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1.
1根据温度判断故障变压器过热主要表现为油温异常升高,变压器的过热对其使用寿命影响很大。变压器绝缘损坏大多是由过热引起,温度的升高降低了绝缘材料的耐压和机械强度。在80~140℃的温度范围内,温度每增加6℃,变压器绝缘有效使用寿命降低的速度会增加一倍。变压器最热点温度达到140℃时,油中就会产生气泡,降低绝缘或引发闪络,造成变压器损坏。当发现油温异常升高时,应进行检查,在正常条件下,油温比平时高出10℃以上或负载不变而温度不 相似文献
2.
正10 kV配电变压器发热不仅不利于其经济运行,还可能破坏其内部的绝缘,严重影响其使用寿命。通过分析查找配电变压器发热的原因,有针对性地采取措施,不仅可以降低配电网损耗,而且可以延长配电变压器的使用寿命。因此,分析10 kV配电变压器的发热原因非常重要。笔者现对配电变压器常见发热原因进 相似文献
3.
为什么农用变压器使用寿命短魏则清山东单县供电局在变压器的寿命问题上，绝缘老化主要是温度引起的．绕组温度愈高，延续作用时间愈长，变压器的绝缘老化速度就愈快，使用期限就愈短。采用Ａ级绝缘的变压器，在恒定温度日下运行时，其使用期限（寿命）可用公式表示：／一... 相似文献
4.
变压器正常使用年限在２０～３０年，但个别变压器由于使用不当，不能正确调整负荷，达不到正常使用年限，造成不必要的经济损失。（１）变压器绝缘温度与使用寿命的关系。我国电力变压器，大部分采用油纸绝缘即Ａ级绝缘。对于Ａ级绝缘的变压器，在正常运行中，当周围空气... 相似文献
5.
<正>1变压器发生火灾和爆炸的原因(1)绕组绝缘老化或损坏,产生短路。变压器绕组的绝缘物,如棉纱、棉布、纸等,如果受到过负荷发热或受变压器油酸化腐蚀的作用,将会发生老化变质,耐受电压能力下降,甚至失去绝缘作用;变压器制造、安装、检修过程中也可能潜伏绝缘缺陷。由于变压器绕组的绝缘老化或损坏,会引起绕组匝间、层间短路,短路产生的电弧使绝缘物燃烧。同时,电弧分解变压器油产生的可燃气体与空气混合达到一定浓度,便会形成爆炸混合物,遇火花便发生燃烧或爆炸。(2)线圈接触不良产生高温或电火花。在变压器绕组的线圈与线圈之间线圈端部与分接头之间如果连 相似文献
6.
对农村用电来讲,每年5～9月雷击损坏变压器时有发生,由此导致的变压器损坏事故比例也是较大的。配电变压器内部损坏的大都是绝缘,绝缘介质在温度、电气化学和机械等作用下而遭到损坏。 相似文献
7.
根据有关统计资料,配电变压器损坏绝大部分都是绝缘老化、变质、失效所造成的。为了帮助农村电工对配电变压器绝缘老化与使用寿命之间的关系有较明确的了解,笔者现就农用配电变压器绝缘老化与其使用寿命等问题,谈点粗浅认识。 相似文献
8.
变压器大多是油浸自然冷却式。这种油是石油的一种产品，闪点约为１４０℃，并易蒸发燃烧，同空气混合能构成爆炸混合物。ｌ 发生火灾危险的主要原因１．１ 变压器的油箱内有“吱、吱”的放电声 变压器内部有“吱、吱”的放电声则是由于绕组或引出线外壳闪络放电，或是铁心接地线断，造成铁心对外壳感应而产生的高电压发生放电引起的，放电的电弧可能会烧坏变压器的绝缘并引起火灾。１．２ 线圈绝缘损坏发生短路 相邻几个线圈匝间的绝缘损坏，将形成一个闭合的短路环路，同时，使一相的绕组减少匝数，在短路环路内流着交变磁通感应出的短路… 相似文献
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11.
