Commercial and industrial buildings commonly have a 208/120 V transformer in a delta-wye configuration to feed receptacles. Single-phase, non-linear loads connected to the receptacles produce triple harmonics, which add up in the neutral. When this neutral current reaches the transformer, it is reflected into the delta primary winding, which causes overheating and transformer failures.
Another transformer problem results from core loss and copper loss. Transformers are typically rated for a 60 Hz phase current load only. Higher frequency harmonic currents cause increased core loss due to eddy currents and hysteresis, resulting in more heating than would occur at the same 60 Hz current.
Transformers supplying nonlinear loads should be checked periodically to verify operation within acceptable limits. Transformers are also critical to the integrity of the grounding system.
The proper grounding of the transformer is critical. There are several solutions for transformer-related power quality (PQ) problems:
Separately derived systems (SDS)
The distribution transformer is the supply for a separately derived system (SDS). The key idea is that the secondary transformer is the new power source for all its downstream loads. This is a powerful concept in developing a PQ distribution system. The SDS accomplishes several important objectives, all beneficial for PQ:
• It establishes a new voltage reference.
• It lowers source impedance by decreasing, sometimes drastically, the distance between the load and the source.
• It achieves isolation.
• A new ground reference is established.
K-rated transformers
Harmonics cause heat in transformers at a greater rate than the equivalent fundamental currents. This is because of their higher frequency. There are three heating effects in transformers that increase with frequency: hysteresis, eddy currents and skin effect.
Application issues with K-factor transformers
K-rated transformers have been widely applied, but there are specific issues with them. Many consultants do not see the need to use transformers with a rating higher than K-13, although K-20 and higher might be supplied as part of an integrated power distribution unit (PDU).
Derating standard transformers
Some facilities managers use a 50% derating as a rule of thumb for their transformers serving single-phase, predominantly nonlinear loads. This means that a 150 kVA transformer would only supply 75 kVA of load. The derating curve, taken from IEEE 1100-1992 (Emerald Book), shows that a transformer, with 60% of its loads consisting of switched-mode power supplies, should be derated by 50%.
Forced air cooling
If heat is the problem, cooling is the solution. Break out the fan, turn it on the transformer, and use forced air cooling. Some experienced hands figure that’s worth 20-30 % on the upside.
Rail electrification and EV solutions for integrated transport systems
Electrical Power & Protection
A comprehensive range of solutions for rail electrification and electric vehicles is available from leading technology provider ABB, contributing significantly to integrated and sustainable transport systems.
Read more...PPS delivers containerised distribution board for Western Cape hybrid power project
Electrical Power & Protection
South African electrical enclosure specialist Power Process Systems has successfully completed the design, fabrication and commissioning of a 4000 A containerised distribution board for a wind/PV solar hybrid renewable energy project on a site in the Western Cape.
Read more...The choice of a thermal carrier is critical for optimal processing
Electrical Power & Protection
Historically steam, fuelled by coal, has been the most prevalent thermal carrier in South African industry. However, times are changing, with the manufacturing and processing sector needing to review the energy reticulation systems and thermal carriers currently in use.
Read more...ABB’s blueprint for a net zero future
Electrical Power & Protection
ABB’s Mission to Zero is a strategic initiative aimed at achieving a sustainable future characterised by zero emissions, zero accidents and zero waste.
Read more...General-purpose 350 W AC-DC power supply
Electrical Power & Protection
The TDK-Lambda GUS350 series of compact single-output general-purpose power supplies addresses the need for an economically priced product while maintaining reliability and quality.
Read more...Ground-breaking battery tester Comtest
Electrical Power & Protection
Midtronics offers the proven MVT handheld battery tester. This revolutionary tool, powered by MDX-AI, is set to redefine the standards of battery diagnostics and testing in the automotive industry.
Read more...Energy audits pave the pathway to sustainability and savings Schneider Electric South Africa
Electrical Power & Protection
Energy audits serve as essential tools for businesses looking to reduce costs and meet environmental targets. By analysing energy consumption across systems such as lighting, HVAC, ICT and water infrastructure, audits identify inefficiencies and quantify carbon footprints, enabling data-driven decisions for operational and financial optimisation.
While every effort has been made to ensure the accuracy of the information contained herein, the publisher and its agents cannot be held responsible for any errors contained, or any loss incurred as a result. Articles published do not necessarily reflect the views of the publishers. The editor reserves the right to alter or cut copy. Articles submitted are deemed to have been cleared for publication. Advertisements and company contact details are published as provided by the advertiser. Technews Publishing (Pty) Ltd cannot be held responsible for the accuracy or veracity of supplied material.
printer friendly version