COOLING SYSTEMS OF LARGE CAPACITY ADJUSTABLE SPEED DRIVE

SYSTEMS

Copyright Material IEEE Paper No. PCIC-2013-17

Manish Verma Douglas Phares Izhak (Ike) Grinbaum James Nanney Member, IEEE Senior Member, IEEE Consulting Electrical Engineer Member, IEEE TMEIC TMEIC Conroe, Texas, 77302 TMEIC 2901 Wilcrest Dr, #210 1325 Electric Road USA 2901 Wilcrest Dr, #210 Houston, TX, 77042 Roanoke, VA, 24018 Houston, TX, 77042 USA USA USA

Abstract – Over the years, there has been a significant evolution in power semiconductor devices from small, low power devices to very high capacity semiconductor devices. As a result, multi-megawatt range adjustable speed drive systems are gaining acceptance in the global onshore and offshore oil and gas industry. Adjustable speed drives are used on liquefied natural gas compressor trains, gas pipeline compressors and other critical rotating machinery. As power levels increase, cooling of a drive system becomes a key application consideration of the overall system safety, reliability, operation, maintainability and design. This paper highlights, contrasts, and compares drive cooling methods, and their operation, materials, fluids and technologies. Advantages and disadvantages of air and liquid cooling are discussed along with a decision making flowchart on which cooling method to use at certain power levels and applications. A comparative study of air versus liquid-cooling and how misapplication of cooling systems can lead to reduced reliability and high operating and maintenance costs. The paper concludes with general guidelines on specifying and evaluating cooling systems for adjustable speed drives. Index Terms – Voltage Source Inverter, Current Source, PWM, Cooling, Variable Frequency, Drives, Adjustable Speed, liquid-cooled, Load Commutated Inverter

I. INTRODUCTION

The global oil & gas industry has experienced a large push

for higher production capacity. This is mainly due to new oil, gas and shale fields being discovered across North America and worldwide. At the same time, government and regulatory restrictions across many nations mandate that industrial installations reduce their carbon footprint. This is due to the “Green” philosophy being adopted by various countries and petrochemical plants producing these fossil fuels.

In a typical petrochemical complex, compressors for oil, gas, and refrigeration are the major consumers of energy. Historically, many compressors have been driven by turbines, largely due to the availability of gas or steam at site. In some cases, the familiarity of operation and maintenance personnel with turbines affects their choice. In recent years, electric motor ratings above the megawatt range have become more common and have become economically competitive with turbines. As a result, some end users are selecting electric motors as compressor prime movers. Even more recently, Adjustable

Speed Drives (ASD) capacities have grown to be able to power very large medium voltage motors. The net result is that more and more new projects require that the main compressor train, and other energy intensive processes, be operated on electric power.

In the 1970s, when direct current ASDs were introduced, they typically used devices such as a diodes or thyristors and were limited to loads of a few hundred horsepower. Cooling of the ASDs was done by convection or forced air. The late 1970s saw the introduction of the Load Commutated Inverter (LCI) ASD with much higher power ratings. These very large ASDs generated heat losses that were very difficult to dissipate with air, hence liquid-cooling became the method of choice.

Table 1 covers the ASDs and cooling methodologies to be highlighted, contrasted, and compared in the remainder of this paper.

TABLE 1

RANGE OF ASDs AND LOADS PRESENTED IN THIS PAPER

Parameter Value

Voltage range(kV) > 2.3 kV Cooling Method Liquid or Air Power Level (kW) 1,500 – 98,000 kW

Motor Type Induction or Synchronous

ASD type Voltage Source Inverter (VSI) Curent Source Inverter (CSI)

II. COOLING AN ADJUSTABLE SPEED DRIVE

A. Sources Of Heat In An ASD System

Fig 1 shows the basic building blocks of an ASD. This will be useful in understanding the major sources of heat in an ASD system. All building blocks shown below are not necessarily needed for all systems. Depending on the ASD topology and the manufacturer, some blocks will be required while some might not be required [1].

Fig. 1 Basic building blocks of ASD system

As shown in Fig. 1, there are several heat sources in ASDs.

These are as follows: 1) Input Isolation Transformer: Heat losses in a

transformer consist of load losses and no-load losses in a ratio of about 4:1 respectively. Typical transformer total efficiencies are in the 98 – 99% range. While these losses are manageable for a 2MW ASD, but they are more difficult to manage with an ASD which will service a 20 MW motor.

2) Converter: The converter section is used to convert utility AC power to DC voltage or current using diodes or power switching devices. Diode converters are typically more efficient than converters applying active semiconductor devices such as Insulated Gate Bipolar Transistors (IGBTs) or Symmetrical Gate Commutated Thyristors (SGCTs).

