In essence, a step-up transformer is a device used to increase the voltage from an incoming line to a level required for further electrical energy transmission. However, the same definition also includes over-voltage, i.e., an abnormal situation when output voltage exceeds the rated one due to improper tap changing or loosening of the power system’s neutral. Although there are two situations described in the paragraph, their nature is quite different, and it may lead a technician to the wrong solution of the problem, like adjusting a tap changer when an open neutral is the root cause. The objective of this manual is to provide a thorough understanding of how a transformer works as a raising voltage device, what happens to electrical current and electrical load while increasing voltage of the line and what effects are produced as a result.
In brief: When a transformer increases the voltage, the voltage at the secondary side becomes higher than the voltage at the primary side according to the ratio of the number of turns — Vs/Vp = Ns/Np — while the current becomes lower by the same ratio. This is because the transformer transfers power but does not generate power. This aspect is the first and the most important consequence of the transformers. After this first consequence, there are four more consequences. The insulation has to withstand increased dielectric stresses, meaning that the winding operating at higher voltage has to have better insulation. Resistive equipment receives more energy due to the square of the voltage growing, meaning that if the voltage rises by 5%, heating in resistive equipment rises by almost 10%. An incandescent lamp will stop glowing if the voltage exceeds the permissible voltage level. The core inductor saturates when the voltage becomes far and beyond the design range.
How a Transformer Raises Voltage
Transformers are devices that are made entirely of stationary components and have no electrical connection between their windings. It works through electromagnetic induction; in other words, in the primary winding, there is an alternating current, and therefore changing magnetic field; then that changing magnetic field creates a voltage in the secondary winding. The voltage that has been generated is equal to the primary voltage multiplied by the ratio of secondary and primary windings.
The voltage generated in the secondary winding of a transformer will depend on the relationship of the turns of its wires to the turns of the primary winding. For instance, if the turns of wire in the secondary winding are double those in the primary, it will hold true to say that the voltage in the secondary coil is also double the voltage in the primary coil. It is this relationship between the turns ratio that gives the ability to a transformer to step voltage up or down and which explains benefits such transformers bring, including using them in a reverse way.
The only limitation of the size and capacity of transformer is determined by the core design. The magnetic flux density within the core grows proportionally with the voltage applied to the coil. Therefore, the higher the voltage, the more saturated the core becomes. Thus, when the core reaches the point of saturation, the line of flux cannot flow through it.Understanding the basic construction that produces these limits is the starting point, and it is set out in our guide to distribution transformer basics.
Where the ratio is fixed at manufacture, the parameter that governs the actual output voltage is the number of turns in circuit on the tapped winding — the reason tap changers exist. Those settings and the rest of the nameplate data are the subject of our explanation of transformer specifications, which is where a buyer should look before assuming a voltage problem is a fault.
The Conservation Rule: Voltage Up, Current Down
The most significant result of increasing voltage is often overlooked: the current decreases.
An ideal transformer conveys energy with no loss and therefore the value of the voltage times the current on the two sides must be identical. When the voltage at the secondary goes up, the secondary current flowing at the same current at the primary goes down correspondingly. In other words, if a transformer gives a current of 100 A at 240 V on the secondary side while consuming 200 A at 120 V on its primary side, the power on both sides is equal since no new current is produced.
| Quantity | Relationship | Consequence of raising secondary voltage |
|---|---|---|
| Voltage | Vs/Vp = Ns/Np | Rises in proportion to the turns ratio |
| Current | Is/Ip = Np/Ns | Falls in inverse proportion |
| Power | VpIp ≈ VsIs | Unchanged, apart from losses |
| Impedance seen by the source | Zp = (Np/Ns)² × Zs | Transformed by the square of the turns ratio |
The impedance row is important to consider because it is the most counterintuitive. A load on the secondary will be seen by the source as not itself but multiplied by the turns ratio squared. With a ten-fold increase in voltage, a 1Ω load looks like 0.01Ω from the primary. This is how impedance matching occurs in audio and RF circuits and is exactly the reason for the phenomenon of a step-up transformer with some amount of fixed load pulling much higher primary current than one may expect from simple calculations.
In real life, a real transformer does not achieve 100% conservation. Both copper losses in the windings and core losses in the core material dissipate in the form of heat, which is why the efficiency is expressed in percentage rather than assumed to be one and the reason transformers become warm under load while they stay cool in an open circuit.The losses themselves are among the mechanisms that eventually limit a transformer’s life, a subject covered in our article on what causes transformers to fail.
