Diagnosis of defects on medium voltage electric energy distribution networks: The case of rural zone’s supply

Electrical Power and Energy Systems 45 (2013) 229–234

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Diagnosis of defects on medium voltage electric energy distribution networks: The case of rural zone’s supply Joseph Voufo 1,⇑, Joseph Kenfack 2, Thomas Tamo Tatietse 3 Ecole Polytechnique, BP 8390 Yaoundé, Cameroun, France

a r t i c l e

i n f o

Article history: Received 23 October 2008 Received in revised form 20 August 2012 Accepted 29 August 2012 Available online 13 October 2012 Keywords: Electricity supply High Voltage A network (HVA) Operation parameters Diagnosis Electricity faults

a b s t r a c t An analysis carried out on the High Voltage A (HVA) electricity power distribution network run by Cameroon’s AES-SONEL company shows that losses are very high due to energy which is produced but not distributed and that the duration of power interruptions as a result of the defects is long due to the time used in searching for these defects, particularly in rural zone’s supply. Given that quick detection of defects is a sure means of improving availability and productivity in any company, we hereby propose a system of real-time diagnosis of the defects on a rural network’s supply of AES-SONEL’s electric power distribution network, the case of. After an inventory of typical defects on electric power networks and the proposal of a tool for their identification, we propose a system for the detection and localization of these various failures. The implementation of the system on a Programmable Logic Controller (PLC) enables the performance of the system to be assessed. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Electricity supply networks are subject to various disturbances because of the means of production, alongside atmospheric conditions and industrial usage which affect them during transportation and the distribution of the energy produced. The control and management of these shortcomings are of primary importance for reasons of reliability, availability, maintainability and effectiveness of the network, as well as for the safety of persons and property. The power network supervision and control in most developing countries is manual. This method of management has a high annual rate of energy interruption (more than 10%) as compared to the rate of energy interruption in developed countries (less than 5%) [2] who use an artificial intelligence based network. Studies made on the AES-SONEL electric power distribution network, with focus on the lower network of the Ngousso station (Yaounde Cameroun) during the period from June 2005 to April 2006 [1], show that the company recorded 1.315.144 kWh of undistributed energy; furthermore it was noticed that the D31 circuit breaker (supply of Monatélé) tripped 54 times for this period, this led to 398 hours of power cuts, an average of 7 hours per power cut [3]. According to the experimental data collected by the Network

⇑ Corresponding author. Tel.: +33 237 77 73 10 56. E-mail addresses: [email protected] (J. Voufo), [email protected] (J. Kenfack), [email protected] (T. Tamo Tatietse). 1 Tel.: +33 237 77 73 10 56. 2 Tel.: +33 237 77 50 00 60. 3 Tel.: +33 237 22 22 45 47. 0142-0615/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijepes.2012.08.059

Load-dispatching Centre (NLC), there is a typically a time lapse of between 50 minutes and 2 hours from the detection of the defect to the beginning of the search for the solution. In the case of loss of phase, it is usually consumers who inform the NLC of the cut [4]. As for the duration of the search for a defect, this depends on the distance from the sub-station to the location of the fault, since the search is done manually. It also depends on the type of network (urban or rural) and on the time of day when the fault occurs. To ensure power availability throughout the network and at all times, AES-SONEL should have a reliable system for the control and management of these defects. This availability requires a reliable system of diagnosis which is an important first step to detection and localisation of defects. Such a system will be of paramount importance in contributing to early and rapid detection, improving availability and productivity of the network equipment as well as the profitability of capital invested [5]. The purpose of this research is to design an automatic system for the diagnosis of defects on the supply’s network of Monatélé. After listing the various potential defects in the electric power network, a tool is proposed for their identification. This is followed by a proposal of an automatic system for the detection and localization of defects as well as the results obtained during system implementation. 2. Types of defects, the characteristic values of the network and identification of defects on electric power network The various types of defects common in electric power networks are listed and the algorithms for the monitoring of characteristic network parameter are explained.

