Furnace Transformer, The Ultimate Guide

In the tender documents of the furnace transformer, the main parameters and components are often imperfectly selected, some of which may lead to product defects and hidden operational problems. Only if the main parameters and components of the oil furnace transformer are selected in a reasonable manner, under the premise of meeting the requirements of the electric furnace smelting process, can the reliable operation of Arc Furnace Power Transformers be ensured.

In this article you will find a detailed description of the main parameters of the furnace transformer, its components and how to service and maintain the furnace transformer.

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Type:Cast resin; Rated Capacity: Up to 25MVA; Rated Voltage: Up to 36KV;

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Frequency: 50/60Hz Rated voltage:10kv, 20kv,30kv Rated Power: 400~2500kva

Why choose daelim as a supplier of electric furnace transformers?

Daelim has extensive experience in the design and production of electric furnace transformers. Several daelim electric furnace transformers have been used in the USA, Venezuela, Nicaragua and Spain, which means that Daelim electric furnace transformers are perfectly suited to the requirements of these regions. Daelim can also design and produce an Arc Furnace Power Transformer specifically for your needs. daelim has its own production facility, which allows us to produce and design a transformer to suit your needs much faster than other companies, usually within 3 months. This can help you save significantly on time costs.

Table of Contents

What are the main parameters of the furnace transformer?

Rated capacity

It is not recommended to mention the model “rated capacity + long-term overload percentage”, it is recommended that only the “rated capacity” of a furnace transformer should be mentioned, but a “load cycle provision” should be proposed. This load cycle curve consists of a number of rectangles, which encompass the maximum load of each smelting period of the furnace, so that the equivalent load current can be calculated when the load changes periodically.

As the winding time constant is only 5~10min, reach stable temperature rise as long as 20~30min, so should be the maximum load current to calculate the winding copper oil temperature difference; and for OF and OD cooling mode oil time constant is about 1.5h, reach stable temperature rise to h, when the equivalent load current can accurately reflect the cycle of the transformer The loss of the transformer, it is equivalent to the impact of the cyclically changing load on the average temperature rise of the oil.

Therefore, the load of the periodically changing furnace transformer can use the equivalent load current to select the cooler capacity.

Such as a 90MVA steelmaking furnace Arc Furnace Power Transformers, it is a smelting cycle of the work system in order: 0 ~ 10min for the rated current In, 11 ~ 35min for 1.2 In, 36 ~ 40min for 1.1In, 41 ~ 45min for IN, 46 ~ 50min for 0In (two furnace interval).

If the equivalent load current according to the choice of 2x400kW cooler can be, and according to 1.2IN to choose 2x500kW cooler to meet the requirements, which is obviously wasteful and unnecessary.

Secondary voltage

Due to the primary voltage value is not selected properly and oil furnace transformer accidents occur from time to time, for different kinds of Arc Furnace Power Transformers primary voltage connotation is not the same.

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In the power Arc Furnace Power Transformers, the output voltage is always set constant, and the primary often has a large voltage adjustment range, such as ±8×1.25%, so that when the primary supply voltage fluctuates within ±10%, the core flux and the voltage per turn are kept constant through the adjustment of the number of turns in the primary winding.

The furnace transformer is the opposite of the Arc Furnace Power Transformers, which always set a constant primary voltage and change the output voltage according to the needs of the smelting process. In the case of furnace transformers other than direct voltage regulation (e.g. those using series or front autotransformers), the primary winding is not tapped and the Arc Furnace Power Transformers may suffer from unacceptable over-excitation if the primary voltage is not taken correctly.

One of the many hazards of over-excitation is an increase in the rate of change of the no-load current A/d and an increase in the cut-off overvoltage.

Secondly, when the three-phase load changes dramatically and asymmetrically, the resulting excessive zero sequence voltage will inevitably cause the discharge of metal structural components (it may be small gaps or poor contact areas), resulting in the characteristic gases from oil decomposition seeping into the oil.

This may also be confusing to the manufacturer’s why the same design products have acetylene in the oil and some do not.

