Getting in Tune with Reduced Harmonics

Editor’s Note: This article is adapted from the author’s presentation at the Energy Efficiency in Motor Driven Systems (EEMODS) 2011 conference, hosted by the National Electrical Manufacturers Association (NEMA), Sept. 12-14, 2011, Washington, D.C. Green Manufacturer thanks the organizations for their cooperation in publishing this article.

Typical 6-pulse variable-speed drives can generate harmonic current distortion, adversely affecting power supply to factories. When excessive, these distortions can affect other equipment and cause energy losses or unintended operation. The good news is that harmonic currents can be limited by relatively simple methods, each with its own advantages and drawbacks.

One new approach to saving energy in VFD motor systems is the use of reduced harmonics technology.

The Problem With Harmonics

An FAA air traffic control facility in Buffalo, N.Y., installed reduced harmonics software to calculate system harmonics. The agency was concerned with potential adverse conditions affecting the electronics in the building causing harmonics on the power lines. Obviously, if the VFD-generated power harmonics were to create problems with the computers and radar for the aircraft controllers,
that could cause very serious air safety problems.

Every electrical circuit is a potential source of electrical interference, particularly those switching reactive loads. Fortunately, most electrical interference is so low that it has no noticeable effect on other branches of electrical equipment.

However, some harmonics, caused by the use of nonlinear loads that produce non-sinusoidal currents, can be troublesome (see Figure 1 VSDs belong to this group of highly nonlinear loads that produce non-sinusoidal currents.

A major consequence of harmonics is increased power draw and deterioration of power quality in the facility supply. These harmonics cause power losses through eddy currents that are proportional to the frequency and magnitude of harmonic current.

The input rectifiers in typical 6-pulse VSDs produce a high level of distortion that can affect the performance of other devices connected to the same electrical supply. Intermittent power flow can cause the power grid or its branches to carry extra power with frequencies of multiples of base 50 or 60 Hz known as harmonic orders.

Although increased power draw and deterioration of power supply quality can remain unnoticed at first, it can have adverse technical and economical results. A typical electric power distribution system consists of several transformers, conductors, and distribution switchgear. Each component of the system presents a certain quantity of impedance (AC circuit resistance) that causes a voltage drop. Because most system elements are inductive, the voltage drop is proportional to the amount of current flowing and its frequency:

Where:

Vd = Voltage drop on system element

^IRMS = Current (with harmonics)flowing through system element

Z = System element impedance (R = element resistance, L = element inductance, f = frequency)

Power losses resulting from harmonics directly affect both capital and operational expenditures related to the system; the components have to be properly sized not only to satisfy end load demand, but also to deliver additional power capacity that is being used because of harmonics inefficiencies.

In a typical electrical system, the voltage that supplies linear loads A and B may be distorted by harmonics generated by drives supplying nonlinear loads C and D (see Figure 2). The current Ipcc2—a sum of currents from all four loads—contains harmonics. The voltage at PCC2 (point of common coupling 2) normally would be sinusoidal; however, it is modified by the voltage drop Vd
that results from Ipcc2 flowing through the impedance of the transformer (Z2), cabling, and switchgear.

Reduced power quality may cause a number of issues:

• Overloads on distribution systems because of increased root mean square (RMS) current
• Overloads on neutral conductors because of the summing of third-order harmonics created by single-phase loads
• Vibrations and premature aging of generators, transformers, and motors
• Premature aging of capacitors in power factor correction equipment
• Distortion of the supply voltage, capable of disturbing sensitive loads
• Disturbances on adjacent communications networks, such as telephone and data networks

In VFD applications, the electrical system typically consists of a voltage supply (usually a transformer), a drive, cabling, and a motor. Because the drive introduces harmonics into the current, some of the power is lost in the transformer because of its impedance and eddy currents. The transformer needs to be properly sized to satisfy demand from both active power and power losses. Transformers define and specify the amount of distortion that they are capable of transmitting with what is known as a K factor rating.

The ratio of that extra power requirement strongly depends on total harmonic distortion (THD) in the current:

Where:

PF = Effective power factor

Cosf = Displacement power factor (motor PF)

THDi = Total harmonic distortion in current

A typical 6-pulse VSD input current waveform contains four pulses per sine period that represent a DC bus capacitor charging cycle (see Figure 3). The RMS current value depends on DC bus filter capacitance; the higher the capacitance used, the higher the current peaks and the more harmonics that are generated.

The power section of a reduced harmonics technology (low-capacitance) drive generates a lower magnitude of harmonics (see Figure 4).

Harmonics Mitigation Techniques

Figure 5 shows six different ways systems harmonics can be decreased at equipment level:

Figure 5
System harmonics can be decreased in six different ways, each applied at the equipment level. The typical 6-pulse technology that is widely used shows no improvement of the current wave shape. Other approaches offer improvement, but have their disadvantages as well.

