Static and Dynamic Stability Analysis of Distributed Energy Resources Components with Storage Devices and Loads for Smart Grids

Abstract

The distributed energy resources (DER) contains several technologies, such as diesel engines, small wind turbines, photovoltaic inverters, etc. The control of DER components with storage devices and (controllable) loads, such as batteries, capacitors, dump loads, are central to the concept of the Smart Grids (SGs). A SG can operate interconnected to the main distribution grid or in islanded mode. This paper presents experimental tests for static and dynamic stability analysis carried out in a dedicated laboratory for research in distributed control and smart grid with a high share of renewable energy production. Moreover to point out, on a laboratory scale, the coupling between DR and storage and to effectively compensate wind fluctuations a number of tests have been done. In order to find out the parameters of various types of DER components for dynamic simulation models a number of tests are required under different operation modes and loads. The testing reporting here includes three modes of operation: stand alone, parallel/hybrid and grid connection.

Full text

International Review on Modelling and Simulations (I.R.E.M.O.S.), Vol. 4, N. 6
December 2011
Manuscript received and revised November 2011, accepted December 2011 Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved
L. Mihet-Popa1, V. Groza2
Abstract – The distributed energy resources (DER) contains several technologies, such as diesel
engines, small wind turbines, photovoltaic inverters, etc. The control of DER components with
storage devices and (controllable) loads, such as batteries, capacitors, dump loads, are central to
the concept of the Smart Grids (SGs). A SG can operate interconnected to the main distribution
grid or in islanded mode. This paper presents experimental tests for static and dynamic stability
analysis carried out in a dedicated laboratory for research in distributed control and smart grid
with a high share of renewable energy production. Moreover to point out, on a laboratory scale,
the coupling between DR and storage and to effectively compensate wind fluctuations a number of
tests have been done. In order to find out the parameters of various types of DER components f or
dynamic simulation models a number of tests are required under different operation modes and
loads. The testing reporting here includes three modes of operation: stand alone, parallel/hybrid
and grid connection. Copyright ©2011 Praise Worthy Prize S.r.l. – All rights reserved.
Keywords: Distributed Energy Resources, Smart-Grid, Static and Dynamic Stability Analysis
Nomenclature: AGC-Automatic Gen-Set Controller; AVR-Automatic Voltage Regulator; DER-
Distributed Energy Resources; DG-distributed Generators; PV-Photo Voltaic; SG-Synchronous
Generator; SOC-State of charge; VRB-Vanadium Redox Battery; WT-Wind Turbine.
I. Introduction
Renewable energy systems are growing up due to not
only environmental aspect but also due to social,
economical and political interest. As the prevalence of
renewable power grows increasing demand is being
placed on maintaining grid stability and fulfilling grid
codes.
One of the challenges of a smart grid is the ability to
cope with intermittent and variable power sources, such
as wind and solar, due to the variable nature of these
systems [1-2].
The electrical power system is facing an evolution
from the traditional concept of energy generation by few
localized power plants interconnected together through a
meshed system to distributed medium and small scale
generators. Some topologies of these generators
embedded into the distribution network are fed by
renewable sources like wind and sunlight. Their main
drawback is their hardly predictable behavior and
uncontrollable output. The presence of energy storage
system may allow a better management of the electric
system allowing the full exploitation of renewable
energy sources. Distribution companies start to
recognize that storage has the unique ability to act as a
buffer between the grid and generation that is either
intermittent or not controlled by the utility [2-5].
The distributed generation is taking importance
pointing out that the future utility line will be formed by
distributed energy resources and micro-grids. The
flexible micro-grid has to be able to import/export
energy from/to the grid, control the active and reactive
power flows and manage of the storage energy [6-7].
With the publication of IEEE Standard for
Interconnecting Distributed Resources with Electric
Power Systems, the electric power industry has a need to
develop tests and procedures for verifying that DER
components meet the technical requirements [7].
