Practical applications of active power filters (IV) [Part 5/5: Low harmonic drives (LHD) for high pressure slurry pumps]

Low harmonic drives

Low harmonic drives (LHD for short), as a special design of shunt active power filters (APF for short), have been around since the 1990s. Description of their topology and operating principle can be found as far back as 1998. They were developed to take care economically of the power quality and energy efficiency problems created by variable speed drives (VSD for short) and electric motors that conventional passive solutions like reactors, passive harmonic filters (PHF for short), multi-pulse VSDs and slim DC link VSDs, or conventional active solutions like active front ends (AFE for short) and matrix inverter VSDs could not handle.

LHDs can be applied to small, medium or large installations in a wide range of segments. They have many low and high voltage potential applications where their use offers many benefits including any kind of machinery or system that needs constant power and/or constant or variable torque.

Functions

LHDs as a special design of shunt active power filters combine the technical advantages of AHFs with the cost-effectiveness of standard 6-pulse VSDs to form a compact economical solution with exceptionally low THD and minimal energy losses without the drawbacks of conventional passive or active solutions.

These specially designed modern LHDs, also known as AHF-based low harmonic drives (AHF-LHD for short), can take care of several power quality problems and support the development of clean energy by combining different control functions in a single device.

Connection

An AHF-based low harmonic drive is a power electronics-based device connected with the equipment generating the power quality problems or that has issues to comply with grid code and energy efficiency requirements. These LHDs can be installed together with new equipment or the VSDs of any existing equipment can be easily upgraded or retrofitted to AHF-based LHDs. They are formed by two components:

• An AHF that behaves as a controlled current source providing any kind of compensation current waveform (in terms of phase, amplitude and frequency) in real time (typical reaction time is under 50 μs and overall response time is under 100 μs).
• A standard 6-pulse VSD (or several VSDs in parallel) controlling the electric motor (or motors).

The most common operating voltage range for AHF-based LHDs is 200 V up to 690 V as the AHFs are built using low voltage IGBT switches. It is possible to connect them to higher voltages using a suitable step-up transformer for the AHF.

LHDs for high pressure slurry pumps

As much as 50% of the material that comes out the ground during coal mining is waste (dirt, rocks and other impurities) that must be removed from the coal before it can be processed and burned. The more of this waste material that can be removed from coal, the lower its total ash content, the greater its market value and the lower its transportation costs.

A coal preparation plant, also known as a wash plant, is a facility that washes coal of soil and rock, crushes it into graded sized pieces, and prepares the resulting pieces for transport to markets. If the whole process and the water and wastewater streams are not well-managed, a coal preparation plant can flood, become dangerous to work in or be forced to shut down.

There are several locations at the plant where pumps are required to handle collected water like in holding ponds fed by runoff from raw and clean coal stockpiles, in basement clean up and conveyor belt head sumps, and in truck-washdown sumps. In most cases, the water to be pumped is a muddy, abrasive slurry laden with coal fines, briquettes and dirt.

High pressure slurry pumps are rugged enough to provide reliable long-term operation in these harsh environments, including applications where the pumps have to handle slurry with a high percentage of solids.

Requirements

Background

A coal preparation plant processes and cleans coal coming from an underground mine. The coal is processed by using a coal extraction technique involving rotary drums that shear coal, shale, and fire clay. After removing large non-coal fragments, water is added to the raw coal to produce a coal slurry to separate the remaining shale, fire clay, and other particles from the coal.

The plant owner wants to optimise the coal washing process. This involves improving the power quality and energy efficiency of the pumps used to transport the slurry while complying with the energy authority’s requirements on harmonic distortion and power factor.

System description

Four pumps rated 500 kW each are connected at the 400 V distribution system. They transport the coal slurry from a common sump to the two classifying cyclones that separate the slurry into two types. The slurry containing smaller mesh particles is discarded while the slurry containing larger mesh particles is dewatered and discharged as clean coal.

These pumps use variable speed drives and induction motors which are the sources of power quality and energy efficiency problems affecting the coal washing process.

Solution

Analysis

To determine the power quality and energy efficiency issues of the installation it is necessary to collect measurement data from a power quality analyser with the pumps operating at different load levels.

The results indicate that the variable speed drives produce high harmonic content on the 5th, 7th, 11th and 13th harmonic orders that cause a high harmonic current distortion in the installation. The induction motors used by the slurry pumps show a low power factor, which contributes to reduce the overall efficiency of the installation.

Proposed solution

Based on the analysis of the measurements, it is possible to dimension a solution that would comply with the requirements of reducing the amount of harmonics to be able to comply with THDi and THDv under 5% and at the same time increasing the power factor. The solution is to upgrade the installation by using a low harmonic drive. In this case, this would be done by keeping the existing 6-pulse VSDs and installing an active harmonic filter rated 1,000 A together with them.

The low harmonic drive can achieve a THDi and THDv of less than 4% at 400 V, below the energy authority’s requirement of 5%. Having a low harmonic distortion level reduces supply currents and allows supply power transformers, protection devices and power cables to be dimensioned according to the actual active power.

This solution also helps to bring the power factor of the installation to unity and is compatible with the existing generators at the plant.

Based on the values monitored, the following functions are proposed for the LHD.

Conclusions

The use of variable speed drives in critical heavy-duty applications is increasing fast. VSDs are nowadays energy efficient but their operation can cause power quality problems in the electric power system ultimately influencing the performance of the controlled equipment. As a result, the VSDs can damage equipment, processes and cause any kind of unexpected problems in installations.

Power quality and energy efficiency improvement solutions, dependable power architecture, and intelligent power monitoring systems are necessary to assure reliability of critical applications. The installation of low harmonic drives at coal preparation plants reduces their harmonic distortion and increases their power factor bringing several benefits:

• Compliance with the harmonic distortion limits of local grid code.
• Over dimensioning of power transformer or power cables is not required.
• Reduced energy losses in the plant.
• Reduction on system complexity.
• Reduction of overall investment costs.
• Optimisation of the use of available space.

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About the author:

Pedro Esteban is a versatile, multicultural and highly accomplished marketing, communications, sales and business development leader who holds since 2002 a broad global experience in sustainable energy transition including renewable energy, energy efficiency and energy storage. Author of over a hundred technical publications, he delivers numerous presentations each year at major international trade shows and conferences. He has been a leading expert at several management positions at General Electric, Alstom Grid and Areva T&D, and he is currently working at Merus Power Plc.