What Happens to Impedance and Load
The nature of the connected load completely governs how the connected load reacts, and thus the result of the rise of voltage is turned from a design specification to a matter of operational concern.
| Load type | Behaviour when voltage rises | Result |
|---|---|---|
| Resistive (heaters, incandescent lamps, kettles) | Current rises proportionally; power rises with the square of voltage | Increased heating; lamp life falls sharply |
| Motors (induction) | Torque rises, but so do magnetising current and core loss; the motor may run cooler on current but hotter in the core | Reduced current draw but increased magnetic losses; overheating if voltage is well above nominal |
| Electronic power supplies | Many are regulated and tolerate a range; those that are not may fail | Depends entirely on the input range; unregulated supplies are vulnerable |
| Capacitor banks | Reactive output rises with the square of voltage | Overcompensation and further voltage rise; possible resonance |
| Transformers | Core flux density rises; saturation and harmonic generation above the design point | Overheating, harmonic distortion, audible noise |
| Electronics with switch-mode supplies | Usually wide input range (100-240 V), so tolerant | Little effect within the rated range |
The resistive row contains an interesting observation. In resistive load, power is related to square of voltage. Therefore, any overvoltage like 5% will correspond to about 10% more heat produced. In case of heating element, it means much shorter life of the element as well higher costs on energy. However, regarding incandescent lamp it means even shorter lifespan due to strong dependence of life from overvoltage. This is also the reason for the importance of voltage regulation for utilities as there is a real, measurable and increasing cost for users because of overvoltage.
Motor row is subtler than it seems at first sight. When motor is getting more voltage, it is consuming less current with the same mechanical output which looks like a good thing but with respect to magnetic circuit the situation is quite different. The reason is that the increased voltage makes core loss rise, the motor overheating internally despite the current decline and at very high voltage the saturation of iron starts.
The Physical Effects of Higher Voltage
In addition to the effect(s) on the load, increases in voltage also put stress on the transformer in four ways.
- Stress on insulation. Insulation stress increases with increased voltage, and as insulation stress increases, lifetime of the insulation decreases. As a consequence, a transformer with a higher winding voltage must also have a higher basic insulation level; also, for the same reason, lightning impulse withstand voltage becomes one of the items in the specification and not a detail.
- Partial discharge. When stress is high enough, a void or defect within the insulation may start discharging, which leads to gradual deterioration of that material. Partial discharge serves as a precursor of a failure but it is not a failure itself, and it is one of the side effects of overvoltage.
- Core saturation and harmonics. When a core is driven above its design magnetic flux density (material and configuration of the core determine the level of the magnetic flux that the core can withstand), the magnetizing current becomes distorted and starts containing some odd harmonics that result in increased losses, increased temperature, and may result in resonances somewhere else in the system.
- Increased audible noise. Magnetostriction–the dimensional change of the core due to the magnetization–also increases with increased flux density; thus, an overvoltage transformer produces significantly more audible sound than the same transformer at normal voltage levels.
These four phenomena have the common feature that makes overvoltage dangerous: they are cumulative and pretty much invisible. It happens so that an overvoltage transformer does not give any signs of trouble, it just ages more quickly, and failure happens sooner than expected.
Unintended Voltage Rise: The Causes
There are four chief reasons why voltage levels will be excessive other than being deliberately raised.
| Cause | Mechanism | How to recognise it | Response |
|---|---|---|---|
| Incorrect tap changer setting | The tap is set for a different supply voltage than the one present | Consistent, predictable offset across all loads; present at all times | Reset the tap to match the actual supply; verify with a measurement |
| Lost neutral | The neutral reference floats, so voltage divides between legs by load impedance | Voltages on two legs sum to the full line voltage; some loads see high voltage while others see low | Emergency: isolate and treat as a supply fault |
| Ferranti effect | On a long, lightly loaded line the capacitive charging current raises the receiving-end voltage above the sending-end voltage | Voltage rises as load falls; worst at night or in low-demand periods | Reactive compensation, or accept it if within limits |
| Capacitor bank resonance | Power factor correction capacitors resonate with system inductance, amplifying voltage at a harmonic frequency | Distortion and voltage rise correlated with capacitor switching | Detune or retune the bank; investigate harmonics |
| PV or generation backfeed | Embedded generation raises voltage at the point of connection when exporting | Voltage rises when generation is high and local load is low | Transformer tap adjustment, voltage regulation, or export limitation |
The simplest cause is a faulty tap changer. If the tap changer is set incorrectly, the transformer will supply the expected voltage all the time, and the solution would entail resetting it correctly after finding out the true input voltage instead of using the nominal voltage value. In the context of tap changers, there exist off-circuit (can be set only when the transformer is not energized) and on-load tap changers that can allow to perform their operation under load and typically function in accordance with a volt-regulating relay.
