Design and applicability analysis of independent double acquisition circuit of all-fiber optical current transformer

1 Introduction

The transformer is the source of data collected for the grid operation.With the growing development of China’s smart grid technology,the installation of electronic transformers in substations,and digitization of data acquisition have already fulfilled the basic technical conditions [1-5].Compared to traditional transformers,the all-fiber current transformer (OCT) bears the advantages of being small in size,having a simple insulation structure,lacking magnetic saturation,the capacity for high measurement bandwidth and precision,and strongly resisting electromagnetic interference [6-10].Additionally,it can easily be integrated with switchgear installation [11],which would promote the development of powerful and highly reliable equipment for miniaturization and integrated automation [12,13].At present,research on the application technology of the all-fiber current transformers mainly focuses on two aspects:structural design and reliability analysis.Structural design mainly focuses on the key components of the optical fiber transformer and the subsystem configuration.The paper [14] proposes a method to improve the measurement accuracy of the all-fiber current transformer based on the open-loop mechanism.The paper [15] is based on digital closed-loop feedback control.Further,a signal processing method for fiber-optic current transformers is proposed.The paper [16-17] propose a phase modulator design method to reduce the complexity of the phase modulation and demodulation system of allfiber transformers.Reliability analysis mainly aims to determine the life and failure probability of key components and subsystems of fiber optic transformers.The paper [18] builds fault modes based on the fault data of the all-fiber current transformer and establishes a fault diagnosis expert system.The paper [19] designed all-fiber self-diagnosis function of current transformers; the common modes of failure of optical components are analyzed in paper [20-21],and performed the accelerated aging test of the lifetime and long-term operational stability of optical components of all-fiber current transformers.The above research mainly focuses on the body of the optical device,and improving the economic feasibility of the all-fiber current transformer with the aim of satisfying the reliability requirement,and thus making it to promote its application.

In this paper,based on the dual sampling requirements of smart substation relay protection,based on closedloop control and open-loop control,a design scheme of an independent dual-acquisition loop of the all-fiber current transformer based on a single optical path is proposed.While satisfying the sampling requirements of relay protection,the acquisition function of two independent all-fiber current transformer products are acquired in an integrated manner on a single all-fiber current transformer,which greatly reduces the cost of applying the all-fiber current transformer in engineering.

2 Sampling requirements for relay protection

According to the Q/GDW 441-2010 “Technical Specification for Relay Protection of Intelligent Substation”,the electronic transformer should be collected by two independent sampling systems.Each sampling system should adopt dual system access,and each output should have two channels.The digital sampled value enters a set of protection devices from the same channel to ensure that the dual protection meets the requirement of being completely independent of each other [22].It is easy for the active electronic current transformer to achieve dual acquisition of one sensing signal because it is a way of collecting data via the unidirectional conversion circuit.For an all-fiber current transformer,there are generally three ways of performing the dual acquisition of one sensing signal.

The first method is to configure four protection sensing elements for each set of the all-fiber current transformers,which are selected through four independent sampling systems,and the two sampling data are entered into the same merging unit.The output values by each merging unit of the two digital samples are entered into a set of protection devices via the same fiber channel,as shown in Fig.1.

Fig.1 All-fiber OCT acquisition mode 1

The second method entails the configuration of two protection sensing elements for each set of the all-fiber current transformers,which are selected through four independent sampling systems.Each sampling system comprises a digital-to-analog converter (D/A) and an analog-to-digital converter (A/D).Both channels of sampled data are entered into the same merging unit,and each merging unit transmits two digital sample values from the same fiber channel into a set of protection devices.This scheme is not only applicable to open-loop controlled allfiber current transformers,but also to closed-loop controlled all-fiber current transformers,as shown in Fig.2.

Fig.2 All-fiber OCT acquisition mode 2

The third method entails the configuration of two protection sensing elements for each set of the all-fiber current transformers,which are collected by four independent A/D sampling systems.The two channels of sampled data are entered into the same merging unit,and each merging unit transmits two digital sample values from the same fiber channel into a set of protection devices,as shown in Fig.3.This scheme is favorable for all-fiber current transformers employing the open-loop control principle.

Fig.3 All-fiber OCT acquisition mode 3

Regarding the three sampling methods above,the first method is applicable in both open loop control and closed loop control; the second method is more suitable for closed loop control,and the third method is more suitable for open loop control.The corresponding sampling loop design schemes are given below for the three dual sampling methods.

