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Connecting wind turbines to the power grid
Precautions to be taken when connecting wind turbines to the power grid: The procedure for connecting wind turbines to an electric distribution network normally consists of 2 steps:
1. First, the HV/LV transformer is energized from the high voltage side,
2. Then, in the right wind conditions and further to wind turbine adjustment tests (initial pole test, pole test sequence, etc.), the turbine is connected to the power grid as follows:
- The rotation of the wind turbine’s blades triggers the aerogenerator (motorgenerator set), which acts as a generator,
- The transformer’s LV winding is energized by the wind turbine’s stator (connected by a star or delta connection) and hence provides electrical energy to the HV network.
However, during this 2-step process, the HV/LV transformer must not, in any event whatsoever, be supplied with high and low voltage currents at the same time. In such an event, there would be a risk of energizing the LV voltage side in opposite phase to the HV side.
The result would be an extremely strong current, the intensity of which would be greater than the brief, 3-phase short-circuit current stipulated in the contract (usually 2 seconds).
General diagram of a wind turbine installation
As the electrodynamic stress on the windings is proportional to the square of the current intensity (F = K.I2), the transformer can not, in general, withstand the extremely intense stress caused by a current greater than the contractual short-circuit current. This type of stress would automatically lead to significant, unacceptable and irreversible mechanical deformation of the LV and HV windings, and the LV connections: hence it would, in due course, totally destroy the transformer.
On-site transformer failures have occurred, as a result of energizing the LV and HV sides at the same time and failing to comply with the phase sequence of the LV network.
The LV winding was subjected to a current much stronger than the contractual 3-phase short-circuit current and, as a result, the transformer was completely destroyed by huge electrodynamic stress.
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Measures to apply in all circumstances…
Power HV/LV Transformer
Therefore, when connecting a wind turbine transformer to a power grid, it is absolutely essential not to energize the LV and HV sides of the transformer at the same time, which may cause the LV winding to be in opposite phase.
Hence, it is extremely important not to interfere with the various tripping sequences, and to comply with the adjustment specifications for the transformer in question.
If the transformer is energized from both sides and, in addition, the phase sequence of the LV network is not respected, the result will be total transformer failure.
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SOURCE: France Transfo
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Transformer Ratings
Transformer size or capacity is most often expressed in kVA. “We require 30 kVA of power for this system” is one example, or “The facility has a 480 VAC feed rated for 112.5 kVA”.
However, reliance upon only kVA rating can result insafety and performance problems when sizing transformers to feed modern electronic equipment.
Use of off-the-shelf, general purpose transformers for electronics loads can lead to power quality and siting problems:
- Single phase electronic loads can cause excessive transformer heating.
- Electronic loads draw non-linear currents, resulting in low voltage and output voltage distortion.
- Oversizing for impedance and thermal performance can result in a transformer with a significantly larger footprint.
It is vital for the systems designer to understand all of the factors that affect transformer effectiveness and performance.
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Thermal Performance
Historically, transformers have been developed to supply 60 Hz, linear loads such as lights, motors, and heaters. Electronic loads were a small part of the total connected load. A system designer could be assured that if transformer voltage and current ratings were not exceeded, the transformer would not overheat, and would perform as expected. A standard transformer is designed and specified with three main parameters: kVA Rating, Impedance, and Temperature Rise.
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KVA Rating
The transformer voltage and current specification. KVA is simply the load voltage times the load current. A single phase transformer rated for 120 VAC and 20 Amperes would be rated for 120 x 20 = 2400 VA, or 2.4 KVA (thousand VA).
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Impedance
Transformer Impedance and Voltage Regulation are closely related: a measure of the transformer voltage drop when supplying full load current. A transformer with a nominal output voltage of 120 VAC and a Voltage Regulation of 5% has an output voltage of 120 VAC at no-load and (120 VAC – 5%) at full load – the transformer output voltage will be 114 VAC at full load. Impedance is related to the transformer thermal performance because any voltage drop in the transformer is converted to heat in the windings.
