Jul 10, 2013 | Archives

Inrush Devices on Residential Distribution Feeders

by | Jul 10, 2013 | Archives

Residential devices that commonly demand high-inrush-power during startup on the distribution system are not new to electric utilities. What is new however is the mass deployment of such devices on the distribution grid due to government incentives and increased desire by the public for energy efficiency. These devices include but are not limited to the following: air conditioners, heat pumps, various types of commercial compressors, power tools, and on-demand water heaters. Air conditioners have been around since the early 1900s and they, along with heat pumps, are already used extensively throughout many parts of the southwestern United States. As thermal conversion technology improves the heat pump is quickly becoming the climate control device of choice in colder climates where it can be used for heating during the winter months and cooling during the summer months.

Heat pump manufacturers advertise up to 40% energy savings over traditional heating sources such as gas or electric furnaces while on-demand water heating suppliers advertise as much as 50% savings in water heating costs. Utilities and/or governments are promoting energy efficiency and conservation employing various incentives and favorable rate structures for consumers who make the switch. The general public views efficiency gains as opportunities for cost reduction, environmental compliance, and greenhouse gas reduction. This combination ensures that installation of such high-inrush-power devices will continue to rise in North America.

The inrush (starting) currents for ac motors can be compared to the locked-rotor currents, which are typically five to seven times the rated full-load current. In case of DC motors, starting currents appear as rectifier loads on the ac power supply side. Depending on acceleration time and load inertia motor inrush currents take around 0.3-3.0 seconds to decay to steady-state values.

Before utilities can reasonably support widespread use of such technologies, we need to fully understand the true energy performance, the impact on customer power quality and utility equipment, and the lifetime projected costs of implementation. In the case of large compressors inherent within heat pumps, air conditioners and various small commercial processes, utilities are already absorbing the costs of upgrading the distribution infrastructure to handle the increased capacity required for short-duration motor starts. The inrush during startup of these devices can cause voltage sags on the utility system, which may be severe enough to damage equipment components. Manufacturers are assigning blame to utilities and in some cases customers seek compensation for damaged equipment.

The main concerns due to increased penetration of high inrush current appliances on distribution feeders include:

  • Flicker (low-frequency voltage fluctuations on the electric power system)
  • Voltage sags/swells (short duration voltage decrease/increase)
  • Fault induced delayed voltage recovery (delayed voltage recovery following faults on the electrical system, which can cause equipment tripping and can even cascade into system collapse)
  • Cold/hot load pickup (re-energizing of the distribution circuit after an extended outage leading to high current levels, which can complicate system restoration)
  • Harmonics (sinusoidal voltages and currents with frequencies that are integer multiples of the fundamental frequency)

We conducted detailed simulations of distribution feeders with a large number of high inrush current devices. Our simulation results show that some currently available mitigation measures (e.g. soft start devices, hard start devices, proper transformer and conductor sizing, etc.) are able to mitigate the negative effects these devices have on the power quality. We also investigated the impact of feeder events, such as voltage sags, on appliances and showed that modern high efficiency and low-inertia compressors are particularly sensitive to these events.

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