transformer engineering design and practice 2_phần 3 pps

Designand manufacture of transformers with the rectifier duty poses certain challenges.Complex winding arrangements, high currents and associated stray field effects,additional losses an

Special Transformers

11.1 Rectifier Transformers

Duties of rectifier transformers serving special industrial loads are more stringentthan conventional transformers Electrical energy in the form of direct current isrequired in electrolytic processes used in aluminum smelters and chemical plants(production of chlorine, soda, etc.) Various methods used for converting AC into

DC in earlier days included use of motor-generator set, rotary converters andmercury arc rectifiers With the rapid development in power electronic convertersand switching devices, transformers with modern static converters (rectifiers) arebeing widely used for current ratings as high as hundreds of kilo-amperes Designand manufacture of transformers with the rectifier duty poses certain challenges.Complex winding arrangements, high currents and associated stray field effects,additional losses and heating effects due to harmonics, necessity of maintainingconstant direct current, etc are some of the special characteristics of rectifiertransformers

11.1.1 Bridge connection

One of the most popular rectifier circuits is three-phase six-pulse bridge circuit asshown in figure 11.1 It gives a 6-pulse rectifier operation with the r.m.s value ofthe secondary current for ideal commutation (zero overlap angle) as

(11.1)

where I d is the direct current For a transformer with unity turns ratio, the r.m.s.value of the primary current is also given by the above expression

The average value of direct voltage is

(11.2)

where E is line-to-line r.m.s voltage.

The secondary winding does not carry any direct current (the average valueover one cycle is zero) The ratings of both primary and secondary windings areequal, which can be obtained by using equations 11.1 and 11.2 as

(11.3)Thus, in the bridge connection the capacity of a transformer is well utilized

because the required rating of (1.047 P d) is the minimum value for a 6-pulseoperation The bridge connection is simple and quite widely used

11.1.2 Interphase transformer connection

When the current rating increases, two or more rectifier systems may need to beparalleled The paralleling is done with the help of an interphase transformer

Figure 11.1 Bridge connection

which absorbs at any instant the difference between the direct voltages of theindividual systems so that there are no circulating currents Two 3-pulse rectifiersystems (operating with a phase displacement of 60°) paralleled through aninterphase transformer are shown in figure 11.2.

Figure 11.2 Arrangement with interphase transformer

The difference between the (instantaneous values of) direct voltages of twosystems is balanced by the voltage induced in the windings of the interphasetransformer, for which they are in series connection Since both the windings arelinked with the same magnitude of magnetic flux, the voltage difference isequally divided between them The output DC voltage at any instant is theaverage value of DC voltages of the two systems Thus, the paralleling of two 3-pulse systems results in a system with 6-pulse performance.

The r.m.s value of the secondary current is given by

(11.5)

The corresponding primary rating is 1.047 P d , the minimum value which can be

obtained for a 6-pulse performance

Since the flux in the magnetic circuit of the interphase transformer isalternating with 3 times the supply frequency when two 3-pulse systems areparalleled or with 6 times the supply frequency when two 6-pulse systems areparalleled, the core losses are higher Hence, the operating flux density in theinterphase transformer is designed to be around 50 to 67% of the value used for theconventional transformer [1],

If a 12-pulse operation is desired, two 6-pulse rectifier systems operating with

a phase displacement of 30° are combined through an interphase transformer Inthis case, the time integral of the voltage to be absorbed is smaller as compared tothe 6-pulse operation (due to smaller voltage fluctuation in the ripple) Also, thefrequency of the voltage is 6 times the supply frequency Hence, the size and cost

of the interphase transformer is reduced When the 12-pulse operation is obtainedthrough one primary winding (usually star connected) and two secondarywindings (one in star and other in delta connection), it may be difficult to get theratio of turns of two secondary windings equal to (because of low number ofturns) In such a case, the 30° phase displacement is obtained by having twoprimary windings, one connected in star and other in delta, and two secondarywindings both connected either in star or delta One such arrangement is shown infigure 11.3