This paper focuses on the static and dynamic
performance and stability of DER components in Smart-
Grids. Dynamic and transient events (wind variation,
connecting/disconnecting sequence etc.) are studied, and
improvement on stability for several cases are presented.
In order to find out the differences between DER
components in power systems and to study the impact
on bus bar voltage and frequency the system will be
tested for different wind speeds and loads.
Static and Dynamic Stability Analysis
of Distributed Energy Resources Components
with Storage Devices and Loads for Smart Grids

L. Mihet-Popa and V. Groza
Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Electrical Engineering, Vol. 4, N. 6,
II. Distributed Energy System-
Experimental Facility
The laboratory under tests is dedicated for research in
distributed control and smart grids with a high share of
renewable energy production. Its experimental facility is
a Wind/PV/Diesel Hybrid Smart-Grid with local storage
and a novel control infrastructure. The facility is spread
across three sites located several hundred meters apart,
as can be seen in Fig. 1.
It includes two wind turbines (11kW and 55kW), a
PV-plant (10kW), a diesel gen-set (48kW/60kVA), an
intelligent office building with controllable loads
(20kW), a number of loads (75kW, 3*36kW) and a
Vanadium Battery of 15 kW/120 kWh. At each of the
three sites there is a switchboard that allows the
components installed at the site to be connected to either
of two bus bars. The two bus bars at each site are
connected to a crossbar switchboard allowing the
flexible setup of the system(s) to be studied. The bus
bars can be either connected to the national grid or can
be part of an isolated system. It allows components and
systems to be in grid connected operation, island
operation, or operation in parallel with wind turbine or
PV-plant. The components are all connected in one
distributed control and measurement system that enables
very flexible setup with respect to experimental
configuration.
Fig. 1. Smart-Grid laboratory for intelligent,
active and distributed power systems.
III. DER Components with Storage
Devices and Controllable Loads
The system under tests contains a Diesel Gen-set of
60 kVA/48 kW, a Wind Turbine of 11 kW, a Dump-
Load of 75 kW, a vanadium-redox battery (VRB) of 15
kW/120 kWh, and a local grid.
All units on the grid – generators, loads, storage
systems, switchgear – are automates and remote-
controllable. Each unit is supervised locally by a
dedicated controller node. The node design combines an
industrial PC, data storage, measurement and I/O
interfaces, backup power and an Ethernet switch inside a
compact, portable container. All nodes are
interconnected via redundant high speed Ethernet, in a
flexible setup permitting on-line changes of topology
and the simulation of communication faults.
III.I. Wind Turbine
The Wind Turbine has 11 KW rated power, 2 bladed
horizontal axis, stall controlled; Rotor diameter has 13
m, rotor speed is keep constant at 56 rpm with a gear
box ratio of 1:18. Starting wind speed is 2.5 m/s, cut in
wind speed is 3.5 m/s, rated wind speed is 9,5 m/s and
cut out wind speed is 25 m/s. The machine driven by
wind turbine is a cage rotor induction generator with
rated power of 11 KW, rated speed of 970 rpm, rated
voltage of 400V and rated current of 21.8/12.7 A; It has
also a capacitor bank of 10 kVAr for self-excitation and
no-load compensation. The consumption of reactive
power at no load is 6.32 kVAr and at full load is 9.7
kVAr [10, 11 and 13].
It has a soft starter in order to reduce the inrush
current during connection. The maximum cut-in current
controlled via soft starter is 22 A, and also max starting
current is 30 A;
III.II. Diesel GenSet
Diesel Gen-set contains a diesel engine with 6
cylinders, a speed governor and a synchronous generator
with an AVR and an AGC.
The governor and the diesel engine system control the
generator speed and provide mechanical power as an
input to the generator. Speed governing is dedicated to
generator response to load changes.
The synchronous machine is a four pole machine with
salient-pole design. It uses brushless excitation and a
digital AVR controller. The AVR controller regulates
the parameters of exciter field to provide a constant
terminal voltage. AVR acts upon the DC Voltage that
supplies the excitation winding of SGs. The variation of
field current in the SGs increases or decreases the emf
(no load voltage) and thus for given load the generator
voltage is controlled.
