Protection of Converter Dense Power Systems

PROTECTION OF CONVERTER
DENSE POWER SYSTEMS
PRESENTED BY:- RAJASHREE SAHOO
REGD NO:- 1241019002
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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
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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.
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The task of a power converter is to process and control the flow of
electric energy by supplying voltages and currents in a form
that 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.
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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).
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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.
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GRID CODE REQUIREMENTS FOR PECs
Fig. 4. Grid Code - PEC ride through requirements for system
voltage depressions

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).”
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POSITIVE IMPACT OF PECS ON POWER SYSTEM
PROTECTION
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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.
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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.
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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.
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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.
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Fig. 8.Energy storage fault current contribution

B. Control of Fault Behaviour of PECs:
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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
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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.
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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” .
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Protection of Converter Dense Power Systems

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