A commonly used and very important standard is IEEE 519-1992, and more recently IEE 519-2014. The standard, among other things, puts two requirements on harmonics; and absolute maximum THDU level, and a variable maximum TDD level. All limits are applied to the Point of Common Coupling (PCC), which is the interface between utility (sometimes called operator) and consumer. The PCC can be located at any voltage level. In some cases, the PCC is considered to be an internal point in a system of particular interest; this is not in line with the original intention of the IEEE 519, which considered only the connection point between operator and user (consumer). These concepts are illustrated in the figure below:
Below is Table 1, from IEEE 519 (2014), p6, “Voltage distortion limits”, as outlined below:
Table 1 (IEEE 519-2014, pg. 7) Voltage Distortion Limits
|Bus voltage V at PCC||Individual harmonic (%)||Total harmonic distortion THD (%)|
|V ≤ 1.0 kV||5.0||8.0|
|1 kV < V ≤ 69 kV||3.0||5.0|
|69 kV < V ≤ 161 kV||1.5||2.5|
|161 kV < V||1.0||1.5a|
a High-voltage system are allowed up to 2.0% THD where the cause is an HVDC terminal whose effects will have been attenuated at points in the network where future users may be connected.
Note that these levels are absolute, and not depending on the size of the operator/utility or the consumer. Also note that the resulting distortion level is the result of the combination of the background distortion and the load distortion created by the consumer.
Following below is an excerpt from Table 2 (IEEE 519-2014, pg. 7, replacing table 10.3, p78, “Current Distortion Limits for General Distribution Systems” in IEEE 519-1992). This table is of importance as it defines target levels to be achieved depending on the short circuit ratio ISC/IL. ISC is the rated short circuit current at PCC, and IL is the maximum demand load current at PCC.
Table 2 (IEEE 519-2014, pg. 7)
Current distortion limits for systems rated 120 V through 69 kV
|ISC/IL||Individual harmonic limits (Odd harmonics) a,b||TDD Required|
|3 ≤ h < 11||11 ≤ h < 17||17 ≤ h < 23||23 ≤ h < 35||35 ≤ h ≤ 50|
a Even harmonics are limited to 25% of the odd harmonic limits above
b Current distortions that result in a dc offset, e.g., half-wave converters, are not allowed
c All power generation equipment is limited to these vales 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
Note the major difference in how harmonics are limited at the current and at the voltage. For voltage harmonics, all requirements are absolute. For current harmonics, the authors of IEEE 519 chose to limit the current harmonics depending on how strong the voltage source is. This is reasonable and understandable; a strong grid will be able to suppress current harmonics to a much larger degree without the voltage being influenced than a weak grid. In very weak grids, voltage distortion and current distortion may have similar values. Hence, it can be argued that current emission requirements must be stricter in weaker grids.
Total Demand Distortion
Total Demand Distortion (TDD) is defined as the ratio of the root mean square of the harmonic content, considering harmonic components up to the 50th order and specifically excluding interharmonics, expressed as a percent of the maximum demand current. Harmonic components of order greater than 50 may be included when necessary.
THDI uses the instantaneous fundamental current as reference. TDD uses the maximum demand current (maximum current) as reference. This means, at 100% load THDI = TDD. The difference between THD and TDD can be quite dramatic, as illustrated below.
TDD, not THDI. Be sure to make it clear if the requirement from the customer is TDD or THDI before specifying your ADF size. Preferably only specify THDI at 100% load or use TDD instead!