Since the two primary windings are displaced by 30°, it is necessary to have anintermediate yoke [2] to absorb the difference between the two limb fluxes (see figure 11.4) The intermediate yoke area should be corresponding tothe difference of the two fluxes (which is about 52% of the main limb area).Under the balanced condition of the two paralleled rectifier systems, thecurrents (average values) in both the windings of the interphase transformer areequal This results in equal flux in the same direction in both the limbs forcing theflux to return through the high reluctance non-magnetic path outside the core (asubstantial portion of DC ampere-turns is absorbed along the non-magnetic returnpath) Other way to explain it is that since net ampere-turns are zero in the window(currents are directed in opposite directions inside the window), flux lines in theclosed magnetic path are absent Hence, the flux density in the core is low underthe balanced operation A slight unbalance in currents of the two systems results in

a non-zero value of ampere-turns acting on the closed magnetic path, which maydrive the core into saturation [3] Thus, the interphase transformer draws a highexcitation current under the unbalanced conditions This is one more reason(apart from higher core losses) for keeping the operating flux density lower ininterphase transformers

Figure 11.3 Twelve-pulse operation

Figure 11.4 Intermediate yoke arrangement

Although the interphase transformer connection has some disadvantages, viz.higher rating of secondary winding and saturation of magnetic circuit due tounbalance between two paralleled systems, it competes well with the bridgeconnection in a certain voltage-current range The application of the interphasetransformers is not restricted to paralleling of two systems; for example with athree-limb core, three systems can be paralleled [1],

If the pulse number has to be further increased (e.g., 24-pulse operation), therequired phase shift is obtained by using zigzag connections or phase shiftingtransformers [1,2]

11.1.3 Features of rectifier transformers

Rectifier transformers are used in applications where the secondary voltage isrequired to be varied over a wide range at a constant current value It is extremelydifficult and uneconomical to have taps on the secondary winding because of itsvery low number of turns and high current value The taps are either provided onthe primary winding, or a separate regulating transformer (autotransformer) is used(feeding the primary of the main transformer) which can be accommodated in thesame tank Various circuit arrangements which can be used to regulate thesecondary voltage are elaborated in [4]

For higher pulse operations, the extended delta connection is shown to be moreadvantageous than the zigzag connection, as it results into lower eddy losses andshort circuit forces [5]

The output connections, which carry very high currents, increase theimpedance of the transformer significantly The increase in impedance due tothese connections can be calculated for a single conductor as per equation 3.80.For go-and-return conductors of rectangular dimensions, the impedance can becalculated as per the formulae given in [6]

For large rating rectifier transformers, the field due to high currents causesexcessive stray losses in structural parts made from magnetic steel Hence, theseparts are usually made of non-magnetic steel

Rectifier transformers are subjected to harmonics due to non-sinusoidal currentduty Hence, sometimes the pulse number gets decided by harmonicconsiderations Due to harmonics, more elaborate loss calculations are requiredfor rectifier transformers as compared to the conventional transformers [7].Sometimes the core of the rectifier transformer supplying power electronicloads is designed to have a small gap in the middle of each limb [5] to limit theresidual flux and keep the magnetizing reactance reasonably constant Thisfeature also limits the inrush current thereby protecting the power electronicdevices Under normal operating conditions, the core flux fringing out in the gapbetween the two core parts hits the inner winding causing higher eddy losses Inorder to mitigate this effect, the windings may also have to be designed with a gap

at the location facing the core gap

Because of possibilities of rectifier faults, special design and manufacturingprecautions are taken for rectifier transformers It is generally preferred to designthe rectifier transformers with larger core area with the corresponding smallernumber of turns to reduce short circuit forces [8] Disk type windings are preferredsince they have better short circuit strength compared to layer windings Quality

of drying/impregnation processes and integrity of clamping/support structureshave to be very good The paper insulation on winding conductors can also bestrengthened

11.2 Converter Transformers for HVDC

There has been a steady increase in High Voltage Direct Current (HVDC)transmission schemes in the world because of many advantages of HVDCtransmission as compared to HVAC transmission [9, 10] The converter transformer

is one of the most important and costly components of HVDC transmissionsystem The converter transformer design has much in common with that of theconventional power transformer except a few special design aspects which areelaborated in this section

11.2.1 Configurations

The standard 12-pulse converter configuration can be obtained using star-star andstar-delta connections with one of the following arrangements, viz 6 single-phasetwo-winding, 3 single-phase three-winding and 2 three-phase two-winding Thearrangements are shown in figure 11.5

Figure 11.5 Configurations of converter transformers

The weight and size of individual transformer are highest and overall cost (withall transformers considered) is lowest in the three-phase two-windingconfiguration, whereas the weight and size of individual transformer are lowestand overall cost is highest in the single-phase two-winding configuration Sincethe cost of spare transformer in the single-phase two-winding configuration islowest (that of only one of the six transformers), it is more commonly used.

