1

Adaptive Instantaneous Overcurrent: a Case Studied

in a Real Brazilian System
F. C. Souza Jr, H. S. Sanca, F. B. Costa, B. A. Souza,

Abstract—Recently, a large number of small generator plants coordination shall to be executed. The traditional coordination
have been building on Brazil as reflex of governmental invest- of power system protection studied, is a difficult and hart task
ments on renewable power source. As known, modifications on [1]. Some authors have been proposing techniques to optimize
power supply may have succeeded by a revision on coordination
protection system settings. However, this task is still make in the process of coordination, reducing the protection engineers
many companies using archaic methods. In other hand, re- effort [2], [3], [4], [5]. However, most of these techniques do
searchers have been shown that an efficient and fast method may not have a good computational performance.
be used to determine the new relays settings, this techniques was To guarantee reliable system operation is necessary esti-
called adaptive protection. In this paper, frequency estimation mating the fundamental power frequency [6]. Several meth-
was used to determine modifications on power supply plant in
a real Brazilian system, and these modification on the system ods have been proposed and, most of them perform well
was used to the adaptive overcurrent protection block determine when the signal is not distorted by harmonics or noises. In
the update on relays settings. The application of the methods distribution systems due to the dynamic load, this system
of frequency estimation to the adaptive overcurrent protection suffers high disturbs and it is usually not balanced, this
presented good results and improvements on the protection of causes high problems in frequency estimation [6]. Recently,
the electric system.
a new protection philosophy propos that relay settings have
Index Terms—Adaptive protection, overcurrent protection, fre- be updated automatically following topological or operational
quency estimation, distributed generation, protection, islanding. modifications [7], [8]. This technique was called adaptive
protection.
In distribution systems the main protection applied is the
I. I NTRODUCTION overcurrent function [10]. On traditional protection techniques,
N actual Power Electric System (PES) scenario, modi- network modifications were not considers on coordination
O fications on topology and operational network are fre-
quently. In electric distribution systems changes have been
studies, such as: blinding of protection and false trip are
frequent [9]. Adaptive protection scheme (APS) provides relay
verified in lot of cases, such as: add new consumers to the settings based on operational conditions. In other words, relay
grid, contingence condition, and more recently, connection of settings suit up to operational conditions.
small power generators. In this paper, an adaptive protection scheme is proposed to
In Brazil, generally are used renewable energy sources, such determine instantaneous overcurrent relay setting. The adap-
as: energy based on sugar cane bagasse, wind energy and solar tive overcurrent protection will monitor system modifications
or photovoltaic energy all these known as green energy. done from the estimation of the fundamental frequency. For
Actually, with the connection of distributed generation (DG) this reason, frequency estimation algorithms have to be robust
in the PES, the system has suffered big changes and dynamic and reliable because frequency estimation is used in the
situations in the operation. During the operation of the system protection.
can occur islanding situation of the DG and the parameters, The frequency of the power system has an important role
such as: voltage, current and frequency can suffer changes. in monitoring and control the operational status of the grid or
Generally, the modification on the PES impact in the per- in DG systems. For reliable operation of the smart grid, it is
formance and reliability of the grid. However, the protection essential to estimate the power system frequency accurately
scheme should also include all these changes and update the [6]. For this reason, many algorithms are presented for funda-
relay setting. So, for each network change, a new setting for mental frequency estimation applied in electric power systems,
for example: zero crossing method [11], [12], adjustment of
—————————————————————–
This work was supported by CAPES (Coordenação de Aperfeiçoamento points to a pure sinusoidal waveform (APPSW) [13], hybrid
de Pessoal de Nı́vel Superior) and CNPq (Conselho Nacional de Desenvolvi- method [14], [15], discrete Fourier transform method (DFT)
mento Cientı́fico e Tecnológico). [16].
F. C. Souza Jr, H. S. Sanca and B. A. Souza are with the
Department of Electrical Engineering/COPELE, Federal University of In this paper, three methods of fundamental frequency esti-
Campina Grande, Campus Univesitário Bodocongó, Campina Grande, mation applied in distribution systems in islanding operation
PB, CEP:58.429-900, Brazil. E-mail: francisco.junior@ee.ufcg.edu.br, huil- of DG were evaluated for adaptive overcurrent protection, such
man.sanca@ee.ufcg.edu.br, benemar@dee.ufcg.edu.br.
F. B. Costa is with School of Science and Technology, Federal University as: (i) Zero crossing, (ii) curve fitting and (iii) hybrid method.
of Rio Grande do Norte, Campus Univesitário Lagoa Nova, Natal - RN, Using a distribute architecture composed for three centers
CEP:59.078-970, Brazil. E-mail: flaviocosta@ect.ufrn.br. [4], [17], proposed method was available in a real Brazilian
Presented at the International Conference on Power Systems Transients
sub-transmission system, owned by Eletrobrás Distributer of
(IPST’2015) in Cavtat, Croatia on June 15-18, 2015. Alagoas (EDAL).
 2

