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Utilization of optical sensors for phasor measurement units
Electric Power Systems Research 156 (2018) 12–14
Contents lists available at ScienceDirect
Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr
Short communication
Utilization of optical sensors for phasor measurement units夽 Wenxuan Yao a,∗ , David Wells c , Daniel King b , Andrew Herron b , Thomas King a,b , Yilu Liu a,b a b c
The University of Tennessee, Knoxville, TN, United States Oak Ridge National Laboratory, Oak Ridge, TN, United States SmartSenseCom, Silver Spring, MD, United States
a r t i c l e
i n f o
Article history: Received 1 August 2017 Received in revised form 21 September 2017 Accepted 5 November 2017 Keywords: Instrument transformer Optical sensor Phasor measurement unit Signal acquisition
a b s t r a c t With the help of GPS signals for synchronization, increasingly ubiquitous phasor measurement units (PMUs) provide power grid operators unprecedented system monitoring and control opportunities. However, the performance of PMUs is limited by the inherent deficiencies in traditional transformers. To address these issues, an optical sensor is used in PMU for signal acquisition to replace the traditional transformers. This is the first time the utilization of an optical sensor in PMUs has ever been reported. The accuracy of frequency, angle, and amplitude are evaluated via experiments. The optical sensor based PMU can achieve the accuracy of 9.03 × 10−4 Hz for frequency, 6.38 × 10−3 rad for angle and 6.73 × 10−2 V for amplitude with real power grid signal, demonstrating the practicability of optical sensors in future PMUs. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Phasor measurement units (PMUs), first developed in the early 1980s, provide synchronized measurements of electric power system with the aid of GPS time synchronization [1]. Due to their high accuracy, PMUs are often used to benchmark the results of critical power system monitoring functions, e.g., state estimation and stability assessment [2]. As all measurement signals are read from instrument transformers, e.g., potential transformers (PTs) or current transformers (CTs), the performance of these instrument transformers significantly impacts the quality of phasor measurement data. For instance, traditional transformers introduce large instrumentation errors due to inherent deficiencies including temperature and electromagnetic interference (EMI) sensitivity, saturation magnetization and nonlinear induction. Furthermore, as most PMUs are installed at high-voltage substations, the traditional CTs and PTs with oil or sulfur hexafluoride (SF6 ) gas requirements for insulation would complicate installation process and bring high maintenance cost especially under conditions of harsh and explo-
sive environments. As an increasing number of PMUs are deployed, the necessity to address these issues arises. Optical sensors technology has proved to be technically mature to the point that they are preferred over their electronic counterparts in numerous applications across the industry. Optical sensors convert light rays into electronic signals and measures the physical quantity of light and then translates it into a form that is readable by an instrument. Based on the principle of electrostriction and Ampere’s law [3], the optical sensors (OSs) can realize high fidelity of voltage signal acquisition by intensity modulation using a non-coherent light source (LED) passing through fiber optic cables without being distorted by any saturation effects. Compared with conventional transformers, OSs have advantages in linearity over dynamic range, seismic performance, EMI immunity, increased safety, low power consumption, size reduction, and low installation and maintenance costs [4,5]. In this paper, the optical sensor technology is incorporated into PMUs. The accuracy of phasor measurements including frequency, angle and amplitude of an optical sensor based PMU is examined via experiment tests. 2. Performance evaluation 2.1. Experiment setup
夽 This work is supported by United States Department of Energy (DOE) under the Grid Modernization Lab Call (GMLC) project. ∗ Corresponding author. E-mail address: [email protected] (W. Yao). https://doi.org/10.1016/j.epsr.2017.11.004 0378-7796/© 2017 Elsevier B.V. All rights reserved.
Frequency disturbance recorder (FDR), which is a member of the PMU family [6], is used as the testbed in this paper. FDR originally uses PT to step down power grid signal for phasor measurement,
W. Yao et al. / Electric Power Systems Research 156 (2018) 12–14
13
Fig. 1. Experiment setup. (a) Photo, (b) block diagram.
