Power Quality Assessment
Power quality has degraded over time due to the increased use of non-linear loads. Drives and other sensitive devices are vulnerable to electrical distortion on the line.
Poor Power Quality can be the cause of costly system-wide problems, including:
- Reduced Energy Efficiency that increases kW Demand and kWh energy use. Causes could be:
- Reduced Distribution System efficiency;
- Transformer and distribution equipment overheating;
- Poor Power Factor that causes billing $ penalties.
- Unscheduled work stoppages and increased Operational and Maintenance costs caused by:
- Random breaker tripping;
- Sensitive electronic equipment may fail completely;
- Drives may trip or fail unexpectedly;
Variable frequency drives (VFDs) are beneficial to a wide range of applications, but contain a power conversion process that creates current and voltage distortion, commonly referred to as Total Harmonic Distortion (THD).
Energy Savings & Cost Reduction Opportunities
Legacy's practice is to begin with metering at every facility. We then take the logged data and perform a preliminary analysis in order to determine if existing Power Quality issues are present and if these issues pose any serious potential risks as energy efficiency project installations progress.
Installation of Reactors to deal with Harmonics may be sufficient to offset common risks. In extreme cases, in locations such as spot-welding shops where Power Quality issues are known to be present, a detailed Power Quality Study may be required in order to investigate and find root causes and to find the appropriate technical solutions.
Power Quality Issues We Attack
- We cancel reactive energy required by the facility loads;
- We cancel harmonic energy demanded by nonlinear loads;
- We reduce iron core losses in the transformer;
- We reduce voltage sags;
- We reduce voltage flickering;
- We flatten facility transients;
- We reduce the power demanded by lighting;
- We reduce the power demanded by chillers and HVAC;
- We reduce the power demanded by Compressed Air Systems.
- Improved electrical system capacity (20%+) and greater utilization of electrical infrastructure.
- Reduced electrical-related down-time;
- Lower overall maintenance costs;
- Increased production capability;
- Reduced kW Demand charges;
- Reduced Power Factor penalties;
- Reduced kWh Energy costs;
- Lower kWh/Unit of Production Costs.
Power Quality Overview
There are fundamental differences between simple DC resistance values of various conducting elements and actual “apparent” AC resistances of the same elements. Motors, lighting, facility wiring, distribution panels, protective devices, transformers and switch gear all experience a wide range of phenomena that combine to create wattage (kWh Energy) losses. Identifying and calculating the total of all losses is an extremely challenging engineering proposition that requires knowledge of all factors that impact operating efficiencies.
The two primary types of transformer losses are core losses and load losses. Core losses occur because a magnetizing current must exist in the primary winding of a transformer. This current is additional to current which flows to balance the current in the secondary winding. The magnetizing current is required to take the core through the alternating cycles of flux at the rate determined by system frequency. In doing so, energy is absorbed. Core-losses are present whenever the transformer is energized.
Transformer load losses occur because of current flow in an electrical system and depend on the magnitude of that current. Load losses are caused by the windings in the transformer, and are only present when loaded. The magnitude of losses is proportional to the load squared. The three categories of load losses that occur in transformers are:
- Resistive Losses: - Often referred to as I2R losses;
- Eddy-Current Losses: - Due to the alternating leakage fluxes;
- Stray Losses: - Occur in leads, core-framework and tank due to the action of load-dependent stray alternating fluxes.
Cables also exhibit the same I2R resistive & heating losses reviewed in the Transformer section above. However, for single conductor cables, where conductors are not operating close to each other, proximity effect can be considered to be negligible. Operating together in a typical industrial conduit-enclosed distribution system, these various line loss factors can sufficiently increase the facility electrical distribution wiring’s apparent AC resistance to more than an order of magnitude above nominal DC resistance values. As a result, typical I2R wiring losses are often far greater than simple chart-based values. It should be noted that I2R losses occur in ALL distribution system conducting components, not only the cable.Hysteresis Losses
Hysteresis losses are heat losses associated with the magnetic properties of an AC motor armature. When an armature core is in a magnetic field, the magnetic particles of the core tend to line up with the magnetic field. When the armature core is rotating, its magnetic field keeps changing direction. The continuous movement of magnetic particles as they try to align themselves with the magnetic field produces molecular friction, causing heat. This heat is transmitted to the armature windings, increasing armature resistance.Skin-Effect Losses
The apparent resistance of a conductor is always higher for Alternating Current (AC) than for Direct Current (DC). The magnetic flux created by AC interacts with the conductor, generating a back Electro-motive Force (EMF), tending to reduce the current in the conductor. The center portions of the conductor are affected by the greatest number of lines of this force. The EMF produced in this manner (self-inductance) varies both in magnitude and phase through the cross-section of the conductor, being greater toward the center and smaller towards the outside. The current, therefore, tends to crowd into those parts of the conductor in which the opposing EMF is a minimum. That is, into the ‘skin’ of a circular conductor or the edges of a flat strip. This phenomenon is known as ‘skin’ or ‘edge’ effect.
