Distributed generation (DG) into a distribution system causes a variety of technical problems, such as local voltage rise, protection coordination, exceeding short-circuit capacity, and harmonic distortion . Distribution power systems have been changed from a radial system to a closed loop or mesh system according to the connection of distributed generation. Therefore, a distribution management system (DMS) should use environmental factors and closed-loop topology due to the output of distributed generation. A new Smart Distribution Management System (SDMS) has been developed to solve the problem of a distribution system for smart grids. As shown in Fig. 1, the role of the SDMS is to provide the real-time situation information and predictive operation of a distribution power system [2-3].
Fig. 1.Role of SDMS 
The accumulation and exchanging of information relative to the fault detection, loading, operation, and efficiency of the distribution system is an important part of the DMS [2-3]. It is not sufficient for the reliability and efficiency of the distribution system to obtain information only at the substation. It would be smarter and more informative if a few select points along the distribution feeder were providing real-time information on the state of the system at those feeder locations . In some instances, real-time data requirements to enhance smart grid operation may mean deploying more sensors on a distribution feeder
Then, in the late 1980s, as the vision of an automated distribution system began to surface , a need was recognized for distribution equipment such as switchgears and reclosers that could detect the voltages and currents of the distribution lines. Almost all the voltage sensors of distribution equipment are capacitive voltage dividers, which is a traditional PT with a transducer, because that is less expensive and less cumbersome to install .
So long as the data measured through the distribution equipment installed here and there is reliable, it may be used to resolve problems arising from connecting distributed generation. Additionally, if only accurate and reliable information about various places in the line can be obtained through the new DMS that is developed in response to the smart grids of the distribution system, it will provide higher performance by removing numerous assumptions in applying the DMS. However, in the case of the voltage measured especially through the distribution equipment, its value is very low, and many errors can occur .
As stated, the reason why the voltage data measured through the distribution equipment installed in the line is fraught with errors is that the distribution equipment is initially installed without measuring the voltage of the real lines where the distribution equipment is to be installed, and the voltage in question is set as the reference voltage of the distribution line. Moreover, even in the case where problems arise with the performance of the voltage sensor due to its deterioration inside the distribution equipment after some time passes, or where some trouble occurs with the process of transmission to the DMS through the Remote Terminal Unit (RTU) from the voltage sensor of the distribution equipment, there is no way of assessing or correcting it with an accurate standard. Hence, a system for periodically assessing and correcting the degree of accuracy of the voltage measurement value measured and transmitted through the distribution equipment installed in the distribution line is required for the smart grids of the distribution system and the new DMS.
First, in this paper, the errors in voltage measurement of the distribution equipment have been analyzed. Later, explanations are given of the concept and methods of the voltage measurement accuracy assessment system of the new system. In addition, this paper has verified the voltage measuring performance of the real distribution line of this new system, and through this process, field tests that can assess the accuracy of this system and verify its performance of correction have been conducted on the distribution equipment installed in the real distribution line.
2. Voltage Measurement Error of Distribution Equipment
Fig. 2 shows a single line diagram of one distribution line. There is no distributed generation or new renewable source in this distribution line; however, a look at the voltage of this line indicated on the DMS may reveal unusual aspects. In the line where only load exists, the voltage of the line must become lower the farther it is from the substation; actually, however, the voltage measured from the distribution equipment of the line considerably far away from the substation is higher than that measured from the line near the substation.
Fig. 2.Single Line Diagram of Distribution Line
Distribution equipment such as overhead and pad-mounted switchgears, reclosers, step voltage regulators (SVR), and line capacitors have sensors for detecting voltages, currents, fault current, active and reactive power, and harmonics. The DMS receives use information such as voltages, currents, fault indicator, and abnormal power quality from an RTU as shown in Fig. 3. The RTU has a higher accuracy of analog data acquisition and also analyzes abnormal power quality status, detects the abnormal power quality, saves the wave of PQ data, and sends the event of PQ status to the DMS .
