Recent Researches in Electric Power and Energy Systems
Digital relays for smart grid protection
FRANCESCO MUZI
Department of Industrial and Information Engineering and Economics
University of L’Aquila
Monteluco di Roio, L’Aquila 67100
ITALY
francesco.muzi@univaq.it
Abstract: - Power system protection in electrical power engineering aims at eliminating a faulted part in an
electric network from an otherwise healthy power system. Protection coordination involves a process of best
timing any faulted currents so as to reduce the extension of outage areas to a minimum after a fault clearance. In
the paper, innovative protection methods and devices widely based on computer relaying techniques and
reporting are presented and discussed. Moreover, some procedures for the coordination and setting of protection
relays are suggested. The main kinds of microprocessor-operated electronic releases analyzed in the study were
overcurrent, directional overcurrent and undervoltage. Finally, possible future developments of digital
protections are presented and discussed.
Key-Words: - computer relaying, digital protections, microprocessor operated releases, smart grids.
relays are designed to operate with the fundamental
frequency of the fault current.
Other relays influenced by significant waveform
distortion are negative-sequence-overcurrent releases
that can accept THD much lower than 20%, [19].
A further issue connected to smart grid protection
must also be considered, namely the automatic
reconfiguration of the network following the
elimination of a faulted line-segment, which is a
necessary premise for electric service restoration.
The network reconfiguration is carried out through
remotely operated reclosers and circuit breakers that
are usually properly driven by a SCADA system or
better by a DMS (Distribution Management System),
for fast recovery actions aimed at reducing the
extension of the outaged area [21].
In order to overcome changes concerning both the
configuration and operating conditions of a smart
grid, it is also possible to apply adaptive protection
procedures,
which
require
a
hierarchical
configuration of communication lines (preferably
optic fibers) to exchange information with network
computers and other intelligent devices [23].
1 Introduction
Smart grids with massive Distributed Generation
(DG) from renewable sources, which are inherently
uncertain as to their times and quantities, are crossed
by bidirectional power flows that may cause changes
in the direction and magnitude of short circuit
currents and therefore engender misunderstandings
at measurement protection relays [8], [11-12], [14],
[16]. Under these circumstances, the main
consequences are:
-
The loss of selectivity [18].
-
The islanding of DG from the main network.
-
The presence of electromechanical transients
and dynamic instability.
The unexpected "islanding", sometimes named
loss of mains, usually entails that an isolated
generator continues to feed local loads.
In addition, DG can generate a voltage increase at
certain network nodes causing problems in
maintaining voltage profiles within admitted limits,
sometimes also in nodes equipped by transformers
with tap-changers [3] which are usually able to
control voltage within a ±10 % range.
The presence of both DG and non-linear loads also
creates problems in power quality, especially when
ISBN: 978-960-474-328-5
2 The protection system architecture
A digital protection consists of subsystems with
well-defined functions, [1-2] [5-7], [9], [15], [24].
In the general scheme, the computational
processor is central, since it is responsible for
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Recent Researches in Electric Power and Energy Systems
processing, storing and sharing data with peripheral
interfaces [20], [2].
Usually, the relay inputs are the signals of
voltages and currents, often acquired from current
and voltage transformers, that must be properly
conditioned and digitalized by suitable analog/digital
converters.
In the following, the main functions implemented
in digital relays are presented and commented with
an aim to correctly set them for an effective smart
grid protection. This protection, which is of the timedelayed type, can be either delay-dependent or delayindependent and can be devised so as to enable the
construction of different operation curves.
Fig. 1 shows the conceptual scheme and the
operation diagram of the function presented.
The function is unipolar and is energized if the
ground current reaches the operation threshold. The
protection function includes a second harmonic
restraint that ensures greater stability during the
transformer energization [10], [13].
This restraint locks the intervention, whatever the
value of the fundamental current. In this protection
as well, operation is possible in either timeindependent or dependent modality with the same
features as previously described. Fig. 2 shows the
block diagram of the 51N function.
B. The under voltage release
In this case, two functions are available, and are
standardized as 27D and 27S. The former triggers if
the direct voltage component Vd of the three-phase
system is lower than the Vsd threshold calibration,
[22]. The latter is triggered instead if one of the
phase voltages is below the threshold.
