Problems Caused by Harmonics in Electrical Plants
Harmonics are popular power quality distortion of recent years that is being spoken more and more everyday and has different distorting impacts in many different facilities. Within scope of this article, distortion affects caused by harmonics on various basic electrical equipments shall be examined.
Harmonics are not available in an electrical system having linear elements fed by a source having purse sinusoidal wave shape at fixed amplitude and frequency. However, non-linear loads such as power electronic elements increasing parallel to advanced technology withdraw harmonic currents over the source and pollute the source. Electrical systems with intense harmonics experience various problems arising from harmonics.
Problems caused by harmonics have many impacts on facilities both in technical and commercial aspects. Correctly analyzing and understanding these impacts has critical importance regarding operation sustainability in facilities and compliance of facilities to national and international directives.
Current and voltage wave shapes expected to be in pure sinusoidal form under normal conditions are distorted in systems with harmonics. Problems generally caused by the wave shape distortion are;
- Source voltage wave shape distortion
- Efficiency loss in transmission and distribution depending on RMS current increase on lines
- Failures in compensation systems
- Overheating in electrical engines and transformers
- Failures in sensitive electronic devices, PLC and CNC devices
- Difficulties and abrasions in insulation levels of equipments
- Increased losses in the system
- Having faulty works in protection and control systems
- Increased voltage reductions
- Resonance risks at high frequencies in systems
Frequency and magnitude of the problem varies depending on factors such as facility type, facility operation voltage, load type / intensity and grid short circuit power.
Failure Analysis Based on Equipment Type
Various electrical elements such as engine, condenser, driver, computer, lamp are used both in industry and commercial businesses. Harmonics have different impacts on these elements. As harmonics are high frequency components with full fold magnitude of basic grid frequency, availability of high frequencies in the grid shall increase resonance risks. Impacts arising from harmonics may remain at one or more element local sometimes while they may cause global problems affecting operation of entire facility. Impacts of harmonics on various electrical equipments are explained below.
1 – Condensers
Condensers, commonly used in electrical systems for compensation purposes, are linear circuit elements and do not withdraw harmonic current from the grid. However, condensers in a system having distortion loads, harmonic currents and harmonic voltages are considerable affected from harmonics.
Condenser impedance is calculated as;
As it can be seen from the equation, increased frequency shall cause reduction in the condenser impedance. For instance, 10 ? condenser impedance at 50 Hz grid frequency shall reduce five folds for 5th Harmonic component frequency 250 Hz and become 2 ?. Accordingly, condensers indicate low impedance to high frequency harmonic components.
Condensers have tendency to withdraw harmonics created by the distortion loads in the system due to their impedances. RMS current shall increase in the condenser withdrawing harmonic currents with the basic grid frequency and start having electrical and mechanical difficulties. IEC 60831 standard has determined a limit for this increase and all condenser manufacturers manufacture their condensers as resistant to 30% more than the nominal current (8 hours / 24 hours).
Another item of relation between condensers and harmonics is the resonance problem. Condensers used in systems which use inductive reactive characterized loads such as transformer, engine create the possibility of serial and parallel resonance with these loads. In order to prevent this situation, it shall be suitable to perform compensation with condensers equipped with serial reactors instead of bare condensers.
2 – Engine and Generators
Most important impact created by harmonics in rotating machines such as engine and generator is overheating in rotor and stator circuits. Copper losses increase due to harmonic currents and iron losses increase due to harmonic voltages. High frequency harmonic components distort the rotating magnetic zone in the grid frequency required to be created for smooth rotation of the machine and cause noisy and vibrated operation of the machine. Efficiency of an engine operating in a system with harmonics reduces.
3 – Transformers
Harmonics create an increasing impact for iron and copper loss on transformers. Increased frequency shall cause increased loss, therefore leveled harmonics may be more effective than low level components.
Harmonics cause noise, abrasion and mechanic difficulties on transformers.
Particularly, a considerable part of loads fed by transformers in industrial facilities being harmonic source revealed the requirement of paying attention during transformer sizing. Harmonics in the transformer increase heat and insulation problems with losses. Therefore, K-factor multiplier is used for special transformers or standard transformers.
When magnetic core of transformers reaches saturation, they also generate harmonic. Thus, transformers are indicated as harmonic sources in the literature.
4 – Switching Elements and Fuses
RMS currents increasing with availability of harmonic currents cause faulty operations on switching elements. Failures may occur in operation of electromagnetic induction coil used by circuit breakers while performing arc extinguishing function due to high frequency components. Faulty operations in the coil affect operation of the breaker. Heat increase arising from harmonics also negatively affects life of switching elements.
Untimely/unnecessary exposals are experienced in fuses connected to RMS current increase, problems occur in protection selection in systems due to changed fuse opening curve characteristics.
5 – Electronic – Sensitive Devices
Electronic control devices operating in systems with harmonics sometimes cause faulty operations. Harmonics distort wave shape of the voltage as well as change and shift zero pass points. Zero pass point is an important reference point in electronic control devices such as PLC and any change in this reference affects operation of entire system.
Electronic card failures are among impacts arising from harmonics. Condensers within the interference filter circuit located at card entrances may withdraw excessive current and explode due to harmonics. In case of card failures of critical loads such as Turning Lathe, tangible provision of this problem reaches serious amounts.
6 – Cable and Busbar
Harmonic components are high frequency and create the impact called as skin-effect in the literature. Resistance of a conductor at alternative current is higher than the resistance at the direct current. This resistance increases more for harmonic components.
While high frequency harmonic currents pass through the conductor, they shall commonly prefer the outer surface of the conductor and that shall cause reduction of the section used by current for flowing and increase of resistance. Conductor losses increase due to skin effect and conductors expose to mechanic difficulties and vibrations. Overheating is one of the negative results of harmonic sourced skin effect on conductors.
In addition to above written issues, many electrical items such as relays, illumination items, transmission and distribution lines are getting affected from harmonics. Through purification of electrical systems from harmonics, commercial revenues acquired for facilities and all system elements can operate for a longer period and without any problems through improvement of facility regarding power quality.