Solid State Protection for Appliances in 220V DC Home Distribution System

978-1-4799-8050-5/14/$31.00 ©2014 IEEE

Solid State Protection for Appliances in 220V DC Home Distribution System

Girish Makarabbi, Kunal Lohia, RakeshBabu Panguloori, and PriyaRanjan Mishra Philips Research India,

Bangalore, INDIA

girish.makarabbi, kunal.lohia, rakeshbabu.panguloori, [email protected]

Abstract This paper presents scheme of solid state protection for electrical appliances used in DC home. The comparison between electromechanical circuit breaker and solid state protection device (SSPD) based on various parameters is being discussed in this paper. This paper discusses about how SSPD tripping characteristics are being matched with that of circuit breaker characteristics. The control block diagram has been presented for SSPD circuit depending upon two tripping zones of the circuit breaker which are thermal trip zone and magnetic trip zone. The simulation and the hardware results of the protection circuit are given and they are matching up to a large extent. An optimized heat sink design is discussed, considering the power dissipation during overload and short circuit conditions.

Keywords: Solid state protection, circuit breaker, Thermal trip, Magnetic trip, Short circuit fault current, Overload current, Heat sink

I. INTRODUCTION

The technology is advancing day-by-day and with the advancement of technology, the usage of non-sinusoidal or direct current (DC) loads is also increasing. All the IT loads like computer, laptops, mobile charger and many more are internally using DC power. The new generation LED lighting also internally uses DC power. Therefore, the concept of DC power distribution is promising and gaining more interest. Hence the distribution of electricity through DC grid will result an increase in overall energy efficiency along with benefits such as economic savings and higher system reliability [1]. But with DC power systems, there comes the need of protection of electrical appliances supplied by DC in the home.

As we know that alternating current (AC) grid systems are protected by AC circuit breakers which are less complex than DC circuit breakers due to natural zero crossing in AC systems. AC circuit breakers which are rated for 230V AC can be used in DC systems up to 50V DC. Since power distribution (in the range 2kW to 5kW) at 50V DC voltage level is inefficient due to higher cable loss [2], power distribution at higher voltage is preferred. Therefore, the need for DC circuit breakers is must. But the conventional DC electromechanical circuit breakers (EMCB) are costly and slower fault clearing response.

The solid state protection device is proposed for the protection of appliances in DC home as it has faster tripping

response when compared to the mechanical circuit breaker. The MOSFET is used as the switching device. The advancement in the semiconductor technology has greatly improved the reliability, voltage and current handling capability and also reduced the ON state resistance of MOSFET. Due to large scale of production, MOSFET has become cost-effective and attractive choice as switching device in the SSPD. However, power dissipation due to ON state resistance heats up the device especially during short circuit and overload condition. This necessitates proper heat sink for the MOSFET to keep its junction temperature within the safe value [3].

In this paper, the control unit of SSPD is designed to match the electromechanical circuit breaker characteristics. Being semiconductor device, SSPD can be easily configured to emulate fuses or classic circuit breaker characteristics or employ completely different protection schemes. But with the present technology, solid state protection device cannot provide the physical isolation equivalent to an air gap available at open mechanical contact. To overcome this, hybrid designs featuring an electrical contact in series with the semiconductor circuitry which remains open in the semiconductor “OFF” state to ensure required physical isolation can be considered.

The paper is organized as follows. The characteristics of 1A mechanical circuit breaker are discussed in Section II. In Section III, the control unit of SSPD and its simulation model is explained. The heat sink design for the SSPD is discussed in Section IV. Simulations and practical results are presented in Section V. Finally conclusions are given in Section VI.

II. STUDY OF ELECTRO MECHANICAL CIRCUIT BREAKER

Electromechanical circuit breakers are used for protection of appliances and will trip whenever there is an abnormal condition like short circuit and overload conditions in the network. There are two operation regions; one is thermal trip region and other one is magnetic trip region [4]. The thermal trip region is due to thermal effect of over current and the magnetic trip region is due to electromagnetic effect of short circuit current. The tripping characteristics of MGN61501 circuit breaker from C60H-DC [5] series is shown in Fig. 1.

From the shown characteristics, the circuit breaker will account thermal trip region if the overload current is less than

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7-10 times the rated current and it will account magnetic trip region if the short circuit current is in the range of 7-10 times the rated current. The thermal tripping time is of the order of seconds and the tripping time decreases with overload. The magnetic tripping time is of the order of few milliseconds and remains almost constant when the short circuit fault current exceeds 7-10 times the rated current.