Fig. 2. Test results with real power grid signals. (a) Frequency, (b) angle (c) amplitude.
which can achieve a measurement accuracy of 5 × 10−4 Hz for frequency, 1 × 10−3 rad for angle, and 5 × 10−2 V for amplitude. To verify the effectiveness of the OS for phasor measurements, one
FDR is modified to connect to a 0.15 class OS (ratio and phase error below 0.1% and 0.053◦ ), referred to as OS-PMU. The OS has a multimode optical fiber cable with wavelength 0.5145 m. The
14
W. Yao et al. / Electric Power Systems Research 156 (2018) 12–14
Table 1 SD of frequency measurements for PT-PMU and OS-PMU with simulated signals (Hz). Input frequency (Hz)
PT-PMU
OS-PMU
59.95 59.98 60.00 60.02 60.05
7.79 × 10−4 6.64 × 10−4 5.58 × 10−4 6.71 × 10−4 8.18 × 10−4
7.01 × 10−4 6.82 × 10−4 6.73 × 10−4 6.79 × 10−4 8.06 × 10−4
Table 2 Accuracy of OS-PMU with power grid signal.
power grid frequency, angle and amplitude over time as PT-PMU. The error of phasor measurement of OS-PMU can be obtained using PT-PMU results as a reference since the “true” frequency, angle and amplitude of real power grid signal are unknown. The accuracy of OS-PMU is listed in Table 2. The errors of OS-PMU are 9.03 × 10−4 Hz for frequency, 6.38 × 10−3 rad for angle and 6.73 × 10−2 V for amplitude, respectively. Total vector error (TVE) of OS-PMU is as small as 0.31%, which is sufficient to fulfill the 1% requirement of IEEE synchrophasor standard C37.118.1 [9]. 3. Conclusion
Frequency (Hz)
Angle (rad)
Amplitude (V)
TVE (%)
9.03 × 10−4
6.38 × 10−3
6.73 × 10−2
3.11 × 10−1
non-linearity of the PT and OS are within 0.2% and 0.15%, respectively. The power grid signal is sensed by an optical sensor head connected with optical fibers. A smart controller is used to generate a beam of light for optical fibers and realize the optical–electric signal conversion using a photodetector [7]. The OS-PMU uses output analog signal from the smart controller instead of PT to perform phasor calculation. Another traditional FDR with PT, referred to as a PT-PMU, is simultaneously running for comparison. The photo and block diagram of experimental setup are shown in Fig. 1. It is noted that both types OS-PMU and PT-PMU equipped the same type of antenna using GPS signals for synchronization throughout the tests as illustrated in Fig. 1(b). With a good GPS signal reception, a GPS receiver can lock 4–12 satellites [8]. The measurement data transmitted to data server is aligned together for accuracy comparison based on unified time stamp. 2.2. Experiment using simulated signals A Fluke 5080A calibrator is used to generate 120 V simulated signals for both PT-PMU and OS-PMU. By changing the output frequency of the Fluke 5080A calibrator, the performance can be evaluated under different conditions. Standard deviation (SD) of frequency measurement error are given in Table 1. Frequency errors of both PMUs are less than 1 × 10−3 Hz, which meet the IEEE standard. Furthermore, the measurement accuracy of the two PMUs is very close to each other, which means OS can provide an equivalently accurate analog signal for synchronized frequency measurement. The SD of OS-PMU is as small as about 8 × 10−3 Hz, which can be explained by the hardware resolution limit posed by a 14-bit analog-to-digital converter used in FDR platform. 2.3. Experiment using power grid signal To further the accuracy and effectiveness of OS for phasor measurement, OS-PMU is tested using real ambient power grid signal. The results of frequency, angle, and voltage magnitude measurements are shown in Fig. 2(a)–(c). It can be seen in Fig. 2 that OS-PMU has the ability to synchronously capture the trends of
