Markus Bakker, Fluke Corporation
It is well known that power quality measurements performed on motors and drives can improve the efficiency of electrical systems and minimize costs. However, there are several hidden issues related to energy use that can significantly increase costs, as well as cause equipment damage and disruptive downtime.
Focusing on these six aspects of power quality can reveal these six hidden problems and reduce costs, as well as improve the performance of the installations as a whole.
Imbalance
In a balanced three-phase system, the voltages and currents in each phase must be equal or have extremely similar values of amplitude and phase. Any imbalance between them can lower performance levels or even cause premature failure. Poor motor performance occurs due to anti-torque and premature motor failure because the imbalance generates excessive heat in the winding.
The highest costs can be caused by motor replacement and lost revenue caused by failure to protect the circuit along with the corresponding downtime and labor costs to fix the problem. But the imbalance also affects energy costs as it reduces motor performance.
One of the best ways to identify voltage unbalance problems in advance is to look at the measured voltage at the utility (inlet) connection. According to the EN50160 power quality standard, the voltage imbalance, being the ratio between the negative and positive sequence components, must be less than 2% at the connection point. If the voltage is not well balanced in the connection, the electrical supply will be unbalanced throughout the installation and must be repaired as soon as possible by the operator of the distribution network.
The imbalance can be present in a single load or in a branch in the internal electrical infrastructure, for example, an electric motor or even a set of motors. That is why it is advisable to check the input voltage and the input current taking into account that the imbalance of these two parameters should not exceed 2% and 6%, respectively. Current imbalance is a direct consequence of voltage imbalance, and if the voltage is balanced, the cause of current imbalance is load imbalance.
Total Harmonic Distortion
Measuring the total harmonic distortion allows us to know what proportion of the voltage or current distortion is due to harmonics in the signal. While some distortion is normal, if it is above 5% at any stage further investigation is required. If this level of distortion is not addressed, it can lead to problems such as high current flowing into neutral conductors, motors and transformers that get hot (affecting insulation life), poor transformer efficiency (or the need to use a larger transformer to accommodate harmonics) and audible noise and vibrations from transformer core saturation (noise and vibration is a waste of energy).
Most of the total harmonic distortion is due to the shortening of the useful life of motors and transformers. Of course, if the affected equipment is part of a production system, revenue can be reduced as harmonics reduce motor and transformer efficiency and performance.
The best way to identify these problems is to make the measurements taking as reference the normal level of the motors, transformers and neutral conductors that serve the electronic loads. It is important to monitor the current levels and temperatures in the transformers to ensure they are not excessive and to understand that the neutral current should never exceed the capacity of the neutral conductor.
Harmonics often originate from certain machines or electrical installations and only occur if these assets are in operation. Therefore, it is very helpful to record the measurements together with the time, so that the intermittent presence of harmonics can be directly related to certain processes.
The harmonics mentioned so far reach up to the 50th harmonic and are derived from the fundamental frequency of the voltage, which is 50Hz. Given the boom experienced by the application of power electronics, such as drives or frequency converters, the most frequent harmonic components can contaminate the network. These components are not related to the fundamental power and are caused by the aforementioned switching. These “supraharmonics” interfere with process control equipment and can even stop these processes.
Transient
Electronic devices are also very vulnerable to transients. These are voltage pulses whose duration is extremely short (less than 10 milliseconds) but their voltage can be very high (up to 6kV). Pulses can be caused by switching large loads, discharging capacitors, and even lightning. When affected by a transient, electronic devices can trip or affect the processes for which they are programmed.
To make sure that the problems have been caused by transients, it is necessary to use a measurement device that has a sample rate high enough to capture the event. It is vital that these devices have a ground connection and the captured event is displayed so that the source of the voltage pulse can be deduced.
The only way for these devices to “come back online” after such an event is to perform a manual reset, which means production processes have to be stopped. In addition, it is necessary to check the quality of all products produced since the event took place. To protect devices against transients, surge arresters can be installed that guide the voltage pulse to ground before leading to electronic devices.
voltage drops
A voltage drop is a temporary decrease in the voltage level that can be caused by added loads without the plant managers being aware of it. These loads can absorb system voltage for a short time if they draw high inrush currents. As a result, electronic equipment can be reset or overcurrent protection is activated. Dips in one or two phases of three-phase loads can cause the other phases to draw a higher current as compensation.
Brownouts can reduce revenue if, for example, a computer or control system is rebooted, a variable frequency drive is tripped, and the life of a UPS is shortened, as a result of power failures. frequent charge cycles. Any preventive maintenance strategy must include supervision measures in motors, UPSs, frequency inverters and electrical panels that feed industrial controls or computer equipment. The obvious consequence of this action would be to minimize downtime and costs.
To assess the severity of a drop it is essential to measure the “depth” of the drop (as a percentage of the nominal voltage) and its length (in milliseconds). From these two parameters it is possible to make a comparison with the limits of the ITIC (Information Technology Industry Council). Electronic equipment can assume voltage drops as long as they are within those limits. If this is not the case, efforts must be made to mitigate these falls. One problem with crashes is that they often occur intermittently, so measurements have to be programmed to capture them automatically. If a previously defined activation level is exceeded, the measuring equipment will start recording the event.
peak demand
The consumption levels of industrial (and commercial) facilities are monitored by power companies several times per hour in order to know their average energy demand. Production plants in particular tend to consume a large amount of energy at start-up and this can influence how utilities are able to calculate their loads based on peak demand (the highest average demand over all intervals in a power cycle). billing).
The way to reduce these costs is to stagger charge cycles to smooth demand and minimize total power consumption at any one time. To do this, it is important to verify which demand interval the power company uses and to measure the power demand over time at the connection using a power quality recorder. It will also facilitate the identification of significant loads that appear concurrently, measuring the demand to verify the readings for each load.
In the event that the installations exceed the contracted levels of peak demand, the electric companies may apply significant fines. Therefore, it is essential to prevent excess spending and regulate energy costs to protect income and reduce expenses.
Power factor
Not all the power generated and transported to the end user is used efficiently and it is the active power (measured in kW) that the end user pays for. The reactive power, which is also part of the electricity supply transported through the infrastructure, is not used and is not charged to the end user, so it can be considered waste. This means that infrastructure elements such as cables, switches, and transformers are sized to carry full power, but only part of this infrastructure is used efficiently. This total power is called apparent power and is measured in kVA.
The ratio between the active power and the apparent power indicates the efficiency in the use of energy, and if it is equal to 1 it means that all the apparent power is used and charged; the lower that number is, the less efficient the use of apparent power. Since power providers cannot charge end-user for reactive power, a limit is set in the contract. If this limit is exceeded there may be a significant fine. The ratio of active power to apparent power is called “cos phi” or “displacement power factor” and should ideally never fall below 0,95.
In addition to the fine, another negative consequence of bad cos phi can be overheating of the infrastructure. To avoid this problem it is necessary to install capacitor banks near high loads such as motors with a power of more than 50kW or at a central point near the distribution board.
Harmonics can also affect power factor. If harmonics are present, compensation by means of capacitors is not enough on its own, so it is essential to resort to filtering to reduce the negative effect of harmonics.
Addressing these six hidden issues related to energy use helps minimize waste, downtime, and equipment damage, while maximizing productivity and efficiency.