Fig. 1. The tripping characteristics of MGN61501 circuit breaker from datasheet [5]

The MGN61501 is a 1A electromechanical circuit breaker. The circuit breaker is tested from overload current 2A to short circuit current 12A. The results are tabulated in Table-1 and illustrated in Fig. 2. It is seen, that the thermal trip zone of the circuit breaker is from 2A to 8A and the magnetic trip zone is between 8A to 10A. Based upon the two tripping zones of the circuit breaker, solid state protection device has been designed. The typical I2t capability of the MGN61501 circuit breaker is taken as 150 A2s. TABLE I. TRIPPING TIME OF EMCB AT DIFFERENT I/IRATED VALUES

Tripping characteristics of C60H-DC CB I/Irated Time I2t

2.14 28.63 s 131.1 3 14 s 126 5 6 s 150 7 4 s 194

7.7 2.43 s 144 9.2 5.5 ms 0.47 12 5 ms 0.76

Fig. 2. Measured tripping characteristics of MGN61501circuit breaker

III. SOLID STATE PROTECTION DEVICE

With the advancement in semiconductor technology, the solid state circuit breaker can be cost effective alternative for the conventional DC circuit breaker. The fast turn-off characteristic of semiconductor switch (like MOSFET) can be used for fast interruption of DC short circuit current. The solid state switch can turn ON and OFF within 2-20 μsec [6], whereas electromechanical circuit breaker relatively takes longer time to clear DC fault and fuses need physical replacement after the operation. The fast clearing characteristic of the semiconductor switch, improves the protection operation speed and this reduces the stress on the appliance. Once the fault is cleared, the SSPD can be made auto reset or remote reset [6].

The solid state protection device is an electronic protection device used for the protection of the electrical appliances in DC home. The SSPD trips for the short circuit current and also for the overload current. However, the trip time depends on the current sensor response and the controller logic of SSPD. The controller unit produces the control signal to the switching device. To achieve fast turn-off and turn-on of the switching device like MOSFET, the corresponding driver is needed. The MOSFET driver helps to remove charge carrier concentration as fast as possible in the conductive semiconductor junction and brought down to zero [6]. The turn-off time of the MOSFET, driven by the driver, is typically in microseconds.

The control block diagram of SSPD is shown in Fig. 3. In this paper, distribution voltage is considered as 220V DC. A current sensor is used in series with the switching device. When the sensed current exceeds 10.Irated value, it corresponds to magnetic trip. The short circuit current will take time to reach its maximum because of line inductance ‘L’ and line resistance ‘R’ in the circuit [7]. If the sensed current is less than 10.Irated value but higher than the rated current then the output of current sensor is squared and integrated to check the I2t capability (equivalent of MGN61501 circuit breaker) for thermal trip action.

Fig. 3. Block diagram of solid state protection device

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There are various operating conditions in the proposed control unit, which are explained below. Normal operation: During normal operation, the current is less than or equal to the rated current of SSPD. The current sensor output is neither greater than rated current nor short circuit current hence the control signal produced by control unit remains high to keep switching device in ON condition. Short circuit condition: During short circuit condition, the sensed current is several times [8] higher than the rated current. The SSPD has to act fast to clear the fault before the fault current reaches to destructible value. If the sensed current exceeds more than 10 times the rated current, the control signal becomes low within a microsecond and the switching device is turned-off to clear the fault. Overload condition: The circuit is in overload condition, if the sensed current is higher than the rated current but less than 7 times the rated current. In this case, the sensed current is squared and integrated to get I2t value. This value is compared with I2t value of 1A MGN61501 circuit breaker. If the measured I2t value exceeds the set value, the control signal becomes low and the switching device is turned-off to clear overload condition. Inrush current: The inrush current of the appliance/load may lead to overload condition and can cause false operation of SSPD. To differentiate between inrush current and overload current, blanking interval is provided. If the sensed current remains higher than the rated current even after the blanking interval, the SSPD considers it as overload condition and turns-off the switching device otherwise SSPD goes to normal operation.

Fig. 4. Simulation model of Solid State Protection Device The simulation model of the proposed control unit is developed using PSIM simulation tool and is shown in Fig. 4. It is designed to have tripping characteristics of 1A MGN61501 electromechanical circuit breaker. The simulation and practical results are presented in Section V.

IV. HEAT SINK DESIGN FOR SSPD

The solid state power switches like MOSFET have higher voltage drop compared to mechanical contacts, and therefore heat sink is needed because of higher power dissipation. The heat sink is required to dissipate the heat generated in the circuit and to protect the MOSFET from junction heating, fusing issues in the case, when high current flows during a short circuit fault. There is a maximum allowable junction temperature of the MOSFET above which it will be blown off.