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Temperature Rise
Steel selection, winding capacity, impedance, leakage current, overall steel and winding design contribute to total transformer heat loss. The transformer heat loss causes the transformer temperature to rise. Manufacturers design the transformer cooling, and select materials, to accommodate this temperature rise.
Transformer Heat Loss
Use of less expensive material with a lower temperature rating will require the manufacturer to design the transformer for higher airflow and cooling, often resulting in a larger transformer. Use of higher quality materials with a higher temperature rating permits a more compact transformer design.
Transformer Insulation Systems
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“K” Factor Transformer Rating
In the 1980′s, power quality engineers began encountering a new phenomenon: non-linear loads, such as computers and peripherals, began to exceed linear loads on some distribution panels. This resulted in large harmonic currents being drawn, causing excessive transformer heating due to eddy-current losses, skin effect, and core flux density increases.
Standard transformers, not designed for nonlinear harmonic currents were overheating and failing even though RMS currents were well within transformer ratings.
In response to this problem, IEEE C57.110-1986 developed a method of quantifying harmonic currents. A “k” factor was the result, calculated from the individual harmonic components and the effective heating such a harmonic would cause in a transformer. Transformer manufacturers began designing transformers that could supply harmonic currents, rated with a “k” factor. Typical “K” factor applications include:
- K-4: Electric discharge lighting, UPS with input filtering, Programmable logic controllers and solid state controls
- K-13: Telecommunications equipment, UPS systems, multi-wire receptacle circuits in schools, health-care, and production areas
- K-20: Main-frame computer loads, solid state motor drives, critical care areas of hospitals
“K” factor is a good way to assure that transformers will not overheat and fail. However, “K” factor is primarily concerned with thermal issues. Selection of a “K” factor transformer may result in power quality improvement, but this depends upon manufacturer and design.
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Transformer Impedance
Transformer impedance is the best measure of the transformer’s ability to supply an electronic load with optimum power quality. Many power problems do not come from the utility but are internally generated from the current requirements of other loads.
While a “K” factor transformer can feed these loads and not overheat, a low impedance transformer will provide the best quality power. As an example, consider a 5% impedance transformer. When an electronic load with a 200% inrush current is turned on, a voltage sag of 10% will result. A low impedance transformer (1%) would provide only a 2% voltage sag – a substantial improvement. Transformer impedance may be specified as a percentage, or alternately, in Ohms (Ω) from Phase- Phase or Phase-Neutral.
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High Frequency Transformer Impedance
Most transformer impedance discussions involve the 60 Hz transformer impedance. This is the power frequency, and is the main concern for voltage drops, fault calculations, and power delivery. However, nonlinear loads draw current at higher harmonics. Voltage drops occur at both 60 Hz and higher frequencies. It is common to model transformer impedance as a resistor, often expressed in ohms. In fact, a transformer behaves more like a series resistor and inductor.
The voltage drop of the resistive portion is independent of frequency, the voltage drop of the inductor is frequency dependent.
Standard Transformer impedances rise rapidly with frequency. However, devices designed specifically for use with nonlinear loads use special winding and steel lamination designs to minimize impedance at both 60 Hz and higher frequencies. As a result, the output voltage of such designs is far better quality than for standard transformers.
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Recommendations for Transformer Sizing
System design engineers who must specify and apply transformers have several options when selecting transformers.
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Do It Yourself Approach
With this approach, a larger than required standard transformer is specified in order to supply harmonic currents and minimize voltage drop. Transformer oversizing was considered prudent design in the days before transformer manufacturers understood harmonic loads, and remains an attractive option from a pure cost standpoint. However, such a practice today has several problems:
- A larger footprint and volume than low impedance devices specifically designed for non-linear loads
- Poor high frequency impedance
- Future loads may lead to thermal and power quality problems
Standard Isolation Transformer
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“K”-factor Rated Transformers
Selecting and using “K”-factor rated transformers is a prudent way to ensure that transformer overheating will not occur. Unfortunately, lack of standardization makes the “K” factor rating a measure only of thermal performance, not impedance or power quality.
