Medium Voltage Cable Networks Research Articles

Influence of overvoltage waves on the development of partial discharges

A theoretical study of the development dynamics of partial discharges in defects in cable insulation of medium voltage networks in the presence of an overvoltage wave has been carried out. Simulation modeling of the development of partial discharges was performed on the basis of a model with three capacitors, supplemented by a model for the appearance of a seed electron. A single-phase short circuit to earth with several breaks in the electric arc was the source of the overvoltage wave. Simulation modeling showed that the number and power of partial discharges significantly increased over the period of the fundamental harmonic. The amplitude of the periodic component of the transient process was already enough to recharge the defect for large insulation defect and it is the highest harmonic that becomes decisive in the formation of partial discharges. This fact may be one of the explanations for a possible trend in medium voltage cable networks, where a failure on one cable is followed by a group of subsequent failures in nearby cable lines within a few days.

Countermeasures for reduction of screen currents due to cross country faults in MV cable distribution networks

• Cross-country faults can involve large currents liable to damage MV cable sheaths. • Methodologies to reduce cross-country fault currents in cable screens are presented. • Methods are compared by means of ATP-EMTP simulations on a 20 kV test network. • Additional ground wires are effective but viable only for newly built cable lines. This paper investigates the reduction of Cross-Country-Faults currents in medium voltage (MV) cable networks in order to decrease thermal stress of cable screens, especially inside joints. Four methods are analyzed: 1) disconnection of cable screens from the earth electrode of primary substation (PS); 2) buried ground wire laid near the cable lines and connected to each earth electrode of secondary substations (SSs) and PS; 3) buried ground wire and disconnection of the bonded cable screens from the earth electrode only at sending/receiving end of each cable line between two consecutive SSs; 4) cross bonding of cable screens between two consecutive SSs. Methods are analysed by ATP-EMTP simulations of a 150/20 kV radial test system formed by eight 5 km long cable lines equipped with 50 mm 2 copper conductors per phase and 16 mm 2 copper screen per cable. Compared to the limiting reactors countermeasure presented in a previous paper by the authors, which seems the solution less burdensome both technically and economically for existing lines, method 3 is more effective but its implementation is limited only to newly built lines, whereas the main drawback of method 1 is the increase of ground potential rise at fault locations.

Tracking, Erosion and Morphological Study of Heat Shrink Anti Tracking Tubes

Components of Medium Voltage (MV) cable network are very much vulnerable under operational stresses and ageing. Heat Shrinkable Anti-Tracking Tube (HSATT), an integral part of the Joints and terminations of the MV cable network system is used to cover and safeguard the power cable joints. In cable system joint and termination are the weakest but the critical part and fail easily under stress. The most threatening source of HSATT failure is electrical tracking. Electrical tracking develops when a conducting path across the HSATT formed under electric stress due to surface discharge. Arcs created from this surface discharge phenomenon burn the HSATT and create carbonized tracks in the long run. This paper reports the electrical tracking performance of three commercially available HSATT samples. Crosslinked polyethylene (XLPE) is the key ingredient for heat shrinkable materials. Electrical tracking using the inclined-plane tracking (IPT) method develops from surface discharge activity followed by erosion under wet and contaminated conditions. So, the material under IPT test faced electrical, environmental and thermal stresses. Among these three HSATT samples two samples failed to withstand these three dimensional stress factors. Dielectric breakdown strength and volume resistance tests were carried out to cross examine the IPT results and the results are identical. The morphology has been studied to understand the failure mechanism of HSATT samples. A morphological model is presented to scrutinize the IPT test failure mechanism and the rate of erosion propagation in the HSATT samples.

An analysis of remote voltage measurement in the medium voltage cable networks

Wydawnictwo SIGMA-NOT wydaje czasopisma fachowe informujące swoich czytelników o najnowszych osiągnięciach naukowych i nowoczesnych rozwiązaniach technicznych w Polsce i na świecie, popularyzuje problemy techniczne oraz poszerza wiedzę i kulturę techniczną.

