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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