农网改造要结合电网结构

农网改造的目的是提高电网供电质量和供电可靠性 ,降低电价 ,以减轻农民负担。网改后实行同网同价 ,农村电价要降到限价以内 ,此时如网损未得到下降 ,势必在一定程度上影响供电企业的经济效益。根据网损的构成 ,主要包括变损和线损。尽管网损率与电网运行方式、负荷分配、无功补偿、运行维护、检修质量、计量管理、电费回收等工作紧密相关 ,但网络结构也从多方面直接影响了配电网络的损耗。1　配电网络的主要评估指标(1)　主、配变电压器容量比 :主、配电变压器的容量比合理范围为 1∶ (2 .5～3.0 )之间。当低于 1∶ 2 .5时 ,说明该供电区主…     

农网无功分散补偿及经济效益

１问题提出 农村１０ｋＶ线路，由于点多，线长，面广，负荷季节性强，配电变压器空载损耗大，以及异步电动机大马拉小车等多种因素，因而农村电网的功率因数特别低，有的竟低至０．４以下，所以认真研究提高农村电网的功率因数是十分必要的。２ １０ｋＶ农网的无功分散补偿 在农村１０ｋＶ电网里，配电变压器的损耗占全部线路损耗的 ８０％以上。而变压器的损耗分为铁损与铜损，铁损与负荷无关，几乎是恒定量，在配电变压器额定负荷时铁损约占１／３。由于农村电网内配电变压器的负荷率很低，经常处于空载运行状态，所以配电变压器的铁损在…     

有载调容智能配电变压器的应用

配电网的节能降损是电网建设中的重要环节,根据不同地区电网结构、用电负荷的特点,合理选用配电变压器,实现多供少损,可以达到提高电网经济效益的目的。笔者现结合河北省任丘市电力局的应用实例,谈一下有载调容智能配电变压器的应用。1有载调容智能配电变压器的功能优势有载调容智能配电变压器具有大小两种额定容量,且两种额定容量运行方式可以     

降损措施与线损管理讲座（连载6）：合理选择变压器的容量和安装位置

1选择配电变压器容量的基本原则（1）要使配电变压器本身的电能损耗最小。比如在同样的用电负荷下，选用一台100kVA的配电变压器，要比选用同型号的两台50kVA配电变压器的损耗相应要小，而且，前者的价格也比后者的价格低廉。（2）要使配电变压器有较高的利用率。这就要求配电变压器尽可能地多带一些性质各不相同、用电时间不同的负荷，通过这些负荷的交替，来提高配电变压器的利用率。（3）要控制配电变压器的备用容量不能过大。如果不这样做，将增加设备投资和涡压设备的容量，而且，将使配电变压器的负载率更低，从而导致它们的无功励…     

合理选择配电变压器 提升农网改造升级水平

在农网改造升级工程中,配电变压器是农村低压电网的核心设备,就如同人的心脏一样重要,所以,配电变压器必须根据负荷需求等因素合理选配与安装。1选择配电变压器容量选择配电变压器容量的基本原则是根据电力发展计划确定,发展计划一般5年为一个周期,并且与国民经济发展相适应。所以,配电变压器容量发展计划年限一般取5年,计划年限确定后,选择容量时就要抓住以     

农网配电变压器容量选择及安装点定位

<正>配电变压器是构成农村电网的主要设备,对其容量的合理选择及安装点的正确定位,直接关系到农村电网的安全可靠运行。山西省朔州供电公司的农网改造实践证明,配电变压器容量选择与安装点定位工作应做到协调一致,有机配合,这样,不仅可以理顺工程流程,便于工程管理,而且对提高工作效率与工程质量、降低工程造价同样起到积极作用。     

配电变压器三相负荷平衡的调整方法

电网三相负荷平衡后,不仅能提高配电变压器利用率和电网末端电压,而且台区各相损耗一目了然,便于指导今后的工作,如反窃电、降损等.现就配电变压器三相负荷平衡调整方法谈谈个人的意见.     