In this paper, the OS was first successfully implemented on PMUs (FDR platform) and its performance for signal acquisition was evaluated in comparison with traditional PMU. The following finds can be obtained: 1) OS can be used in PMU for measuring signal acquisition with GPS synchronization to replacing traditional voltage transformer. 2) The frequency accuracy of OS-PMU is stable under different frequency conditions. 3) The OS-PMU is capable of achieving the accuracy of 9.03 × 10−4 Hz, 6.38 × 10−3 rad and 6.73 × 10−2 V for frequency, angle, and amplitude, respectively, with power grid signal, sufficiently meeting requirement of synchrophasor standard. Future work includes dynamic performance evaluation, comparison of optical sensors with current transforms, field test and reduction of manufacturing cost. References [1] A.G. Phadke, J.S. Thorp, et al., A new measurement technique for tracking voltage phasors, local system frequency, and rate of change of frequency, IEEE Trans. Power App. Syst. 102 (5) (1983) 1025–1038. [2] A.G. Phadke, Synchronized phasor measurements in power systems, IEEE Comput Appl. Power 6 (2) (1993) 10–15. [3] Institute Electric Power Research, Fiberoptic sensors: advanced optical sensing technology shows potential for improved power system monitoring and diagnostic applications, IEEE Power Eng. Rev. 13 (8) (1993) 9. [4] K.T. Grattan, B.T. Meggitt (Eds.), Optical Fiber Sensor Technology, vol. 1, Chapman & Hall, London, 1995. [5] P. Niewczas, J.R. McDonald, Advanced optical sensors for power and energy systems applications, IEEE Instrum. Meas. Mag. 10 (1) (2007) 18–28. [6] Y. Liu, L. Zhan, et al., Wide-area measurement system development at the distribution level: an FNET/GridEye example, IEEE Trans. Power Deliv. 31 (2) (2016) 721–731. [7] SmartsenseCom, EPC Optical Power Monitoring System, http://www. smartsensecom.com/uploads/4/7/1/7/47174183/ssc epc optical power monitoring system - brochure.pdf. [8] W. Yao, D. Zhou, L. Zhan, et al., GPS signal loss in the wide area monitoring system: prevalence, impact, and solution, Electr. Power Syst. Res. 147 (2017) 254–262. [9] IEEE Standard for Synchrophasor Measurements for Power Systems, IEEE Standard C37.118.1, (2011).
Contents lists available at ScienceDirect
Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr
Short communication
Utilization of optical sensors for phasor measurement units夽 Wenxuan Yao a,∗ , David Wells c , Daniel King b , Andrew Herron b , Thomas King a,b , Yilu Liu a,b a b c
The University of Tennessee, Knoxville, TN, United States Oak Ridge National Laboratory, Oak Ridge, TN, United States SmartSenseCom, Silver Spring, MD, United States
a r t i c l e
i n f o
Article history: Received 1 August 2017 Received in revised form 21 September 2017 Accepted 5 November 2017 Keywords: Instrument transformer Optical sensor Phasor measurement unit Signal acquisition
a b s t r a c t With the help of GPS signals for synchronization, increasingly ubiquitous phasor measurement units (PMUs) provide power grid operators unprecedented system monitoring and control opportunities. However, the performance of PMUs is limited by the inherent deficiencies in traditional transformers. To address these issues, an optical sensor is used in PMU for signal acquisition to replace the traditional transformers. This is the first time the utilization of an optical sensor in PMUs has ever been reported. The accuracy of frequency, angle, and amplitude are evaluated via experiments. The optical sensor based PMU can achieve the accuracy of 9.03 × 10−4 Hz for frequency, 6.38 × 10−3 rad for angle and 6.73 × 10−2 V for amplitude with real power grid signal, demonstrating the practicability of optical sensors in future PMUs. