Measuring harmonics

Harmonics can cause significant damage to equipment and power systems. This article explains how harmonics are generated and why they should be regularly monitored.

Utilities generate virtually pure sinusoidal power. Nonlinear loads, such as frequency inverters, switching power supplies and UPSs, consume nonlinear power. When the switch is connected, there is a voltage drop due to the supply line impedance. The result is that pure voltage in the source leads to distorted voltage and current in the main service.

In order to analyse the influence of the harmonics, a Fast Fourier Transform (FFT) is performed. FFT divides the waveform into several waveforms, each one in another frequency which is a multiplication of the fundamental one. Each waveform has its own amplitude and phase shift. In order to enable analysis of the harmonic distortion, the parameter THD (total harmonics distortion) is defined as:

There are two different THD values for the voltage (THDv) and for current (THDi). Comprehensive research, conducted at 1400 sites across eight countries in Europe, found that 20% experienced the following:

• Computer lockouts (20%)
• Light flickering (22%)
• Electronic card failures (18%)
• Power factor correction system failures (17%)
• Failures in high load switching (16%)
• Neutral conductor overheating (12%)
• Unexpected breaker operation (11%)
• Power meters inaccurate readings (6%)

And, of course, excess losses and downtime.

Various standards specify limits for the THDv and THDi. While the voltage is defined as an absolute value in each standard/network (typically between 3% to 8%), the current is a more fluctuating value - this presents another challenge. For example, a site with computer servers causing 100 A distorted current out of a total of 1000 A has 10% THDi. At night, most of the loads are down with total current of 200 A but the computers continue to work, resulting in 50% THDi. It would seem that at night the problem is worse, but it is the other way around. The level of polluted current is the same, but since the network is less loaded at night, the case is better than during daytime.

Using another parameter enables estimation of the current harmonics during different loading conditions. TDD (total distortion demand) is defined as:

It is defined for the current only and its common levels are similar to the voltage (3% to 8%).

Transformers

Transformers are a major source of losses in the electrical network. There are four different types of losses:

• No load losses (core losses) that are fixed and do not change with the load
• Copper losses, equals I²R (all harmonics have the same effect)
• Winding eddy current losses, linear with the square of the harmonic index P_ECαI²h² 
• Stray losses (in clamps, tanks, etc) - includes various losses and in estimation is linear with the 0.8 power of harmony P_SLαI²h^0.8 

As evident from the above formulas, there is a difference in the losses caused by the current in each harmonic order. Since THD and TDD parameters provide equal importance to each harmonic order, they are not suitable of analysing the losses. There are several formulas to estimate the losses and help with designing the network for harmonics. The most common one is K factor, which is calculated as:

 

Specially designed K-transformers are used to cope with harmonics, according to the K factor. For example, a K4 transformer works with harmonics up to K factor of 4, similar to a regular (K1) transformer with pure sinusoidal waveforms.

The additional losses in the transformer are converted to heat and increase their operational temperature. According to Arrhenius law, each 10°C reduces the life expectancy by 50%, which means that harmonics shorten the lifetime of transformers and other loads.

Motors

Three-phase motor (squirrel cage) design is based on the rotation of the three-phase network. Harmonic voltage runs N times faster (N - order of harmonic). The result is that they create a force reversed to the motor force that is generated by the fundamental harmony and slow down the motor. The harmonic current increases the motor heat, reducing its life expectancy.

Power factor capacitors

At high frequencies, a capacitor acts as a short circuit. Power factor correction capacitors are designed for the fundamental harmony. In presence of harmonics, their impedance is lower - this results in increasing the amount of harmonics, capacitor overheating, potentially permanent damage to capacitors and resonance between the capacitors and transformers. The solution is to install a series reactor to each capacitor that limits this phenomenon but increases the system losses.

Direction of harmonics

As the rule of thumb above, utilities deliver voltage harmonics to the consumers and consumers inject current harmonics towards the power source. However, voltage and current harmonics increase each other, particularly in situations of resonance. This makes it complicated to identify which part has a higher responsibility for the higher harmonics. The source of harmonics is analysed on the basis of impedances and network simulation. However, there are two practices that allow reasonable evaluation of the direction:

• Monitoring the harmonic energy flow direction (negative sign means load generates harmonics)
• Comparing the THDv and TDD - If THDv > TDD the source is the utility

Recommendations

Any electrical installation must take into account the harmonics. It is important to continuously monitor the harmonics and take action if they are exceeding certain limits.

Recommendations for new installations:

• Estimate the level of harmonics
• Install power meters that can measure accurately at least 40 harmonics, THD, TDD and K-Factor
• Specify limits for alerts and configure the power meters accordingly
• Consider harmonics filtration solutions (low harmonic pollution loads, passive filers, active filters, retuned power factor etc)

Recommendations for existing installations:

• Perform harmonics study for a week and compare to international standards. Check the levels of at least 40 harmonics, THD, TDD and K-Factor. It is possible to use temporary handheld meters. However, permanent installation is preferred.
• Specify limits for alerts and configure the power meters accordingly.
• Consider harmonics filtration solutions (low harmonic pollution loads, passive filers, active filters, retuned power factor etc).

All SATEC meters from PM130EH and up provide details about the harmonics, including: measurement of THD, TDD and K Factor; measurement of individual harmonics; automatic comparison to international power quality and harmonics standards; programmable controller logic on harmonics such as in cases of high harmonic level, PF control application or high losses detection.

Amir Broshi, B.Sc in Electrical Engineering (with honors), began his career in 1988. With more than 20 years of experience, Broshi has performed enormous power quality measurements, analysis and improvement projects.