Power Quality Software
Yamila Comejo
Electric power quality is the degree to which the voltage, frequency, and waveform of a power supply system conform to established specifications. Good power quality can be defined as a steady supply voltage that stays within the prescribed range, steady AC frequency close to the rated value, and smooth voltage curve waveform (which resembles a sine wave). In general, it is useful to consider power quality as the compatibility between what comes out of an electric outlet and the load that is plugged into it.[1] The term is used to describe electric power that drives an electrical load and the load's ability to function properly. Without the proper power, an electrical device (or load) may malfunction, fail prematurely or not operate at all. There are many ways in which electric power can be of poor quality, and many more causes of such poor quality power.
The electric power industry comprises electricity generation (AC power), electric power transmission and ultimately electric power distribution to an electricity meter located at the premises of the end user of the electric power. The electricity then moves through the wiring system of the end user until it reaches the load. The complexity of the system to move electric energy from the point of production to the point of consumption combined with variations in weather, generation, demand and other factors provide many opportunities for the quality of supply to be compromised.
It is often useful to think of power quality as a compatibility problem: is the equipment connected to the grid compatible with the events on the grid, and is the power delivered by the grid, including the events, compatible with the equipment that is connected? Compatibility problems always have at least two solutions: in this case, either clean up the power, or make the equipment more resilient.
Ideally, AC voltage is supplied by a utility as sinusoidal having an amplitude and frequency given by national standards (in the case of mains) or system specifications (in the case of a power feed not directly attached to the mains) with an impedance of zero ohms at all frequencies.
An uninterruptible power supply (UPS) can be used to switch off of mains power if there is a transient (temporary) condition on the line. However, cheaper UPS units create poor-quality power themselves, akin to imposing a higher-frequency and lower-amplitude square wave atop the sine wave. High-quality UPS units utilize a double conversion topology which breaks down incoming AC power into DC, charges the batteries, then remanufactures an AC sine wave. This remanufactured sine wave is of higher quality than the original AC power feed.[5]
Modern systems use sensors called phasor measurement units (PMU) distributed throughout their network to monitor power quality and in some cases respond automatically to them. Using such smart grids features of rapid sensing and automated self healing of anomalies in the network promises to bring higher quality power and less downtime while simultaneously supporting power from intermittent power sources and distributed generation, which would if unchecked degrade power quality.
A power quality compression algorithm is an algorithm used in the analysis of power quality. To provide high quality electric power service, it is essential to monitor the quality of the electric signals also termed as power quality (PQ) at different locations along an electrical power network. Electrical utilities carefully monitor waveforms and currents at various network locations constantly, to understand what lead up to any unforeseen events such as a power outage and blackouts. This is particularly critical at sites where the environment and public safety are at risk (institutions such as hospitals, sewage treatment plants, mines, etc.).
Nisenblat et al.[9] proposes the idea of power quality compression algorithm (similar to lossy compression methods) that enables meters to continuously store the waveform of one or more power signals, regardless whether or not an event of interest was identified. This algorithm referred to as PQZip empowers a processor with a memory that is sufficient to store the waveform, under normal power conditions, over a long period of time, of at least a month, two months or even a year. The compression is performed in real time, as the signals are acquired; it calculates a compression decision before all the compressed data is received. For instance should one parameter remain constant, and various others fluctuate, the compression decision retains only what is relevant from the constant data, and retains all the fluctuation data. It then decomposes the waveform of the power signal of numerous components, over various periods of the waveform. It concludes the process by compressing the values of at least some of these components over different periods, separately. This real time compression algorithm, performed independent of the sampling, prevents data gaps and has a typical 1000:1 compression ratio.
To the untrained eye, problems in electrical distribution systems or the equipment connected to the circuit may not be recognizable as power quality problems. When no immediate problem is apparent, it may be written off as just an old breaker that needs replacing or a one-time nuisance reset. You might look at types of loads on the system and monitor for harmonics, unbalance, or disturbance patterns.