Secondly, steelmaking arc furnaces often experience a sudden loss of electrode load, which is similar to a generator Arc Furnace Power Transformers load shedding situation, which will result in overvoltage at the Arc Furnace Power Transformers terminals. The more severe the overexcitation, the higher the overvoltage of the dumped load will be, and its maximum value will be much more severe than that of the power Arc Furnace Power Transformers.

The above analysis shows that most furnace transformers do not have the ability to regulate over-excitation like the electric Arc Furnace Power Transformers, and can only rely on a slightly lower magnetic density in the core design, which will inevitably lead to increased load losses and costs.

As severe over-excitation will have a number of fatal consequences for the oil furnace transformer, the primary voltage must be determined in line with the maximum voltage that the oil furnace transformer can actually withstand, with the maximum no-load overvoltage not exceeding 5% of the rated voltage, and any further value should be within the range of voltage regulation allowed by the front step-down transformer and other loads. The maximum no-load overvoltage should not exceed 5% of the rated voltage, and any further values should be within the range of voltage regulation allowed after the front step-down transformer has taken into account other loads. If the user insists that the furnace transformer has 10°.

Over-excitation ability, oil furnace transformer core magnetic density should be lower than the power change of about 5%, but the load loss and cost will certainly increase, otherwise oil furnace transformer safety operation is difficult to ensure.

Learn more about What You Need To Know About Power Transformer On Pole

Secondary voltage regulation range

The smelting process requirements of the secondary voltage range, the appropriate extension of the upper and lower is necessary, but the extension of too large will be counterproductive: if the common voltage work in the position of the highest voltage far from the secondary, oil furnace transformer actual operation of the short-circuit impedance will increase, resulting in oil furnace transformer common level output power reduced.

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Users often feel the oil furnace transformer “powerful” or “no power”, in addition to the short network reactive power and three-phase impedance imbalance is too large, but also with the common level of work position. Therefore, blindly increase the secondary voltage regulation range, the Arc Furnace Power Transformers output power, product manufacturing costs are not conducive to.

Short-circuit impedance

From the voltage regulation rate and reduce the reactive power aspects, the smaller the short-circuit impedance, the better.

However, the short-circuit current multiplier is inversely proportional to the short-circuit impedance, and the winding will suffer a huge electrodynamic shock and generate a high winding temperature during a short-circuit, so in order to limit the short-circuit current, the oil furnace transformer is expected to have a large short-circuit impedance. Both too low and too high an impedance will increase the cost of manufacture for core Arc Furnace Power Transformers.

The situation now is that the short-circuit impedance of a furnace transformer with a large short-circuit opportunity is much smaller than that of a power Arc Furnace Power Transformers with a small opportunity: for example, for a 110 kV power Arc Furnace Power Transformers, the short-circuit impedance The standard value is 10.5%, in order to improve the reliability of the city network power supply, double-winding power Arc Furnace Power Transformers are often used to grade the high impedance Arc Furnace Power Transformers, whose short-circuit impedance can be 10.5%, 12.5%, 145%, 16.5%, 23% five kinds, which This undoubtedly ensures the reliable operation of the Arc Furnace Power Transformers in the city network.

The short-circuit impedance requirements of various furnace transformers vary.

Ultra-high power electric arc furnace generally use long arc operation, natural power factor close to the limit value of 0.866, in order to stabilize the arc combustion, steel furnace Arc Furnace Power Transformers second highest voltage short-circuit impedance requirements of about 8%, usually also add a series reactor to maintain the main circuit cosp 0.75 ~ 0.85 between.

Obviously, the short-circuit resistance of the Arc Furnace Power Transformers is easily ensured.

In the submerged arc furnace, 110kV calcium carbide furnace Arc Furnace Power Transformers secondary highest voltage when the short-circuit impedance standard of 6% ~ 8%, yellow phosphorus furnace Arc Furnace Power Transformers for 7 ° ~ 9%.

In reality, many users unilaterally pursue low short-circuit impedance, some even require constant power at the lowest secondary voltage short-circuit impedance of 6% or so, so that the secondary maximum voltage short-circuit impedance is only about 3 ° ~ 4%.

