This article will take an in-depth look at isolation transformers.
The article will look at topics such as:
This chapter will explore the operation, design, and customization of isolation transformers, as well as the key factors to consider when selecting one.
An isolation transformer is a stationary device designed to separate primary and secondary windings, ensuring physical and electrical isolation between circuits. It transfers electrical energy through magnetic induction, using a magnetic field to induce an electromotive force (EMF) in a secondary circuit while maintaining the original frequency.
In transmission and distribution systems, isolation transformers are employed to adjust voltage levels by stepping up or down, ensuring that voltage and current capacities match between the coils. A crucial role of these transformers is to mitigate voltage spikes in supply lines, which could otherwise disrupt service or damage equipment if they reach the load.
When placed between power supply lines, an isolation transformer helps diminish voltage spikes before they impact the load. Additionally, it prevents grounding issues on the secondary side, thereby reducing ground loop interference and minimizing noise effects in the load equipment.
An isolation transformer ensures electrical isolation between its primary and secondary coil circuits. This design allows for the safe transfer of electrical power from an alternating current (AC) source to equipment or devices while keeping the equipment electrically separate from the power source. This isolation is primarily used for safety purposes or to reduce electrical transients and harmonics. Typically, in step-up transformers, the secondary coil has more turns than the primary coil, whereas, in step-down transformers, the primary coil has more turns than the secondary coil.
Isolation transformers adjust voltage output by altering the voltage input according to the required voltage, current, and turns ratio. In single-phase AC systems, voltage fluctuations occur simultaneously, making it less suitable for powering large motors and industrial machinery. In contrast, three-phase power uses three distinct signals, each with a separate peak time, which helps to eliminate the oscillations present in single-phase systems. This results in smoother operation, reduced vibration, and simplified equipment design, making three-phase electricity more effective for powering large motors and heavy industrial equipment.
An isolation transformer serves to physically and electrically separate two circuits, offering protection from mains electrical shock to both electronic circuits and individuals. It utilizes magnetic coupling to transfer electrical energy from the primary to the secondary side.
The primary function of an isolation transformer is to mitigate voltage spikes in the supply lines. Voltage spikes, which can be caused by illumination, static electricity, or sudden changes in voltage, are brief increases in voltage levels that last for a short duration (typically 3 nanoseconds or more). The isolation transformer helps to reduce these spikes and protect connected equipment from potential damage.
Voltage spikes can carry extremely high voltages, ranging from a few to several thousand volts. If these high voltage spikes reach the load, they could lead to service disruptions or cause damage to the equipment. By installing an isolation transformer between the power supply lines, these spikes can be significantly reduced before they impact the load.
When a high-voltage spike with fast changes occurs on the primary side of the isolation transformer (the supply side), the transformer handles it as follows: The spike, which passes through the primary winding, is resisted by the inductor. This resistance results in an exponential rather than an instantaneous shift in current. As the current increases, the flux rises, which in turn increases the voltage on the secondary side.
The inductive properties of both the primary and secondary windings prevent the spike from transferring directly to the secondary side. The resistance to current flow increases with the rate of change, meaning that because a voltage spike involves rapid changes, the generated resistance is also higher. Consequently, the spike's impact on the secondary or load circuit is significantly reduced, protecting the load equipment from potential damage.
Additionally, an isolation transformer prevents the grounding of load equipment or the secondary side, eliminating ground loop interference and noise effects. This feature makes isolation transformers essential for protecting sensitive equipment in various applications, including measurements, laboratories, and medical devices, from voltage spikes, ground loops, and other power line distortions.
An isolation transformer is constructed similarly to a standard core-type transformer, but with additional features. It incorporates electrostatic shields that completely separate the secondary winding from the primary winding, effectively suppressing noise and interference. Unlike isolation transformers, autotransformers—where the primary and secondary windings are electrically connected—cannot provide the same level of isolation. This is because autotransformers lack the necessary physical separation between the windings, which is crucial for true isolation.
