Fecha de Recepción: 31/may/2021 Fecha de Aceptación: 10/jul/2021 DOI: 10.47187/perspectivas.vol3iss2.pp54-61.2021
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Analysis of Power System Stability of a Hybrid
Microgrid for On-grid and Off-Grid Operation by
using ETAP software
Análisis de Estabilidad del Sistema de Potencia de una
micro-red Híbrida para Operación Conectado y No-conectado
a la red Utilizando el Software ETAP
Holguer Noriega
, Victor Herrera-Perez
, Mayra Pacheco-Cunduri
,
Esteban Guevara-Cabezas
§
, Fernando Vaca-Urbano
, Iván Ortiz-Parra
,
Facultad de Ingeniería en Electricidad y Computación, Escuela Superior Politécnica del Litoral, Ecuador
,,§
Escuela Superior Politécnica de Chimborazo, Riobamba, Ecuador
Investigador Independiente, Riobamba, Ecuador
Email:
hnoriega@espol.edu.ec,
isaac.herrera@espoch.edu.ec,
mayra.pacheco@espoch.edu.ec,
§
esteban.guevara@espoch.edu.ec,
fearvaca@espol.edu.ec,
vanchos_mop@hotmail.com
Abstract— In this article, we worked under a hybrid
microgrid design that includes photovoltaic generation and elec-
trical network to perform a stability analysis of the microgrid
response considering off-grid and on-grid operation scenarios.
The methodology used for stability analysis was developed using
the ETAP software, from the modeling and simulation of 4 cases
corresponding to different operating scenarios of a 400 kW
concentrated electrical load micro-grid, in which possible stability
failures were identified. Finally, the worst type of failure that
occurred was tested and resolved, determining that the
photovoltaic system does not influence the stability in off-grid
operation, assigning the instability to the auxiliary devices of the
network, and to the speed of their response to the failures. It was
concluded that the critical clearance time and the critical
clearance angle of the fault are crucial to know if an electrical
power system will be able to return to a stable condition or become
unstable.
Keywords Power System Stability, Hybrid Microgrid,
ETAP, Fault clearance, Prosumer
Resumen— El este artículo se trabajó bajo un diseño de
micro-red híbrida que incluye generación fotovoltaica y red
eléctrica para realizar un análisis de estabilidad de la respuesta de
la micro-red considerando escenarios de operación aislada y
conectada a la red red. La metodología de análisis de estabilidad
se desarrolló utilizando el software ETAP, a partir del modelado y
simulación de 4 casos correspondientes a distintos escenarios de
operación de la micro-red de carga concentrada de 400 kW, en los
que se identificó las posibles fallas de estabilidad. Finalmente se
probó y resolvió el peor tipo de falla ocurrida, determinando que
el sistema fotovoltaico no influye en la estabilidad en operación
aislada, adjudicando la inestabilidad a los dispositivos auxiliares
de la red, y a la rapidez de respuesta de los mismos a las fallas. Se
concluyó que el tiempo de despeje crítico y el ángulo de despeje
crítico de la falla son cruciales para saber si un sistema de energía
eléctrica podrá volver a una condición estable o volverse inestable.
Palabras Clave Estabilidad de Sistema de Potencia, Mi-
crorred Híbrida, ETAP, Despeje de Falla, Prosumer.
I. INTRODUCTION
It is well known that “Power System Stability” has become
an important and trendy topic used in the generation and
transmission of power, according to [1]. here is a field that has
gained notoriety in electrical generation systems called
microgrids which allow bidirectional power flow (from suppli-
ers to consumers, and vice versa), using digital technology and
encouraging the incorporation of renewable generation sources
[2].
Microgrids are on-site generation and storage resources that
serve a localized electrical load and also can be con-nected/
disconnected from a larger electrical grid in cases of outage or
instability [3]. Hybrid systems incorporate gen-eration assets,
often in the form of renewable energy like wind generation,
photovoltaic generation and battery-based energy storage [4].
The implementation of a Smart grid and microgrids need the
development of electrical equipment such as, smart meters,
protection relays, special transformers and so on. For instance
[5] proposed a novel transformer which permits the integration
of renewable sources direct to the grid avoiding the cost of
high-power converters, as well as allowing the bidirectional
power flow.
Keeping the balance between power supply and load has
become problematic for usability in recent years [5]. Microgrid
implementation improves the control of the power supply-
load balance by offering storage and generation services to
the main grid [6]. Therefore, it is imperative to study and
evaluate the behavior and stability of these systems to identify
failures, elements of the network that cause them and predict
future results and implement new technologies that serve as
improvements to guarantee the optimal functioning of the
system in terms of reliability. and stability [7]. This type of
analysis is carried out in the present work through the
modeling and simulation of a microgrid using the ETAP
software, in which a hybrid microgrid between photovoltaic
and electrical generation with a configured load of 400 kW is
considered.