Relationship between stable carbon isotope discrimination and water use efficiency under regulated deficit irrigation of pear-jujube tree 总被引：2，自引：0，他引：2
Ningbo Cui Taisheng Du Shaozhong Kang Fusheng Li Xiaotao Hu Mixia Wang Zhijun Li 《Agricultural Water Management》2009,96(11):1615-1622
To investigate the relationship between stable carbon isotope discrimination (Δ) of different organs and water use efficiency (WUE) under different water deficit levels, severe, moderate and low water deficit levels were treated at bud burst to leafing, flowering to fruit set, fruit growth and fruit maturation stages of field grown pear-jujube tree, and leaf stable carbon isotope discrimination (ΔL) at different growth stages and fruit stable carbon isotope discrimination (ΔF) at fruit maturation stage were measured. The results indicated that water deficit had significant effect on ΔL at different growth stages and ΔF at fruit maturation stage. As compared with full irrigation, the average ΔL at different growth stages and ΔF at fruit maturation stage were decreased by 1.23% and 2.67% for different water deficit levels, respectively. ΔL and ΔF among different water deficit treatments had significant difference at the same growth stage (P < 0.05). Under different water deficit conditions, significant relationships between the ΔL and WUEi (photosynthesis rate/transpiration rate, Pn/Tr), WUEn (photosynthesis rate/stomatal conductance of CO2, Pn/gs), WUEy (yield/crop water consumption, Y/ETc) and yield, or between the ΔF and WUEy and yield were found, respectively. There were significantly negative correlations of ΔL with WUEi, WUEn, WUEy and yield (P < 0.01) at the fruit maturation stage, or ΔL with WUEi and WUEn (P < 0.01) over whole growth stage, respectively. ΔF was negatively correlated with WUEy, WUEn and yield (P < 0.05), but positively correlated with ETc (P < 0.01) over the whole growth stage. Thus ΔL or ΔF can compare WUEn and WUEy, so the stable carbon isotope discrimination method can be applied to evaluate the water use efficiency of pear-jujube tree under the regulated deficit irrigation. 相似文献
12.
A 3-year study was carried out to assess the root biomass production, crop growth rate, yield attributes, canopy temperature
and water-yield relationships in Indian mustard grown under combinations of irrigation and nutrient application for revealing
the dynamic relationship of crop yield (Y) and seasonal evapotranspiration (ET). Three post-sowing irrigation treatments viz. no irrigation (I
1), one irrigation at flowering (I
2) and two irrigations one each at rosette and flowering stage (I
3), three nutrient treatments viz. no fertilizer or manure (F
1), 100% recommended NPK i.e., 60 kg N, 13.1 kg P and 16.6 kg K ha−1 (F
2) and 100% recommended NPK plus farmyard manure @ 10 Mg ha−1 (F
3) were tested in a split-plot design. Root biomass was significantly greater in I
3 than I
2 and I
1, and in F
3 than F
2 and F
1. The I
3 × F
3, I
2 × F
3 and I
3 × F
2 combinations maintained significantly greater crop growth rate, plant height, yield components, ET and crop yield and better
plant water status in terms of canopy temperature, canopy-air temperature difference (CATD) and relative leaf water content
(RLWC). Number of siliqua plant−1 and seeds siliqua−1 were the major contributors to the seed yield. Marginal analysis of water production function was used to establish Y–ET relationship. The elasticity of water production (E
wp) provides a means to assess relative changes in Y and ET, and gives an indication of improvement of Y due to nutrient application. The ET–Y relationships were linear with marginal water use efficiency (WUEm) of 3.09, 4.23 and 3.95 kg ha−1 mm−1 in F
1, F
2 and F
3, respectively, and the corresponding E
wp were 0.63, 0.71 and 0.61. This implies that the scope for improving yield and WUE with 100% NPK was little compared with
100% NPK + farmyard manure. The crop yield was highest in I
3 × F
3 combination, and the similar yield was obtained in I
2 × F
3 and I
3 × F
2 combinations. Application of organic manure along with 100% NPK fertilizers maintained greater crop growth rate, better water
relation in plants, yield attributes and saved one post-sowing irrigation. 相似文献
13.
