Technical Paper The case for the ABB AVC; An alternative ...

Power Quality

Technical Paper

The case for the ABB AVC; An alternative to the UPS

Technical Paper2

The case for the ABB AVC; An alternative to the UPS

Twenty years ago UPS’s were primarily targeted at the most sensitive applications to PQ (power quality) issues. These were then recognised as “commercial” applications with the target markets being summarised as banking, data, IT and commerce related computer systems.

Now, if you are constructing a modern industrial plant with reasonable levels of automation, to not automatically include a large number of UPS’s would be unthinkable, such is the role placed on PQ and guarantee of supply and the role of the UPS. The author has visited semiconductor plants in Asia with over 75MW of UPS backed up systems on one site!

The Problem

“Dips or sags have been identified as the most common disturbance effecting a typical manufacturing plant. ”This fact has been documented and reported by so many independent studies, that it is difficult to dispute.

Fig 1: Scatter Plot of Voltage Sag Event Data for Major Industrial Sites.

A couple of important points to note from Figure 1 are that sags are hugely prevalent, that complete outages (or sags to zero volts) are nearly non-existent, and that most sags are only 5-50 cycles duration.

Causes and Effects

The most common cause of faults can be summarised below:

Lightning: At the point of impact a major over-voltage occurs, but interestingly, the most common distributed effect is a single-phase sag.Storms: The blowing of debris into transmission line systems can cause single-phase ground faults and phase to phase faults. Typical transmission line operating procedures implement multi-reclosures to attempt to clear a fault and give rise to multiple sag events in quick succession.Vehicle Accidents: Whereby vehicles run off the road and take down the local power lines.Snow and Ice: Accumulations of snow and ice predominantly cause single-phase flash-overs and faults.Cable Faults: Caused from accidental contact while digging, from poor maintenance and insulation failure. Misc: There are numerous other common causes for faults, such as bird strike, animals gaining access to conductors by climbing across insulators, fire initiated arc-overs, salt spray pollution, cable and transformer faults and many more.

The most common distributed effect from lightening is a single-phase sag

Types of Faults

It has been determined the probability of any given fault being a single phase fault is 80 percent, a two phase and ground fault is 10 percent, a two phase fault is 5 percent and a three phase fault is also 5 percent.

As the majority of faults are remote from your actual plant, it is worth noting that the likelihood of a complete outage is slim. Only if the fault is internal to your plant or on your specific feeder is the fault likely to increase to a complete outage. All other events will create a sag event.

Disturbance DataAll Semiconductor Sites

The UPS has become one of the most significant power quality solutions to be applied to Industrial plants.

However there are other options worthy of investigation and the AVC (Active Voltage Conditioner) from ABB Power Quality is making huge inroads into the domain of the more traditional solutions.

Vernon Pryde presents the case for the ABB AVC...

ABB AVC; An alternative to the UPS 3

Looking at Traditional UPS Solutions

The actual UPS itself is a significant piece of electronic equipment and complete with its battery, occupies significant real estate. Batteries in the short-term are the cheapest, easiest solution for storing large amounts of energy. However in the larger picture batteries have some major shortcomings. These include:

Lifetime: It is usual to select either a 5-year life battery or a 10-year life battery with prices to reflect the longevity:Temperature: Battery life is significantly reduced by operation at temperatures above recommendation. Optimum life is at 20 degrees Celsius and lifetime is halved for every 10 degrees rise. The author has been in many electrical switch rooms where the batteries are quite warm to touch!Maintenance: Unless a well-organised maintenance plan is in place and being implemented, failure of one cell in a multi-string system can have disastrous consequences for the related batteries.Cost: In a large UPS system the batteries can typically be more than 50 percent of the capital cost of the system. If any of the issues given above are deviated from, replacement of the battery system is often after a far shorter lifetime than initially predicted. Therefore total cost of ownership can be significantly higher than initially projected.Disposal: In many countries, disposal of old batteries is becoming an environmentally sensitive issue and this is predicted to grow.

The cost of a battery to back up a plant appears to be initially low, but often in reality the costs are significant over a 5 or 10 year lifetime.When installing larger UPS systems distortion is also a consideration, especially if diesel generator back-up is included.

Efficiency figures vary significantly amongst manufacturers but anywhere from 90 percent (realistic) to more than 95 percent (under very defined conditions!) make the operating cost of large UPS’s a significant budget item.

When checking the PQ records within plants and it becomes apparent that complete outages are actually a rare event. Records ABB Power Quality sighted for a large production plant, showed that it had not had a complete (uncontrolled) outage in over two years. So while the UPS does correct for the voltage sags, the battery is an expensive energy source that is grossly under-utilised.

The AVC Active Voltage Conditioner

The ABB AVC is a system consisting of a voltage source inverter, bypass circuit and an injection transformer connected in series between the utility and the loads to be protected”.

The AVC monitors the incoming supply voltage and when it deviates from the set point, it injects an appropriate compensating voltage using a modern IGBT based inverter and series injection transformer.

The important point of differentiation from a UPS is that the energy is actually sourced from the network, not from a separate storage device. Remember that when the voltage sags by 30 percent, there is still 70 percent left to reconstruct the missing component. The AVC regulates any voltage sag with a sub-cycle response rate, thus eliminating the voltage sag from the delicate load.

Speed of Response

The AVC detects any voltage deviation from set point within 1 to 2msecs and starts applying a correction factor immediately. Therefore, the speed of response is fast enough to protect even the most sensitive loads, such as total semiconductor manufacturing plants in Korea!

Maintenance

The most significant maintenance requirements tend to centre on the storage medium. With the AVC there is no storage medium so the majority of maintenance requirements are eliminated.

Efficiency

The lifetime cost of ownership of a PQ device is something that should not be overlooked but commonly is. Replacing consumables (i.e. batteries) is a significant overhead for alternative solutions. For the AVC, which is continuously on line, power flow is straight through the series connected injection winding, not through any multiple power conversion stages - so efficiencies of 98 percent (worst) to more than 99 percent (typical) is achievable.

Size

The AVC connects to the load via a series connected injection transformer. The system only requires to be rated to correct for the typical sag depth, say 50 percent single-phase. Therefore the actual kVA rating of the injection transformer need only be 30 percent (three-phase) rated as well. Continued over page...

Fig 2. Block diagram of the ABB AVC

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ABB LimitedPower Quality Automation Products

Ph: +64 6 843 1400Fax: +64 6 843 0398Email: [email protected]/powerquality

Vernon Pryde is International Sales Manager for ABB power Quality.

He has over twenty years involvement in the application, installation and commissioning of high power inverter based systems around the globe.

So for a typical 1MVA system, the injection transformer and inverter are physically the size of a 300kVA system, hence making for an extremely compact footprint.

Reliability

The AVC includes a high power inverter stage within its design, however a very important point is that the inversion stages are not between the load and the utility. The only item between the utility and the critical load you are protecting is the injection winding. The AVC is probably the most secure PQ device available on the market today because it is not possible for it to actually “drop” the load.