Critical winder—one of many types of applications that require power ride through to sustain proper operation. (Photo courtesy of Allen-Bradley.)

Lightning, momentary high-voltage faults, highvoltage switching, and numerous other “events” in power distribution networks frequently interrupt the continuous flow of power to adjustable-speed drives. In most cases, these outages are short, one or two cycles (17 to 33 msec), and are generally undetected. But even these short interruptions can create serious problems in critical processes such as winding, unwinding, processing fine filaments, and mixing critical compounds. In unprotected applications, interruptions of a few milliseconds may cause a drive to shutdown then go through a restart procedure. This disruption can be costly because of lost and damaged product produced during a shutdown and restart. In addition. some unexpected “events” can present safety problems.

You can avoid these problems by equipping the drive with some form of ride-through capability. In many critical installations, avoiding just one unplanned outage may pay for the investment.

Types of outages

Interruptions to electric power are usually cause by two types of actions: planned and unplanned. For maintenance, line de-icing, and many other reasons, it is common practice for electric utility personnel to make planned grid changes, that is switch the main feeder circuits from one grid to another. During this planned switching process, your power may dip some or a lot for two to four cycles (33 to 67 msec). This goes unnoticed in most operations, although the lights may give a slight flicker. However, such an outage may be costly for some operations that use solidstate controls, including adjustable-speed drives. Generally considered uncommon, some planned grid changes produce outages lasting 12 to 20 cycles (200 to 333 msec). These durations present new challenges, the magnitude of which depends your process.

Unplanned outages, typically caused by lightning, wind storms, and equipment failure, often last a few seconds to several minutes.

There are two methods for determining the severity of such interruptions. One is to get the information from the power company. Some companies will give the information willingly, others will not. If this fails, you can use a power line monitor to record interruptions with the time and magnitude of each occurrence. Such instruments can be rented from firms specializing in these devices.

To overcome the problems cause by these interruptions, you can purchase drives with a wide range of ride-through capabilities. In many respects, selecting the capability needed is similar to selecting an insurance policy — you optimize the need-cost relationship.

Ride-through capabilities

Two major types of equipment are available for sustaining process operation. The uninterrupted power supply (UPS) can be sized to maintain an ac power supply for several minutes. This method, Figure 1, uses rectified incoming power to maintain a charge in a large battery. This, in turn. supplies power to an inverter that produces constant-voltage, constant-frequency single or three-phase power. This power flows independently of the ac line as long as the battery has sufficient charge. Alternate methods rely on stored magnetic or mechanical energy (see boxes).

These rather expensive systems are often installed with computers and drives on critical systems and processes that justify significant “insurance.”

For shorter interruptions — in the millisecond and second category — ridethrough capability is relatively easy to include in PWM (pulse-width modulated) adjustable-frequency drives, ac or dc.

Generally speaking, any drive that uses a fixed-voltage dc bus between the rectifier and the final power section that converts constant-voltage dc to adjustable- voltage ac or dc can offer some type of ride-through capability, Figure 2.

It is in the filtering section that extra capacitors are added to prolong the presence of a dc voltage, if the ac power should fail. The amount of capacitance determines the duration that a sufficient voltage can be maintained for a given load. The more the capacitance (microfarads), the more the stored energy. Of course, the more the load, the faster the stored energy is consumed.

For extended ride-through capabilities, some drive manufacturers connect a battery bank (in place of extra capacitors) across the dc bus to sustain operation during a power outage.

In most drive designs, power from this dc bus feeds two sections: power for the inverter section, and control power (an ampere or two) for the regulator and control circuits.

Also, by using a regenerative inverter, a decelerating, high-inertia load can transfer energy to the dc bus, thus increasing the time that the bus maintains sufficient voltage to maintain control power.

To prevent attempted operation while a dc bus voltage is too low for proper operation, most PWM drives have an undervoltage trip circuit that shuts the drive down, in a safe sequence, when the dc bus voltage dips to a specified level. Should the drive operate when the dc voltage is below the specified level, the inverter section would be unable to maintain the required volts per Hertz, a condition that can damage both the inverter and the motor.

For drives other than PWM, the UPS is usually the only workable solution; because, for example, in the six-step inverter design, the dc bus voltage changes with the required motor speed, unlike the PWM that uses a constant dc bus voltage.

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