Adaptive protection system was built using ATP/MODELS determining of the overcurrent relays. This choice was made
[18], [19]. The three centers that make up the APS system because protective device would have not need high processing
were: substation control center (SCC), control operational power. As indicated in [26], the adaptive protections system
center (COC) and the Inteligent Electric Device (IED). SCC can be implemented using digital signal processor, such as:
center performs monitoring and topological/perational detec- DSP or FPGA chips. Finally, forming the third center, the
tion. COC center is responsible to execute all mathematic IED does traditional digital relays and also proposes adaptive
operations to determine a new instantaneous instantaneous task.
overcurrent relay settings. IED has the same architecture as
proposed on literature [20], [21] with one difference, relay
SCC
parameters do not loaded on device, it is provided from COC
center, through a communication channel.
COC
Results showed that in some cases, traditional protection
scheme does not provide a correct performance when topo-
logic modification occurred. In some cases yet miscoordination
was verified. However, when the proposed APS was apply, in
all cases protection devices concerned a good performance
for network, including when topological and/or operational
Power Electric
modifications occurred. System

Other paper contribution is respect to eliminate the coordi-

Fig. 1. Adaptive Protection System Architecture.
nation difficults, making studied coordination process simpler
and more efficient than the traditional.
A. Substation Control Center (SCC)
II. A DAPTIVE P ROTECTION S YSTEM The SCC has as main function the monitoring and topolo-
The adaptive power system protection is a mathematic gical change detection on electrical grid. In Fig. 2 contains a
toolkit which keeps protection coordination even when the detailed flow chart of SCC.
network passes from topological or operational changes [22].
Researches have been studied several ways to do this func-
tionality. In [2], suggests a linear programming to optimal
coordination maintenance of overcurrent relays, opposite to-
pological and operational changes. In these papers traditional
off-line tools techniques as: power flow and short circuits to
determine time dial setting of time overcurrent relay.
Regarding the system architecture, [23] proposes divide it
into five areas with free access to each others information,
operating on a transmission network with a strong presence of
DG. On the same hand [24] uses a Multi Agent Strategies with Fig. 2. Substation Control Center (SCC).
a Power Protection System (PPS) composed for three layers.
In [24], only instantaneous overcurrent units are controlled by In this paper, SCC center is implemented by frequency
adaptive protection. The necessary parameters to calculate the estimation as will describe on section III.
instantaneous overcurrent reach must be obtained using [25]. To detect a modification on the system, the frequency of the
In [17], uses the same architecture that [24]. However the power network grid is estimated and respected value compared
setting of overcurrent relays determination does not calculate with nominal frequency. To detect a modification on the grid,
on real time. A preliminary step, held in off-line mode, is the method monitor the variation of the frequency respect to
necessary to do all operational network possibilities will be any modification of the system.
considered on the coordination study and, so, adjusts settings To make a correct identification of an operational change,
for each operational scenario has be calculated. Thus, APS is is necessary differ this event of a fault scenario. To do this,
responsible to detection and identification network topology SCC architecture provide a communication channel to receive
scenario. This detection will be used as input of digital relays fault detection of relays. So, an operational modification only
on the PES. With known topology IED will adopt the specific will be indicate if the event do not represent a fault.
adjustment relay settings group to the current topology. When estimated frequency is different of the nominal fre-
Based on [24], [4], a three interconnecting centers composes quency and any fault was detected, a digital message will
the PPS architecture, as shown in Fig. 1. The first center, be send to COC center that will be interpreted as a flag for
Substation Control Center (SCC), is responsible for electrical calculation of new settings.
network monitoring and topological changes identification.
The second center or Control Operational Center (COC), is B. Control Operational Center (COC)
responsible for mathematics determination of setting over- The COC is the layer where all mathematics process nec-
current devices. It is opted for the decentralization of APS essary to setting adjustments of protective devices are made.
 3

