Consequences of high harmonic distortion levels

The total harmonic distortion(THD) is a measurement of the harmonic distortion present in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Distortion factor, a closely related term, is sometimes used as a synonym.

The plant engineer’s worst fear…

Just as high blood pressure can create stress and serious problems in the human body, high levels of harmonic distortion can create stress and resultant problems for the utility’s distribution system and the plant’s distribution system, as well as all of the equipment that is serviced by that distribution system.

The result may be the plant engineer’s worst fear – the shutting down of important plant equipment ranging from a single machine to an entire line or process.

Equipment shutdown can be caused by a number of events. As an example, the higher voltage peaks that are created by harmonic distortion put extra stress on motor and wire insulation, which ultimately can result in insulation breakdown and failure. In addition, harmonics increase rms current, resulting in increased operating temperatures for many pieces of equipment, greatly reducing equipment life.

Table below summarises some of the negative consequences that harmonics can have on typical equipment found in the plant environment.

Negative Consequences of Harmonics on Plant Equipment

While these effects are categorised by problems created by current and voltage harmonics, current and voltage harmonic distortion usually exist together (current harmonic distortion causes voltage harmonic distortion) .

"Harmonic distortion disrupts plants. Of greatest importance is the loss of productivity, throughput, and, possibly, sales"

These occur because of process shutdowns due to the unexpected failure of motors, drives, power supplies, or just the spurious tripping of breakers. Plant engineers realize how costly downtime can be and pride themselves in maintaining low levels of plant downtime. In addition, maintenance and repair budgets can be severely stretched.

For example, every 10°C rise in the operating temperatures of motors or capacitors can cut equipment life by 50%.

Impact of Voltage Dip on Power Quality

A voltage sag or voltage dip is a short duration reduction in rms voltage which can be caused by a short circuit, overload or starting of electric motors. A voltage sag happens when the rms voltage decreases between 10 and 90 percent of nominal voltage for one-half cycle to one minute.

Its also explained as short, temporary drop in the voltage magnitude in the distribution or customer's electrical system. It may be caused by various faults in the transmission and distribution networks, faults in the connected equipment or high inrush and switching currents in the customer's installation.

What is causing voltage dips?

Figure 1 shows the sketch of a voltage dip, together with the associated definitions. The major cause of voltage dips on a supply system is a fault on the system, that is sufficiently remote electrically that a voltage interruption does not occur.

Other sources are the starting of large loads (especially common in industrial systems), and, occasionally, the supply of large inductive loads.

Figure 1 - Voltage dip sketch

Voltage dips due to the latter are usually due to poor design of the network feeding the consumer. A voltage dip is the most common supply disturbance causing interruption of production in an industrial plant.

Faults on a supply network will always occur, and in industrial systems, it is often practice to specify equipment to ride-through voltage dips of up to 0.2s. The most common exception is contractors, which may well drop out if the voltage dips below 80% of rated voltage for more than 50-100ms.

Motor protection relays that have an under voltage element setting that is too sensitive is another cause. Since contactors are commonly used in circuits supplying motors, the impact of voltage dips on motor drives, and hence the process concerned, requires consideration.

Figure 2: multiple-voltage-dip emerich

Other network-related fault causes are weather–related (such as snow, ice, wind, salt spray, dust) causing insulator flash over, collisions due to birds, and excavations damaging cables. Multiple voltage dips, as illustrated in Figure 2, cause more problems for equipment than a single isolated dip.

The impact on consumers may range from the annoying (non-periodic light flicker) to the serious (tripping of sensitive loads and stalling of motors). Where repeated dips occur over a period of several hours, the repeated shutdowns of equipment can give rise to serious production problems.

Figure 3 shows an actual voltage dip, as captured by a Power Quality recorder.

Figure 3 - Recording of a voltage dip

Typical data for under voltage disturbances on power systems during evolving faults are shown in Figure 4.

Figure 4 - Under voltage disturbance histogram

Disturbances that lie in the front right-hand portion of the histogram are the ones that cause most problems

How Harmonics effects Switch-gear and relays

Harmonic currents in switch-gear will increase heating and losses in switch-gear in the same way as has been discussed for power cables above. Similarly, voltage distortions can cause problems for voltage transformers (VT) and connected relays, while current distortion can do the same for current transformers (CT) and current-operated relays.

In particular, electromechanical over-current relays exhibit a tendency to operate more slowly, while static under-frequency relays that use zero crossing for frequency measurement are susceptible to substantial changes in operating characteristics in the same way as noted above for electronic control circuits.

Harmonics can also impair the speed of operation of electromagnetic-type differential relays.

🔺 Modern digital relays use filtering techniques to produce the fundamental frequency only so that the fault current and voltage measurement circuits are not significantly affected by harmonics. Correct filtering requires the relay algorithm to track the system frequency, and most relays have a limited frequency range over which they are designed to operate.

Although the fault-measuring capability may not be compromised, its ability to correctly measure and detect overloading conditions depends on its ability to measure the heating effect accurately, i.e., the full rms values.

Although corrections can be made in the settings when the harmonic mix is known, it is important to know the cut-off frequency of the relay to be able to fully compensate and protect devices, such as high-frequency filters.