3) DC Link: The DC link is the energy storage element in an ASD system. Voltage source inverters (VSI) use capacitors as the DC link. Capacitors generate very little heat. Current Source Inverters (CSI) use an inductor as the energy storage element and heat is generated by current flow through the resistance of the coils.

4) Inverter: The inverter section converts DC voltage to variable voltage, variable frequency AC. The inverter section produces a significant share of the heat losses in the overall ASD system. These losses come from semiconductor switching losses, conduction losses, and power component losses from capacitors, and suppressor networks. The heat contribution by the converter/inverter (rectifiers plus power bridge) is approximately 75% of the ASD system heat loss.

B. Why Is An ASD Cooling System So Critical? In today’s economic climate, project time lines have become

shorter and more critical. During the engineering & procurement stage, concerns for ASD safety details, reliability, footprint, price, weight and delivery can overshadow the cooling system aspects of the ASD. Since cooling system issues are often seen as life cycle issues, they may not be included in system evaluations with the same level of detail. In the South West Research Institute [SWRI] study [2] referenced, the authors carried out a multi-year study and developed a guideline for electric motor-driven centrifugal compressors in pipeline applications. They investigated operational reliability and life cycle costs of various commercially available electric motor drive technologies. They concluded that one of the

highest sources of large ASD outages were from ASD cooling system issues, including coolant leaks. This result shows the central role played by ASD cooling systems in overall reliability. In performing its critical operation, a well selected ASD

cooling system provides three major functions: 1. The cooling system removes the heat from the power

semiconductor devices in the inverter, converter, and from their auxiliary components. This is critical to be done efficiently [3]. This heat is produced as power semiconductors conduct, and high currents generate losses in the devices from internal device resistance.

2. The cooling system maintains the overall ASD temperature for long system life.

3. The cooling system, when optimized, allows the ASD to deliver its rated power, with the smallest equipment footprint.

By contrast, poorly designed or misapplied cooling systems can result in:

1. Premature failure of the semiconductors. 2. Risk of operating the ASD beyond thermal limits of the

components, which could be a safety hazard or reduce component life.

3. Additional unnecessary expense over the entire life cycle of the installation by choosing a cooling type (air versus liquid) that is not optimized for the application.

4. Leaks from sub-standard cooling system material selection (such as pipes, hoses, fittings, connections, etc.). This compromises personnel and equipment safety, long term reliability, and operation.

C. Types Of Cooling Systems Available At the present time, there are two methods of cooling an

adjustable speed drive. They are air cooling and liquid cooling. Depending on the ASD manufacturer, the power range of air-cooled and liquid-cooled may vary. Fig. 2 shows a typical power range of air-cooled & liquid-cooled ASDs.

Fig. 2 Power vs. voltage for air-/liquid-cooled ASDs

III. AIR-COOLED ADJUSTABLE SPEED DRIVE

Air-cooling of ASDs is the most basic and fundamental form

of cooling. A. Principles of Air-Cooling Air-cooling of ASDs works on the principle that heat transfers

from hot devices and component surfaces to the mass of air flowing over or past them. Most air-cooled ASDs use fans to force air through the ASD to dissipate heat. Typically, such air-cooled ASDs use industrial fans located on the top of the enclosures to pull the required flow of air through the ASD to cool its various components. Fig. 3 shows a front view of an air-cooled ASD.

Fig. 3 Typical air-cooled ASDs and air flow

It should be noted that some specialized outdoor ASD

applications use natural air flow from convection to cool the ASD. These may also employ a combination of natural convection and forced air blown across external heat sinks for cooling. B. Advantages of Air-Cooling Air cooling of ASDs is perceived as being simpler than liquid

cooling and has the following advantages: 1. An air-cooled ASD cooling system is self-contained

and simpler than a corresponding liquid-cooled drive. There are no pumps, hoses, piping, filters, deionizers, or heat exchangers, which help to avoid any potential problems with leaks, contamination, freezing, along with other characteristic problems.

2. When air-conditioner system repairs are needed, it is usually easier to find a knowledgeable HVAC service expert than a corresponding expert for a liquid-cooled ASD system repair. This can result in faster restoration of equipment to service.

3. Specially designed air-cooled ASDs can be rated up to 50°C and built into NEMA 3R enclosures for outdoor operation.

4. When used for starting duty only, smaller air-cooled ASDs can often be applied to large motors. This may be done more economically that applying a liquid-cooled ASD.

C. Disadvantages of Air-Cooling Corresponding disadvantages of air-cooled ASD include: 1. Air-cooled units usually have a larger footprint

compared to a similar size liquid-cooled ASD. This requires more space to house the ASD in an industrial control building.

2. Since blower fans are used for cooling the ASD, the noise levels produced during operation can be high. Typical noise levels for medium voltage ASDs are 79 – 82dB (measured at 1 meter).