An isolation transformer can also come in a toroidal, or donut-shaped, configuration. Toroidal transformers offer several advantages, such as their compact size and lightweight design, making them suitable for a variety of applications. The windings in a toroidal transformer are distributed evenly around the core, which passes through its center. The core itself can be made from materials like silicon iron or nickel-iron alloys, contributing to the transformer’s efficiency and performance.
For higher frequency applications, amorphous alloys and iron powder are better alternatives for the core material. Additionally, toroidal transformers can reduce audible noise and stray field radiation. Toroidal isolation transformers can be fitted with a metal band to further limit stray magnetic fields. An isolation transformer may have extra insulation for equipment like patient monitoring systems that don't allow much room for interference.
Various kinds of isolation transformers are available, including:
The ultra isolation transformer features a distinct design that eradicates various forms of electrical interference, especially common mode noise. By isolating the primary and secondary sides and providing a neutral ground on the secondary side, it helps create a separate power source to address current loops. It effectively diminishes transverse mode noise and reduces common mode noise through the use of advanced insulating materials and specialized shielding methods.
Ultra isolation transformers are designed for critical and sensitive equipment such as computers, medical devices, digital communication systems, CNC machinery, and more. They mitigate disturbances caused by noisy equipment that can interfere with the power supply. These transformers are specified with high voltage ratings between 1000 to 4000 volts between their windings and are equipped with robust insulation separating the primary and secondary sides.
These transformers are particularly beneficial for wireless communication stations and advanced medical devices. They feature low coupling capacitance and are built with advanced shielding techniques to ensure high noise attenuation. Their sophisticated design ensures they are highly effective, durable, and suitable for various operating voltages.
The constant voltage transformer (CVT), which employs ferroresonant technology, is a 1:1 transformer that operates at a high point on its saturation curve. It provides an output voltage that remains largely unaffected by variations in the input voltage. The CVT uses a tank circuit consisting of a high-voltage resonant winding and a capacitor to transform a variable input into a nearly stable average output. Typically, the input winding functions at relatively low flux linkage levels.
The output winding features inherent energy storage that, in conjunction with the primary capacitor, generates a self-contained AC flux field that is somewhat isolated from the input winding. Unlike traditional stabilizers, the CVT does not use relays that could briefly interrupt the output voltage, making it a preferred choice. The CVT provides comprehensive protection against voltage spikes by maintaining a regulated output voltage.
Galvanic isolation in electrical systems involves the physical and electrical separation between the input and output power circuits. This is typically achieved using an isolation transformer, which ensures that the output power wiring remains completely separated from the input wiring. Personal computers already incorporate galvanic isolation between their power supply and internal logic circuits as mandated by international safety standards to minimize shock hazards. Thus, adding an additional transformer for this purpose is generally unnecessary.
While galvanic isolation is often believed to address noise on the ground (earth) wire, in reality, isolation transformers only separate the power wires and do not affect the ground wire, which continues to pass through without obstruction. Although some uninterruptible power supply (UPS) systems provide galvanic isolation, many online UPS models do not. For example, the online UPS systems from Exide, Unison, and ON-LINE (Pheonixtec) do not feature galvanic isolation.
Isolation is a feature that can be incorporated into any UPS system, regardless of its type. The main benefit of adding an isolation transformer is the substantial reduction of common mode noise affecting the computer. Noise filters, such as those available in the APC Smart-UPS series, can also reduce common mode noise effectively. These filters can be as efficient as isolation transformers, especially at high frequencies where computers and networks operate. However, isolation transformers tend to be more effective at very low (audio) frequencies.
Audio frequency noise on the power line generally does not affect computers or their peripherals, meaning that in such applications, an isolation transformer offers no additional benefit compared to filters. Additionally, isolation transformers can generate extra heat, which may shorten the lifespan of nearby UPS batteries. Another consideration is that incorporating an isolation transformer adds significant weight to the UPS system.
Drive isolation transformers supply power for both AC and DC variable frequency drives by shifting the voltage to the required level for SCR (Silicon Controlled Rectifier) Drives, while providing magnetic isolation between the incoming power line and the motor drive. SCR drives demand strong mechanical and electrical design and testing due to the mechanical stresses, voltage distortions, and harmonics they generate. While various types of motor drives exist, they all share a common requirement: incoming power must be rectified to achieve a DC level.
