The basics of transformers, UPS and switchgear
On continual basis, it is imperative for engineers to learn about vital electrical equipment like transformers, switchgear and UPS systems when performing design, installation or maintenance activities
- Become familiar with general information about transformers, uninterruptible power supplies and switchgear.
- Learn about their basic construction, operations and significant applications.
- Understand the applicable standards and code associated with the equipment.
Electrical engineers must focus on the basic construction and operation of electrical equipment while also considering the application. The use of any equipment is driven by its application, cost, availability of space, voltage level requirements or other specific design requirements.
A transformer is a static device that inductively transfers electrical energy from one alternating current circuit to another. It consists of coiled wires around a ferrous metal core; it has no moving parts. The transformer is one of the most significant pieces of electrical equipment: it was invented during the 19th century and is universally used in electrical systems.
All transformers operate on the principle of Faraday’s law of electromagnetic inductions. The law states that if the flux linking a loop (or turn) varies as a function of time, a voltage is induced between its terminals and the value of the induced voltage is proportional to the rate of change of magnetic flux in a magnetic field. The law simply illustrated how a transformer works on the principle of mutual induction.
For example, if two coils are coupled inductively and applied alternating current through one coil, voltage is induced in the other coil in form of electro-motive force. The intensity of the electromagnetic field around the coils is proportional to the permeability of the transformer medium or core that provides a path for the magnetic line of flux.
The medium can be air or iron. A core iron produces a significant amount of voltage compared to air as a medium. The first coil can be described as the primary winding, connected to an AC power source while the secondary winding (the second coil) receives electrical energy from the primary winding. A circuit (load), connected to the terminal of the second winding, results in current flow across the load (see Figure 1).
The ratio of the voltage induced will depend on the number of turns, the coil of wires is simply called the transformer winding. This determines high- and low-voltage windings. The transformer windings can be two or three, even more depending on the application.
It is important to note that the input power source to any transformer is always AC because direct current cannot create a changing magnetic field. An AC can easily be converted to a direct current if required by subjecting the AC waveform to a bridge rectifier and other electronics to create a DC voltage level.
From the basic operation mentioned, the construction of a transformer consists of an iron core, wound with two or more coils. The iron core is typically made from very thin steel laminations, each coated with insulation, to reduce losses. The steel core provides a low resistance path for magnetic flux. The transformer windings (primary and secondary) are insulated from the core and from each other and the connection points (leads) are brought out through insulating bushings. Figures 1 and 2 show a basic working principle of a single-phase transformer, shell and core types.
A three-phase transformer typically has a core with three legs and has both high-voltage and LV winding around each leg. In the core type construction of a single-phase transformer, the windings are cylindrical former wounds, mounted on the core limbs while in shell type, the primary and secondary are wound in layers directly on a rectangular form.
There are a variety of types of transformers and their applications can be customized to fit specific needs.
Typical use of transformers include:
- Step-up at electricity generating stations for transmission on the grid.
- Step-down to reduce transmission (400, 230, 138 and 69 kilovolts) and nominal utility voltages (38 and 15 kilovolts) to service voltages (4,160, 480/277, 120/208 and 120/240 volts).
Other uses include:
- Step down (120 to 48, 24 or 12 volts) for the operation of LV devices such as electronics and signal circuits.
- Isolation for isolating a piece of equipment from the source power for safety purposes.
- Impedance-matching transformers to match the impedance of a source and that of its load.
- Current regulation.
- Harmonic mitigation.
In the electrical utility industry, step-up transformers revolutionized the industry by allowing energy — higher voltage and lower current — to be transmitted from remote electrical generating plants over great distances. The higher voltage reduces size and cost of transmission lines and reduces transmission losses. Conversely, step-down transformers receive energy at a higher voltage and distribute it safely at a lower voltage to homes and factories.
The primary or HV used in transmission lines is typically greater than 69 kilovolts. At the local distribution level, the primary voltages are usually between 4,160 volts and 38 kilovolts and are typically 15-kilovolt class equipment.
At the local distribution level, transformers are being used to step down the primary voltage to LV use such as 480/277 volts or 208/120 volts 3-phase, depending on the commercial application. Commercial and residential distribution transformers are typically pole-top or pad-mounted, depending on the kilovolt-ampere rating required.
Most of the LV transformer types used in the building are indoor (dry) types, (cast coil or vacuum pressure impregnated/encapsulated) while the HV power transformers are outdoor (wet) types (oil). For indoor transformer installations, space requirements and heat-generating properties should be given special considerations. Indoor transformers (600 volts and less) are subject to stringent regulations outlined by NFPA 70: National Electrical Code Article 450 to ensure the safe installation of the transformers.
IEEE’s National Electrical Safety Code addresses interior and exterior electrical installations and maintenance for transformers and other equipment in the utility industry using substantially high voltage, typically above 15,000 volts.
