Variable Frequency Drive Pumps

The Variable Frequency Drive pump acts as a variable speed Rotodynamic Pump. VFD pumps are intended for use with a Real Time Control (RTC).

When defining the pump characteristics, the Switch On Level and Switch Off level define when the pump switches on and off. Once the pump is running, its speed is controlled by the RTC scenario.

These values are set on the Pump Grid Window of the Links Grid or in the Pump Property Sheet.

The following elements are required to control a variable frequency drive pump using real time control.

• A range, to define when the pump should increase or decrease in speed.
• A rule, to define the speed or the rate of change of speed of the pump.

The speed or rate of change of speed of the pump, defined in the RTC, always takes priority over that defined as part of the pump definition when defining the network. The Positive Change in Speed and Negative Change in Speed fields defined on the Pump Grid Window of the Links Grid or the Pump Property Sheet are only used if those in the RTC are left blank.

If the actual speed of the pump is not defined by RTC, the pump will operate at nominal speed during the simulation.

Head-Discharge Curve

Similarly to the Rotodynamic Pump, flow characteristics are determined from a head-discharge curve.

The relationship between discharge and head at nominal speed is represented as a table, defined in a Head Discharge Table object. The first discharge value, corresponding to the closed valve head, must be zero. Intermediate values are interpolated linearly from the table. Values outside of the table range are linearly extrapolated from the last two table entries.

Mechanical power values are also defined in the Head Discharge Table allowing the calculation of an efficiency curve and electric power curve that can be viewed on a graph when analysing pump operation (see below).

Minimum and maximum speed for the pump is defined on thePump Grid Window of the Links Grid or in the Regulator Section of the Pump Property Sheet. The head discharge table associated with the pump applies to the pump at nominal speed. The head discharge relationship is adjusted depending on the actual speed using the Affinity Laws:

(2)   (3)   (4) where: Q = Discharge through pump station n = Rotational speed of impeller HP = Head P = Power 'o' indicates rated value

Analysing Pump Operation

The operation of a variable frequency drive pump can be analysed by graphing the curve for the pump including display of all the operating points for the pump during the simulation.

To display a graph of pump operation:

• Open simulation results in a GeoPlan Window
• Click on the pump to be analysed using the Pick Graph tool. The Graph Pick Selection Dialog will be displayed.
• Select the Pump Operation option from the Graph Pick Selection Dialog. A graph of pump operation will be displayed. The graph displays curves for minimum and maximum speed as well as for actual speed.

Pump Operation Graph