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Voltage dips/sags & momentary interruptions defined.

An interconnected power system will, in the normal course of operation, experience short duration voltage dips/sags and momentary interruptions. A momentary voltage dip or sag can be defined as a reduction in the rms value of the supply voltage with duration of up to 3 seconds. A momentary interruption can be defined as a complete loss of supply voltage with duration of up to 3 seconds.

 Causes of momentary Voltage Dips/Sags & Interruptions.

These events may occur naturally as a result of lightning strikes on high tension lines, flash-overs caused by fires under the line, dirty insulators, snow storms, overgrown vegetation or animals. Disconnecting the supply to extinguish the arc and then reconnecting normally clears these types of fault. This process is known as auto re-closing and results in a momentary supply interruption. The most common artificial causes are heavy load switching, starting large motors and internal short circuits.

 SEMI F47 standard.

The semiconductor industry formulated the F47 standard in order to define a common level of voltage sag immunity for semiconductor manufacturing equipment.  The standard requires the equipment to function correctly when subjected to a variable depth supply voltage sag with a maximum depth of 50% and a duration of 1 second. Manufacturers of semiconductor tools design the equipment to meet the F47 sag immunity standard.

 The ITIC Curve

The Computer and Business Equipment Manufacturers Association (CBEMA) collected data and a curve was developed as a reference for the ride through capability of data processing equipment. Subsequently The Information Technology Industry Council (ITIC) updated this curve. The curve shows that a voltage sag of 30% (supply drops to 70% of nominal value) will adversely affect equipment operation.

 Problem overview

Power Quality studies have been carried out for some time and a large volume of data has been collected and analyzed. Briefly, the data shows that a distribution customer experiences over 50 events per annum where the supply voltage sags below 90% of nominal. The majority of sags have a duration of 166ms (10Hz) or less with a depth of 20 to 30% (supply drops to 80-70% of nominal value).  It should be noted that the number and magnitude of events experienced can vary greatly from one location to another also distribution customers will be subjected to considerably more events than a customer fed directly from the transmission grid or via a premium grid. It is important to note that even premium grid users experience momentary sags and interruptions. Faults anywhere in the power system will cause momentary sags with a duration determined by fault clearing devices. Circuit breakers can take up to 100ms to clear a fault in a transmission system and auto re closing from 300ms to 3 seconds. Fault clearing in a distribution system can take considerably longer, the actual time depending on the substation protection equipment settings. It is clear that all power users will experience power quality events and if their business depends on continuous production processes they can expect process disruptions due to voltage dips and momentary interruptions.

We offer cost effective solutions to these problems!

DPI Range>DPI & VDC Ranges

Switching Systems Voltage-Dip Proofing Inverters offer a low cost  maintenance free solution to production line stoppages caused by short power interruptions. Energy storage capacitors eliminate the need for battery maintenance and hazardous waste disposal.

 The inverters can withstand large surge currents and are therefore ideally suited for contactor operation. When specifying an inverter only the nominal load VA needs to be considered, in contrast a UPS system must be specified for the peak load.

With a power factor range of cosФ from 1 to 0 the inverters can supply highly inductive loads such as contactors, starters and relays.

 The inverters are easy to incorporate during the manufacture of new switchgear and, due to their small physical size, existing switchgear can be retrofitted without difficulty.

 The Voltage-Dip Proofing Inverter can be used in any continuous process plant which is susceptible to short power interruptions or voltage dips and, in particular, when the restart of such process is complex and costly.

A single phase voltage dip on a three phase system would have little effect on the  motors, however if the motor controls are fed by this phase, then the contactors would drop out causing the plant to shut down unnecessarily.

When compared to other products that offer a solution, such as PLC’s or DC contactors, the Voltage-Dip Proofing Inverter is up to five times more cost effective and offers much simpler integration into existing systems. Compared to a UPS system, long term costs are lower, the DPI is more robust and better suited to coping with industrial environments. Compared to CVT's, the DPI is more tolerant of variable loads, provides longer ride-through and more accurate control.

Voltage Dip Compensator

The reliability of electrical power to industry is in general very high, nevertheless, voltage sags or dips do occur. These instabilities are caused by short circuits, lightning strikes on overhead power lines and heavy load switching. The duration of such faults is generally shorter than one second. Power Quality data shows that voltage sags with a maximum depth of around 50% constitute 92% of all events.

 Most plant can ride through such voltage dips by virtue of their mechanical and electrical inertia. However, this is not the case with electrically held-in contactors and relays that control the machinery. Contactors typically drop out from 5ms to 20ms after power is removed. Each short voltage dip now becomes a power failure and the plant must be restarted. This can be complicated, time-consuming and costly.

 Our Voltage Dip Compensators are designed to maintain the switchgear control voltage during voltage sags, effectively keeping the plant connected. The stored electrical and magnetic energy is allowed to flow, supporting the mechanical inertia of the machinery. When the supply recovers after a short voltage dip, the plant is still running at near synchronous speed, the inrush currents will be small and the stress to the system minimal. The VDC provides an economic solution for users who need their equipment to comply with the Semi F47 standard for voltage sag immunity.

 Historically, this problem has been addressed by using DC contactors, latched contactors and intelligent controls such as PLC’s. These systems are complex and expensive and do not provide a solution for equipment already in service. The current approach to this problem has been to employ intelligent control systems that provide a curative solution. In contrast, the Voltage Dip Compensator, is a preventative solution.

Power Quality Solutions:

Voltage Dip-Proofing Inverter

Power Quality Problems:

A brief introduction