The Ferranti effect is another cause that presents a surprise. It is based on the fact that a long-way transmission or power supply line has capacitance distributed along its length, and when the very line is not loaded much, the capacitive current traveling through the inductance of the line will cause the voltage to increase instead of decreasing. The nature of the effect is proportional to the line length and inversely proportional to load capacity meaning that the effect is the most severe in case of long lines and low loads that is why a voltage drop problem can happen at night rather than when consumption is higher.
The Lost Neutral Case
This missed depth is the reason why there may not be general distinctions between this cause and other such reasons, as the answer is different in nature, viz. a crisis rather than an adjustment.
In the case of a split-phase supply, the centre tap of the transformer provides the neutral, which keeps both the line-to-neutral voltages at their normal levels. If the neutral connection is loose, corroded, or open at any point, such as at the transformer, in the service drop, at the weatherhead, at the meter base, or in the panel, the reference is lost. The two line conductors plus the loads connected between them and the neutral are in series for the whole line voltage, and the voltage is distributed depending on the impedances of the loads on either side.
The resulting effect is that a voltage rises on one side and falls on the other side. Thus, loads in the lightly loaded leg will take voltages almost equal to the full line voltage, whereas devices on the heavily loaded leg will note nearly no voltage changes. The voltage differences are due to the nature of the electrical appliances, since whenever any electric device is switched on, the balance is altered. The key symptom is burnt devices on one side and underperforming devices on its opposite side in parallel.
This is why a lost neutral should be viewed as an emergency of the supply and not as a regulation issue.The protective arrangements that limit damage in such events, and the role of the neutral in the grounding scheme, are covered in our guide to transformer grounding basics.
How Voltage Is Controlled
Voltage control is integrated into the system at multiple stages as both the expected and unexpected voltage variations are important.
- Tap changers. Off-circuit tappers set the voltage ratio and on-load tap changers change the ratio without breaking the electrical circuit under the supervision of the automatic voltage regulator.
- Automatic voltage regulators and compensators. They measure the voltage in the system and change tap to maintain the voltage in the range with dead zone to avoid hunting.
- Reactive compensation. Capacitors and reactors regulate the reactive energy which causes voltage variation due to load, detuned battery eliminates the resonance issue.
- Voltage measurements and protection. The overvoltage relay breaks the circuit or indicates the occurrence of the voltage level outside the acceptable range—last opportunity for voltage protection.
- Surge arrestors. They eliminate overvoltage coming from lightning and switching action and do not allow the insulation to operate beyond its impulse level.
- Proper calculation in the beginning. Choosing the transformer of the needed insulation class and impedance as well as the tap changing range for real supply conditions prevents most of the issues described in this article because this is the reason why the specification is the most important step.
For a site installing or replacing a distribution transformer, the practical step is to specify against the measured supply voltage and the expected load profile rather than against nominal figures. Our distribution transformer range illustrates the parameters that should be pinned down — rated voltages, tap range, impedance, insulation level and cooling class — because each of them determines how the unit behaves when the supply is not exactly nominal.
Voltage Limits and Standards
Utilities don’t attempt to keep voltage consistent; instead, they keep it within a band and the extent of that band defines what kind of variations equipment can withstand.
In North America, the main specification used is ANSI C84.1, which specifies for example the so-called Range A utilities (roughly ±5% of nominal) as a range within which equipment is expected to work and which is referred to as Range B or (approximately ±5.5%) range that is expected to be tolerated but not sustained. If we take 120 V service we get roughly 114-126 V for Range A and if we take 240 V services we get roughly 228-252 V.
In Europe, EN 50160 specifies the voltage characteristics of voltage in use and which states nominal voltage of 230 V and allowed variations in the surrounding space about ±10% during most of the time and also with the restrictions on harmonics, flicker and unbalance. Able equipment in the circuits designed for the European market will be able to work with these band characteristics of the voltage.
Practically this means that nominal voltage is probably a more exactly defined mean of the voltage which is required from the transformer. The transformer should be chosen in such a way that it provides correct voltage (nominal voltage) at some point in the band of available supply voltage and for the insulation to work normally at the top level of this band and for the load to be compatible with calculating this variation. In case something is happening in the supply and if some company’s supply has always been near a band edge, it would be better to adjust tap.
