Kit media
par Markus Bakker, Fluke Corporation
It is well known that power quality measurements on motors and variable speed drives can improve the energy efficiency of electrical systems and keep costs low. However, there are several invisible problems related to the use of energy in facilities, which are likely to result in significant additional costs, while causing damage to equipment and generating disruptive untimely shutdowns.
By focusing on the following six aspects of power quality measurement, it is possible to uncover some invisible problems and reduce costs, while improving the overall performance of the installation.
Imbalance between phases
In a balanced three-phase system, the voltages and phase currents must be either identical or extremely close in terms of amplitude and phase. Any imbalance in any of these areas can lead to reduced performance or even premature failure. Motors can be less efficient because of the strong torque, while their failure can be caused prematurely by excessive overheating of the windings.
The biggest costs likely to occur are related to engine replacement, and the loss of revenue generated by triggering the protection circuits, as well as the downtime and labor costs associated with resolving the problem. However, the imbalance also affects energy costs as this imbalance reduces engine performance.
One of the best ways to identify potential voltage imbalance problems is to measure the voltage at the customer's connection to the public power grid (electricity meter). According to the EN50160 standard on energy quality, the voltage imbalance (the ratio between the negative and positive components) must be less than 2% at the meter. If the voltage is not properly balanced at the meter, the power of the entire installation is unbalanced and must be corrected as soon as possible by the distribution system operator.
The imbalance can exist at the level of a single charge, or an entire branch of the internal electrical infrastructure, for example for an electric motor or even a group of motors. It is therefore advisable to check the voltages and phase currents at the input, it being understood that the imbalance must not exceed 2% in voltage and 6% in current. Current imbalance is a direct consequence of voltage imbalance. If the voltages are balanced, the current imbalance is caused by an imbalance of loads.
Total harmonic distortion
Total harmonic distortion (THD) is the amount of distortion of the voltage or current due to the harmonics of the signal. While it is normal to observe some distortion of the current, any value greater than 5% over a 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 that heat up (which reduces insulation life), poor transformer performance (or the need to use larger transformers to account for harmonics), audible noise or vibration, due to the saturation of the transformer core (noise and vibration represent a waste of energy).
The biggest costs associated with THD are the reduction in the life of motors and transformers. Of course, if the equipment concerned is part of a production system, revenues can also be impacted, since harmonics reduce the efficiency and performance of motors and transformers.
The best way to identify these problems is to perform measurements from the normal level on motors, transformers and neutral conductors that feed the electronic loads. It is important to monitor the current levels and temperatures inside transformers to ensure that they are not overloaded, and also to understand that neutral current should never exceed the capacity of the neutral conductor.
Harmonics are often caused by certain specific electrical machines or installations, and only occur if these elements are energized. It is therefore very useful to record the measurements with a timestamp to be able to directly link the intermittent presence of certain harmonics to certain processes.
The harmonic frequencies in question go up to the50th, and 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 grid. 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 "supraharmonics" 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 switching heavy loads, discharging capacitors, or even lightning. If impacted by a transient, electronic devices can shut down or disrupt the processes for which they are programmed.
To ensure that transients are causing a problem, a measuring device with a sufficiently high sampling rate to capture the event should be used. It is imperative that these devices have a ground connection, and that the captured event can be displayed, so as to allow the identification, or at least the presumption of the origin of the voltage pulse.
The only way to put these devices back into service after such an episode is to perform a manual reset, which involves suspending production beforehand. In addition, the quality of all products manufactured since the incident will have to be checked. To protect devices from transients, surge arresters can be installed, which send the voltage pulse to earth before it reaches the electronic devices.
Voltage drops
A voltage drop is a temporary reduction in the voltage level, which can be caused by adding 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 may cause some electronic equipment to be reset, or trigger overcurrent protection features. A voltage drop over 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 cause loss of revenue, for example, if a computer or control system resets. Income losses can also occur due to the activation of certain variable frequency drives (VFDs), or due to the reduction in the life of uninterruptible power supplies (UPS) due to too frequent charge cycles. Any preventive maintenance strategy should include monitoring measures for motors, inverters, variable frequency drives, and distribution panels powering industrial controls and IT equipment. The obvious benefit of such monitoring is to minimize downtime and reduce costs.
To assess the severity of a voltage drop, it is essential to measure the intensity of this drop (as a percentage of the rated voltage) and its duration (in milliseconds). Thanks to 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 it remains 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; Measurements must therefore be triggered automatically in order to capture them immediately. If a predefined trigger level is reached, the measuring equipment shall start recording the event.
Peak demand
Energy suppliers monitor the power consumption levels of industrial (and commercial) facilities several times an hour, to determine the plant's average energy demand. At start-up, generation facilities in particular tend to consume a large amount of energy, which can impact how energy providers calculate their rates based on peak demand (the highest average demand of all time intervals in a billing cycle).
In order to reduce these costs, load switching should be staggered to reduce demand and minimize maximum instantaneous consumption. To do this, it is important to determine the demand interval used by the networked distributed service and to measure the energy demand over time at the meter level using a power quality recorder. It is also useful to identify if large loads are connected at the same time, by analyzing demand measurements to identify measurements corresponding to different loads.
If facilities exceed contractually agreed peak demand levels, they face significant financial penalties from energy suppliers. Therefore, preventing overruns and regulating energy costs is essential to protect revenues and reduce expenses.
Power factor
Not all electricity produced and transported to the end user is used efficiently; it is actually the effective power (measured in kW) that the end user has to pay. Reactive energy, which the infrastructure must also transport, is not used and is not charged to the end user. It can therefore be considered wasted energy. This means that infrastructure, including cables, switches and transformers, must be sized to carry the full amount of energy, but only part of this infrastructure is used efficiently. The total power carried is called bulk power (measured in kVA).
The ratio of effective power 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 may be charged. The ratio of effective power to apparent power is called "phi cosine" or "displacement power factor" and, ideally, should never be less than 0.95.
In addition to the possible financial penalty, a bad cosine phi can lead to overheating of the infrastructure. To avoid this, installations must have capacitor banks installed near large loads, such as motors with an output greater than 50 kW, or at the central level, near the main switchboard.
Harmonics can also have an effect on power factor. In case of presence of harmonics, the only compensation by capacitors 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 on www.fluke.com.
