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The Role of Harmonics and Non-Sinusoidal Loads in a Wind Farm
Wind turbine transformers should be designed for the additional heating caused by harmonic loading and have an electrostatic shield between windings, neither of which are provided by conventional "off the shelf" transformer designs. Read on for more information on the role harmonics play in wind farm transformer design.
Harmonics Basics
Transformer design is based on the principal of creating a fluctuating magnetic field from a uniform sinusoidal input alternating voltage source to induce current flow in, and voltage potential across, an electrically separate conductor in that fluctuating field.
A purely uniform sinusoidal wave form is possible only theoretically. In real world transmission and distribution power systems, voltage and current waves get distorted from the ideal. In fact, total harmonic distortion (THD) of 1 to 2 percent is common at the point of generation. Also, non-linear loads such as switching actions, rotating machinery, variable frequency drives, and electronic devices of all types add further distortion to the ideal wave shape.
The cumulative distortions repeat every cycle, adding peaks that ride on the voltage and current waves and occur at other than the fundamental frequency of 60 Hertz. The harmonics creating these distortions are multiples of the fundamental frequency and are referred to by their multiple, for example, 3rd, 5th, and 7th.
Dangers of Harmonics
The danger from harmonics is that they increase the eddy and stray losses within the transformer. Eddy and circulating currents in a winding conductor causes additional heating which must be addressed with additional cooling. Otherwise, the additional heat can cause insulation degradation and lead to premature transformer failure.
Harmonics in Wind Turbine Generators
Wind turbine generators, like conventional generation sources, will produce generator-caused distortions, which result in harmonic wave forms. In addition to these harmonics caused by the generators, the turbine step-up transformers are managed with solid state controls that contribute their own form of damaging harmonics.
Rectifier/Chopper Circuits
Generator systems using rectifier/chopper circuits present particular problems for the transformer. Since harmonic losses for this configuration are multiples of the transformer's inherent winding eddy current losses, design steps are required to reduce the eddy losses to compensate for the harmonic currents.
For rectifier/chopper turbine controllers, the transformers should be designed for harmonics similar to those for furnace or rectifier transformer applications. If not addressed by the transformer designer, the combined non-sinusoidal wave forms from the turbines and the switching induced harmonic wave forms will create excessive heating in the transformer. Shortened life spans and premature failures can result from using conventional distribution transformers which are not designed for this type of duty
Harmonics contributed by rectifier/chopper type controllers contain high frequencies that can also affect other equipment on the grid if permitted to pass through the transformer. One example is that protective equipment may see this as a fault condition and attempt to disconnect the turbine. Though harmonic filtering is not specifically a function of the turbine step-up transformer, electrostatic shields located between the primary and secondary windings will act as a filter to prevent the transfer of dangerous harmonics onto the collector bus. Thus, an electrostatic shield should be considered mandatory for a turbine step-up transformer.
Switching Surges and Transient Over-Voltages in Wind Farms
The long cable runs and frequent switching operations found in multi-tower wind farms puts the wind turbine step-up transformer at greater risk than a conventional distribution or power transformer installations. Carefully locating the wind farm, along with using transformers with fault ride through capability, grounding transformers, surge arrestors, and properly rated transformer bushings, are among the strategies that can be used to counter the risks from switching surges and over-voltages at wind farms.
Locating Transformers at Wind Farms
The general rule of thumb for locating a transformer is to reduce the costs of large copper cables by placing the transformer in a manner that reduces the length of low voltage, high current cables. When this consideration is applied to wind farms, it follows that the wind generator and its associated transformer should be as close together as possible.
For land based sites, the turbine step-up transformer can either be located as near to the tower base as possible, or alternately, within the tower or nacelle. For off shore sites, the latter is the only realistic choice available. It should be noted that while liquid filled pad-mounted transformers are the normal for location adjacent to the turbine tower, dry-type transformers are normally used for nacelle mounted transformers. Of course, the placement of the transformer is decided by the construction of the wind generator manufacturer.
The problem with large sprawling turbine arrays is that the need for connecting the individual turbine step-up transformers to the "collector" bus results in very long cable runs. This in turn results in increased voltage drops, cable related resistive and capacitive losses, and the increased potential for cable ground faults. The extensive use of cable in wind farms coupled with their "daisy chain" connection pattern leads to two primary systems problems: cable faults and voltage stress caused by single or double line-to-ground fault.
