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Describe a Flexible Connection.

A flexible conduit connection is used to mitigate the transfer of noise and vibration through the conduit.

A flexible conductor connection is used to connect rigid bus duct between the transformer and switchgear.

  • Describe a Flexible Connection.
  • What Output Problems Can Occur with Variable Frequency Drives (VFD or VSD) and How Can You Mitigate These Issues?

      A voltage-sourced Variable Frequency Drive (VFD) uses Insulated-Gate Bipolar Transistors (IGBTs) to rapidly switch voltage on and off to form a Pulse Width Modulated (PWM) voltage source for the motor. The PWM simulates a sine wave voltage source to the motor and it operates as if it was being powered by a sine wave.  The PWM wave allows the VFD to change the fundamental frequency of the PWM waveform and simulate sine waves.  Since the speed of a motor is directly related to the fundamental frequency of the sine wave, a VFD can control speeds from a fraction of a hertz to hundreds of hertz.

      Output reactors, dV/dT filters or drive isolation transformers can be used to help mitigate some issues caused by the PWM output.  PWM outputs cause rapid switching transitions which can cause over-voltages due to parasitic capacitance and inductance in the motor’s leads. The parasitic currents and voltages can be determined by the equation of V = L × (Δi/Δt).  VFD’s switching frequencies (the amount of pulses used to simulate the sine wave) generally range from 1,000-20,000 pulses per second.  IGBT’s produce an almost perfect square wave which produces a very high Δv/Δt.   High Δv/Δt can cause higher surge currents in the leads. This then causes high voltage pulses across the parasitic inductances.  Therefore the faster the pulses switch, the greater the impact of cable capacitance and inductance. These voltage pulses stress the motor’s windings causing higher audible noise, heat and possibly premature failure of the insulation. There is also capacitance in the motor’s bearings.  The combination of lubrication and air gaps prevent direct and continuous contact of the bearings to the metal traces that contain them.  Parasitic currents [I = C × (Δv/Δt)] causes current to flow through the bearings.  The amount of current will increase as the VFD output switching speed increases. These currents can cause micro pits to form in the bearings and eventually will lead to premature bearing failure.

  • How does the sound level differ between Buck-Boost and isolation transformers?

      Buck-Boost transformers, connected as autotransformers, will be quieter than an equivalent isolation transformer that is rated for the same load. The isolation transformer would have to be physically larger than the buck-boost transformer, and smaller transformers are quieter than larger ones. For example, a 10kVA is 35dB and a 75kVA is 50dB.

  • What is ANSI C57.12.91?
  • What is ANSI C57.12.51?

      IEEE Standard for Ventilated Dry- Type Power Transformers, 501 kVA and Larger, Three-Phase, with High- Voltage 34.5 kV to 601 V and Low- Voltage 208Y/120 V to 4160 V covering General Requirements. The current standard was updated in 2008.

      This standard is intended to set forth characteristics relating to performance, limited electrical and mechanical interchangeability, and safety of the equipment described, and to assist in the proper selection of such equipment. Specific rating combinations are described in the range from 750/1000 to 7500/10 000 kVA inclusive, with high-voltage 601 to 34 500 volts inclusive and low-voltage 208Y/120 to 4160 volts inclusive. Part I of this standard describes certain electrical and mechanical requirements and takes into consideration certain safety features of 60-Hz, two-winding, three-phase, ventilated dry-type transformers with self-cooled ratings 501 kVA and larger, generally used for step-down purposes. Part Il describes other requirements or alternatives which may be specified for some applications and lists forced-air-cooled ratings for certain sizes.

  • What defines the audible noise levels in a dry type transformer?

      NEMA ST-20 (2014) defines the noise level in transformers up to 1.2 kV and up to 1000 kVA.

      NEMA TR-1 defines the sound level for medium voltage transformers above 1.2 kV class up to 7500 kVA.

  • What is NEMA ST 20?
  • What is ANSI C57.12.01?
  • When is Sound Level an issue in the design?

      Sound needs to be considered when transformers are located in close proximity to occupied areas. All energized transformers emanate sound due to the alternating flux in the core. This normal sound emitted by the transformer can be a source of annoyance unless it is kept below acceptable levels. There are ways of minimizing sound emission as discussed in the HPS “Field Service Guide”. HPS  Transformers are built to meet the latest ANSI, CSA and UL standards. These standards are outlined in the accompanying table.

  • When is audible sound level an issue in the design?

      Sound needs to be considered when transformers are located in close proximity to occupied areas. All energized transformers emanate sound due to the alternating flux in the core. The larger the kVA, the louder the audible noise. This normal sound emitted by the transformer can be a source of annoyance unless it is kept below acceptable levels. There are ways of minimizing sound emission as discussed in the HPS “Field Service Guide”.

      HPS Transformers are built to meet the latest ANSI, CSA and UL standards. Audible noise is typically define by NEMA ST-20.

  • What Seismic ratings Does HPS Use?

      HPS units meet Risk Category IV (Ip=1.5) for Sds=2.0 per ASCE 7-16 (and equivalent factors in NBCC 2015) for ground-level installations only (z/h=0) for all locations in North America.

      HPS units can be designed to meet California OSHPD (Office of Statewide health Planning and Development) requirements. Please see the California OSHPD site for a listing of HPS transformers which have been tested and certified.