3 All-fiber current transformer sampling circuit design

3.1 Design of the acquisition loop for the closed loop control

The all-fiber current transformer adopting the closed-loop control mode has a D/A conversion circuit in addition to the A/D conversion circuit.The phase modulator has only one pair of modulation electrodes that are connected externally,and is connected by a D/A conversion circuit.The output of the D/A conversion circuit is connected to the ground modulation electrodes of the phase modulator through a ground line and a modulation signal line,as shown in Fig.4.Since the current value output by the D/A conversion circuit is integrated processing data,it is difficult to achieve double sampling by adding an A/D conversion circuit.Therefore,if the closed-loop control all-fiber current transformer acquisition loop shown in Fig.4 is used,it needs to have four sets of independent products installed and assembled into a set of Q/GDW 441-2010 “Intelligent Substation Relay Protection Technical Specifications”.This method is derived from a complete dual acquisition scheme.Four sets of expensive optical components:the phase modulator,optical splitter,light source,and detector are prepared.When the cost of the equipment increases significantly,the configuration scheme becomes complicated and bulky.The installation process is cumbersome,making its adaptation by many users and the promotion of its application difficult.

3.2 The independent double acquisition loop design for closed loop control

For the optimization of the closed loop control acquisition loop,a single optical path is used to construct an independent dual acquisition loop,and a dual feedback modulation digital closed loop control loop.A dual feedback modulator is then used to achieve independent acquisition and D/A feedback of the dual acquisition circuit.The solution includes an optical path portion and a circuit portion as shown in Fig.5.The optical path portion includes a light source,beam splitter,polarizer,phase modulator,sensing fiber ring,and detector.The circuit part is usually integrated into the circuit board,called the signal processing board,which comprises the analog and digital signal processing circuits.A sensing fiber ring which comprises a quarter wave plate and a sensing fiber is disposed into the optical path,and one end of the sensing fiber is fitted with a mirror,and detector,phase modulator and the circuit portion in the optical path to form a closed loop control.The circuit portion includes two A/D conversion circuits,two D/A conversion circuits,and a digital signal processing unit.The signal processing circuit structure is as shown in Fig.6.Two A/D conversion circuits are connected to the detectors in the optical path,and the two A/D conversion circuits independently transmit the output of the detector and the output of the two sampling data to the digital signal processing unit which is based on time division.The two channels of sampled data are subjected to time-division demodulation processing to generate two demodulation processed signals and sequentially transmit output to corresponding D/A conversion circuits.Both D/A conversion circuits are connected to the phase modulators in the optical path,thereby respectively utilizing the two D/A conversion circuit performs phase time division modulation and closed loop feedback of the optical signal on the phase modulator,and the digital signal processing unit respectively obtains two measurement current data outputs to the same merging unit to realize two independent dual acquisition loops.The problem can be solved by preventing the impact of a circuit failure on the relay protection,which is suitable for the all-fiber current transformer configuration scheme shown in Fig.2.

Fig.4 Closed-loop controlled all-fiber OCT acquisition loop schematic

Fig.5 Closed-loop controlled all-fiber OCT independent dual acquisition loop schematic

3.3 Independent double acquisition loop design for open loop control

All-fiber current transformers are called open-loop control when only square-wave modulation as opposed to step-wave feedback modulation is used.The open-loop controlled all-fiber current transformer is suitable for the all-fiber current transformer configuration shown in Fig.3 because it does not have closed-loop feedback.The scheme also constructs an open-loop independent double-sampling loop based on a single optical path,double-sampling,and separate demodulation,including the optical path portion and the circuit portion,as shown in Fig.7.Similar to the scheme in Fig.5,the optical path portion also includes a light source,a beam splitter,a polarizer,a phase modulator,a sensing fiber ring,and a detector,and the circuit portion is integrated into the circuit board.A sensing fiber ring is also disposed into the optical path.The sensing fiber ring includes a quarter wave plate and a sensing fiber,and a mirror is disposed at one end of the sensing fiber.In contrast,the phase modulator in Fig.7 belongs to a component of the phase modulation module,and the phase modulation module includes a square wave control circuit,a D/A conversion circuit,and a phase modulator that are sequentially connected.

