Excitation Rectifier Transformers Selection and Calculation

Excitation rectifier transformers Selection and calculation 2011-12-22 by seoer1 1 Overview In the field of small hydropower supporting the selection of equipment to upgrade or repair work, power users often encounter problems rectifier transformers parameter calculation. Many electrical design manual are designed to provide a rectifier transformer formula, but formula is applicable to the application of standard conditions, and small hydropower has the actual differences in operating environment, whereby the design of the transformer may not be realistic. While the grass-roots small hydropower is also a lack of professional and technical personnel, users usually feel rectifier transformer sizing is difficult. Therefore, the primary user to propose a simple method of calculation is necessary. 1.1 rectification of choice: the current use of self-motivation are basically low-pressure unit-type static thyristor excitation mode. The three-phase rectifier way to have full-wave half-controlled rectifier and three-phase half-wave rectifier two (Figure 1) full-wave rectifier transformer efficiency is relatively high (95%), the waveform is better. Half-wave rectifier silicon fewer components, but the transformer secondary winding DC current through, the efficiency is relatively low (74%), large waveform distortion, with the rectifier circuit in less than 10kW, but some of the early design of the larger unit is semi- wave rectifier. Two types of rectifier transformer formula is different. 1.2 rectifier transformer in the form of: epoxy dry-type Transformer Power. Usually in the 10100kVA capacity within a nominal voltage (network side) 400V, secondary voltage (valve side) 100V or less, within the current 100-300A. As the capacity is relatively small, with the rectifier set a power strip with the body. Rectifier transformer cooling method is self-cooling, the plate side of the closure plate is not installed, the cooling conditions are good. 1.3 Insulation and Cooling: the use of small hydropower dry epoxy transformer insulation levels are generally Class B, the maximum temperature insulation system for the 130 ℃, so the transformer full-load temperature hot when the appearance is normal. If forced air-cooled transformer to be effective, the output power can be increased by 10% to 30%. Conversely, if the transformer is the work of distribution in a sealed box, poor heat dissipation, it is necessary to reduce the current capacity to 10% or more. 1.4 Impedance voltage: the generator excitation system, there may exist the DC rectifier circuit short-circuit breakdown or other factors, so the rectifier transformer short-circuit impedance voltage of the transformer is higher than normal in order to limit excessive short-circuit current . Short-circuit impedance voltage from the transformer plant design parameters, we will not discuss, but users will need to specify when ordering factory is the thyristor rectifier transformer. 2 wiring group Rectifier transformer with thyristor connection group must control the phase matching requirements. If the new design, the following principles can be considered.

Generally use the D, y11 way that the network side (a) using △ connection, valve side (secondary) using y connection. This connection of the secondary phase voltage than the first phase voltage in phase behind the 30 °. D, y11 to adapt the access method for the simultaneous three-phase half-wave rectifier and full wave two kinds of forms. If the existing rectifier transformer connection group is Y, d11, it can also be used, but not for three-phase half-wave rectifier. As for Y, y of the connection group is not recommended. We know that three-phase controlled rectifier generates the third harmonic voltage is very high, can Daji Bo value of more than 50%, while the D transformer can be connected in the third harmonic flux to offset the impact to a minimum. However, if a Y, y wiring group, the rectifier circuit of the third harmonic of the fluxfree closed-loop can not be offset. High third harmonic waveform distortion will be too large, affecting the instrument transformers and generators and other electrical equipment running. Proposed power plant order from the manufacturer data, should indicate clearly the transformer connection groups, the first and second voltage (also required an indication of phase or line voltage). 3 the choice of a line voltage U1 Small unit-side rated line voltage is 400V, but the small hydropower stations in the grid are generally remote from the substation impedance of the line is very long. Caused by the end of the network voltage is too high, especially in the wet period and peak power generation, the net voltage (-side) is often as high as 460V or more. If at this time or in accordance with a voltage of 400V to design, it will withstand over-voltage transformer, the loss increases, the heat excessive. Rectifier transformer iron loss than its withstand voltage multiplier into 4 th power of relationships, such as the design of the rectifier transformer according to 400V, in 1.2-fold (480V) voltage operation, the iron loss increased (480/400) 4 = 2.07 times. These losses are eventually converted to heat within the transformer, the transformer temperature rise substantially. Moreover, when the high voltage reaches a certain level, the transformer core flux density will enter the saturated zone, so that one side of the current surge in the coil burnt. Some of the rectifier transformer design and manufacture due to cost considerations, the selected core values ??of the magnetic flux density Bm high voltage winding and a still select 400V, so the area burned in the high voltage transformer network examples are not uncommon. In this regard it should be appropriate to increase the primary winding voltage, so +20% increase in net voltage transformer can handle the work. There are generally 5% of the voltage transformer overload, so we can use empirical formulas to select the primary winding rated line voltage U1 = 0.95U1 (MAX),

