The All-digital Electrical Energy Metering System Based on Optical Current and Voltage Transducer
Yang Huayun, Jiang Bo, Jiang Wei, Zhang Jiamin , Jiang Yinxia and Liu Kun
Electric Power Metering Center
Sichuan Electric Power Test & Research Institute
Chengdu, P.R. China yhy12485@126.com
Abstract—The communication protocol structure of digital substation in smart grid has changed greatly, and the electrical energy metering system is changed greatly too. In this paper, an all-digital electrical energy metering system based on pure optical voltage transducer (OVT) and optical current transducer (OCT) is introduced. The advanced communication protocol IEC61850-9-2 is adopted in the all-digital electrical metering system firstly. And in this paper, the difference between traditional electrical energy metering system and all -digital electrical energy metering system is analyzed and compared in principle, and the benefits of all-digital electrical energy metering system are described. The realization of the proposed all-digital electrical energy metering system on a 220kV voltage grade power system is described. The results of running show that the developed all-digital electrical energy metering has the performances of high accuracy, high reliability and high safety.
all-digital electrical energy metering system on a 220kV voltage grade power system is described. The study of the all-digital electrical energy metering system based on OCT and OVT following the IEC61850-9 -2 communication protocol possess great value for the development of the digital substation.
II. THE CHARACTERISTIC OF ALL-DIGITAL ELECTRICAL POWER METERING SYSTEM
Keywords-optical current transducer; optical voltage
transducer; all-digital electrical metering; IEC61850-9-2; digital substation; smart grid
Fig. 1 shows a diagram of traditional electrical energy metering system. The traditional electrical energy metering system is made up of electromagnetic stimulant current transformer (CT), voltage transformer (VT) and electrical energy meter by cables. Suppose that the degree of accuracy of current transformer, voltage transformer and traditional electrical energy meter is 0.2 grade separately, and the cable error which is caused by the voltage transformer secondary voltage drop is 0.1%. Then the integrated uncertainty for the tra ectrical ener em is 0.7%. I. INTRODUCTION
With the development of digital substation of smart grid, the all-digital electrical energy metering system becomes more and more important and necessary [1-3]. In order to fit the development of the digital substation, it is necessary to adopt advanced technologies to development a novel all-digital electrical energy metering system which satisfied the demand of digital substation.
Fig.
m of
In this paper, a novel all-digital electrical energy metering system based on OVT and OCT is introduced. The proposed all-digital electrical energy metering system consists of these parts: 1) three adaptive optical current transducers [4]; 2) three adaptive optical voltage transducers [5]; 3) a merging unit (MU); 4) an industrial Ethernet switch; 5) an all-digital multi-function power meter; 6) GPS. In the proposed all-digital electrical energy metering system, the communication protocol is following the IEC61850-9-2 one -to-many communication protocol. The IEC61850-9-2 [6] is the most advanced communication protocol in the digital substation. In this paper, the all-digital electrical energy metering system is given out in detail, and the difference between traditional electrical energy metering system and all-digital electrical energy metering system is analyzed. In the last the realization of the proposed
onal electri y metering system Fig. 2 Diagram of all-digital electrical energy metering system In the all-digital electrical energy metering system, all-digital electrical energy meter receiving digital current and voltage
instantaneous values is a full digital processing system.
978-1-4244-4813-5/10/$25.00 ?2010 IEEE
Because the electrical energy is calculated based on digital processing principle, no error is brought to the all-digital electrical energy meter on theory. So there is no accuracy grade set in all-digital electrical energy meter. The digital current and voltage instantaneous values are transported bye optical fiber and Ethernet, and can’t be interfered by electromagnetic wave or secondary voltage drop. So there is no additional error when the digital current and voltage instantaneous signals are transmitted on the optical fiber. The errors of the all-digital electrical energy metering system are determined by the electronic current transformer (ECT) and the electronic voltage transformer (EVT).
Fig. 2 shows a diagram of the integrated uncertainty
for all-digital electrical energy metering system. Suppose that the accuracy of EVT and ECT is 0.2 grade separately, the integrated uncertainty of all-digital electrical energy metering system is 0.4 grade, and it is better than traditional electrical energy metering system.
III. THE ALL-DIGITAL ELECTRICAL ENERGY METERING
SYSTEM BASED ON OCT AND OVT Fig. 3 shows the structure of the proposed all-digital electrical energy metering system. As shown in Fig. 3, all- digital electrical energy metering system is made up of three adaptive optical current transducers (AOCT), three adaptive optical voltage transducers (AOVT), a merging unit (MU), an industrial Ethernet switch, all-digital multi-function power meter, GPS ,master cl c.
Fig. 3 Structure of the proposed all-digital electrical
energy metering system A.
AOVT and AOCT
The AOCT is based on the Faraday magneto-optic effect principle which has good measurement linearity and can
measure not only variant current but also the steady state current. Obviously, the optical current transducer following Faraday magneto-optic effect principle hasn’t measurement frequency band problem, and it has good harmonic measurement accuracy, it can meet specification of electric energy quality measurement and monitor. In this paper, the adopted AOCT developed by Beijing Xuji Power Optics Technology Co. Ltd has solve two difficult problem which are temperature drift problem of measuring accuracy and stableness problem in long-time running. The structure of adaptive optical current transducer is as shown in Fig. 4.
The measurement principle of AOVT is based on Pockels effect. In this paper, the adopted AOCT developed by Beijing Xuji Power Optics Technology Co. Ltd too. The AOCT and AOCT are made up of optical sensor head and secondary converter. The secondary converter provides the optical source for optical sensor and receives the analog optical signal. Then the secondary converter collects the voltage or current signal and realizes the synchronization according to received the synchro
Conductor Primary Little CT
Optical Sensor
Optical ADC
cell
Optical
Fiber Post
Isulator
Optical Laser Secondary Fiber Converter Unit Fig. 4 The structure of adaptive optical current transducer
B.