3 The overcurrent release
The overcurrent function is available in four models,
furtherly subdivided into two banks, each available
in two groups, named Group A and Group B,
respectively. These groups can be arranged in two
different modalities through an appropriate
configuration of specific parameters. Fig. 1 shows
the general scheme of the overcurrent function.
C. The earth directional overcurrent release
This function has two settings banks, each
available in two models, and there are three types of
operations.
Type 1 determines the projection of the I0 residual
current on the characteristic straight line whose
position can be fixed by adjusting the θ0
characteristic angle with respect to the residual
voltage.
A. The maximum earth overcurrent release
The maximum earth current function (51N) is
characterized by two thresholds.
Signal exceeded threshold
logic selectivity
Timed output
Timed output
Signal exceeding threshold
Trip
Value of internal
timing counter
Fig. 1 Conceptual scheme and operation diagram of the overcurrent function
ISBN: 978-960-474-328-5
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Recent Researches in Electric Power and Energy Systems
This projection is compared with the Is0 threshold.
The timing is always time-independent. Fig. 3 shows
the block diagram for the type-1 modality.
The Type 2 function acts as an overcurrent
protection to which the concept of direction was
added. It is useful for single-ring configurations or
with grounded neutral.
Fig. 4 shows the block diagram for a type-2
modality. The protection also allows to set a T1
release time, as shown in Fig. 5.
Finally, Fig. 6 shows that Type-3 modality acts as
a zero-sequence overcurrent protection to which a
criterion of angular direction was added.
It is worth to note that the sequence network
theory must be properly revised for its application on
complex smart grids [22], [27].
Signal exceeding
threshold logic selectivity
Timed output
Fig. 2 Block diagram of the 51N function
memory reset
Line-bars
choice
Timed output
memory
Pick-up signal logic
selectivity
Fig. 3 Block diagram for the Type 1 modality for the earth overcurrent release
line-bars
choice
Timed output
Pick-up signal logic
selectivity
External VT
Fig. 4 Block diagram for the Type 2 modality for the earth overcurrent release
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Recent Researches in Electric Power and Energy Systems
-
MODBUS PLUS with a fast dedicated
channel.
As concerns the OSI Application Layers, Fig. 7
and Table 1 show the Modbus communication stack
and layer levels respectively.
trip
Value of internal
timing counter
Fig. 5 Operation diagram for the Type 2 modality
Fig. 7 Modbus communication stack
Line-bars
choice
Table 1 Comparison between OSI and Modbus
levels
Timed
output
Pick-up signal logic
selectivity
External VT
LAYER ISO/OSI
MODEL
7
Application
MODBUS MODEL
6
Presentation
Empty
5
Session
Empty
4
Transport
Empty
3
Network
Empty
2
Data link
Modbus Serial line Protocol
1
Physical
Modbus
Ethernet, etc.
Modbus Application Protocol
Threshold
Trip zone
5 Possible future developments
Fig. 6 Operation diagram of Type 3 modality
In the field of electric energy distribution the time
for radical changes in efficiency, reliability and
security has come, and industry is now ready to
invest in developing the advanced technologies
necessary to improve the new smart grids. As a
matter of fact, these must be managed and controlled
by a set of advanced, digitally-based technologies,
which include phasor measurement, centralized and
integrated voltage and VAR controls, grid
automation, advanced monitoring and diagnostics.
The expected improvements will affect network
power quality and security, and be verified by means
of remote, real-time inspection performed through
new telecommunication systems using advanced
encryption [25].
In combination with the new challenges coming
from the need to safeguard the environment while at
same time assuring the required energy, a myriad
new technologies will soon reshape the traditional
4 Communication protocols
Normally, communications between different
sensitive devices installed takes place with a
MODBUS protocol. This is a protocol placed at level
7 of the OSI (application Layer). The different
implementation types of a Modbus protocol are
shown in the following:
-
TCP/IP –ETHERNET.
-
Serial asynchronous transmission applied
with various supports (EIA/TIA 232 E,
EIA/TIA 422, EIA/TIA 485 A, optical fiber,
radio, etc.).
ISBN: 978-960-474-328-5
Master/Slave,
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Recent Researches in Electric Power and Energy Systems
fundamentals of electrical distribution, including
power protection systems. Some of the technologies
required to implement smart grids are already
available on the market.
