VFD multidrives: Investment cost

VFD multidrives and investment cost

Continuing our popular series on benefits of multidrives [1] we will talk about multidrive investment cost, especially when comparing it with alternative based on single drives.

Introduction

VFD multidrive solution offers plenty of benefits. Several of them had been discussed and explained in our previous posts, such as further boost of energy efficiency [2], reduced VFD footprint and overall system footprint [3], better harmonic performance on the grid side [4] and so on. Last, but definitely not least we will talk about multidrive investment cost. How does CAPEX of a multidrive benchmark with solution based on single drives? If you wan to know the answer keep reading this article.

1. Cost of system components

In this section we will compare cost of major drive system components, such as switchgear, input transformer, harmonic filter and VFD in case of single drive and multidrive solution. The motors are not discussed as they shall be exact same for both solutions.

(i) Circuit breaker, resp. switchgear

An upstream circuit breaker is an essential component in a drive system. It is required for the operation (e.g. starting sequence) and especially for protection. A medium voltage switchgear for a single drive (one incomer, one feeder) costs a 5 digit figure in EUR or USD. Having three single drives the user can install a switchgear with one incomer and three feeders. The price will roughly double. For a multidrive a single feeder is sufficient making the switchgear remarkably cheaper.

For input voltages up to roughly 24 kV class the cost of a switchgear is relatively small compared to other drive system components. Ssay that you save 30 kEUR on the swichgear which is nice, but in % of a 1’500 kEUR drive system it becomes less significant. However, as the grid voltage increases, the switchgear becomes more costly and its portion in the drive system increases.

(ii) Input isolation transformer

Almost every medium voltage VFD requires a dedicated input transformer. Exception are direct to-line configurations that are rather rare. The purpose of such transformer was explained in [5]. The price of the transformer does not scale linearly with the power rating. A 10 MVA transformer will be significantly cheaper than sum of two 5 MVA transformer units. Consequently, VFD multidrive with just one input transformer saves investment cost also on the transformer. The gain gets bigger when the application uses power regeneration through the dc bus. Then the input transformer of a multidrive can be sized significantly smaller than the sum of ratings of single drive transformers.

(iii) Variable frequency drive

VFD multidrive is more cost effective than the sum of individual motor drives. The savings heavily depend on the configuration. There is typically cost reduction on the water cooling unit and control cubicles as minimum. With power regeneration there are savings on the input rectifier. If some “options” are needed, such as e.g. braking chopper, the cost benefit of multidrive becomes even more attractive.

(iv) Re-cooler

Cost of the re-cooler depend on the cooling capacity (i.e maximum heat losses plus design margin) as well as the number of cooling units. One larger cooler will always be cheaper than several smaller units. Moreover, the multidrive likely has lower losses than the total losses of all single drives. The piping will be shorter in most cases, too.

(v) Power cables

Power cables interconnect the system components. Multidrive configuration typically leads to reduced total cable length (same amount on motor side, lower amount on transformer side). Besides cables the user also saves on cable shows, terminations and other “installation material”. This aspect is covered in installation cost further below in this article.

In next section we will compare the total cost of drive system components for single motor drive versus multi-motor drive (multidrive). Since prices might be subject to confidentiality and we want to be compliant, the absolute prices are expressed in bananas (a random conversion factor to USD or EUR). Anyway, the real price depends on the manufacturer, selected options or region. More important is the comparison in %.

Case A: 2x 7 MW asynchronous motors without braking chopper

This is the same model case as used in the footprint comparison [2]. There are two identical asynchronous motors, each rated 7 MW. We compare two single drives versus one common multidrive. In this case a diode rectifier is sufficient. The multidrive has a 24-pulse rectifier while each single drive is just 12-pulse and both drives create a quasi 24-pulse system when working together.

We consider grid voltage in the range 11 to 20kV (input voltage has impact on switchgear and transformer cost).

Table 1: Cost comparison of 2 x 7 MW asynchronous motor drive system (diode rectifier, no chopper)

* Prices in bananas (‘b’)

From table 1 we can observe that dual 7 MW motor application realized with single drives costs approx. 14.4% more than a multidrive or that changing from individual drives to a multidrive saves 12.6% of the total cost of system components (1 – 100/114.4).

Case B: 2 x 7 MW asynchronous motors with braking chopper

Case B is almost identical to case A. We just add a braking chopper that might sometimes be required for infrequent braking. Each single drive obviously needs its own braking chopper while multidrive has a common dc bus and gets along with only one chopper.