Fig.6 Signal processing circuit structure diagram

The circuit portion includes a first A/D conversion circuit,a second A/D conversion circuit,a double sampling data processing unit,a square wave control circuit and a D/A conversion circuit in the phase modulation module.The first A/D conversion circuit and the second A/D conversion circuit are both connected to the detector and the double sampling data processing unit is synchronized with the timing signal in the square wave control circuit.The first A/D conversion circuit and the second A/D conversion circuit respectively perform independent sampling on the output of the detector,and then input two sampling data into the double sampling data processing unit,and the double sampling data processing unit separately demodulates and calculates the two sampling data.The respective differences in light intensity are obtained to reveal two current values as open-loop demodulation processing data,and current compensation processing is respectively performed,and the double-sampled data processing unit also sends a synchronization signal to the square wave control circuit,and the square wave control circuit generates square wave modulation.The signal is input to the phase modulator via the conversion circuit for phase modulation.The double sampling data processing unit yields the two compensated measured current values.

Fig.7 Open-loop controlled all-fiber OCT independent dual acquisition loop scheme

The scheme double AD samples and demodulates separately,synchronously cooperates with the square wave to control the structure of the DA conversion,and yields two mutually independent transformer values without interference.The solution enables the integration of the functionality of two independent all-fiber current transformer products.

4 Reliability analysis

4.1 Reliability model

Since the independent dual acquisition loops are based on a single optical path design,the structure shown in Fig.5 is selected for reliability modeling.The system reliability block diagram of the all-fiber current transformer mainly includes a high-voltage side fiber sensitive ring,a transmission fiber optic cable,a low-voltage side optical path unit.A low-voltage side signal processing unit and a low-voltage side power supply unit.As shown in Fig.8,a series model reliability system is constructed.

Fig.8 System reliability block diagram of all-fiber OCT

The high-voltage side fiber sensitive ring mainly includes key optical components such as a sensing fiber ring,a mirror,and a quarter-wave plate.Additionally,it constitutes a series model.This part can be wholly analyzed as an optical component.Fig.9 shows the reliability block diagram of the high-voltage fiber sensitive ring.

Fig.9 Reliability block diagram of high-voltage side fiber sensitive ring

The low-voltage side optical path unit mainly includes key optical components such as a light source module,a beam splitter,a polarizer,a phase modulator,and a detector,and also constitutes a series model.Fig.10 shows the reliability block diagram of the low-voltage side optical path unit.

Fig.10 Reliability block diagram of the lowvoltage side optical path unit

The splitter and the polarizer belong to a passive optical device,and the light source module,the phase modulator and the detector belong to the active optoelectronic device.The reliability data of this part of the optics can be obtained from the relevant fiber optic current transformer manufacturers and estimated with reference to the national military standard GJB/Z 299C-2006 [23].

The low-voltage side signal processing unit mainly includes an electronic circuit such as an AD1 acquisition module,an AD2 acquisition module,a digital signal processing unit,a DA1 feedback module,or a DA2 feedback module,and constitutes a combined serial system.The reliability block diagram of the low-voltage side signal processing unit is shown in Fig.11.

Fig.11 Reliability block diagram of the low-voltage side signal processing unit

4.2 Failure rate calculation

According to the reliability block diagram in Fig.8,the reliability calculation of the all-fiber current transformer includes a high-voltage side fiber sensitive ring,a transmission fiber cable,a low-voltage side optical path unit,a low-voltage side signal processing unit,and a lowvoltage side power supply unit.

The calculation of the failure rate of the high-voltage side fiber-sensitive ring can be regarded as an entire optical component.The failure rate of the high-voltage side fiber sensitive ring can be predicted using its average lifespan which is assumed to be 50 years.To simplify the analysis,the approximate distribution is estimated according to the exponential distribution.The relationship between the average life expectancy of the exponential distribution q and the failure rate lP is as shown in formula (1):

According to formula (1),the work failure rate of the high-voltage side fiber sensitive ring can be calculated as shown in formula (2):

The calculation of the work failure rate of the transmission fiber can be based on the data given in paper [25].The work failure rate of a single fiber is shown in formula (3):

The light source module,the beam splitter,the polarizer,and the detector in the reliability diagram of the low-voltage side optical path unit are all one type of optoelectronic device,and the working failure rate of the optoelectronic device in the imported component can be given according to the national military standard GJB/Z 299C-2006 (People’s Liberation Army,n.d.).The model [23] is computed as shown in formula (4):

In formula (4),λB is the basic failure rate; πT is the temperature stress coefficient; πE is the environmental coefficient; and πQ is the mass coefficient.According to the calculation of equation (4),the parameters of the component failure rate of the low-voltage side optical path unit are as shown in Table 1.