Cable Faults
A radial wind farm configuration typically connects ten to twelve transformers in a daisy chain fashion. The pad-mount transformers are configured with a loop-feed bushing arrangement, in which the transformer at the end of the radial line is connected to the next transformer in line. The second transformer from the end is connected to the third and so forth until the first transformer in line is connected to the collector bus. Since a cable fault can happen anywhere along this radial line, the transformer must be able to handle fault currents from a fault at any location The fault ride through requirement becomes critical, since clearing a fault would require disconnecting a complete radial line, approximately 20 to 30 MVA of generation.
Single or Double Line-To-Ground Fault Voltage Stress
In addition to the fault currents that occur during a fault, a single or double line-to-ground fault causes voltage stress on the transformer. A single phase cable fault, on the Delta-connected HV winding, causes one phase to ground, putting phase-to-phase voltage between the other two phases and ground. The resulting voltages overstress the transformer's insulation system. Finding and clearing the cable fault is exacerbated by the longer cable runs and the wind farm layout.
The use of grounding transformers at critical locations within the wind farm helps alleviate this type of dielectric stress. The grounding transformer provides a zero sequence impedance to support the voltage on a faulted leg during a single line-to-ground fault. By holding the faulted phase above ground, this impedance acts to limit the resulting overvoltage on the un-faulted phases. Most grounding transformers have a thermal fault rating in the range of 10 seconds to one minute. This may give an indication of the expected length of a fault. Fault duty due to either voltage or current are serious concerns that need to be addressed by both the wind farm developer and the transformer manufacturer.
Increased Lightning Exposure and Voltage Surges from Repeated Switching
Further concerns for the reliability of the turbine step-up transformer center on the increased exposure to lightning strikes and transient voltage surges resulting from the repeated switching.
Wind farms are located in remote areas, typically at higher elevations or exposed plains, where wind patterns are unobstructed by surrounding terrain and man-made or natural obstacles. Unfortunately, this increases the transformer exposure to storm events and lightning strikes, especially considering the geometry of the turbine and tower combination. Surge arresters should figure prominently in the wind turbine step-up transformer protective equipment list.
Perhaps an even greater concern is transient over-voltages cause by switching. Changes in wind strength are directly translated into a varying load profile for the step-up transformer. As the wind speed increases, the turbines are brought on line. When the wind strength begins to wane, the loading drops and ultimately the turbine is switched off line.
This can happen multiple times within a 24-hour period as surface wind is affected by diurnal heating cycles or incoming weather patterns. It can also happen when a feeder breaker opens and disconnects the turbine step-up radial line from the collector circuit. Breaker operations introduce a transient recovery voltage (TRV) wave into the wind turbine step-up transformer voltage circuit. This phenomenon is exacerbated by the present day extensive use of vacuum breakers and their extremely rapid switching times.
The TRV surges associated with breaker switching operations on either the HV or LV side of the transformer can combine with cable capacitance and produce standing waves and ringing that are many times the original voltage levels. These extreme voltages can lead to transformer dielectric failures. When the frequency content of the high voltage, fast rise-time, TRV surges coincide with the internal resonant frequencies of the winding, the circuit can resonate and elevate the electric stress in the windings beyond the dielectric withstand strength of the windings. A recent IEEE Standard addressed the interaction between breaker switching and transformer response.
A Final Thought - The Importance of Padmount Transformer Bushings
One of the most common specification errors occurs on padmount transformers. On 34.5 kV rated windings, the highest rated dead front bushing is rated at 150 kV BIL. Thus most specifications also specify 150 kV BIL as the insulation level of the associated winding. Instead, a good practice would be to specify a full 200 kV BIL for the winding, while leaving the bushing at 150 kV BIL. It is much easier to replace a failed bushing than a failed transformer.
Transformer Applications in a Modern Wind Farm
Today's modern utility wind farm consists of a collection of wind turbines distributed in an array that provides the greatest exposure to the local wind flows. Often the array is composed of a series of radial lines of turbines connected in parallel to feed a common "collector" bus. The number of transformers on each radial and the number of radials varies with the site terrain, available area, and individual switching and protection schemes. See Figure 1 for a typical wind farm array layout.
Depending upon the design, wind farms may use transformers for six unique functions.
Wind Turbine Step-Up Transformers
Each turbine is equipped with a transformer to step-up the turbine generator output voltage to the collector system voltage. This transformer also serves as a source for the turbine's auxiliary power requirements when the turbine is off line or generating insufficient power. Proximity to the turbine is critical to limit the length of costly and inefficient high current, low voltage, cables. The typical distribution transformer is often ill-suited to the requirements of a wind farm. We have found that the mismatch between requirements and typical transformer design contributes to the present high rate of step-up transformer failures at wind farms.