III.III. Data Acquisition and Control System
The data acquisition and control system (hardware
and software) is responsible for the supervision and
control of the research platform for distributed
intelligent energy systems with a high penetration of
renewable energy. The supervisory software code was

L. Mihet-Popa and V. Groza
Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Electrical Engineering, Vol. 4, N. 6,
written in Java and is able to manage the data
acquisition, processes the data and executes the control
loop and outputs the control variables. The sensors
outputs are connected to a signal conditioning board,
which in turn is connected to the data acquisition (DAQ)
board based on a PC (SCADA System).
Some loads can be controlled by the central building
controller which receives data and events from wireless
switches and sensors. In one room, a small touch-screen
user interface can be used to influence the controller
policy (Fig. 2). Through its own grid control node, the
building controller can get information on the status of
the power grid, and adapt its control strategy
accordingly. Active policies, measurement data and user
settings can be communicated back to the grid.
A dedicated controller node is collocated with each
of the components. The nodes combined an X86-based
computer, local disk storage, analogue measurement
hardware, field-bus interfaces, status display backup
power and an Ethernet switch inside a portable rack.
Fig. 2. Graphical user interface for control system.
III.IV. Storage Components
The system under tests contains also a number of
loads (75 kW, 3x36 kW), an intelligent office building
with controllable loads and a Vanadium Flow Battery of
15 kW/120 kWh as a part of the distributed energy
systems experimental facility.
A vanadium battery stores energy in two electrolytes.
The capacity of the battery is determined by the size of
the tanks and its power is determined by the size of the
cell stack. The VRB uses different ionic forms of
Vanadium in a dilute sulphuric acid for both half cells,
eliminating the possibility of cross contamination. The
two acid electrolytes are separated from each other by a
PEM (Proton Exchange Membrane). The different redox
couples are V5
+ (VO2
+) and V4
+ (VO2
+) for the positive
half-cell and V3
+ and V2
+ ions for the negative half-cell.
At discharge the negative half-cell V2
+ is oxidized to
V3
+. The freed electron passes through the circuit and
participates in the reduction of VO2
+ to VO2
+. During
charge the reaction occurs in the opposite direction [12].
The open cell voltage is 1.4 V at 50% SOC and 1.6 V
at 100% SOC. By power electronics the DC voltage is
transformed to 400 VAC. The supplier of the vanadium
battery specifies high efficiencies (>70%) and long
cycle life (more than 13000 cycles for the battery). Also,
it should not degrade under prolonged periods of
discharge [12].
The battery package is an interesting option for
storing excess energy from the hybrid grid (wind
intermittency) for later use. It may also act as a peak
shaving unit and thereby contribute to a stronger grid
[12-13, 20].
The propose of the energy storage system is to be
coupled with a wind generation system in order to
realize different tasks, such as: to have the generation
output power smoothed and to grant no power transfer,
for a certain period on distribution system operator
request, at a point of common coupling (PCC) in any
battery state of charge condition. The idea is to control
the battery charging and discharging in order to control
the whole power plant.
IV. Component Testing under Different
Modes of Operation
The testing reported here include three modes of
operation: stand alone, parallel/hybrid and grid
connected.
In order to find out the differences between DER
components in power systems and to study the impact
on bus bar voltage and frequency the system will be
tested for different wind speeds and loads.
To test the static and dynamic characteristics of a
VRB a set of charge/discharge tests and a step response
tests at different SOC levels have been done. The
proposed tests have been carried out to characterize the
battery from a power system point of view and to assess
it with respect to integration of wind energy in power
system and to evaluate its potential role in the future
energy systems. Also the tests have been done to find
the main parameters of the battery and to test the system
response for parameter tuning of the controller.
