6 Ways to Control the Flow of Harmonic Currents and Reduce Harmonic Distortion Limits
Controlling Harmonic Techniques
The first technique used in industry to control harmonic-related problems involved the substantial use of single-tuned filters to provide a low-impedance path to harmonic currents. Interestingly, in an industry operating without a harmonic filter, it is not difficult to find loads that produce harmonics in the megavolt to ampere range. This is a difficult issue for utilities to control because current standards are often more like a reference guide for the industry than a regulatory statement.
Large harmonics generators, often in the industrial sector, may be the only ones to adopt harmonic filtering methods to reduce multiple distortions that can start to affect sensitive equipment and processes, which can occur beyond the point of measurement if it goes wrong. Due to the high cost, this method is not common practice in commercial and residential facilities.
Unfiltered harmonic currents are allowed to propagate freely upstream and downstream from the PCC, following natural propagation laws. They can reach adjacent installations and sometimes even go to the auxiliary substation.
It is common to see utilities and harmonic current generating customers in a constant search for alternative methods to address and hopefully overcome the increasing levels of harmonic distortion.
For example, special application transformers connected to variable frequency drives (VFDs) and therefore highly exposed to the extreme heat of harmonic current are often specified as special K-factor transformer designs. These special types of transformer structures prevent the transformer from operating below the nominal values. Type-K transformers are basically designed with improved windings and low loss iron cores that reduce the amount of additional heating produced by harmonic currents.
Note that harmonic currents on the source side of the converter are not controlled or removed in any way in the windings. Some cancellation of harmonics can occur, for example, in phase-shifted transformers that provide a 30° shift between two 6-pulse converters. One is delta connected and the other is fed from the star connected secondary windings of the transformer.
This technical article explains some techniques used in the industry to control the flow of harmonic currents produced by nonlinear loads in power systems. Most relevant are the following six:
Reconfiguration of the network
Increasing the short-circuit current ratio
Static multi-pulse power converters with phase shift transformers
sequential reactors
Phase load balancing
Unbalance of phase voltage
Effects of unbalanced phase voltage
load grouping
1. Reconfiguration of the network topology
One measure that is often useful to reduce the effect of unfiltered harmonics is network reconfiguration. In this application, it is necessary to identify the users and sectors in the installation that give a large amount of harmonic current to the system and to characterize the frequency content.
As is often the case in residential installations, redistributing loads through the same wiring or additional circuits can provide an economical solution to greatly reduce faults.
It is useful to divide the largest non-linear loads between different distributors, such as balancing single-phase loads in three-phase systems. This measure will reduce the overvoltage drops caused by harmonic currents that would otherwise be carried in a single path.
If harmonic filters are not an option to consider, mixing linear and non-linear loads in a feeder can allow reducing harmonic distortion as linear loads act as natural attenuators of parallel resonance peaks. This precaution should not be considered for linear loads, sensitive electronics or industrial processes that may be interrupted by a slight increase in THD.
2. Increased supply mode stiffness
Increasing the ratio between the current short-circuit current and the rated load current creates a stronger supply node. This happens when power companies increase the size of their substations. It also occurs when industrial customers add some cogeneration to the supply bus to aid operation during peak demand.
Rigid AC sources increase the available short-circuit current, for which the ratio between short-circuit and load currents is often used as a measure of weld stiffness. Strong supply nodes can be better absorbed in the network and mitigate the effects of sudden large transformer currents, cable energizing and large motor loads starting.
The same applies to harmonic currents reaching the substation. This is because the lower impedance of a solid source produces smaller voltage drops not only for steady state but also for higher frequency currents.
High short-circuit currents are then associated with low impedance sources, which in turn
They are inverse functions of the maker size. This can be demonstrated by calculating the impedance change when an "old" transformer rated MVA1 is replaced with a "new" transformer rated MVA2.
3. Harmonic cancellation using multi-pulse converters
Single-phase converters are used in small load applications. For lower initial costs, half-wave rectifiers can be applied when current requirements are small. Half-wave rectification produces a DC component that saturates the transformers. It is recommended to use full-wave rectifier converters to limit the former.