Grounding Transformers
Under normal conditions, the common collector bus will be connected to the substation main transformer. Depending on the system configuration, grounding transformers may be required to provide a system ground as protection circuitry operates. Also, grounding transformers themselves provide a protective function by preventing a faulted phase from staying at ground potential and limiting overvoltage conditions on the un-faulted phases. Grounding transformer sizing is based on the ampere capacity of the radial collector circuit and the time required for de-energization of the turbine and collector circuits when a fault occurs.
Substation Main Transformers (Sometimes Known as Collector Transformer)
Voltage from the collector system is stepped-up to the sub-transmission or transmission level voltages by the substation main transformer. This intermediate step helps limit the transformation ratio required by the individual wind turbine step up transformers. Some installations might use more than one substation main transformer to limit its size or take advantage of site logistics, depending upon station design philosophy.
Transmission Auto-Transformers
These transformers provide the flexibility to interconnect to multiple transmission lines with dissimilar voltages.
Station Service and Auxiliary Power Transformers
Because wind farms are usually located far from developed urban or residential lands, they must be self-sufficient and not require power from external sources. This is particularly important for off-shore installations. Thus, power for such local functions as lighting, heating, switching, tripping, relaying, metering, and communications is provided by the station service transformer connected to the main transmission lines. It is possible to provide some of these functions from a grounding transformer, but this approach is not popular with designers and would not provide continuing power in the case of a fault.
Voltage Conditioning Equipment
Dynamic-VAR compensation equipment can be used to limit damage to the turbine equipment due to under-voltage conditions and to provide system stability. This equipment requires integral transformers of sizes as high as 8 MVA. The transformers are integral to this equipment, so they are not normally ordered separately or specified by the system designers, as the case is for the other type of transformers in the wind farm environment.
Introduction to Busway
Commercial and industrial distribution systems use several methods to transport electrical energy. These methods may include, heavy conductors run in trays or conduit. Busway, a grounded metal enclosure containing factory-mounted bare or insulated conductors can be an effective method of distributing power. A bus bar is a conductor that serves as a common connection for two or more circuits. Standard bus bars in busway are commonly made of aluminum or copper.
A busway is used in numerous applications and can be found in industrial installations as well as high-rise buildings. Busway used in industrial locations can supply power to heavy equipment, lighting, and air conditioning. Busway risers (vertical busway) can be installed economically in a high-rise building where it can be used to distribute lighting and air conditioning loads. Busway provides flexible power distribution solutions for a variety of applications where change and adaptation are important.
NEMA Definition
National Electrical Manufacturers Association (NEMA) defines busway as a prefabricated electrical distribution system consisting of bus bars in a protective enclosure, including straight lengths, fittings, devices, and accessories. Busway includes bus bars, an insulating and/or support material, and housing.
A major advantage of a busway is the ease with which its sections are connected together. Electrical power can be supplied to any area of a building by connecting standard lengths of busway. It is typically faster to install or change than cable and conduit assemblies. The total distribution system frequently consists of a combination of busway, cable and conduit. In many cases busway can be used in lieu of the cable/conduit feeders at a lower cost.
Benefits of a Busway
If you need to add load to or extend power from an existing distribution system, a Busway systems may be the answer. First introduced in 1932, busway solved the automotive industry's need for a flexible power distribution system to serve its linear layouts. Since that time, this product has grown to serve many other types of loads.
Where and How to Apply Busway
You can install busway in most applications where you'd normally use cable and conduit. Busway manufacturers produce systems ranging from 100A to 6500A. Low-amperage applications include high-technology firms, such as computer manufacturers and test laboratories. The automotive industry and other heavy assembly industries require high-amperage busway systems.
Recognizing the need for flexibility, manufacturers developed elbows and offsets to make directional changes easy. With these fittings, busway offers extensive layout versatility. When new loads develop, it's easy to meet changing conditions by adding tap-off units and/or new sections. However, busway is not the best solution for every application. For example, if the situation requires low current to a specific source, you're better suited for cable and conduit, which is best for underground applications. Also, Sec. 364-4(b) of the National Electrical Code (NEC) says you cannot install busway where it's subject to severe physical damage or corrosive vapors.
Benefits to End Users
Busway provides an effective means of distributing power in a building. Since it requires easy maintenance procedures and is flexible, it's relatively simple for accommodating changing load situations. Maintenance primarily consists of annual inspection of joint fittings using an infrared heat gun, and then using a torque wrench to tighten appropriate connecting bolts. If required, it is possible to cut power to only a portion of a busway run so one can perform any required maintenance without risk of injury or equipment damage.