浅议农村电网降损改造

农村电网降损关系到农电企业的经济效益。如何降低线损，提高能源利用率，笔者谈点看法，供参考。１ 对配网进行升压改造 取消６ ｋＶ电压等级的供电网络，将其升压为１０ｋＶ电压运行。在电网绝缘水平允许和符合电压质量标准的前提下，也可在原有电压水平的基础上，采取适当的措施，用提高中枢点的电压来减少电压降和降低可变损失。这是降低线损的有效途径，其效果也是显而易见的。根据计算，电网升压改造后的降损效果如表１：２ 改变变压器结构 选择新型节能型变压器，可以大大降低配电变压器的有功损失。从表２可以明显地看出其降损效果…     

提高用电设备功率因数的方法及效益

统计资料和理论计算表明,农村电网无功负荷主要集中在网络末端,配电变压器和异步电动机等用电设备的无功消耗占总无功负荷的90%.由于配电变压器和异步电动机负载率低,造成电网在低功率因数下运行.1 提高自然功率因数的方法1.1 正确选用异步电动机的型号和容量     

S7型配电变压器更新换代问题的探讨

变压器在变压和传递电功率过程中，本身要消耗电网有功功率和无功功率，其对网络损耗影响大小随电网情况不同而异。通过对福建省莆田市农村51条10kV配电线路理论线损计算的结果统计分析，我们发现，配电变压器的电能损失占总理论损耗的74％。因此，如何降低配电变压器的损耗，尤其是降低配电变压器的铁损已成为农村10kV配电线路降损的主攻方向。     

Application of drag reducing commercial and purified guargum for reduction of energy requirement of sprinkler irrigation and percolation rate of the soil

Drag reducing polymers reduce the drag in a turbulent flow while increase the drag in a laminar flow, due to an increase in shear viscosity. This feature of drag-reducing polymers has been utilised in reducing the energy requirements of sprinkler irrigation system and increasing the area of coverage as well as reducing the percolation loss of water added with drag reducing polymers. Two types of polymers at various concentrations were studied at the Indian Institute of Technology, Kharagpur. Two different methods of injection of polymers were also tested to determine the effect of polymer on mode of injection. The concentrations of 100, 250, 300 and 450 ppm of commercial guargum and 50, 100 and 150 ppm of purified guargum were used for homogenous injection (T₁). The concentrations of 100, 300, 600, 1000 and 1500 ppm of commercial guargum and 50, 100, 200, 350 and 500 ppm of purified guargum were used for injection at the suction side of the pump(T₂). The infiltration characteristics of the soil with polymer added water was tested for 300, 1000 ppm of commercial guargum and 100, 500 ppm of purified guargum. For T₁, the maximum power reduction of 28% was obtained in case of 300 ppm commercial guargum and approximately the same percentage was obtained in case of 100 ppm purified guargum. The maximum drag reduction was 35.5% at 300 ppm of commercial guargum and 38% at 100 ppm of purified guargum. The maximum increase in the radius of coverage at 500 ppm commercial guargum was estimated to be 33 and 37% at 150 ppm of purified guargum. For T₂, the maximum power reduction at 1000 ppm of commercial guargum and at 500 ppm of purified guargum was 31.25%. The maximum increase in radius of coverage at 1000 ppm of commercial guargum and at 500 ppm of purified guargum was 37 and 38.2%, respectively. The maximum drag reduction of 40% was obtained at both 1000 ppm of commercial guargum and 500 ppm of purified guargum. The reduction in the infiltration rate at 1000 and 300 ppm of commercial guargum was found to be 56.52 and 27%, respectively, whereas the reduction in the infiltration rate at 100 and 500 ppm purified guargum is 15 and 22.8%, respectively. The study reveals great potential of using drag reducing polymers for irrigation water management.     

Estimation of reference evapotranspiration for southern region of Saudi Arabia

The reference crop evapotranspiration (ET_r) for four areas in Saudi Arabia was estimated using five different methods: FAO-Penman, Jensen-Haise, Blaney & Criddle, pan evaporation, and calibrated FAO-Penman under local conditions (Penman_-SA). Comparison was also made between the estimated ET_r and the measured ET_r of alfalfa grown in lysimeters in the Riyadh area. Regression analysis revealed that estimated ET_r values were highly correlated with measured ET_r values. In addition, linear regression relationships between ET_r values estimated by the Penman_-SA method and other methods were determined. The results of this study indicated that the calibrated Penman_-SA method can be transferred successfully to other locations, and this method could be used for the estimation of ET_r values in all areas in the southern region of Saudi Arabia. Received: 16 January 1998     