When specifying a transformer for a particular application, the voltage ratings, frequency, rating in kilovolt-amperes, voltage taps, impedance (base rating), basic impulse insulation level and winding connections (either delta or wye) should be considered for the design applications mentioned.
Typical distribution transformer efficiencies are greater than 98%. Transformer cores are laminated to prevent the formation of eddy currents, which reduces energy loss and increases efficiency. Copper loss is due to the power loss in a transformer caused by the impedance of the copper (or aluminum) wire used to make the windings. As the current rises, the resistance in the windings produces heat and the temperature increases.
The rating or capacity of a transformer is limited by a temperature that the insulation can tolerate. This can be done by improving the transformer insulation or by increasing the rate of heat dissipation.
- Ambient air/forced air.
- Liquid cooled/ambient air.
- Liquid cooled/forced air.
- Forced-liquid-cooled/forced air.
- Forced-liquid-cooled/ forced water.
Each method can have an impact on the ventilation required of the transformer to dissipate the heat.
With the LV dry-type transformer is gaining popularity over liquid-type transformers due to its lower cost, improved safety and higher efficiency, the efficiency levels for LV dry-type transformers are regulated by the U.S. Department of Energy, which requires manufacturers to design their products to increase the efficiency levels.
The efficiency level is technically known as CFR Title 10 Chapter II Part 431. The DOE standards for transformer efficiency are optimized to 35% per-unit load for LV dry-type transformers and 50% per-unit load for medium-voltage and liquid-immersed transformers.
Invariably, the downside to this rule is the decreasing of the transformer impedance while increasing the efficiency, which will tend to increase the flow of inrush and fault current. This will result in the generation of high incident energy, consequently causing another issue with mitigating of arc flash occurrences. This law supersedes DOE-TP-1 efficiency levels implemented in 2007.
In IEEE standards, switchgear is expressed as a general term that describes switching and interrupting devices, either alone or in combination with other associated control, metering, protective and regulating equipment, that are assembled in one or more sections. The switchgear functions to regulate, protect and isolate a power system with a variety of controls housed in a metal enclosure. This facilitates switchgear to be used for both primary service and distribution equipment.
The requirements for distribution equipment are described in NEC Article 408: Switchboards, Switchgear and Panelboards. The electrical equipment referenced in Article 408 basically accepts power from a source and distributes it to other protective devices or equipment. Unlike switchgear and switchboards that are applicable in larger facilities, panelboards operate on smaller scales and serve as final distribution points, feeding the branch circuits that contain electrical devices.
LV switchboards offer a compact footprint and cost-effective solutions for service entrance applications rated at or below 1,000 volts. Switchgear operates in a wide range of voltage ratings, from below 1,000 volts up to 500 kilovolts in the U.S. It is generally the most robust in construction and expensive compared to other electrical distribution equipment.
LV, MV and HV switchgear are the major types of switchgear; this article will focus on LV switchgear. LV switchgear has the following ratings:
- Maximum voltage: 1,000 volts.
- Bus current rating: 1,600 to 6,000 amperes.
- Short-circuit withstand current: up to 200 kiloamperes.
- Short-time withstand current: typically up to 100 kiloamperes, 30 cycles.
- Breaker configuration: draw-out.
- Insulation level: typically 2.2 kilovolts.
- Power frequency: 50 or 60 hertz.
LV switchgear is designed for high power handling capacity and its protection devices are used to de-energize equipment to safely carry out testing or maintenance work and to clear faults downstream. It provides the benefits for safety and reliability for a stable and constant power supply. Features like manual control provision, fault operation, absolutely certain discrimination and complete reliability are essential to switchgear operations.
When it comes to low voltages — 1,000 volts or below — the terms “switchgear” and “switchboard” are often used interchangeably. LV switchgear is tested UL 1558 and ANSI C37.20.1. LV switchboards are constructed to UL 891. UL 1558 incorporates a number of requirements that enhance the reliability, durability and maintainability above the requirements of UL 891. Both can operate together in tandem, to provide maximum protection and coordination (see Figures 3 and 4).
A typical section of LV switchgear consists of breakers, bus and cable compartments. Each vertical section can hold up to four power circuit breaker compartments and each power circuit breaker is individually isolated from other breakers. The extensive compartmentalization of LV switchgear is designed to increase the safety, reliability and serviceability of the switchgear by preventing accidental contact with certain conductors such as the main bus or circuit breakers in adjacent cells, which can happen while performing maintenance or during installation. In the event of a short circuit inside a compartment, the resulting energy will be contained within the compartment and isolated from other breakers, bus and cable compartments.
A collection of one or more of these sections or structures is called a switchgear line-up or assembly. A switchgear line-up consists of a complete assembly of one or more of the overcurrent protection devices. Switchgear uses draw-out breakers with front and rear access, with each breaker in its own separate compartment described above. All the draw-out breakers can be detached from the bus and removed for inspection, maintenance or replacement without shutting down the main or disrupting the other breakers in the switchgear assembly.