The parts of the paper are organized as follows: in section
II Power System Stability. Section III brief description of the
Modeling and Simulation used. Section IV the details of the
methodology while in section V the performance assessment
of methodologies is reported. Conclusions are given in section
VI.
II. POWER SYSTEM STABILITY
The power system is a highly nonlinear system that oper-
ates in a constantly changing environment; generator outputs,
operating parameters, and loads mutate continually. When
disturbances occur, the stability of the system depends on the
initial operating condition and the nature of the disturbance
[8].
Power system stability is the capacity of an electric power
system, for a given initial operating condition, to recover equi-
librium of operation after being exposed to a large disturbance
(sudden load changes, switching operations in power elec-
tronic devices, faults in the system, etc.) with most operation
variables controlled, so, practically the entire system remains
undamaged [9,10].
According to the literature, the power system stability is
classified into Steady State, Transient and Dynamic Stability.
Steady State Stability studies are constrained to small and
gradual changes in the system operating conditions. The
attention here is limiting the bus voltages close to their
nominal values. At the same time guaranteeing that phase
angle between two buses are not too large and performing
evaluations for the overloading of the equipment and
branches. These evaluations are usually performed by
power flow studies [11].
Transient Stability involves the study of the power system
following a major disturbance. Following a large distur-
bance a synchronous generator response to fast changes
in electromechanical swings and during these changes the
rotor angle stabilizes at a new value or the rotor angle
gradually increases which may lead the system to a loss
of synchronism [12].
Dynamic Stability is the capacity of a power system to
keep stability under continuous small disturbances
because of unplanned fluctuations in loads and generation
levels [13].
A. The Swing Equation
Under normal operating conditions, the relative position of
the rotor axis and the resultant magnetic field axis is fixed. The
angle between the two is known as the power angle or
torque angle. During any disturbance, the rotor will decelerate
or accelerate with respect to the synchronously rotating air
gap Mmf, and a relative motion begins [14,15]. The equation
describing this relative motion is known as the swing equation,
represented as follows:
M
d
2
dt
2
= P a = Pm P e
(1)
Where,
M =
2H
ωs
(2)
P a is the accelerating power,
Pm is the mechanical power,
P e is the electrical power output,
ωs is the synchronous angular velocity of the rotor, δ
is the synchronous machine rotor angle,
M is the inertia constant coefficient.
H is the inertia related constant.
B. The Power-Angle Equation
The simplest form of the power angle equation and is basic
to an understanding of all stability problems. The relation
shows that the power transmitted depends upon the transfer
reactance and the angle between the two voltages [16-18].
P e =
E
|V |
X
sinδ (3)
Where,
P e is the electrical power output,
E
represents the transient internal voltage of the generator.
V is the voltage at the receiving end and is regarded as that of
an infinite bus or as the transient internal voltage of a
synchronous motor whose transient reactance is included in the
network.
X is the transfer reactance between E
and V .
C. Equal-area criterion
This method is a graphical explanation of the energy stored
in the rotating mass and help to know the keeping of the
stability of the machine after a disturbance. The colored areas
Al and A2 must be equal, and similarly.
From Figure 1, a critical clearing time could be calculated,
which is the maximum elapsed time from the initiation of the
fault until its isolation such that the power system is transiently
stable. Also, the critical clearing angle can be found through
the following expression:
(4)
δ
cr
= cos
1
[(π 2δ
0
) sinδ
0
cosδ
0
]
Where,
δ
0
is the generator power angle pre-fault condition.
The critical clearing time is:
t
cr
=
4H(δ
cr
δ
0
)
ωs
(5)
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Figure 1. Equal-area criterion- sudden change of load [12].
Where,
ωs is the synchronous angular velocity of the rotor,
δ
0
is the generator power angle pre-fault condition,
δ
cr
is the critical clearing angle.
III. MODELING AND SIMULATION
In order to perform a power system stability analysis, it is
necessary to develop an adequate model of a hybrid microgrid,
with capabilities to operate off-grid and also on-grid. The
hybrid system designed consists of six buses, the power grid, a
diesel generator, and a photovoltaic plant as power sources
connected at a main distribution bus, along with four power
transformers servicing 40 users represented by a static load
with power factor of 1. A simplified diagram of the system is
shown in Figure 2 and Figure 3.
The input data regarding all the elements that comprise the
system is detailed in the following tables:
IV. METHODOLOGY
A stability analysis of the diagram shown in Figure 2 was
conducted by ETAP software based on the next conditions:
Figure 2. Box Diagram of the System.
Table I
POWER GRID.
ID
Rating %Z 100MVA Base
MVAsc KV R X X/R
Grid 1
1000 34.5 0.99504 9.95037 10
Figure 3. Case of study 1: Hybrid microgrid single line diagram.
Table II
GENERATOR.
ID
Rating %Z
%P.F. RPM Poles H
MW KV Xd”
Gen 1
2 0.48 19 80 1800 4 0.384*
*Value automatically calculated by ETAP
Table III PHOTOVOLTAIC
PANELS.