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S ¼ P= cos u

2.1. Various types of defects

The algorithm of the calculation of S is established. Once kept in memorized, this data is scrutinized every 20 ms to detect a possible overflow of the preset thresholds or an abnormal operation of the installation. The above samplings enable us to identify the defects on the electric network of power. The sampling of voltage and current can be taken in a specific point of the network by the Phasor Measurement Unit (PMUS). The PMUs are the devices capable of measurement synchronous real-time voltage and current phasors in power system [12]. The measurements in real time of the PMUs must be synchronized with the GPS. Unfortunately, the use of the GPS is not even current in our country.

A defect occurs when there is a discrepancy between the reference value and the measured value for any given network parameter [6,7]. In this regard, the electric power network is subjected to four main types of faults: short-circuits, over voltages, overloads and line ruptures (loss-of-phase). 2.2. Collecting characteristics parameters of the network With the help of current and voltage transformers, samples are taken at a specific point of the network, at predetermined intervals. According to Shannon’s theorem, a sampling rate that is at least equal to twice the frequency of the original signal is good enough. The voltage between phases is sampled as follows: Effective voltage is obtained by getting the average of the squares of samples taken at regular intervals. Then the square root of this average is calculated. A sample is taken every 20 ms, the square root is calcuN lated and kept in memory. After 20 ms which correspond to the period, the average of all the samples taken is calculated, then the square root of this average gives us an approximate value p offfiffiffithe effective phase-to-phase. For N = 10 and uðtÞ ¼ 15; 000 2 sin 100pt, Table 1 can be obtained. This is within the 5% tolerance rating stipulated by AES SONEL. For a higher degree of accuracy, the sampling interval is fixed at 1 ms (N = 20). The algorithm for the calculation of phase-to-phase voltage is established.

2.3. Identification of defects on electric power network This identification shall involve defects on overload, homopolar current and the loss of phase. 2.3.1. Identifying line overload defects We must define the various thresholds of current intensity and times at the end of which a defect indication must be issued. We have three thresholds of overload. The first corresponds to operating current higher than 140% of the nominal intensity during 500 ms; the second is that in which the current is higher than 180% the nominal intensity during 100 ms; the third corresponds to the current of minimal short-circuit during an instantaneous time. The calculation algorithm for the identification of an overload defect is shown in Fig. 1.

2.2.1. Sampling of phase current I Given that the current signal period is identical to that of the voltage, the methodology is exactly the same as that used in sampling the voltage. By replacing U with I in the calculation algorithm value of U, one obtains the calculation algorithm of the value of I.

2.3.2. Identifying homopolar current defects Homopolar current is the algebraic sum from the instantaneous values of electrical currents on the three phases. The threshold of the homopolar current is selected between 130% from I0 and 10% the nominal intensity. I0 is the value of the current due to the ground-phase capacities at the upper level of the network. The calculation algorithm for the identification of homopolar current defect is shown in Fig. 2.

2.2.2. Sampling of active power P This is a signal with a period of 40 ms. This requires 40 samples of the current and the voltage to be obtained simultaneously. The sampling interval is therefore 1 ms. The sampling method is that of two wattmeters, since the neutral is not being distributed. An approximate value of the active power is given by the average of the products of samples taken. The current is taken on phases 1 and 3. The voltage is taken between phases 1 and 2, then 2 and 3. The power P12 is calculated between phases 1 and 2 and then the power P23 between phases 2 and 3. The power P = P12 + P23. The algorithm of the calculation of P12 and P are established.

2.3.3. Identifying loss- of- phase defects There is loss of phase when at the upper level the three phases are live, whereas two phases only are having current. The calculation algorithm for the identification of loss-of-phase defect is shown in Fig. 3. The measurements of PMUs [13], Artificial Neural network (ANN) [14] and the complex space-phasor and Hilbert–Huang transform [15] can be used to identify the various types of defects.

