The high price of poor power quality
John Walker, engineering director, JPW Consulting Group, and Ian C Evans, managing director, Sentinel Power Quality Group
Power quality and harmonics have been a developing and growing concern as buildings have become more reliant on technology, the ever-increasing utilisation of digital control systems, and the drive towards more efficiency. In recent years, harmonics from buildings have caused issues to supply networks, and authorities now require owners of large buildings to take measures to limit harmonic distortion being imposed on to the utility grid network, and to protect their own internal power network.
Harmonics are generated by almost all equipment connected to the power system (plant motors, fans/pumps, variable speed drives, elevators/escalators, lighting control gear/lamp types, lighting control systems, UPS equipment, IT equipment, PC/laptops/LED screens, security/general equipment, etc.). Most buildings will be full of harmonic generating equipment.
One of the key issues and consideration for building owners is the impact of variable speed drives (VFDs) on the power quality of the buildings. JPW has been involved in a number of survey projects where VFDs and the associated active filters were causing significant power quality issues within the development.
VFDs are an integral part of MEP installations and benefits include controllability and substantial energy savings; however, they produce harmonic voltage distortion.
The cost of harmonic mitigation is expensive and designers rarely investigate all options. Instead, they tend to concentrate on a single solution of parallel active filters and the so-called “low harmonic drives.” Unfortunately, the lack of expert knowledge and experience, regarding the practical application of these technologies results in system and equipment failures and additional project costs.
IEEE 519 (1992), the most popular project harmonic standard, is now not suitable for current equipment. The standard specifies a maximum total harmonic voltage distortion (Uthd) of 5% in LV systems in the GCC, and 3% for special applications such as hospitals and airports.
Harmonics are limited to the 50th order in the above standard, but in reality, extend well beyond this level. The majority of standard analysers only measure to the 50th order. Additional “high frequency” voltages and currents, in the case of inadequately selected active filters, can cause early life failure of both transformers and generators with resultant disruption and cost implications.
Active filters current ratings are often specified incorrectly. The calculations do not take into account the actual harmonic current drawn from the filters when operating which can be 25-160% more than the calculated value, depending on the value of reactance in each VFD. This has serious cost and performance implications.
VFDs are installed to save money, but the running costs rarely factor in the selection of the equipment. For example, “low harmonic drives” have the lowest overall (with motors) efficiency of all the VFD/mitigation options. (See Table 1.) The difference in the running costs per annum between, for example, a 6 pulse VFD with series passive wide spectrum filter and low harmonic drives at 2.1% translates to real kWhr savings of US$ millions per annum or money wasted, usually the latter.
Both VFDs and active filters need to be installed in strict compliance with manufacturers’ recommendations concerning the minimisation of EMI (electromagnetic interference). In the experience of the authors, this rarely is the case in the GCC. As a result, for example, the switching frequency of the VFD(s) is superimposed on the phase to ground voltage (i.e., common mode voltage) and “seen”throughout the facility via the ground conductor. The result can be the disruption of susceptible equipment and the destruction of bearings in VFD fed motors, as the common mode current ground path is the motor. The cost and inconvenience of excessive common mode voltage problems can be extremely expensive.
Specifications and guidance in this crucial area are often out of date and inadequate due to lack of specialist subject knowledge, in this age of new electronic technology within buildings.
These issues with their associated costs can be avoided by engaging specialist consultants to take responsibility for all aspects of power quality, thus maximising equipment lifecycle.
Prior to the introduction of electronic switching, harmonics and common mode voltage did not exist. The recent increase in failure of plant and equipment can, almost entirely, be attributed to this new age phenomenon.
JPW Consulting Group in conjunction with Sentinel Power Quality Group can provide:
Specifications and peer reviews for all aspects of power quality.
Management of power quality with regard to the optimum and correct selection of equipment to ensure that all equipment is installed in compliance with EMC requirements and standards.
Carry out verification testing of power quality before handover.
Power quality surveys and troubleshooting.
The generation of harmonics and EMC issues are here to stay and without the correct mitigation measures unexpected plant, equipment, and control failures will continue to occur.