Understanding and Mitigating Harmonic Distortion

Understanding Harmonics

VSD Working Principle

VSDs consist of three sub-systems: the Diode bridge converter, DC Bus, and IGBT output inverter. The Diode bridge input rectifier rectifies the incoming AC voltage into a DC voltage. The DC Bus system utilises DC capacitors to help smooth the rectified AC voltage and provide voltage storage for the system. The IGBT output system utilises the DC Bus voltage and creates the variable frequency and voltage output to the motor, using a pulse width modulated (PWM) control.

VSD Causes of Harmonics

VSDs contribute to harmonics in 2 ways as follows (Line and Load Harmonics):

Line Harmonics (Input/Grid/Supply/Mains)

Any VSD using a bridge rectifier on the input stage inherently cause current and voltage harmonics on the power supply (mains/utility) that can create power quality issues for other sensitive electrical loads. This is due to the VSD’s non-linear load produced by the rectifier. A load connected to an AC input voltage is a linear load if the current draw is in the same form as the voltage (resistive, inductor, or capacitive load, for example). A non-linear load draws a current that is not in the same form as the AC voltage waveform (non-sinusoidal). In a VSD, once the DC Bus capacitors are charged, the input current to the capacitors will flow only when the incoming AC voltage is greater than the DC Bus voltage. This is near the top of the arc of the sine wave voltage waveform. As the voltage of the sine wave drops below the DC Bus level, the current will stop flowing. This nonlinear capacitor current results in a current pulse on the main incoming voltage line. Significant voltage distortion can create operational problems for other electrical equipment connected to the utility. Certain types of electrical equipment are more susceptible to voltage distortion issues. Harmonics can also create larger RMS currents which increase conductor heating, stress the system’s electrical and mechanical components, and increase the likelihood of system resonances. Harmonic distortion also impacts Power Factor (PF) as follows:

• Power Factor (PF) and how it relates to Harmonics: Power factor (PF) is sometimes used to refer to the amount of harmonics in a system. The total PF represents the ratio between the power being put into useful work (active power) at the fundamental frequency and the total electrical power being transferred. A unity PF (1.00) is ideal and indicates that all power being delivered is being usefully consumed by the load. A low power factor is to be avoided because it requires electrical components to be oversized to accommodate the transfer of unused reactive and/or harmonic power. Two components make up the total power factor: the Displacement Factor and the Distortion Factor (PF Total = PF Displacement * PF Distortion). The PF Displacement is the ratio of the used Active Power (measured in watts) and the Reactive Power (measured in VoltAmps) at the fundamental frequency. The displacement factor is affected by the impedance of the load and calculated by measuring the phase shift (expressed as ø) between the applied voltage and load current. A standard six-pulse VSD will have a relatively high PF Displacement, usually greater than 0.9. PF Distortion is the component of power factor which is affected by harmonics. It represents the ratio between the rms current at the fundamental frequency and the total rms current at all frequencies. In short, a PF Distortion value close to 1.00 represents a system with very little harmonic distortion. The further the Distortion PF value moves away from 1.00 the more harmonic distortion is present. However, a PF measurement by itself is somewhat lacking in that it gives no information to the magnitude of contributing harmonic distortion relative to the size of the utility supply.

Load Harmonics (Output/Motor)

Load Harmonic distortion comes from a VSD sending current to a motor. The switching action of the VSD IGBTs creates a pulse-width modulated (PWM) waveform (that allows variable control of the speed and torque of a motor), which contains high-frequency harmonics. The higher the switching frequency of the VSD, the higher the order of harmonics produced. The downside of this harmonic distortion is that it can produce negative effects on a motor (can damage insulation and increase motor temperature). These harmonics typically worsen as the cable length between the VSD and Motor increases, with the impact/damages also being influenced by cable/motor quality/type/age etc.

Harmonic Filters

Undesirable harmonic distortion generated by Variable Speed Drives (VSDs) can be mitigated effectively by installing harmonic filters. Harmonic Filters are used to limit and control VSD distortion and its adverse effects on the motors and other nearby electronics, and ultimately increases the reliability, performance and efficiency of VSD systems and extends the life of both the VSD and motor.

For Harmonic interference on the supply/grid/input line, Line Reactors/Input Chokes are used.

Input Chokes (Line Reactors)

Connected in series with the incoming power supply line and is recommended when the power supply is unbalanced or when power factor is poor in order to:

• protect the VSD from transient overvoltage conditions typically caused by utility capacitor switching;
• reduce the harmonic distortion and imbalance of the power supply current;
• reduce over-voltage trips caused by transient voltage spikes and power line surges;
• protect input rectifiers from in-rush current caused by sudden power supply surges and sags;
• extend the life of the DC bus capacitor bank by reducing the internal heating caused by ripple currents.

For Harmonic interference on the load/output/motor line, Load Reactors/Output Chokes are used, or depending on the level of distortion and cable length, SineWave Filters are used.

Output Chokes (Load Reactors)

Used for operating ‘non-VSD duty’ motors or when the length of wiring between the VSD and motor exceeds 50 metres (can consider from 25 metres). This is to:

• protect the motor insulation against VSD short circuits and IGBT reflective wave damage;
• allow the motor to run cooler by ‘smoothing’ the motor current waveform;
• reduce the effects of high motor wiring capacitance and ‘soften’ the dv/dt (high rates of change of voltage) applied to the motor windings;
• suppress the capacitive charging current of the cable between the VSD and motor;
• protect motor windings.

Sine Wave Filters

AC motors operate at their peak efficiency when they run on clean sinusoidal power. VSDs produce square waves that are modulated so that the pulse width of the square waves can be varied continuously (known as pulse width modulation – PWM). VSDs maintain fine control over the width and amplitude of the square waves so that they take on the appearance (as viewed on an oscilloscope) of good approximations of a sine wave. Yet, even the most sophisticated VSDs fail to match the purity of a true sine wave. Installing a sine wave filter between the VSD and the motor itself ‘smooths’ the VSD’s PWM signal and converts it into a nearly perfect sinusoidal power wave form. This then:

• reduces motor insulation stress and eliminates switching acoustic noise from the motor;
• prevents disturbing pulses from being transmitted to the motor;
• minimises eddy current losses in the motor, resulting in a cooler motor and thus extended motor lifetime;
• reduces capacitances in screened motor supply cables which cause high oscillating circuit currents through motor bearings, vaporising lubricant, and causing damage to the bearings (especially in motors above 50 kW);
• provides protection for the VSD (due to the lower pulse load being reflected in lower semiconductor losses).

To be installed for applications using older motors, applications in ‘aggressive’ environments, applications requiring frequent braking, 690V applications with general purpose motors and applications where cable lengths between the VSD and motor is 150 to 500 metres (can reach 3000 metres).