Percent Impedance
Some manufacturers achieve a good “K” factor using design techniques that lower impedance and enhance power quality, others simply derate components and temperature ratings. Only experience with a particular transformer manufacturer can determine if a “K” factor transformer addresses both thermal and power quality concerns.
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Transformers Designed for Non-Linear Loads
Transformers designed specifically for non-linear loads incorporate substantial design improvements that address both thermal and power quality concerns. Such devices are low impedance, compact, and have better high frequency performance than standard or “K” factor designs. As a result, this type of transformer is the optimum design solution.
This type of transformer may be more expensive than standard transformers, due to higher amounts of iron and copper, higher quality materials, and more expensive winding and stacking techniques. However, the benefits of such a design in power quality and smaller size justify the extra cost, and make the low impedance transformer the most cost effective design overall.
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Maintenance Of High Voltage Circuit Breakers
Most manufacturers recommend complete inspections, external and internal, at intervals of from 6 to 12 months.
Experience has shown that a considerable expense is involved, some of which may be unnecessary, in adhering to the manufacturer’s recommendations of in ternal inspections at 6 to 12 month intervals. With proper external checks, part of the expense, delay, and labor of internal inspections may be avoided without sacrifice of dependability.
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Inspection schedule for new breakers
A temporary schedule of frequent inspections is necessary after the erection of new equipment, the modification or modernization of old equipment, or the replication of old equipment under different condi tions.
The temporary schedule is required to Correct internal defects which ordinarily appear in the first year of service and to correlate external check procedures with internal conditions as a basis for more conservative maintenance program thereafter. Assuming that a circuit breaker shows no serious defects at the early complete inspections and no heavy interrupting duty is imposed, the following inspection schedule is recommended:
| .6 months after erection | .Complete inspection and adjustment |
| .12 months after .previous inspection | .Complete inspection and adjustment |
| .12 months after .previous inspection | .Complete inspection and adjustment |
| .12 months after .previous inspection | .External checks and inspection; if checks are .satisfactory, no internal inspection |
| .12 months after .previous inspection | .Complete inspection and adjustment |
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Inspection schedule for existing breakers
The inspection schedule should be based by the interrupting duty imposed on the breaker. It is advisable to make a complete internal inspection after the first severe fault interruption. If internal conditions are satisfactory, progressively more fault interruptions may be allowed before an internal inspection is made. Average experience indicates that up to five fault interruptions are allowable between inspections on 230 kV and above circuit breakers, and up to 10 fault interruptions are allowable on circuit breakers rated under 230 kV.
Normally, no more than 2 years should elapse between external in spections or 4 years between internal inspections.
External Inspection Guide
The following items should be included in an external inspection of a high-voltage breaker.
- Visually inspect PCB externals and operating mechanism. The tripping latches should be examined with spe cial care since small errors in adjustments and clearances and roughness of the latching surfaces may cause the breaker to fail to latch properly or increase the force neces sary to trip the breaker to such an extent that electrical tripping will not always be successful, especially if the tripping voltage is low. Excessive “opening” spring pressure can cause excessive friction at the tripping latch and should be avoided. Also, some extra pressure against the tripping latch may be caused by the electro magnetic forces due to flow of heavy short-circuit currents through the breaker.
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Lubrication of the bearing surfaces of the operating mechanism should be made as recommended in the manufacturer’s instruction book, but excessive lubrication should be avoided as oily surfaces collect dust and grit and get stiff in cold weather, resulting in excessive friction.
. - Check oil dielectric strength and color for oil breakers. The dielectric strength must be maintained to pre vent internal breakdown under voltage surges and to enable the interrupter to function properly since its action depends upon changing the internal arc path from a fair conductor to a good insulator in the short interval while the current is passing through zero. Manufacturer’s instructions state the lowest allowable dielectric strength for the various circuit break ers. It is advisable to maintain the dielectric strength above 20 kV even though some manufacturer’s instructions allow 16 kV.
