a review paper on the protection schemes of the low

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A REVIEW PAPER ON THE PROTECTION SCHEMES OF THE LOW

VOLTAGE DC NETWORKS

Puneet Dwivedi*1, Prof. Parikshit Bajpai*2

*1Student, Department Of Electrical Engineering, SRIT, Jabalpur, India.

*2Guide, Department Of Electrical Engineering, SRIT, Jabalpur, India.

ABSTRACT

Low voltage DC (LVDC) applications find their applications in many field including telecom data centre, vehicle

power system, etc. In this paper the LVDC system and their configurations are reviewed first. Four types of

Architecture of DC Micro grid are discussed. Further the types of the fault that occurs in the DC microgrid is

reviewed further. The existing DC protection schemes and four previous done protection methods are reviewed

in detail.

Keywords: LVDC, Protection Schemes, Microgrid.

I. INTRODUCTION

Climate change and carbon emission are today’s major concern. EU’s plan for 2050 has moved the researches to

focus more toward non-renewable source of energy [1,2]. EU’s framework has plans to cut the carbon emission

to 80 to 95% by the year 2050 and at least 27% usage by 2030. Major challenges faced by the researchers is in

the integration of several non-renewable sources. This problem results the development of smart grids. In [3], it

has been shown that the integration will result in the reduction of usage of fossil fuels and also CO2 emission.

Several energy sources connected together forms Distributed Energy Sources (DES). Integration of several DES is

termed as microgrid [4]. Further, the microgrids can be classified as the AC microgrid and the DC microgrid. Most

of the devices runs on the DC. To prevent the unnecessary power conversion, DC microgrid is proposed for a

distribution system. Focus on the usage of renewable sources and the Electric vehicle gave the concept of

distributing the power at the lower voltage level and this is commonly known as Low Voltage DC (LVDC).

Low and Medium voltage DC were already in use in the applications [5,6] like telecom data center, vehicle power

systems, railway traction system, etc. The development of protection scheme is still the focused research area

and it needs to be explore more [7].

II. ARCHITECTURE OF DC MICROGRID

Many DC microgrid architecture have been proposed considering specific design and the applications [8-11].

DC microgrids can be classified into following four categories.

a) Single bus configuration

b) Multi-bus configuration

c) Ring configuration

d) Interconnected configuration

Single bus configuration: Figure shows the single line diagram of the single bus configuration [12-13]. Single bus

topology is used in the telecom sector to supply 48VDC [14,15]





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Figure 1: Single Bus Configuration

Multibus configuration: Multibus topology makes the system more flexible by providing different amplitude DC

voltages to the different buses. This also improves the power supply reliability [16]. DC Bipolar multibus

configuration is shown in the figure

Figure 2: Multibus Configuration

Ring Configuration: In ring configuration, there are multiple paths present between the source and the load as

shown in the figure. Ring topology are generaly suggested to urban and industrial applications [17].





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Figure 3: Ring configuration

Interconnected configuration: In interconnected configuration, several DC microgrids are connected to the

existing AC Network as shown in the figure. Powerflow can be the birectional according to the requirements.

This configuration is mainly used in the applications where the power reliability need is more for example

unmanned aerial vehicle, shipboards and aircraft [18].

Figure 4: Interconnected configuration

III. PROTECTION IN LVDC SYSTEM

For the system reliability, protection in the LVDC system is very much critical. Whenever a fault occurs, there

must be a protection scheme fot the to protected the connected devices from this fault [19,20]. Following types

of fault may occur in the LVDC systems

a) Overvoltage fault

b) Under voltage fault

c) Short circuit fault





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Short circuit fault is further classified into two types of fault.

a) Line to Line fault

b) Line to Ground fault

In [21], paper addresses the problem of over and under voltage as well as short circuit in Low Voltage DC

(LVDC) system. Disturbance caused by these faults can result in damage of components, loss of energy and can

heavily effect the operation of delicate medical and industrial equipment. This paper describes the protection

schemes, fault calculations and circuit modeling against over and under voltage as well as short circuit in

LVDC system. Solid state switches are used as breaker in circuit model. Finally, validity of the calculations and

results are verified by using MATLAB/Simulink software.

In [22], the protection scheme for the LVDC distribution system is presented. The possible fault cases and

protection requirements are also presented. The future challenge is the integration of protection functions to

the power electronic devices to reduce system costs and to decrease protection system complexity

IV. EXISTING PROTECTION SCHEME

Mehdi Monadi [23], proposed a centralized protection scheme for MVDC (Medium Voltage DC) type

microgrids. The proposed scheme has a communication assisted mechanism for fault detection, along with a

centralized coordinator for the protection. It also has a fault isolation technique that uses the minimum number

of DC circuit breakers. The proposed scheme not only has fault detection and fault isolation techniques but also

a backup protection is there. A backup protection gets activated when the communication falls. The main

components of the proposed scheme are following

1. For fault detection and location, differential based relay is used.

2. To protect the VSC connected to the host network and DG (Distributed Generator), overcurrent relay was

being used.