3. Keeping the ASD area clean, dry and free of dust is critical for uninterrupted service. Frequent air-filter checks and change outs are required.

4. Where reliability is critical, redundancy is required for both; the ASD blower fans and the HVAC system. This will add cost and complexity to the product life cycle.

5. HVAC power requirements can be 8 – 9 times higher than the pumping power required for liquid-cooled ASDs. This must be considered when doing compete life cycle costing analysis.

D. Application Consideration When Applying Air-Cooled ASDs Applying an air-cooled ASD requires consideration of the

service requirements and the environment in which the ASD will be operated. At the present time, air-cooled adjustable speed drives are available in the following configurations:

1. ASDs with an integral isolation transformer: In this configuration, the isolation transformer is a part of the ASD line-up and appears like a single unit when installed.

2. ASDs with an external isolation transformer: In this configuration, the isolation transformer is external to the ASD line-up and, in many cases, can be placed in an outdoor environment to reduce the indoor heat load.

3. ASDs with no isolation transformer: This design configuration eliminates an isolation transformer and its associated indoor heat load.

This paper will review the application considerations for the first two types of ASDs. The sample case air-cooled ASD study shown in Figs 4, 5

and 6 is based on a 2.24MW, 4.16kV air-cooled ASD. The study will show the details to make a decision of whether to opt for an external transformer or an integral isolation transformer.

Fig. 4 Typical heat loads of air-cooled ASD with integrally

mounted transformer Fig. 4 lists the specific application data for an integral indoor

isolation transformer type of ASD. As mentioned earlier in this paper, typically 3 to 4% of the ASD actual output is lost as heat. Of this, 75% is related to the converter-rectifier and inverter, and 25% to the transformer. Using an average of 3.5% losses as the basis, yields a conservative ASD heat loss of 3kW per 74.6kW of motor power. The total ASD heat loss in Fig. 4 is calculated to be 90kW (3 x 2240/74.6 = 90). This requires approximately 26 tons of HVAC [that portion related to heat extraction]. Other factors affecting the decision also apply here. Since

the transformer is integral to the lineup, the building will also be larger and more expensive. This would be offset by avoiding cable and transformer installation labor, and the cables connecting to the transformer themselves. An integral lineup would mean that no outdoor transformer space and pad is required and no related outdoor fire protection. A non-financial factor is that an ASD with integral transformer is typically tested as a complete unit at the factory. Now consider the configuration with an outdoor mounted

transformer as shown in Fig. 5. By moving the transformer outdoors, 25% of the heat loss is shifted outside the building. Using the same assumptions as above, the heat contribution to the building is reduced by 6.5 tons to 19.5 tons. However, now the costs mentioned above must be included for transformer installation, project coordination, and buying and installing cables between the transformer and ASD.

Fig. 5 Typical heat loads of air-cooled ASD with outdoor

mounted transformer Fig. 6 shows an estimated cost comparison between an

outdoor transformer ASD and indoor transformer ASD.

Fig. 6 Cost comparisons between air-cooled ASDs with

integrally mounted vs. outdoor transformer

E. Maintenance of Air-Cooled ASDs Air-cooled ASDs are designed for low maintenance

operation. However, since air is moved through the ASD, any suspended dust or dirt particles can reduce ASD reliability, possibly reducing the life of the ASD and even jeopardizing safety. Major maintenance items include:

1. Inspect and clean air filters. In most ASDs, this can be done during operation.

2. Inspect and check torque on bolts at major bus bar and cable connections (can only be done at a shutdown time).

3. Inspect and verify HVAC operation for proper cooling, internal buildup of dust or contaminants and other visible damage.

4. Verify that the ASD location continues to maintain an atmosphere free from dust, moisture and humidity.

IV. LIQUID COOLING OF ADJUSTABLE SPEED

DRIVES

The liquid cooling systems of liquid-cooled ASDs are more complex than their air-cooled ASD counterparts. Liquid-cooled systems are engineered specifically for the application, considering outdoor ambient temperature, cooling water availability, criticality of the driven process and the level of redundancy required. By nature, liquid-cooled ASD systems are applied to large, multi-megawatt levels of motors and loads. Since these processes are mission critical, they demand the highest levels of safety, reliability, availability, and efficiency. The following sections describe the various aspects of liquid-

cooled systems. A. Principles Of Liquid-Cooled ASDs Liquid-cooled ASDs include a pump cooling panel that

includes electronic controls, electrical pumps and mechanical equipment to move the liquid through the ASD. This pump cooling panel has a similar role as the industrial fans that draw air into and through air-cooled ASDs. The liquid used in most ASDs is either de-ionized water or a mixture of de-ionized water and ethylene glycol for low ambient temperature applications. The de-ionization is required to maintain a low electrical conductivity of the fluid; a requirement imposed by the high voltages that appear on the heat producing components.

