Resolving inconsistencies in three-phase current measurements

It is widely assumed that any unbalance between phase currents in 4-wire distribution circuits will equal the neutral current; that is that the sum of currents is zero. In practice, however, measurements made at distribution substations often show significant inconsistency. Several possible explanations for this are explored, showing that calculations of the voltage drop and losses are more accurate if harmonics are included. More generally, feeder configurations are described in which the currents may not sum to zero

Demand response behaviour of domestic consumers with photovoltaic systems in the UK: an exploratory analysis of an internet discussion forum

Background: Domestic consumers with photovoltaic (PV) systems in the UK can benefit financially by time-shifting their electricity demand to coincide with the output of the PV. This behaviour is a form of demand response and can provide insights into demand response behaviour more generally. This paper investigates whether people with PV in the UK engage in demand response, what appliances are used, and whether benefitting from free, self-produced electricity appears to influence their behaviour. Methods: To achieve this, the approach presented here consists of an exploratory text analysis of an internet discussion forum frequented by consumers with PV in the UK. Results: Data was gathered on 105 forum participants with PV, of which 45 mentioned engaging in demand response, for example by changing cooking or cleaning practices. Washing machines, dishwashers and electric space and water heaters were the most commonly used appliances for demand response. Six participants engaged in demand response and yet received no direct financial benefit from this behaviour, while 14 participants specifically mentioned the influence of free electricity. Conclusions: The results illustrate novel demand response behaviour compared to previous studies and indicate that while price may be an effective initiator for demand response, there are additional factors beyond price that can enhance responses. The discussion considers the application of these factors to the development of innovative demand tariffs for low-carbon futures

Impact of wind curtailment and storage on the Irish power system 2020 renewable electricity targets: a free open-source electricity system balancing and market (ESBM) model

The All-island power system, representing the electrical grids of the Republic of Ireland and Northern Ireland, has a target of supplying 37% of electricity with wind power by 2020. This presents a considerable integration challenge, in particular associated with the increasing number of periods when there is too much wind power and not enough demand on the system, requiring wind power to be turned off or ‘curtailed’. The authors previously estimated potential curtailment on the All-island system in 2020 using a novel model. The model was validated using actual wind output and curtailment data from 2011, and produced results for 2020 indicating curtailment levels ranging from 5.6% to 8.5% - consistent with previously published estimates. This paper expands the previously published model to include: simulation of dispatch of conventional generation based on merit order; a representation of variable prices within a wholesale electricity market; and the operation of electrical energy storage within the system. The model is used to estimate the installed wind capacity required to supply 37% of electrical demand and the potential for storage to reduce the capacity required to meet this target. Finally, the model has been adapted to MS Excel and made available to download for free

Impacts of demand data time resolution on estimates of distribution system energy losses

Copper losses in low voltage distribution circuits are a significant proportion of total energy losses and contribute to higher customer costs and carbon emissions. These losses can be evaluated using network models with customer demand data. This paper considers the under-estimation of copper losses when the spiky characteristics of real customer demands are smoothed by arithmetic mean averaging. This is investigated through simulation and by analysis of measured data. The mean losses in cables and equipment supplying a single dwelling estimated from half-hourly data were found to have significant errors of 40%, compared to calculations using high resolution data. Similar errors were found in estimates of peak thermal loading over a half-hour period, with significant variation between results for each customer. The errors reduce as the demand is aggregated, with mean losses for a group of 22 dwellings under-estimated by 7% using half-hourly data. This paper investigates the relationship between the demand data time resolution and errors in the estimated losses. Recommendations are then provided for the time resolution to be used in future measurements and simulation studies. A linear extrapolation technique is also presented whereby errors due to the use of averaged demand data can be reduced

Integrated simulation of photovoltaic micro-generation and domestic electricity demand: a one-minute resolution open source model

Domestic photovoltaic (PV) generation can partially offset the electricity demand within an individual dwelling. The net demand may be readily estimated on an annual basis but modelling its import and export, with respect to time, is more complex. A key issue is that domestic electricity demand, particularly lighting, is significantly influenced by the outdoor light level, which of course also has a direct effect on PV generation. Thus, realistic time-step simulation of the net demand requires that the two components are modelled with respect to a common representation of the solar irradiance. This paper presents the construction of an integrated model that provides data at a oneminute time resolution, built upon a fully validated high-resolution electricity demand model. An open-source software implementation of the integrated model in VBA within Microsoft Excel is described and is available for free download