A study was conducted to determine the relationship between midday measurements of vine water status and daily water use of
grapevines measured with a weighing lysimeter. Water applications to the vines were terminated on August 24th for 9 days and
again on September 14th for 22 days. Daily water use of the vines in the lysimeter (ETLYS) was approximately 40 L vine−1 (5.3 mm) prior to turning the pump off, and it decreased to 22.3 L vine−1 by September 2nd. Pre-dawn leaf water potential (ΨPD) and midday Ψl on August 24th were −0.075 and −0.76 MPa, respectively, with midday Ψl decreasing to −1.28 MPa on September 2nd. Leaf g
s decreased from ~500 to ~200 mmol m−2 s−1 during the two dry-down periods. Midday measurements of g
s and Ψl were significantly correlated with one another (r = 0.96) and both with ETLYS/ETo (r = ~0.9). The decreases in Ψl, g
s, and ETLYS/ETo in this study were also a linear function of the decrease in volumetric soil water content. The results indicate that even
modest water stress can greatly reduce grapevine water use and that short-term measures of vine water status taken at midday
are a reflection of daily grapevine water use. 相似文献
14.
A preliminary study of an alternative controlled drainage strategy in surface drainage ditches: Low-grade weirs 总被引：3，自引：0，他引：3
This study examined hydrological characteristics of low-grade weirs, an alternative controlled drainage strategy in surface drainage ditches. Chemographs of vegetated and clear scraped (control) replicates of weir vs. non-weir treatments were compared to determine differences in time to peak (Tp) and time to base (Tb). Drainage ditches Tp and Tb were affected by both vegetation and weir presence. The order of treatment efficiency for Tp was observed to be: non-vegetated non-weir < vegetated non-weir < non-vegetated weir < vegetated weir. Furthermore, Tb for each ditch was the reverse relationship from Tp where vegetated weir > non-vegetated weir > vegetated non-weir > non-vegetated non-weir. Low-grade weirs increase chemical retention time (vegetated and clear scraped), the average time a molecule of contaminant remains in the system. Future research in water quality improvement and weir management will yield useful information for non-point source pollutant reduction. 相似文献
15.
Summary One means of using infrared measurements of foliage temperature (T
f
) for scheduling irrigations requires the use of meteorological data to predict the foliage-air temperature difference for a comparable well-watered crop (T
f
*
– T
a
). To determine the best method for making this prediction, parameters for models of increasing complexity for predicting (T
f
*
– T
a
) were derived for wheat using two sets of field data collected in 1982 and 1983.The simplest model with vapor pressure deficit (VPD) as the sole predictor accounted for 64% of observed variance in (T
f
*
– T
a
). The next model with both VPD and net radiation (R
n
) as predictors accounted for 74%. The most complex model predicted (T
f
*
– T
a
) from the crop energy balance. In addition to VPD and R
n
it included parameters for the effects of air temperature (T
a
), aerodynamic resistance (r
a
) and the canopy resistance of a well-watered crop (r
cp
) and accounted for 70% of the variance.Accuracy of these alternative models was tested against an independent set of field data collected in 1984. The single variable model with VPD as sole predictor accounted for 17% of the variance in observed values of (T
f
*
– T
a
). This increased to 47% when the effect of R
n
was included by using the two variable model and was increased further to 65% when the additional variables of T
a
, r
a
and r
cp
were included by use of the energy balance model. When the complexity of the model was measured by its number of variables there was a close relationship between complexity and the accuracy of the predictions. Reasons for the residual variability are discussed. The need for improved instrumentation for meteorological measurements was indicated. 相似文献
16.