Liquid-cooled ASDs exploit the high thermal conductivity of

flowing liquid and high thermal absorption of liquid. Thermal conductivity is a material’s ability to conduct heat. The thermal conductivity of liquid such as pure water is over 24 times that of air at atmospheric temperature (25°C). This means that liquid can remove heat much faster than air. Also, the specific heat capacity of liquid such as water is roughly four times that of air. This high specific heat capacity of liquid allows it to absorb more heat per degree rise in liquid temperature as compared to air. The combination of high conductivity and high heat capacity makes liquid cooling well suited to cooling large ASDs. B. Liquid-Cooling Systems Configurations There are two major configurations of ASD liquid cooling

systems and shown in Fig. 7. 1. Liquid-cooled ASDs that have an internal liquid-to-

liquid heat exchanger (Fig. 7(a)). 2. Liquid-cooled ASDs that are outfitted with an outdoor

liquid-to-air heat exchanger for heat dissipation (Fig 7(b)).

Fig. 7 ASD liquid cooling system arrangements

Fig. 7 pictorially describes the two major arrangements of

ASD liquid cooling systems. Here are some details: 1. In the liquid-to-liquid system, Fig 7(a), the cooling loop

“A” is always a closed loop carrying de-ionized fluid internal to the ASD through a liquid-to-liquid heat exchanger. The second liquid cooling loop (loop B) carries the plant cooling water which takes away the heat picked up from the internal exchanger. Liquid- cooled ASDs that have an internal liquid-to-liquid heat exchanger as shown in Fig. 7(a).

2. In the liquid-to-air system, Fig 7(b), the cooling loop “A” and the cooling loop “B” are actually one common loop carrying de-ionized fluid. The fluid path passes through the hot drive parts, through the pump panel and out to the external liquid-to-air heat exchanger where it is cooled and returned. Because the fluid leaves the industrial control building, it must be tolerant of all anticipated outdoor ambient temperatures, and may need to include glycol in a mixture with de-ionized water to inhibit freezing.

C. Cooling System Arrangement Details Liquid-cooled ASDs as shown in Fig. 7(a) are fitted with an

internal liquid-to-liquid heat exchanger. The two cooling loops in the system are; the ASD loop (Loop A) which is the de-ionized liquid loop, and the plant liquid loop (Loop B). A specific plant liquid ASD inlet temperature range is required for optimum heat transfer by the cooling system.

Liquid-to-Liquid systems are less expensive than Liquid-to-air systems, and more compact. However, liquid-to-liquid systems do require the plant to provide high quality cooling liquid to the ASD at the required flow rate and temperature. They are best applied at plant sites where cooling liquid is also used to cool other parts of the process such as motors, vessels, and so forth. The ASD then efficiently uses the available cooling liquid supply.

As shown in Fig. 7(b), ASDs with liquid-to-air heat exchangers typically use outdoor radiator type forced air-cooled exchangers to transfer the heat from the liquid. This is a similar arrangement to a car radiator. One of the key benefits is that no plant cooling liquid is required to cool the ASD, making it a very attractive solution for remote ASDs that might be applied to gas-pipeline compressor stations or existing facilities with no available coolant liquid. High reliability is achieved by using redundant cooling fans and pumping systems, with long life filters and deionizer that can be changed without shutting down the ASD.

Liquid-to-air systems are more expensive than liquid-to-liquid designs. The higher costs include the exchanger itself, the space and mounting pad for the exchanger, piping and the ongoing expense of the fan operation. Operation depends on an outdoor ambient temperature being cool enough to remove the heat efficiently and a low enough temperature for full rating. In practice, ambient temperatures of up to 40°C can easily be accommodated by this type of system. High ambient temperatures common in some installations may require de-rating of the ASD, or even the use of chilled liquid from a refrigeration based system to cool the ASD and achieve full power output. D. Liquid-Cooled ASD Components, System Details And

Functions. ASD cooling systems typically include the following

equipment and instrumentation: 1. Motor driven pumps 2. Control system 3. Instrumentation and sensors for conductivity,

temperature, pressure & flow 4. Coolant reservoir 5. Heat Exchanger 6. Deionizer cartridge & filter 7. Pipes, valves, & actuators

Figs. 8(a) and 8(b) show a typical liquid cooling system

process & instrumentation diagram. This may vary from manufacturer to manufacturer and even from drive to drive for a particular manufacturer. However, the general concept remains the same.

