Introduction

One of the earliest methods for addressing harmonic-related issues involved the use of single-tuned filters, which provided a lower impedance path for harmonic currents. Interestingly, identifying a harmonic-producing load in the megavolt-ampere range within industries that operate without harmonic filters is not difficult. Major sources of harmonic distortion, particularly in industrial settings, may still rely on traditional harmonic filtering techniques to control disturbances that extend beyond the system’s metering point, ultimately affecting sensitive processes and equipment. However, this filtering method often comes with significant financial costs, making it less practical for residential and commercial applications. This article will explore various technical approaches to control harmonics and reduce the distortion they cause in power systems.

Technique 1: Network Reconfiguration

Network reconfiguration is one strategy to reduce harmonics. This process begins by identifying the users or sectors that contribute significant harmonic currents to the power system and categorizing them based on the characteristics of their frequency content.

If the use of harmonic filters is not considered, mixing both linear and non-linear electrical loads on the same feeder can help reduce harmonic distortion. Linear loads act as natural attenuators, mitigating the parallel peaks of resonant frequencies.

Technique 2: Increasing Supply Node Stiffness

Increasing the ratio between the short circuit current and the rated load current results in a stronger power supply node. This approach is commonly seen when power suppliers upgrade the capacity of their substations. It also occurs when large power consumers, such as industrial clients, install additional cogeneration units on the main supply bus to manage peak demand during operations.

The ratio between the short circuit current and the load current determines the “stiffness” of the power system. A stiffer AC supply increases the likelihood of available short-circuit current.

A stronger supply node offers better absorption of transient disturbances as they arise.

Expect low impedance sources with high short-circuit currents, which typically form an inverse relationship with the size of the transformer. This can be illustrated by calculating the impedance change when an old transformer rated at MVA1 is replaced by a new transformer rated at MVA2.

Using the fundamental expression for transformer impedance:

This equation shows the impedance ratio between the new and old transformers. For instance, a 60-MVA transformer will have half the impedance of a 30-MVA transformer, effectively doubling the short-circuit current, assuming both transformers have the same leakage current characteristics.

At harmonic frequencies, the capacitive and inductive impedances of the power system change according to the frequency function:

This function is composed of the feeder and substation components.

For short feeders, the source impedance is typically the dominant component. In these cases, harmonic currents are likely to reach the system’s substation, resulting in harmonic distortion. Stiffer systems, however, generally exhibit less harmonic distortion.

Technique 3: Use of Multi-pulse Converters for Harmonic Cancellation

One method to reduce harmonic distortion is through full-wave rectification.

The six-pulse converter is the simplest polyphase converter, while the 12-pulse converter is designed to eliminate lower-order harmonics such as the 5th and 7th harmonics.

Figure 1: 12-Pulse Converter Connections

If additional harmonic currents need to be reduced, phase multiplication can be used. For example, a 24-pulse converter can be constructed by combining four full-wave rectifier bridges, each with a 15-degree phase shift relative to the others. This is achieved using phase-shifting transformers with zigzag-connected additional windings, as shown in the figure below.

Figure 2: 24-Pulse Converter Connection

The conditions for harmonic elimination using a six-pulse rectifier are as follows:

• The transformers used in the configuration must have identical leakage impedances and transformation ratios.
• The load should be evenly distributed among the available converters.
• All converters must have similar firing angles.
• The phase difference between transformers should be 60/N degrees, where N is the number of sections.

The equation for calculating the characteristic harmonic reduction is:

 

Technique 4: Use of Series Reactors

Series reactors have been widely used to control short circuits in various industries, including smelting, power substations, and steel plants. They are also effective in attenuating harmonics.

Technique 5: Phase Balancing

Imbalances in single-phase electrical loads can cause unequal currents in the three-phase conductors, resulting in voltage drops and inducing unbalanced phase-to-phase voltages.

This unbalanced voltage can be harmful to the distribution feeder, particularly when there are insufficient compensatory measures in place. While achieving perfect phase balance is difficult, maintaining balanced phases can significantly reduce harmonic distortion.

Phase Voltage Unbalance

To determine phase voltage unbalance, you must first calculate the deviation using the formula below:

 

 

Technique 6: Load Grouping

In electrical networks, non-linear loads often have differing spectral characteristics. Grouping these loads by their harmonic spectrum can optimize the location, installation, sizing, and selection of harmonic filters.

Summary

This article has introduced six techniques that engineers can use to minimize harmonics in power systems. These include:

• Network Reconfiguration
• Increasing Supply Node Stiffness
• Adding Multi-pulse Converters for Harmonic Cancellation
• Use of Series Reactors
• Phase Balancing
• Load Grouping

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