1. Typical 6-pulse—This is a baseline technology used by most manufacturers. The drive consists of a rectifier, DC bus with capacitor filters, and the inverter section that supplies output voltage using pulse width modulation (PWM) driven IGBT (insulated gate bipolar transistors).

2. Reduced harmonics technology—Reduced harmonics technology is based on very low DC bus filter capacitance. With this technology, advanced drive motor controllers employ an algorithm that monitors ripples on the DC bus and attenuates inverter PWM power to generate undistorted output current. Such loopback systems correct the power factor, while the controller maintains the input current in phase with supply voltage. Using custom PWM techniques, the motor is supplied with additional voltage should the input power line sag or become distorted due to disturbances. Since the energy is not stored in large capacitors on the DC bus, this technique is typically recommended only for use on variable-torque centrifugal loads such as fans and pumps.

3. Line reactors—Inductors installed in series with drive input and/or DC bus provide impedance that increases with the frequency of current harmonics. This method is used with 6-pulse drives and can be implemented externally without changes to drive circuitry. Drive input chokes add impedance and are most suitable for filtering higher-order harmonics, while low-order harmonics typically are of the highest current magnitude.

4. Passive filters (broadband)—An LRC (filters containing inductors and capacitors tuned for a specific frequency) installed in parallel with the device that generates harmonic distortion. The filter shorts out the harmonic currents, preventing the flow of harmonics to the power source. This technique can be implemented after commissioning by analyzing the power line to identify the frequency of the largest harmonic currents.

5. Multipulse input rectification—A 12- or 18-pulse transformer feeds multiple rectifiers that feed the same DC bus of a drive, while each of the transformer output voltages is phase-shifted. Phase shift minimizes the ripple in the DC bus voltage so it requires minimal filtration and provides high and constant power to the inverter. Although this method provides high system efficiency and reliabilty, it can be costly, heavy, and large.

6. Active front end—This method is based on active, controlled input rectifiers using IGBTs that are controlled from an intelligent microprocessor. This system monitors power quality on the drive input and controls the input rectifier accordingly to minimize the amount of harmonics in the input current. While one of the most effective harmonics-reduction techniques, this method can be costly and usually is designated only to systems requiring the highest power quality. Since the controlled IGBT switching frequency is usually above 4 kHz, these types of rectifiers often require filters to mitigate these high-order harmonics. Line conditioning using an inductor is also often required in front of these rectifiers to ensure proper operation.

Active filters (power conditioners) are systems employing power electronics, installed in series or in parallel with the nonlinear load to generate harmonics-canceling currents and thereby prevent power source distortion. In most cases, this method is used at the installation level, filtering harmonics coming from multiple branches of nonlinear loads. While it provides very good results, this technique requires frequent maintenance because of the deterioration of built-in capacitors.

The Low-capacitance, Reduced Harmonics Option Reduced harmonics technology features include:

• THDi (Total harmonic distortion in current) of less than 30 percent without any external filters.
• Compliance with IEC-61000 requirements.
• High power factor.
• Low cost of harmonic mitigation.
• Small capacitors for reduced drive size.
• Low-resistance power section with no line chokes or DC bus chokes, which helps minimize energy loss.

The Electrical Power Research Institute (EPRI) recently performed research on six products from various leading manufacturers. Drives were tested at 16 different speed and load combinations using the manufacturers’ suggested settings. The drive employing reduced harmonics technology maintained the current in phase with supply voltage, so the power factor was close to unity. The power factor is represented in values between 0 and 1, in which 1 equals no phase difference between current and voltage and 0 equals full 90-degree displacement.

In general, the reduced harmonics technology drive generated low harmonics of rank 5, 7, 11, and 13. At low motor speeds, performance and efficiency dropped because of the relatively high level of harmonics compared to the current being delivered to the motor and the relatively high switching losses on the drive inverter.

While the product was tested with a carrier frequency of 12 kHz, suggested by the manufacturer to reduce motor audible noise, this high-carrier frequency caused an efficiency drop of about 2 percent compared to a 6-kHz carrier setting. The higher carrier frequency also resulted in an increased r level of higher-order harmonics: 17, 19, 23, and 25.

The testing showed reduced harmonics technology drives to be viable VSD technology for use with high-inertia loads that are not sensitive to intermittent changes in the supply. They require lower-rated wire and smaller protective devices, when compared to the use of external equipment such as line chokes.

A high power factor helps to convert the energy efficiently, and low-voltage harmonics reduce stress on other devices connected to the same power distribution circuit. In industrial environments, high harmonic current levels can be a major cause of energy loss.

Reduced harmonics technology, built into standard drives, can accomplish both objectives while reducing overall initial equipment and installation costs. Addressing this harmonic issue when selecting VFDs—regardless of the mitigation devices selected—will result in fewer power problems and can actually increase overall system efficiency.

NEMA, www.nema.org
EEMODS, www.eemods.org