IV.I. Stand-Alone Operation
In this mode of operation the Diesel Gen-set is
connected together only with a dump load, as can be
seen in Fig. 1.
The voltage and frequency are controlled by the gen-
set and the load is provided by diesel generator. The
ability of the diesel generator to respond to frequency
changes is affected by the inertia of the Diesel Gen-set,
the sensitivity of governor and the power capability of
the Diesel Engine and Synchronous Generator. The
response time of AVR and excitation system will show
the ability of the machine to control the voltage.
The objective of the test is to investigate static and
dynamic stability that means the property of
synchronous generator to remain in synchronism at a

L. Mihet-Popa and V. Groza
Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Electrical Engineering, Vol. 4, N. 6,
very slow and quick variation of load and to test the
system response for parameter tuning of the AVR and
governor controllers.
In Fig. 4 the diesel gen-set was running and at t=1sec
was applied a large step in load (about 80% of nominal
load) from zero to 40 kW and back to zero at t=7 sec.
The droop voltage was about 10-12 V and the settling
time around 3 sec.
Fig. 3. Details about Smart-Grid architecture (plant components)
in islanding mode.
1 2 3 4 5 6 7 8 9 10
200
220
240
Time(s)
Vph(V)
Phase Current and Voltage of Diesel Generator under Dump-Load step-up & down
1 2 3 4 5 6 7 8 9 10
0
20
40
Voltage
Current
Fig. 4. Phase Voltage and Current as a function of time during
switch-on and off of a large step in load. The data was acquired by an
Oscilloscope with a sampling frequency of 1kHz.
IV.II. Hybrid Mode of Operation
In this mode of operation the Diesel Gen-set is
connected together with the Wind Turbine, a dump load
and a battery, as can be seen in Fig. 5. The voltage and
frequency are controlled by the gen-set and the load is
shared between DG and gen-set.
The main objective of the tests has been to study the
voltage, active and reactive power and frequency and
speed variations during variable wind conditions and
during start-up and shut-down of the wind turbine and
during changes in load.
Another objective was to investigate if the Gen-set
unit maintains synchronism during these transients. This
includes the dynamic performance (Fig. 6) and the long
term conditions, too (Fig. 7).
In Fig. 6 is depicted a start-up sequence of Wind
Turbine at a wind speed higher than nominal (ws=12
m/s > wsN=9.5 m/s), with the Diesel Gen-set and a load
of 12 kW already connected (see Fig. 3). The WT was
connected at t=1 sec, then a step-up in load was applied
at t=4sec and a step-down at t=7sec from 15kW to
20kW. During these changes in load the voltage
response of the synchronous generator controller was
very fast. The droop voltage was the same as in
standalone operation when we applied a large step in
load but the settling time in this case was larger (about 4
sec). Also the current has risen from 18 A to 42 A.
Fig. 5. DG components and loads under parallel operation mode.
In Fig. 6b) is shown the voltage response when the
Wind Turbine was disconnected with Diesel Generator
and a load of around 12 kW still connected.
The Figures 7-8 presents the dynamic performance of
the system in terms of active and reactive power
distribution between the components and voltage,
frequency and speed response of the system during
different transient, such as connection and disconnection
of the wind turbine, and during a large step in load.
The variations in wind turbine output are
compensated by diesel gen-set. The voltage and
frequency variations are very small during dynamic
changes due to the automatic voltage regulator (AVR) of
the synchronous machine exciter and speed governor of
the diesel engine. The frequency variations are between
(49÷51) Hz and speed variations are between
(1480÷1520) rpm, as can also be seen in Fig. 7. In Fig. 8
the battery was used as a load with wind turbine and
Diesel Gen-set connected together.
Frequency band of AVR controller is within 2-3 Hz
and of speed governor control is less than 2 Hz, in
general. SGs operating in a power system have speed
droop controllers to allow for power sharing between
various units. Speed droop is typically (4-5) % [9].

