General Information – Harmonic Distortion
It is important to note that THD (Total Harmonic Distortion) levels above 5% can shorten the life of the electrical equipment and contribute to other issues with electronics such as unexplained glitches on computers and test & monitoring equipment and/or systems.
These issues can have harmful and/or costly effects on the overall health and functionality of your electrical distrbution system and it’s connected equipment.
To help understand the impact and significant role harmonic distortion can play, a bit of information concerning THD is warranted.
What is harmonic distortion?
Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency being 50 or 60Hz (50Hz for European power and 60Hz for American power). For example, if the fundamental power frequency is 60 Hz, then the 2nd harmonic is 120 Hz; the 3rd is 180 Hz, etc. In modern test equipment today, harmonics can be measured up to the 63rd harmonic. When harmonic frequencies are prevalent, electrical power panels and transformers become mechanically resonant to the magnetic fields generated by higher frequency harmonics. When this happens, the power panel or transformer vibrates and emits a buzzing sound for the different harmonic frequencies.
What causes harmonic distortion?
Harmonics are caused by and are the by-product of modern electronic equipment such as personal or notebook computers, laser printers, fax machines, telephone systems, stereos, radios, TVs, adjustable speed drives and variable frequency drives, battery chargers, Uninterruptable Power Supplies, and any other equipment powered by switched-mode power supply (SMPS) equipment. The above-mentioned electronic SMPS equipment is also referred to as non-linear loads. These types of non-linear loads or SMPS equipment generate the very harmonics they are sensitive to and that originate right within your building or facility. SMPS equipment typically forms a large portion of the electrical non-linear load in most electrical distribution systems. There are two basic types of non-linear loads: single-phase and three-phase. Single-phase non-linear loads are prevalent in modern office buildings while three-phase non-linear loads are widespread in factories and industrial plants.
What does harmonic distortion do to the power at my facility?
In today’s environment, all computer systems use SMPS that convert utility AC voltage to regulated low voltage DC for internal electronics. These non-linear power supplies draw current in high amplitude short pulses. These current pulses create significant distortion in the electrical current and voltage wave shape. This is referred to as a harmonic distortion and is measured in Total Harmonic Distortion (THD). The distortion travels back into the power source and can affect other equipment connected to the same source.
Example: To give an understanding of this, consider a water piping system. Have you ever taken a shower when someone turns on the cold water at the sink? You experience the effect of a pressure drop to the cold water, reducing the flow of cold water. The end result is you get burned! Now imagine that someone at a sink alternately turns on and off the cold and hot water. You would effectively be hit with alternating cold and hot water! Therefore, the performance and function of the shower is reduced by other systems. This illustration is similar to an electrical distribution system with non-linear loads generating harmonics. Any SMPS equipment will create continuous distortion of the power source that stresses the facility’s electrical distribution system and power equipment.
What other effects besides distorting the shape of the voltage and current sinusoids, do harmonics cause?
Since non-linear loads produce harmonic currents with frequencies considerably higher than the power system fundamental frequency, these currents encounter much higher impedances as they propagate through the power system than does the fundamental frequency current. This is due to “Skin Effect” which is the tendency for higher frequency currents to flow near the surface of the conductor.
Since little of the high-frequency current penetrates far beneath the surface of the conductor, less cross-sectional area is used by the fundamental current. As the effective cross section of the conductor is reduced, the effective resistance of the conductor is increased. The higher resistance encountered by the harmonic currents will produce a significant heating of the conductor, since heat produced — or power lost — in a conductor is I2R, where “I” is the current flowing through the conductor. This increased heating effect is often noticed in two particular parts of the power system: neutral conductors and transformer windings.
Typical problems, together with before mentioned overheating in neutral conductors, transformers, or induction motors, include:
- Malfunctioning of microprocessor-based equipment.
- Deterioration or failure of power factor correction capacitors.
- Erratic operation of breakers and relays.
- Pronounced magnetic fields near transformers and switchgear.
- Large load currents in the neutral wires of a 3 phase system. Theoretically the neutral current can be up to the sum of all 3 phases therefore causing overheating of the neutral wires. Since only the phase wires are protected by circuit breakers of fuses, this can result in a potential fire hazard.
- Overheating of standard electrical supply transformers which shortens the life of a transformer and will eventually destroy it. When a transformer fails, the cost of lost productivity during the emergency repair far exceeds the replacement cost of the transformer itself.