Domestic photovoltaic systems, battery storage, and the economic impact of time-of-use electricity pricing

Time-of-use electricity pricing is characterised by high 'peak' prices, generally throughout the day and evening, and low 'off-peak' prices, generally at night. Consumers can benefit from time-of-use pricing provided their ratio of peakto- off-peak electricity consumption is less than a ratio of the relative prices of the two tariffs. To alter their consumption ratio, consumers can time-shift their demand, known as demand response. Consumers with grid-connected PV systems, however, already have reduced net demand during the day-time peak, due to the PV generation. The first question of interest to this paper is whether consumers with PV systems would benefit financially from switching to timeof- use pricing even if they do not engage in demand response. There remains the concern, however, of high prices during the evening peak, when the PV is not generating. Consumers unwilling or unable to engage in demand response during these periods can install battery storage systems, which are charged during the day and discharged during the evening. Two additional questions are therefore: what is the additional financial benefit of battery storage to PV systems with time-ofuse pricing and are batteries financially viable for domestic consumers with PV? These questions are answered using data from real dwellings with PV in the UK and simulating power flows using a published lead-acid battery model. Economic impacts are measured for a range of time-of-use pricing tariffs from the UK and Ireland. Results indicate that PV has little effect on the financial benefit of time-of-use pricing with day period prices that are similar to the flat rate price. For tariffs where the day period price is greater than the flat rate price, PV improves the benefit, but not enough to make it an economic choice for the average consumer. Battery storage improves the financial return, but this is not enough to make the business case positive. Even using optimistic assumptions, such as lossless batteries and high electricity price increase, system costs need to be lowered by at least 33.5% for lead-acid systems, and 195% for lithium ion systems

Assumptions and approximations typically applied in modelling LV networks with high penetrations of low carbon technologies

Uncertainties in the assessment of LV network capacity to accommodate PV and other low-carbon technologies can lead to installation constraints or costly network reinforcements that may not be entirely necessary. This paper reviews the numerous assumptions often used in such assessments and highlights those relating to time resolution of demand models, harmonics, network grounding and impedance modelling as being particularly questionable. In many cases, the individual assumptions may be low risk, but there is greater uncertainty when assumptions are applied in combination

Series impedance of distribution cables with sector-shaped conductors

Models of power distribution networks require accurate cable impedance data. For unbalanced networks, both the self-impedances and the mutual impedances are needed. However, published studies use differing approaches to determine cable impedances, leaving uncertainty over the level of detail required. This paper compares impedances provided by the manufacturer with those from several analytical methods, showing the impact of modelling the non-circular geometry and of including corrections allowing for the AC resistance. The analysis is compared to results from a freely available finite element solver where the current distribution is modelled in detail, taking account of eddy currents and the rotation of the cores relative to the neutral due to the cable lay. At 50 Hz, the analytical methods provide a good approximation but the finite element results show that eddy currents affect the impedance at harmonic frequencies. The results also show the impact of including the ground path in the impedance calculation. The current distribution in the ground has a wide cross-sectional area, suggesting that the assumption of a perfect multi-grounded neutral is inappropriate for LV networks with short cable lengths

Accurate determination of distribution network losses

Distribution losses can be difficult to measure when the loss power has a similar proportion to monitoring equipment sensor tolerances. Measurements for the Western Power Distribution Losses Investigation project have addressed this problem by using high resolution demand measurements combined with loss calculations with an I2R method. This is combined with additional measurements to verify accuracy of the network database used to calculate losses in each network branch

Modified operation of a small scale energy recovery device for seawater reverse osmosis

The Clark pump reciprocating pressure intensifier is a well established mechanism for highly efficient brine stream energy recovery in small scale seawater reverse osmosis (SWRO) desalination systems. This paper describes operation of a modified Clark pump in which the roles of the two pairs of chambers are reversed and the general arrangement of the complete RO system is substantially altered. In particular, the low-pressure motorised pump that feeds into the standard Clark pump is replaced by a high-pressure motorised pump that sits in parallel with it. A conceptual comparison of the original and modified arrangements is presented, followed by a discussion of the practical modifications made to a standard Clark pump in order to test the concept. The initial tests were successful and results indicating specific energies in the range 3.5 to 4.5 kWh/m3 are presented