User's Manual

ANSI Electric Meter v3.1 User’s Guide 31
configured by the user. Phase loss also causes a diagnostic code to be
activated.
 Total harmonic distortion (THD): Records three types of total harmonic
distortion: voltage total harmonic distortion (VTHD), current total harmonic
distortion (ITHD) and VA (volts-amperes) total harmonic distortion
(VATHD). THD is a ratio of the voltage or current at harmonic frequencies to
the voltage or current at the fundamental frequency for the meter, expressed
as a percentage. A THD event occurs when the value of any THD
measurement exceeds the defined threshold for the duration specified for that
threshold. For ANSI meters, THD measurements are made for the system
and not per phase, meaning that if you have different harmonics on separate
phases, the THD value reported by the ANSI meter will be a combination of
those values.
The number of total harmonic distortion events, the date/time of the last
occurrence, the average value of the last occurrence, and the maximum value
of the last occurrence are stored as power quality data, and can be recorded
in the meter event log. A THD event occurs when the value for any THD
measurement goes above the defined threshold for that THD value for the
duration specified for that threshold.
Note: ITHD calculations will be disabled when the current is lower than 200 mA,,
because the current wave form will be distorted under these circumstances and
the ITHD calculation will be invalid. VATHD calculation will also be affected
by this threshold since its calculation is related to ITHD.
Note: THD calculations may be impacted by the meter’s optical or PLC
communications, so the time required to perform each THD calculation may
vary.
THD Calculations
There are two methods of calculating the VTHD and ITHD. The method used is user-
configurable. The first method, shown below, is defined in IEC 61000-4-7.
componentlfundamentaofrmsV
harmonicorderNthofrmsV
VTHD
N
2
10
2
2



componentlfundamentaofrmsI
harmonicorderNthofrmsI
ITHD
N
2
10
2
2



The second method used to calculate VTHD and ITHD uses the RMS value to
calculate the THD.
componentlfundamentaofVrms
harmonicorderNthofVrms
VTHD
N



10
2