- High voltage distortion exceeding IEEE Standard 1100-1992 “Recommended Practice for Powering and Grounding Sensitive Electronic Equipment” and manufacturer’s equipment specifications.
- High current distortion and excessive current draw on branch circuits exceeding IEEE Standard 1100-1992 “Recommended Practice for Powering and Grounding Sensitive Electronic Equipment” and manufacturer’s equipment specifications.
- High neutral-to-ground voltage often greater than 2 volts exceeding IEEE Standard 1100-1992 “Recommended Practice for Powering and Grounding Sensitive Electronic Equipment.”
- High voltage and current distortions exceeding IEEE Std. 519-1992 “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.”
- Poor power factor conditions that result in monthly utility penalty fees for major users (factories, manufacturing, and industrial) with a power factor less than 0.9.
- Resonance that produces over-current surges. In comparison, this is equivalent to continuous audio feedback through a PA system. This results in destroyed capacitors and their fuses and damaged surge suppressors which will cause an electrical system shutdown.
- False tripping of branch circuit breakers.
Again as you can see harmonic distortion can be destructive and costly to your electrical system, equipment and devices as well as your businesses bottom line.Read More
What is the difference between lagging power factor and leading power factor?
Power factor is the ratio of watts (true power) to VA (volt-amperes, also called apparent power). Where the load is resistive only, the power factor is one, or unity, because the voltage waveform and the current waveform are in phase. Thus, for resistive loads only, true power and VA are the same. Where the load is reactive, the load stores energy, releasing it during a different part of the cycle. This shifts the current waveform so that it is offset, or out of phase with the voltage waveform.
Reactive loads can be inductive (electric motors), capacitive, or non-linear (rectifier power supplies). When the load is inductive, the inductance tends to oppose the flow of current, storing energy then releasing it later in the cycle. The current waveform lags behind the voltage waveform. When the load is capacitive, the opposite occurs, and the current waveform leads the voltage waveform.
Therefore, lagging vs. leading is another way of saying the net reactance is either inductive or capacitive. This is slightly simplistic, and what we are talking about above is really DPF, or Displacement Power Factor. Non-linear loads do not really shift the current waveform, they distort it. The current waveform starts to look like a square wave, and square waves contain harmonics. So non-linear loads add harmonic distortion, and this tends to mimic a capacitively reactive load, adding some leading power factor.
Therefore, when we say power factor, we really must include DPF plus harmonic distortion in total. One memory aid that may help to remember all this is: ELI the ICE man The L in ELI means inductance. The E (voltage) comes first, then the I (current) lags behind. Inductive reactance produces a lagging power factor. The C in ICE means capacitance. The I (current) comes first (leads) then the E (voltage) comes later. Capacitive reactance produces a leading power factor.
Remember, it’s always the current waveform that is affected by the reactive load, so you have to think about whether the current is leading or lagging. Most reactive loads are inductive, so at most sites the PF is lagging. One cool tidbit is that capacitive reactance cancels out inductive reactance. So if we have a building full of motors, we can add a bunch of capacitors to improve our power factor, meaning we get as close to unity as we can. Thus, power factor correction capacitors are made just for this purpose.Read More
This document covers; The Concepts of Reactive Power, Low Power Factor and Methods of Power Factor Improvement.
Power factor is defined as the ratio of real power to apparent power. This definition is often mathematically represented as kW/kVA, where the numerator is the active (real) power and the denominator is the (active+ reactive) or apparent power. Though the definition is very simple, the concept of reactive power is vague or confusing even to many of those who are technically knowledgeable.
Explanation for reactive power says that in an alternating current system, when the voltage and current go up and down at the same time, only real power is transmitted and when there is a time shift between voltage and current both active and reactive power are transmitted. But, when the average in time is calculated, the average active power exists causing a net flow of energy from one point to another, whereas average reactive power is zero, irrespective of the network or state of the system. In the case of reactive power, the amount of energy flowing in one direction is equal to the amount of energy flowing in the opposite direction (or different parts -capacitors, inductors, etc- of a network, exchange the reactive power). That means reactive power is neither produced nor consumed.
But, in reality we can measure reactive power losses, many different types of devices, equipment and systems can be introduced to manage or mitigate reactive power. These types of compensation are to reduce electricity consumption and cost.