Basic SCADA (Supervisory Control And Data
Acquisition) systems have actually evolved towards
DMS (Distribution Management System), and
Geospatial information systems (GISs) can be
integrated with Outage Management Systems
(DMSs). New advanced sensors allow accurate, realtime evaluations of network performances.
Advanced Metering Infrastructures (AMIs) in
combination with Fault Detection, Isolation and
Recovery (FDIR) systems represent a powerful
means to reduce both the SAIFI (System Average
Interruption Frequency Index) and SAIDI (System
Average Interruption Duration Index), [17]. On the
other hand, differently from the estimated,
approximated methods used in the past, the present
accuracy of outage reports may increase the CAIDI
(Customer Average Interruption Duration Index),
while quick automatic service restorations may cause
a shift of events from SAIFI into MAIFI
(Momentary Average Interruption Frequency Index).
This may lead to further investments in research
targeted at improving grid power quality and
reliability.
[3] D. Westermann, P. Bretschneider, H. Rüttinger,
A Novel Approach for Distribution System
Operation Utilization State of the Art
Communication Technology, IEEE Power and
Energy Power Society General Meeting 2009,
Calgary, Canada, 2009.
[4] F. Muzi, Real-time Voltage Control to Improve
Automation and Quality in Power Distribution,
WSEAS Transactions on Circuits and Systems,
Issue 4, Volume 7, April 2008.
[5] Schneider Electric, Sepam user-manual – 2012.
[6] F. Muzi, A. De Sanctis, P. Palumbo, Distance
protection for smart grids with massive
generation from renewable sources, 6th
IASME/WSEAS International Conference on
Energy & Environment (EE'11), Cambridge
(UK), 2011.
[7] M. Cerullo, G. Fazio, M. Fabbri, F. Muzi, G.
Sacerdoti, Acoustic signal processing to
diagnose transiting electric-trains, IEEE
Transactions on Intelligent Transportation
Systems, Vol. 6, No. 2 June 2005
[8] C. Buccella, C. A. Canizares, C. Cecati, F. Muzi,
P. Siano, Guest Editorial for the Special Section
on Methods and Systems for Smart Grids
Optimization, IEEE Transactions on Industrial
Electronics, Vol. 58, Number 10, ITED6,
October 2011.
[9] F. Muzi, Distance relays in conjunction with a
new control algorithm of inverters for smart
grid protection, 2011 CIGRE International
Symposium, Bologna, Italy, 2011.
[10] M. Gong, X. Zhang, Z. Gong, W. Xia; J. Wu, C.
Lv, Study on a new method to identify inrush
current of transformer based on wavelet neural
network, Electrical and Control Engineering
(ICECE), 2011.
[11] F. Muzi, M. Barbati, A real-time harmonic
monitoring aimed at improving smart grid
power quality, 2011 IEEE International
Conference on Smart Measurements for Future
Grids (SMFG), Bologna, Italy, Nov. 14-16,
2011.
[12] G. Houlei, P. Qingle, A. Yanqiu, Z. Baoguang,
Q. Xiaosheng, W. Yuanbo. T. Chun, New type
of protection and control method for smart
distribution grid, Developments in Power
Systems Protection, 2012, 11th International
Conference on Digital Object Identifier, 2012.
[13] F. Muzi, R. Dercosi Persichini, An analysis of
overvoltages in large MV-Cable installation,
15th IEEE-ICHQP International Conference,
Hong Kong, 17-20 June 2012.
6 Conclusions
An important, as yet unsolved problem with smart
grids involves the present criticality of network
infrastructures for renewables and distributed
generation. Actually, the intermittency of large scale
renewable energies in combination with a necessarily
distributed-type generation introduces another
challenge, namely the management of power flowing
back and forth along the grid. This problem also
affects fault currents and consequently the operation
of protection systems. Possible solutions may surely
be found with an improvement of digital protections.
In this paper the some new kinds of microprocessoroperated releases used to protect smart grids are
presented, analyzed and discussed.
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ISBN: 978-960-474-328-5
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Recent Researches in Electric Power and Energy Systems
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ISBN: 978-960-474-328-5
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