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Phasor measurement units (PMUs), first developed in the early 1980s, provide synchronized measurements of electric power system with the aid of GPS time synchronization [1]. Due to their high accuracy, PMUs are often used to benchmark the results of critical power system monitoring functions, e.g., state estimation and stability assessment [2]. As all measurement signals are read from instrument transformers, e.g., potential transformers (PTs) or current transformers (CTs), the performance of these instrument transformers significantly impacts the quality of phasor measurement data. For instance, traditional transformers introduce large instrumentation errors due to inherent deficiencies including temperature and electromagnetic interference (EMI) sensitivity, saturation magnetization and nonlinear induction. Furthermore, as most PMUs are installed at high-voltage substations, the traditional CTs and PTs with oil or sulfur hexafluoride (SF6 ) gas requirements for insulation would complicate installation process and bring high maintenance cost especially under conditions of harsh and explo-
sive environments. As an increasing number of PMUs are deployed, the necessity to address these issues arises. Optical sensors technology has proved to be technically mature to the point that they are preferred over their electronic counterparts in numerous applications across the industry. Optical sensors convert light rays into electronic signals and measures the physical quantity of light and then translates it into a form that is readable by an instrument. Based on the principle of electrostriction and Ampere’s law [3], the optical sensors (OSs) can realize high fidelity of voltage signal acquisition by intensity modulation using a non-coherent light source (LED) passing through fiber optic cables without being distorted by any saturation effects. Compared with conventional transformers, OSs have advantages in linearity over dynamic range, seismic performance, EMI immunity, increased safety, low power consumption, size reduction, and low installation and maintenance costs [4,5]. In this paper, the optical sensor technology is incorporated into PMUs. The accuracy of phasor measurements including frequency, angle and amplitude of an optical sensor based PMU is examined via experiment tests. 2. Performance evaluation 2.1. Experiment setup
夽 This work is supported by United States Department of Energy (DOE) under the Grid Modernization Lab Call (GMLC) project. ∗ Corresponding author. E-mail address: [email protected] (W. Yao). https://doi.org/10.1016/j.epsr.2017.11.004 0378-7796/© 2017 Elsevier B.V. All rights reserved.
Frequency disturbance recorder (FDR), which is a member of the PMU family [6], is used as the testbed in this paper. FDR originally uses PT to step down power grid signal for phasor measurement,
W. Yao et al. / Electric Power Systems Research 156 (2018) 12–14
13
Fig. 1. Experiment setup. (a) Photo, (b) block diagram.
Fig. 2. Test results with real power grid signals. (a) Frequency, (b) angle (c) amplitude.
which can achieve a measurement accuracy of 5 × 10−4 Hz for frequency, 1 × 10−3 rad for angle, and 5 × 10−2 V for amplitude. To verify the effectiveness of the OS for phasor measurements, one
FDR is modified to connect to a 0.15 class OS (ratio and phase error below 0.1% and 0.053◦ ), referred to as OS-PMU. The OS has a multimode optical fiber cable with wavelength 0.5145 m. The
14
W. Yao et al. / Electric Power Systems Research 156 (2018) 12–14
Table 1 SD of frequency measurements for PT-PMU and OS-PMU with simulated signals (Hz). Input frequency (Hz)
PT-PMU
OS-PMU
59.95 59.98 60.00 60.02 60.05
7.79 × 10−4 6.64 × 10−4 5.58 × 10−4 6.71 × 10−4 8.18 × 10−4
7.01 × 10−4 6.82 × 10−4 6.73 × 10−4 6.79 × 10−4 8.06 × 10−4
Table 2 Accuracy of OS-PMU with power grid signal.
