Harmonic currents in the selection of busbar trunking systems (busways)
From Electrical Installation Guide
Contents |
Introduction
Harmonic current is generated by most modern electronic loads, which can be found in all sectors of Industrial, Commercial, and domestic facilities. These electronic loads use power electronic devices which are responsible for generating harmonic currents. Common non-linear loads:
- Industrial equipment (Soldering machines, Induction furnaces, bridge rectifiers and battery chargers)
- Variable Speed Drives (VSDs) with AC or DC motors
- Uninterruptible Power Supplies (UPS)
- Information Technology Equipment (computers, monitors, servers, copiers, printers, etc.)
- Domestic equipment (TV sets, microwave ovens, fluorescent lamps, light dimmers, etc.).
Today’s electronic loads share a common element: electronic power supplies.
The benefits of the electronic power supply are its cost, efficiency and the ability to control its output. For this reason, they are found in a wide variety of common single and three-phase electrical equipment. Harmonic currents are a natural by-product of the manner in which electronic power supplies draw current. In order to be more efficient, these devices draw current for only a small portion of the electrical cycle.
Installations where these devices can be found in great number are computer centers, banks, Internet Data Centers etc.
Harmonic currents generated by these loads present some problems:
- Voltage distortion responsible for failure of some types of electrical equipment
- Increased losses, the rms current being higher than the fundamental design current
- Risk of resonance when power factor correction capacitors are present.
Third harmonic currents (150/180 Hz) or multiple of 3 (triple-n harmonics) are specifically responsible for increased neutral currents in three-phase, four-wire systems.
That the reason why it’s important to select optimum busbar design for office buildings, where neutral conductor overload is a major concern.
Neutral current in three-phase, four-wire systems
Figure E48 represents the non-linear phase currents and resulting non-linear neutral current, in a three-phase, four-wire system, supplying identical single phase loads.
Fig. E49: Examples of applications where the level of harmonics (THD) is either negligible or high, depending on the proportion of loads generating harmonics versus classical loads
The harmonic spectra of the phase and neutral currents are represented in Figure E49 and E50. It can be seen that the neutral current only includes third or triple-n harmonics (i.e. 3, 9, 15, etc). The amplitude of these currents are equal to three times the amplitude of the phase currents. In the neutral current measurements, third harmonic has the greatest magnitude and the other triple-n’s (9, 15, 21, etc.) decrease significantly in magnitude so do not contribute significantly to the rms value.
In this example, the rms value of the neutral current is equal to 1.732 (√3) times the rms value of the line current. This theoretical value is only obtained with loads absorbing a current similar to the one represented on Figure E48.
When the loads include partially linear circuits (such as motors, heating devices, incandescent lamps), the rms value of the neutral current is strictly less than √3 times the rms value of the phase currents.
Load factor of the neutral conductor
Simulations have been carried out to assess the influence of the 3^{rd} harmonic level on the neutral conductor current. Figure E51 represents different line current waveforms for different amounts of non-linear load. The same active power was maintained (linear loads are assumed purely resistive).
The neutral current is then calculated and compared to the line current for different levels of third harmonic. The load factor of the neutral conductor (ratio of the neutral current to the line current) is represented in Figure E52.
In installations where there are a large number of single-phase electronic non-linear loads connected to the same neutral, a high load factor can be found in that neutral.
In these installations the neutral current may exceed the phase current and a special attention must be given to sizing the neutral conductor. This prevents the installation of a reduced size neutral conductor, and the current in all four wires should be taken into account.
The diversified power absorbed by such a group of loads is generally limited, and even if the neutral current exceeds the line current, then the neutral conductor capacity is only exceeded in extreme circumstances if its size is equal to the line conductor's.
A common practice in these conditions is to use a 200 % neutral conductor. This does not form part of the electrical/ building regulations, but is encouraged by organizations such as the Copper Development Association.
In high power installations (>100 kVA or >150 A), various factors contribute to reduce the neutral conductor load factor:
- More and more high quality IT equipment (work stations, servers, routers, PC, UPS, etc.) include Power Factor Correction circuits, reducing considerably the generation of 3^{rd} harmonic currents
- HVAC equipment in large buildings are supplied by a three-phase network, and as such do not produce triple-n harmonic currents
- Fluorescent lighting equipment (with magnetic or electronic ballast) generates triple-n harmonic currents which are phase shifted with harmonic currents generated by PCs, giving a partial vector cancellation.
Except in exceptional circumstances, the 3^{rd} harmonic level in these installations does not exceed 33 %, so the neutral current does not exceed the line currents. It is not therefore necessary to use an oversized neutral conductor.
