6 minute read
Energy efficiency
Improving the efficiency of HV motors and generators
Demand for replacement HV coils, to help improve the efficiency and operational life of HV motors and generators, is increasing. James Stevens explains what needs to be considered when designing and manufacturing diamond coils to meet these demands.
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The discussion around repairing or replacing High Voltage (HV) motors and generators has been on-going for many years. This is an in-depth subject, often influenced by factors such as whether a machine is known to be faulty, the end-user can plan an outage for repair or replacement of the machine(s), or whether the existing machine has particularly low efficiency, or if the machine is operating continuously making an efficiency-based ROI realistic amongst many other factors.
As the industry strives to gain a clear understanding of the effect of repair vs replacement, the full carbon footprint of each option, and contribution to the circular economy should certainly be included in the overall picture. Much like the debate about whether running a ten-year-old car with reduced efficiency is a better environmental prospect than purchasing a new car and absorbing the carbon footprint of manufacturing and transporting it. In calculating this comparison, it is key to consider the efficiency of old vs new, as well as the carbon footprint of manufacturing vs repair.
There is an option to improve the efficiency of HV motors and generators when repairing, and this subject is one that would benefit from further research.
As a manufacturer of diamond coils, Preformed Windings is often approached by repair companies, whether OEM’s or independent repair facilities, to supply coils to rewind HV motors and generators. This repair ethos is typically driven by factors such as; the end-user has this machine on a frame and does not want to redesign to suit a new layout, or integration of equipment down or upstream of the machine in question. The lead time of a new machine could be longer than that of a repair to their existing equipment. New equipment could be perceived to be less durable as the material content may be reduced. Regardless of the reason, there are key steps that can be taken to improve the operational efficiency of the equipment being repaired along with contributing to the circular economy, and we are seeing increased demand from the industry for HV coils that both improve efficiency and offer a longer life. It takes experience and knowledge to understand the influencing or limiting factors, therefore often coil manufacturers in the repair market supply like for like replacement coils, rather than optimising designs to improve machine characteristics at little or no extra cost.
There have been studies showing that typically the largest loss component of three-phase AC equipment is I 2 R losses. An effective way to reduce I 2 R losses in a diamond coil is to reduce the resistance of the coil, which can be done by reducing the length of the conductors or increasing the crosssectional area (CSA).
COIL GEOMETRY
When repairing a machine, the geometry of an HV coil is influenced by the voltage, power and speed of the machine, as well as the limitations of the existing core dimensions such as slot width, height and length, support ring location, and baffle location. The height and width of the slot ultimately limit the overall cross-sectional dimensions of a coil in the slot portion, along with manufacturing tolerance and slot packing. HV machines tend to have a long service life if well maintained, and since the time of original manufacture, advances in insulation material technology often allow for a reduction in insulation thickness while maintaining or even improving the dielectric properties, in turn, allowing improved efficiency and lengthening service life. As manufacturing technologies improve and R&D is carried out, the tolerances to which we manufacture coils are improved, allowing for conductors with a larger CSA to be used, resulting in reduced I 2 R losses.
As the slot length is generally fixed, there is limited opportunity to reduce the conductor length by reducing coil length as it usually involves altering the endwinding geometry, which is critical. Experience when redesigning the endwinding is key as reducing the overhang length can make winding of the coils more difficult as well as effecting machine operating conditions. If the overhang length of the coil is reduced, it affects the space between coils which is important to the effective cooling of the machine. The operating temperature of the machine directly affects the life of the insulation, and therefore careful consideration should be made when designing the overhang portion of the coil. Additionally, reducing the overhang length may introduce smaller radiuses at the point where the slot portion joins the end winding, which is a stress concentration point during the installation of the coils. This is where experience and best practices become particularly important.
EDDY CURRENTS
Eddy currents in the slot are caused by cross-slot leakage flux introducing a potential difference in voltage between the top and bottom of conductors and across the slot. This can be minimised by effective design of the conductor stack, laminatingthe conductors with conductor insulation and turn insulation. Another effective way of reducing the eddy current losses is to put a transposition or roebel in the coil. Preformed Windings often manufactures coils with transpositions in, while redesigning the conductor stack to take advantage of increased CSA’s, using state of the art manufacturing facilities including a conductor pay-off that rotates through the axis of the copper allowing for in-line turn taping and transposing at the looping stage. Our ability to design in and apply dedicated turn insulation allows us to manufacture coils suitable for variable frequency drives further enhancing efficiency.
PARTIAL DISCHARGE
A subject that is becoming more prevalent in the repair industry is partial discharge (PD). PD is small electrical discharges occurring in localised areas in coils between two electrodes typically occurring in voids within the coil. Typically, in Resin Rich (RR) coils it is caused by poor manufacturing, while in Vacuum Pressure Impregnated (VPI) coils, it can be poor process control during the vacuum process. PD accelerates the breakdown of insulation and shortens the life of the coil while also reducing efficiency. Manufacturing control during the production of RR coils is critical to ensure a homogenous insulation stack, and through R&D Preformed Windings is at the forefront in the development and supply of low PD coils, supplying coils with under 100Pc (Qm value at 8.8kV) in response to an increase in specifications calling for low void content and low PD.
The careful design and manufacture of high-quality diamond coils are critical to improving the efficiency of HV machines and keeping assets in service for longer. Although coils from various manufacturers may look similar from the outside, it is important to design the conductor stack carefully and implement quality manufacturing processes to reduce the various losses highlighted as well as reduce the number, and size of voids, which in turn increases the life of coils and the efficiency of the machine. HV coil losses can be significantly reduced, and we would welcome further collaboration and whole-industry research into efficiency gains and the circular economy related to the repair of HV machines.