Fig. 8 (a) ASD liquid-to-liquid cooling system

Resin

Bed

FM Flow

Meter

Strainer

Pump

1

Pump

2

Make-Up

Resevior

TSTemp &

Conductivity

Sensor

Resin

Trap

PSPressure

Switch

LS

Filter

FM

LS

Liquid – to – Air

Heat

Exchanger

Semi conductors Devices

HOT Liquid

COLD Liquid

De Ionizer

Loop Fig. 8 (b) ASD liquid-to-air cooling system

Liquid is passed through the heat sinks at various points within the adjustable speed drive. The heat sinks are in contact with the power semiconductors, diodes, resistors and other heat generating devices. As the ASD starts delivering power to the motor, the power devices get hot. Their heat is transferred from the device to the heat sink and then to the liquid flowing through the pump. This hot liquid is pumped from the ASD and passes through the heat exchanger. Depending on the selection, heat from the hot liquid is either transferred to the plant cooling water (Fig. 8 (a)) or to the air (Fig. 8(b)). The cooled liquid then flows back again to the ASD and the whole process is repeated again. Since the ASD cooling loop liquid must be de-ionized, a small

amount of liquid is always passed through a de-ionizer DI-resin bed. This maintains the required ultra-low conductivity of the water. Several parameters such as liquid temperature, conductivity and flow are monitored at all times. Each pump shown in Figs 8(a) or 8(b) is sized for 100% full

load capacity. This enables the ASD to continue running even if one of the pumps is out of operation. The entire pumping process is managed by a programmable logic controller that is user configurable. Fig. 9 shows a typical cooling system pumping and control panel. Typical liquid temperatures in the ASD are between 45°C

and 52°C, and flow rate ranging from 300 to 700 liters/minute.

Water Reservoir

Redundant Pumps

De-ionizer

Water to water Heat Exchanger

Pump Control panel

Fig. 9 Typical cooling system panel

E. Advantages of Liquid-Cooled Systems As ASD power levels increase above 3.5 MW, liquid-cooled

ASDs become more and more widely used and available. Key advantages of using a liquid-cooled system over air-cooled systems include the following:

1. Liquid-cooled ASDs are designed to dissipate the high levels of heat, typical of large ASDs.

2. A liquid-cooled ASD footprint is smaller per kilowatt of delivered power.

3. Electrical houses are smaller, resulting in lower building and real estate costs.

4. Electrical house noise levels are lower. 5. Reduced exposure to airborne based pollution gives

higher environmental tolerance. 6. HVAC costs are 80-90% lower compared to an air-

cooled ASD of the same power output.

7. Redundancy is often designed in both the pumping system and the fans.

F. Disadvantages of Liquid-Cooled Systems While the advantages with a liquid-cooled system often

outweigh the disadvantages, there are a few limitations of liquid-cooled systems:

1. For installations with an outdoor air-cooled heat-exchanger, fan noise can be high and may require sound barriers or special fans to mitigate the noise. Typical fan noise is about 85db at 1 meter.

2. Liquid temperature, resistivity & flow must be monitored and controlled at all times.

3. Filter and deionizer cartridges typically need to be replaced every 5 years or when clogged. For some designs this can be done while the ASD is in operation.

4. 10 – 15% of the ASD heat loss is still dissipated into the electrical room, requiring a corresponding level of HVAC.

V. COMPARATIVE SELECTION POINTS OF AIR-

COOLED VERSUS LIQUID-COOLED ASDS

This section of the paper will use what had been presented to

this point and will compare air-cooled versus liquid-cooled in several areas including life cycle operating costs, footprint, noise, HVAC and building costs, and reliability. A. ASD Cooling System Selection Process Based on ASD

Output Power In every ASD application, the goal of the owner is to

minimize initial cost including installation and to maximize process payback and energy savings. Knowing the environment in which the ASD will be located, motor power requirements, and the type of load to be driven are key items in accomplishing these goals.

Fig. 10 combines these elements into a decision making flow chart to illustrate the ASD cooling system selection. While eventually it becomes impractical to build very large ASDs with air-cooling, a hard and fast number for ASD power is not used as a decision making point (first branch in the flowchart). Instead the term “very large” is used to represent the size of decision parameter. Individual user preference and changing technology affects this point, as was described earlier.

START

What is the motor power?

Is the motor

power very

large?Specify Air-cooled ASD

Select Liquid

Cooled ASD

YES Request ASD vendor for

heat-load for HVAC sizing

Proceed!

Is cooling water

available on site?

Specify liquid-liquid Heat

Exchanger

Specify liquid-air Heat

Exchanger

YES Is

Outdoor Ambient

temp. less

than 0°C

NO

YES

NO

Specify Antifreeze (Glycol)

as coolant

Specify water as

coolant

NO

Fig. 10 Flowchart on selecting air-cooled vs. liquid-cooled ASD B. ASD Cooling System Selection Effects on ASD Footprint Adjustable Speed Drive footprint and weight are an important

consideration for oil & gas complexes. For on-shore installations, real estate might be inexpensive and widely available. However, if ASDs are applied on off-shore facilities such as FPSO’s, FSO’s, and platforms, then space is at a premium and the weight of the ASD is extremely critical. Fig. 11 shows a footprint comparison for a 7.5 MW (~10,000HP) ASD

Fig. 11 Footprint and weight of a 7.5MW air-cooled and

liquid-cooled adjustable speed drive.