11.2.2 Insulation design

Simplified schematic diagram for bipolar (double) 12-pulse operation is shown infigure 11.6 The windings connected to converter and that connected to AC sideare generally termed as valve and AC windings respectively Since the potentials

of the valve winding connections are determined by the combination ofconducting valves at any particular instant, the entire valve winding has to befully insulated Also, unlike the AC winding, both the terminals of the valvewinding experience the full DC voltage of the bridge to which it is connected.Hence, the end insulation is higher resulting into greater radial leakage field at thewinding ends The winding eddy loss due to radial leakage field can be muchhigher than the conventional transformer, if the conductor dimensions are notchosen properly

Thus, in addition to the normal AC voltage, the valve windings are subjected to

a direct voltage depending on their position with respect to ground Under an ACvoltage, potential distribution is in inverse proportion to capacitance or electricstress is inversely proportional to permittivity in a multi-dielectric system Sincethe permittivity of oil is about half of solid insulation, the stress in oil is moreunder the AC voltage condition in the conventional transformers Since the

Figure 11.6 Schematic diagram of bipolar 12-pulse operation

dielectric strength of the oil is quite less as compared to that of the solidinsulation, the insulation design problem reduces mainly to designing of oil ducts

as elaborated in Chapter 8 Contrary to AC conditions, under DC voltageconditions the voltage distribution is in direct proportion to resistance or electricstress is directly proportional to resistivity At lower temperatures the resistivity ofsolid insulating materials used in transformers is quite high as compared to that ofthe oil The ratio of resistivity of a high quality pressboard to that of the oil isabout 100 at 20°C, which reduces to as low as 3.3 at 90°C [11] This is because thefall in the resistivity of the pressboard with temperature is much higher than that ofthe oil [12] Such a large variation in the ratio of the two resitivities increases thecomplexity of insulation design

Thus, under DC conditions at lower temperatures, most of the voltage getsdistributed across the solid insulation and stress in it greatly exceeds that in theoil The oil ducts have practically only AC voltage across them, whereas solidinsulations (barriers, washers, supporting and clamping components, etc.)generally have preponderance of DC voltage with a certain amount ofsuperimposed AC voltage Therefore, the pressboard barriers tend to be more in theconverter transformers as compared to the conventional transformers However,the proportion of solid cannot be higher than a certain value because thecomposite oil-solid system has to withstand AC voltage tests as well

Let the symbols ε and ρ denote relative permittivity and resistivity With a

voltage V applied across two parallel plates shown in figure 11.7, under AC field

conditions the stresses in oil and solid insulation are

(11.6)

Figure 11.7 Oil-solid insulation system

and under DC field conditions the stresses are

(11.7)

For non-uniform field conditions involving complex electrode shapes, thetechniques described in Chapter 8 should be used to calculate the stresses.Under steady-state DC conditions, space charges get accumulated at theboundary of the oil and solid insulation When there is a polarity reversal, in

which the applied DC voltage changes from +V to -V, an equivalent of the voltage difference 2V gets applied As the time required for reversing the polarity of the

applied voltage is much shorter than the space charge relaxation time [13], thevoltage due to space charge is not affected during the time of polarity reversal.Therefore, using equations 11.6 and 11.7 the stresses in the oil and solidinsulation under the polarity reversal condition can be given by

(11.8)

(11.9)

Thus, under the polarity reversal condition, the oil gap is stressed more

It can be easily seen from the above equationsthat the smaller the stress across the oil gap before the polarity reversal, the morethe stress is across it after the polarity reversal The voltage distribution undervarious conditions is shown in figure 11.8 The voltage across the oil gap is muchhigher during the polarity reversal condition (Case 3) as compared to Case 2 ofsteady-state DC voltage condition (prior to the polarity reversal)

For the oil-solid composite insulation system, a relatively low DC voltagesuperimposition on AC voltage has very little effect on the partial dischargeinception voltage [12–15]; this is due the fact that most of the DC voltage getsdropped across the solid insulation, which has a high DC withstand voltage If,however, the DC voltage magnitude is within the range of the breakdown DCvoltage, the breakdown behaviour of the entire system is determined by the DCvoltage Although the converter transformers are stressed by combined AC and