A basic polyphase converter is a six-pulse unit. Theoretically, the 12-pulse unit shown in the figure below will eliminate the lower order harmonics (5th and 7th) that will appear in the 11th and 13th rows. Since 17th and 19th harmonics are not characteristic, the following pair of harmonics will be 23rd and 25th.
4. Series reactors as harmonic attenuators
Series reactors have long been used in industry as a way to provide some control over short-circuit current levels. We see them in iron and steel or smelting plants and power substations, or in the neutral-to-earth connections of generators or power transformers. Series reactors are also used to some extent as harmonic attenuators in industrial applications.
Typically, 5% impedance reactors are seen installed upstream of power converters in a number of applications.
As an energy storage device that resists rapid changes in current, a series reactor theoretically provides a bidirectional attenuation against ripple and harmonic currents generated on either side. This means attenuation of harmonic currents from the converter (or any non-linear load) to the AC source and from harmonic currents from neighboring customers or from fluctuations produced in the distribution system towards the converter.
This seems attractive as a way of providing some relief to power line side transients or subtransients that, in addition to attenuation of harmonic currents, occur during switching capacitor banks or transients during long cables or line faults.
5. Phase balancing
Some electric power companies, residential installations, municipal street lighting, etc. It uses four-wire distribution systems with a primary grounded wye and single-phase transformers that provide phase-to-earth voltage to single-phase loads such as Variations in single-phase loads can create unbalance. Currents in three-phase conductors produce different voltage drops in the three-phase and cause a phase-to-phase voltage imbalance.
Maximum phase-to-phase or phase-to-ground voltage imbalance can be more critical at the far end of a distribution feeder, where the voltage may experience a significant drop during heavy load conditions, especially in the absence of appropriate voltage profile compensation measures.
“A perfectly balanced system is difficult to achieve because single-phase loads are constantly changing, creating a constant imbalance in phase voltages and eventually causing unequal and uncharacteristic harmonics.
5.1 Phase voltage unbalance
The simplest method of determining voltage unbalance is to calculate the largest deviation of the phase-to-phase voltage from the average voltage:
For example, if a 480 V application shows VAB, VBC, and VCA voltages equal to 473, 478, and 486 V, respectively, with an average voltage (473 + 478 + 486)/3 = 479 V, the voltage imbalance is:
5.2 Effects of unbalanced phase voltage
When unbalanced phase voltages are applied to three-phase motors, it causes additional negative sequence currents to circulate in the motor windings, increasing the heating losses. The most severe condition occurs under an open phase condition.
All motors are sensitive to phase voltage imbalance. Certain types of motors, such as those used in hermetically formed compressors in air-conditioned units, are more susceptible to this condition. These motors operate with high current densities in the windings due to the total effect of refrigerant cooling.
When a motor is suddenly shut down by a protection system, the first step consists of determining the cause of the disconnection and checking the operating current after recommissioning to ensure the motor is not overloaded. The next step consists of measuring the voltage across the three phases to determine the amount of voltage imbalance.
The figure below shows a motor overheating when the voltage imbalance exceeds 2 to 3% for full load operation. Computer operation may be affected by a voltage imbalance of 2% to 2.5%.
In general, single-phase loads should not be connected to three-phase circuits that power sensitive equipment. A separate circuit must be used for this.
6. Load grouping
Extensive electrical networks can have nonlinear loads with different spectral content. Whenever possible, calculate loads by harmonic spectrum type.
Upscaling (eg 6-pulse converters, 12-pulse converters, arc-type devices, fluorescent lighting, etc.) can optimize the installation, position and sizing of harmonic filters.
While this is a difficult task to accomplish especially when comparable load types are not in the same place, the idea should be considered as a way to reduce the number of harmonic filters to be installed.
Load grouping can also help reduce telephone interference by trying to keep telephone lines as far away as possible from places carrying high order harmonic currents.