Protection of Converter Dense Power Systems

PROTECTION OF CONVERTER DENSE POWER SYSTEMS

PRESENTED BY:- RAJASHREE SAHOO

REGD NO:- 1241019002

1

Contents

Introduction Definition of PECs PECs Fault Response Grid Code Requirements for PECs Positive impact of PECs Negative impact of PECs Potential Protection Solutions Conclusion Reference

2

INTRODUCTION

The number of Power Electronic Converter(PECs) utilised in power systems throughout the world is increasing.

PECs are found in a huge range of applications, in power systems they are used to interface Distributed Generation (DG) to the main power system and for fault current limiting/interruption applications.

These PEC interfaces generally have a low tolerance to overcurrent and rely on extremely fast acting protection which is integral to the PECs’ control systems.

The widespread introduction of PEC-interfaced energy sources has both positive and negative implications for network protection.

3

4

The task of a power converter is to process and control the flow of electric energy by supplying voltages and currents in a formthat is optimally suited for the user loads.

An ideal static converter controls the flow of power between the two sources with 100% efficiency. Power converter design aims at improving the efficiency.

What are PECs?

Image Courtesy : Ref[8]

CONTD.. There are two types of sources: voltage and current sources. Any

of these sources could be a generator or a receptor (load).

The principle of operation of a converter is based on the switch mode action of its switches. Commutations of the switches generate very fast current and/or voltage transients so that the transient behaviour of the sources is fundamental for converter design.

The transient behaviour of a source is characterized by its ability or inability to withstand steps generated by the external circuit in the voltage across its terminals or in the current flowing through it. Then new definitions could be stated:

5

CONTD. a) A source is a voltage source if the voltage across its terminals can not undergo a discontinuity due to the external circuit variation.

b) A source is a current source if the current flowing through it can not undergo a discontinuity due to the external circuit variation.

Minimizing the losses in the switches maximizes the efficiency of the converter.

These switches must have a voltage drop as low as possible in the ON-state, and a negligible leakage current in the OFF-state.

6

PEC FAULT RESPONSE A PEC’s response is governed by its control system i.e., how it

should behave (in terms of fault current contribution during external faults). how it should behave (in terms of fault current contribution during external faults).

7

Fig. 2. PEC RMS Current Fault Response

Image Courtesy: Ref[2]

Fig. 3. PEC RMS Voltage Fault Response


…CONTD PECs typically share some general characteristics: they are

often configured to attempt to “ride through” external faults, sometimes at the expense of providing significant levels of fault current.

A typical fault response for a PEC during a fault is shown in Fig. 2 and Fig. 3.

8

9

GRID CODE REQUIREMENTS FOR PECs

Fig. 4. Grid Code - PEC ride through requirements for system voltage depressions Image Courtesy: Ref[2]

CONTD.. The UK grid code specifies that a PEC should conform to the

following parameters for faults on the transmission system: “PECs should remain transiently stable and connected for

faults up to and including close-up solid three-phase short circuit faults for up to 140ms. For faults in excess of 140ms a PEC should remain transiently stable and connected for voltage depressions anywhere on or above the heavy black line shown in Fig. 3. For faults involving voltage depressions, and corresponding times, below the black line, it is acceptable for the PEC to trip.”

During the period of the fault the PEC should provide maximum reactive current without exceeding its transient rating limit.

Duration and fault clearance times should be specified in a bilateral agreement between the PEC installer and the Distribution Network Operator (DNO).”

10

POSITIVE IMPACT OF PECS ON POWER SYSTEM PROTECTION

11

While this research focuses primarily on the negative impact PECs have on power system protection there are a number of benefits to having PEC-dense power systems. PEC-interfaced DG normally limits the fault current contribution from the DG and in doing so limits the risk of damage during over current conditions. They also help mitigate certain protection issues:-

1. Reduction of Protection Blinding- Blinding of protection occurs when a protection device is unable to detect a fault due to fault current not being detected by its measurement transformer .

Fig.5. Blinding of Protection


2.