Table1 Component failure rate parameter table of lowvoltage side optical path unit

Component λP λB πE πQ πT Light source module 4.05 4.5 1.0 1.0 0.9 Beam splitter 0.0331 0.036 1.0 1.0 0.92 Polarizer 0.0306 0.036 1.0 1.0 0.85 Detector 0.46 0.5 1.0 1.0 0.92

In addition,the components in the low side optical path unit also include a phase modulator.The phase modulator is determined based on the Y-waveguide.The calculation of the work failure rate of the Y-waveguide can be referenced from the data given in paper [25].At a temperature of 22° C,the average working life of the Y-waveguide is q=1.6 ×107 hours.According to the approximate distribution of the exponential distribution,the working failure rate of the Y-waveguide can be obtained using formula (5):

According to the failure probability of the light source module,the beam splitter,the polarizer,the radio and television detector in Table 1,the failure rate of the transmission fiber of formula (3) and the failure rate of the (5) Y waveguide,each component is considered as one set.According to the series structure,the overall failure rate of the low-voltage side optical path unit can be calculated,as shown in formula (6):

The components in the reliability block diagram of the low-voltage side signal processing unit include electronic circuits such as the AD1 acquisition module,AD2 acquisition module,digital signal processing unit,and DA1 and DA2 feedback modules.The components of this part of the module are refined,and can be divided into an A/D pre-processing circuit,an A/D acquisition circuit,a D/A conversion circuit,and other partial circuits.The A/D pre-processing circuit includes an operational amplifier,a resistor,and a capacitor.The A/D acquisition circuit mainly comprises an A/D converter while the D/A conversion circuit mainly comprises a D/A converter.Other parts of the circuit mainly comprise a voltage reference chip,a field programmable gate array chip (FPGA),and digital signal processing (DSP).

The operational amplifier,AD converter,DA converter,FPGA,and DSP are all semiconductor integrated singlechannels,which can be calculated according to the work failure rate model of the semiconductor monolithic integrated circuit in the imported components given by the national military standard GJB/Z 299C-2006 [22],as shown in formula (7):

In formula (7),C1 is the circuit complexity failure rate while C2 is the package complexity failure rate.

The work failure rate model of the voltage reference chip is calculated as shown in formula (8):

In formula (8),πA is the resistance application coefficient.

The work failure rate model of the resistor is calculated as shown in formula (9):

In formula (9),πR is a resistance coefficient.

The working failure rate model of the capacitor is calculated as shown in formula (10):

In formula (10),πCV is a capacitance coefficient; while ppCH is a surface mount coefficient.

According to the relevant parameters that paper [23] outlines in formula (7)-(10) and national military standard GJB/Z 299C-2006,the failure rate of each component in the low-voltage side signal processing unit can be calculated,as shown in Table 2-3.

Table2 Component failure rate parameter of low-voltage side signal processing unit Table 1

Component λP λB πE πQ πT πA C1 C2 Operational Amplifier 0.0606 - 1.0 6.0 0.513 - 0.0191 0.0003 AD converter 0.0949 - 1.0 6.0 0.36 - 0.0431 0.0003 DA converter 0.0949 - 1.0 6.0 0.36 - 0.0431 0.0003 Voltage reference chip 0.348 0.058 1.0 6.0 - 1.0 - -FPGA 0.1113 - 1.0 6.0 0.274 - 0.0137 0.0148 DSP 0.1107 - 1.0 6.0 0.171 - 0.0056 0.0175

Table3 Component failure rate parameter of low-voltage side signal processing unit Table 2

Component λP λB πE πQ πR πCV πCH Synthetic resistor 0.007 0.007 1.0 1.0 1.0 - -Film resistor 0.0112 0.0112 1.0 1.0 1.0 - -Film capacitor 0.00213 0.00285 1.0 1.0 - 0.5 1.5 Aluminum electrolytic capacitors 0.00608 0.0054 1.0 1.0 - 0.75 1.5

According to Table 2-3,the failure rate of each part of the circuit of the low-voltage side signal processing unit can be calculated separately.The A/D pre-processing circuit includes two operational amplifiers,10 synthetic resistors,6 thin film resistors,4 thin film capacitors,and 2 aluminum electrolytic capacitors.According to the series structure,the overall failure rate is as shown in equation (11) below.