FAQ
How does a transformer increase the voltage?
Utilizing electromagnetic induction and turns ratio for an electric transformer. It generates a magnetic field in the magnetic core through the process of inducing current. In case the secondary winding of the transformer has more turns than the primary winding, the voltage induced is proportionately high, meaning that the ratio Vs/Vp = Ns/Np applies. There is no logical connection between the branches and energy is not produced and thus the transformer performs the transfer of electrical power in a different circuit with an alteration of voltage – current ratio at the same time.
What would cause a transformer to have high voltage?
There are four main reasons. A tap changer that is incorrectly adjusted will give rise to a constant voltage shift regardless of the situation and can be corrected by resetting the tap based on the supply voltage measured. An interrupted neutral will cause the floating of the reference neutral thus resulting in a rise of voltage in one phase but a fall in another, meaning it is a supply emergency instead of adjustment. Ferranti effect will result in an increase of the receiving end voltage due to capacitive charging in long unloaded lines, reaching the peak in minimal loading. The resonance of the capacitor bank and the inductiveness of the system will boost the voltage. The embedded generator project will export power in unloaded systems as well making it a more frequent cause of voltage rise nowadays.
What happens if the voltage increases?
The scenario can be described as follows: In the case of a transformer operating under a constant load under which the current rises, the current will be inversely proportional for this load as a transformer can provide power without generating it. Constant loads will consume power as resistive load obtaining approximately 10 percent more heat under 5 percent overvoltage. Power varies in direct proportion to voltage in the power square; therefore, resistive load will make heating element life less. Core losses in motors and magnetising current increase while running current smay decrease, therefore motor will be hotter centrally. Insulation will be under greater electric voltage and will undergo wear faster, therefore with prolonged overloading there are chances of getting through the discharges. Saturation takes place in transformers and motors, so they generate harmonics as well as heat. And in any case, the transformer behind the affected one will have louder humming at high flux density.
How does a transformer affect voltage?
The electrical transformer manages the proportion of voltages in the primary and secondary circuits through the turns ratio, and in this mechanism, it performs three major functions. First, the transformer steps up or steps down the voltage, depending on which circuit has a bigger number of turns. Next, it performs voltage regulation not too well, because when the load increases, the output voltage decreases due to the drop in impendant voltage in relation to current value, which is expressed in the percentage of impedance on the nameplate. Finally, the transformer has taps to regulate voltage either off the circuit during installation or when the relay comes to operate. The voltage at the output of the transformer will depend not only on its operation but also on the load profile and state of supply network.
Does raising the voltage reduce losses?
The answer depends on where the losses are taken into account, either in the transmission line or at the receiving end. For any fixed amount of power, increasing the voltage lowers the current, and the resistive losses in a conductor depend on the square of the current, thus leading to significant decrease in the transmission losses. However, this effect takes place only in the transmission part of the line. Once the voltage is increased when the power is being consumed, the resistive load will be consuming more power and increasing the losses in the load. It is important to distinguish between these two conditions; therefore, high voltage can be regarded as efficient in transmitting the power and at the same time overvoltage leads to energy losses and even shorter service lifetime of the equipment used.
References
- IEEE — Transformer Loading and Insulation Coordination Standards
- International Electrotechnical Commission — IEC 60076 Power Transformers and IEC 60038 Standard Voltages
- ANSI — ANSI C84.1 Electric Power Systems and Equipment Voltage Ratings
- CENELEC — EN 50160 Voltage Characteristics of Electricity Supplied by Public Networks
- U.S. Department of Energy — Power Quality and Distribution System Guidance
Conclusion
When it comes to increasing the voltage in a transformer, it is evident that the output voltage will be dependent on the winding ratio. The output current will also decrease at the same ratio as the voltage increase due to the fact that transformers transfer power instead of generating it. After encountering the higher output voltage, the next step is to consider what effect it will make in the equipment operated with it. Resistive installations will require more power with voltage squared and produce more heat. Motors will operate with less current and increased core losses. Higher voltage will result in more quick ageing of insulation, more noise, and excessive harmonics if the voltage exceeds its threshold value. When it comes to the increase generation of this high voltage, the whole system is designed according to these pack of parameters like insulation, winding ratio, and tap. When the generation of high voltage is unintentional, its root cause determines what solution to apply. For instance, switching the faulty tap back to default or compensating for Ferranti effect.