Strictly, an uninterrupted voltage and current phasor transfer communication channel to receives adjustments settings that
from COC to the relays would result in the necessity of a are provided using an online method by COC.
very fast communication channel. For this fact, decides for a
duplication on COC for a relays similar architecture describes
in [21], i.e., all steps blocks responsible for the tasks until the III. D ESCRIPTION OF METHODS APPLIED IN THIS WORK
phasor estimation are present as on the relays devices as in FOR FREQUENCY ESTIMATION
the COC. Fig. 3 shown the COC operation architecture.
According to Fig. 3 presents the system must be able to In this section, the description of many methods applied
operate with current and voltage signals in more than one in this work for the fundamental frequency estimation is
point of the grid. This necessity is due to the fact that presented.
the determination of the equivalent network depends on the
knowledge of such signals at various points in the system.
Despite having the same structure of digital relays, COC A. Method based on signal zero-crossings
dont need made phasor estimation for all samples. The win-
This algorithm is based on the measurement of the time
dowing process continues to be done without interruption,
but the following steps: phasor estimation, calculation of interval between two zero-crossings of the sampled signal.
The exact time of the zero-crossing is obtained by linear
the equivalent network and protective devices adjustments
interpolation between two consecutive samples of different
settings, are only performed if the SCC detected any change
sign [13] is given by:
in the electrical network.
The setting adjustments of adaptive protection devices are
extremely dependent of phasor estimation and network equiva- tk−1 Vk − tk Vk−1
tzc = , (1)
lents routine. According [9] network equivalent determination Vk − Vk−1
must be obtained using probabilistic relationships between
voltage and current. Through routines oriented tests was were k denotes the instant of the sample that follows the zero
verified a shorter error value of equivalent network routines crossing; and k − 1 denotes the instant the sample before the
when a minimum 40 samples/cicle of voltage and current are zero crossing, (Vk , Vk−1 ) and (tk , tk−1 ) are the voltage and
used. As IED and COC uses 16 samples/cycle, sets equivalent time in the instant (k) and (k − 1) respectively.
network determinations routine to use the triple samples hate During the time interval between two zero-crossings, it is
value, i.e., the mathematics calculation will be realized once possible to assume that the frequency value is equal to the last
every three complete cycles of samples on phasor estimation. calculated value. The frequency in the instant (k) is calculate
After determination of new adjustments settings, these pa- by:
rameters will be transferee to IEDs thought communication
channel. 1
fk = , (2)
2 ∗ (tzcnext − tzcprevious )
C. Inteligent Electric Device (IED)
were fk denotes the fundamental frequency, in Hz, calculated
A lot of paper on technical literature suggest a relay model
in the instant k by zero-crossing; tzcnext denotes the time
to most kind of analysis on the PES [20], [21], [27]. However,
of the next zero-crossing; tzcprevious denotes the time of the
actually there is no defined a model that provide the adaptive
previous zero-crossing.
protection techniques implementation. For this reason, a new
relay model it is necessary to effectuate adaptive tasks.
[17] describes a device with adaptive characteristics. Ho-
wever, proposed relay model needs a high processing per- B. Adjustment of points to a pure sinusoidal waveform method
formance, this fact makes the proposal commercially non- This method was applied in [13], in this method is used
viable. Furthermore, the IED will concentrate all adaptive trigonometric relations to find the frequency value. The
protection tasks. This fact should be strenuously avoided since method based on three consecutive samples (Vk−2 , Vk−1 , Vk ),
the probability of failure in the system would increase rather. spaced by a time interval △t, was used. In such conditions,
So, an adaptive protective device may have, beyond the the pure sinusoidal wave fulfils the following relationship is
functions of any protective device, the possibility of adjust- given by:
ment settings change with no stop analysis of voltage and
current process. Actually, most recent relay have a logic
Vk−2 + Vk
control process that use digital switch equipments, such as cos(2πf △t) = . (3)
2Vk−1
circuit breakers, status to change relay adjustments settings
[28]. However, to do this process an offline step is necessary.
The frequency (f), is obtained for voltage calculated in the
To torn possible this characteristic, a new step was add
instant (k) and the time interval ∆t is given by:
to the basic relay model proposed in [21]. This step will be
responsible to automaticly relay setting permute.  
In Fig. 4, the proposed device model was presented. As Vk−2 + Vk 1
fk = cos−1 . (4)
can seen in Fig. 4, on the new relay model there is a 2Vk−1 2π△t
 4

