Chapter 7 Overload Capacity of Power Transformers

Guide for electrical design engineersPower Quality Katarzyna Strzalka-Goluszka Doctoral Student of Faculty of Electrical Engineering, Automatics, IT & Electronics AGH University of Science & Technology [email protected] Overload Capacity of Power Transformers t Load factor K2 K1 0 Time of day 24 h Power Quality described by: • • • equivalent initial load Sp.leonardo-energy. These principles are presented in the course book [3]. and examples of practical calculations and transformers sizing are provided in the course book [4]. The above equivalent load values should be computed as a root mean square average according to the relation: S= where: ∑ S Δt ∑ Δt 2 i i i (1) Si – load over the time interval Δti. Δti – the considered time interval. since the early seventies. The initial and final equivalent loads are determined from the representative load: Kp = Sp S nt Kk = 2 Sk S nt (2) . duration of the final load tk. The basis for consideration in standard [1] was a representative two-step load cycle determined from the known or expected ordered 24-hour load curve of a transformer. the principles of sizing oilimmersed transformers and evaluation their overload capacity have been defined in standard PN-71/E-81000 [1].org 1. The basic criterion adopted in standard [1] for determining power transformers loading limits is the thermal life of insulation. The standard defines transformer overload capacity assuming nominal transformer insulation life and reduced insulation life expectancy corresponding to overloading a transformer under disturbed conditions.Power Quality www. Introduction For over three decades. equivalent final load Sk. 4.0 duration of the final load tk = 0. Both the loading and temperature rise above the rated values will result in risk of premature failure of a transformer that may occur either immediately or after certain time. Loading beyond nameplate rating and/or higher ambient temperature involves a risk and results accelerated insulation ageing.Overload Capacity of Power Transformers www. The standard gives guidelines on transformer loading in relation to the operating temperature rise and thermal ageing of insulation. in which the cooling modes shall be considered. where the effects of stray leakage flux are significant and the consequences of failure are severe. Large power transformers (exceeding 100 MVA). applicable to loading beyond nameplate rating. the insulation relative ageing rate is assessed using the hot spot temperature. Since the transformer sensitivity to overloading depends evidently on its size. assuming the following parameters: • • • ambient temperature ϑo = . 2.org where: Snt – transformer nominal power. 1. The most important changes are discussed further in this paper. due to deterioration of the transformer components. 2. 8. • • For each category the standard defines separate requirements. 0.5. Table 1 shows limit currents and temperatures for the above transformer categories. Main assumptions of the standard PN-IEC 60354 methodology The normal service life of transformer is a conventional reference basis for continuous operation in normal ambient temperature under nominal operating conditions.10. The permissible equivalent final load Kk = f(Kp.25÷1. 12 and 24h. for which only the hot-spot temperature and thermal deterioration have to be considered.leonardo-energy. Standard PN-IEC 60354 [2] implemented in 1999 considerably alters the principles of determining the overload capacity of oil-immersed transformers. tk) is given in standard PN-71/E81000 [1] in the form of curves and tables. 20 and 30ºC equivalent initial load Kp = 0. Medium power transformers (not exceeding 100 MVA). 10. the standard specifies three categories of transformers: • Distribution transformers (with maximum power of 2500 kVA). 3 . 