Fig. 11 compares examples of footprints of an indoor air-cooled ASD and a liquid-cooled ASD of corresponding power level with an external transformer. The air-cooled system indoor footprint is almost double the liquid-cooled footprint. Different technologies, topologies and different manufacturers will give different ratios. However, liquid-cooled ASDs deliver a higher kW/ft

2 for a given load rating.

The economic effect of a larger footprint will vary. If the ASD is installed in a new portable industrial control building, the cost for a cooled building for an integral transformer, air-cooled drive can be $300-$500/ft

2 or more, including HVAC. The per-square

foot cost is significantly less for liquid-cooled drives in the order of $100 – $200/ft

2. This is mainly due the savings in space due

to compact liquid-cooled ASD. C. ASD Cooling System Selection Effects On HVAC And

Noise Except for drives packaged for outdoor use, ASDs are

designed for use in a temperature controlled environment. This makes HVAC sizing and reliability critical factors in evaluating air-cooled versus a liquid-cooled systems. Consider again the example of a 7.5MW ASD in Fig. 11. For the air-cooled drive, with 96.5% efficiency, it is safe to use a realistic heat load of 3kW of heat loss per 74.6kW (100 HP) of motor output power. For the 7.5MW example, 300kW of heat will be dissipated. This translates to approximately 86 tons of HVAC rating required. HVAC noise adds to the noise generated by the VFD fans themselves. Compare this to a liquid-cooled ASD where typically between

85 – 90% of the heat is removed via the heat exchanger with only 10-15% dissipated in the electrical building. For the example in Fig. 11, with an external transformer, the total converter-inverter loss is 75% of the total or 225 kW. The 10-15% remainder is between 22.5 to 33.75 kW which is dissipated in the control building via the HVAC system. Since the ASD must have a climate controlled environment

(20°C – 40°C, 95% non-condensing humidity, varying from supplier to supplier), the reliability of the whole ASD system directly depends on the reliability of the HVAC. Many ASD installations are specified with redundant HVAC equipment, requiring “N+1” supply of cooling units. If one of the HVAC units is down for maintenance or repair, the ASD system can continue operation. This HVAC redundancy adds incremental cost. D. ASD Cooling System Selection Effects On Operating And

Lifetime Costs The upfront capital and installation costs associated with an

ASD system are only part of the lifetime costs. When comparing an air-cooled versus liquid-cooled solution, it is important to look at the overall package rather than the initial costs only. Appendix A presents a sample life cycle costing (LCC)

analysis for a 4.4MW (6000HP) ASD.

VI. ASD COOLING SYSTEM REQUIREMENT

SPECIFICATIONS

Requirement specifications written to include ASDs must be

as clear as possible about the considerations relating to the choice of cooling systems. What follows are some examples intended to help clarify and provide examples of the specification task. A. Environmental Specification Areas When specifying cooling systems for adjustable speed

drives, it is important to clearly define the environment in which the ASD is expected to operate. An example cut from a typical requirements specification is shown in Fig. 12.

Fig. 12 Sample ASD environmental requirements

Based on the specification in Fig 12, an ASD supplier might

assume that the end user would be supplying a climate controlled environment for the ASD. However, this specification may only indicate the environment that the ASD must tolerate, but not be an indication that the user will supply such an environment. This is why it is important to clearly define who is providing the climate controlled environment for the ASD. Although the requirements above are very important, they are

not sufficient to specify the ASD cooling method or system. The information above refers to the indoor, climate controlled environment requirements for the ASD, but the outdoor environmental conditions must also be clearly specified. Fig. 13 shows a specification example that rightly relates the

outdoor environments and the provision of the environmentally controlled space for the ASD. Such clear requirements will help the ASD vendors to propose the type of ASD cooling system that will work best. In this case, the ASD vendor would be expected to supply the climate controlled building to house the ASD.

Fig. 13 Sample ASD outdoor and electrical house environment

requirements

For an air-cooled solution, the ASD manufacturer must know the altitude of the site and the maximum and minimum outdoor temperatures. He could then propose a building with sufficient heating and cooling, and the HVAC systems would be sized suitably. If a high level of redundancy is required then a N+1 redundancy in HVAC may be included and specified.

