THDi – New Standards And The Impact For The UPS Industrial Application

The objective of this article is to explain the THDi for the UPS industrial application, mainly regarding the new statements of the revised standard, which will become valid soon.

Disturbances Injected into The Mains Power Supply

Generally speaking, the Uninterruptible Power Supply (UPS) can be considered as a conversion system and the technology used for the input stage of the UPS determines the type and quantity of harmonic pollution generated and injected into the upstream mains power supply.

Total harmonic distortion (THD) is a measure of the harmonic distortion present in a signal and is defined as the ratio between the sum of the powers of all harmonic components and the power of the fundamental frequency. See below for current (I):

In a double conversion UPS for example, the AC/DC converter element of the UPS (or rectifier as it is otherwise known) is connected directly to the mains power supply. For a Silicon Controlled Rectifier (SCR) thyristor-based rectifier, if the converter devices are constantly conducting (SCRs fully on), then the total harmonic distortion generated on the current waveform (THDi) during normal operation is ≤30%. With phase angled control of the SCRs (reduced conduction of the SCRs), the distortion level increases. This can introduce and create serious problems for the installation:

  • Distortion amplification (series and parallel resonance).
  • Reduction in use of sources.
  • Instability of power generating units.
  • Dielectric ageing.
  • Poor equipment functioning.
  • Reduced System Efficiency.
  • Thermal affects (on transformers, cables, motors etc).

Non-linear loads (in this case a UPS) inject the generated harmonic currents into the upstream supply circuit and depending on the level of THDi and the impedance of the upstream circuit, will cause a distortion of the voltage supply. Because of this, EMC Standards define and impose limits to permissible voltage distortions in low voltage grids. These restrictions are not fixed but are changing and evolving over the time.

In general, a current distortion total that causes a voltage distortion level to not exceed 8% at the point of common coupling (PCC) is considered reasonable, this in accordance with EN 50160 (THDv<8% in public grids – low voltages), IEC 61000-2-2 (THDv<8% in public low voltage networks) and IEC 61000-2-4 (THDv: <5% labs; <8% public; <10% heavy industrial).

  

In order to calculate this effect, the upstream impedance value, the generator power and the level of harmonics created by the UPS must be known. These values can be used to identify the voltage distortion caused by the UPS at the PCC, if the UPS is considered as the only load powered by the upstream source. In the case that other loads besides the UPS are supplied by the source, the harmonic voltage distortion must be calculated by including the harmonic pollution generated by the other loads also.

Table 1:
Compatibility levels for the individual harmonic voltages in low voltage networks (IEC 61000-2-2)

Odd Harmonics Non-Multiple of 3

Odd Harmonics Multiple of 3

Even Harmonics

Harmonic Order h

Harmonic Voltage %

Harmonic Order h

Harmonic Voltage %

Harmonic Order h

Harmonic Voltage %

5 6 3 5 2 2
7 5 9 1,5 4 1
11 3,5 15 0,4 6 0,5
13 3 21 0,3 8 0,5

Note: The levels given for odd harmonics that are multiple of three apply to zero sequence harmonics. Also, on a three-phase network without a neutral conductor or without load connected between line and ground, the values of the 3rd and 9th harmonics may be much lower than the compatibility levels, depending on the unbalanced of the system.

 


In terms of impedance, it is the case that in low power rated supply systems, the impedance is high. The impedance decreases as the power of the supply system increases. Based on this, the effect of the harmonics is potentialized in systems of lower power ratings, because the voltage is distorted more, and this increases the harmful effects on the grid. This is the main reason why in the new statement for the standard IEC 62040-2:2016 (referenced later in this document), that the harmonic tolerance increases depending on the power ratio (Grid/UPS) of the system.