5 160 115 As can be seen from table 1.] [°C] [°C] 1.e. hot-spot temperature rise ΔΘhr = 78ºC.u. long-time emergency cyclic loading.8 160 115 1.leonardo-energy.Power Quality www.5 140 105 LARGE POWER TRANSFORMERS 1.] [°C] [°C] 1. cyclic loading. thus at the temperature Θh = 104ºC V = 2.8 150 115 1. As the basis for analysis of transformer insulation thermal ageing the rule of 6ºC is taken. expressed in terms of "normal" days.] [°C] [°C] 2. i. Standard [2] provides the method for calculation of transformer daily loss-oflife.org Table 1 : Currents and temperature limits applicable to loading beyond nameplate rating TYPE OF LOADING Normal cyclic load: Current Hot-spot temperature Top-oil temperature Long-time emergency cyclic loading: Current Hot-spot temperature Top-oil temperature DISTRIBUTION TRANSFORMERS [p. i. and at Θh = 110ºC V = 4. Assuming the relative rate of ageing V at the hot-spot temperature Θh = 98ºC equals unity V = 1.5 140 105 MEDIUM POWER TRANSFORMERS 1. the standard recommendations apply to three types of transformer loading: • • • continuous loading.e.u. the rate of insulation ageing doubles for every increment of approximately 6ºC.u.5 140 115 1.3 130 115 Short-time emergency loading: Current Hot-spot temperature Top-oil temperature [p. equivalent days of operation with rated power at the ambient temperature 20ºC. 4 . Standard [2] provides the method for determining thermal behaviour of transformers with various cooling modes and comprises computation results in the form of thermal characteristics for adopted assumptions. among which the most important are: • • ambient temperature Θa = 20ºC.0 - 1.3 120 105 [p. Method of representing an actual load by an equivalent two-step load cycle t Load factor K2 K1 0 Time of day 24 h Fig.Overload Capacity of Power Transformers www. In the case of a load cycle with one peak the value of t should be selected on the equal areas basis as indicated in Fig. 2. 5 . Load cycle with one peak In the case where there are two peaks of nearly equal amplitude but different duration (Fig.leonardo-energy. 1. Fig. Equivalent two-step load cycle The load steps in figure 1 shall be K1 and K2. Standard [2] describes the method of determining this duration for different shapes of actual load cycles. the value of K1 is selected to correspond to the average off-peak load. For the off-peak portion of the load cycle. where K2 is the peak load. The duration of the peak load is t hours. the value of time t is determined for the peak of a longer duration and the value of K1 is selected to correspond to the average of the remaining daily load.org 3. 3). 2. Load cycle with peaks in close succession 4.leonardo-energy.org Fig. Determining permissible transformer load for various types of loadings For a normal continuous loading. Load cycle with two peaks of equal amplitude and different duration For the load cycle where two peaks occur in close succession (Fig. Table 2 gives an acceptable load factor K = K24 for continuous duty and different ambient temperatures. 6 . Fig. 4.Power Quality www. 4). the standard [2] recommends the use of a constant equivalent load current. the value of t is made long enough to enclose both peaks. 3. and K1 is selected accordingly to the average load during the remaining portion of the day. which shows no pronounced variation over a day. 92 0.22 1. 5.91 0.17 1.08 1. °C Hot-spot temperature rise.83 0.28 1.08 1.06 1. 0.30 1. 20. Permissible loading of distribution transformers at the ambient temperature 20°C 7 .82 0. If the ambient temperature value falls between two values. OF and OD cooling) AM BIENT TEM PER ATU RE .92 0. -10. Figure 5 shows an example of relations K2 = f(K1.22 1.81 0. and the ambient temperature Θa = +20°C. -20.24 1. the standard recommends interpolation between the two nearest curves.00 1.17 1.00 1.09 1. 10.14 1.94 0.org Table 2 Acceptable load factor K24 for continuous duty at different ambient temperatures (ON.00 0.15 1.33 1. °C -25 123 -20 118 -10 108 0 98 10 88 20 78 30 68 40 58 K24 Distribution transformers Medium and large power transformers ONAN 1.11 1.21 1.leonardo-energy. t) for distribution transformers with ONAN cooling.25 1. Fig.33 1.31 1. 30 and 40°C) standard PN-IEC 60354: 1999 [2] gives curves that can be used to determine the permissible peak load K2 for a given duration t and a given initial load K1.37 1.87 ON OF OD For normal cycling loading and various types of transformers and eight different ambient temperatures (Θa = -25.Overload Capacity of Power Transformers www.00 1. 