3. For the supervision of the adaptive devices and making them adaptive to the operating conditions, a

centralized protection unit is implemented.

4. Combination of DCCB and the isolators are there for the Practical Computation of d i /d t for High-Speed

Protection of DC Microgrids he fast fault interruption.

The above mentioned technique have been HIL simulated, and the result obtained found to be optimally fast for

the safety of the VSCs. The time required to restore the network is found to be with 100ms to 300ms.

Chunpeng Li et al [24] in the paper shows that the major challenge that the DC systems has in the short

circuited faults. This problem can be addressed by isolating the faulty networks just before the discharge of the

DC side capacitors. A new method has been proposed to get the new high speed distance protection schemes

using the rate of change of current. In this method, two techniques are presented that optimizes the rate of

change of fault current transients using the MATLAB model of DC microgrid with injected measurement noise.

1. The first method derives the optimized selection of the sampling frequency of the current measurement

signal and at the same time minimizing the noise pickups. In this method the effect of noise is constrained.

Computational step is M times the sampling period ∆T , the equation of computation is represented as

equation 1

[ ] [ ]

…………………..(1)

have been set of 25, 100 and 400 micro sec. to test the sensitivity of the approach. As shown in the

figure . the effect of noise is reduced as the sampling time increases.





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Figure 5: Effect of increased sampling time

The larger time steps are good for the noise suppression but not for the accuracy. It gave rise to the tradeoff

between noise suppression and accuratc. An optimum sampling time is to be used. A method to get the

optimum time step is also discussed in the paper. According to the discussed method the optimum sampling

interval was found to be just greater than the 19.6 micro second.

2. The second method uses the Finite Impulse Response filter, for conditioning of the signal just before the

di/dt computation. The RLC current and voltage response is given below

………….(2)

α is the damping factor, ωd is the damped resonant frequency. VCf is the voltage across the capacitor. Appling

a fourier transform on the above mentioned equation will give rise to the frequency distribution of the fault

current signal.

………… (3)

The obtained result in the paper shows that the main frequency content is found to be in lower band. While

designing a low pass filter the cut-off frequency is found to be

………… (4)

The designed FIR filter gives the following frequency response





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Figure 6: FIR filter response

Further the paper shows that the derivative based over current protection is found to be faster than the

threshold based over current protection.

Khaled et al. [25] uses the travelling wave approach for the fault detection and location finding. In this paper a

travelling wave is studied that used to be generated from the different types of fault in the MVDC circuits. The

proposed method analyses the frequency of the generated travelling wave rather than the arrival time. The

microgrid has been modelled in the PSCAD/ EMTDC. The underground cables has been modelled as the

distributed parameters that depends on the frequency. The telegrapher’s equation has been used to describe

current and voltage travelling wave in the frequency domain

………… …(5)

…………………(6)

Vtw and Itw are the functions of the frequency w. These are the Fourier transform of the voltage and currents

respectively. Solution of Vtw and Itw are given in following equation

…………. (7)

…………. (8)

Vi , Vr, Ii and Ir are the incident and reflective voltages and currents respectively. X is the distance travelled by

the wave. ϒ is the propagation constant. The proposed scheme based on the frequency analysis of the fault

generated travelling wave detects the fault within 128 micro second. The proposed scheme does not uses any

communication systems and only the local measurements has been taken, this process avoid any issues that

may arise due to the communication systems. In this paper a comparison has been done with the previous

methods and the proposed method has been found to be better in terms of selectivity, sensitivity and the

accuracy.

Dong Wang et al. [26] proposes a method based on the both voltage change and the current change. The main

focus of the proposed method is to reduce the level of the fault current within the distribution network. The

result of the multiplication of the both, current change and voltage change is used as a coordination for the

protection on the different relays. The model is implemented in PSCAD/ EMTDC simulator. The used

distribution network is shown in the figure





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Figure 7: Distribution network under test

Limiting the fault current level also give the more time to the overcurrent based protection schemes to operate.

This limited level also support the system to withstand the thermal impact. Flow chart of the proposed scheme

is shown in the figure

Figure 8: Flow chart of the proposed scheme

The paper also gave the future scope of the current limiting techniques to be widely used.

V. CONCLUSION

The application of Low Voltage DC networks are vast and it is the main component of the modern day

microgrid. Different type of fault may occur inside such networks. This review paper highlighted the basic

architecture of the DC network. Faults in the DC networks were also reviewed. Over-voltage, under voltage and

short circuit fault were reviewed. Result of previous protection scheme for DC network were done. In future,

more advance controllers can be design to cate the faults present in the LVDC Networks.

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a review paper on the protection schemes of the low

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