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If the oil is carbonized, filtering may remove the suspended particles, but the interrupters, bushings, etc., must be wiped clean. If the dielectric strength is lowered by moisture, an inspection of the fiber and wood parts is advisable and the source of the moisture should be corrected. For these reasons, it is rarely worthwhile to filter the oil in a circuit breaker while it is in service.
. - Observe breaker operation under load.
. - Operate breaker manually and electrically and observe for malfunc tion. The presence of excessive friction in the tripping mechanism and the margin of safety in the tripping function should be determined by making a test of the minimum voltage required to trip the breaker. This can be accomplished by connecting a switch and rheostat in series in the trip-coil circuit at the breaker (across the terminals to the remote control switch) and a voltmeter across the trip coil. Staring with not over 50 percent of rated trip-coil voltage, gradually in crease the voltage until the trip-coil plunger picks up and successfully trips the breaker and record the mini mum tripping voltage. Most breakers should trip at about 56 percent of rated trip-coil voltage.
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The trip-coil re sistance should be measured and compared with the factor test value to disclose shorted turns.
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Most modern breakers have trip coils which will overheat or burn out if left energized for more than a short pe riod. An auxiliary switch is used in series with the coil to open the circuit as soon as the breaker has closed. The auxiliary switch must be properly adjusted and successfully break the arc without damage to the contacts.
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Tests should also be made to deter mine the minimum voltage which will close the breaker and the closing coil resistance.
. - Trip breaker from protective relays.
. - Check operating mechanism adjustments. Measurements of the mechanical clearances of the operating mechanism associated with the tank or pole should be made. Appre ciable variation between the value found and the setting when erected or after the last maintenance overhaul is erected or after the last maintenance overhaul is usually an indication of mechanical trouble. Temperature and difference of temperature between different parts of the mechanism effect the clearances some. The manufacturers’ recommended tolerances usually allow for these effects.
. - Doble test bushings and breaker.
. - Measure contact resistance. As long as no foreign material is present, the contact resistance of high-pres- sure, butt-type contacts is practically independent of surface condition. Nevertheless, measurement of the electrical resistance between external bushing terminals of each pole may be regarded as the final “proof of the pudding.” Any abnormal increase in the resistance of this circuit may be an indication of foreign material in contacts, contact loose in support, loose jumper, or loose bushing connection. Any one of these may cause localized heating and deterioration.
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The amount of heat above normal may be readily calculated from the increase in resistance and the current.Resistance of the main contact cir cuits can be most conveniently measured with a portable double bridge (Kelvin) or a “Ducter.” The breaker contacts should not be opened during this test because of possible damage to the test equipment.
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Table 1 gives maximum contact resistances for typical classes of breakers.
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. - Make time-travel or motion-analyzer records. Circuit breaker motion an alyzers are portable devices designed to monitor the operation of power circuit breakers which permit mechanical coupling of the motion an alyzer to the circuit breaker operating rod. These include high-voltage and extra- high-voltage dead tank and SF6 breakers and low-voltage air and vac uum circuit breakers.
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Motion analyzers can provide graphic records of close or open initiation signals, contact closing or opening time with respect to initiation signals, contact movement and velocity, and contact bounce or rebound. The records obtained not only indicated when mechanical difficulties are present but also help isolate the cause of the difficulties. It is preferable to obtain a motion-analyzer record on a breaker when it is first installed. This will provide a master record which can be filed and used for comparison with future maintenance checks.
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Tripping and closing voltages should be re corded on the master record so subsequent tests can be performed under comparable conditions. Time-travel records are taken on the pole nearest the operating mecha nism to avoid the inconsistencies due to linkage vibration and slack in the remote phases..
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Internal Inpection Guide – Lines
An internal inspection should include all items listed for an external inspection, plus the breaker tanks or contact heads should be opened and the contacts nd interrupting parts should be inspected. These guidelines are not intended to be a complete list of breaker maintenance but are intended to provide an idea of the scope of each inspection.
A specific checklist should be developed in the field for each type of inspection for each circuit breaker maintained.