The motor drive converts power through an AC-DC-AC to AC-DC process using a three-phase rectifier bridge and an SCR. This conversion generates electrical noise and harmonics, which can be particularly problematic for transformers used in isolation drive applications, as they can cause significant heating and mechanical stress at higher frequencies. Drive isolation transformers are specifically designed to withstand the thermal, voltage, and mechanical stresses associated with motor drives. They help mitigate issues like motor bearing currents, line-to-ground voltage transients, and other noise problems caused by common-mode voltages.
Grounding the secondary side of the transformer can effectively eliminate common-mode noise, thereby enhancing both system reliability and safety. Delta-wye drive isolation transformers are capable of establishing a ground reference on the secondary side. In a motor drive system, high-frequency currents often return to the transformer winding, causing wave distortion on the line. These currents can significantly increase eddy-current losses in the windings, which should be accounted for when selecting a new transformer to ensure appropriate temperature rise calculations. Eaton's drive isolation transformers are specifically designed to manage high-frequency currents, incorporating ThermoGuard protection within the coils. This protection system alerts users to high temperatures that could otherwise reduce the transformer's lifespan or lead to failure. ThermoGuard features a set of “N.O.” dry contacts to signal such conditions.
Dry isolation transformers are encased in a sealed, pressurized container or epoxy resin, which protects the core and windings. These transformers require minimal maintenance and are reliable, making them ideal for applications where safety is critical, such as in schools, hospitals, factories, and the chemical industry.
There are two main types of dry isolation transformers: cast resin and vacuum pressure. Cast resin transformers have their windings encapsulated in epoxy resin for protection. Vacuum pressure transformers, on the other hand, feature windings enclosed in a vacuum-sealed box with moisture protection to guard against the effects of moisture.
Some of the benefits of dry isolation transformers are:
This chapter will explore the origins and impacts of electrical noise in isolation transformers, as well as the maintenance practices associated with them.
Key factors contributing to electrical noise in isolation transformers include:
A power transformer is the most expensive and crucial piece of equipment in an electrical substation. To maintain the transformer's high performance and long functional life, it is desirable to carry out various preventative maintenance tasks. Measurements and tests of the transformer's numerous properties are among the routine maintenance procedures needed for a power transformer.
Transformer maintenance can be categorized into two types: routine preventative maintenance, which should be performed regularly, and reactive maintenance, which is carried out as needed. Additionally, emergency or breakdown maintenance is only undertaken when absolutely necessary. However, regular preventative maintenance significantly reduces the chances of requiring emergency interventions.
Condition maintenance involves the regular inspection and upkeep of transformers to prevent emergencies and breakdowns. By focusing on thorough condition maintenance, technical staff can minimize the need for emergency repairs, as ongoing maintenance helps ensure the equipment remains in good working order and prevents unexpected failures.
Daily maintenance should include checking the transformer's cleanliness, winding temperature, oil temperature, and load hours. It is important to monitor readings from both the main tank and the conservator tank’s Magnetic Oil Gauge (MOG). Additionally, the color of the silica gel in the breather should be inspected. If the MOG indicates a low oil level, the transformer needs to be refilled with oil, and the transformer’s tank should be checked for any leaks. If any oil leaks are found, appropriate measures must be taken to address and repair them.
Monthly maintenance should include checking the oil level in the cap located beneath the silica gel breather. If the oil level in the cup is below the recommended mark, the transformer oil must be replenished. Additionally, the silica gel breather's ventilation holes should be inspected weekly and cleaned if necessary to ensure proper operation. For transformers with oil-filled bushings, manually check the oil levels in each bushing using the connected oil gauge on a monthly basis. If needed, top off the bushing oil to the correct level. Note that oil filling should be done during a planned shutdown.
Annually, the cooling system's automatic, remote, and manual functions—including the oil pumps, air fans, and their control circuits—should be thoroughly inspected. If any issues arise, evaluate the physical condition of the pumps and fans as well as the control circuitry. Additionally, soft cotton cloths should be used to clean the transformer bushings each year, and while cleaning, inspect the bushings for any signs of cracking.
