The primary limit on individual customers is the amount of harmonic current that they can inject into the utility network. The current limits are based upon the size of the consumer relative to the size of the supply. Larger customers are restricted more than smaller customers.
The relative size of the load with respect to the source is defined as the short circuit ratio (SCR), at the point of common coupling (PCC), which is where the consumer’s load connects to other loads in the power system. The consumer’s size is defined by the total fundamental frequency current in the load, IL, which includes all linear and nonlinear loads. The size of the supply system is defined by the level of short-circuit current, ISC, at the PCC.
These two currents define the SCR:
A high ratio means that the load is relatively small and that current limits will not be as strict as limits that pertain to a low ratio. This is demonstrated in 1, which lists recommended, maximum current distortion levels as a function of SCR and harmonic order. The table also identifies total harmonic distortion levels. All of the current distortion values are given in terms relative to the maximum demand load current. The total distortion is in terms of total demand distortion (TDD) instead of the more common THD term.
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Table 4.1 shows ocomponents as well as total harmonic distortion. For example, a consumer with an SCR between 50 and 100 has a recommended limit of 12.0% for TDD, while for individual odd harmonic components with orders less than 11, the limit on each is 10%. It is important to note that the individual harmonic current components do not add up directly so that all characteristic harmonics cannot be at their individual maximum limit without exceeding the TDD.
Table 4.1. IEEE 519 Current distortion limits for conditions lasting more than one hour (Shorter periods increase limit by 50 %)
Harmonic current limits for non-linear load at the point-of-common-coupling with other loads, for voltages 120-69,000 volts.
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Maximum odd harmonic current distortion in % of fundamental harmonic order.
Harmonic current limits for non-linear load at the point-of-common-coupling with other loads, for voltages > 69,000 – 161,000 volts.
Maximum odd harmonic current distortion in % of fundamental harmonic order.
Harmonic current limits for non-linear load at the point-of-common-coupling with other loads, for voltages > 161,000 volts.
Maximum odd harmonic current distortion in % of fundamental harmonic order.
Even harmonics are limited to 25% of the odd harmonic limits above.
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*A11 power generation equipment is limited to these values of current distortion, regardless of actual ISC/1L.
Where ISC = Maximum short circuit current at point-of-common-coupling.
And IL = Maximum demand load current (fundamental frequency) at point of common coupling.
TDD = Total demand distortion (RSS) in % of maximum demand.
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It is important to note that Table 4.1 shows limits for odd harmonics only. IEEE 519 addresses even harmonics by limiting them to 25% of the limits for the odd orders within the same range. Even harmonics result in an asymmetrical current wave (dissimilar positive and negative wave shapes) which may contain a dc component that will saturate magnetic cores.
Guidelines for Utilities:
The second set of criteria established by IEEE 519 is to voltage distortion limits. This governor the amount of voltage distortion that is acceptable in the utility supply voltage at the PCC with a consumer. The harmonic voltage limits recommended are based on levels that are low enough to ensure that consumers’ equipment will operate satisfactorily. Table 4.2 lists the harmonic voltage distortion limits from IEEE 519.
Table 4.2. Voltage distortion limits from IEEE 519
(For conditions lasting more than one hour. Shorter periods increase limit by 50%)
Note- High voltage systems can have up to 2.0% THD where the cause is a high voltage DC terminal which will attenuate by the time it is tapped for a user.
As for current, limits are imposed on individual components and on total distortion from all harmonic voltages combined (THD). What is different in this table, however, is that three different limits are shown. They represent three voltage classes, up to 69 kV, 69 to 161 kV, and equal to or greater than 161 kV. Note that the limits decrease as voltage increases, the same as for current limits.
Again only odd harmonic limits are shown in the table. The generation of even harmonics is more restricted since the resulting dc offset can cause saturation in motors and transformers. Negative sequence current can cause heating in generators. Individual even harmonic-voltage is limited to 25% of the odd harmonic limits, the same limit as currents.
Often utility feeders supply more than one consumer. The voltage distortion limits shown in the table should not be exceeded as long as all consumers conform to the current injection limits. Any consumer who degrades the voltage at the PCC should take steps to correct the problem. However, the problem of voltage distortion is one for the entire community of consumers and the utility. Very large consumers may look for a compromise with the utility over resolution of a specific problem, and both may contribute to its solution.