The resultant non-uniform current density has the effect of increasing the apparent resistance of the conductor, causing increased losses. Harmonic loads amplify skin effect losses by the square of the increase in frequency above nominal line frequency. Because of this, harmonics are the cause of substantial energy losses in any facility with nonlinear equipment loads, such as VFDs, DC drives, rectifiers, induction heaters or other arcing or switching power supply devices.Proximity Effect Losses
Proximity effect exists when conductors are close together, particularly in low voltage equipment, where the interaction between the magnetic fields of conductors causes further distortion of current density. In the same way as an EMF can be induced in a conductor by its own magnetic flux, another conductor can produce an EMF in any other conductor. If two such conductors carry currents in opposite directions, their electromagnetic fields are opposite, tending to force one another apart. This results in a decrease of flux linkages around the adjacent parts of the conductors and an increase in the more remote parts. This forces a larger concentration of current to the adjacent parts where opposing EMF is at a minimum.
If the currents in the conductors move in the same direction, the above action is reversed. This effect, known as the ‘proximity effect’, (or ‘shape effect’), increases the apparent AC resistance. If the conductors are arranged edgewise to one another, the proximity effect increases. As an additional note, in many cases the proximity effect will also tend to increases distribution network stresses under short-circuit load conditions.Eddy Current Losses
Flux will flow in any electrical system component comprising an iron or steel frame and an electrical coil as a result of the alternating current in the coil. The flux in the steel will itself induce an EMF in the material following the basic laws of induction. Since the material is essentially an electrical circuit itself, the induced EMF will cause a circulating electrical current called an eddy-current. Its total magnitude is dependent on the value of EMF and on the resistivity of the current path. As in any other electrical circuit, the losses can be calculated as the square of the current times the resistance. In a similar manner to hysteresis losses, the eddy-current loss manifests itself as heat, and contributes to the maximum operating temperature limit of the device. Eddy current losses occur in protective circuit breakers, lighting ballasts, power supply transformers, magnetic motor starters, voltage reduction or isolation transformers, current overload relays, control contactors and relays, and motor windings. They can also exist in facility wiring if it is in proximity to steel or iron structures such as electrical enclosures, distribution panels, or terminal or distribution blocks.
Innovations Designed To Counter Losses in an Electric Network
It is evident that components of an electrical system are contributors to energy losses in any facility. Measures can be taken to reduce current and harmonics in many facilities that will help minimize distribution losses and save energy. I2R resistance and heating losses in many facilities contribute from 1 - 3% of a facility’s overall kW usage. Hysteresis and skin-effect losses are greatly impacted by current harmonics, and in facilities with high harmonic content, can add 1 – 5% to overall facility kW usage.
These types of losses can be effectively reduced by utilizing Legacy's Power Quality Solutions and Reactive Energy Correction Systems such as the Legacy-Liner & the Legacy Power-Liner that are specifically designed to filter Harmonics and reduce any facility's current usage.By Correcting Segmentation Losses, significant energy savings can be achieved:
- Skin effect losses 2% - 8%
- Proximity effect losses 1% - 3%
- Line losses 1% - 3%
- Hysteresis losses 2% - 5%
- Eddy-current losses 1% - 3%
- Transformer losses 1% - 2%
The addition of a reactor will reduce harmonic content, which reduces the total RMS current with many benefits as follows:
- Improved total power factor;
- A voltage transient, commonly caused by capacitor bank switching (or other issues), sends a current surge into the VFD bus capacitor. The additional current raises bus voltage, thus causing a drive fault (trip). In addition, this overvoltage condition will cause the drive to shut down in order to protect its components. The additional impedance offered by a Line Reactor slows down the current surge, thus reducing the likelihood of the drive tripping offline. This means reduced expensive down-time;
- Input voltage phase unbalance may prevent the drive from performing due to subsequent overcurrent conditions which cause the drive to cease operating. The addition of a reactor to the input of every drive will help balance the drive input line currents;
- Line reactors reduce harmonic current which increases life of drives and protects sensitive equipment;
- Motor life is extended due to reduced motor heating;