12

Reduction of False Tripping -False tripping may occur when two or more lines are connected to a common bus. The PEC interface reduces the likelihood of this problem occurring by limiting the fault current from the DG as shown in Fig. 6. With the PEC interface in place the protective device on line 2 will measure a higher fault current than the device on line 1 and should trip faster.

Fig.6. False Tripping


NEGATIVE IMPACT OF PEC ON POWER SYSTEM PROTECTION

PECs have a low tolerance to overcurrent and their protection must therefore operate extremely quickly to avoid damage to their semiconductors during a fault.

13

Fig. 7. Fault on PEC Dense Power System


For a fault, as shown in Fig.7, there is a risk that the PEC protection will operate before downstream relays, either tripping or limiting the fault current.

Neither of these responses are desirable from a PEC/network protection coordination perspective.

Tripping before network protection relays will result in an unnecessary loss of load; fault current limiting may result in a sustained fault current condition wherein the fault current is not high enough to trip the overcurrent relays on the network.

14

POTENTIAL PROTECTION SOLUTIONS There are a number of potential solutions to the problems

created for power system protection by PEC-interfaced sources:-

A. Incorporation of Energy Storage Devices: One of the most significant problems with PEC interfaces is their lack of fault current contribution. Energy storage devices could be used to contribute to fault current and thus enable fault detection by conventional overcurrent relays as shown in Fig. 8.

15

Fig. 8.Energy storage fault current contributionImage Courtesy: Ref[2]

B. Control of Fault Behaviour of PECs:

16

Fig. 9.PEC Fold Back Response


Fig.10. PEC Brickwalling Response


Fold back is a method for controlling the fault current provided by the PEC. All of the control is carried out internally by the PEC’s control system.

Directly after the fault, the control system allows the PEC current output to peak; the peak being dependent on the capabilities of the PEC switches and the source supplying the PEC.

This peak fault current level is sustained for a short period of time in an attempt to operate overcurrent protection devices while limiting damage to the PEC’s power electronics.

The brickwalling response differs from fold back in several ways: the fault current output is sustained at a lower level for a longer duration and the output current isn’t reduced to a level less than load current to facilitate switchgear interruption

17

CONCLUSION A description of the behaviour of PECs during faults, both in

terms of fault current and response time has been presented and the positive and negative impacts.

One of the main objectives of this research is to determine the level of PEC proliferation at which the overall protection system may begin to experience problems and to investigate solutions that could be used to mitigate such problems.

18

REFERENCES[1] L. Freeris and D. Infield, "Renewable Energy in Power Systems," 1st

ed Chichester: John Wiley & Sons Ltd, 2008, pp. 122, 133, 192, 144-145.

[2] J. Morren and S. W. H. de Haan, "Impact of distributed generation units with power electronic converters on distribution network protection," in Developments in Power System Protection, 2008. DPSP 2008. IET 9th International Conference on, 2008, pp. 664669.

[3] The Grid Code, Issue 4 Revision 1, Connection Conditions, N. G. E. Transmission, 2009.

[4] H. Wan, K. P. Wong, and C. Y. Chung, "Multi-agent application in protection coordination of power system with distributed generations," in Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century, 2008 IEEE, 2008, pp. 1-6.

[5] J. R. S. S. Kumara, A. Atputharajah, J. B. Ekanayake, and F. J. Mumford, "Over current protection coordination of distribution networks with fault current limiters," in Power Engineering Society General Meeting, 2006. IEEE, 2006, p. 8 pp. 19

REFERENCES[6] S. M. Blair, A. J. Roscoe, C. D. Booth, G. M. Burt, A. Teo, and C. G.

Bright, "Implications of Fault Current Limitation for Electrical Distribution Networks," Developments in Power System Protection, pp. 1-5, 2010.

[7] K. Kauhaniemi and L. Kumpulainen, "Impact of distributed generation on the protection of distribution networks," in Developments in Power System Protection, 2004. Eighth IEE International Conference on, 2004, pp. 315-318 Vol.1.

[8] F. Bordry CERN, Geneva, Switzerland , “Power converters: definitions, classification and converter topologies” .

20

21

THANK YOU

Technology

Protection of Converter Dense Power Systems