L. Mihet-Popa and V. Groza
Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Electrical Engineering, Vol. 4, N. 6,
1 2 3 4 5 6 7 8 9 10
200
220
Time(s)
Diesel Gen-set and Gaia Wind Turbine during a start-up and a load step
1 2 3 4 5 6 7 8 9 10 0
50
Voltage
Current
a)
1 2 3 4 5 6 7 8 9 10
225
230
235
240
Time(s)
Diesel Gen-set and Gaia Wind Turbine during a shut-down sequence
1 2 3 4 5 6 7 8 9 10
10
20
30
40
Voltage
Current
b)
Fig. 6. Phase current and voltage acquired by an Oscilloscope a)
when the wind turbine was connected at 1 sec and an increase and a
decrease step in load was applied at 4 and 7 sec. respectively, and b)
during a shut-down sequence. The WT was connected together with
the Diesel Gen-set and a dump load.
4.41 4.42 4.43 4.44 4.45 4.46
x 107
-5
0
5
10
15
20
25
30
35
Time(s)
P(kW), Q(kWAr)
Active and Reactive Power during a connection, step change in load and disconnection of GAIA Wind Turbine connected on the same isolated grid with a Diesel Genset
P
Q
a)
4.41 4.42 4.43 4.44 4.45 4.46
x 107
390
395
400
405
410
Voltage(V)
Voltage, Frequency and Synchronous generator speed in parallel operation mode
4.41 4.42 4.43 4.44 4. 45 4.46
x 107
49.2
49.4
49.6
49.8
50
50.2
f(Hz)
4.41 4.42 4.43 4.44 4.45 4.46
x 107
1440
1460
1480
1500
1520
Time(s)
ng(rpm)
b)
Fig. 7. a) Active and reactive power and b) voltage, frequency and
speed as a function of time when the WT was connected together with
Diesel Gen-Set and a battery (t=4.41 sec), then a step in load was
applied at t=4.42 sec and the WT was disconnected at around t=4.45
sec.
720 730 740 750 760 770
-15
-10
-5
0
PAC, PDC(kW)
720 730 740 750 760 770
100
200
300
400
UAC, UDC(V)
720 730 740 750 760 770
25
30
35
40
45
50
Time(min)
f(Hz), SOC(%)
PAC
PDC
f
SOC
UAC
UDC
Fig. 8. AC and DC Power and Voltage, SOC level and Frequency
of the battery terminals when the Wind Turbine was connected (t=720
min)/disconnected (t=775 min) to/from the hybrid system and a step
in load was applied at 738 min and again at 760 min.
IV.III. Grid Connection Mode of Operation
In this mode of operation the Wind Turbine was
connected to the local grid together with a Vanadium
Battery, as can be seen in Fig. 9. The main objective of
the tests has been to study the active power and
generator speed variations during variable wind
conditions and during start-up and shut-down of the
wind turbine.
During the start-up sequence the current of the
induction generator increased 3 times more than nominal
current (iN=22 A) and the start-up time was around 2
second, as shown in Fig. 10 a). The wind speed was
between 10-15 m/s and the active power produced by
wind turbine between 9-12 kW, as can also be seen in
Fig. 10 b).

L. Mihet-Popa and V. Groza
Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Electrical Engineering, Vol. 4, N. 6,
In Fig. 11 is shown a dynamic response test of VRB
during charge/discharge cycle and a step response test at
different SOC levels. The battery-inverter system can
function as a power source when it is discharging
energy, and as a load as it is storing energy.
Fig. 9. The configuration during grid connection mode.
2 4 6 8 10 12 14 16 18 20
233
234
235
236
Time(s)
Vph(V)
Gaia Wind Turbine during connection to the grid
2 4 6 8 10 12 14 16 18 20
0
20
40
60
Voltage
Current
a)
835 840 845 850 855 860 865 870 875
8
10
12
14
ws(m/s)
Wind speed, Generator speed and Active power
835 840 845 850 855 860 865 870 875
0
200
400
600
800
1000
nG(rpm)
835 840 845 850 855 860 865 870 875
-5
0
5
10
Time(min)
P(kW)
b)
Fig. 1. a) Generator Phase Voltage and Current during a start-up
sequence to the local grid, acquired by an Oscilloscope and b) Wind,
Generator Speed and Active Power, acquired by SCADA system with
a sampling frequency of 1Hz.