2.2.3. Sampling power-factor cos u This follows from three preceding measurements when none of them is not on zero. Because

pffiffiffi cos u ¼ P= 3UI

ð2Þ

3. Proposing a system for the detection and localization of defects on an electric energy network

ð1Þ

To identify any defect, the processing unit needs to have all the characteristics of the network in real time. These characteristics are furnished to the treatment unit by detection modules placed in a specific point of the network and which must fulfil the following requirements:

The algorithm of the calculation of cos u is established. 2.2.4. Sampling of power S This follows from the measurement of P and cos u

Table 1 Evaluation of the effective value of U(T) starting from samples taken every 2 ms. I

1

2

3

4

5

6

7

8

9

10

U[i] (kV) Square (U[i]) Total U (kV)

0 0 224,986 14,9994

12,469 155,475

20,174 406,990

20,174 406,990

12,469 155,475

0 0

12,469 155,475

20,174 406,990

20,174 406,990

12,469 155,475

J. Voufo et al. / Electrical Power and Energy Systems 45 (2013) 229–234

231

Fig. 1. Identification process of high intensity default.

3.1. Localizing the defects To meet the requirements prescribed for the detection modules, we propose as in the case of the supply’s network of Monatélé that the portion of search for defect (distance between two detection modules) is limited to 1 km in the portion without nodes and that the detection modules is placed to every network nodes. For the sending end D31 (supply’s network of Monatélé), which we use as example, forty-five detection points were defined. They are indicated in Fig. 4. Many studies have been done for the placement of the detection modules in the case of the use of the PMUs [16]. 3.2. Current and voltage sensors

Fig. 2. Identification process of homopolar current default.

– Be able to forward to the NLC the exact geographical position of the defects which they detected. – Be able to disconnect and accurately isolate the defective portion of the network, and as such as limit the number of customers not having electricity. – Be able to delimit the portion where technicians shall look for the defects.

Sensors are used to provide electric current and voltage to the treatment system in real time. Three voltage transformers are used to record voltage at each point where the detection module is placed. These transformers are assembled together between phases. The transformation ratio depends on the input voltage of the treatment system. To protect the voltage entries, one thus uses a resistance adaptation of Ra whose value depends on the output voltage of the transformer U, on the input impedance of the voltage module Re and on the maximum entry voltage of the module Umax.

Ra ¼

ReðU  U max Þ U max

ð3Þ

As for the current magnitude, the current transformer with two windings is used on each phase: the first winding is shunted and

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Fig. 3. Identification process of loss phase default.

Fig. 4. Positions of detection modules.

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Fig. 5. Module diagram for the detection of current and voltage.

 Calculating the other parameters of operation of the part of the network.  Deducing the operating condition of the part of the network (be able to indicate if the portion is under normal operation or if a defect was detected thereon).  Identifying the defect in of the part of the network and communicating it to the NLC.

connected to the treatment module to record current by phase, while the second winding is dedicated to the detection of homopolar current. All the transformers are star-connected. The transformation ratio now depends on the load. The shunt resistance Rs, depends on the output current of the current transformer I, on the input impedance of the module Re and on the maximum value of the input current of module Imax.

Rs ¼

Imax  Re I  Imax

The number of detection modules, the distances between these modules, as well as the distance between the modules and the NLC, all put together do not still enable us to establish a physical connection between the modules on the one hand, and between the modules and the NLC on the other hand. A wireless communication must still with a general access point at the level of the NLC. The data processing system at the point of detection must then have a data treatment unit for analogue inputs/outputs, for numerical inputs/outputs, and a device for wireless communication. It can then be done by ADLS MODEM, radio or GSM. The different possibilities offered for the implementation of the system processing include microprocessors, microcontrollers, Programmable Logic Controller [17], or all other systems able to make undertake such processing.