power grid frequency, angle and amplitude over time as PT-PMU. The error of phasor measurement of OS-PMU can be obtained using PT-PMU results as a reference since the “true” frequency, angle and amplitude of real power grid signal are unknown. The accuracy of OS-PMU is listed in Table 2. The errors of OS-PMU are 9.03 × 10−4 Hz for frequency, 6.38 × 10−3 rad for angle and 6.73 × 10−2 V for amplitude, respectively. Total vector error (TVE) of OS-PMU is as small as 0.31%, which is sufficient to fulfill the 1% requirement of IEEE synchrophasor standard C37.118.1 [9]. 3. Conclusion
Frequency (Hz)
Angle (rad)
Amplitude (V)
TVE (%)
9.03 × 10−4
6.38 × 10−3
6.73 × 10−2
3.11 × 10−1
non-linearity of the PT and OS are within 0.2% and 0.15%, respectively. The power grid signal is sensed by an optical sensor head connected with optical fibers. A smart controller is used to generate a beam of light for optical fibers and realize the optical–electric signal conversion using a photodetector [7]. The OS-PMU uses output analog signal from the smart controller instead of PT to perform phasor calculation. Another traditional FDR with PT, referred to as a PT-PMU, is simultaneously running for comparison. The photo and block diagram of experimental setup are shown in Fig. 1. It is noted that both types OS-PMU and PT-PMU equipped the same type of antenna using GPS signals for synchronization throughout the tests as illustrated in Fig. 1(b). With a good GPS signal reception, a GPS receiver can lock 4–12 satellites [8]. The measurement data transmitted to data server is aligned together for accuracy comparison based on unified time stamp. 2.2. Experiment using simulated signals A Fluke 5080A calibrator is used to generate 120 V simulated signals for both PT-PMU and OS-PMU. By changing the output frequency of the Fluke 5080A calibrator, the performance can be evaluated under different conditions. Standard deviation (SD) of frequency measurement error are given in Table 1. Frequency errors of both PMUs are less than 1 × 10−3 Hz, which meet the IEEE standard. Furthermore, the measurement accuracy of the two PMUs is very close to each other, which means OS can provide an equivalently accurate analog signal for synchronized frequency measurement. The SD of OS-PMU is as small as about 8 × 10−3 Hz, which can be explained by the hardware resolution limit posed by a 14-bit analog-to-digital converter used in FDR platform. 2.3. Experiment using power grid signal To further the accuracy and effectiveness of OS for phasor measurement, OS-PMU is tested using real ambient power grid signal. The results of frequency, angle, and voltage magnitude measurements are shown in Fig. 2(a)–(c). It can be seen in Fig. 2 that OS-PMU has the ability to synchronously capture the trends of
In this paper, the OS was first successfully implemented on PMUs (FDR platform) and its performance for signal acquisition was evaluated in comparison with traditional PMU. The following finds can be obtained: 1) OS can be used in PMU for measuring signal acquisition with GPS synchronization to replacing traditional voltage transformer. 2) The frequency accuracy of OS-PMU is stable under different frequency conditions. 3) The OS-PMU is capable of achieving the accuracy of 9.03 × 10−4 Hz, 6.38 × 10−3 rad and 6.73 × 10−2 V for frequency, angle, and amplitude, respectively, with power grid signal, sufficiently meeting requirement of synchrophasor standard. Future work includes dynamic performance evaluation, comparison of optical sensors with current transforms, field test and reduction of manufacturing cost. References [1] A.G. Phadke, J.S. Thorp, et al., A new measurement technique for tracking voltage phasors, local system frequency, and rate of change of frequency, IEEE Trans. Power App. Syst. 102 (5) (1983) 1025–1038. [2] A.G. Phadke, Synchronized phasor measurements in power systems, IEEE Comput Appl. Power 6 (2) (1993) 10–15. [3] Institute Electric Power Research, Fiberoptic sensors: advanced optical sensing technology shows potential for improved power system monitoring and diagnostic applications, IEEE Power Eng. Rev. 13 (8) (1993) 9. [4] K.T. Grattan, B.T. Meggitt (Eds.), Optical Fiber Sensor Technology, vol. 1, Chapman & Hall, London, 1995. [5] P. Niewczas, J.R. McDonald, Advanced optical sensors for power and energy systems applications, IEEE Instrum. Meas. Mag. 10 (1) (2007) 18–28. [6] Y. Liu, L. Zhan, et al., Wide-area measurement system development at the distribution level: an FNET/GridEye example, IEEE Trans. Power Deliv. 31 (2) (2016) 721–731. [7] SmartsenseCom, EPC Optical Power Monitoring System, http://www. smartsensecom.com/uploads/4/7/1/7/47174183/ssc epc optical power monitoring system - brochure.pdf. [8] W. Yao, D. Zhou, L. Zhan, et al., GPS signal loss in the wide area monitoring system: prevalence, impact, and solution, Electr. Power Syst. Res. 147 (2017) 254–262. [9] IEEE Standard for Synchrophasor Measurements for Power Systems, IEEE Standard C37.118.1, (2011).