DC voltages during service conditions, it is not considered necessary to testthem with superimposed voltages [16] Conventional power frequency andimpulse tests are generally sufficient besides pure DC voltage tests and thepolarity reversal test

Like in the case of AC insulation design (Chapter 8), the stresses under DC can

be calculated accurately by numerical methods The field distribution is generallycalculated for the worst case situation, say at 20°C, since at this temperature theratio of resistivity of the solid insulation to that of the oil is high resulting in highstress in the solid insulation The density of equipotential lines in barriers/cylinders is much higher as compared to the oil, necessitating an increase in theirthickness as compared to the conventional transformers The shape and placement

of barriers and the width of the resulting oil ducts would have already beendecided by the requirements of AC design and the thermal considerations [11].Hence, it is obvious that the high strength of the solid insulation cannot be fullyutilized from the DC design consideration if the stress in the oil gap (having alower strength) has to be kept low under the AC and polarity reversal conditions[17] Discontinuities in solid insulations result into higher tangential (creep)stresses along the solid-oil interfaces, and hence these should be properly looked

at while finalizing the insulation design

The insulation design of the converter transformers is complicated by the factthat the ratio of resitivities of the solid insulation and oil, which variesconsiderably as explained earlier, is greatly influenced by a number of factors[16], viz temperature, field strength, moisture, time and aging (contrary to theconventional transformers where the corresponding ratio of permittivities doesnot exceed about 2 and is practically independent of external factors)

The volt-time characteristics of the oil under DC voltage application arereported in [13,18] for plane-plane electrode; the DC breakdown voltage shows arapid rate of decrease for stressing time till 100 seconds, after which there is hardlyany decrease The long-time breakdown voltage (t→∞) is 70 to 80% of the one-

Figure 11.8 Voltage distribution under various conditions

minute breakdown voltage [18] Under combined AC-DC voltage, the ACbreakdown voltage of the oil decreases as the DC voltage increases The DC 1-minute withstand voltage of the oil gap is about 20 to 30% lower than the AC 1-minute withstand voltage [14,18,19]; on the contrary the oil-impregnated paper/solid insulation withstands more DC voltage as compared to AC voltage [14] Thehigher DC strength of the solid insulation can be partly explained by the absence

of partial discharges which lower the strength in the case of AC voltage Even ifthere are oil voids in the solid insulation, the stress in them is too low under DCvoltage to initiate partial discharges

It is reported in [20] that the dielectric strength of the converter transformerinsulation under the polarity reversal condition is similar to that under switchingimpulse stresses An equivalent AC power frequency voltage test has beensuggested for the polarity reversal test

From the typical schematic shown in figure 11.6, it is clear that when a number

of converters are connected in series, the line (AC side) windings are connected inparallel across the same lines and the inductively transferred overvoltage toground increases cumulatively from one converter bridge to the next higherbridge because the valve (DC side) windings are connected in series (althoughvoltages across the valve windings remain almost the same for all the converters)

11.2.3 Other design aspects

On-load tap changer (OLTC): The OLTC of a converter transformer plays a

crucial role in HVDC transmission system The OLTC tap position is adjusted toget a voltage which minimizes the reactive power requirement of HVDCconverters (firing angle of converters is kept as minimum as possible) Hence,the OLTC is an important constituent of the HVDC control scheme The number

of OLTC operations in the converter transformer is usually much higher thanthat in the conventional power transformer for the same reason The OLTC isused for effective control of the power flowing through HVDC line and the DCvoltage

Leakage impedance: The leakage impedance of the converter transformer is

the principal component of commutating reactance, which limits the rate of rise ofloop current during the small overlap period when the current is transferred fromone valve (thyristor) to another Thus, the leakage impedance helps in preventing

instantaneous current transfer which otherwise would result in high di/dt value

damaging the valves The leakage impedance value has to be judiciouslyselected; a higher value reduces the rate of rise of the loop current during thecurrent commutation process, but increases the overlap angle and reactive powerdemand of converters The permissible tolerance on the impedance value of theconverter transformers is usually lower than the conventional transformers so thatthe distortion in DC voltage waveform and the non-characteristic harmonics arereduced