ð4Þ

The assembly diagram of these transformers is as in Fig. 5. 4. System for the processing of data and implementing the system of diagnosis 4.1. System for the processing of data This data processing system with is defined for a well defined portion of the network; it shall be capable of:  Recoding current and voltage for that part of the network. CT1 400/1A

I 630 A

L1

CT2 400/1A

L2

0

BO S1

1

S4 S2 S3

KA1

KA1

+IV3

+IV2

5/TSX DEY 16A4

COM3

19/TSX DEY 16A4

6/TSX DEY 16A4

4/TSX DEY 16A4

2/TSX DEY 16A4

3/TSX DEY 16A4

TSX P57

100V~/48V-

Fig. 6. The system diagnostic test.

KA1

KA3

20/TSX DEY 16A4

19,8MΩ

19,8MΩ

KA2 9/TSX DEY 16A4

1/TSX DEY 16A4

COM1

19/TSX DEY 16A4

19/TSX DEY 16A4

3,56Ω

COM2

19/TSX DEY 16A4

+IV1

+IC3

3,56Ω

TP3 15kV/ 100V

100V

S5

0,16Ω

+IC2

+IC1 3,56Ω

VT215kV/

100V

19,8MΩ

0,16Ω

VT1 15kV/

COM3

19/TSX DEY 16A4

0,16Ω

CT3 400/1A

COM2

19/TSX DEY 16A4

COM1

19/TSX DEY 16A4

L3

+Vcc

D1 48V

D2

COM

RS232

MODEM +Vcc RS232 COM

KA2

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The installation of the fibre optic started in Cameroun. At the end of this installation, the communication between modules will be able to be done by fibre optic.

4.2. Implementing the system of diagnosis The diagnostic system was implemented using the TSX Premium Programmable Logic Controller (TSX P57352) available in our laboratory. The module processor of this equipment authorizes a connection by modem via a RJ45 port and a RS232 port. For the collection of the electric input quantities, we used analogue input modules, while for the control of the actuators, we used output modules with Direct current [8–11]. All these elements enabled us to carry out the system diagnostic tests (Fig. 6). The PLC permanently scans the network data and calculates its operation parameters. When a defect is detected, the information on the said defect, its position and the type of defect are sent to the NLC. The operator at the NLC activates the circuit breaker before the defect, thereby disconnecting the section where the defect is found. If a module identifies a defect, all the modules which precede it should not be activated and all the modules surrounding the defect zone must be opened by the network supervisor, thereby isolating the faulty section permanently. The circuit breakers of unaffected areas with blackout are re-closed.

5. Conclusion From a preliminary analysis, we highlighted the link between voltage and current during defects which usually occur in medium voltage distribution networks. These two parameters were used in setting-up a system for the diagnosis of defects. The results from the processing of such elements by a PLC have shown that it is possible to determine the state of the supervised zone and also the actions to take. We have shown that it is possible to reduce the durations of power failures resulting from defects in the MV distribution network, by equipping the network with modules that can detect the presence of a defect, identify it, locate it and isolate the affected zone. The reduction of the losses is significant in terms of time. With every tripping of the circuit breaker, the network controller will have all the necessary information in real time. The implantation of such detection modules is salutary for the population of Monatélé and a company which produces and sells electric energy such as AES-SONEL. The next use of the fibre optic will permit to reduce these losses and the cost of the installation of the detection modules. However, the coordination of these modules is another aspect in the setting-up of an automatic network management system. There will inevitably be a problem of communication between the PLC placed in the network and the supervisory PC placed at the NLC, as well as the different protocols having to govern these exchanges. Thus, for a better performance, this aspect could be handled within the scope of another study.

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Glossary ADSL: Asymmetrical Digital Subscriber Line ANN: Artificial Neural network GPS: Global Positioning System GSM: Global System Mobile HTA: High Voltage A MODEM: Modulation–Demodulation MV: Medium Voltage network NLC: Network Load-dispatching Centre PC: Personal Computer PLC: Programmable Logic Controller PMUs: Phasor Measurement Unit