Harmonics Problems Generated from Nonlinear Load and Power Electronics Devices

Electronic equipment (switching power supplies) draws current differently than nonelectronic equipment [4]. Instead of a load having a constant impedance drawing current in proportion to the sinusoidal voltage, electronic devices change their impedance by switching on and off near the peak of the voltage waveform. Switching loads on and off during part of the waveform results in short, abrupt, non-sinusoidal current pulses during a controlled portion of the incoming peak voltage waveform. These abrupt pulsating current pulses introduce unanticipated reflective currents (harmonics) back into the power distribution system. As mentioned earlier, these currents operate at frequencies other than the fundamental 50/60 Hz.

The harmonics problem in electrical circuits has been an issue since the establishment of the AC generators, where distorted voltage and current waveforms were observed in 20th century. For more than 100 years, harmonics have been reported to cause operational problems to power systems. From the above-mentioned harmonics in power systems, we find that harmonics are mainly generated from both the component level (nonlinear magnetic or dielectric components) and the circuit level (including power electronic circuits) [4, 5]. At the component level, the presence of harmonics in electromagnetic waves leads to a distorted signal for the E field or H field while, at the circuit level, the presence of harmonics in the voltage or current waveform leads to a distorted signal for voltage or current [6].

The harmonics generated from nonlinear electromagnetic fields (on nonlinear components) can cause voltage and current waveform distortion, due to the induced voltage and current carrying distorted waveforms and then propagating to circuits. Obviously, the source, or harmonics generator, is from the nonlinear component, and coupled with linear electric circuits. Figure 2.3 shows the nonlinear magnetic and nonlinear dielectric material, and the harmonics are generated from the B-H magnetic materials. The corresponding excitation current is non-sinusoidal, due to the nonlinear B-H relationship of the core, as shown in Figure 2.3(a). When only the fundamental component of the current is considered, however, the relationship between the phasors of voltage and current can be determined by a resistor (equivalent resistance of the core loss) in parallel with a lossless inductor (self-inductance of the coil), as illustrated in the diagram.

It is well known that matter can be classified by its electrical conductivity into conductors, semi-conductors, and insulators. Insulators, also called dielectrics, do not conduct electric current under the influence of an electric field. On the other hand, however, dielectrics can be polarized by an electric field. In a macroscopic sense, the same amount of positive and negative surface charge is induced on one surface side, and the opposite side perpendicular to the field, respectively. Microscopically, electrically charged particles constituting the materials (such as atomic nuclei, electrons, and ions) cannot be freely moved, but are displaced from their equilibrium position due to Coulombic force. This phenomenon is called dielectric polarization, as shown in Figure 2.3(b).

The amount of displacement is macroscopically characterized by so-called dielectric constants, and these are different in different materials. If the field is weak, the displacement is proportional to the intensity of the field. In this case, the dielectric constant can be considered as a proportional constant connecting the displacement and intensity of the field. However, if the field has a larger intensity, the situation can be different.

Figure 2.3 Nonlinear magnetic and nonlinear dielectric materials, (a) the B-H hysteresis loop of the magnetic material, and (b) the direction of the polarization (D-E) hysteresis loop of ferroelectric material

Actually, literalistic treatment of a dielectric constant as ‘constant’ is a kind of first-order or linear approximation of a nonlinear electric response. In reality, the dielectric constants are not constants, but vary depending on the external field. Table 2.1 presents the current distortion due to nonlinear load and power electronics devices, where the different waveform represents different load characteristic [1].

Some of the major effects include are listed below [1, 2]. These effects depend, of course, on the harmonic source, its location on the power system, and the network characteristics that promote propagation of harmonics.

Table 2.1 Current distortion due to nonlinear load and power electronics

Type of Load	Typical Waveform	Current Distortion	Weighting Factor
Single phase power supply		80% (high 3rd)	2.5
Semi-convertor		High 2nd, 3rd, 4th at partial load	2.5
Six pulse converter with capacitor smoothing, no inductance		80%	2
Six pulse converter with capacitive smoothing, with series inductor >3%, or DC drive		40%	1
Six pulse converter, with large inductor for current smoothing		28%	0.8
12 pulse converter		15%	0.5
AC voltage regulator		Varies with firing angle	0.7
Fluorescent lighting		0.05	0.5

• • Capacitor bank failure from dielectric breakdown or reactive power overload.
• • Interference with ripple control and power line carrier systems, causing operational failure of systems which accomplish remote switching, load control, and metering.
• • Excessive losses in - and heating of - induction and synchronous machines.
• • Over-voltages and excessive currents on the system from resonance to harmonic voltages or currents on the network.
• • Dielectric breakdown of insulated cables resulting from harmonic over-voltages on the system.
• • Inductive interference with telecommunications systems.
• • Errors in induction kWh meters.
• • Signal interference and relay malfunction, particularly in solid-state and microprocessor-controlled systems.
• • Interference with large motor controllers and power plant excitation systems (reported to cause motor problems, as well as non-uniform output).
• • Mechanical oscillations of induction and synchronous machines.
• • Unstable operation of firing circuits, based on zero voltage crossing detection or latching.

A large portion of the nonlinear electrical load on most electrical distribution systems comes from power electronics equipment, such as DC/DC converters or switching mode power supplies (SMPS) [7]. For example, all computer systems use switching mode power supplies that convert utility AC voltage to regulated low-voltage DC for internal electronics. These nonlinear power supplies draw current in high-amplitude short pulses that create significant distortion in the electrical current and voltage wave shape - harmonic distortion, measured as total harmonic distortion (THD). The distortion travels back into the power source, and can affect other equipment connected to the same source.

Most power systems can accommodate a certain level of harmonic current, but will experience problems when harmonics become a significant component of the overall load. As these higher-frequency harmonic currents flow through the power system, they can cause communication errors, overheating and hardware damage, such as:

• • Overheating of electrical distribution equipment, cables, transformers, standby generators and so on.
• • High voltages and circulating currents caused by harmonic resonance.
• • Equipment malfunctions due to excessive voltage distortion.
• • Increased internal energy losses in connected equipment, causing component failure and shortened lifespan.
• • False tripping of branch circuit breakers.
• • Metering errors.
• • Fires in wiring and distribution systems.
• • Generator failures.

• • Crest factors and related problems.
• • Lower system power factor, resulting in penalties on monthly utility bills.