Harmonic distortion issues and voltage and current anomalies represent the areas where common problems in power quality occur. Voltage and current anomalies can produce problems; however, many can be corrected before they damage equipment. Spot the symptoms:
Power Quality Loggers make it easy to profile energy usage in your facility. They deliver power-measurement data so you can discover and mitigate sources of electrical energy waste or overloaded circuits. These tools are great to have in-hand for routine load studies and preventive maintenance programs.
Power Quality Recorders are the tool to grab for advanced power quality analysis. Uncover the root cause of equipment malfunctions with load studies and transient analysis so you can later go back and make necessary repairs or develop baseline information for routine load studies.
Electrical Measurement Windows are an additional tool to make measurements with these tools easier. An electrical measurement window is permanently installed into the face of a cabinet with the necessary connections ready to go. Instead of having to open the cabinet, you can just take measurements at the window, making it a safer process for routine preventive maintenance or load studies.
The role of power conditioning systems in the past was to protect the customer's equipment from power quality problems that occurred external to their facility. Today we must also deal with power quality problems caused by their own equipment.
While power requirements are decreasing for individual pieces of equipment, the electrical distortion caused by the newer, more efficient power supplies degrade the performance of the electrical system both inside and outside the facility. Utilities are unable to provide the high quality and reliability in electrical power required to meet the ever increasing power quality standards of newer equipment.
In the 1970's and 1980's, the problems were most felt in large data centers using sensitive computers. Power quality problems were addressed with Uninterruptible Power Systems, Power Distribution Units, and on site Power Generation. In the 1990's, these problems have increased and moved into factories, offices and anywhere solid-state devices are used. The question now is "Can these systems deal with the new types of critical load?" In many cases the answer is, "Not without minor, and in some cases, major design changes."
The issues are becoming more technical and much harder to explain to the design Engineers as they are not aware of changes that will occur after the original build. This document will explain the problems associated with harmonics.
Power quality is a very important issue that should be addressed as poor power quality costs money and in some cases downtime. We will look at some direct and indirect costs attributed to power quality.
Direct cost is the loss of production due to a voltage problem, which trips motor and control devices that stop the manufacturing process. It is the loss of products not produced and the labor charges for removing any damaged materials as well as employee wages paid while waiting for the process to restart.
Indirect cost is the replacement of other equipment that becomes stressed by changing electrical voltages. As an example, a solid-state motor drive fails due to voltage spikes over time. These are commonly caused by power factor capacitors switching on and off line to correct varying power factors. However, the failure and subsequent damage to the machine will be untimely because is not caused by any one spike but by numerous spikes occurring and over a period of time. With this type of power quality damage, it is impossible to avoid the outage.
Nonlinear Loads:
Nonlinear loads cause harmonic distortion, which is the most difficult problem to explain. We need to establish some background on electrical power.
Electrical power in the United States is supplied at 60 hertz (Hz) and is produced by rotating generators. The prime mover for these generators can be hydro (water), steam from coal, oil or nuclear. In all cases, the prime mover changes one type of energy into rotational energy (torque). This rotational energy is then applied to the generator, which turns a magnet in an electrical field. The speeds that the magnet turns determine the frequency of the electrical voltage produced. In the U.S, it is 60 turns each second or 60 Hz. In other countries it may be 50 turns a second (50 Hz).
Each cycle or turn produces a voltage that starts at zero volts and increases to some maximum positive value at of a revolution (90 ) and then decreases back to zero volts at a revolution (180). It then increases in a negative direction to a maximum value of revolution (270), then decreases back to zero one complete revolution. This is one cycle and is repeated 60 times each second. The voltage waveform is a sine wave and contains 360 electrical degrees.
With a linear load, the current drawn by the load is dependent on the voltage. A linear load is also known as a resistive load. Some examples of resistive loads are heaters and lights.