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Medium-voltage control (MVC) equipment starts and stops electrical loads and is used to drive productivity, process performance and energy savings. Medium-voltage-control equipment includes a variety of starters and adjustable frequency drives that range from 2,300 to 13,800 volts. The motor loads can be as low as 50 horsepower, with the upper limit of approximately 8,000 horsepower, depending on voltage.
Typically, this equipment controls motors used in pumping, fans and chiller applications. Transformers and capacitors can also be energized by medium-voltage control as well as other equipment powering oil and gas, water and wastewater, utility generation and other applications.
Standards for medium-voltage control are defined by UL and NEMA in North America and by IEC in Europe and other parts of the world.
Full-voltage starters are the most commonly used medium-voltage control. Also known as across-the-line starters, this equipment applies line voltage to the motor in a single action. The motor will typically draw six times its normal running current while it accelerates.
Upon start-up, reduced-voltage starters apply a low voltage to the motor and switch to full voltage once the motor comes up to speed. During starting, the motor will draw less current (typically, about three times running current), so it is less likely to cause a voltage sag. Starting torque is also reduced, which means it is important to ensure the motor is able to start its load under reduced torque conditions.
Reduced-voltage-solid-state starters provide smooth acceleration by limiting starting current as voltage is ramped up to the motor. Once the motor has accelerated, the bypass contactor closes applying full voltage to the motor. RVSS technology reduces starting torque, similar to reactor and autotransformer starters, but with the ability to fine-tune start and ramp settings. It is important to ensure the motor is able to start the load under reduced torque conditions.
Variable frequency drives (VFDs) can substantially reduce energy usage and related costs. Drives match power output to actual process requirements, rather than running processes at full voltage when less than full load is required.
Drives control motor speed without using mechanical devices like dampers and valves. Learn more about how VFDs can help save energy in water industry applications.
Drives dramatically reduce electrical and mechanical stress on the motor due to a reduction of in-rush currents at startup. Using a drive, the motor can provide above full load torque at zero speed to slowly ramp up, avoiding abrupt voltage and current transients that can cause premature damage to the motor. In other words, the drive can provide full-load power while ramping up the motor without voltage disturbances to the system.
Functionally, drives rectify the main AC power source and convert the DC to variable frequency AC. The motor is smoothly accelerated with starting currents no greater than running currents. Starting torque is also unaffected, eliminating concerns related to having sufficient torque to accelerate.
Conventional motor control is built to UL or CSA guidelines in North America or IEC standards in Europe and other parts of the world. Around the world, motor control equipment is designed to enhance safety and provide reliable performance. However, conventional motor control is not designed to withstand the enormous energy released during arcing fault conditions.
As defined by IEEE, arc-resistant gear encompasses all equipment designed to withstand the effects of an internal arcing fault. Motor control certified as arc-resistant is designed to contain and redirect arc flash energy.
Typically, arc-resistant equipment diverts arc flash energy through a plenum to an area where it can be released and is intended to avoid harming personnel and other equipment. Learn more about why arc-resistant needs to be considered when evaluating risk.
Arc-resistant motor starters are designed to contain an arc until venting occurs, which means that internal damage to the equipment itself is often more severe than for non-arc equipment.
Ductless Ampgard AR uses arc-absorbing technology to absorb energy and contaminants created during an arc fault. This design meets the standards for arc-resistant construction on the latest revision of IEEE C37.20.7 without the need for external exhaust ducts or external blast zones.
Arc-sensing devices can be integrated into both standard and arc-resistant starters and drives to reduce the internal equipment damage of an arc flash event. Arc flash relays reduce arc flash energy available by quickly clearing the current feeding the fault.
Arc-resistant equipment designs are classified into two types, depending on their level of accessibility. Equipment (including medium-voltage control and drives) with arc-resistant designs that protect only the front of the equipment are assigned a Type 1 category. Type 2 category design will provide arc-resistant protection at the front, back and sides of the unit.
In addition to the type classification, additional suffixes (A, B, C, or D) may be added depending on specific sections that are covered by the IEEE C37.20.7 guideline. For example, a Type 2B arc-resistant medium-voltage drive will meet all the Type 2 testing requirements with the low-voltage compartment door open.
Integrating variable frequency drives into motor control centers has proven advantages: easier and faster installation, more compact equipment, reduced maintenance and fewer cable connections. Traditionally, drives were installed individually and treated as independent systems. That is no longer necessary nor the most productive approach for medium-voltage control.
Synchronous transfer systems control multiple motors with one variable frequency drive. In multi-motor or soft-start applications, a synchronous transfer system is designed to ramp up multiple motors in a series, transfer the load to adjacent bypass contactors and operate the motors at full speed. The system can ramp the motors down in series the same way. The drive can be used to throttle the last motor brought on-line to provide process control and energy savings.
Starters are most commonly built in a lineup with a main bus to distribute medium-voltage power to each starter in the lineup. Stand-alone starters without a bus are a less common configuration.
Full-voltage starters include an isolation switch, power fuses, and main contactor, as well as control and protection devices. As its name implies, the isolation switch isolates medium voltage from the starter compartment, allowing access to the starter while avoiding exposing personnel to medium voltage.
