By Peter Cox
Like other types of installations, power generation facilities know the benefits of matching the right equipment to the job. Loads on equipment such as boiler fans and pumps can change with environmental conditions and operational requirements, therefore demand on the motors that run them will vary. In such situations, variable frequency drives (VFDs) become an effective design strategy. VFDs allow equipment such as fans and pumps to operate at optimal efficiency under a variety of conditions.
Benefits for the power industry include higher reliability and reduced wear and tear on motors, both critical in an industry where investments are made for the long haul.
Along with the benefits, VFDs present challenges; they are capable of generating unwanted noise in the drive-and in the drive system cable. When a variable frequency drive is deployed in a power plant, choosing the right cable can make a large difference in installation, operation and maintenance. Electrical energy flowing in the cable contains frequencies as high as 30 MHz. If this energy is not contained within the cable, it can radiate out to interfere with the proper operation of nearby equipment, disrupting mission-critical operations. Noise from a VFD can affect plant Ethernet communications, particularly when the components are not suitably hardened for industrial environments. Instrumentation cables can also be affected.
When disruptions occur, it is difficult to track down and eliminate noise. In one example, equipment was operating erratically. The situation was corrected only when proper VFD cables were installed on a system one story below.
|A simple representation of a VFD system.|
Variable Frequency Drive Cables Impact System Performance
Four key cable-related issues that commonly affect variable frequency drive systems are:
• Common mode current,
• Capacitive coupling and cable charging,
• Reflected wave voltage, and
• Installation safety and reliability.
Common mode current (or current noise) is defined as any current that leaves the drive on the primary motor leads and returns through any ground path. The correct VFD cable will offer the lowest possible impedance ground path, creating an attractive path for the potentially harmful currents to return to the drive. This minimizes disturbance to the surrounding networks and instrumentation.
Capacitive coupling and cable charging can consume drive power and result in reduced motor torque, drive overload trips and induced voltages on adjacent cable systems. Capacitive coupling can be reduced by having the lowest practical cable capacitance and ensuring the motor lead sets are effectively shielded from each other. Capacitive charging losses are proportional to the length or run of the cable, and to the number of conductors that a given set of motor leads can capacitively interact. A worst-case scenario occurs when multiple motor leads with high capacitance (e.g., thermoplastic high heat-resistant nylon-coated wire) are ganged together in a conduit.
Reflected wave voltage is a problem common in VFDs with longer motor leads. A mismatch between the cable and motor impedance creates a voltage reflection at the point where the leads enter the motor windings. This reflection causes a standing wave effect and a potential doubling of motor terminal voltages. Higher voltages increase the probability of motor and drive failures.
Installation safety and reliability requires selecting cables that can withstand harsh environments and electrical challenges, as well as ones that will provide the highest degree of safety. Unsuitable cable selection can lead to induced voltages and system failures that are a risk to personal safety and can impact operational reliability.
Choosing a Variable Frequency Drive Cable
There are no standards for what can be called a VFD cable. No governing body determines a cable's suitability for operation with VFDs. It is, therefore, buyer beware when shopping for VFD cables.
At the highest level, there are two key commandments:
1. Understand the requirements of the application, size the cable accordingly, and select a cable that meets the specific needs of the environment. Evaluate potential challenges such as: Is this environment sensitive to common mode current? Are instruments, networks, safety circuits or other sensitive equipment in proximity of the drive cabling? Are multiple motor leads run in parallel? What life span or mean time between failure (MTBF) is needed from this application? What are the drive manufacturers' cabling recommendations?
2. Know the difference between construction-grade and high-performance VFD cable, and choose according to the requirements of the job. The right cable can reduce or avoid noise emissions, voltage reflections and motor cable failures. The wrong cable can result in a failure of the entire system, with ensuing costly production downtime
|Purpose-built, high-performance VFD cables are designed to address cable-related issues in power gen applications.|
Construction-grade vs. High-performance Variable Frequency Drive Cable
The distinction between construction-grade and high-performance VFD cables is most marked in the following three areas:
• Stranding: High-performance VFD cables tend to be constructed with flexible high-strand-count tinned conductors; construction-grade products tend to have only the seven or 19 strands of bare copper required by construction standards. The high-performance cable is more flexible and more thermally stable at the connection points. It is more attractive for the high frequency drive output components because its conductor surface area is four to eight times larger than construction-grade cable.
• Insulation: Construction-grade VFD products are usually cross linked polyethylene (XLPE) insulated, which means they are sufficient from an insulation value perspective; most comply only with the minimum wall thickness requirements of UL 1277 (tray cable). High-performance VFD cables often significantly exceed the specified minimum wall thickness. The benefits are greatly reduced capacitance and increased permissible cable distances.
• Grounding and shielding: Construction-grade products typically use a single helically applied copper tape in contact with three segmented grounds. These constructions simply meet the minimum requirement of the National Electrical Code (NEC) and may contain less than one equivalent full-sized ground. High-performance VFD cables do a better job of attracting and containing common mode currents because they have considerably more copper at ground potential. High performance four-conductor cables with foil braid and heavy drains can have more copper in the ground and shielding system than they do at circuit potential (the equivalent of three full sized grounds and >300 percent of the NEC requirement). This extra ground copper ensures the lowest ground impedance and the lowest common mode current emissions.
As drive systems become larger, the ratio of common mode current to working current tends to be reduced. As a result of this effect, the ratio of ground potential copper to circuit conductor can be reduced as the size of the cable is increased. It is practical to move from foil braid shielded cables to designs with dual copper tapes in contact with full-sized equivalent symmetric ground conductors. Dual copper tapes offer maximum surface area and superior high-frequency performance compared to a thicker single copper tape. Belden's research suggests that the use of symmetric design to prevent internally generated ground currents begins to be a design factor for motors above 50 hp with long runs, but is typically more significant in motors greater than 100 hp.
The Right Equipment Tomorrow is the Right Equipment Today
As might be assumed from the previously mentioned comparison, high-performance cable is more expensive to purchase than construction-grade cable. That is not the issue, however. The application is the issue. The right cable for the job may cost more. Correct installation to ensure proper grounding is achieved at the cable terminations might take more time. Choosing the right cable for VFDs within power generation facilities, however, will mean higher reliability, less wear and tear on the equipment, and reduced risk to personnel safety. In the end, choosing cable that fits the demands of the application means looking at the total lifecycle of the application and opting for the larger long-term benefits.