Part 2

Purchasing Decisions

What to look for when purchasing medium voltage drives

There are many considerations that need to be taken before purchasing medium voltage drives. Other than technical issues, which will be described in Part 6, prospective purchasers and users of such drives would be advised to consider the following:

Reliability

What is reliability?

Medium voltage drives are often used in applications where high reliability and availability are paramount, such as feedwater pumps, draft fans in power stations and process compressors in the chemical and petrochemical industry.

The calculations, which can be carried out on problems of reliability, are based on statistics obtained from long-term testing of components and from service experiences.

In order to use the results of long-term testing, appropriate mathematical models must be used. For the main circuit components which are found in a medium voltage drive such as power semiconductors, capacitors, resistors and their associated failure mechanisms - the exponential distribution is appropriate.

How is reliability measured?

The reliability of a component or a system can be defined by the simple equation:

R = e- y t

where

t = the time that any unit operates failure free

y = failure rate of a component or system

for example, the probability that the unit is still operational after one year is:

R = e - 8760hr / MTBF

where

t = 8760 hours

R = reliability

MTBF = Mean Time Between Failure

 

Furthermore, the availability of a component or system can be calculated according to the following equation:

= MTBF/ MTBF+ MTTR

where

MTTR = Mean Time To Repair

A = Availability

High reliability is the best way to achieve high availability. Availability is influenced by failure rate and the ability to repair.

 

What is the reliability of today's medium voltage drives?

A high calculated MTBF value is naturally a figure of merit for a piece of equipment and knowing the failure rate of different components is important to the designer.

However, care must be taken when comparing MTBF values from different manufacturers as the way the calculations have been carried out may differ considerably. Therefore, making meaningful direct comparisons between various manufacturers is not always straight forward.

Notwithstanding this, if the basis of the calculation is understood, it can be useful to compare values for MTBF.

Improveements in design and serviceability mean that users of medium voltage drives should be expecting availabilities in the order of 99.9%.

 

How is reliability achieved with today's modern technology?

Advanced power semiconductors have significantly reduced the parts count of inverters.

IGCT technology, for example, (see part 3) requires a maximum of one-third the number of parts of high voltage IGBT technology. IGCT - based medium voltage drives are more reliable than medium voltage drives based on IGBTtechnology. Furthermore, lower losses mean less cooling equipment and higher inherent reliability.

Lower parts count brings a reduction in complexity and a smaller medium voltage drive. This, in turn , ensures maximum installation flexibility and offers particular advantages for retrofits.

Electronic speed control can bring less maintenance since drives often replace a mechanical transmission system.

The mechanical stresses on the machines, bearings, and shafts are lower, which prolongs the service life of the equipment. A reduction in the bearing speed dramatically increases the bearing lifetime.

Because of the low starting current, the thermal stress on the machine itself and the electrical stress on the power supply are substantially reduced. All these factors contribute to high reliability and very low downtime of the machinery.