Fig. 3.(a) Switchgear and (b) Remote Terminal Unit (RTU) 
Voltage and current are measured at the installation points of the distribution equipment and transferred to the central device of the DMS. In the case of measured voltage, its value is very low, and many errors can occur. Thus, it is currently very difficult for an operator to monitor the voltage status .
This paper decided to verify the voltage measurement errors with respect to the distribution equipment. It selected the switchgear that is installed and operated on the test distribution line in the KEPCO PT Center to check its voltage measurement errors. As shown in Fig. 4, it measured the values transmitted from the voltage sensor to the RTU of the switchgear by dividing voltage. Almost all distribution equipment is designed to transmit 4[V] to the RTU when the distribution standard operating voltage is 13.2[kV] by capacitive voltage divider. The standard operating voltages of the distribution system at the Korea Electric Power Corporation (KEPCO) are line to line 22.9[kV] and phase to ground 13.2[kV] at present.
Fig. 4.Measurement of switchgear voltage error
It measured 3.17[V] as the transmission value of the switchgear phase C to the RTU, as shown in Fig. 5. At the same time, it is measured 13,170[V] at the phase C distribution line by the standard voltage transformer, which has a 0.2[%] precision class. By switchgear voltage dividing ratio, when it is measured at 13,170[V], it transmits 3.99[V] to the RTU, not 3.17[V]. Therefore, it is explained that there is a 20[%] error in voltage measurement by the voltage sensor of the switchgear.
Fig. 5.Graph of switchgear voltage measurement data
The reason why voltage measurement errors have occurred with the switchgear is that real line voltage is not measured when the distribution equipment is installed at the distribution line first so that it is set to measure the standard operating line voltage uniformly. And after distribution equipment is installed at the distribution line, there are no methods to measure and assess the voltage measurement accuracy of the distribution equipment on the live line, despite the aging of the voltage sensor and controller in the distribution equipment by In addition, distribution equipment voltage measurement errors may occur even in the transmission process from the RTU, which picks up analog signals from the voltage sensor rather than the voltage sensor and the control section of the distribution equipment, and sends them to the central unit of the DMS; however, the actual state of things is that there is also no way of measuring or assessing them.
3. Voltage Measurement Accuracy Assessment System for Distribution Network
First of all, it is important to receive accurate real-time line voltage data for the reliability and efficiency of a distribution power system in a smart grid. Currently, only the existence of voltage is checked by a voltage detector and phase comparator for the distribution line or distribution equipment during maintenance. Therefore, it is necessary that the system assesses and calibrates the accuracy of the distribution equipment voltage measurement and transmission data. The new system is called a voltage measurement accuracy assessment system. It has to make a highly precise measurement of the distribution line voltage on live lines safely, easily and directly . In addition, it can measure the voltage of the distribution equipment installed in the field distribution line and check its transmission performance.
3.1 Main concept
The voltage measurement accuracy assessment system is designed as shown in Fig. 6. It is used to measure the voltage of the live distribution line part that is close to the installed distribution equipment with high precision safely, easily and directly. The voltage sensor of the system is like a live stick for the safety of the crew. In addition, the sensor communicates with the main assessment system by ZigBee wireless networking. It simultaneously receives the transmission voltage data measured by the distribution equipment sent to the distribution management system from the RTU of the distribution equipment with serial communication. Therefore, this system can assess the accuracy of the distribution equipment voltage measurement data based on direct voltage measurement data. In other words, the voltage measurement accuracy assessment system compares the values from the voltage sensor of the system to the data from the RTU of the distribution equipment.