The A/D acquisition circuit includes one A/D converter,and the failure rate is as shown in formula (12):

The DA acquisition circuit includes one D/A converter,and the failure rate is as shown in formula (13):

The other parts of the circuit comprise one voltage reference chip,one FPGA and one DSP.According to the serial structure,the overall failure rate is as shown in formula (14):

The main component in the low-voltage side power supply unit is a DC-to-DC converter.According to the data given [26],the key components affecting the reliability of the DC-to-DC power supply module are vertical doublediffused metal-oxide-semiconductor field-effect transistors (VDMOS) and schottky barrier diodes (SBD).The average lifespan of the two are 1.47×107 h and 4.3×107 h.Calculate the work failure rate according to the series relationship as shown in formula (15):

According to the above calculation results,the reliability of the closed-loop controlled all-fiber current transformer can be further compared when the complete dual acquisition loop configuration and the independent dual acquisition loop configuration are adopted,respectively.The difference between the two schemes is mainly dependent on whether the configuration of each module is double-set.The two sets of configuration parts are considered in parallel,and the failure rate is calculated as shown in formula (16):

In formula (16):m is the number of parallel components.The module configuration number and reliability calculation results of the two schemes are shown in Table 4.

Table4 System failure rate calculation results for different configuration schemes

Name Failure Rate Number of dual acquisition loop modules Number of independent dual acquisition loop modules High voltage side fiber sensitive ring 2.2831 2 1 Low voltage side optical path unit 4.6502 2 1 AD pre-processing circuit 0.27908 2 2 AD acquisition circuit 0.0949 2 2 DA acquisition circuit 0.0949 2 2 Other circuit 0.57 2 1 Low side power module 0.0173 2 1 System failure rate - 5.3263 7.8332 Average life (year) - 21.3 14.6

The calculation results in Table 4 show that when the all-fiber current transformer adopts the independent dual acquisition loop scheme,the system failure rate increases by about 32% compared with the dual acquisition loop scheme,but the service life expectation is close to 15 years.Thus,the requirements for application in engineering are met.

5 Economic analysis

Further comparison and analysis of the closedloop control of the all-fiber current transformer,and independent dual acquisition loop configuration along with the economic feasibility of its use was carried out.The price of each component is quoted by the current mainstream manufacturer,and the calculation results are shown in Table 5.

Table5 Cost calculation results for different scenarios

Number of independent dual acquisition loop modules Polarization-maintaining fiber delay ring 2100 2 1 Name Unit price (yuan)Number of dual acquisition loop modules Elliptical core polarizationmaintaining fiber 145 4 2 Circular fiber 210 20 10 Guaranteed fiber optic mirror 450 2 1 SLD light source module 2900 2 1 Polarization-preserving coupler 1600 2 1 PIN-FET detector 1600 2 1 Phase modulator 6600 2 1 Fiber polarizer 1100 2 1 Fiber optic cable 16 20 10 Pre-processing circuit module 200 2 2 AD conversion circuit module 200 2 2 DA conversion circuit module 200 2 2 FPGA+ DSP 3000 2 1 Low voltage side power module 500 2 1 Total cost (yuan) 46000 23600

The calculation results in Table 5 show that when the all-fiber current transformer adopts the independent dual acquisition loop scheme,the total cost is reduced by 48.7% compared with the dual acquisition loop scheme.

6 Conclusion

Presently,most of the smart substations in China use conventional current transformers.The number of intelligent substations using all-fiber current transformers is less than 5%.On the one hand,the application of new technologies needs to be gradually piloted,and on the other hand,the high cost of applying all-fiber current transformers makes it difficult to promote them in largely developed areas.Considering the all-fiber current transformer acquisition loop as an example,this paper proposes a single optical path-based independent optical fiber current transformer independent dual acquisition loop design scheme,which actualizes the acquisition function of two independent all-fiber current transformer products in an integrated manner.Reliability and economic analysis show that the solution can significantly reduce the cost of all-fiber current transformers,and only slightly reduce the reliability of high products,thereby significantly improving the economics of all-fiber current transformers in engineering applications.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No.U1866203).