For a liquid-cooled solution with an external liquid-to-air heat exchanger, the maximum & minimum site outdoor temperatures are the most significant factors. Altitude also has an effect, since the external exchanger may need to be larger at higher elevations with its less dense atmosphere. For an outdoor liquid-to-air heat exchanger design, the ‘liquid’ in the system must be mixed with polypropylene glycol to allow operation below the liquid freeze point, 0°C in case of water. On the high side, the outdoor heat exchanger design must also be sufficient to cool the ASD in the worst case, 45°C environment given in Fig. 13. B. Related Specification Areas There are other factors and requirements that need to be

included within specifications defining ASDs in relationship to the selection of liquid- or air-cooled designs. Some of these are listed below. They are categorized for air-cooled and liquid-cooled designs. For air- and liquid-cooled solutions: 1. If the end user is supplying the ASD building, then the

ASD supplier shall provide heat dissipation data necessary to design all auxiliary cooling systems and utilities. In particular, the following data must be asked by the purchaser:

a. ASD air flow requirements. b. Heat dissipation by the ASD and all

associated auxiliaries. c. Max ASD operating ambient temperature. d. Humidity and air quality requirements.

2. In case the end user decides to split the scope of the building and the ASD, there shall be very close communication between the building supplier and the ASD vendor to make sure that proper operating conditions are maintained for a safe and reliable ASD operation.

3. Air-cooled ASDs that are supplying critical plant motors should have 100% redundant fans and automatic switchover in the event of a fan failure.

Example specification inclusions for liquid-cooled ASD

systems: 1. Liquid-cooled ASDs shall be provided with 100%

redundant pumps and automatic switchover in the event of a pump failure.

2. Liquid-cooled ASD system shall be designed so that a failed pump can be safely isolated and repaired while the ASD system remains in service.

3. Pump motors shall have an L10 bearing life of at least 50000 hours [4].

4. When an end user is to supply cooling liquid to an ASD’s liquid-to-liquid heat exchanger, the liquid quality, maximum pressure, and maximum temperature of this liquid should be provided in the specification data.

5. When an outdoor liquid-to-air heat exchanger is to be installed outdoors, specification should clearly define the responsibilities between the client and the ASD vendor for items such as mounting, piping and running power for this exchanger.

6. Quick disconnect fittings shall be provided at each connection between the header and the supply hose.

7. Dissimilar metals shall be completely avoided in liquid cooling piping and fittings. De-ionized water can be corrosive to copper, aluminum and other materials. Thus careful selection of material must be made to avoid corrosion.

8. Liquid-cooled ASDs shall maintain system coolant temperature within a safe minimum and safe maximum temperature to avoid thermal shock and/or condensation.

9. Liquid-cooled ASDs shall include conductivity monitoring and a shutdown circuit. Also, there should be alarm annunciation upon loss of cooling medium.

10. Failure Mode and Effect Analysis (FEMA) of the cooling system shall be performed in order to obtain the reliability of the system.

11. Close communication and exchange of data shall be done between the owner, engineering firm and manufacturer to confirm that the ASD will operate as intended.

Specifying the cooling system using some or all of the requirements above will help in selecting, procuring, installing and operating a safe, reliable and high quality ASD and cooling system.

VII. EVALUATION AREAS OF ASDS AND ASD

COOLING SYSTEM DESIGNS

Investing time and resources in selecting and evaluating a

particular cooling system offered by an ASD manufacturer is a must in order for the ASD to function as intended. This will be beneficial during the installation, commissioning and operational stage of the ASD. Some of the key things to review are:

1) Redundancy: Most air-cooled ASDs are offered with a single set of blower fans unless redundant fans are requested (for an additional cost). When evaluating liquid-cooled systems, based on the criticality of the service, it is almost always necessary to have redundant pumps. Instrumentation and auxiliary equipment should be able to be safely serviced and maintained while the ASD is in operation.

2) Data Sheets: The ASD vendor must provide a list of all materials (such as copper, stainless steel, plastic, rubber, etc) that are used in the cooling system which come in contact with the liquid. Materials used for piping and fluid compatibility are essential for a safe and reliable cooling system. For example: de-ionized liquid is extremely corrosive to certain materials, but it is compatible with stainless steel piping.

3) Serviceability and Maintainability: Most liquid-cooled ASDs are expected to run uninterrupted for a period of at least 5 years. However, certain components within the cooling systems do require periodic maintenance. Filters and de-ionizer cartridges typically need to be replaced while the ASD is running. Hence, the ASD must be designed in a suitable way to enable personnel to safely perform the service necessary. That includes replacing pumps, motors, filter cartridges, etc. with the equipment in operation.

4) Availability: The ASD being considered must also be evaluated for availability. In large oil & gas plants, it could be up to several hours or even a day or two by the time maintenance personnel could get a permit,

approval and clearance to service the ASD in the event of an unexpected shut down.

5) Safety: All instrumentation and detection components in the ASD system must be of the fail-safe design. This implies that any predictable failure must result in the shutdown of the ASD.