The Proposal of a New Tool for Condition Assessment of Medium Voltage Power Cable Lines

A medium voltage (MV) cable network is a substantial component of the distribution network. Present management of this grid segment is mainly based on the failure rate analysis, i.e., a rise in the number and kind of faults on the actual line means that its technical condition is getting worse. The efficiency of the power system is low and additional costs of repair works, supply interruption, difficulties in the investment planning and operation and maintenance works are necessary. The aim of the R&D works done in the realised project is to implement the management of the MV cable network based on the estimated condition of the individual cable line, obtained from diagnostic measurements. The diagnostic investigations of the cable lines are the reference. Many years of research work have led to the development of the Health Index based on diagnostic, technical and service data.

THERMAL PHYSICAL ASPECTS OF DETERMINATION OF LOAD CAPACITY MEDIUM VOLTAGE POWER CABLES WITH CROSS-POLYETHYLENE INSULATION IN STATIONARY OPERATING MODES

Currently, the development trends of global medium voltage cable networks are aimed at the introduction of cables with cross-linked polyethylene insulation. Such cables have a higher operating temperature, which allows to increase their throughput, and increased stability when working in conditions of overload and short circuits. It is possible to determine the allowable heat and electrical loads only if the heat balance is observed. Fundamentally important for solving the problems of heat balance is the significant dependence of thermal conductivity and heat capacity of polyethylene insulation on temperature. It is established that the capacity of the cable, determined by regulatory models and average environmental parameters, provides only preliminary information about its heating. For certain requirements and for innovative cable designs, a necessary condition for determining the load capacity in real stationary modes of operation is the use of the heat balance model. A system of mathematical equations is proposed, which due to the combination of structural and thermophysical parameters of the cable with the conditions of the external environment allows to determine the parameters of the stationary thermal regime of the cable at any load current.

Influence of heating dynamics of polymer insulation of medium voltage power cables on their performance

Currently, global trends in the development of medium voltage cable networks are aimed at the introduction of cables with cross-linked polyethylene insulation. Such cables are innovative products of domestic cable manufacturers. Most often, single-core cables are laid in parallel in one plane. The article analyzes a number of significant features of cables with cross-linked polyethylene insulation, which affect the thermal regime both in working condition and in emergency operation, among which - the dependence of thermal conductivity and heat capacity of polymer insulation on temperature. Cable throughput, which is determined by regulatory models and average environmental parameters, provides only preliminary information about its heating, useful for comparing its throughput with other cables of the same type. For certain requirements and for innovative cable designs, it is necessary to determine the capacity according to thermal constraints in specific operating conditions. Thermal and electrical asymmetry must also be taken into account in the test conditions of cables intended for laying three single-core cables in the plane. The heating of medium voltage cables with crosslinked polyethylene insulation was studied [1], an experimental assessment of the influence of the distance between the cables on the asymmetry of currents and temperatures was performed using three single-phase single-core cables АПвЭгаПу 1х70/16–35with the same current direction. It is established that in the absence of longitudinal currents in electrically conductive screens, the current in the middle core can be practically taken as a basis. It is proposed to use the method of concentrated heat capacities for the analysis of cable heating by overload current, which allows to significantly reduce the test time in production conditions. The study aims to create a method for determining the allowable current of medium voltage cables in specific operating conditions.

Quality Evaluation of Voice Transmission Using BPL Communication System in MV Mine Cable Network

This article presents results of a quality evaluation study, considering voice transmission in a 6 kV medium voltage cable network using the BPL (Broadband over Power Line) communication system. The tests are carried out under real mining conditions for the selected power cable without voltage, earthed at both sides. Such a method of monitoring work conditions is of great importance, especially during a disaster. Power cables are potentially resistant to mechanical damage and, thus, ensure continuity of transmission, also, in cases of electricity stoppage and partial damage (interruption) of the phase conductors. The voice transmission quality is tested for a relatively low bitrate of 8 kbps–48 kbps using both induction and mixed induction-capacitive coupling with the power cable. Such a solution should provide both reliable and high-quality services, considering clear and understandable voice messages to people. The quality evaluation is carried out on a group of 15 people aged between 25–35 years old. The tested signal samples consist of voice messages in British English, German, and Polish. On the basis of the investigated results, respective conclusions are formulated.