Expected yield losses as a function of quality and quantity of water applied for irrigation are required to formulate guidelines for the effective utilisation of marginal quality waters. In an experiment conducted during 2004-2006, double-line source sprinklers were used to determine the separate and interactive effects of saline and alkali irrigation waters on wheat (Triticum aestivum L.). The study included three water qualities: groundwater (GW; electrical conductivity of water, ECw 3.5 dS m−1; sodium adsorption ratio, SAR 9.8 mmol L−1; residual sodium carbonate, RSC, nil) available at the site, and two synthesized waters, saline (SW; ECw 9.4 dS m−1, SAR 10.3 mmol L−1; RSC nil) and alkali (AW; ECw 3.7 dS m−1, SAR 15.1 mmol L−1; RSC 9.6 meq. L−1). The depths of applied SW, AW, and GW per irrigation ranged from 0.7 to 3.5 cm; the depths of applied mixtures of GW with either SW (MSW) or AW (MAW) ranged from 3.2 to 5 cm. Thereby, the water applied for post-plant irrigations using either of GW, SW or AW ranged between 15.2 and 34.6 cm and 17.1 and 48.1 cm during 2004-2005 and 2005-2006, respectively and the range was 32.1-37.0 and 53.1-60.0 cm for MSW or MAW. Grain yields, when averaged for two years, ranged between 3.08 and 4.36 Mg ha−1, 2.57 and 3.70 Mg ha−1 and 2.73 and 3.74 Mg ha−1 with various quantities of water applied using GW, SW and AW, respectively, and between 3.47 and 3.75 Mg ha−1 and 3.63 and 3.77 Mg ha−1 for MSW and MAW, respectively. The water production functions developed for the two sets of water quality treatments could be represented as: RY = 0.528 + 0.843(WA/OPE) − 0.359(WA/OPE)2 − 0.027ECw + 0.44 × 10−2(WA/OPE) × ECw for SW (R2 = 0.63); RY = 0.446 + 0.816(OPE/WA) − 0.326(WA/OPE)2 − 0.0124RSC − 0.55 × 10−4(WA/OPE) × RSC for AW (R2 = 0.56). Here, RY, WA and OPE are the relative yields in reference to the maximum yield obtained with GW, water applied for pre- and post-plant irrigations (cm), and open pan evaporation, respectively. Crop yield increased with increasing amount of applied water for all of the irrigation waters but the maximum yields as obtained with GW, could not be attained even with increased quantities of SW and AW. Increased frequency of irrigation with sprinklers reduced the rate of yield decline with increasing salinity in irrigation water. The sodium contents of plants increased with salinity/alkalinity of sprinkled waters as also with their quantities. Simultaneous decrease in potassium contents resulted in remarkable increase in Na:K ratio. 相似文献
17.
Effects of deficit irrigation on the yield and yield components of drip irrigated cotton in a mediterranean environment 总被引：1，自引：0，他引：1
Mustafa Ünlü R?za KanberD. Levent Koç Servet TekinBurçak Kapur 《Agricultural Water Management》2011,98(4):597-605
A field study on cotton (Gossypium hirsutum L., cv.) was carried out from 2005 to 2008 in the Çukurova Region, Eastern Mediterranean, Turkey. Treatments were designated as I100 full irrigation; DI70, DI50 and DI00 which received 70, 50, and 0% of the irrigation water amount applied in the I100 treatment. The irrigation water amount to be applied to the plots was calculated using cumulative pan evaporation that occurred during the irrigation intervals. The effect of water deficit or water stress on crop yield and some plant growth parameters such as yield response, water use efficiencies, dry matter yield (DM), leaf area index (LAI) as well as on lint quality components was evaluated. The average seasonal evapotranspiration ranged from 287 ± 15 (DI00) to 584 ± 80 mm (I100). Deficit irrigation significantly affected crop yield and all yield components considered in this study. The average seed cotton yield varied from 1369 ± 197 (DI00) to 3397 ± 508 kg ha−1 (I100). The average water use efficiency (WUEET) ranged from 6.0 ± 1.6 (I100) to 4.8 ± 0.9 kg ha−1 mm−1 (DI00), while average irrigation water use efficiency (WUEI) was between 9.4 ± 3.0 (I100) and 14.4 ± 4.8 kg ha−1 mm−1 (DI50). Deficit irrigation increased the harvest index (HI) values from 0.26 ± 0.054 (I100) to 0.32 ± 0.052 kg kg−1 (DI50). Yield response factor (Ky) was determined to be 0.98 based on four-year average. Leaf area index (LAI) and dry matter yields (DM) increased with increasing water use. This study demonstrated that the full irrigated treatment (I100) should be used for semiarid conditions with no water shortage. However, DI70 treatment needs to be considered as a viable alternative for the development of reduced irrigation strategies in semiarid regions where irrigation water supplies are limited. 相似文献
18.