Each year, check the condition of the OLTC oil by collecting a sample from the diverter tank’s drain valve. The oil should be tested for dielectric strength (BDV) and moisture content (PPM). If the BDV is low or if moisture levels exceed recommended limits, the OLTC oil should be either replaced or filtered.
All marshaling boxes should be cleaned thoroughly at least once a year. Additionally, inspect all lighting, space heaters, and terminal connections for control and relay wires annually, ensuring that all connections are secure and tightened as necessary.
The relays, alarms, and control switches within the relay and control panels, including the remote tap changer control panel, should be cleaned with appropriate cleaning agents. Verify the functionality of the Buchholz relay and pressure release device each year. Test the operation of relays in the remote panel by momentarily short-circuiting trip and alarm contacts with a short wire. Use a battery-operated megger with a 5 KV range to measure transformer insulation resistance and polarization index annually. Also, perform annual measurements of earth connection resistance with a clamp-on earth resistance meter. For transformers, conduct Dissolved Gas Analysis (DGA) annually for 132 KV transformers, every two years for those below 132 KV, and twice every two years for transformers over 132 KV.
This chapter will explore the various applications and advantages of isolation transformers. The discussion will cover:
Isolation transformers offer robust protection against power issues for various electrical devices. Voltage fluctuations and sudden electrical surges can damage critical components, disrupting the normal operation of equipment. By isolating the equipment from the power source, isolation transformers help prevent such risks, thereby extending the lifespan and reliability of the equipment.
In medical settings such as hospitals, where electronic devices are crucial for diagnoses, treatments, and patient care, the risk of sudden equipment failures is significant. Isolation transformers help mitigate these risks, safeguarding not only expensive medical equipment but also the safety and well-being of patients and medical staff.
Power surges can inflict significant damage on electrical devices, even though these spikes in voltage are brief. Isolation transformers offer protection against such damage by providing galvanic isolation, which shields the equipment from these surges. By isolating the DC power lines, these transformers effectively prevent any potential harm that might result from power surges.
Audio systems often experience noticeable noise interference when signals from amplifiers are transmitted to the speakers. Isolation transformers can effectively address these issues by reducing noise and improving the performance of audio devices. These transformers incorporate Faraday shields, which are designed to block electromagnetic interference by preventing disruptions in the electric field. This design helps minimize electromagnetic noise and is valuable in various industries. Reliable isolation transformers are especially crucial for the proper functioning of telecommunications, CNC machinery, remote control systems, and other critical equipment.
Electronic motors are used in a variety of industrial machinery types, and they cause harmonic voltage distortions. The equipment breaks down as a result of these harmonic changes. The best options for harmonics correction are isolation transformers. As a result, they are great protectors of industrial electric and electronic machinery.
One significant benefit of isolation transformers is their ability to prevent grounding failures. By using these transformers, there is no direct conductive link between the ground and the secondary side. Additionally, the Faraday shields incorporated in these transformers enhance their efficiency and performance.
Isolation transformers reduce the risk of current leakage, which enhances the quality of power supplied to machinery. This improvement in power quality indirectly contributes to increased equipment longevity.
Isolation transformers offer great flexibility and adaptability for various industrial power supplies, communication hubs, data acquisition systems, and similar applications. Selecting the right isolation transformer depends on the specific requirements and intended use. A thorough assessment of the application helps in choosing the most suitable transformer for optimal performance.
Understanding the different types of isolation transformers can help in selecting the most appropriate one for your particular application:
An isolation transformer, just like typical transformers, is a non-moving device that transmits electrical energy from one circuit to another without requiring any physical contact. It works on the idea of magnetic induction, which uses a magnetic field to induce EMF in another circuit without affecting the frequency. Transformers are used in transmission and distribution networks to step up and down voltage levels. An isolation transformer is a type of transformer that provides electrical isolation between two circuits (primary and secondary) without modifying the secondary properties (voltage, current, and frequency levels). Therefore, it’s important to be mindful of the type, characteristics, applications, and advantages of an isolation transformer before selecting one.
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