Выключатели постоянного тока для многоподстанционных сетей постоянного тока

The article considers trends relating to the development of multisubstation DC (MSDC) networks in the form of cable and overhead power lines along with current and/or voltage converters, with DC circuit breakers serving as their key components. The requirements, circuit solutions, and main characteristics of capacitor-type DC circuit breakers were analyzed, and it has been shown from the performed analysis that it is possible to make them for medium-voltage devices. The DC circuit breaker test circuit arrangements need standardization and differ from each other for different manufacturers, a circumstance that adds difficulty to comparison of different makeup versions of these circuit breakers for selecting the optimal technical solutions. For application in medium-voltage cable MSDC networks, it is recommended to use cheap and well-matured domestically produced capacitor-type DC circuit breakers equipped with vacuum arc quenching chambers and an induction-dynamic drive, which satisfy the requirements of using them in MSDC networks.

Accurate on-line fault location and PD activity location results obtained with SCG – a long-term utility experience

In recent years, Alliander (one of the largest network owners in the Netherlands with almost 40,000 km of medium-voltage (MV) cable (polymer insulated lead covered and cross-linked polyethylene) installed a number of smart cable guard (SCG) systems to verify its effectiveness in preventing failures or, if there is a breakdown, to reduce the repair time (outage time). Alliander concluded that SCG is indeed a very effective tool for on-line detection and location of (intermittent) faults and partial discharges in MV power cables. These phenomena are located with an inaccuracy better than 1%, based on travelling waves. On average, each SCG system could have prevented about 6150 customer minutes lost per year. Preventive repair also allows seeing the cause of degradation, helping to develop counter measures. SCG can be used in all types of MV cable networks and replaces fault passage indicators as well. Based on this, Alliander will implement more SCG systems in the upcoming years.

Partial discharge diagnostic system for smart distribution networks using directionally calibrated induction sensors

Asset management and optimization is becoming increasingly adaptable for distribution utilities with the help of developments in the digital and wireless technology. Medium voltage cables, cable accessories (joints and terminations) and distribution transformers are most prone to the insulation defects. This paper presents an online partial discharge (PD) diagnostic system to detect and locate the weak insulation spots developed in the network components spread over the distribution area. The PD pulses originating and propagating from the defect site arrive at feeder joints, terminals and T-splices and are reflected from there. This makes the interpretation of the measured signals ambiguous in order to find the location of PD faults, while using conventional location techniques. An improved technique is introduced to monitor multi-section multi-branched cable network. Identification of the faulty location is carried out by using direction of arrival (DOA) of PD signals. The DOA of PD signals is obtained by using directionally calibrated Rogowski coil induction sensors. Polarity of captured PD signals with reference to supply voltage, determines their DOA. Rogowski coil along with its directional sensing feature is simulated in ATP-EMTP. The DOA technique is integrated over a medium voltage (MV) cable network and its performance is evaluated using ATP-EMTP environment. Frequency-dependent JMarti cable model is used for simulation. An intelligent algorithm is proposed for practical implementation of DOA technique for PD monitoring and automated location. Proposed scheme can be adopted in distribution automation system to improve proactive diagnostic capabilities of the network.

Impact of MV Connected Microgrids on MV Distribution Planning

The planning and development of distribution networks with a substantial penetration of microgrids connected to the medium voltage (MV) network form the main themes of this paper. The impact of microgrids is assessed in terms of their effect on optimal network topology, losses, reliability, reserve connections, network upgrade and expansion savings. The earning base of the distribution system operator also comes under scrutiny. A suburban MV cable network is planned using a network planning algorithm developed by the authors, first with optimal routing for demand-only nodes and then with a 33% penetration of randomly located microgrids. The network is then expanded to meet the requirements of a future planning horizon, in order to compare the expanded and upgraded optimum MV network topology with and without microgrids. Apart from visually depicting the topological differences, the savings such microgrids can give to the long term distribution network investment and running costs are quantified in terms of the investment costs, loss costs and interruption costs. When networks are planned with optimal rather than full backup, the introduction of microgrids is shown to have a considerable saving impact on all cost components except the cost per unit power transfer in the distribution network.