So there is a strange phenomenon, the work of short-circuit for the normal state of the steel furnace Arc Furnace Power Transformers are running very smoothly, the electrode current is relatively small fluctuations in the submerged arc furnace Arc Furnace Power Transformers damage is common.

Obviously, in order to unilaterally pursue the low impedance of the oil furnace transformer, at the expense of weakening the short-circuit resistance of Arc Furnace Power Transformers is not worth the loss, because the increase in short-circuit impedance and increased reactive power can be fully compensated by capacitive compensation.

The authors recommend that the submerged arc furnace Arc Furnace Power Transformers secondary maximum voltage, its short-circuit impedance should be used in the corresponding standard provisions of the upper limit, so that the winding short-circuit resistance can easily be guaranteed.

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What are the main components of the furnace transformer?

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Selection of the maximum rated current (/um) through the on-load tap-changer

Experience has shown that the on-load tap-changer is an important link in the reliability of the oil furnace transformer, so the on-load tap-changer should be selected correctly so that the characteristics of the switch do not limit the load capacity of the furnace transformer.

The rated current through the on-load tap-changer is not less than the maximum tap current generated by the normal continuous load on the tap winding in all cases.

Most electric furnaces have a fixed period of over nameplate rating operation during each furnace smelting cycle, for the on-load tap-changer this should not be considered an overload condition, at which point the on-load tap-changer should ensure that it can carry and open this overload current, even the operating short-circuit current.

This is due to the fact that working short circuits during the melting of the steel furnace are often unpredictable and sudden. When the load at the moment of regulation seems to fluctuate little and the working short circuit occurs suddenly after the movement of the energy accumulator past the dead point, the diverter switch inevitably overloads the switching. Unfortunately, Arc Furnace Power Transformers manufacturers do not always determine the maximum rated current through the switch according to the maximum overload current when selecting the switch, and most procurement tenders do not explicitly require this.

The “Rules for the Design, Maintenance and Operation of Electric Arc Furnace Power Systems” prepared by the French Iron and Steel Technology Association stipulates that the maximum rated through current (/um) of the on-load tap-changer should be (1.5~1.7) times the rated current (/N) of the Arc Furnace Power Transformers and states that no electrical problems have ever been reported with this provision.

The authors recommend Arc Furnace Power Transformers Ium > 1.3IN for submerged arc furnaces, Arc Furnace Power Transformers /um > 1.5IN for ladle furnaces, and Arc Furnace Power Transformers Ium > 1.7IN for steelmaking furnaces.

Although this increases the cost of Arc Furnace Power Transformers, it is insignificant compared to the losses caused to the user due to the cost of handling each switching incident.

Find more about Ultimate Transformer for Guide

The choice of water cooler type and cooling water quality management

Water cooler due to “oil-water mixing” caused by the furnace transformer accidents occasionally occur, which both the quality of the cooler defects caused by the corrosion of the cooling water, so the choice of water cooler type and cooling water quality management is essential.

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The water cooler is divided into two types of strong oil circulation water cooler and spiral plate type strong oil circulation water cooler.

The tube type is divided into single tube type and two heavy tube type, its water contact with the liner tube are pure copper; spiral plate type of the body is divided into stainless steel and carbon steel two.

Because of the lower cost of the spiral plate type, so now a lot of applications.

As the copper has a better function of inhibiting microbial growth and corrosion resistance compared to carbon steel and stainless steel, so the column tube type strong oil water cooler should be preferred, and for large capacity furnace transformer it is recommended to use the more reliable operation of the two-tube type strong oil circulation water cooler.

Water quality has a great impact on the life of the cooler, poor quality cooling water will accelerate tube corrosion, scale and impurities to reduce cooling efficiency, and even lead to oil furnace transformer due to a sudden increase in temperature and shutdown, so the management of water quality is very necessary, I hope that the supply of cooling water using a fully closed system, that is, cooling water through the water / water or water / air heat exchanger cooling after recycling, so that can This will ensure that no problems with the cooler will cause an accident in the operation of the oil furnace transformer.