L.G. Hou H.L. Xiao J.H. Si S.C. Xiao M.X. Zhou Y.G. Yang 《Agricultural Water Management》2010,97(2):351-356
Based on successive observation, fifteen-day evapotranspiration (ETc) of Populus euphratica Oliv forest, in the extreme arid region northwest China, was estimated by application of Bowen ratio-energy balance method (BREB) during the growing season in 2005. During the growing season in 2005, total ETc was 446.96 mm. From the beginning of growing season, the ETc increased gradually, and reached its maximum value of 6.724 mm d−1 in the last fifteen days of June. Hereafter the ETc dropped rapidly, and reached its minimum value of 1.215 mm d−1 at the end of growing season. The variation pattern of crop coefficient (Kc) was similar to that of ETc. From the beginning of growing season, the Kc value increased rapidly, and reached its maximum value of 0.623 in the last fifteen days of June. Afterward, with slowing growth of P. euphratica, the value dropped rapidly to the end of growing season. According to this study, the ETc of P. euphratica forest is affected not only by meteorological factors, but by water content in soil. 相似文献
19.
Impacts of climate variability on reference evapotranspiration over 58 years in the Haihe river basin of north China 总被引：3，自引：0，他引：3
Physically, evaporative demand is driven by net radiation (Rn), vapour pressure (ea), wind speed (u2), and air temperature (Ta), each of which changes over time. By analyzing temporal variations in reference evapotranspiration (ET0), improved understanding of the impacts of climate change on hydrological processes can be obtained. In this study, variations in ET0 over 58 years (1950-2007) at 34 stations in the Haihe river basin of China were analyzed. ET0 was calculated by the FAO Penman-Monteith formula. Calculation of Kendall rank coefficient was done by analyzing the annual and seasonal trends in ET0 derived from its dependent climate variables. Inverse distance weighting (IDW) was used to analyze the spatial variation in annual and seasonal ET0, and in each climate variable. An attribution analysis was performed to quantify the contribution of each input variable to ET0 variation. The results showed that ET0 gradually decreased in the whole basin over the 58 years at a rate of −1.0 mm yr−2, at the same time, Rn, u2 and precipitation also decreased. Changes in ET0 were attributed to the variations in net radiation (−0.9 mm yr−2), vapour pressure (−0.5 mm yr−2), wind speed (−1.3 mm yr−2) and air temperature (1.7 mm yr−2). Looking at all data on a month by month basis, we found that Ta had a positive effect on dET0/dt (the derivative of reference evapotranspiration to time) and Rn and u2 had negative effects on dET0/dt. While changes in air temperature were found to produce a large increase in dET0/dt, changes in other key variables each reduced rates, resulting in an overall negative trend in dET0/dt. 相似文献
20.