The indisputable law of conservation of energy states, “energy can neither be created nor be destroyed”; yet we talk about Conservation of Energy!! The confusions erupt when we yell out the theory of conservation ignoring other theories of thermodynamics – like one, which states that entropy (low quality energy) is ever increasing. Mathematical sum of total energy has no meaning to an energy user, and hence he must be concerned about the efficiency of conversion and conservation of energy. Similarly, though we can mathematically prove that loss in reactive power is no real loss and no reactive energy is lost, we have several other reasons to be concerned about reactive power improvement. This can be better explained by physical analogies.
Suppose I want to fill a water tank with water, one bucket at a time. Only way is to climb a ladder, carrying a bucket of water and pouring the water into the tank. Once I fill up the tank, then I have to go down the ladder to get more water. In this one cycle of going up the ladder and coming down I have done some “work” or “the energy required to go up is more than the energy required for coming down.”
If I had climbed the ladder with an empty bucket, and I had come down with the same bucket I am not doing any work. The energy for upward and downward motion is the same. Though I have not done any work – worth paying for- I require some energy.
That is, the energy that it takes to go up and down a ladder carrying nothing either way requires reactive power, but no real power. The energy that it takes to go up a ladder carrying something and come down without carrying anything requires both real power and reactive power.
The analogy can be extended for explaining 3 phase system; it is like putting 3 ladders going up to the tank and having 3 people climb up in sequence and pouring their water into the tank such that there is always a steady flow.
Here is a simplistic analogy called, the “Beer Mug analogy”
Power Factor = Active power/Apparent power = kW/kVA
= Active power/ (Active Power+Reactive Power)
The more foam (higher kVAr) indicates low power factor and vice versa.
(In Electrical terms kW, kVA, and kVAr are vectors and we have to take the vector sum).
What causes low power factor in Electrical System
Various causes, which can be attributed for low PF, may be listed as follows.
- Inductive loads. Especially lightly loaded induction motors, and transformers.
- Induction Furnaces
- Arc Lamps and arc furnaces with reactors.
- Fault limiting reactors
- High Voltage.
- Harmonic distortion up to 63rd harmonic
The reactive power required by these loads increases the amount of apparent power in the distribution system and this increase in reactive power and apparent power results in a lower power factor.
How to improve Power Factor
Power factor can be improved by adding consumers of reactive power in the system like Capacitors or Synchronous Motors. It can also be improved by fully loading induction motors and transformers and also by using higher rpm machines. Removing or reducing the harmonic distortion. Improving and regulating of the voltage sine wave. Usage of automatic tap changing system in transformers can also help to maintain better power factor.
Question: Under which circumstances may power factor corrections..
A) reduce electricity consumption in a plant
Answer: Power factor improvement in plant, by adopting any one of the aforementioned options, will generally compensate for the losses and reduce current loadings on supply equipment, i.e.; cables, switchgear, transformers, generating plant, etc. That means power factor corrections – whenever there is scope for correction- will reduce electricity consumption in the plant and in turn the electricity cost. Many of these losses are not properly monitored in many industries and hence the savings are not quantified. This may be one of the reasons for the argument that PF improvement reduces only electricity costs; in case the power utility is offering a tariff where a reactive power demand charge are part of the monthly electricity bill.
Power factor improvement will lead to reduction in electricity consumption, when it is done at the equipment level or at the Control Center level (case studies have shown the savings in both these instances)
B) reduce electricity costs only
Answer: Power factor correction will reduce electricity cost only, when the plant receiving power from a common grid carries out the correction at the supply voltage/incoming voltage level, just to compensate for the reactive power drawn from the grid. But, even this improvement in PF may not always reduce the electricity cost as the contract demand in a plant is very often fixed on a fictitious consumption in the plant. On many occasions contract demand is fixed based on the future expansion plans, and based on the high diversity factor taken during design stages. In most of the cases the Utilities charge for a minimum contract demand irrespective of the consumption and a reduction in kVA may not produce any benefit as long as the contract demand is re-fixed to actual value.
Generally PF is improved to 0.95-0.98, as improving PF further to unity (1.0) may lead to higher payback periods.
C) reduce both electricity costs and electricity consumption
Answer: In all other cases, other than the above mentioned exception, whenever improvement of power factor is carried out, it will eventually lead to reduction in electricity consumption and hence electricity cost.
However, payback on investment due to power factor correction depends on the type of installation and various other factors like power tariff, loading pattern of equipment, method of power generation/utilization, operating philosophy of the plant etc.Read More