Design of Power Supply for On-line Monitoring System of Transmission Lines

This paper uses CT to gain the energy directly from the high-voltage transmission line, to address the problem of power supply for monitoring system in high voltage side of transmission line. The draw-out power coil can induce voltage from the transmission line, using single-chip microcomputer to analog and output PMW wave to control the charging module, provides a stable 3.4 V DC voltage to the load, and solve the problem of easy saturating of core. The power supply based on this kind of draw-out power coil has undergone the overall testing, and it is verified-showing that it can properly work in a non-saturated status within the current range of 50 1000 A, and provide a stable output. The equipment also design protection circuit to improve the reliability to avid the impacts of the impulse current or short-circuit current. It effectively solves the problem of power supply for On-line Monitoring System of Transmission.


Introduction
The monitoring devices need to be installed directly in the transmission lines, so how to provide electrical power for the devices is one of the key issues we need to resolve.The power supply of study has important practical value.Now the most widely used method is the solar energy [1], but this method is susceptible to affected by climatic conditions, and lack of long-term maintenance-free capacity.The laser energy has been applied in the electronic current transformers and active optical current transformer [2,3], but such power is not suitable for work in the field.Using the capacitive bleeder is the use of a capacitor to obtain energy from the high-voltage transmission, but the stability and reliability of this method is poor, and the power of the method is limited [4].The best way of power supply is using CT to take energy directly from the transmission lines.In this paper, a new power supply used in high-voltage buses is designed and verified by experiments.

Basic Schematic of CT Inductive Power
The basic schematic of the power supply is shown in Figure 1.
The basic principle of Using CT to obtain energy is based on the Faraday's law of electromagnetic induction.
Within the current range of 50 -1000 A, using CT to induce from the high-voltage transmission, then after rectifying, voltage reduction, it can finally output stable 3.4V-DC voltage for on-line monitoring equipment.

The Theoretical Analysis of Draw-out Coil
The draw-out coil works like a transformer, its no-load equivalent model is shown in Figure 2. According to the knowledge of the electromagnetic theory [5,6], the secondary voltage valid value is: where E 2 is the magnetic induction electromotive force RMS; f is the power frequency; N 2 is the secondary-side winding turns; Φ m is the magnetic flux amplitude, the flux amplitude is: where B m is saturation magnetic induction; k is laminated coefficient.
According to Ampere's circuital law: where S is the cross-sectional area of the core; I is the average magnetic path length; I 1 is the exciting current; N is the primary winding turns, which takes one.The relation of magnetic induction peak and magnetic field strength peak is: where 0  is the vacuum permeability; r  is the relative permeability of magnetic core; H m is the peak of the magnetic field strength.By the formulas (1)-( 4) can be known: Form equation (5) we can be known that secondaryside output voltage of the coil is related to the primary side of the excitation current Im, and has no relation with the secondary-side current.
As is shown in Figure 3, from region 1 to region 4, the magnetic induction B is approximately proportional to field strength H, growing with the increasing of H.But in the saturation region, when Hm increases, Bm slows down or even not increases.The formulas (1) and (2) show that, If B m does not change, the secondary-side voltage RMS E 2 unchanged.So E 2 does not grow with the increasing of wire current in the saturation region.
The literature [7] and experiment results show that the waveform of induced voltage distorts seriously and becomes into a narrow pulse waveform in deep saturation, which is a big challenge for the follow-up circuit.To solve this problem, to increase the magnetic saturation current ,the ways such as adding air gap to the power coil [8] and using feedback compensation control [7] have certain effects.But the air-gap width is hard to control and the structure is too complex, both of which have a bad effect on the reliability of the power.

Front-end Protection Module
When the transmission line being struck by lightning or short circuit, especially the lightning stroke, the relay protection device could not work timely, it will cause the lethal threat to the power apparatus circuit.The impulse current does not only electrical harm but also mechanical harm to the power apparatus.On the one hand, the impulse current can cause the draw-out coil inducing a transient high voltage.On the other hand, it can cause huge electric force to destroy the draw-out coil.In this paper, the double polarity transient suppression diode (TVS) and the voltage dependent resistor are paralleled as front-end protection, and filling with a soft buffer layer between the core and winding to reduce the impact of the electric force.