1 0. the loading is not permissible.leonardo-energy.0055 30°C 20°C 10°C 0°C -10°C -20°C -25°C In order to determine whether a daily load diagram characterized by particular values of K1 and K2 is permissible and to evaluate the daily loss of life entailed. Table 3 Conversion factor related to the ambient temperature 40°C AMBIENT TEMPERATURE°C Conversion factor kp 10 3. The below example table 4 determines relative ageing rates V and winding hotspot temperature rise ΔΘh for distribution transformers with ONAN cooling and duration t = 4 h.5 to 24 h) and four types of transformers.75. Figure 1 shows the line for K2/K1 = 1.32 0. For emergency cyclic loading the standard PN-IEC 60354: 1999 [2] gives tables that can be used to ascertain whether a load characterized by particular values K1 and K2 is permissible for a given ambient temperature.66 for duration t = 8h. (3) 8 .15 and K1 = 0. and determine a daily loss of life expressed in "normal" days.2 1 0.If the resulting hot-spot temperature exceeds the limit stated in table 1.From table 4 find the hot-spot temperature rise ΔΘh and determine the hot-spot temperature Θh from the formula: Θh = ΔΘh + Θa where: Θa . the following steps should be preceded: .ambient temperature .01 0. t) can also be used for determining the rated power of a transformer (with normal life duration) for a given rectangular load profile defined as the ratio K2/K1. which can be found by marking corresponding points on ordinate K2 = 1 and abscissa K1 = 1.org If the supply voltage remains constant the curves K2 = f(K1.Power Quality www. These tables correspond to six duration values t (0. which allows determining the factors K2 = 1.032 0. For this purpose it is necessary is to find the intersection of the curve corresponding to the duration of the load K2 with the line of constant slope K2/K1. 040 56 0.178 83 0.00 99 8.2 1.056 64 0.080 46 0.3 1.8 1.020 61 0.2 130 93.50 0.677 100 2.154 87 0.30 1.236 89 0.20 1.11 111 7.010 59 0.04 95 2.8 121 43.7 114 90.6 1.8 137 146 149 587 162 2500 176 + + 0.76 119 28.039 70 0.00 125 38.36 106 6.056 77 0.7 128 66.8 0.093 57 0.005 51 0.0 144 282 157 1220 171 5540 184 0.9 1.72 89 5.9 132 69.7 0.117 66 0.7 142 334 155 1300 167 5410 180 + + 3.00 78 1.50 115 14.45 109 4.70 0.8 140 246 153 971 166 4080 179 + + 1.170 74 0.294 84 0.9 2.1 134 211 146 694 159 1560 172 + + + + 64.0 K1 V Δθh V Δθh V Δθh V Δθh V Δθh V Δθh V Δθh V Δθh V Δθh V Δθh V Δθh V Δθh V Δθh V Δθh 0.5 1.09 122 25.56 104 4.7 123 76.10 1.4 127 50.0 1.38 117 19.7 1.091 73 0.023 68 0.Overload Capacity of Power Transformers www.091 73 0. kp (4) where: kp – the conversion factor related to the ambient temperature according to table 3.455 98 1.org .6 125 160 137 371 149 110 161 3830 174 + + + + 302 127 431 139 790 151 1950 163 6110 176 + + + + 1510 141 2200 153 4190 166 + 179 + + + + 9 .377 76 0.5 112 34.50 0.292 67 0.4 1.13 110 16.From table 4 find relative ageing rate V and determine the daily insulation loss of life L from the formula: L = V .12 103 3.leonardo-energy.32 87 2.1 1.417 92 1.1 139 187 151 745 164 3150 177 + + 0.566 86 1.06 97 4.40 1.02 108 9.3 147 394 160 1690 173 + + 0. Table 4 relative ageing rate V (daily loss of life) and winding hot-spot temperature rise Dqh for distribution transformers with ONAN cooling and duration t = 4 h.36 114 11.032 48 0.9 101 20.7 132 135 144 470 157 1790 169 7370 183 + + 14.94 120 17.80 0.00 1.621 94 1.012 53 0.003 43 0.90 1.5 134 97. K2 0.25 0. The tables and curves. [3] Strojny J. 10 . [4] Strzałka J.leonardo-energy. Conclusion The standard PN-IEC 60354 implemented in 1999. An essential advantage of the recommended methods of verification of overloading capacity of transformers is that the size and cooling modes of transformers are considered. AGH-UST Publisher. Loading of oil-immersed transformers.: Electric Power Equipment ― Problem Book. provided in this standard. (in Polish). [2] IEC 60354: Loading guide for oil immersed transformers. AGH-UST Publishers Krakow 2005.org 5.Power Quality www. being the result of thermal calculations allow determining permissible loading of transformers under different operating conditions. introduced considerable changes to the principles of assessing thermal effects of transformers overloading under various types of load.: Design of Electric Power Equipment. (in Polish). References [1] PN-71/E-81000 Transformers.. Strzałka J. SU1674. Krakow 2001. SU1609.