Industrial Control Transformers
Industrial Control Transformers are intended for industrial applications where higher single-phase voltages need to be converted down to usable AC line voltages. Even before selecting an Industrial Control Transformer, various specifications must be kept in mind like load, minimum voltage required for operation, inrush load power factor etc.
Industrial Control Transformers are designed with very low temperature rise, exceptional voltage regulation and great overload capacity of high momentary in-rush current demand by the contactors, relays and solenoids. The voltage drop under high transitory in-rush current is also very low, which guarantees satisfactory operation of contactors, relays and solenoids.
Features of an Industrial Control Transformer
An Industrial Control Transformer is usually,
Encapsulated in epoxy which seals the transformer coils against moisture, dust, dirt and industrial contaminants for maximum protection in hostile and industrial environments
Fuse clips for most models. Factory mounted for integral fusing on the secondary side to save panel space, save wiring time and save the cost of buying an add-on fuse block or kit
Integrally molded barriers between terminals and transformer protects against electrical creepage. Up to 30% greater terminal contact area permits low-loss connections. Extra-deep barriers reduce the chance of shorts from frayed leads or careless wiring
Terminals molded into the transformer are difficult to break during wiring. A full quarter-inch of thread on the 10-32 terminal screws prevents stripping and pull out
Two jumper links are standard with all transformers which can be jumped
Operation of an Industrial Control Transformer
Industrial control circuits and motor control loads typically require more current when they are initially energized than under normal operating conditions. This period of high current demand, referred to as an inrush, may be as great as ten times the current required under steady state or normal operating conditions, and can last up to 40 milliseconds. A transformer in a circuit subject to inrush will typically attempt to provide the load with the required current during the inrush period. However, it will be at the expense of the secondary voltage stability by allowing the voltage of the load to decrease as the current increases. This period of secondary voltage instability, resulting from increased current, can be of such magnitude that the transformer is unable to supply sufficient voltage to energize the load. The transformer must therefore be designed and constructed to accommodate the high inrush current, while maintaining secondary voltage stability. According to NEMA standards, the secondary voltage would typically be 85% of the rated voltage.
Selecting an Industrial Control Transformer
Should your organization be looking to invest in a industrial control circuit transformer, the following is what you need to understand:
Inrush VA is the product load voltage (V) multiplied by the current (A) that is required during circuit start-up. It is calculated by adding the inrush VA requirements of all devices, which will be energized together.
Seated VA is the product of load voltage (V) multiplied by the current (A) that is required to operate the circuit after initial start-up or under normal operating conditions. It is calculated by adding the sealed VA requirements of al electrical components of the circuit that will be energized at any given time.
Primary Voltage is the voltage available from the electrical distribution system and its operational frequency, which is connected to the transformer supply voltage terminal.
Secondary Voltage is the voltage required for load operation which is connected to the transformer load voltage terminals.
Primary Fuse Kit this kit includes a 2-pole class CC fuse block, instructions and all associated mounting and wiring hardware.
Industrial control transformers from Pacific Crest Transformers
Pacific Crest Transformers builds industrial transformers specifically designed to meet client needs to provide feeder voltages to the single phase control transformers described in this article. We also offer Custom Design Transformers to meet the most demanding industrial situations without compromising on quality and performance levels. Custom designed transformers are manufactured keeping in mind specifications given by the clients and industry standards. Our transformers are built to handle the roughest industrial situations.
Acceptance Testing and Maintenance of Power Transformer
An energy transformer is a rather critical piece of equipment and this makes its installation a monstrous task that needs to be well backed by a good testing and inspection program.
Power transformer testing and inspection should ideally start with the installation of the transformer and continue throughout its life. The initial acceptance inspection, testing and start-up procedures are extremely critical. The inspections, both internal and external, will reveal any missing parts or items that were damaged in transit. It will also help you verify that the transformer is constructed exactly as specified. The acceptance tests will also reveal manufacturing defects if any and establish baseline data for future testing.
The start-up procedures should ensure that the transformer is properly connected, and that no latent deficiencies exist before the transformer is energized. Ensuring that the transformer starts off as it should is the best way to guarantee dependable operation throughout its service life.
Manufacturers recommend a wide range of acceptance and start-up procedures and it is best to follow them strictly.
Power transformer maintenance
A power transformer is a fa ...
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