VIII. CONCLUSION

There is a growing need for installing larger capacity

adjustable speed drive systems in the oil & gas Industry that are safe and reliable. This paper presented the details of ASD cooling systems and why they are a critical consideration during the engineering and design stages of a project. Various types of cooling systems that are available at the present time were discussed. One of the key recommendations is that careful evaluation of each and every ASD application be done across a range of parameters such as: safety, reliability, maintainability, and life cycle costs. This will allow the best decision of air versus liquid cooling before a final decision is made. Specifying the ASD correctly from the beginning will lead to safe operation, lower operating costs and long term reliability.

IX. ACKNOWLEDGEMENT

This paper has been completed with utmost effort. Every

detail has been taken care of to maintain a high level of accuracy. The authors would like to thank Mr. William Horvath of TMEIC Corporation for his useful insights, industry experience, and comments on the paper. Without his support, this would not have been possible.

X. REFERNCES

[1] Gardell, Jon, et al. "Adjustable Speed Drive Motor

Protection Applications and Issues." IEEE PES Power System Relaying Committee (2008).

[2] Melissa Wilcox, Brandon Ridens, Grant Musgrove,

Nathan, Poerner, Richard Baldwin, “Electric motor drive reliability review and life cycle cost analysis.” SWRI, Gas Electric Partnership, Houston, TX, 2013,

[3] Masao Yano, Shigeru Abe, Eiichi Ohno, “History of power

electronics for motor drives in Japan,” IEEE Conference on History of Power electronics, 2004.

[4] IEEE, 2006, IEEE Standard 1566: Standard for

Performance of Adjustable Speed AC Drives Rated 375 KWand Larger. Piscataway, NJ: IEEE.

XI. VITA

Manish Verma (Manish.Verma@tmeic.com) graduated in

2006 from Virginia Tech with BSEE. He began his career with TMEIC in 2006 while continuing his professional education. In 2009 he completed his MSEE with concentration in power. After a broad exposure and education in the various TMEIC business units, he joined the global drives division, with concentration on sales and application engineering, new product developments and drive enhancements. His responsibilities include power systems and electronics, adjustable speed drives, and technical

training. He is an active member of IEEE and has authored technical papers for several conferences. Douglas Phares (Douglas.Phares@tmeic.com) holds a

BSEE and a Masters in Computer Science from Virginia Tech. He worked for GE and now TMEIC Corporation for over 30 years in the Variable Speed Drives business. Doug began his career in system engineering and moved through roles in quality control, manufacturing, and sourcing. For the past 15 years, Doug has worked as a Sr. Sales/Application engineer and Marketing Leader, focusing on medium voltage variable frequency drive systems for cement, utilities, petrochemical and mining. He serves on the cement industry committee and is the Vice Chair of the Portland Cement Association. Izhak (Ike) Grinbaum, P.E (izgrinbaum@yahoo.com)

graduated from Howard University in Washington D.C, with a BSEE. He is a registered professional engineer in the state of Texas. He has over 40 years of experience in electrical engineering, and over the years has been involved in the design of power systems, maintenance, construction, commissioning and start-up of industrial facilities in USA, UK, France and South Korea. He has extensive knowledge of large engineered variable speed drives. He has worked with Procter & Gamble, Scott Paper, and ExxonMobil. Ike retired from ExxonMobil and presently works as a Consulting Electrical Engineer. James Nanney (James.Nanney@tmeic.com) graduated in

1979 from Texas A&M with a bachelor’s in industrial distribution. He has been involved with the design, manufacture, application and selling of low and medium voltage motors with Baldor Electric, Hyundai Ideal and World Wide Electric. Currently, he is working with TMEIC Corporation as a Technical Sales Manager. His responsibilities include specifying and applying medium voltage motors for critical oil & gas applications such as: turbo compressors, pumps, fans, extruders etc.

XII. APPENDIX A

Comparision

ParameterLiquid-Cooled Drive (4.4MW) Air-Cooled Drive (4.4MW)

1Base Cost of the

ASD$700,000 $600,000

2 HVAC Unit Costs $4,000 $60,000

3HVAC Annual Operating

costs ($0.04/kWh)$400 $7,000

4HVAC Life Cycle

Cost (20 yr)$18,000 $290,000

5 Spare Parts Cost $100,000 $90,000

6Annual Maintainence

Cost$1,300 $4,000

7Training/Learning

Cost$5,000 $4,000

8Downtime Costs (over

20 life) per year$1,000 $5,000

9Xfmr + Xchgr

Installation cost $15,000 $0

10 Commissioning Cost $9,000 $60,000

11Building cost (Per

VFD sqft ONLY)$8,000 $13,000

GRAND TOTAL

(Per VFD)$861,700 $1,133,000

Fig. A-1 Comparative analysis of air-cooled versus liquid-cooled.