LV switchgear breakers are typically LV power circuit breakers with integral trip units, designed to meet UL 1066. LV power circuit breakers are rated to withstand a short circuit condition for up to 30 cycles (0.5 seconds). This is in contrast to LV switchboard breakers, which are designed to withstand a short circuit condition for up to three cycles (0.05 seconds). Ordinarily, switchboard construction comprises molded-case individual or group mounted breakers.
Switchgear versus switchboards
In a main-tie-main configuration, switchboards can use draw-out main and tie-breakers to save the cost of procuring switchgear. The breaker trip settings for a switchboard could be thermal-magnetic or electronic while electronic is only applicable in switchgear.
Although the cost of switchgear is higher compared to a switchboard, both units have different capabilities and functions. The decision to use a switchboard, switchgear or both will depend on the facility’s power requirements. These requirements will dictate space consideration in the equipment selection and should be established early in the design stage. Working space around all types of electrical equipment must meet the requirements of NEC Article 110.
Both switchgear and switchboards require periodic maintenance including cleaning, lug torquing and lubrication of the moving parts of draw-out breakers. Proper switchgear maintenance and testing regimes, performed by qualified personnel, will ensure a constant power supply is not disrupted.
Uninterruptible power supplies
A power supply used to provide alternating current power to a load for some period of time in the event of a power failure is defined in NEC Article 100 as an uninterruptible power supply. When a facility’s critical loads require stored energy to maintain operations, a UPS typically is employed to ensure seamless transfer of critical load from normal to backup supply in milliseconds such that no loss of data or computer malfunction results.
An added benefit of using a UPS to overcome outages and brownouts is the UPS’s power conversion process (AC to DC and DC to AC) provides a power conditioning process eliminating incoming voltage spikes, voltage fluctuations, frequency noise, frequency variation or harmonic distortion, creating a smooth AC output waveform.
A UPS keeps connected equipment operational or allows its safe shutdown. The transfer time — which is the amount of time a UPS takes to switch from the utility supply to the battery-derived supply when there is a main failure — varies among different schemes and manufacturers.
However, the maximum tolerance duration of power failure stated in IEEE Standard 446: Emergency and Standby Power, varies from 1 millisecond to 1 cycle, depends on the equipment manufacturer. This is the minimum period of an outage that computers must tolerate without disturbance and to prevent program loss or maintain the normal operation.
All static UPSs use batteries as storage systems such as valve-regulated lead-acid, vented lead-acid and lithium-ion batteries. The marketplace is trending to lithium-ion batteries due to their longer improved life cycle costs. The battery, which is a collection of one or more cells whose chemical reactions create a flow of electrons in a circuit, allows connected equipment to continue running in the event of power source failure or voltage drops to an unacceptable level.
Other UPS applications include the use of rotary UPS systems that use the inertia of a high-mass spinning flywheel to provide short-term power protection. However, with the advancement of modern static UPS technology, the rotary UPS systems have been replaced with static battery UPS. There are a variety of topologies that categorized UPS technologies available, such as standby, line-interactive and double conversion.
Standby is the most basic UPS topology. It allows the load to run off utility power until the UPS detects power loss and switches to battery power. This type of topology is designed for security systems and other basic electronic equipment.
A line-interactive UPS incorporates technology that regulates voltage to correct minor power fluctuations without switching to a battery. Line-interactive UPS models are typically ideal for applications where protection from abnormal power is required.
A double conversion (online UPS) provides consistent, clean power regardless of the condition of incoming power by isolating equipment from raw utility power, converting power from AC to DC and back to AC (see Figure 5). Unlike other topologies, double-conversion provides zero transfer time to the battery for sensitive equipment. Double-conversion UPSs are designed to protect mission critical information technology equipment, data center installation and advanced network equipment from damage caused by utility-supplied problems.
Thus, the following information is required for specifying or designing a UPS system:
- Load (in kilovolt amperes).
- Input voltage and output voltage.
- Voltage range: +10 to -15% of nominal with no battery contribution.
- Number of phases.
- Starting current, associated with motor and transformer.
- Power factor.
- Battery backup time.
To establish appropriate power consumption, identifying the loads that the UPS is required to serve should be examined first. While UPSs are typically designed to back up IT systems they can sustain other loads, like dimmable lighting systems, laser printers, motors or compressors, but there are limitations due to high starting currents that will drain the batteries faster if these short-term starting loads are not accounted for in the battery calculations. It is typically recommended to keep these loads off UPS and alternate means are found to keep these loads running, e.g., a backup generator.
Knowing the UPS run-time will help determine its battery cabinet footprint, which allows the appropriate space planning for the installation.
Capital costs of the UPS system are driven by battery type and by the reliability required of the load, i.e., N, N+1, N+2, etc., where additional equipment is provided to enable the UPS to reach the reliability required.