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Typical Internal Breaker Problems
The following difficulties should be looked for during internal breaker inspections:
- Tendency for keys, bolts (espe- cially fiber), cotter pins, etc, to come loose.
- Tendency for wood operating rods, supports, or guides to come loose from clamps or mountings.
- Tendency for carbon or sludge to form and accumulate in interrupter or on bushings.
- Tendency for interrupter to flash over and rupture static shield or resis tor.
- Tendency for interrupter parts or barriers to burn or erode.
- Tendency for bushing gaskets to leak moisture into breaker insulating material.
Fortunately, these difficulties are most likely to appear early in the use of a breaker and would be disclosed by the early internal inspections. As unsatis factory internal conditions are corrected and after one or two inspections show the internal conditions to be satisfactory, the frequency of internal inspections may safely be decreased.
Influence Of Duty Imposed
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Influence of light duty
Internal inspection of a circuit breaker which has had no interruption duty or switching since the previous inspection will not be particularly beneficial although it will not be a total loss. If the breaker has been energized, but open, erosion in the form or irregular grooves (called tracking) on the inner surface of the interrupter or shields may appear due to electrostatic charging current. This is usually aggravated by a deposit of carbon sludge which has previously been generated by some interrupting operation.
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If the breaker has remained closed and carrying current, evidence of heating of the contacts may be found if the contact surfaces were not clean, have oxidized, or if the contact pressure was improper. Any shrinkage and loosening of wood or fiber parts (due to loss of absorbed moisture into the dry oil) will take place following erection, whether the breaker is operated or not. Mechanical operation, however, will make any loosening more evident. It is worthwhile to deliberately impose several switching operations on the breaker before inspection if possible. If this is impossible, some additional information may be gained by operating the breaker several times after it is deenergized, measuring the contact resistance of each pole initially and after each operation.
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Influence of normal duty
The relative severity of duty imposed by load switching, line dropping, and fault interruptions depends upon the type of circuit breaker involved. In circuit breakers which employ an oil blast generated by the power arc, the interruption of light faults or the interruption of line charging current may cause more deterioration than the interruption of heavy faults within the rating of the breaker because of low oil pressure. In some designs using this basic principle of interruption, distress at light interrupting duty is minimized by multiple breaks, rapid contact travel, and turbulence of the oil caused by movement of the contact and mech anism.
In designs employing a mechanically driven piston to supple ment the arc-driven oil blast, the performance is more uniform. Still more uniform performance is usually yielded by designs which depend for arc interruption upon an oil blast driven by mechanical means. In the latter types, erosion of the contacts may appear only with heavy interruptions. The mechanical stresses which accompany heavy interruptions are always more severe.
These variations of characteristic performance among various designs must be considered when judging the need for maintenance from the service records and when judging the performance of a breaker from evidence on inspection. Because of these variations, the practice of evaluating each fault interruption as equivalent to 100 no-load operations, employed by some companies, is necessarily very approximate although it may be a useful guide in the absence of any other information.
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Influence of severe duty
Erosion of the contacts and damage from severe mechanical stresses may occur during large fault interruption. The most reliable indication of the stress to which a circuit breaker is subjected during fault interruptions is afforded by automatic oscillograph records. Deterioration of the circuit breaker may be assumed to be proportional to the energy dissipated in the breaker during the interruption.
The energy dissipated is approximately proportional to the current and the duration of arcing; that is, the time from parting of the contacts to interruption of the current. However, the parting of contacts is not always evident on the oscillograms, and it is sometimes necessary to determine this from indicated relay time and the known time for breaker contacts to part. Where automatic oscillograph records are available, they may be as useful in guiding oil circuit breaker maintenance as in showing relay and system performance.
Where automatic oscillographs are not available, a very approximate, but nevertheless useful, indication of fault duty imposed on the circuit breakers may be obtained from relay operation targets and accompanying system conditions. All such data should be tabulated in the circuit breaker maintenance file.
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SOURCE: HYDROELECTRIC RESEARCH AND TECHNICAL SERVICES GROUP
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