Guide to Harmonics
with AC Variable Frequency Drives
- Basics of the harmonics phenomena
- Harmonic distortion sources and effects
- Harmonic distortion calculation by using DriveSize software
- Circuit diagram for the calculation example
- Input data for motor load
- Motor selection
- Inverter selection
- Inverter supply unit data
- Network and Transformer data input
- Calculated harmonic current and voltage
- Calculated harmonic currents in graphical form
- Part of the printed report
- Standards for harmonic limits
- EN61800-3 (IEC1800-3) Adjustable speed electrical power drive systems
- IEC1000-2-2, Electromagnetic compatibility (EMC)
- IEC1000-2-4, Electromagnetic compatibility (EMC)
- IEC1000-3-2, Electromagnetic compatibility (EMC)
- IEC1000-3-4, Electromagnetic compatibility (EMC)
- IEEE519, IEEE Recommended practices and requirements for harmonic control in electrical power systems
- Evaluating harmonics
- How to reduce harmonics by structural modifications in the AC drive system
- Factors in the AC drive having an effect on harmonics
- Table: List of the different factors and their effects
- Using 6-pulse diode rectifier
- Using 12-pulse or 24-pulse diode rectifier
- Using phase controlled thyristor rectifier
- Using IGBT bridge
- Using larger DC or AC inductor
- Other methods for harmonics reduction
- Tuned single arm passive filter
- Tuned multiple arm passive filter
- External active filter
- Summary of harmonics attenuation
- 6-pulse rectifier without inductor
- 6-pulse rectifier with inductor
- 12-pulse rectifier with polygon transformer
- 12-pulse with double wound transformer
- 24-pulse rectifier
- Active IGBT rectifier
Standards for harmonic limits
The most common international and national standards setting limits on harmonics are described below. Figure 5.1 is shown as an example for harmonic distortion limits.
Part 3: EMC product standard including specific test methods
The countries of the European Economic Area (EEA) have agreed on common minimum regulatory requirements in order to ensure the free movement of products within the EEA. The CE marking indicates that the product works in conformity with the directives that are valid for the product. The directives state the principles that must be followed. Standards specify the requirements that must be met. EN61800-3 is the EMC product standard of adjustable speed electrical power drive systems (PDS). Meeting the requirements of this standard, is the minimum condition for free trade of power electronics converters inside the EEA.
EN61800-3 states, that the manufacturer shall provide in the documentation of the PDS, or on request, the current harmonic level, under rated conditions, as a percentage of the rated fundamental current on the power port. The referenced values shall be calculated for each order at least up to the 25th. The current THD (orders up to and including 40), and its high-frequency component PHD (orders from 14 to 40 inclusive) shall be evaluated. For these standard calculations, the PDS shall be assumed to be connected to a PC with Rsc = 250 and with initial voltage distortion less than 1%. The internal impedance of the network shall be assumed to be a pure reactance.
In a low voltage public supply network, the limits and requirements of IEC1000-3-2 apply for equipment with rated current � 16 A. The use of the future IEC1000-3-4 is recommended for equipment with rated current > 16 A. If PDS is used in an industrial installation, a reasonable economical approach, which considers the total installation, shall be used. This approach is based on the agreed power, which the supply can deliver at any time. The method for calculating the harmonics of the total installation is agreed and the limits for either the voltage distortion or the total harmonic current emission are agreed on. The compatibility limits given in IEC1000-2-4 may be used as the limits of voltage distortion.
IEC1000-2-2, Electromagnetic compatibility (EMC)
Part 2: Environment – Section 2: Compatibility levels for low frequency conducted disturbances and signalling in public low-voltage power supply systems
This standard sets the compatibility limits for low-frequency conducted disturbances and signalling in public lowvoltage power supply systems. The disturbance phenomena include harmonics, inter-harmonics, voltage fluctuations, voltage dips and short interruptions voltage inbalance and so on. Basically this standard sets the design criteria for the equipment manufacturer, and amounts to the minimum immunity requirements of the equipment. IEC1000-2-2 is in line with the limits set in EN50160 for the quality of the voltage the utility owner must provide at the customer’s supply-terminals.
IEC1000-2-4, Electromagnetic compatibility (EMC)
Part 2: Environment – Section 4: Compatibility levels in industrial plants for low frequency conducted disturbances
IEC1000-2-4 is similar to IEC1000-2-2, but it gives compatibility levels for industrial and non-public networks. It covers low-voltage networks as well as medium voltage supplies excluding the networks for ships, aircraft, offshore platforms and railways.
IEC1000-3-2, Electromagnetic compatibility (EMC)
Part 3: Limits – Section 2: Limits for harmonic current emissions (equipment current <16 A per phase)
This standard deals with the harmonic current emission limits of individual equipment connected to public networks. The date of implementation of this standard is January 1st 2001, but there is extensive work going on at the moment to revise the standard before this date. The two main reasons for the revision are the need for the standard to cover also the voltage below 230 V and the difficulties and contradictions in applying the categorisation of the equipment given in the standard.
IEC1000-3-4, Electromagnetic compatibility (EMC)
This standard has been published as a Type II Technical report. Work is going on to convert it into a standard. It gives the harmonic current emission limits for individual equipment having a rated current of more than 16 A up to 75 A. It applies to public networks having nominal voltages from 230 V single phase to 600 V three phase.
The standard gives three different stages for connection procedures of the equipment. Meeting the individual harmonic limits of Stage 1 allows the connection of the equipment at any point in the supply system. Stage 2 gives individual harmonic current limits as well as THD and its weighted high frequency counterpart PWHD. The limits are classified and tabulated by the short circuit ratio. The third stage of connection is based on an agreement between the user and the supply authority, based on the agreed active power of the consumer’s installation. If the rated current is above 75 A, Stage 3 applies in any case.
The structure of this standard is generally seen to be good, but it may justly be questioned whether single and threephase equipment should have different limits in Stage 2. It is very probable that the structure of the standard will remain as it is, but the version having the status of actual standard, will contain different limits for single and three-phase equipment.
IEEE519, IEEE Recommended practices and requirements for harmonic control in electrical power systems
The philosophy of developing harmonic limits in this recommended practice is to limit the harmonic injection from individual customers so that they will not cause unacceptable voltage distortion levels for normal system characteristics and to limit overall harmonic distortion of the system voltage supplied by the utility. This standard is also recognised as American National Standard and it is widely used in the USA, especially in the municipal public works market.
The standard does not give limits for individual equipment, but for individual customers. The customers are categorised by the ratio of available short circuit current (Isc) to their maximum demand load current (IL) at the point of common coupling. The total demand load current is the sum of both linear and non-linear loads. Within an industrial plant, the PCC is clearly defined as the point between the non-linear load and other loads.
The allowed individual harmonic currents and total harmonic distortion are tabulated by the ratio of available short circuit current to the total demand load current (Isc/IL) at the point of common coupling. The limits are as a percentage of IL for all odd and even harmonics from 2 to infinity. Total harmonic distortion is called total demand distortion and also it should be calculated up to infinity. Many authors limit the calculation of both the individual components and TDD to 50.
The table 10.3 of the standard is sometimes misinterpreted to give limits for the harmonic emissions of a single apparatus by using Rsc of the equipment instead of Isc/IL of the whole installation. The limits of the table should not be used this way, since the ratio of the short circuit current to the total demand load current of an installation should always be used.