Pirjo Heine

A method for estimating the frequency and cost of voltage sags

The annual number and cost of voltage sags were determined for five Finnish distribution companies. The method of fault positions was applied for the calculation of voltage sag frequency. The economic consequences were assessed by multiplying the sag frequency and cost by the number of customers. The cost of a single sag was taken from a survey that had been carried out in three Nordic countries in the mid 1990s. The paper proposes a method for retrieving sag-related information from statistics originally prepared for other purposes. In addition, the paper provides an estimate of the total annual sag-related cost for each of the companies considered in this study and for each customer category. The total cost per company appeared to be much higher than had generally been assumed.

Clustering of Connection Points and Load Modeling in Distribution Systems

The lifetime of transmission and distribution power systems is long and thus, long-term plans are needed for their successful development. In generating long-term scenarios, the starting point is the analysis of the present electricity consumption. The data of electricity consumption will become more exact by the end of 2013, when hourly based automated meter reading (AMR) consumption data will be received from each customer in Finland. The amount of data is huge and powerful analysis methods are needed. This paper presents a method for clustering the electricity consumptions using principal component analysis (PCA) and K-means clustering. AMR data of 18 098 customers from two city districts of Helsinki, Finland is applied for a case study reported in this paper. A multiple regression analysis is also carried out on the two largest clusters to find the most important explanatory factors for the load modeling. The interpretations of the clusters and the plausibility of the regression coefficients are considered very important. Five distinct and meaningful clusters are found. The regression models give interesting insights into the explanatory factors behind electricity consumption. The models of the main customer groups assist the distribution system operator (DSO) in the long-term development of the power system.

Voltage sag analysis taken into account in distribution network design

Voltage sag distributions were calculated for six different 20 kV electricity distribution systems. Results are highly dependent on the distribution system. The percentage of deep severe sags is much higher in urban systems while the annual number of sags is bigger in rural areas. AM/FM-GIS has proved to be an efficient tool in distribution system planning. Often costs are included in these studies. As a new development step, sag analysis was introduced to be integrated to these tools. A pilot software has already been developed in this project.

Neutral point treatment and earth fault suppression

Earth fault management in medium voltage networks involves assessing of customer interruptions, challenges in fault clearing processes and safety issues. The major advantage by allowing sustained earth faults is the avoidance of customer interruptions. However, the network operation with sustained earth faults does not only include welfare. The paper will generally discuss the advantages and disadvantages of operating the network with sustained earth faults. In addition, in the paper, the Helsinki medium voltage network and the developments in neutral point treatment and network operation during earth faults will be outlined. The paper presents the earth fault analysis in the Helsinki 10 kV network and compares the results to a reference analysis. Based on the analysis, the paper presents the possibilities and limitations of a sustained operation with earth faults. The paper also presents the strategy steps of managing sustained earth faults as well in the 10 kV and 20 kV areas and the required long term development steps.

Measured faults during lightning storms

Lightning storms may only last some hours, but the number of faults during these storms can represent a considerable share of the annual disturbances. In this paper, six identified lightning storms were the main interest. In general, single-phase earth faults are the most typical fault type. However, in the lightning storms we researched, short circuits represented the majority of faults. The majority of faults, both during lightning storms and in normal conditions, are cleared by high-speed auto reclosure. In addition, it was observed that during lightning storms coincident faults and fault types that develop into another fault type before the opening of the circuit breaker occurred repeatedly. When observing the influence of overvoltage protection type on faults, the results encourage the use of surge arresters.

Cost-causation based approach in forming power-based distribution network tariff for small customers

This paper discusses the development of electricity distribution network tariffs. In the paper, the present state of distribution network tariffs of small customers is discussed together with key future challenges of the distribution network business. As a potential solution to these challenges, power-based tariffs of the small customers are discussed and preliminary results of a case study are presented where tariffs are formed by applying data of one Finnish Distribution System Operator (DSO).

Overvoltage protection, faults and voltage sags

Distribution transformers in rural networks have to cope with frequent lightning induced overvoltages and thus must be protected using spark gaps or surge arresters. The overvoltage protection type has an influence on the system behaviour and on the interruptions and voltage sags experienced by power system customers. In this paper, measurement results showing the effect of overvoltage protection type on established faults in an unearthed medium voltage overhead line network are presented. Detailed results of fault frequencies and fault types are shown. Further, a probability function is established to describe the influence of overvoltage protection type on voltage sags experienced in the network. This latter aspect is important when limiting the effects of interruptions and voltage sags for sensitive customers.

Economic analysis of implementing novel power distribution automation

Three solutions for novel distribution automation implemented in medium voltage networks were developed as a theoretical study. The distribution automation studied included intelligent local automation arrangements also useable for ring-type networks. The profitability of the new technique was evaluated by comparing the costs of the automation investments to the savings obtained in the economic impact of interruptions and voltage sags calculated for four different networks. The results showed that the new technique offers promising possibilities in limiting the effects of interruptions and voltage sags. The degree of automation must be adapted to each network construction. The selection of the adequate degree of automation assures the operational capability of the network company.

Short interruptions caused by lightning in overhead line medium voltage networks

Lightning strokes represent a considerable cause of short interruptions in overhead line medium voltage networks. The overvoltages caused by lightning can not be avoided but their influence can be limited by appropriate overvoltage protection. This paper presents the results of theoretical studies about the number of short interruptions caused by lightning in various environmental circumstances. The results will be compared to the fault statistics collected from Kainuun Energia. In the calculations, the environmental conditions, ground flash densities and various overvoltage protection types of power distribution transformers are taken into account. At the end, further research is discussed.

Probability of Voltage Sags Depending on the Characteristics of Medium Voltage Networks

This paper presents how the characteristics of medium voltage (MV) networks affect the probability of the voltage sags experienced by a low voltage customer. Faults occurring in the MV level are typically an important origin of voltage sags. Thus, when aiming to lower the sag probability the characteristics of the local MV network have an important role. This paper concentrates to various conventional solutions of network components and connections. An own primary transformer and an MV network with underground cables of minimum feeder lengths is a superior solution for a sag-sensitive customer. In a case of overhead line MV networks, surge arresters should be used for overvoltage protection of pole-mounted power distribution transformers. Surprisingly, not necessarily all stages of the overall development of power distribution networks result to a lower probability of voltage sags. Mixed- overhead/underground cable-MV networks and some stages of distribution automation included characteristics that need a careful sag analysis