How Harmonics effects an Electronic equipment?!

Power electronic equipment is susceptible to mis-operation if there are significant levels of harmonic distortion. Some of the control systems for power electronic devices use zero crossing detection to control switching.

Harmonic distortion can result in shifting of the voltage zero crossing points, and these changes can be critical for many types of electronic control circuits. Also, if incorrect switching occurs, more harmonics can be produced, compounding the problem.

Medical Instruments

Malfunctioning medical instruments present the most serious negative effect of harmonics to electronic devices. This is because it may place a person’s life in jeopardy. For this reason, many medical instruments are provided with line-conditioned power and protected by proper power quality devices.

In addition, less dramatic interference effects of harmonics can sometimes be observed in radio and television equipment, as well as in video recorders and audio reproduction systems.

Computers and Other Electronic Devices

Computers and allied equipment like programmable controllers typically require AC sources that have no more than a 5% harmonic voltage distortion factor, with the largest single harmonic being no more than 3% of the fundamental voltage.

Harmonic distortion levels that exceed standard limits may result to malfunctioning equipment, which in some cases, have serious consequences. It must be noted that electronic devices can be disturbed by the transmission of AC supply harmonics via the equipment power supply or through magnetic coupling of harmonics into equipment components.

Moreover, other electronic instruments can be affected by harmonics by giving incorrect data or unpredictable performance (i.e. digital energy meters).

Voltage Notching

Most electronic devices are installed at the low voltage level of its associated power distribution system. As a result, they also become exposed to the effects of voltage notching. Voltage notches often introduce frequencies, both harmonic and non-harmonic, which are much higher than those exhibited in 5 kV and higher voltage distribution systems.

Subsequently, these frequencies are in the radio frequency (RF) range, which can lead into detrimental effects associated with spurious RF, such as signal interference introduced into communication or logic circuits. Sometimes, the voltage notching effect is of adequate power to overload electromagnetic interference (EMI) filters and similar high-frequency sensitive capacitive circuits.

Source:

IEEE 519-1992 Recommendation & Practice

How Harmonics Effect Capacitors?!

Any capacitance in an AC network can produce a risk of resonance with the inductive parts of the network. Although electrical networks are designed not to have any resonances at fundamental frequencies, when the multiple frequency effects of harmonic distortions are considered, there is always the possible risk of system resonance.

Effects of harmonics on capacitors and capacitor banks are as follows:

• Resonance imposes considerably higher voltages and currents in capacitors.
• The capacitor bank acts as a sink for higher harmonic currents, which increases the heating and dielectric stresses.
• The losses in a capacitor are proportional to the reactive output (kVAR), which, in turn, is proportional to the frequency. These losses are increased, and the overall capacitor life is shortened with increasing harmonics.

🔺 To avoid or minimize such problems, capacitor banks can be tuned to reject certain harmonics by adding reactance.

In most industrial harmonics power systems, the primary objective for installing capacitors is to meet the utility power factor requirements as expressed in its tariff rates. Additional benefits are better voltage regulation and lower losses.

Commonly used locations are shown in Figure 2 below.

 Figure 2 – Typical SLD  for an industrial system

Any capacitor bank can be a source of parallel resonance with the system inductance.

  

Avoiding resonance problems

The best approach to avoid resonance problems is to install large capacitor banks at the main bus. 

This solution offers the following advantages:

1. More available reactive power to the system as a whole
2. Easier control of harmonic voltages and currents
3. Lower capital costs, as large banks are more economical in terms of purchase cost
4. Reactors can be added to shift the resonant frequency away from the characteristic harmonic frequency of the plant

Capacitors can also be combined with reactors to develop harmonic filters at the troublesome resonance harmonic frequencies.The resonant frequency at the capacitor bus can be calculated by: 

Where:

fr is resonant frequency

fs is system frequency, 50 Hz

kVAsc is three-phase system fault level in kVA

kVAc is three-phase capacitor-bank rating in kVA

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How immoral harmonics influence the work of motors & Generators

Generators and motors are adversely affected by harmonics in the networks to which they are connected.

 Typical effects are:

1. Increased heating due to iron and copper losses at the harmonic frequencies
2. Higher audible noise emission as compared with sinusoidal excitation
3. Harmonic currents in the rotor

The harmonic currents noted above are caused by harmonics in the stator winding, which will produce harmonic currents in the rotor, e.g., 5th- and 7th-order stator harmonics will produce 6th-order rotor harmonics, while 11th- and 13th-order stator harmonics will produce 12th-order rotor harmonics.

These rotor harmonic currents will result in increased rotor heating and pulsating or reduced torque.

It should also be noted that system unbalance (standing unbalance or ground faults), expressed as negative-sequence currents, can also reflect into the rotor as harmonic currents, which add to those noted above.

Generators can also produce harmonics and, in particular, triple harmonics that can circulate through adjacent Wye-grounded transformers when generators are directly connected to a load bus. The use of the Delta-connected generator transformers can control this.

FOR POWER QUALITY STUDY & AUTOMATED REPORT:  

Connect with Emerich Energy

Please write to us info@emerich.in Call: 044-48577667