Where, U1 (MAX) is the net power (equivalent to-side) of the maximum voltage value according to the results if less than 400V 400V selection. Select the value of a voltage increase, the secondary voltage should be increased by the same ratio to maintain than the same transformer, in order to maintain the terminal voltage excitation voltage and the same proportion to the increase or decrease, because the higher the voltage generator, the excitation power on the need be. Improve the practice of a voltage, equivalent to increasing the number of turns per volt, are intended to reduce the transformer core flux density. Prevent access to the saturation flux density curve segment. Benefits have reduced the Distribution Transformers load current and iron loss. Of course, this has some negative effects, due to the winding turns Cadogan, the transformer resistance increases, the current loss (copper loss) increased slightly, but the normal operation of the transformer does not constitute any effect. Voltage adjustment factor for the n = U1/400 Simple calculation, you can pass to take U1 = 440V, can adapt to most network conditions (400V-470V) requirements. 4 Calculation of the secondary voltage U2 Secondary voltage of the selected values ??related to the excitation system of the top value (strong excitation) voltage, the maximum excitation current, the thyristor conduction angle and harmonic distortion, power factor rectifier circuit and so on. According to the specifications, the excitation circuit to provide 1.6 to 1.8 times the strong excitation voltage, the transformer secondary voltage that is required is a rating of 1.6 to 1.8 times. But in fact, very few of China's small hydropower units have become isolated from the power grid, most are incorporated into the sale of electricity to run a large grid, the grid does not provide strong excitation power to the needs and capabilities - Notice a very large network capacity large, a single small hydropower units of its impact is negligible. If you press to improve the value of 1.6 to 1.8 times to select the secondary voltage, high voltage rectifier, thyristor excitation system is a long period when the state of deep control, thyristor conduction angle is small, the waveform distortion increases, power factor variation, short-circuit fault current increases, these factors have on the transformer and unit equipment to run negative. While in the same capacity under the power transformer, high voltage current will inevitably lead to reduced cross-sectional area of ??the wire coil winding down, the current consumption is also increased. In our experience, select the maximum rectified voltage is 1.3 times the rated excitation voltage is more moderate, in addition to dealing with high voltage to avoid the shortcomings of the outside, but also to retain a certain amount of rectifier power margin to meet the changing demands of operating conditions. Phase voltage calculation of U2

Three-phase full-wave rectifier U2 = 1.3 * 1.06 (nUE +2.5) / 2.34 = 0.59nUE +1.47 Three-phase half-wave rectifier U2 = 1.3 * 1.06 (nUE +1.7) / 1.17 = 1.18nUE +2.0 The last type of explanation: UE-generator rated excitation voltage (V); Factor of 1.3-As mentioned earlier, is the excitation voltage margin value; Coefficient of 1.06 at full load-current resistance of the transformer leakage reactance voltage drop caused by the compensation value. Here's a simple fixed value instead of complex calculations, the error is not too large; n-voltage adjustment factor, see above section; Coefficient of 2.34 (or 1.17) - three-phase full-wave (or half-wave) rectifier element full turnoutput DC voltage input AC phase voltage ratio, that is UE/U2 = 2.34 (or UE/U2 = 1.17); Figure 2.5 (or 1.7) - excitation circuit the sum of the voltage drop, including the rectifier forward voltage drop components (1.5V or 0.75V), and the feed wire and the carbon brush collector ring voltage drop (1.0V). Transformer ratio K = U2/U1. Simple calculations, the results can be put on the first type, two combined, with U2 = 0.71UE (full wave), or U2 = 1.4UE (half-wave) 5 current calculation A three-phase full-wave rectified current I2 = 0.816KIE, secondary phase current I1 = 0.816IE A three-phase half-wave rectified current I2 = 0.472KIE, secondary phase current I2 = 0.577IE I1 value has not been considered here, the efficiency of the transformer. 6 Calculation of power Excitation power: PE = UEIE (W) Transformer secondary side power: Full-wave rectifier P2 = 3U2I2 = 3 (0.59nUE +1.47) * 0.816IE = 1.45nPE +3.60 IE (W) Half-wave rectifier P2 = 3U2I2 = 3 (1.18nUE +2.0) * 0.577IE = 2.04nPE +3.46 IE (W)