Keep on reading The Complete Guide to Pad-mounted Substation

Capacitor compensation

The oil furnace transformer is divided into two types of parallel and series compensation. The shunt compensation technology is mature, the combination of shunt capacitor and series reactor also has the 3rd or 5th harmonic management function, his disadvantage is that the compensation has nothing to do with the load state, always work with constant power; series compensation has the advantage of instant compensation, but the capacitor failure will affect oil furnace transformer’s normal operation, and no harmonic control function, its operating experience has yet to be accumulated.

Comprehensive evaluation results, may parallel compensation than series compensation is superior.

Shunt compensation is also divided into four types of compensation, such as high voltage compensation, medium voltage compensation, low voltage compensation, high school and low voltage part of the combination of compensation, etc. This paper recommends high voltage direct parallel compensation, because at this time the capacitor throwing transient and other factors that may endanger the safe operation of the oil furnace transformer will no longer exist.

Such as a request furnace power for 21150kW of three-phase ferroalloy furnace Arc Furnace Power Transformers, furnace natural power factor cos column = 0.78, the power factor after the request capacitor compensation cos% = 0.94.

(1) When the high voltage is directly compensated.

Arc Furnace Power Transformers a, secondary winding capacity: S1 = 21150 / 0.78 = 27120kVA.

Capacitor capacity: Qc = 21150 [tan (cos10.78) – tan (cos “10.94)] = 9290kvar

(2) when single compensation winding (i.e. medium voltage supplement)

Compensation winding capacity: Sb = Qc = 9290km.

Primary winding capacity: 21150/0.94 = 22,500kVA.

Secondary winding capacity: 21150/0.78 = 27120 kVA.

Arc Furnace Power Transformers structure capacity: S2 = (9290 + 22,500 + 27,120) / 2 = 29,455 kVA.

(3) low-voltage and complementary when.

Parallel compensation Arc Furnace Power Transformers capacity: 9290kVA.

Arc Furnace Power Transformers first and second winding capacity: 21150 / 0.94 = 22,500 kVA.

Two Arc Furnace Power Transformers capacity and did not = 929f22,500 = 31790kVA.

So the high voltage direct replenishment is the lowest cost in three ways, the loss of the smallest program. As for some high-capacity electric furnace direct supplementation occurs arc pressure is not normal, electricity consumption is not stable and other phenomena, that is the lack of large furnace manipulation experience, should not be related to the compensation method.

Read on Application of Pad-Mounted Substation in Railway

What is a furnace transformer?

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When a high voltage is applied to the high voltage winding of the main transformer, a constant voltage is generated in the low voltage winding of the main transformer by electromagnetic induction and a constant voltage is generated in the regulating winding of the main transformer. The voltage at all levels of the main transformer regulating winding is applied to the series transformer high voltage winding by means of a tap changer, where the series transformer low voltage winding also varies with the voltage of the high voltage group. furnace transformer uses a tank and is assembled as a whole.

The furnace transformer is likely to fail for the following reasons

Short circuit faults in the line

Arc Furnace Power Transformers failures preceded by electrodes coming off the slag surface and failure to detect external factors causing the short circuit. After the Arc Furnace Power Transformers had been coring, two short-circuit fault points were found in the coil of the regulating section of the B-phase winding due to insulation damage.

Long-term overload operation and severe insulation deterioration

I use 6300KVA Arc Furnace Power Transformers, while the actual use can only reach a maximum capacity of 3000KW. It is unlikely that the damage to the Arc Furnace Power Transformers was caused by an overload.

Moisture in the winding insulation

The windings were sealed and operating normally without moisture. The fault occurred in March, which is a dry season with low ambient humidity.

Poor contact at winding joints or tap changer joints

No poor contact at the joints was found after core extraction, but when checking the on-load regulator switch, a burned contact was found on the B-phase on-load regulator switch.

Lightning wave intrusion, causing winding overvoltage

The fault occurred in March and no lightning or lightning waves caused by lightning occurred at the time of the accident.

On-load breakers

Arc Furnace Power Transformers blackout was for the purpose of lowering the electrode or replacing it, according to the site inspection the electrode was not in contact with the slag surface, excluding the possibility of sending power with load.