* *
vL , v C vL , v C vLd , vCd VLd , VCd
Analog A/D
iL , iC Filter
*
iL , iC
*
iLd , iCd Buffer ILd , ICd
Conversor

^ ,V
V ^
C L

SCC Network S Phasor Thevenin

modifications?
Estimation Equivalent
^
IC , ^
IL ^ ^
E1 E 2
ZL
Z1 Z2

New IOC IED

setting
determination

Fig. 3. Internal architecture of Control Operational Center.

ADAPTIVE RELAY

*
v v v vd Vd
Auxiliary Analogical A/D Phasor
i Filter i* id Buffer Id
i PTs e CTs Conversor Estimation

From COC
Circuit-breakers status Digital
Inputs
^ ^
V I

To SCC
Digital Relay Relay
Comparison
Outputs logic Settings
Open circuit-breakers flag

Fig. 4. Adaptive relay model.

C. Combined frequency estimation technique: hybrid method where: (k) represents the sample number; f rk denotes the
This method was presented by [15], [14] and combines value of frequency estimated by the APPSW algorithm; f0k
two traditionally techniques. The method combines the zero- denotes the value of frequency estimated by the zero-crossing
crossings method and the APPSW method. If T2 is the time algorithm; and fk denotes the final output of the frequency
length of the latest tree cycles. The precision of the zero- estimation algorithm. Results obtained from simulation studies
crossings is corrupted by effect of noise and harmonics and indicate that an appropriate value for T h2 is 0.05 Hz [15].
for minimize this problem is can use the average of the
frequencies of the last three cycles of the signal, given by: IV. A DAPTIVE I NSTANTANEOUS OVERCURRENT
Instantaneous overcurrent setting is most frequently defined
3 by pick-up current unit [1] . However, some authors have been
f2 = . (5)
T2 shown that, also distance protection, instantaneous overcurrent
The equation (5), this equation provides greater immunity relays may act based on local fault where short-circuit current
to signal distortions. However, is less sensitive to frequency was equal to pick-up current setting [10].
variations, for this reason is used f1 (frequency obtained by To obtain the reach of directional instantaneous overcurrent
zero-crossing in one cycle) or f2 (frequency obtained by zero- relay settings, [25] propose the following equation:
crossing in tree cycles), given by: ZLT − ZS × (k1 − 1)
h = , (9)
k1 × ZLT
If |f2 − f1 | < T h1 ,
then : f0 = f2 , (6) where:
else : f0 = f1 , ZLT - line impedance,
ZS - equivalent impedance,
where f0 denotes the final value estimated by the sample count k1 - reliable coefficient.
and interpolate method; and T h1 denotes a threshold value In this paper, system equivalent impedance in an online
equal to the maximum error due to noise and harmonic effects method based on [9], was obtained. To estimate system
in (5) and the value used is 0.01Hz [15]. Thevenin equivalent circuit, load voltage and current samples
The second method APPSW have the same problem that are required.
the zero-crossing. The specified conditions are as follows:
V. C ASE STUDY
f rk−1 + f rk−2 + f rk−3
If − f rk < T h2 , To evaluate the proposed method, a real Brazilian sub-
3
transmittion system was used. This network composes part
then : fk = f rk , (7) of the EDAL system and has 11 buses and 20 lines. The total
installed load has 242.6 MW and 107 MVAr. According to
else : fk = f0k , (8) Fig. 5, two units of DG were connected on the grid by CPC
 5