Table 2: Cost comparison of 2 x 7 MW asynchronous motor drive system (diode rectifier, with chopper)

* Prices in bananas (‘b’)

In this case the solution with individual drives costs 17.4% more compared to the multidrive investment cost. When changing from single drives the multidrive cost reduction is 14.8% (1 – 100/117.4).

Case C: 2 x 7 MW asynchronous motors with active rectifier

The system now incorporates VFDs with an active rectifier (active front end, AFE). Each single drive has a 6-pulse active rectifier. To eliminate potential harmonic issues, the 6-pulse VFDs have integral input filter (included in VFD cost). In contrast, the multidrive has a 12-pulse active rectifier that has better harmonic performance hence input filter is not required.

Table 3: Cost comparison of 2 x 7 MW asynchronous motor drive system (active rectifier)

* Prices in bananas (‘b’)

In case C the cost savings reach already 18% when changing from individual drives to a multidrive solution. The cost of system components of single drives make 121.9% of the system cost of multidrive concept.

Case D: 3 x 7 MW asynchronous motors with active rectifier and power regeneration through dc bus

In this case there are three motors, each having still 7 MW rating as before. Drives have active rectifiers. In addition, the application allows power regeneration through dc link (meaning that one machine is in generator mode while another one is in motor mode). This is the most favorable case for multidrive investment cost when compared to single drives as we will see in below Table 4.

Table 4: Cost comparison of 3 x 7 MW asynchronous motor drive system (active rectifier; regeneration through dc bus)

* Prices in bananas (‘b’)

Now the multidrive solution is already 26% cheaper compared to three individual single drives. We have still dimensioned the rectifier of a multidrive quite conservatively (with regards to power regeneration through dc link) so there is still space for potential cost reduction. Also, if you have several options and features, they make the difference even larger in favor of a multidrive (as seen earlier on example of braking chopper).

From above cases A – D we clearly recognize the benefit of lower CAPEX, i.e. reduced multidrive investment cost on component level. But that is not all. Multidrive allows you savings on infrastructure and installation cost as well.

2. Infrastructure cost

Besides the component cost there is also the cost of infrastructure. How can multidrives contribute to reduce this portion of cost? There are several ways.

Reduced footprint

As described in detail in previous article [3] multidrive solution helps to reduce the footprint. In turn, customer saves money on electric room or a pre-fabricated building (container) due to overall smaller dimensions.

For illustration we repeat here the footprint comparison of two 7 MW single drives versus one 2 x 7 MW multidrive.

Since multidrive VFDs are inherently modular drives they often allow great flexibility in the mechanical layout as well. That might be crucial to accommodate the VFD into an existing building and avid extra expenses for civil engineering and building modification.

Less cabling

Multidrive tendetially has less cabling in between the main system components. That reduces the cable cost itself as well as related infrastructure (cable trays, cable supports). Maybe you need to break a wall to do the interconnecting cabling. It will be easier with a multidrive concept requiring less cables.

Less losses

Recently we described how multidrives further boost the VFD efficiency as well as system efficiency [2]. That translates into reduced operation cost. Well, it is true that most energy savings come from the speed control itself. Whenever the motor is operated at reduced speed, it consumes less power compared to fix speed (direct on-line) application. The improved efficiency of the VFD comes on top. It is rather small gain in percentage, but if you translate it into cost savings over 20-25 years of lifetime of the equipment (eventually longer with some upgrades) the figure starts looking quite interesting. This is something not visible in the investment cost, but definitely something to consider in the lifecycle cost.

Moreover, better efficiency also means less losses into the ambient. And less heat dissipation means smaller re-cooler inside the building (e.g. smaller HVAC or less fans for ventilation). The air conditioning is generally not so efficient and you need quite significant amount of power to evacuate certain amount of kW losses from the room.

3. Installation cost

Installation cost is another area where a multidrive solutions can contribute to cost savings.

Easier material handling

Smaller dimensions make the material handling somewhat easier and faster. The loading and offloading to and from a truck or cargo waggon is less time consuming.

Re-assembly on site

Larger VFDs are shipped in modules (transport units) to be re-assembled on site. Multidrive tends to have less transport units than the sum of all transport units of individual drives.

Less cabling

Reduced amount of cables was already mentioned. Besides the material cost it also means reduction of manpower associated with cable laying, grounding, testing etc.

Conclusion on multidrive investment cost

This article demonstrates how a multi-motor VFD (multidrive) enables lower capital investment. Interesting savings can be achieved in all major categories, such as system components (switchgear, transformer, VFD etc), infrastructure and installation.