Fig. 6.Configuration of the voltage measurement accuracy assessment system
If the data between the voltage sensor and RTU is not equal, it is necessary for this system to correct the errors. The data errors are caused by the voltage divider in the equipment and the RTU. As shown in Fig. 6, the calibrator detects transmission voltage from the voltage sensor of the equipment sent to the RTU and sends them to the main assessment system. Then, the system analyzes transmission voltage errors and provides them to the calibrator. The calibrator changes the resistance of the voltage divider of the equipment according to the errors until the main system assesses that the exact transmission voltage is delivered from the equipment to the RTU. After that, if errors between the voltage sensor data and transmission data of the RTU sent to management remain, the main assessment system directly corrects the errors of the RTU by serial communication.
Therefore, the voltage measurement accuracy assessment system can evaluate and calibrate the accuracy of the voltage measurement data of the distribution equipment based on comparing and analyzing the data from the voltage sensor, the calibrator and the RTU.
3.2 Prototype of system
The voltage measurement accuracy assessment system consists of a voltage sensor, main assessment system, calibrator and operating software. The voltage sensor is used to measure the medium voltage distribution line directly. The sensor can measure in a 0~25[kV] voltage range and has a 0.5[%] precision class. The measuring voltage data, which is converted from analogue to digital by the sensor, is sent to the main assessment system by wireless communication. The main assessment system performs a comparative analysis with the line voltage data from the voltage sensor of the system, the transmission voltage from the calibrator, and the voltage measurement data of the distribution equipment from the RTU.
The calibrator corrects voltage errors that are transmitted from the voltage sensor of the equipment to the RTU by using an automatic rolling device based on the analysis from the main assessment system. In addition, the operating software of the system carries out the following functions: network communication, calculation of the error ratio, monitoring of the communication condition and display. The function of the network communication is to acquire data between the main system and the voltage sensor, the calibrator and the RTU by ZigBee wireless and serial communication method. In addition, the operating system estimates the accuracy of the voltage measurement according to the error ratio from a comparison analysis of the three data sources. And it can monitor the condition of the communication with the voltage sensor, calibrator and RTU and display the output of the system.
As shown in Fig. 7, a prototype of the voltage measurement accuracy assessment system has been developed. The voltage sensor measures and displays the RMS values of the voltage. It has the ZigBee protocol and displays the connection condition with the main assessment system. It receives Global Positioning System (GPS) signals from the main system and synchronizes the time periodically and consistently.
Fig. 7.Prototype of the voltage measurement accuracy assessment system
The main assessment system is used to determine the degree of voltage measurement precision based on a calculation of the error ratio with the comparison analysis of the three data sources from the voltage sensor, calibrator and RTU. And it monitors the communication connection of each device consistently. The assessment and calibration process of the system is performed with a Human Machine Interface (HMI), and the results data are saved as a type of history data in the system according to time and equipment. This would help the crew to maintain the distribution equipment. It could minimize human error and maximize the reliability and the efficiency of maintenance for the distribution equipment. In addition, this system is portable and can run on batteries.
4. Field Test
Tests were carried out to ascertain the performance of the new system proposed in this paper. First, the basic performance of this system, the ability to accurately measure the voltage of the real line was confirmed in the test line at the KEPCO PT Center, thereby the voltage measurement accuracy of the distribution equipment installed in the distribution line on service in the field was verified, and finally, by correcting the site where errors in voltage measurement were found, the performance of this system was substantiated.
4.1 Voltage measurement test
Voltage measurement tests on live lines are the most important part of system performance. Therefore, the voltage sensor of the new system and a standard voltage transformer, as shown as Fig. 8, which has a 0.2[%] precision class, both measured the same spot of a distribution line at the same time. Then, the measurement data was compared.
Fig. 8.Standard voltage transformer
The voltage sensor of the new system made up for the weak points of the electromagnetic shielding to block the effect of other lines during the field test. In addition, safety tests for the sensor were carried out before the field test, because this system will be applied to live distribution lines. The safety tests were conducted based on IEC 61243-2, IEC 61481, IEC 60855-1, and IEC 61000-4-2, which are the standards of the voltage detector, portable phase comparators, and insulation solid rod, since they are not the same products [8-11].
