P. 8
Even harmonics cancel out due to the symmetry The Point of Common Coupling (PCC) with the
between the positive and negative halves of a consumer/utility interface is the closest point on the
periodic wave & majority of non-linear loads produce utility side of the customer’s service where another
odd harmonics. utility customer is or could be supplied. For the
CESC LT distribution system, the Feeder Pillar Box
➤ Uneven current draw between the positive & (bus-bar) may be considered as the PCC.
negative halves of one cycle of operation can
generate even harmonics, but the presence of LIMITS ON SYSTEM HARMONICS
even harmonics in a power system can normally be
neglected for all intents and purposes. IEEE std 519-2014 gives us the allowable Voltage and
current harmonics limits in a power system. Voltage
➤ The major point of interest is the odd Triplen harmonic limits are based on the THD values whereas the
harmonics i.e. with frequencies in odd multiples of current harmonic limits vary based on the short circuit
3 (h = 3,9,15,…). Other harmonics are either positive strength of the system they are being injected into.
or negative sequenced, thus the effective current
in Neutral is zero under balanced condition but for Table 1: IEEE std 519-2014 voltage harmonic limits
triplen harmonics, the currents in three phases add
up in the neutral increasing the neutral current. Voltage Distortion Limits

MEASUREMENT OF HARMONICS Bus Voltage Individual Total Voltage
at PCC Voltage Distortion
Harmonics in the system need to be quantified before THD (%)
further analysis. Two factors are defined for this purpose: Distortion (%)

1. THD (Total Harmonic Distortion):TheTotal Harmonic Below 69 kV 3.0 5.0
Distortion (THD) is an indicator of the distortion of a
signal. For a signal y, the THD is defined as: 69 kV to161 kV 1.5 2.5

Thus, THD is the ratio of the r.m.s. value of all the 161 kV and above 1.0 1.5
harmonic components of the signal y, to the
fundamental Y1. Note: High-voltage systems can have up to 2.0% THD where the cause
2. TDD (Total Demand Distortion): TDD is the per is an HVDC terminal that will attenuate by the lime it is tapped for a user.
phase harmonic current distortion against full load
demand of the system. It may be expressed as: Table 2: IEEE std 519-2014 Current harmonic limits

IL is defined as the maximum demand load current Currant Distortion Limits for General Distribution Systems
(fundamental frequency component) at PCC (Point (120 VThrough 69000 V)
of Common Coupling). This would be the maximum
current averaged over a demand interval for a given Maximum Harmonic Current Distortion in Percent of IL
customer. TDD = THD at full load.
Individual Harmonic Order (Odd Harmonics)

ISC / IL <11 11≤h< 17 17≤h<23 23≤h<35 35≤h TDD
<20* 4.0 2.0 1.5 0.6 0.3 5.0

20<50 7.0 3.5 2.5 1.0 0.5 8.0

50<100 10.0 4.5 4.0 1.5 0.7 12.0

100<1000 12.0 5.5 5.0 2.0 1.0 15.0

>1000 15.0 7.0 6.0 2.5 1,4 20.0

Even harmonics are limited to 25% of the odd harmonic limits above.

Current distortions that result in a dc offset, e.g. half-wave converters,
are not allowed.

* All power generation equipment is limited to these values of current
distortion, regardless of actual ISC / IL.

ISC = maximum short-circuit current at PCC.

IL = maximum demand load current (fundamental frequency
component) at PCC.

TDD = Total demand distortion (RSS), harmonic current distortion
in % of maximum demand load current (15 or 30 min

PCC = Point of common coupling.

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