Effects of harmonic currents on circuit conductors
The circulation of harmonic currents produces additional heating within the conductors for several reasons:
- Heat is produced as a result of the additional high levels of triple-n harmonic currents, compared with the relatively minimal current flowing in the neutral for normal balanced linear loads.
- Additional heating of all conductors by increase of the skin effect and eddy current losses due to the circulation of all harmonic orders.
Modeling separately the power losses created by each harmonic order reveals the impact of harmonic currents in busbar trunking systems. Heat measurements performed on busbar trunking systems with circulation of harmonic currents of different frequencies has been also been considered.
The same approach has been used to compare two different type of busbar construction both with the same total cross sectional area (c.s.a.) of active conductors, a 200 % neutral and a standard 100 % neutral. This can be seen in Figure E55.
Placed in the same conditions, a busbar trunking system with 4 identical conductors will have a lower temperature rise than a 200 % busbar with the same total c.s.a.
It is then perfectly adapted to this situation. Of course, the selection of the size of the conductors must take the possible current flowing through the neutral conductor into account.
Simplified selection procedure
The first step in the selection procedure for busbar trunking systems is to assess the phase currents and 3^{rd} harmonic current level.
Note: the 3^{rd} harmonic current level has an impact on the neutral current, and consequently on the rating of all components in the installation:
- Switchboard,
- Protection and dispatching switchgear,
- Cables and busbar trunking systems.
Depending on the estimated 3^{rd} harmonic level, 3 cases are possible:
A) 3^{rd} harmonic level below 15 % (ih3 ≤ 15 %):
The neutral conductor is considered as not loaded. The size of the phase conductors is only dependant on the phase currents. According to IEC rules, the neutral conductor size may be smaller than the phase conductors', if the cross section area is higher than 16 mm^{2} for copper, or 25 mm^{2} for aluminium.
B) 3^{rd} harmonic level between 15 and 33 % (15 < ih3 ≤ 33 %)
The neutral conductor is considered as current-carrying conductor.
The practical current shall be reduced by a factor equal to 84 % (or inversely, select a busbar with a practical current equal to the phase current divided by 0.84. Generally, this leads to the selection of a busbar trunking system, which the current rating is immediately superior to the requested capacity.
The size of the neutral conductor shall be equal to that of the phases.
C) 3^{rd} harmonic level higher than 33 % (ih3 > 33 %)
The neutral conductor is considered as a current-carrying conductor.
The recommended approach is to adopt circuit conductors with equal size for phase and neutral. The neutral current is predominant in the selection of the size of conductor.
Generally, this leads to the selection of a busbar trunking system which current rating is higher than the requested capacity (generally by a factor of two).
Example for KT Schneider-Electric offer:
Rating (A) | No harmonic | Usual harmonic level | Very high level |
---|---|---|---|
1000 | KTC1000 | KTC1000HRB | KTC1350HRB |
1350 | KTC1350 | KTC1350HRB | KTC1600HRB |
1600 | KTC1600 | KTC1600HRB | KTC2000HRB |
2000 | KTC2000 | KTC2000HRB | KTC2500HRB |
2500 | KTC2500 | KTC2500HRB | KTC3200HRB |
3200 | KTC3200 | KTC3200HRB | KTC4000HRB |
4000 | KTC4000 | KTC4000HRB | |
5000 | KTC5000 |
Conclusions
Office buildings are often subject to the circulation of high levels of triple-n harmonics in particular 3rd harmonic current. These are responsible for possible overload of the neutral conductor.
The performance of standard construction busbar trunking system with circulation of harmonic currents has been analyzed in depth.
A simplified procedure has been proposed for selection of busbar trunking systems adapted to the circulation of harmonic currents, and particularly in the neutral conductor.
A 200 % neutral conductor is not the optimum solution.
Busbar trunking systems with equal size for all conductors are perfectly adapted to harmonic distortion. The design is valid as long as the design for a realistic neutral overload is taken into consideration and is applied to the whole system.
The raw material and performance optimization for more guarantees
Figure E58 shows the comparison between 2 busway constructions. The test conditions are the same for both cases:
- Phase current: IL = 1600 A
- 3rd harmonic level: ih3 = 33%
- Neutral current: IN = 1520 A
Placed in the same conditions, a busbar trunking system with 4 identical conductors will have a lower temperature rise than a 200 % busbar with the same total c.s.a.
It is then perfectly adapted to this situation. Of course, the selection of the size of the conductors must take the possible current flowing through the neutral conductor into account.
Fig. E58: Comparison between double-neutral busway solution and properly selected single-neutral solution
Coherent system approach
The approach on busway dedicated to harmonics network performance is a solution approach. The busway is optimized but completely in accordance with the electrical devices connected on it:
- Tap-off unit
- Circuit breakers
- Number of cables.
Fig. E59: Coherent system approach for all components of the electrical installation