Load shifting

At that time before the power outage was not adjusted to gear, when the power supply has been adjusted to 18 gears required for the process, no need to adjust the gear.

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How can I prevent the furnace transformer from malfunctioning?

Closing the gate in low gear

Short-circuit impedance: In addition to the load current producing load losses, it also produces a voltage drop in the winding, so that the output voltage of Arc Furnace Power Transformers with a load is lower than the no-load voltage. This voltage drop, expressed as a percentage of the constant voltage, is called the impedance voltage of the winding. The impedance voltage includes a voltage drop component on the equivalent resistance corresponding to the load losses and a voltage drop component on the leakage resistance corresponding to the leakage flux coupled to the winding. The values of these two components cannot be measured separately and directly. Instead, they are calculated separately after the load loss and impedance voltage have been measured.

The impedance voltage, short-circuit impedance and load loss tests are carried out by applying an approximately sinusoidal current to one winding of the Arc Furnace Power Transformers (usually the higher voltage winding of the tested series) in the main tap position, at rated frequency, and short-circuiting the other winding with a conductor of sufficient cross-section to gradually increase the supply voltage so that the rated current flows through the winding. The power input to the Arc Furnace Power Transformers at this point is the load loss of the pair of windings, and the voltage applied to the output of the excitation winding (line voltage for three-phase Arc Furnace Power Transformers) is the impedance voltage of the pair of windings, or short-circuit impedance when expressed in ohms per phase. Short circuit impedance.

After the Arc Furnace Power Transformers were repaired, factory tests were carried out to measure the impedance of the Arc Furnace Power Transformers. This resulted in a low impedance for the HKSSPZ-6300/35 shell type Arc Furnace Power Transformers used at our factory. According to the measured impedance values, the higher the gear, the lower the impedance, and the Y-connection method has a higher impedance than the D-connection method.

However, according to the actual use, our plant furnace transformer belongs to the submerged arc furnace type furnace transformer, but the actual use of the process requires the use of high voltage between 13-15 gear, so a long time to use the high-voltage D-type connection, and the higher the gear the lower the impedance, the more impact on Arc Furnace Power Transformers The higher the gear, the lower the impedance, the greater the impact on the Arc Furnace Power Transformers, especially during power outages when the impedance is smaller will impact on the high-voltage winding, strengthening the high-voltage impact on the high-voltage winding. Due to production and process needs, the furnace transformer needs to be shut down and fed 2-3 times a day. With frequent shutdowns and low impedance, it is a challenge to ensure the long-term stability of the Arc Furnace Power Transformers.

During the Arc Furnace Power Transformers test, the Arc Furnace Power Transformers were shut down and fed at different speeds: 1, 5, 10 and 18. The test results showed that the high voltage side current did not fluctuate when closing in gear 1, the high voltage side current fluctuated slightly when closing in gear 5, the high voltage side current fluctuated greatly when closing in gear 10 and the high voltage side current fluctuated strongly when closing in gear 18.

Therefore, the Arc Furnace Power Transformers will be least affected by the use of the lower gears. Therefore, it is specified that each time the power is shut down and delivered, the 5th gear is used. As gear 1 is only 1% higher impedance than gear 5, gear 5 was chosen.

The furnace transformer uses an on-load tap-changer, which means that the voltage level on the low voltage side can be adjusted during the operation of the furnace transformer, so can the on-load tap-changer be used between frequent gear changes.

The on-load tap-changer is mainly used for oil-immersed power, rectifier and furnace transformer with a capacity of 63000 kVA or less, rated voltage 35~110kV, maximum rated through current of 350A for three phases, 350A for single phase, 50Hz frequency, three-phase Y-connected neutral voltage regulation, bell-type or box-top type. The on-load regulator can be shifted during operation.

Can the three-phase current or single-phase current be less than 350 A if shifting at load? The impact of shifting should be the same as the impact of a power failure. If shifting with load is the same as sending power with load, the impact on the high-voltage coil winding is very high, with the instantaneous current and voltage exceeding 3-3.5 times the rated voltage (and 105KV-122KV).