Modelling of Discolouration of Dried Powder of Carrots

A model to represent the discolouration of dried powder of carrots as a function of temperature was developed. Two types of carrots, Asian and European, containing large amount of lycopene and β-carotene, respectively, were tested. The dried powder was stored under an atmosphere with O₂and at three different temperatures, 5,20 and 35°C. The surface colour of the powder was monitored during storage by a Hunter colour ratioa/b. In the proposed model, the rate of oxidation was assumed to be represented by the product of the fraction of concentration of oxidized pigment at arbitrary time and a specific rate of oxidation of pigment. The rate parameters in the proposed model were calculated by fitting the experimental data with smooth curves by trial and error at the end of the induction period but before discolouration took place. The frequency constant and activation energy, expressing the temperature dependence of the rate parameters, were expressed by the Arrhenius relation. The simulation results of a Hunter colour ratioa/bof dried powder at 25°C showed good agreement with the experimental data, suggesting that the proposed model was effective for the prediction of discolouration of dried powder of both types of carrot. The use of the induction period in the manner described may enable the shelf life of dried powder to be predicted.     

Economics of drip irrigation of olives in Turkey

Agricultural growers need investment and cost guidelines for drip irrigation to evaluate the economics of getting crops into production as quickly as possible and to minimise economic losses from drought during the productive life of an olive orchard. The benefits of irrigation may include; better olive survival, earlier crop production, greater yields, efficient nutrient distribution, less plant stress, reduced yield variability and improved crop quality.This research was conducted to help olive growers make decisions regarding investments in drip irrigation systems. This analysis was aimed at the farm business level to provide an economic rationale for investing in drip irrigation systems.The net present value (NPV) criterion was used to determine the discounted break-even investment results from published responses to drip irrigation systems. Growers with typical drip irrigation systems can expect investments of US$ 2244 ha⁻¹ with 1.6 ha blocks of olives. Analysis of survey findings indicate that net present value was US$ 3464 ha⁻¹ after an initial investment of US$ 2244 ha⁻¹.     

Evaluation of a crop water stress index for irrigation scheduling of bermudagrass

This study was conducted to assess crop water stress index (CWSI) of bermudagrass used widely on the recreational sites of the Mediterranean Region and to study the possibilities of utilization of infrared thermometry to schedule irrigation of bermudagrass. Four different irrigation treatments were examined: 100% (I₁), 75% (I₂), 50% (I₃), and 25% (I₄) of the evaporation measured in a Class A pan. In addition, a non-irrigated treatment was set up to determine CWSI values. The status of soil water content and pressure was monitored using a neutron probe and tensiometers. Meanwhile the canopy temperature of bermudagrass was measured with the infrared thermometry. The empirical method was used to compute the CWSI values. In this study, the visual quality of bermudagrass was monitored seasonally using a color scale. The best visual quality was obtained from I₁ and I₂ treatments. Average seasonal CWSI values were determined as 0.086, 0.102, 0.165, and 0.394 for I₁, I₂, I₃, and I₄ irrigation treatments, respectively, and 0.899 for non-irrigated plot. An empirical non-linear equation, Qave=1+⌊6[1+(4.853 CWSIave)^2.27]^−0.559⌋$Q_{\text{ave}}=1+⌊6{[1+{(4.853\text{CWS}{\text{I}}_{\text{ave}})}^{2.27}]}^{-0.559}⌋$, was deduced by fitting to measured data to find a relation between quality and average seasonal CWSI values. It was concluded that the CWSI could be used as a criterion for irrigation timing of bermudagrass. An acceptable color quality could be sustained seasonally if the CWSI value can be kept about 0.10.     

