• • • RENEWABLE TECHNOLOGY • • •
transformer to accommodate harmonics) and audible noise and vibrations due to saturation of the transformer core (noise and vibration are a waste of energy). The largest THD expenses relate to the
shortening of the operational life of motors and transformers. Of course, if the equipment affected forms part of a production system, income can be reduced there as well since harmonics reduce motor and transformer efficiency and performance. The best way to identify such issues is to
carry out measurements against the normal level for the motors, transformers and neutral conductors that serve the electronic loads. It is important to monitor current levels and temperatures within transformers to ensure they are not overburdened and to understand that neutral current should never exceed the capacity of the neutral conductor. Harmonics are often caused by specific
machines or electric installations and happen only if these assets are switched on. Therefore, it is very helpful to log measurements with a time stamp so that the intermittent presence of harmonics can be related directly to certain processes. Harmonics discussed so far go up to the 50th
harmonic and all are derived from the fundamental frequency of the voltage, which is 50Hz. With the emerging application of power electronics such as frequency drives and converters, higher frequent harmonic components can pollute the network. These components have no relation with the fundamental power and are caused by the switching mentioned above. These so-called ‘supra harmonics’ interfere with process control equipment and can even bring processes to a halt.
Transients Electronic devices are also highly vulnerable to transients. These are impulsive voltages that are extremely short in time (less than 10 milliseconds) but may be very high in voltage (up to 6kV). The impulses can be caused by switching heavy loads,
discharging capacitors and even lightning strikes. Once impacted by a transient, electronic devices may switch off or disrupt the processes that they are programmed to deliver. To be sure that the problems have been caused
by transients, a measurement device must be used that has a sufficiently high sampling rate to capture the event. It is vital that these devices have a connection to ground, and the captured event is displayed so that the origin of the voltage pulse can be surmised. The only way to get these devices ‘back online’
after such an episode is to carry out a manual reset, which means production processes have to be stopped. In addition, the quality of all the products that were produced since the strike took
place will have to be checked. To protect devices against transients, surge arresters can be installed that guide the voltage pulse to ground before it hits the electronic devices.
Voltage dips A voltage dip is a temporary reduction in voltage level which can be caused by loads being added without plant managers being aware. These loads can draw down system voltage for a short moment if they draw high inrush currents. This can result in resets on electronic equipment or overcurrent protection trips. Dips on one or two phases of three-phase loads can lead to the other phase(s) drawing higher current to compensate. Income can be lost through voltage dips if a
computer resets itself, for example, or through control system resets, variable frequency drive (VFD) trips and reductions in the life of uninterruptible power supplies (UPSs) as a result of frequent charge cycles. Any preventive maintenance strategy must involve tracking measurements on motors, UPSs, VFDs and panels serving power to industrial controls or computer equipment. The obvious consequence of taking this action would be to minimise downtime and costs. To assess the severity of a dip, it’s essential to
measure the ‘depth’ of the dip (in percentage of nominal voltage) and its length (in milliseconds). With these two parameters it’s possible to carry out a comparison against Information Technology Industry Council (ITIC) limits. Electronic equipment is able to cope with dips as long as they stay within those limits. If not, efforts to mitigate these dips must be taken. One problem with dips is that they often occur intermittently so measurements need to be triggered to capture them automatically. If a previously defined trigger level is exceeded, measurement equipment would start to record the event.
36 ELECTRICAL ENGINEERING • NOVEMBER 2022
electricalengineeringmagazine.co.uk
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