The power quality of electrical power systems has a severe influence on control and utilization of power. Electrical power systems behave like nonlinear loads, creating a deformed waveform that is made up of voltage and current harmonics. Voltage and current harmonics created by these nonlinear loads cause problems such as increasing power losses, degrading the conductors, and as a result have negative effect on the distribution systems and other electrical segments. It is therefore essential to evaluate the complete effect of these harmonics.
The sum total of the various harmonics present in a system is called Total Harmonic Distortion (THD). THD gives the opportunity to evaluate the extent of distortion in a system. The phenomenon of total harmonic distortion on distribution networks has been discussed below for the benefit of end-users.
What is Total Harmonic Distortion (THD)?
THD is a very complicated phenomenon to understand. Yet, it becomes simpler when it is divided into the fundamental definitions of harmonics and distortion. Other switching devices like solar PV inverters can cause distortion in the system’s voltage as well as abnormal unusual conditions of sensitive loads.
In spite of the economic difficulties, electricity utilities and end-users are still acquiring fresh spaces and buying extra loads. Other facilities with limited or no loads will soon be reoccupied and fixed into the grid in the next few years. The vast competition in several businesses would cause more loads to be added to the system. The increase in load will cause other power quality issues. New power quality issues will appear. Harmonics on power distribution networks is one of the power quality problems.
This relates to the increasing installation of solar PV panels. The incorporation of a power inverter in the front-end power conservation networks will affect the power quality relating to voltage harmonics. Similar to harmonic distortion are voltage sags and surges. Most end-users might or might be the cause of these problems.
Let’s take this for example: a power system containing an AC source and a load.
Power System with AC source and electrical load
The electrical load could be linear or non linear; and will affect the quality of power of the system in one way or the other. Linear loads attract sinusoidal currents and as a result do not cause any distortion.
Idea Sine wave with zero harmonics
Nonlinear loads, on the other hand, draw current that is not sinusoidal. This current waveform diverges from the sine wave and as a result distortion of the voltage is formed. This distortion will severally change the behavior of the sinusoid as shown. This is known as harmonics.
Harmonics consist of frequencies that are functions of the waveforms basic frequency. For instance, if 6Hz basic waveform is present, the second, third, fourth and fifth harmonic elements will be equal to 120Hz, 180Hz, 240Hz and 300Hz respectively. An ideal sinusoidal wave has zero harmonic, and as a result the wave cannot be distorted.
Harmonics associated with Power factor correction
The presence of power factor correction in voltage and current harmonics is quite a confusing phenomenon. The overall installation current is made of two elements, namely, the real power as a function of the voltage and the reactive power component, which is given by.
Itotal2 = Ipower2 + Ireactive power2
The power factor is therefore given by
New South Wales Service and Installation regulations (March 1999), for instance, has a requirement that end-users should keep their power factors between 0.9 lagging and unity. Other power sectors have a tariff policy which entreats end-users to maintain their power factors as unity as possible thereby limiting the reactive current. This can be achieved by installing a required shunt capacitor.The presence of harmonics causes the frequency component added to the current to rise and the previous equation is modified as follows:
Itotal2 = Ipower2 + Ireactive2 + Iharmonic2
In several scenarios like the installation of computer, Ireactivegets close to zero, however, Iharmonic is huge and the power factor becomes lower than unity. Should the end-user install power factor correction capacitors, the reactive power component will increase as a result of the capacitor current, thereby causing the total installation current Itotalto increase, thereby making the power factor poor/worse.
How to calculate the Total Harmonic Distortion
End-users should consider two factors of harmonics, namely, equipment susceptibility and equipment emission.Equipment susceptibility is the extent at which harmonic distortion can destroy or damage to equipment and it is a function of the harmonic voltage tolerated by the equipment. Equipment emission is the rate at which the supply is affected by the equipment and it is a function of the harmonic current drawn.
The total sum of all harmonics of either voltage or current waveform divided by the basic component of the current and voltage sinewave is termed as THD.
The percentage THD determines the extent of distortion present in the signal.
For instance: if the harmonic voltage of 20 volts is added to a sinewave of 240 volts, the rms figure of both voltages is given by:
V2 = V12+ VH 2
V2 = 2402 + 202 = 24O.8 volts
The THD is then expressed as a percentage, and is given by:
Power quality field measurements on PV inverters enable the evaluation of their behaviour under real operating conditions
How to analyse Harmonics
- Get data from the supply system. This information is provided in a short-circuit current form that can be used to evaluate the impedance of an equipment
- Evaluate the main harmonic sources within the installation.
- Based on every harmonic order, model the electric power network and installation. Inductive reactance increases with increase in frequency, whereas capacitive reactance reduces and resistances unchangeable.
- Evaluate and analyse the voltage at the point of common coupling from the harmonic current and the harmonic impedance which has been calculated
How to reduce Harmonics in a system
In any situation, when you intend to install large distorting loads, harmonics in relation to that of Australian standards, AS 2279.2 should be considered. In case you have no experience with calculation of total harmonic distortion, you can contact a local provider or a consultant. In case of any excessive harmonic after calculation, you can consider the following alternatives.
- Request for a design of the smaller harmonic current from the local supplier
- Installation of surplus devices that can draw several harmonic currents and prevent it from spreading into the power supply network. You can use a harmonic filter- which consists of capacitors, resistor and inductor.
- In case the harmonic distortion is which is as a result of enlargement by power factor capacitor, an appropriate detuning inductor must be connected in series with it, preventing the capacitor from absorbing large harmonic currents
- When the installation is very big, the local power system provider could modify the system to decrease the impedance of the power system.
The aforementioned factors and analysis can also be implemented during commercial installations.