As shown in Fig. 9, the voltage measurement test of the voltage sensor was performed at the distribution line. Simultaneously, the standard voltage transformer was measured at the same spot. The results of the voltage measurement test are as shown in Fig. 10. The comparative voltage measurement tests on the three phases of a, b, c of the distribution equipment installed in the Gochang test line showed that all were measured within the margin of error of 0.5% as compared with the standard voltage sensor. These tests demonstrated the performance and field usability of the voltage sensor of this system, which sets a standard for assessment and correction of the voltage measurement accuracy of the distribution equipment through this newly devised system. The standard voltage transformer was simultaneously measured at the same spot.
Fig. 9.Performance test of voltage measurement
Fig. 10.Test results of voltage measurement
4.2 Voltage measurement accuracy assessment
Based on the foregoing voltage measurement test results, a voltage measurement accuracy performance assessment was performed on nine pieces of distribution equipment installed in the distribution line being actually operated by the KEPCO business establishment using the prototype of this system made as shown in Fig. 7.
By comparing the values transmitted to the central unit of the DMS through the system’s voltage sensor and the RTU of the distribution equipment from each site for 5 minutes, the voltage measurement accuracy of each piece of distribution equipment was assessed as shown in Fig. 11. The assessment results are shown in Table 1. Based on the 5 minutes of assessments, a comparative analysis and evaluation was made to obtain maximum, minimum and average errors between the voltage value of real line measured by this system and the value transferred from RTU. As a result, despite the voltage sensor of the distribution equipment having a specification performance accuracy of 1%, it could be ascertained that a maximum of 7.9% of errors in voltage measurement and transmission occurred, as shown in Table 1. As mentioned in the foregoing, this is attributable to uniformly setting the measurement voltage of the distribution equipment to the standard operating voltage rather than to the actual line voltage at the initial installation of the line, and therefore, the occurrence of errors over time due to the deterioration of the voltage sensor of the distribution equipment and of those in transmission from the RTU leads to the production of distorted line data with which it is difficult to utilize the DMS.
Fig. 11.Field test for assessment at KEPCO branch office distribution line
Table 1.Results of assessment test
4.3 Field Test for Correction
Finally, voltage measurement accuracy assessment and correction tests under this system were conducted on the distribution equipment installed in the real distribution line. The voltage measurement accuracy assessment system evaluated every phase of the distribution equipment at the KEPCO branch office’s distribution line and corrected their errors.
When comparing voltage measurement data from the voltage measurement accuracy assessment system with the transmission data from the RTU sent to the distribution management system, all phases of the equipment had few errors, as shown in Table 2. Fortunately, this site was found to have no errors that may occur through data transmission from the RTU to the DMS. Then, the calibrator corrected errors using the automatic rolling device for changing voltage divider resistance based on the main assessment system’s analysis. After that, as shown in Table 2, every phase had transmission errors of voltage measurement under 0.5[%]. This field test verified the performance of the voltage measurement accuracy assessment system and its field applicability.
Table 2.Results of correction test
The voltage measurement data of the distribution equipment is an important part of a distribution system that has been changed to a smart grid. This paper has proposed a system for assessing and correcting the voltage measurement accuracy of the distribution equipment that is of such importance. Based on the measurement of the voltages of the real distribution line in a state of live-line working, this new system enables the identification and assessment, as well as correction, of errors in the voltage sensor of the distribution equipment and of those in data transmission from the RTU. This study analyzes voltage measurement errors by measuring the voltage data of switchgear installed on a test distribution line. Then, a voltage measurement accuracy assessment system was developed to evaluate and calibrate the voltage data accuracy of the distribution equipment. A field test of the voltage measurement accuracy assessment system was conducted to verify the performance of the system. The test results showed that the proposed system can provide solutions to problems with sensitivity. Moreover, the proposed system is deemed to enable the establishment of a new maintenance strategy that can increase the reliability and efficiency of the DMS.