AHF applications in commercial power systems

Introduction

As innovative and advanced as modern power systems are becoming, they're not without their challenges. The prevalence of nonlinear loads, stemming from advanced equipment like variable speed drives, computer switch mode power supplies, and LED lighting, brings one of those challenges into the spotlight - harmonic distortions. These deviations in current and voltage sinewaves can adversely impact power quality and place undue stress on essential electrical components.

That's where Active Harmonic Filters (AHFs) come in. They're sophisticated electronic devices strategically designed to counteract the effects of harmonics. AHFs, through active monitoring of harmonic content, dynamically inject equal but opposite harmonic currents, effectively neutralising the unwanted distortions. This cutting-edge tech is essential for maintaining the overall power quality of commercial power systems, ensuring voltage and current waveform integrity, reducing losses, and safeguarding sensitive equipment.

Let's take a closer look at the practical applications of AHFs in commercial settings, and the impact they have on power quality, energy efficiency, and the optimisation of power systems.

Fig 1. AHF with SVG

What are the issues associated with elevated harmonic levels?

Total Harmonic Distortion (THD) is at its most basic a measure of how much your load is distorting the perfect voltage waveform provided by your electricity supplier. THD is always present in current and voltage but too much distortion can cause problems.

We all know that non-linear loads, such as variable speed drives, draw currents at multiple frequencies (harmonics) of the fundamental frequency – in the case of NZ the fundamental supply frequency is 50Hz. These harmonic currents are drawn by the connected device, through the low voltage circuit and equipment connecting that device to the low voltage side of the transformer. These harmonic currents are then drawn from the generator thru the medium and high voltage supplies. The magnitude of harmonic current remains the same as a percentage across the transformer.

Problems that remain localised to site

The obvious problem of carrying additional currents is overloading of cables, control, protection, and distribution. It is common to see cables operating hotter and burnt crimp lugs. Invariably this problem only affects the circuit passing the harmonic currents so is localised to the site with the offending equipment.

Fig 2. A solder lug overheated by high harmonic currents has melted the solder in the lug

Problems that affect other consumers
As electricity is distributed through the network there is an impedance between the generating source and the consumer. Typically, the further you are away from the generator the higher your supply impedance will be. The harmonic currents interact with the impedance of the electrical network to create voltage distortion.

The voltage waveform is common to other consumers that are joined to the network at a point of common coupling. This can be either the low voltage side of the transformer if you are sharing a transformer with others, or if your site has its own transformer(s), the medium voltage network connecting the primary of the transformer. A distorted voltage waveform can affect:

• Devices designed to work on a sinusoidal waveform, particularly devices relying on electromagnetism like motors, transformers, coils, induction hobs operate inefficiently and can burn out.
• Devices containing solid state rectifiers, in particular SCR, can operate unreliably and fail prematurely as the zero crossing point of the waveform can be shifted.
• Devices using the 50Hz waveform as a timing circuit can no longer function accurately. For example, clock radios and oven clocks often loose time or run fast for no apparent reason.

Problems that affect the network company
Like consumers the network company is negatively affected by increased harmonic currents. Their network must transport these harmonic currents. This takes up valuable network capacity because they are reactive power only and do no useful work. This causes additional volt drop, heating in their distribution equipment and a reduction in true power factor. They are also regularly involved in handling the fall-out of customers who are being supplied a badly shaped voltage waveform because of someone else’s harmonic producing load.

When does the problem get to a magnitude that I am likely to experience THDV issues on site?
To preserve the quality of electricity, the level of allowable voltage distortion which may be introduced into an electricity supply system by a consumer’s installation is governed by the Electricity Regulations and NZ ECP36. This states that the total harmonic voltage distortion at any point of common coupling with a nominal system voltage of less than 66 kV shall not exceed 5 percent. Some electrical networks have introduced local regulations which are stricter than this and state a maximum level of THDi that may be drawn from the secondary side of the transformer.

Complying to any of these regulations does not guarantee that you will be free of issues created by THDV. You really need to measure THDV on your low voltage bus. Our experience shows that once you delve into the range of 8% to 10% THDV on the low voltage bus you can expect unexplained equipment failure, particularly electronics and power supplies, and the significant increased heating of electromagnetic devices.

Fig 3. A voltage trace of a supply showing in excess of 15% THDV. This site was experiencing random electronics failures and unexplainable production outages.

Implementing AHFs effectively

Installation involves connecting the AHF in parallel with the loads generating harmonics. Proper integration is crucial for the effective functioning of AHFs within existing commercial power systems. This requires careful consideration of the specific harmonic profile of the system. Coordination with existing power quality measures, such as capacitors and transformers, is essential for optimal AHF performance.

During installation, specific current transformers are strategically placed to measure the load. The solid-state AHF can then respond and apply full-rated compensation in under 5 milliseconds. Best practices for effective implementation include regular monitoring of harmonic content, ensuring the AHF is properly sized, and coordinating its operation with other power quality equipment.

Ensuring compatibility is a key aspect of AHF implementation. This involves understanding the specific harmonic issues present in the system and configuring AHFs accordingly. Collaboration with power system experts during the design phase is crucial for compatibility, especially in new builds where harmonic requirements can often be calculated out. For retrofitting or upgrading existing installations, data collection is generally required to tailor the AHF configuration to the system's unique characteristics.

Cost efficiency and long-term savings

The cost-effectiveness of AHFs hinges on factors like the severity of harmonic distortions, the sensitivity of connected equipment, and the overall power system configuration. While the initial investment in AHFs may be higher, the long-term benefits typically result in a favourable payback period.

AHFs contribute significantly to long-term savings by addressing key aspects of commercial power systems:

• Reduced maintenance costs - by actively mitigating harmonics, AHFs lessen the wear and tear on connected equipment, leading to lower maintenance costs over time
• Enhanced equipment lifespan - lower stress from harmonics ensures a prolonged lifespan for sensitive electronic devices and machinery, contributing to overall equipment reliability
• Energy efficiency gains - AHFs help maintain a clean and sinusoidal power supply, optimising the performance of the electrical system. This results in potential energy efficiency gains and reduced energy consumption, translating to cost savings over the life of the equipment
• Demand reduction - AHFs can contribute to reducing site-nominated demand, leading to decreased electricity bills for commercial entities

While the initial investment may appear higher, the holistic approach of AHFs toward improving power quality and system efficiency positions them as a strategic investment with substantial long-term savings potential for commercial applications.

Conclusion - AHFs are a proven and effective solution

AHFs actively monitor and neutralise harmonic distortions, safeguarding commercial power systems from the detrimental effects of unwanted harmonic currents. They also reduce stress on connected equipment and minimise downtime, and they contribute to reduced maintenance costs, extended equipment lifespan, and potential energy efficiency gains. By considering AHFs in the design and management of your power systems, you can strengthen your operations against the adverse impacts of harmonic distortions and benefit from enhanced power quality and operational resilience.

If you're interested in learning more about the future of power quality correction in New Zealand, make sure you check out this blog.