There are two modes of operation for heat sink which are called static (steady during overload current) and dynamic (transient during short circuit current) mode [9]. The thermal equivalent structure of the heat sink is shown in Fig. 5. In static mode, the thermal capacitance is in open circuit mode and does not cause any temperature change. In dynamic mode, the thermal capacitance accounts for temperature change. Choosing the right heat sink (Rth and Cth) for a particular rated circuit with given parameters ‘Tj’, ‘Ts’, ‘P’ and ‘t’ is very important.

Fig. 5. The thermal equivalent circuit of a heat sink

Typically, Rjh and Cj are small compared to Rth and Cth values. Therefore

Tj – Ts = P*Rth*(1- exp(-t / RthCth))

Where Tj = Junction temperature of the MOSFET Ts = Allowable temperature of the surrounding (40°-50°C) P = Power dissipation in the MOSFET = I2

*Rds Rth = Thermal resistance of the heat sink Cth = Thermal capacitance or thermal mass of the heat sink

During static mode or steady mode or overload mode:

Tj – Ts = P*Rth = I2*Rds*Rth (1)

Where, ‘I’ is the overload current.

The thermal resistance of the heat sink is dependent on the thickness ‘d’, cross sectional area ‘A’ and thermal conductivity ‘L’ of the heat sink.

Rth = d / L A (2)

which implies Rth is inversely proportional to cross sectional area of the heat sink. If the area ‘A’ increases, then Rth decreases which will decrease Tj in the overload mode or steady mode [10]. During Dynamic mode or transient mode or short circuit mode:

Tj – Ts = P*t / Cth = I2*Rds*t / Cth (3)

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Where, ‘I’ is the short circuit current which is much greater than the overload current, and t is the short circuit time interval.

The thermal capacitance Cth or the thermal mass of the heat sink is dependent on the mass ‘m’ and specific heat ‘c’ of the heat sink [9]

Cth = m*c (4)

Tj is inversely proportional to Cth, which implies more the thermal mass less will be the junction temperature. Therefore, for a good heat sink the cross sectional area should be large to limit Tj in steady mode and the thermal mass should be more to limit Tj in transient mode.

In the current design, MOSFET IPW60R075CP [11] is chosen which has Rds of 0.075 Ω. Maximum juntion temperature is taken as 125°C and surrounding temperature as 50°C. The worst case current for 1A SSPD is 10A (overload). From equation (1). Rth = = 10°C/W

From equation (2) Cth= With I2t rating of 438A2s, Cth =0.438 J/K So, WV-DT2-101E heat sink is chosen which has Rth = 9°C/W and Cth = 19.8 J/K.

V. SIMULATION AND TEST RESULTS

The circuit shown in Fig. 4 is simulated and the simulation results at different loading conditions are shown in the Figures 6-10. The SSPD is designed for a rated current of 1A. For the Fig. 9, the short circuit fault was given at time t = 3sec. From the simulation results, it can be concluded that SSPD thermal trip time is more than CB thermal trip time and SSPD fault clearing time (magnetic trip time) is less than CB fault clearing time. SSPD provides more overload capability and faster fault clearance than circuit breaker which is more desirable.

Fig. 6. At rated load current of 1A

Fig. 7. At overload current of 2A

Fig. 8. At overload current of 5A

Fig. 9. During short circuit fault at t = 3 sec

Fig. 10. Zoomed view of Fig. 9 at instant of fault

Hardware Prototype:

220V DC is obtained from DC power conditioning unit (PCU), which takes power from solar panel or batteries or grid based on priority. The 220V DC is fed to the load through SSPD as shown in Fig. 11. The SSPD device monitors load current and controls the MOSFET to connect load during normal operation and disconnect the load during overload and short circuit conditions.

Fig. 11. Illustration of SSPD device in a typical DC distribution system

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A hardware prototype is built to verify the proposed control method and is shown in Fig. 12. The components used are DC supply for 220V DC, current sensor IC (fhs40psp600), quad comparator IC (TLC274AI), two multipliers IC (MPY634KP - one for amplifying the current and the other one for squaring the current), signal MOSFET (2N7002 to reset the integrator) and power MOSFET (IPW60R075CP) as switching device.

Fig. 12. Hardware prototype of SSPD

The test results at different loading conditions are shown from figures 13-16. It is seen from Fig. 15, the SSPD short circuit fault clearing time is 30µsec. The tripping characteristics of SSPD and measured data of 1A MGN61501 circuit breaker are plotted in Fig. 17 for comparison. It is observed that SSPD provides more overload capability and faster fault clearance than circuit breaker which is desirable.

