by Markus Bakker, Fluke Corporation
It is well known that power quality measures performed on motors and drives can improve the energy efficiency of electrical systems, and keep costs down. However, there are several unseen problems associated with energy use in facilities that can result in significant cost overruns, while causing damage to equipment and disruptive nuisance shutdowns.
By focusing on the following six aspects of power quality measurement, it is possible to uncover some of the invisible problems and reduce costs, while improving the overall performance of the facility.
Imbalance between phases
In a balanced three-phase system, the phase voltages and currents must be either identical or extremely close in magnitude and phase. Any imbalance in either of these areas can result in reduced performance or even premature failure. Motors may perform less well due to resistive torque, while failure may be caused prematurely by excessive winding overheating.
The most significant costs that can occur are related to motor replacement and lost revenue from tripped protection circuits, as well as the downtime and labor costs associated with fixing the problem. However, the imbalance also affects energy costs in that the imbalance reduces motor performance.
One of the best ways to identify potential voltage unbalance problems is to measure the voltage at the customer's connection to the public power grid (electric meter). According to the EN50160 power quality standard, the voltage unbalance (the ratio of negative to positive components) must be less than 2 % at the meter. If the voltage is not well balanced at the meter, the power of the whole installation is unbalanced and must be corrected as soon as possible by the distribution system operator.
The unbalance may exist at a single load, or at an entire branch of the internal electrical infrastructure, for example for an electric motor or even a group of motors. It is therefore a good idea to check the phase voltages and currents at the input, with the understanding that the unbalance should not exceed 2 % in voltage and 6 % in current. The current unbalance is a direct consequence of the voltage unbalance. If the voltages are balanced, the current imbalance is caused by an imbalance of the loads.
Total harmonic distortion
Total harmonic distortion (THD) is the amount of distortion in the voltage or current due to harmonics in the signal. While some distortion of the current is normal, any value greater than 5 % on any phase warrants further investigation. If this level of distortion is not corrected, it can lead to problems such as high current flowing through neutral conductors, motors or transformers getting hot (reducing insulation life), poor transformer efficiency (or the need to use larger transformers to accommodate harmonics), audible noise or vibration, due to saturation of the transformer core (noise and vibration are a waste of energy).
The most significant costs associated with THD are reduced motor and transformer life. Of course, if the equipment involved is part of a production system, revenues may also be impacted, as harmonics reduce the efficiency and performance of motors and transformers.
The best way to identify these problems is to take measurements against the normal level on motors, transformers and neutral conductors that supply electronic loads. It is important to monitor the current levels and temperatures inside the transformers to ensure that they are not overloaded, and also to understand that the neutral current should never exceed the capacity of the neutral conductor.
Harmonics are often caused by specific electrical machines or installations, and only occur when these elements are energized. It is therefore very useful to record the measurements with a time stamp to be able to directly link the intermittent presence of certain harmonics to certain processes.
The harmonic frequencies in question go up to the 50èmeThe harmonics are all derived from the fundamental frequency of the voltage, which is 50 Hz. With the increasing use of power electronic systems, such as inverters or frequency converters, higher frequency harmonics can pollute the network. These harmonics have no relation to the fundamental frequency (50 Hz) and are caused by the switching of the power electronic systems mentioned above. These "super harmonics" interfere with process control equipment and can even cause it to stop.
Transients
Electronic devices are also very vulnerable to transients. These are extremely short pulse voltages (less than 10 ms) but can be very high in magnitude (up to 6 kV). These pulses can be caused by heavy load switching, capacitor discharge, or even lightning. If impacted by a transient, electronic devices may shut down or disrupt the processes for which they are programmed.
To ensure that transients are the cause of a problem, a measuring device with a high enough sampling rate to capture the event must be used. It is imperative that these devices have a ground connection, and that the captured event can be displayed, so that the origin of the voltage pulse can be identified, or at least presumed.
