Here is an overview of the VFD operation:
As the quality of the heat to the evaporator source improves seasonally, the total Btu/hr. output of the heat pump improves.
The sizing of the heat pump array should be designed for the lowest quality heat the evaporator source will experience seasonally.
Adding a VFD to the compressor allows slower compressor operating speeds, resulting in lower total BTU/hr. output of the heat pump, lower energy usage and an improved COP.
With improved evaporator source heat, the heat pump array performance can be reduced to the original design output,  by either reducing the number of units active  in the array, or slowing the compressor speeds with a VFD across all or selected units in the array.
A combination of a reduced number of units in the array and reduced compressor speeds may be utilized to maximize the efficiency of the system.