Summary A coupled soil-vegetation energy balance model which treats the canopy foliage as one layer and the soil surface as another layer was validated againt a set of field data and compared with a single-layer model of a vegetation canopy. The two-layer model was used to predict the effect of increases in soil surface temperature (T
s
) due to the drying of the soil surface, on the vegetation temperature (T
v
). In the absence of any change in stomatal resistance the impact of soil surface drying on the Crop Water Stress Index (CSWI) calculated from T
v
was predicted. Field data came from a wheat crop growing on a frequently irrigated plot (W) and a plot left un watered (D) until the soil water depletion reached 100 mm. Vegetation and soil surface temperatures were measured by infrared thermometers from tillering to physiological maturity, with meteorological variables recorded simultaneously. Stomatal resistances were measured with a diffusion porometer intensively over five days when the leaf area index was between 5 and 8. The T
v
predicted by the single-layer and the two-layer models accounted for 87% and 88% of the variance of measured values respectively, and both regression lines were close to the 11 relationship. Study of the effect of T
s
on the CWSI with the two-layer model indicated that the CWSI was sensitive to changes in T
s
. The overestimation of crop water stress calculated from the CWSI was predicted to be greater at low leaf area indices and high levels of stomatal resistance. The implications for this bias when using the CWSI for irrigation scheduling are discussed.List of Symbols
C
Sensible heat flux from the soil-vegetation system (W m–2)
-
c
l shade
Mean stomatal conductance of the shaded leaf area (m s–1)
-
c
l sun
Mean stomatal conductance of the sunlit leaf area (m s–1)
-
c
max
Maximum stomatal conductance (m s–1)
-
c
0
Minimum stomatal conductance (m s–1)
-
C
p
Specific heat at constant pressure (J kg–1 °C–1)
-
C
s
Sensible heat flux from the soil (W m–2)
-
C
v
Sensible heat flux from the vegetation (W m–2)
-
c
v
Bulk stomatal conductance of the vegetation (m s–1)
- CWSI
Crop Water Stress Index (dimensionless)
-
e
a
Vapor pressure at the reference height (kPa)
-
e
b
Vapor pressure at the virtual source/sink height of heat exchange (kPa)
-
e
0
*
Saturated vapor pressure at T
0 (kPa)
-
e
s
Vapor pressure at the soil surface (kPa)
-
e
v
*
Saturated vapor pressure at T
v
(kPa)
-
G
Soil heat flux (Wm–2)
- GLAI
Green leaf area index (dimensionless)
- GLAIshade
Green shaded leaf area index (dimensionless)
- GLAIsun
Green sunlit leaf area index (dimensionless)
-
k
Extinction coefficient for photosynthetically active radiation (dimensionless)
-
k
1
Damping exponent for Eq. A 5 (m2 W–1)
- LAI
Leaf area index (dimensionless)
-
LE
Latent heat flux from the soil-vegetation system (W m–2)
-
LE
s
Latent heat flux from the soil (W m–2)
-
LE
v
Latent heat flux from the vegetation (W m–2)
-
p
a
Density of air (kg m–3)
- PARa
Photosynthetically active radiation above the canopy (W m–2)
- PARu
Photosynthetically active radiation under the canopy (W m–2)
-
r
a
Aerodynamic resistance (s m–1)
-
r
b
Heat exchange resistance between the vegetation and the adjacent air boundary layer (s m–1)
-
r
c
Bulk stomatal resistance of the vegetation (s m–1)
-
R
n
Net radiation above the canopy (W m–2)
-
R
s
Net radiation flux at the soil surface (W m–2)
-
r
st
Mean stomatal resistance of leaves in the canopy (s m–1)
-
R
v
Net radiation absorbed by the vegetation (W m–2)
-
r
w
Heat exchange resistance between the soil surface and the boundary layer (s m–1)
-
S
Photosynthetically active radiation on the shaded leaves (W m–2)
-
S
d
Diffuse photosynthetically active radiation (W m –2)
-
S
0
Photosynthetically active radiation on a surface perpendicular to the beams (W m–2)
-
T
a
Air temperature at the reference height (°C)
-
T
b
Temperature at the virtual source/sink height of heat exchange (°C)
-
T
0
Aerodynamic temperature (°C)
-
T
s
Soil surface temperature (°C)
-
T
v
Vegetation temperature (°C)
-
w
0
Single scattering albedo (dimensionless)
-
Psychrometric constant (kPa °C)
-
0
Cosine of solar zenith angle (dimensionless) 相似文献