why is a harmonic filter may not good enough for the modern electrical system?

Electrical noise from classical electrical loads: 

The classical systems used bridge rectifiers for operation, and the resultant noise was predominant in the first lower order frequency spectrum. The switching noise from the bridge rectifiers has predominant 3rd, 5th, 7th, 9th, and 11th order. The noise decay over the higher-order frequencies from its 11th order. 

noise from classical system

Noise spectrum from the classical electrical load

Electrical noise from modern digital equipment:

However, the modern system uses SMPS topologies with high-frequency switching components such as MOSFETs, IGBT’s, etc., at 50-300kHz range. The resultant switching noise from the SMPS is in the higher-order harmonics spectrum, as shown in the graph below. 

noise from modern load

Noise spectrum from the modern digital load

The other factors that contribute to an increase in high-frequency frequency/oscillations in the system are:

  1. Increasing use of low-loss distribution transformers, which reduces the network damping substantially.
  2. Increasing use of power electronics with minimal ohmic loads such as electrical heaters – causing a reduction in the network damping. 

Are harmonics filters good enough for the system? 

May not be. Harmonics filters are an excellent solution for the classical system, removing the electrical noise in the lower order frequencies. However, the harmonic filters may not effectively remove the electrical noise generated in the higher-order harmonics from modern digital equipment. They are not tuned to remove the higher-order frequencies

To remove the higher-order harmonics and the switching noise, the facility must need specifically custom-tuned powerful electrical filters to remove the higher-order frequency noise. A well-designed low pass filter with surge suppression capabilities can attenuate the higher-order frequency, as shown in the following picture. 

Noise before filtration

Noise before adding high-frequency noise filters

Noise after filtration

Noise removal after adding high-frequency noise filters

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Switching noise generated from your computerized loads

We no longer live in a world of an incandescent bulbs. The digital revolution introduced us to many more electronics, including LEDs, VFDs, PLCs, electric vehicles, and other computerized loads, aka nonlinear loads. Nonlinear loads, the modern equipment is digital, programmable, and automated. The nonlinear loads are fast, efficient, and smart.

The operating principle of the nonlinear load is to “switch” AC to DC, DC to AC frequently. The switching process generates “switching noise” – an unwanted electrical signal in the system. The switching noise is also called higher-order harmonics (3-100kHz) or high-frequency noise.

Each nonlinear load uses hundreds of switching components such as MOSFETs, IGBTs, ICs in its circuit. The components use switching frequency from 25-300kHz with their harmonics noise in 1MHz. 

The following two graphs show the difference between a linear and a nonlinear load. The first picture shows a clean sinusoidal waveform from the liner load, while the second picture shows the distorted waveform with the noise on the peaks and zero-crossing. 


Linear load waveform

Noise free waveform from the linear load

Distorted waveform from the nonlinear load

Switching noise is the higher-order harmonics noise that may not be filtered out using traditional harmonics filters. Custom-designed low pass filters tuned at the high-frequency noise are the only solution to remove and protect your facility from the higher-order harmonics. 

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How does electrical noise impact your system’s performance?

The noise from the internally generated transients (switching noise) adversely affects the system’s performance. The effects cause protection and performance issues to the system, costing thousands of dollars to the facility. Here are some of the examples of adverse effects from the higher-order harmonics from the switching transients.

  1. Switching noise increases the eddy current and hysteresis losses in the magnetic material, resulting in increased heat and inefficient operation of the motor. The noise ultimately results in slip losses, vibrations, and insulation failures. 
  2. Switching noise malfunctions the behavior of electronics in a sensitive load, such as fluorescent and LED lighting, resulting in premature failures. 
  3. The noise feeding to IC’s results in improper functioning of the timing circuits, causing improper switching of MOSFETs and IGBTs, resulting in erratic behavior and random load failure. 
  4. Transients and noise combined to degrade the contacting surfaces of the switches, disconnects, and circuit breakers. Intense transient activity can produce “nuisance tripping” of breakers by heating the breaker and “fooling” it into reacting to a non-existent current demand 
  5. The noise increases the skin effect on the wire. The flow of electrons tends to be on the skin of the wire, leaving a hollow cylinder on the cross-section of the wire for no use. This phenomenon limits the wire’s amperage (load) capacity and therefore becomes less efficient to carry currents. The wire eventually runs hotter, with its insulation prone to higher temperatures. The noise increases the system’s impedance, and thus the ohmic losses (i2R losses) increase, causing the equipment to run at a hot temperature. For every 10C rise in temperature, the equipment loses its life by half. Therefore, the noise causes premature death of electrical loads. 

There are many other adverse effects of higher-order harmonics in the system. These are just a few.

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What are electrical noise and its sources?

Electrical noise is unwanted high-frequency oscillations generated in the electrical system. The electrical noise distorts the fundamental voltage and current waveforms, resulting in power quality issues. The poor power quality causes:

  • Permanent damage to the sensitive electronics loads 
  • Malfunction and erratic behavior of computerized loads
  • Increased maintenance and energy usage of the loads
Damage from noise
Noise causing damage to the load

Internally generated noises is about 70-85%, while the externally generated noises is about 15-30%

The electrical noise is classified into two categories based on its origin of generation 1) external and 2) internal.

Internally generated noises constitute about 70-85% of the total transients in a facility. The sources of the internally generated transients are: 

  1. Load switching 
  2. Nonlinear load behavior such and VFDs and PLC 
  3. AC-DC power conversion 
  4. Starting and stopping loads 
  5. Arcing faults (ground) 
  6. Discharge of inductive loads such as transformers, motors 
  7. Contactors, relays, and circuit breaker operation
Loads adding internally generated noise to the system
Lightning is an example of externally generated noise

Externally generated noises remain at 15-30% of the transients. The sources of externally generated surges are: 

  1. Lightning – both direct and indirect hits 
  2. Utility initiated grid and capacitor switching 

Again, the internally generated noise is 85%, while the externally generated noise is 15%. Learn more about the switching noise from the nonlinear loads here.

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