Some examples of different topologies currently used for AC/DC converters in three-phase input UPS are shown below. Fully controlled 6 pulse rectifier, 12 pulse rectifier and 12 pulse rectifier with harmonic filter:

Harmonic​

6 pulse​

12 pulse + filter​

1​

100%​

100%​

2​

0%​

0%​

3​

0%​

0%​

4​

0%​

0%​

5​

26%​

4%​

6​

0%​

0%​

7​

5%​

4%​

8​

0%​

0%​

9​

0%​

0%​

10​

0%​

0%​

11​

4,5%​

4,5%​

12​

0%​

0%​

13​

3%​

3%​

14​

0%​

0%​

15​

0%​

0%​

16​

0%​

0%​

17​

0,5%​

1,8%​

18​

0%​

0%​

19​

1,6%​

1%​

20​

0%​

0%​

THDi %​

28%​

8,5%​

Table 2: Comparison between 6 pulse and 12 pulse with harmonic filter 

  

Harmonic​

6 pulse​

12 pulse​

1​

100%​

100%​

2​

0%​

0%​

3​

0%​

1,7%​

4​

0%​

0%​

5​

26%​

3,6%​

6​

0%​

0%​

7​

5%​

2,3%​

8​

0%​

0%​

9​

0%​

0%​

10​

0%​

0%​

11​

4,5%​

4%​

12​

0%​

0%​

13​

3%​

7%​

14​

0%​

0%​

15​

0%​

0%​

16​

0%​

0%​

17​

0,5%​

2%​

18​

0%​

0%​

19​

1,6%​

2%​

20​

0%​

0%​

THDi %​

28%​

9,8%​

Table 3: Comparison between 6 pulse and 12 pulse without filter  

                 

New THDi Requirement According To IEC 62040-2:2016

On 9th July of 2021, the transition time from standard EN IEC 62040-2:2006 to standard EN IEC 62040-2:2016 will end.

In terms of harmonic requirements, IEC 62040-2:2006 is only applicable for UPS with input currents less than 16A​. However, for IEC 62040-2:2016 this range was increased​:

  • Current harmonics of UPS with input current < 16A as well as between 16-75A are defined.
  • Current harmonics of UPS with input current ≥ 75A is not required yet

 

According to IEC 61000-3-12, the requirements of current harmonics are shown as below:

  • Input current between 16A and 75A.

  • Max. limits of current harmonics for symmetrical 3-phase system are given in Table 4.

  

Table 4 – Current emission limits for balanced three-phase equipment:

Minimum Rsce

Admissible individual Harmonic current  Ih/Irefa

Admissible harmonic parameters %

 

I5

I7

I11

I13

THC/Iref

PWHC/Iref

33 10,7 7,2 3,1 2 13 22
66 14 9 5 3 16 25
120 19 12 7 4 22 28
250 31 20 12 7 37 38

≥ 350

40 25 15 10 48 46

The relative values of even harmonics up to order 12 shall not exceed 16/h %. Even harmonics above order 12 are taken into account in THC and PWHC in the same way as odd order harmonics.

Linear interpolation between successive Rsce values is permitted

a Iref =Reference current; Ih = harmonic current component 

 

Rsce is defined as the short-circuit ratio and is described as the ratio between the short-circuit power of the grid relative to the apparent power of the UPS. Summarizing therefore, for power ratings up to approximately 50kVA (400V 3 phase systems), we will have to comply with the harmonic levels as defined for Rsce≥33 to cover all kinds of grid, which means an IGBT technology AC/DC converter on the UPS front-end. 

However, for the industrial applications, where the short-circuit power of the grid is usually higher than 5kA, this can be partially complied with, using the harmonic levels as defined for Rsce≥66.

   

Representing this information in diagram form, whereby device power is proportional to its block size, we have the following diagram.

Please see below a working example of the Rsce calculation, based on an industrial UPS (typical values):

  • Mains voltage (L-L): 400V.
  • Mains short circuit current: 5kA.
  • UPS output apparent power: 50kVA.
  • UPS output power factor: 0,80.
  • UPS efficiency (AC to AC): roughly 0,90.
  • UPS input power factor: roughly 0,85.

 

Mains Short Circuit Power: 

UPS Output Active Power:  

UPS Input Apparent Power:  

So, the Rsce will be:

 

 

The typical Rsce will not be lower than 66 for industrial applications, because 50kVA is the biggest commercial rating covered by the new standard (in 400V and 3 phase systems) and the mains short circuit current is usually >5kA.

Therefore, technologies of 6 pulse or 12 pulse (plus filters) can comply with the limits of Rsce≥66 and also comply with the new standard.

Based on that explanation, I hope to have helped a better understanding of the changes on the standard regarding the harmonic distortion, and the impact mainly for the UPS industrial application.

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Author


Henrique Baldin Braga

Product Manager AC Industrial
AEG Power Solutions