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Power Design Pro: Load Characteristics

Power Design Pro: Load Characteristics

10/31/2014

This is the fourth article in our series about Power Design Pro™ Software and it highlights the options users can make to load characteristics.

When configuring a load within Power Design Pro™, the software automatically populates key sizing characteristics. These characteristics change when the user selects configurable options. This provides the user with immediate net-effect feedback of their load configuration choices. The example below is the characteristics for a typical 100 hp motor starting across the line.

Starting Load Characteristics
Starting KVA (sKVA) is the apparent power that the load draws while starting. This is most prevalent for motor loads though all loads have this characteristic. When a motor starts it draws a large amount of current – locked rotor amps (LRA). This current draw is captured within the program as sKVA which is the product of locked rotor amps and rated voltage. The load’s sKVA will directly impact the system voltage dips and may influence the selection of the alternator that is matched onto the genset.

Starting kW (sKW) is the real power that the load draws during starting. This is also most prevalent for motor loads. When a motor starts, it draws a large amount of current at a low power factor. Most of the current flows as kVAR, but a significant amount is also seen as kW loading on the genset engine. The sKW creates a load step on the engine which will result in a system frequency dip. Starting kW values are additive to any previous running kW load establishing the peak loading for a given load step.

Running Load Characteristics
The selection of different load configurable options will also directly change the running kVA (rKVA) and running kW (rKW). These values are totaled for all entered loads to establish the generators maximum running load and running power factor. The rKW information is used to select the genset size and determines the load level on that genset configuration.

Harmonic Current Distortion Characteristics
In addition to traditional starting and running calculations, Power Design Pro™ is the only sizing program in the industry to perform harmonic analysis. Harmonic analysis is the process of taking a given load’s harmonic current signature and projecting it through the alternators source impedance. The goal is to calculate the resulting total harmonic voltage distortion (THVD) for the system. The load characteristic below is the typical harmonic current signature of a 100 hp motor with an unfiltered, 6 pulse variable frequency drive (VFD).

Power Design Pro™ automatically selects an estimated harmonic current distortion (THID) and spectrum based on the type of non-linear load device selected. The harmonic spectrum is the percentage of each frequency created by the non-linear load. In this example the 5th harmonic (300 hertz) is at 26.1% and the 7th harmonic (420 hertz) is at 10.4% of the measured 60 hertz, fundamental value. The user can also increase or decrease the THID to match a manufacturer’s specific data sheet information. For applications with a specific measured harmonic current distortion spectrum, the user can enter that exact spectrum as a user defined load.

The non-linear aspect of the load is summarized by the continuous and momentary THID. Loads that only have a momentary THID produce harmonics exclusively during the starting phase. Most non-linear loads have similar starting and running characteristics. The one exception is soft starters that have significant harmonics during starting and no harmonics when running.

Load Characteristics Summarized
The load characteristic information is a consistent set of load data that is summarized at the bottom of each load input screen. It is designed to provide the user with immediate feedback resulting from the selected configurable choices. This information is also rolled up within load steps and subtotaled to provide the program with the information necessary for selecting the correct genset configuration. Once the genset is selected, this information will also determine voltage and frequency dips and total harmonic voltage distortion for the system.
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