Fig. 13. At rated current of 1A (CH1 is 50V/div, CH2 is 1A/div, and CH3 is 5V/div and time is 5second/div)

Fig. 14. At overload current of 2A (CH1 is 50V/div, CH2 is 1A/div, and CH3 is 5V/div and time is 5second/div)

Fig. 15. At overload current of 5A (CH1 is 50V/div, CH2 is 1A/div, and CH3 is 5V/div and time is 5second/div)

Fig. 16. During short circuit current of 10A (CH1 is 50V/div, CH2 is 1A/div, and CH3 is 5V/div and time is 20microsecond/div)

CH 1 Drain to Source voltage (Vds)

CH 2 Load current (I)

CH 3 Gate signal (Vgs)

31 seconds

CH 1 Drain to Source voltage (Vds)

CH 2 Load current (I)

CH 3 Gate signal (Vgs)

CH 1 Drain to Source voltage (Vds)

CH 2 Load current (I)

CH 3 Gate signal (Vgs)

15 seconds

CH 1 Drain to Source Voltage (Vds)

CH 2 Load current (I)

CH 3 Gate signal (Vgs)

Short circuit appiled at this

point

Fault clearing time is 27 microsecond

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Fig. 17. Comparison of tripping characteristics of SSPD and EMCB

VI. CONCLUSIONS

Solid state protection device is presented to protect electrical appliances in DC home. SSPD rated for 1A is designed to match the tripping characteristics of 1A MGN61501 circuit breaker. The control unit of SSPD is tuned to provide more overload capability and faster fault clearance than conventional circuit breaker. By modifying the current thresholds, I2t capability and heat sink size, the same SSPD can be used for other ratings such as 3A, 5A etc.

Being semiconductor device, SSPD can be easily

configured to emulate fuses or to employ completely different protection schemes. Also in future, intelligence to monitor and measure energy, power consumption can be incorporated.

ACKNOWLEDGEMENT

The authors would like to acknowledge Philips Innovation Campus for providing valuable support and Dr. Narendranath Udupa & Geetha Mahadevaiah for their encouragement.

REFERENCES

[1] Panguloori, R.-B.; Mishra, P.; Makarabbi, G.; Gavade V., "Compatibility and Performance Study of Home Appliances in a DC Home Distribution System " accepted for the India Conference (PEDES), 2014 IEEE international conference.

[2] Friedeman, M., van Timmeren, A., Boelman, E. and Schoonman, J. (2008) Concept for a DC-low voltage house, in Smart & Sustainable Built Environments (eds J. Yang, P. S. Brandon and A. C. Sidwell), Blackwell Publishing Ltd, Oxford, UK.

[3] Swart, J.; Pienaar, C., "Mounting DE-Series MOSFETs - a comparison of two recognised techniques," AFRICON 2007 , vol., no., pp.1,6, 26-28 Sept. 2007.

[4] Wei Liu; Huang, AQ., "A novel high current solid state power controller," Industrial Electronics Society, 2005. IECON 2005. 31st Annual Conference of IEEE , vol., no., pp.5 pp.,, 6-10 Nov. 2005.

[5] http://www.schneider-electric.cl/documents/pdf/c60hdc/cataloguec60hdc.pdf.

[6] http://www.topandtail.org.uk/publications/AE_GBProtecting_the_last_mile.pdf.

[7] Berizzi, A; Silvestri, Andrea; Zaninelli, D.; Massucco, S., "Short-circuit current calculations for DC systems," Industry Applications, IEEE Transactions on , vol.32, no.5, pp.990,997, Sep/Oct 1996.

[8] Mehl, Richard; Meckler, Peter, "Comparison of advantages and disadvantages of electronic and mechanical Protection systems for higher Voltage DC 400 V," Telecommunications Energy Conference 'Smart Power and Efficiency' (INTELEC), Proceedings of 2013 35th.

[9] Wasserrab, Andreas; Balzer, Gerd, "Calculation of Short Circuit Currents in HVDC Systems," Universities' Power Engineering Conference (UPEC), Proceedings of 2011 46th International , vol., no., pp.1,6, 5-8 Sept. 2011.

[10] Chaudhari, M.B.; Puranik, B.; Agrawal, A, "Heat Transfer Characteristics of a Heat Sink in Presence of a Synthetic Jet," Components, Packaging and Manufacturing Technology, IEEE Transactions on , vol.2, no.3, pp.457,463, March 2012.

[11] ftp://ftp.efo.ru/pub/efo-ftp/TMP/pub/power/Discretes%20MOSFETs/pdf/IPW60R075CP_rev2+3_PCN.pdf

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Simulation result of SSPD

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