PCA CPC
DG

TBM

MCO-CHESF

MCO-CHESF CTO
PNO

CZA

BBE

PJA

DG

Fig. 5. Maceió region power system.

and CZA bus. CPC and CZA generators supply about 40% of the main generator greater than when some DG is get out.
total load system. However, to relay R3, near CPC and CZA buses, greater
Four cases were analysed on this paper. On the first, any value was obtained. This occurs because the contribution of
generation was connected on the grid, so all installed load main generator for fault near R3 is less than when no DG
was supplied by main generator. In second scenario, CPC is connected. In Fig. 7 reach of instantaneous directional
and CZA DG were on the grid, and on the following cases, overcurrent is present with all DG are connected on the grid.
generators were disconnected at network, one by one. In each
case, a stream of three relays was coordinated using adaptive
72.5 Instantaneous Reach (% of LT) 62
Instantaneous Reach (% of LT)

technique proposed. 64

Instantaneous Reach (% of LT)

h (R1) h (R2) h (R3)
72 61.5 63.5
Only one side (local relay) instantaneous directional over- 71.5 61 63
current unit was simulated in this paper. The relays was placed 71 60.5 62.5

on the following lines: 70.5 60 62

70
• R1 - MCO-CHESF Line 1 0 50 100 150 200
59.5
0 50 100 150 200 61.5
0 50 100 150 200
Time (ms) Time (ms) Time (ms)
• R2 - TBM-PNO Line 1 (a) (b) (c )

• R3 - PNO-CTO
Fig. 7. Reach of instantaneous directional overcurrent unit when CPC and
The first relays (R1) may protect a line with only 500 m. CZA distributed generator it is on the grid.
On the second protective device (R2), the line protected has
10,34 km and, R3 relay acts on a line with 3,014 km.

A. Without distributed generation C. Out of CPC distributed generator

On this case, all installed load was supply by main genera- CPC distributed generator unit supplies 60% of total GD
tor. As may be seen in Fig. 6, for all relays adaptive proposed of the system. In effect of CPC get out of the grid, main
method obtain feasible results for the reach of instantaneous generator increases its contribution to supply the loads. So, for
directional overcurrent unit. relays installed near main generator, a reduction on the reach
21.5 61 52
of instantaneous directional overcurrent unit were observed.
Instantaneous Reach (% of LT)
Instantaneous Reach (% of LT)
Instantaneous Reach (% of LT)

h (R2) h (R3)
21
h (R1)
60.5 51.5
For the R3 relay, near CPC bus, was verified that a large
20.5 60 51 variation on Thevenin equivalent system. In addiction to this
20 59.5 50.5 variation, reach of overcurrent instantaneous unit was verified,
19.5 59 50 see Fig. 8.
19 58.5 49.5
0 50 100 150 200 0 50 100 150 200 0 50 100 150 200
Time (ms) Time (ms) Time (ms) 71.9 60.62 55
Instantaneous Reach (% of LT)

Instantaneous Reach (% of LT)

Instantaneous Reach (% of LT)

(a) (b) (c ) 71.8

h (R1) h (R2) 50 h (R3)
60.6
71.7 45
71.6 60.58 40
Fig. 6. Reach of instantaneous directional overcurrent unit when no distributed 71.5
35
60.56
generator it is on the grid. 71.4
30
60.54 25
71.3
20
71.2 60.52
15
71.1 60.5 10
0 50 100 150 200 0 50 100 150 200 0 50 100 150 200
Time (ms) Time (ms) Time (ms)
(a) (b) (c )
B. With CPC and CZA distributed generators
In this scenario, the influence of total DG was analysed. In Fig. 8. Reach of instantaneous directional overcurrent unit when CPC
distributed generator get out of the grid.
this cases, contribution of power generations to fault current
make reach of instantaneous overcurrent unit of relays closer
 6

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