1250 1300 1350 1400
-5
0
5
10
15
PAC & PDC(kW)
1200 1250 1300 1350 1400
100
200
300
400
UAC & UDC(V)
1250 1300 1350 1400
75
80
85
90
95
100
105
Time(min)
SOC(%)
PAC
PDC
UAC
UDC
Fig. 2. AC and DC Power and AC and DC Voltage during a
charge/discharge cycle at different SOC levels. The data was acquired
by SCADA System at battery terminals.
V. Discussion and Conclusion
The modeling of DER components in power systems
and the relative control architecture are an important
part for the introduction of relevant quantity of
renewable energy in the future smart grid. Therefore it is
a strong necessity to have proper validated models to
help operators to perform better studies and to be more
confident with the results.
In order to find out the parameters of various types of
DER components for dynamic simulation models a
number of tests are required under different operation
modes and loads. It is essential to find generic
parameters and the approach to obtain their values in
order to facilitate the modeling of a system. Effects of
static and dynamic stability, effects of disturbances on
power system equipment and network should be
analyzed.
AC power system satisfactory operation is obtained
when frequency and voltage remain nearly constant or
vary in a limited and controlled manner when active and
reactive loads vary. Active power flow is related to
prime mover’s energy input and thus to the speed of
synchronous generator. On the other hand, reactive
power control is related to terminal voltage. When a
generator acts alone on a load, or it is by far the
strongest in an area of a power system, its frequency
may be controlled via generator speed, to remain
constant with load. Integration of a diesel gen-set in an
isolated power system can improve the system behavior
during large changes in load.
Automatic generation control distributes the
generation task between AVR controller and speed
governor. Speed (frequency) control quality depends on
the speed control of the SG but also on the other
induced influences, besides the load dependence on
frequency. SG voltage control quality depends on the
SG parameters and excitation power source dynamics.

L. Mihet-Popa and V. Groza
Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Electrical Engineering, Vol. 4, N. 6,
The battery package is an interesting option for
storing excess energy from the hybrid grid (wind
intermittency) for later use. It may also act as a peak
shaving unit and thereby contribute to a stronger grid.
A series of tests has been done for different
operational scenarios and during several modes of
operations to find out the parameters for dynamic
modeling of DER components in power systems and
also to implement and test different control strategies for
the combined system. Moreover to point out, on a
laboratory scale, the coupling between wind generation
and storage and to effectively compensate wind
fluctuations a number of tests have been done during
charge/discharge cycles.
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Authors’ information
1 Dept. of Intelligent Energy Systems, RISO-DTU, Denmark.
2 Dept. of Automation and Applied Informatics, University of Ottawa
(Canada).
L. Mihet-Popa received the B.S. degree, M.S.
degree and Ph.D. degree from the
POLITEHNICA University of Timisoara,
Timisoara, Romania, in 1999, 2000 and 2003,
respectively, all in electrical engineering. He is
currently working as Scientist in the
Department of Intelligent Energy Systems,
RISO-DTU, Denmark. Dr. Mihet-Popa
received in 2005 the second prize paper award
of the IEEE Industry Applications Society. His research interest
includes control and modeling of DER components in smart grids,
electrical machines and drives, detection and diagnosis of faults,
especially for wind turbine applications.
Voicu Groza received the B.S. degree and Ph.D.
degree from the POLITEHNICA University of
Timisoara, Romania, in 1972 and 1985,
respectively. He is currently working as Associate
Professor in School of Information Technology
and Engineering, University of Ottawa. His
research interest includes real-time embedded
systems, distributed intelligent instrumentation
and smart sensors networks.