The control of Voltage THD in MV power systems

Harmonic standards give limits for individual harmonics and for voltage THD. Guidelines are given for the allocation of individual harmonic current but no allowance is made for the control of THD in IEC standards. The paper shows how voltage THD can be controlled within its limit by means of the authors´ previously proposed Voltage droop approach. Two main concepts are required to develop an allocation strategy: (i) WITHD - a modified THD for current in which components are weighted depending on the harmonic frequency, (ii) a diversity law for THD based on the Summation law used for individual harmonics. The proposed allocation strategy gives an additional allocation of WITHD for an installation and is illustrated with a case study. It is shown that a WITHD allocation must be considered for large installations if voltage THD limits are to be met.