Harmonic compliance assessment and mitigation design in renewable energy networks

We were pleased to contribute a paper to CIDER 17, Cigre’s “Conference on the Integration of Distributed Energy Resources”  for the Asia Pacific region held in Sydney, 15 and 16 August 2017. The analysis approach for harmonic emission compliance studies in distributed generation systems is shaped by a regulatory framework that requires a priori resolution of several performance requirements. This approach is steeped in risk avoidance and, like most risk avoidance, results in conservative outcomes that are not necessarily in the best interest of the market. Our article describes this analysis approach and demonstrates how the regulation and associated analysis result in increased rather than decreased risk to the stakeholders. A suggestion is made for a pragmatic approach that will result in better outcomes for network owners as well as generation proponents. The recommended approach provides for an initial risk assessment followed by field measurements that quantify the extent of compliance and assist in the design of mitigation systems.

Damped filter in a railway application

Mass rail transport systems in modern urban centres require reliable, safe, and high quality electrical energy. Urban rail systems include power supplies to rolling stock and to stations. Rolling stock is commonly supplied with direct current from rectifiers that generate significant harmonic distortion, and this distortion is distributed throughout the network. High voltage distortion can result in communication failures and malfunctioning electrical and electronic equipment, with consequent danger or inconvenience to passengers and rail personnel. High voltage distortion contribute to network losses and can also result in non-compliance with network supply arrangements and reduce equipment life.

This note describes an approach that ensures voltage distortion on sensitive loads is limited to within acceptable levels.

Specifying capacitor discharge time

When capacitors are disconnected from the supply, a DC voltage persists across the terminals of the capacitor unit. Safe handling of capacitor units after de-energisation requires that the stored electric charge in the capacitor unit should be removed to avoid the risk of electric shock to personnel. Any stored charge should be removed gradually — shorting the terminals of a capacitor unit to remove the charge will result in very rapid discharge of a substantial amount of energy that can endanger personnel and result in damage to the capacitor units themselves.

Capacitor units are therefore supplied with a discharge device capable of reducing the voltage between the terminals practically to zero, within a given time, after the capacitor has been disconnected from a network. The question of what this discharge time should be is of interest in this article.

The decision of a shorter discharge time for a capacitor bank may seem safe and harmless. In fact such a decision results in possible long term damage to the the capacitor units, and will always have a significantly higher carbon footprint than the default IEC60871 discharge time.

Calculating component ratings in detuned capacitor banks

Shunt capacitor banks are expected to operate for many years in harsh electrical and environmental conditions. Conservative designs are therefore preferred from a technical perspective. On the other hand, commercial preference is for least cost designs.

This note reviews the requirements for capacitor voltage rating and reactor current rating in two common situations. The purpose is to determine component ratings according to the requirements of standards such as IEC 60871 that will result in reliable performance under foreseeable operating conditions.

We are experts in reactive power compensation design, manufacture, delivery and operation. Feel free to contact us for more information or assistance.


Choosing reactors for shunt capacitor banks

Shunt capacitor banks are installed for a variety of reasons in industrial, distribution and transmission systems. A common thread to all installations is the question of what, if any series reactor should be installed with the capacitor bank.

Series reactors are used with capacitor banks to limit the effect of transients during capacitor switching, and to control the natural frequency of the capacitor bank and system impedance to avoid resonance or to sink harmonic current.

Choosing between damping, detuning or tuning reactors in any capacitor application requires an understanding of the consequences of such a decision. This application note describes these consequences in the context of a practical example.

Visit our downloads area for much more information.

Selecting current transformers for capacitor bank protection

Medium and high voltage capacitor banks require protection against faults in the capacitor units and in the associated equipment such as feeder cables, series reactors, overhead structures and conductors connecting the various parts of the capacitor bank.

Faults inside capacitor units are generally cleared by internal fuses. The operation of these fuses must be monitored to allow for appropriate alarm and trip instructions to be issued based on voltage stress levels in the capacitor units. The same applies for unfused units with unbalance current providing an indication of the number of shorted elements. Such protection depends on very sensitive detection of relatively small currents in the star points of capacitor banks. External faults are either phase to ground or phase to phase and require more conventional over current and earth fault detection.

Both of these types of protection require current transformers to provide the necessary detection of current in the primary circuit. Appropriate selection of protection current transformers is therefore essential for safe operation of capacitor banks. This selection must be done to assure correct operation of protection while minimising cost and avoiding any false operation of protection, i.e. ensuring continuity of operation wherever possible.

ONE has prepared a brief application note describing this selection process to assist our clients in this process. Please contact us if more information is required.

Training for Vestas Singapore

In early December 2013 we presented a training course on harmonic filters to specialist engineers from Vestas in Singapore. The intention of the course was to extend the knowledge of engineers on practical and theoretical aspects of harmonic analysis, filter design, and component ratings. The course was attended by a truly international audience, with Singapore, Australia, New Zealand, China, India, Venezuela and Vietnam represented. The training lasted three days and included real world examples of filter design for wind farm connections.

We are passionate about our field of expertise and welcome any opportunity to share our knowledge with customers and their consultants. Contact us if you would like to know more.

ONE participates in the DIgSILENT User Group meeting in Sydney on 5-6 September 2013

Having the right tools for the job is essential, and when the job is to carry out complex harmonic filter design studies then the choice of the best power system analysis tools is critical. At Optimised Network Equipment we make extensive use of DIgSILENT PowerFactory for network analysis, allowing us to offer services ranging from compliance studies, conceptual design of solutions, and all the way to detailed engineering design covering aspects of harmonic filter component rating, switching transient analysis and stability analysis. Our infinite bus license enables us to tackle any problem, no matter how complex.

As a demonstration of our commitment to this aspect of our business and to share our experience with other users of the software we recently presented a paper on harmonic filter design to the bi-annual user group meeting in Sydney.

The paper can also be found in the Download section of the site. We would welcome any comment to this contribution.


Call for registration: Training course on harmonic filter design

Optimised Network Equipment is conducting a series of courses on reactive power compensation covering shunt capacitor banks, series and shunt reactors.

We will start the series with a specialised course in the design and application of harmonic filter on 19–20 November 2012, and we invite you to join us for training in this highly specialised but commonly used technology.

The training is aimed at providing experienced system analysts and designers with the necessary skills to carry out comprehensive harmonic impact studies and to come up with suitable harmonic filter designs to meet the requirements for their various applications.

An important aspect of the course is to provide an understanding of the practical implications of filter selections from a manufacturing and specification perspective.

The training will take place at the Novotel Hotel in Creek Street in Brisbane. The course will be run over two days starting at 9 AM and finishing approximately 4 PM on each day.

The cost of the training course is $1 850 per person, including lunch and refreshments.

Please confirm your interest in this course by contacting us before 19 October 2012.