How to Handle Harmonics in Electrical Power Systems
While musical harmonics can be pleasing to the ear, electrical current harmonics are an increasing problem in power systems. Equipment and cables can overheat, motors can be damaged, and your electric bill could be a lot higher than it needs to be.
Without going into Fourier equations or requiring an electrical engineering level of knowledge, let’s look at the cause of harmonics in today’s power systems, and how a power engineering review of your facilities can help solve the problem.
Harmonic waves compared to normal alternating current and voltage waves
The dictionary definition of “harmonics” is: A wave whose frequency is a whole-number multiple of another wave.
The usual waveform of alternating current in most electric power circuits is a sine wave that changes directions at a specific frequency, usually 50 or 60 hertz. A linear electrical load draws current at the same frequency and sinusoidal wave shape as the voltage frequency and wave shape, although the timing or phase between current and voltage may change.
Examples of linear loads include incandescent lighting and electric heaters. The load for constant-speed AC motors is almost linear, and the load current is a sine wave with very little distortion because linear loads are relatively steady and don’t produce any new frequencies (harmonics).
Non-linear loads, however, cause harmonics because they draw current at frequencies other than 60 Hz, in abrupt and uneven pulses. These non-sinusoidal distortions in the current and voltage waveforms then layer themselves as multiple frequencies upon the fundamental frequency. These multiple frequencies are called harmonics.
Offices have many sources of nonlinear loads that are harmonic generators, such as computers, printers, copiers, and LED drivers for fluorescent lighting. Industrial sources of nonlinear loads include uninterruptible power supplies, rectifiers, variable frequency/speed drives for motors, programmable controllers, and fluorescent lighting.
Harmonics can creep into your power system
The percent of non-linear loads in office buildings and industrial facilities has increased over the last decade as incandescent lighting is replaced by fluorescent lighting, as more computers and programmable controllers are connected to the electrical system, and as more electric motors use variable frequency drives (VFDs). Harmonic disturbances can also be transmitted from the network if you have an unfiltered power factor correction capacitor on your incoming power.
Third harmonics are caused by power supplies in computers and electronic ballasts, and can result in a neutral current greater than the phase currents.
The fifth and seventh harmonics produced by the three phase bridge rectifiers in the power supplies of six-pulse VFDs create pulsating torques that can cause shaft vibrations, and damage motor bearings and couplings.
Higher harmonic distortions can also lead to the nuisance tripping of circuit breakers, the overheating of cables and equipment, and the premature failure of electrical equipment. Transformers can quickly overheat and fail after load changes due to replacing or adding electrical equipment, installing VFDs, and upgrading electronic ballasts. Plant manufacturing safety may be at risk.
How bad are your harmonics?
IEEE standard 519 provides guidelines for acceptable values of total harmonic distortion (THD). Its focus is on the point of common coupling. The standard also seeks to limit damage to power factor correction (PFC) capacitors and harmonic filter systems from excessive harmonics, and to prevent series or parallel resonance in the electrical system.
The goal is to limit the THD to less than 5% where an industrial power system is connected to the utility network. If your THD values exceed the IEEE 519 values, it’s important to determine whether harmonics are causing problems.
Harmonics and a poor power factor could be costing you money
Harmonics and power factor are not directly related but taking action to cure power factor could enhance the problem of harmonics. The power factor is the ratio (from -1 to +1) of the actual power used by the load compared to the apparent power available in the circuit. A change from unity or power factor of 1 to some other number leading or lagging means that current is shifted relative to the voltage applied.
A power factor of 1 means that no current is lost or distorted. A power factor of 1 says that all of the current is going into doing useful work. Any number less than 1 indicates that some of the applied voltage goes into magnetic or other forms of energy that are not lost but when it is given back it shifts the current. A low power factor means that you are not fully using the electric power you are buying when your local utility charges a “power factor penalty” or bills you for your kVA usage instead of your KW demand. This current is increased and causes system losses and this is what the utility is trying to cover these losses with higher demand charges or power factor penalties.
To understand the power factor, let’s look at a glass of beer. The glass represents the apparent power, or kVA. The actual beer in your glass is the actual power or kW. Any foam at the top of your glass is the reactive power, or kVAR. It doesn’t perform useful work, but is used to maintain a magnetic flux in motors, for example.
The more foam you have, the less beer you have, but you still paid for a full glass. Similarly, your utility company will charge you for the apparent power, which is the sum of the actual power plus the reactive power.
Reducing harmonics and raising your power factor
A thorough energy audit by an electrical engineer can yield impressive paybacks on power factor improvements. But without expert guidance, attempts to improve your power factor may make your harmonics worse. Installing components like capacitors for power factor correction may actually amplify the harmonic resonance voltage distortion. This can cause high spikes of voltage at capacitor terminals and lead to their premature failure. The very item installed to reduce power factor penalties (the capacitor) could self-destruct because the harmonic currents are amplified by just the right system electrical conditions.
If you do not have more than 20% of your electrical load from harmonic generators and there are no power factor correction capacitors, you should not have significant harmonic issues.
However, if non filtered power factor correction capacitors are installed on a system with significant harmonic generators, Matrix can analyze the susceptibility of your facility’s electrical power system to total harmonic distortion. We can also review the myriad utility rates and determine if your costs are significantly impacted by a poor power factor.
Matrix Technologies is one of the largest independent process design, power systems engineering, industrial automation engineering, and manufacturing operations management companies in North America. To learn more about our electrical power system capabilities and manufacturing process control solutions, contact Vince Trejchel, PE.
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