Transformer capacity is calculated: Full-wave rectifier S1 = P2 / (0.8 * 97%) = 1.29P2 = 1.29 (1.45nPE +3.60 IE) = 1.87nPE +4.64 IE (VA) Half-wave rectifier S1 = P2 / (0.8 * 97%) = 1.29P2 = 1.29 (2.04nPE +3.46 IE) = 2.63nPE +4.46 IE (VA) Where 0.8 is the transformer rated power factor, 97% of the efficiency of the transformer primary side. Simple calculations, the results can be put on the first type, two combined, there S1 = 2.2PE (full-wave use), or S1 = 3PE (half-wave use) In the actual order, we will sometimes find that some manufacturers to reduce costs, transformer manufacturing materials tight, so that the high temperature transformer operation in this case, for insurance purposes, it is best to order the transformer capacity to increase 10% to meet, this time value does not change the voltage transformer, the first and second current according to the percentage increase. 7 Calculation Example : Need to order one of a small hydropower excitation rectifier transformers, generator parameters are: Generator power 400kW, rated voltage 400V, generator excitation voltage UE = 49.4V, excitation current IE = 153A. Working conditions: excitation device for three-phase full-wave half-controlled thyristor rectifier, power unit and a large run, the net maximum power voltage (-side) 460V. Solution as follows : 1) connected group D, y11 2) excitation power PE = UE x IE = 49.4 × 153 = 7558.2 (W) 3) a line voltage U1 = 460 × 0.95 = 437 (V), take the 440V Voltage adjustment factor n = U1/400 = 1.1 4) The second phase voltage U2 = 0.59nUE +1.07 = 0.59 × 1.1 × 49.4 +1.07 = 33.13V, optional 34V; 5) The transformer ratio K = U2/U1 = 34/440 = 0.0773

6) The secondary current I2 = 0.816IE = 0.816 × 153 = 124.85 (A), optional 125A. 7) a current I1 = 0.816KIE = 0.816 × 153 × 0.0773 = 9.65 (A), optional 9.7A 8) Power Capacity S1 = 1.87nPE +4.64 IE = 1.87 × 1.1 × 7558.2 +4.64 × 153 = 16257 (VA), optional 17 kVA Finally, fill in the order list: Three-phase rectifier transformers ordering data Transformer capacity: 17 kVA connection group: D, y11 Voltage ratio: 440V (line) / 34V (phase) current (current from time to time reported): 9.7A/125A Power Factor: 0.80 working system: long-term work According to the results of this paper, when the transformer at rated load conditions was 0.77, in the state of the economy. Rated excitation current output rectifier thyristor control angle of about 65 °. We ms excel (spreadsheet) software development, computer program, the calculation only to three parameters: excitation voltage UE, current IE and the highest network voltage U1 (MAX) input, the program automatically calculates all the rectifier transformer data, to the order list. This program in Shenzhen Hopewell Electric Company's website (szpwr.com) can download, or the site can act as the calculation 8. Simple calculation method Generator parameters with the former, excitation voltage UE = 49.4V, excitation current IE = 153A. Excitation power: P = 40.3 × 161.4 = 7558.24 (W) Take a pass line voltage: U1 = 440V Three-phase half-wave rectifier Secondary phase voltage U2 = 1.4UE = 1.4 × 49.4 = 69.2V, 70V optional Transformer capacity S1 = 3 × 7558 .= 22674 (VA), optional 23kVA Three-phase full-wave rectifier

Secondary phase voltage U2 = 0.71UE = 0.71 × 49.4 = 35.1 (V), optional 35V Transformer capacity S1 = 2.2 × 7558.2 = 1662 (VA), elected 17kVA Section on the calculation results with the more visible, simple method of calculating the error is not large, you can put to use. In the excitation voltage UE is large, simple method of U2 value slightly too large, but the impact of affect on actual operation. keywords:distribution transformer