Partial discharge parameters to evaluate the insulation condition of on-line located defects in medium voltage cable networks

Partial discharge analysis has a long record as reliable diagnostic tool to assess the integrity and the quality of electrical insulation of power systems. On-line partial discharge monitoring systems for live medium voltage cable connections have recently been introduced in Dutch grids and also a few worldwide. To enhance the applicability of on-line cable diagnostics there is a need to develop a PD interpretation approach suitable for on-line monitoring. This paper discusses various PD parameters, including PD charge magnitude, PD occurrence rate and PD charge density and their related statistical values such as mean and maximum, used to interpret the PD activity. Trend watching of these parameters is employed to study the degradation state of cable connections and their components. The approach is applied to field data obtained over a year through monitoring several live circuits. In this paper, three examples are presented. Weak spots were detected and failures were prevented by early warning.

Influence of Ring Main Units and Substations on Online Partial-Discharge Detection and Location in Medium-Voltage Cable Networks

Online partial-discharge (PD) monitoring of medium-voltage power cables is traditionally performed on a cable between two consecutive ring main units (RMUs). PD location is achieved using a sensor at each cable end. Monitoring multiple consecutive cables, with RMUs in between, using a single monitoring system is more efficient. In addition, substations and RMUs without possibilities for sensor installation can be circumvented by installing the sensor at the next RMU. This paper studies how RMUs and substations along the cable under test affect online monitoring, including their influence on detection sensitivity, location accuracy, and charge estimate accuracy. Single-sided PD measurement, including PD location, with an RMU or substation at the far end is also considered. Models for RMUs and substations are proposed and verified by measurements. The performance of online PD monitoring is studied for a number of network configurations.

On-line partial discharge monitoring in medium voltage underground cables

For many years incipient partial discharge (PD) faults in power cables have been identified through off-line investigation techniques. More recently, in an effort to allow pro-active asset management of the medium-voltage (MV) cable network to be carried out, continuous on-line monitoring systems are being installed with the aim of reducing unexpected failures. This study presents work on the analysis and handling of acquired data, from the point of view of asset management and the PD activities observed in an on-line cable monitoring systems. Initially, a review of on-line against off-line cable PD monitoring is presented, in terms of their setups and their respective advantages and disadvantages. The study then presents the authors’ experience of applying wavelet-based denoising techniques [both the discrete wavelet transform (DWT) and the second generation wavelet transform (SGWT)] to PD data denoising. Results of a study on the on-line PD-based monitoring of MV underground cables that are presented in the following section demonstrate that PD activity which is observed in on-line monitoring also has an associated high level of electrical noise that must be removed to allow proper identification of the PD signals and the denoised PD activity is seen to vary with time. Finally, the necessity of developing a means by which knowledge rules can automatically be acquired from on-line condition monitoring data, to reduce reliance on human expertise, is discussed.

A new combinatorial algorithm for the optimal planning of distribution urban medium voltage cable networks

AbstractIn this paper a new combinatorial algorithm for the optimal planning of distribution urban medium voltage cable networks is presented. Realistic modeling requirements and an improved combinatorial method have resulted into an interactive software package with the proven performance on some actual distribution cable networks.

Loss determination for long three‐phase high‐voltage submarine cables

AbstractFor the planned, great offshore‐wind farms with extended medium‐voltage cable networks and distances to the onshore grid connection point of 100 km or more, the costs of the cables and their losses play an important role. In this paper basic equations are developed to determine the losses of long three‐phase AC cables, taking into account the longitudinal distributions of current and temperature. It is shown, that the determination of cable losses by means of the line equations based on the nominal ohmic resistance, i.e. in neglecting the temperature‐dependence, may lead to essential errors, with the risk of counterfeited valuations of the installation devices under technical and economical aspects. The relations derived in this paper allow a fast determination of the load‐ and temperature‐dependent cable losses by means of a few, only cable‐specific parameters.

Comparative analysis of security concepts for urban medium voltage cable distribution networks

The paper compares various security concepts with respect to urban medium voltage cable distribution networks, providing the minimum total annual costs including the outage costs. New models for the estimation of the duration of an interruption due to faults on underground cables, as well as for the estimation of the mathematical expectation of annual outage costs due to forced and scheduled outages, are developed. These models consider failures on all underground cable elements as well as various activities of the repair crew that seek and isolate the faulted section. The effects of different procedures for seeking the faulted section, including various types of fault indicators and automatic network control, are analyzed and compared. The proposed method is applied to several examples from practice for illustration.