Estimation of Moisture Diffusivity Coefficient and Thermal Properties of Alfalfa Pellets

Heat and mass transfer characteristics of alfalfa pellets are needed in the optimization of coolers for freshly-made pellets and in managing storage schedules of the pellets in silos and bins. Moisture diffusivity and thermal properties are important parameters used to characterize the heat and mass transfer ability of a material. In this study, experimental thin-layer data on (a) moisture desorption, (b) moisture absorption and (c) rate of heating of alfalfa pellets were collected. By applying the inverse theory and using second order mass transfer and heat transfer equations in cylindrical coordinates, the moisture absorption and desorption data were used to estimate the moisture diffusivity as a function of moisture content of the pellets while the heating rate data were used to estimate the thermal properties (thermal conductivity and specific heat) as a function of pellet temperature.Better estimates were obtained when moisture diffusivity of the pellets was exponentially related to moisture content in comparison with a linear relation between moisture diffusivity and moisture content. Moisture diffusivity during desorption (2·40×10^-9to 4·12×10^-9 m²/s) was about three times that of the values of diffusivity during moisture absorption (7·50×10^-10to 1·26×10^-9 m²/s). A good fit to the experimental heating rate data was obtained when thermal conductivity and specific heat of the pellets were linearly related to temperature. Over a temperature range of 2 to 110°C, estimated particle thermal conductivities and specific heats of the pellets were in the range of 0·04 to 0·19 W/m K and from 962 to 2114 J/kg K respectively.     

Design of microirrigation subunit of minimum cost with proper operation

Matlab software named PRESUD (Pressurized Subunit Design) was developed to identify the optimum microirrigation subunit design using the annual water application cost per unit of irrigated area (C _T). This is defined as the cost per cubic meter of water applied to the soil for crop use, calculated as the sum of investment, maintenance, energy, and water (C _w) costs. In this study, only rectangular subunits are considered, using an iterative method for calculating the lateral and manifold pipelines. The infrastructure necessary for water delivery to the subunit inlet was taken into account in the price of water. The results indicate that C _w is the most important factor in C _T, which includes the investment and energy costs for moving water from the source to the subunit inlet. Other important factors, in order of importance, are the emission exponent (x), coefficient of variation of emitter manufacturer (CV_qmf), and emitter spacing (s _e). The minimum water application cost for a typical subunit to irrigate vegetable crops such as pepper is obtained with a subunit of 0.3–0.5 ha, with 80 m of paired lateral pipe length of 16 (13.6 mm) PE 0.25 MPa diameter, and 50 (44 mm) PE 0.4 MPa of manifold pipe diameter. The cost of a typical drip irrigation subunit design for a crop, such as grapevines on trellises, is equivalent to 25 % of the C _T of a typical subunit to irrigate vegetable crops, such as pepper.     

The effect of degradation of phalaris + white clover pasture on soil water regimes of a Brown Chromosol on the Northern Tablelands of NSW,Australia

The soil water regimes of a Brown Chromosol in response to drying and wetting are reported under three pastures types that were grazed all year long. The study was conducted at the Big Ridge 2 site near Armidale, on the Northern Tablelands of New South Wales (NSW) between 1994 and 1998. The three pasture types were degraded pasture (dominated by annual species), a phalaris dominant pasture, and an improved pasture containing phalaris + white clover. This study was conducted to assess the hydrological implications of losing perennial pasture species from the high rainfall (summer dominant) zone of south-eastern Australia. Pasture active rooting depth, water use and extraction during drying periods, and the ability of the soil profile to store water during wet periods were evaluated for each pasture type.Pasture active rooting depth, which affects water use, varied with season and water availability. During a typical autumn drying period between 1 and 22 March 1996, the phalaris + white clover pasture with an active rooting depth down to 100 cm used 46 ± 3.9 mm of water, with 16% of this extracted from the 55–130 cm zone. In the same period, the degraded and the phalaris pasture with active rooting depths of ≤60 cm used 30.7 ± 5.2 and 23.6 ± 7.9 mm of water, respectively, all from the surface 0–55 cm zone. However, under extreme drought conditions such as those in spring and early summer 1997 and autumn 1998, no differences in water use were detected between pastures.Pasture water use during dry periods affects the amount of water that can be stored in the soil profile and the potential amount of water loss during subsequent wet periods. In any wet period, the increase in soil water storage was greater in the 0–55 cm depth than in the 55–130 cm zone. For example, between 24 January and 14 February 1997 with total rainfall of 203 mm, water storage in the 0–55 cm zone was increased by 104.4 ± 6.7 mm under the phalaris + white clover pasture compared with 86.4 ± 4.3 and 84.4 ± 3.3 mm for the degraded and the phalaris pastures, respectively. The water storage increase in the 55–130 cm zone was not different between pastures (<12 mm).It was concluded that without appropriate grazing management and the presence of the legume component, the phalaris based pasture became unstable and failed to persist. The decline in the phalaris pasture caused invasion of annual species and weeds resulting in low water use, similar to that of the degraded pasture. In contrast, the combination of white clover and phalaris pasture showed a greater potential to maintain the phalaris component and a greater total biomass, and so was able to extract more water and from deeper parts of the soil profile. The vigorous phalaris + white clover pasture has greater potential to store more water than the degraded pasture and the phalaris pasture without legumes in the summer dominant rainfall area of temperate Australia. Therefore, maintaining pasture in good condition should be the main objective for sustainability of a grazing system in this region.     