Voltage Control Circuit
The preceding analysis shows that, when the transmission-line current is higher than a certain value, the drawout coil becomes saturated.The wave of secondary induced voltage become a peaked wave or even a highamplitude pulse wave, so measures must be taken to limit the excessive voltage to prevent the DC-DC module from damage.Equation (5) shows that，when the draw-out core parameters and the turns of the secondary winding is a fixed value，the coil's secondary-side output voltage E 2 is only related to the primary side of the excitation current I m .The higher the voltage E 2 ，the greater the excitation current I m .In order to suppress the draw-out coil into saturation, we can reduce the secondary-side voltage of draw-out core to control the input voltage of the DC-DC circuit within the certain range.The voltage control circuit is shown in Figure 4, comprising a control signal circuit and a step-down circuit.
Step-down circuit is mainly composed of NMOS transistor Q 1 and charge-storage capacitor C. When the DC-DC module input voltage is too high, Q 1 will work and discharge excess energy, to achieve the purpose of reducing voltage.Control signal circuit is constituted by the microcontroller of programmable counter array.When the voltage of the energy storage capacitor C exceeds the normal operating range, the Q 1 will be switched by the control of the microcontroller.Specifically, as shown in Figure 4, the normal operating range of energy-storage capacitor C voltage V c is disposed as V c1-V c2.When V c is higher than maximum normal operating voltage V c2 , the microcontroller outputs a high level signal to close the NMOS transistor Q 1 .The excess energy is discharged through Q 1 and the load is powered by C, capacitor C supplies power for the load and its terminal voltage V c decreases.When V c is lower than minimum normal operating voltage V c1 , the control signal circuit have no output and the Q 1 open.The output power of the rectifier circuit directly supply to the load and charge-storage capacitor C.
The protection circuit includes a voltage regulator tube W 1 , current limiting resistor R 1 and the grounding resistance R 2 .When the voltage V in exceed the breakdown voltage of W 1 , W 1 is turned on, the current flowing through W 1 and producing a voltage drop on R 2 .Therefore, the NMOS transistor Q 1 discharges the excess energy.
To verify whether the circuit can achieve the purpose of reducing voltage, we use the signal generator to replace the control circuit.It loop outputs different dutycycle PWM wave to control the step-down circuit, using 47 Ω resistors R as the load.As shown in Figure 5, the transmission-line current is 200 A, after Q 1 is completely turned on, the voltage of R reduce from 30V to almost 0 V.As shown in Figure 6, when the transmission line current are 300 A, 400 A and 500 A,with the increase of the duty cycle of the control signal, the voltage across the resistor R gradually reduce.When the duty cycle of the control signal is about 90%, the voltage is about 10 V.The purpose of the step-down can be achieved.

Experimenting Platform
We choose the U93-MnZn ferrite U-shaped magnetic core which is produced by the Ferroxcube Company.after the calculation and experiments, the turns is chosen as 400, the diameter of enameled wire is chosen as 1mm.The draw-out coil can set in the transmission line with special design shell.
The experimenting platform is shown in Figure 7.The input of voltage regulator connect to 220V/50Hz AC, the current-generator output is short connecting for producing high current.It can change the output current of the current-generator by regulating the voltage regulator.For facilitating to measure the current of the draw-out coil,   the high-accuracy current transformer is used to detect the current.

Testing Result and Analysis
When in start current 50 A, we need to test weather draw-out coil can provide enough power.We Use a variable resistor to simulate the changing load to test the coil output power.Experiments show that the coil output is related to the load.As is shown in Figure 8,with the current-generator output current I 1 =50 A, when the load resister R is 150 Ω, the coil output power P 2 reach the maximum, as high as 653 mW.It can supply sufficient power for the load.
The design requirements of the draw-out power are that the starting current is 50 A, it can provide more than 500mW power the load (the normal operating power of the transmission line monitoring device with a GPS module is about 500 mW) and it can work stably within the range of 50 A to 1000 A of the transmission-line current.
Using 47 Ω resistors R as load, the power apparatus based on the above parameters is tested in the experiments.Form Figure 9, we can find that the maximum voltage of storage-capacitor C is 12.0 V, minimum voltage is 9.0V.When the NMOS transistor Q 1 close, the energy-charge capacitor C directly supply energy for load and its terminal voltage V c decrease.When Q 1 open, the output power of the rectifier circuit directly supply to the load and charge capacitor C, the voltage V c of C will increase.In Table 1, V max is the maximum input voltage of DC-DC, V min is the minimum voltage of DC-DC, V out is the output voltage.
After the voltage-control circuit, DC-DC module input is 9.0 -12.0 V and out a 3.4 V voltage within the range of 50 A to 1000 A bus current.With bus current 300A， the power apparatus continue working 300 minutes, the ripple wave within 300 minutes of DC-DC output voltage V out is shown in Figure10.After the DC-DC module, the power apparatus can provide a stable 3.4 V voltage.

Conclusions
In this paper, we study the issue about the power for the high-voltage on-line monitoring device.The analysis shows that using CT to induce energy is a low-cost, practical and viable way.
To save the problems of easily saturating and highpower consumption of the exiting power supply, the paper introduces a new design of the high-voltage-side induces power apparatus.A large number of experiments show that, the power apparatus of the paper can prevent the magnetic core from saturating in high current, steadily work in non-saturated and low-power consumption status within the range of 50 A to 1000 A transmission-line current.

Figure 1 .Figure 2 .
Figure 1.The basic schematic of the power supply.

Figure 3 .
Figure 3.The magnetization curve of the magnetic material.

Figure 5 .
Figure 5.The voltage cross the load R.

Figure 6 .
Figure 6.The relation diagram between the voltage of R and the duty cycle.

Figure 8 .
Figure 8.The relationship curve between the output power and the load.

Figure 9 .
Figure 9.The voltage cross energy-shortage capacitor C.

Figure 10 .
Figure 10.The ripple wave within 300 minutes of DC-DC output voltage.