The isolation switch is a non-load-break device, so mechanical and electrical interlocks are supplied to ensure the main contactor is open before the switch is operated. Power fuses are supplied to provide protection from a high-current short-circuit fault. The main contactor (typically a vacuum contactor) connects and disconnects the starter load during normal or motor-overload operation. Control and protection devices help ensure the motor is disconnected if an extended overload or fault conditions occur.
Reduced-voltage starters add additional components to apply reduced voltage during startup, plus use a run or bypass contactor to short the reduced-voltage devices after the motor is accelerated.
The incoming section of a medium-voltage drive includes the disconnecting means, usually a VFD controlled contactor and short circuit protection provided by current limiting fuses. Depending on the manufacturer, this section may be included as part of a standard offering or it may be up to the user to provide separate isolation and inter-connection between the drive and disconnect
Isolation transformer has three primary purposes:
Rectifier circuit
The rectifier circuit is responsible for converting the incoming AC voltage into DC. Its performance is measured by the number of pulses it uses to rectify the AC waveform. For a medium-voltage drive, anything below 24 pulses would create levels of harmonics above acceptable industry standards, while providing a greater number of pulses does not provide sufficient benefits to justify higher costs
The DC bus is where the DC energy is stored before reaching the inverter section. On a voltage-source drive, which comes with active and passive front-end configurations, capacitors are used in order to smooth out ripples caused during the rectification.
The inverter is composed of power electronics components and converts DC voltage stored in the DC bus into an alternating pulse width modulated waveform, which can be controlled to manipulate the motor’s speed.
The vast majority of drives installed today use voltage-source drive technology. This type of drive enables various types of rectifier and inverter configurations.
Capacitors are used in the DC link to store energy and filter ripples in the rectified signal. A soft-magnetization system can be used to avoid high transformer in-rush currents that can damage rectifier components and DC link capacitors while energizing.
Passive front-end drive (PFE) technology enhances reliability and costs more than active front end technology, while using fewer components and avoiding the need for complex controls. Passive technology uses phase-shifting, 18-pulse or 24-pulse transformers and a rectifier. These converters employ fused diodes in a full wave rectifier arrangement.
Instead of passive front-end drive technology, some applications use active front-end (AFE) technology, which incorporates active components like insulated-gate bipolar transistors (IGBTs) to convert the AC voltage coming from the line into DC. This arrangement involves complex control schemes, which are needed in regenerative applications where power can be transferred from the load back to the utility. To accomplish this power transfer, many components are needed: transistors, gate drivers, power supplies, voltage and current sensors, and more.
o 5kV class starters are typically applied at 2400V, 3300V, or 4160V
o 7.2kV class starters are typically applied at 6600V or 6900V
o 15kV class starters are typically applied at 10kv, 11kV, 12.5kV or 13.8kV
o Factory dielectric-withstand tests are performed at 2.25 x rated voltage + 2000 volts; for a 4160V starter that voltage is 11,360 volts
o Dielectric tests performed after the equipment is shipped from the factory should be done at lower levels to prevent long-term damage to the starter insulation system, typically 2 x rated voltage.
o The testing is performed by applying a 1.2 x 50 high voltage waveform with peak value equal to the equipment’s BIL rating
o BIL testing is destructive to the equipment’s insulation system and is done on a representative sample of the equipment during qualification testing; it is not conducted as part of the factory production testing
o Typical BIL ratings for 5kV and 7.2kV class equipment are 45kV BIL or 60kV BIL
o In a 60kV test, the waveform goes from 0V to 60kV in 1.2 micro-seconds, and decays to 30kV in 50 micro-seconds; Lightning arrestors may be added to the system to improve the BIL performance of the starter.
Short-time-withstand current for medium voltage control
The short-time-withstand current is the maximum symmetrical RMS value of the short-circuit current that the main bus and ground bus can withstand. Per ANSI standards, the rated time for this short-circuit current is 10 cycles. Longer short-time-withstand ratings may be offered by the starter manufacturer as an option. The typical withstand current rating is 50kA.
Momentary-withstand current
The momentary-withstand current is the maximum total current that the main bus and ground bus can withstand. It is based on the peak value of the symmetrical short-time-withstand current plus DC offset. Industry standards state that the peak current value be 2.6 times the short time withstand current. The typical momentary withstand current is 130kA.
Starter-peak-withstand current
The peak-withstand current is equal to the peak let-through of the main power fuse measured in the starter withstand test. To establish this value, a 50kA symmetrical (130kA peak) available current is applied to the starter with the output terminals shorted. The main fuses clear and the peak current is measured and becomes the starter rating. Fuses with higher peak let-through should not be used since the performance of the starter with these higher currents has not been verified.
Contactor-interrupting current
The interrupting current is the RMS symmetrical current that the contactor has been proven to interrupt. Per ANSI standards, the contactor must perform three close-open operations at the rated current.
Starter-rated or continuous current
This rating reflects the current that the starter can carry on a continuous basis without exceeding allowable heat rise per industry standards. The rating can vary depending on starter configuration (1-high, 2-high, etc.) and enclosure type. Starters are typically supplied with fuses, current transformers, cables, and other components to match the specified load of the starter.
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