Phenology based irrigation scheduling and determination of crop coefficient of winter maize in rice fallow of eastern India

Vast rainfed rice area (12 million ha) of eastern India remains fallow after rainy season rice due to lack of appropriate water and crop management strategies inspite of having favourable natural resources, human labourers and good market prospects. In this study, a short duration crop, maize, was tried as test crop with different levels of irrigation during winter season after rainy season rice to increase productivity and cropping intensity of rainfed rice area of the region. Maize hybrid of 120 days duration was grown with phenology based irrigation scheduling viz., one irrigation at early vegetative stage, one irrigation at tassel initiation, two irrigation at tassel initiation + grain filling, three irrigation at early vegetative + tassel initiation + grain filling and four irrigation at early vegetative + tassel initiation + silking + grain-filling stages. Study revealed that one irrigation at tassel initiation stage was more beneficial than that of at early vegetative stage. Upto three irrigation, water use efficiency (WUE) was increased linearly with increased number of irrigation. With four irrigations, the yield was higher, but WUE was lower than that of three irrigations, which might be due to increased water application resulted in increase crop water use without a corresponding increase of yield for the crop with four irrigations. The crop coefficients (K_c) at different stages of the crop were derived after computing actual water use using field water balance approach. The crop coefficients of 0.42–0.47, 0.90–0.97, 1.25–1.33, and 0.58–0.61 were derived at initial, development, mid and late season, respectively with three to four irrigation. Study showed that leaf area index (LAI) was significantly correlated with K_c values with the R² values of 0.93. When LAI exceeded 3.0, the K_c value was 1. Study revealed that the K_c values for the development and mid season stage were slightly higher to that obtained by the procedure proposed by FAO, which might be due to local advection.     

Assessment of the effect of tie-ridging on smallholder maize yields in Malawi

Tie-ridging is being promoted in Malawi as an on-field rainwater harvesting technique to ensure a maize (Zea mais L.) crop during a dry or drought year. Resource-poor smallholder farmers are likely to take up tie-ridging if it increases and not decreases maize yield in most years. A numerical study was conducted to calculate the expected maize yield gain due to tie-ridging taking into account the probability of occurrence of drought, dry, normal and wet years (climatic uncertainty). Mean yields due to tie-ridging in drought, dry, normal and wet years at different N levels were derived from observed smallholder maize yield data using a linear nitrogen response model and field-observed retained rainwater amounts in tie-ridged fields. Simulation results indicate that tie-ridging will result in hybrid maize yield gain in a drought year (1050 kg ha⁻¹) and dry year (560 kg ha⁻¹). There will be a hybrid maize yield loss in a normal year (350 kg ha⁻¹) and wet year (700 kg ha⁻¹). For local maize, there will be a yield gain in a drought year (500 kg ha⁻¹), dry year (220 kg ha⁻¹) and normal year (120 kg ha⁻¹). There will be a slight yield loss in a wet year (60 kg ha⁻¹). Considering observed probability of the occurrence of drought, dry, normal and wet years in Malawi, the study reveals that there will be no hybrid maize yield gain in any coming year with tie-ridging. For local maize, the expected yield gain in any coming year was positive (133.3 kg ha⁻¹) but this gain is less than the minimum gain required considering the opportunity cost of labour (142.5 kg ha⁻¹). Thus under the smallholder conditions and climate of Malawi, the expected yield gain in any coming year due to tie-ridging is likely to be minimal and uneconomic.