The only way to put these devices back into service after such an episode is to perform a manual reset, which implies suspending production beforehand. In addition, the quality of all products manufactured since the incident will need to be verified. To protect devices from transients, surge protectors can be installed, which send the voltage pulse to ground before it reaches the electronic devices.
Voltage drops
A voltage drop is a temporary reduction in voltage level, which can be caused by the addition of loads without the knowledge of plant managers. These loads can cause the system voltage to drop for a short time if they generate high inrush currents. This can cause some electronic equipment to reset, or trigger overcurrent protection functions. A voltage drop on one or two phases of a three-phase load can cause the other phase(s) to draw a higher current to compensate.
Voltage drops can result in lost revenue, if a computer or control system resets, for example. Loss of revenue can also occur due to tripping of some variable frequency drives (VFDs), or shortened uninterruptible power supplies (UPSs) due to frequent load cycling. Any preventive maintenance strategy should include monitoring of motors, inverters, variable frequency drives, and distribution panels supplying industrial controls and IT equipment. The obvious benefit of such monitoring is to minimize downtime and reduce costs.
To evaluate the severity of a voltage drop, it is essential to measure the intensity of this drop (in percentage of the nominal voltage) and its duration (in milliseconds). With these two parameters, it is possible to make a comparison with the limits set by the American consortium ITI (Information Technology Industry Council). Electronic equipment is able to withstand voltage drops as long as they remain within these limits. If this is not the case, efforts must be made to mitigate these voltage drops. The problem with voltage drops is that they often occur intermittently, so measurements must be automatically triggered to capture them immediately. If a predefined trigger level is reached, the measurement equipment should start recording the event.
Peak demand
Energy providers monitor the power consumption levels of industrial (and commercial) facilities several times an hour to determine the average energy demand of the plant. During start-up, production facilities in particular tend to consume a large amount of energy, which can affect how energy providers calculate their rates based on peak demand (the highest average demand of all time intervals in a billing cycle).
To reduce these costs, load switching should be staggered to lower demand and minimize the maximum instantaneous consumption. To do this, it is important to determine the demand interval used by the networked distributed service and to measure the power demand over time at the meter using a power quality recorder. It is also useful to identify if large loads are connected at the same time, by analyzing the demand measurements to identify the measurements corresponding to the different loads.
If facilities exceed contracted peak demand levels, they face significant financial penalties from energy suppliers. Therefore, preventing overruns and regulating energy costs is critical to protecting revenues and reducing expenses.
Power factor
Not all of the electricity generated and transmitted to the end user is used efficiently; it is actually the effective power (measured in kW) that the end user must pay for. The reactive energy, which the infrastructure must also transport, is not used and is not charged to the end user. It can therefore be considered as wasted energy. This means that the infrastructure, including cables, switches and transformers, must be sized to carry the full amount of power, but only a portion of it is used efficiently. The total power carried is called apparent power (measured in kVA).
The ratio of effective to apparent power indicates the efficiency of energy use. If this ratio is equal to 1, all apparent power is used and billed. The lower the ratio, the less efficient the use of apparent power. Since energy suppliers cannot charge the end user for reactive power, a limit is set in the contract. If this limit is reached, a significant financial penalty can be charged. The ratio of effective to apparent power is called "cosine phi" or "displacement power factor" and ideally should never be less than 0.95.
In addition to the potential financial penalty, poor cosine phi can cause the infrastructure to overheat. To avoid this, facilities should have capacitor banks installed near large loads, such as motors larger than 50 kW, or centrally near the main distribution panel.
Harmonics can also have an effect on the power factor. If harmonics are present, compensation by capacitors alone may be insufficient, so it is essential to implement filtering to reduce the negative effect of harmonics.
By addressing these six invisible power consumption issues, facilities will be able to minimize unnecessary expenses, downtime and equipment damage, while maximizing productivity and energy efficiency. More information is available at www.fluke.com.