Practical applications of active power filters (I) [Part 5/5: Low harmonic drives (LHD) for centrifugal 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 centrifugal?pumps

Industrial pumps are essential devices required in every phase of oil and gas operations helping to transfer process fluids from one point to another. The process fluids to be handled can range from easy to difficult, so depending on the nature of the fluid to transfer and the required flow rate, a suitable pump needs to be selected.

Different types of pumps are used for fluid transfer in the oil and gas industry. They can be classified based on their design and construction and generally fall into six categories: Centrifugal pumps, reciprocating plunger pumps, progressive cavity pumps, gear pumps, diaphragm pumps and metering pumps.

Centrifugal pumps are the most common type of pumps used in the oil and gas industry. The owners and operators of oil and gas installations using centrifugal pumps look nowadays to balance initial procurement cost with longer-term operational concerns including equipment reliability, enhancing production efficiency and reducing energy consumption. Proper selection of a pump system with high energy efficiency and able to operate smoothly overcoming power quality disturbances can save a huge amount of money in operating costs over the lifetime of the system and improve dramatically the profitability of the operations.

Requirements

Background

Centrifugal pumps used in offshore oil and gas installations include submersible motor pumps for sea water lift, process pumps to move and handle oil, gas and water, high-pressure pumps for sea water injection, high-pressure pumps for crude oil transportation and utility and fire protection pumps.

Power quality and energy efficiency problems usually present in these installations like harmonics, interharmonics, unbalances, voltage fluctuations (flicker) and low power factor cause that operating centrifugal pumps do not comply with international power quality and energy efficiency standards and requirements, and they ultimately influence their performance. As a result, the pumps can damage or stop processes and cause any kind of unexpected problems in installations. For example, harmonics and unbalance are some of the most common causes of premature failure of electrical motors and in general of rotating machinery. Power quality and energy efficiency issues are very critical in oil and gas applications that require constant high reliability and availability.

System description

Centrifugal pumps consist typically of two main parts, the pump and an electrical induction motor which converts electrical energy into rotational mechanical energy. This mechanical energy is used to drive the pump and draw fluid into the intake of the pump and force it through the discharge section via centrifugal force.

Variable speed drives of 6, 12, 18 or 24 pulses are commonly used to control the induction motors of these centrifugal pumps. They enable adjustments to be made to production parameters and output when operating conditions change. They also protect pumps and motors by reducing electrical stress during start-up.

Solution

Analysis

The induction motors used by centrifugal pumps usually have a low power factor, which contributes to reduce the overall efficiency of the installation.

Variable speed drives?can produce some power quality problems in installations and their energy efficiency can be low depending on manufacturer selected and their design. They can produce high harmonic content that can damage the connected pump or other nearby machinery. They also offer a poor performance if operating with any system unbalance, and sometimes, their power factor can be lower than the minimum required by the installation.

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 pump operating at different load levels. In this certain case, the power factor of the motor is only 0.9 and the VSD is producing harmonics of the 5th, 7th, 11th and 13th harmonic orders that cause a high harmonic current distortion (THDi = 45%) in the installation.

Proposed solution

Based on the analysis of the measurements, it is possible to dimension a solution that would comply with customer’s requirements. The solution is a low harmonic drive installed in series with the centrifugal pump.

LHDs can take care of the power quality problems related to harmonics, interharmonics, unbalances or voltage fluctuations (flicker) affecting centrifugal pumps used in offshore oil and gas installations. In addition, they can improve the power factor of the installation where they are connected, and they can also improve the output voltage of motors fed from unbalanced power supplies.

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

Conclusions

LHDs offer a better performance and have an smaller layout and lower losses than other possible solutions that could be considered for this application like multi-pulse VSDs, active front ends or VSDs with passive harmonic filters. An additional advantage of using LHDs is that they can easily be scaled up at a later stage allowing easy expansion of the offshore oil and gas facility.

The main drivers behind the widespread adoption of LHDs are stricter regulations on power quality and energy efficiency requirements, the increasing demand for smart devices and wireless connectivity technology for industrial equipment (including the integration of the IIoT in drives) and the increasing need for more compact drives.

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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.