This can easily cause the winding of the furnace transformer to fail, resulting in damage to the Arc Furnace Power Transformers. This means that the shifting of the furnace transformer needs to be carried out at no load so that the impact on the coil and Arc Furnace Power Transformers is minimised and the regulating section coil is gradually energised during the shifting, reaching the required gear and the required voltage.

The number of times the on-load regulator switch is used and maintained: 3 times a day the power needs to be stopped and delivered, each time the gear is used at 15, so when the power is stopped the on-load regulator switch needs to be adjusted to 5 gears and the on-load regulator switch moves 10 times, when the power is delivered the on-load regulator switch needs to move from 5 gears to 15 gears 10 times, a stoppage and delivery of power continues to move the on-load regulator switch 20 times. If the power is stopped and delivered 3 times a day, 60 operations are required. This means that 21,900 operations are required over 365 days a year.

According to the manufacturer’s manual, the number of times the on-load regulator switches are operated is between 100,000 and 150,000 times. It is recommended that the on-load tap-changer should be serviced after a certain period of operation. It is recommended that the on-load tap-changer be overhauled once a year by drawing the core, checking the contacts of the on-load tap-changer and replacing damaged parts so that the on-load tap-changer can operate normally.

The arcing time of the electrodes should not exceed 10 seconds

Three electrodes cannot be inserted into the furnace at the same time because it is a manual operation. If two electrodes are inserted into the furnace, the Arc Furnace Power Transformers will work out of phase, so the operator is required to insert one electrode first and then two other electrodes at the same time, and the other two electrodes must be inserted within 10 seconds after one electrode is inserted.

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How do I maintain Arc Furnace Power Transformers?

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The high voltage side and high voltage vacuum circuit breakers of Arc Furnace Power Transformers require routine inspection and maintenance to ensure that the high voltage side is insulated and the low voltage side is free from short circuits, to ground and insulation.

A full test is carried out once a year to assess the condition of the Arc Furnace Power Transformers and if the standards are not met then the Arc Furnace Power Transformers should be considered for coring and maintenance.

The purpose of the Arc Furnace Power Transformers test is to verify that the Arc Furnace Power Transformers are performing in accordance with the relevant standards and technical conditions and that there are no defects that could affect normal operation (e.g. short circuits, broken circuits, overheating, etc.).

In addition, the results of the tests are analysed to identify directions for improving and enhancing the quality of operation.

Quarterly oil samples are taken from Arc Furnace Power Transformers for testing. Annual testing of Arc Furnace Power Transformers. Variable voltage ratio test, insulation performance test, externally applied high voltage test, induction high voltage test and coil DC resistance test.

The variable voltage ratio is an important data for the parallel operation of Arc Furnace Power Transformers. A 1% difference in the ratio of two small to medium sized Arc Furnace Power Transformers can result in a circulating current of 10% of the rated current when they are connected in parallel, so it is important to limit the error in the ratio. The ratio test shows whether the voltages are the same for each phase of each gear. The purpose of the insulation test is to verify that the insulation is good enough to decide whether to put it into operation or to continue it.

R60” is the DC voltage acting on the insulation material, a weak current will flow through the insulation material, which consists of three parts: charging current, absorption current and leakage current.

The charging current quickly disappears, then the absorption current gradually decreases to 0 and finally stabilises at the leakage current, which basically stabilises at the leakage current within 60 seconds. Leakage current and the overall moisture or insulation moisture, surface soiling and the degree of insulation defects, is the method to determine the insulation good or bad.

Dielectric loss is the loss of energy on the medium proportional to the square of the applied voltage, proportional to the frequency of the voltage, proportional to the capacitance of the medium under test and proportional to the tangent of the dielectric loss angle. The tangent of the dielectric loss angle indicates the relative magnitude of the dielectric loss.

At a certain voltage and frequency, the tangent of the dielectric loss angle can reflect the state of the insulation, and is sensitive to the overall defects.

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The world is currently facing the opportunity of the great development of electric furnace steel, submerged arc furnace is also facing the development of large capacity and industrial concentration, I hope that users can reasonably choose the main parameters and components of the furnace transformer, so that the reliability of the oil furnace transformer to a higher level.

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