin with, with the same power rating, costs tend to be higher than for step-down transformers as the high-voltage winding uses insulated parts and clearance materials that are connected with voltage parameters rather than power output. Next comes the risk that is connected with the potential failure of insulation systems due to the greater power of possible failures in the case of using high voltage equipment. Thirdly, when one uses a step-up transformer supplying a long cable, it is necessary to address voltage rise at the cable ends due to low voltage load conditions. Despite the fact that step-up transformers are quite reliable under normal conditions, all of the indicated factors affect the process of their specification and protection.
Can a step-down transformer be used as a step-up transformer?
In terms of functionality, the transformer can certainly operate in a reverse mode as there are situations where this is done in emergency situations. However, a device that has been intentionally built for use as a step-down transformer will usually not be able to achieve its full capacity when connected to the line in reverse mode because of numerous factors including but not limited to the design of the transformer in terms of winding construction, number of taps in use, and cooling arrangements. In reverse operation, the taps will usually remain on the primary (high voltage) side which may be regarded as inappropriate for operation. It is always advisable to check the reversibility option with the manufacturer and the devices that are made specifically for this purpose should be installed rather than recompleting the one that was not made for reverse operation.
How to tell if it’s a step-up or step-down transformer?
Examine the rating plate, which provides the rated voltage for each winding and is conclusive. In the absence of the rating plate, compare the bushings; the side with the bigger bushings, greater spacing and larger clearance from earth is said to be the high-voltage side, meaning that it is the side to which the transformer steps up and down from. In instances where the conductors are visible, noting the sizes of the conductors is the second clue; since the larger conductor is used for the winding with the higher voltage and lower current. Then follow the supply line to find out which of the windings is in fact the primary, as transformers with various taps may not always be installed in the configurations that results in the effective ratio being same as in the nameplate specifications.
Which transformer is used in a substation?
In both cases, the substation serves the reciprocal function. In the primary substation, transmission voltage is lowered to the level of distribution voltage. Therefore, all transformers in this type of substation are called step-down transformers. In a generation facility, the substation serves a different purpose since it is raising the voltage produced by the generator to the level of voltage transmission by using a step-up transformer. When it comes to interconnection substations, they might work in both modes, as they may contain both step-up and step-down units depending on the direction in which electricity flows most frequently and switching the flow if the flow does not change at the generation and demand points.
References
- International Electrotechnical Commission — IEC 60076 Power Transformers
- IEEE — Transformer Connections, Voltage Regulation and Loading Guides
- ANSI — ANSI C57 Series Power Transformers, Regulators and Reactors
- NFPA — National Electrical Code, Article 450 Transformers and Transformer Vaults
- U.S. Department of Energy — Transformer Efficiency and Distribution System Losses
Conclusion
Step-up and step-down transformers are different sides of the same coin. The only difference is the number of turns in each transformer. In step-up transformers the secondary winding has more turns than the primary winding; thus, the transformer raises voltage and lowers current. Electricity transmission at high voltage is far more effective than doing so at low voltage — which is why the step-up transformer was invented in the first place. The step-down transformer on the other hand has fewer turns on the secondary winding and lowers voltage while raising current. Basically, there is no such thing as a transformer that could be connected to the transmission line. Both transformers transfer the same amount of power except for losses involved, and they obey the same formula Vs/Vp=Np/Ns. They are built in the same manner — the only thing that differentiates them is their position in the electric power transmission system and that makes step-up and step-down units price different. The step-down unit can be used as a step-up transformer but its efficiency will be the lowest because it is designed to work only in one way.