ID
Rating
KW V A
481.1 450.6 402.05 1120
Table IV
INVERTER.
ID
DC Rating AC Rating
MW V A MVA kV A
Inv 1
0.5 400 1250 0.45 0.48 541.3
Table V
TRANSFORMERS.
ID
MVA
Pr kV
Rating
SeckV
%Z
X/R
Vector group
T1 20
im.
10 20 Dyn11
T2 2.5 0.48 13.8 6.25 6 Dyn11
T3 1 13.8 0.48 5 3.5 Dyn11
T4 1 13.8 0.208 5 3.5 Dyn11
Table VI
BUSES.
ID kV
Bus 1
34.5
Bus2, Bus3
0.48
Bus 5
0.208
Bus 4
13.8
DC Bus 1
0.4
Table VII
CIRCUIT BREAKERS.
ID kV
34.5
13.8
0.48
CB1
CB4, CB5, CB6, CB7
CB2, CB3
CB8, CB9
0.208
From the diagram depicted in Figure 2, four cases will
be analyzed. Case 1 (Figure 3) consists about the system in
normal operation, all circuit breakers closed, Grid1 in swing
34.5
13.8
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![t]
Table VIII
TRANSIENT STABILITY PARAMETERS.
Method of solution Newton-Raphson
99
0.00010000
60
Maximum number of iterations
Precision of the solution System
frequency
Units
metric
operation, Gen1 in voltage control operation and Inv1 in
voltage control operation, load1 at 100% and all transformers
operating.
Case 2 (Figure 4) consists in the system in operation with
only two sources, CB1 and CB4 open, Grid1 out of service,
Gen1 in swing operation and Inv1 in voltage control operation,
load1 at 100%, T2, T3 and T4 transformers operating, T1 out
of service. Case 3 (Figure 5) consists in the system in operation
with only one source of power. In this case we consider the
system is operating in off-grid mode. The photovoltaic plant
is the prime source of power. CB1, CB4, CB2 and CB5 open,
Grid1 and Gen1 out of service, Inv1 in swing operation, load1
at 100%, T3 and T4 transformers operating, T1 and T2 out of
service.
Finally, in case 4 (Figure 6) consists in the system in
operation with only two sources, CB2 and CB5 open, Gen1 is
out of service, Grid1 in swing operation and Inv1 in voltage
control operation, load1 at 100%, T1, T3 and T4 transformers
operating, T2 out of service.
In each of the proposed cases, a 3-phase fault and sub-
sequent clearance events are introduced in Bus4 in order to
determine the behavior and stability of the system. The events
are as follows:
Figure 4. Case of study 2: Hybrid microgrid single line diagram without the
Public grid.
Figure 5. Case of study 3: Hybrid microgrid single line diagram with only
photovoltaic generation.
Figure 6. Case of study 4: Hybrid microgrid single line diagram without
synchronous machine generation.
Table IX
TRANSIENT STABILITY PARAMETERS.
ID Time (s) Device Action
Event 1
1.0
Bus 4 3 phase fault
Event 2
1.5
Bus 4 clear fault
Some performance curves of the following elements were
plotted:
For the Generator Gen1: Power angle (relative), Power
Angle (absolute), Speed, Mechanical Power (MW) and
Electrical Power (MW).
For the buses Bus1, Bus2, Bus3, Bus4 and Bus5: Voltage,
Voltage Angle and Frequency.
V. RESULTS
A. Case 1
When the 3-phase fault occurs in 1.0s, both the relative and
the absolute angle of the Gen1 begins an oscillatory process,
the speed increases and the electrical power decreases, as in
the case of simulation, the control element (governor) for the
mechanical power is not considered and this value remains
constant.
At the moment of clearing the fault, the relative angle of
the Gen1 rotor remains in oscillation but manages to quickly
stabilize at 2.0s at its pre-fault values. The same happens for
the values of speed and electrical power delivered.
In the case of Bus1, Bus2, Bus3, Bus4 and Bus5, they
recover the voltage, angle and frequency almost immediately
when the fault clears in 1.5s, the bus that represents the
greatest variation until it recovers its initial condition is Bus2
corresponding to the connection point of Gen1 and followed by
Bus3, connection point of Inv1 and the photovoltaic plant
system, it presents an angle compensation due to its voltage
control operation. In general lines, the system represented in
case 1 remains stable after the events introduced after 2.0s,
meaning for Gen1 that the clearance angle (δ
c
) in 1.5s is within
the initial angle (δ
0
) and the critical clearance angle (δ
cc
),
under the criterion of equal areas, the area A1 would be equal
to the area A2 and therefore Gen1 returns again to its initial
state.
B. Case 2
In this case, after the failure in 1.0s and its clearance in
1.5s, the rotor angle of the Gen1 and its speed begin a steep
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