MU,Industrial Ethernet Switch and Master Clock
In the all-digital electrical energy metering system and digital substation, one of the most important issues is the synchronization. The MU provides synchronization of sampling pulses within one period T0 = 1/ f0 with respect to the synchronization signal. The synchronization is performed periodically on once a second (1 pulses/s) as required in [7]. One pulse per second can be generated by the master clock which is a global positioning system (GPS) satellite-controlled clock. The master clock can provide the pulse per second signal to the MU, Ethernet switch and protection to ensure the
synchronization of sampling measured value. With a synchronization once a second, the number of samples of AOVT and AOCT in that interval is 200 ? rat f = 50 Hz, which is less than 216
, or 20 000
r-1, enough to uniquely identify samples.
According to IEC60044-8 [7], a MU is consist of secondary exchangers of 7 current transformators and 5 voltage transformators , all the measurands are output as serial digital quantity. The 12 electric parameters in a interval can be concurrent processed by MU, including the three phase current for measuring , the three phase current for protection, the current of no-slip point, three phase voltage, voltage of no-slip point and voltage of the bus-bar. The 12 digital output signals are packed as a frame by MU, then the signals of protecting frame and the measuring frame are delivered to the industrial Ethernet switch through the 100Mbit/s optical fiber following the communication protocol of IEC61950-9 -2. And the secondary devices such as electrical energy meter, line protection, fault recorder and so on can receive the information on 100MB Ethernet following IEC 61850-9-2 one-to-many-one-way communication protocol from the industrial Ethernet switch according its requirement. This can insure the rapidity, reliability and anti-interference capability of sampling data transmission.
MU is one of most important devices in the all-digital
electrical energy metering. The function of MU is as following: 1) synchronism function module. After the synchronous pulse per second (PPS) clocked signal imported is accurately identified ( be the output from GPS receiver), the synchronous sampling signal is generated. And the synchronous sampling signal is used to re-sample the current and voltage digital signal received from AOVT and AOCT.
2) Multi-channel data acquisition and functional processing module. As main functional modules connecting with the secondary converter unit of AOCT and AOVT, the output data from AOVT and AOCT at most can be simultaneously received and examined. And the received data are re-sampled by MU.
3) the function module of serial port transmission. It is used to taking the re-sampled data into a frame following IEC61850-9-2, and sending the frame to the industrial Ethernet switch with 100Mbit/s optical fiber.
C. Electrical energy measuring
When data packet is received from Ethernet switch by electrical energy meter, the data packet following IEC61850-9- 2 is parsed, and sampling values of three phase current and voltage for measurement are gained. And the amplitude and phase position of voltage and current of fundamental wave and every harmonic component are got with the arithmetic of DFT or FFT. Then active power can be got by the following formula
P = 1 T uidt = N
T ∫ ∑ U I cos .
(1)
0 n n n
And the
0
formula
reactive power can be obtained by the following
N
Q = ∑0Un In sin n .
(2)
Then the active electrical energy and the reactive electrical energy can be obtained. In the all-digital electrical energy meter, the electrical parameters are calculated with the digital current and voltage instantaneous values, and no error generates when the electric energy is calculated based on digital signal processing principle in theory [7]. IV. T
HE RRE
EEALIZATION AND UNNING OF DIGITAL LECTRICAL
NERGY METERING SYSTEM BASED ON OCT AND OVT
An all-digital electrical energy metering system is manufactured in a 220kV substation based on the configuration as shown in Fig. 1. The rated current of adopted AOCT is 1200A, and the accuracy ade is 0.2sgrade. The rated voltage of adopted AOVT is 220/ 3 kV and the accuracy grade is 0.2 grade. And the 220kV AOCT installed in the 220kV substation is shown in the Fig. 5. The digital output used for the measurement of AOVT and AOCT is 2D41H. The all-digital electrical energy metering system is put into operation is a 220kV substation in Sichuan, China in Apr. 2009. Up to the present, the all-digital energy is running well currently. The error of the active power energy compared with the additional electrical energy metering installed in the 220kV substation is about 0.3%. The communication protocol IEC61850-9-2 is used in the 220kV substation firstly.
Fig. 5 The AOCT installed in the 220kV substation
V. CONCLUSION
In this paper, the all-digital electrical energy metering system based on optical current and voltage Transducer following IEC61850-9-2 is introduced in this paper. The integrated error of the all-digital electrical energy metering system is determined by the measurement error of the AOCT and AOVT. The measurement data are digital and transmitted by the optical fiber. The all-digital electrical energy metering system has the characteristics of accuracy, reliable and safety of measurement. The IEC61850-9-2 communication protocol is used firstly in the all-digital electrical energy metering system
of 220kV digital substation that possesses great value for the development of the digital substation and the all-digital electrical energy metering system.
R
EFERENCES
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BIOGRAPHIES
Yang Huayun was born in china in 1974. He received the B.S. degree in electrical engineering from Chongqing University, Chongqing, China, in 1998.
From 1998 to 2002, he was an assistant electrical engineer in China Gezhouba Group Co. Ltd, Hubei, China. From 2002 to 2008, he has been working towards the M.S. and Ph.D degree in electrical engineering in Huazhong University of science technology, Wuhan, China. Since 2008, he has been working in Sichuan Electric Power Test & Research Institute. His main rearch is the digital substation and electrical power energy metering.
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