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A CONTROL HIERARCHY OF INTERCONNECTED MINI-GRIDS
w. Sinsukthavorn*, E. Ortjohann*, M. Lingemann*, S. Jaloudi*, D.
Morton**
*South Westphalia University of Applied Sciences/Division Soest,
Ltibecker Ring 2, 59494 Soest, Germany E-mail:
[email protected]. [email protected]
**The University of Bolton, Deane Road, Bolton, BL3-SAB,
U.K.
Keywords: Distributed Generation, Distributed Energy Resource,
Hierarchy Control, Conventional Power System, Droop Control
Function.
Abstract
Distributed generation (DG) is recently integrating conventional
power systems to assist main power plants to satisfy customer need.
A control hierarchy to interconnect DG systems to conventional
power systems is required to improve reliability and quality of
power supply systems. The main task is to control the frequency and
voltage of the systems. This task is currently done by using
synchronous generators in many interconnected power systems.
Therefore, their control strategy of conventional power systems can
be based and adapted to be implemented into other DG technologies
through their interface unit to the grid, namely the inverter. The
inverter is the primary interface unit between the energy source
and the grid. This paper presents a flexible control hierarchy of
interconnected mini-grids based on inverters.
1 Introduction Recently, DG systems have become an option to be
integrated into the conventional power systems. DG has been
increasingly interesting to customers, thus, many electrical
providers are increasing their budgets for DG technologies.
Therefore, the penetration of DG at medium and low voltages is
expected to play a major role in future power systems. Implementing
distributed energy resources (DERs) such as wind turbines,
photovoltaic, gas turbines and fuel cells into interconnected grids
could be part of the solution to meet the rising electricity demand
[I, 3, 5, 7]. DG technologies are currently investigated and
developed in many research projects to form smart grids.
This paper presents an adaptable and flexible hierarchy control
strategy of interconnected mini-grids based on inverters. The
proposed strategy is based on the conventional power control
structure and is therefore able to handle not only modern DG
sources, but also conventional sources. The operational hierarchy
control structures of interconnected power systems are analyzed and
the functions are identified. All control functions are examined
regarding their ability to support future sources and power system
architectures. These hierarchical control levels are the primary
control at unit level, the secondary control at local level and the
tertiary
control at supervisory level. They lead to an implementation
strategy, especially with the focus on DG systems connected to the
grid by inverters. The proposed strategy is modular, flexible, and
reliable and can be easily integrated in interconnected grids.
Mini-grids, containing inverter-based distributed energy resources
(DERs), can be linked to interact and operate in parallel [1 - 4].
The main control functions are structured and hierarchical control
levels are defined as shown in Fig. I.
The proposed control strategy enables maintenance of the grid
voltage and frequency by its primary control and secondary control
at unit and local levels respectively. It also manages power
sharing between the sources along with user settings, rated power
and meteorological forecasting. The border of each grid contains
measurement units to measure the power and to observe the power
flow. These communicating signals of the measurement units are sent
to the tertiary control at supervisory level through the secondary
control. The tertiary control uses information from the secondary
control to optimize and control power dispatching and load sharing
for the entire power system.
This paper is structured as follows: First, the control
methodology of a grid is introduced including the role of
inverters. Second, the hierarchy control strategy of a conventional
power system is described. Third, the proposed control strategy of
distributed generation in interconnected grids is described.
Finally, the proposed strategy is validated through the simulation
of interconnected grids based on inverters in grid forming mode and
grid supporting modes.
Fig. I. Overview control strategy in interconnected grids.
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2 Inverter control methodology A mini-grid is generally composed
of five main components, which are energy conversion systems
(ECSs), energy storage systems (ESSs), information and
communication technologies (lCTs), connection lines and power
electronics interfacing units (e.g. inverter). The above are
discussed in more detail in [5]. A general philosophy to supply
electric energy in isolated power systems through power electronic
inverters, which are key elements at the grid side of mini-grid, is
introduced in [2, 7]. Normally, the power produced by ECSs is DC
power. This is fed to the grid through an inverter that produces an
AC output of a specific voltage magnitude and frequency. This
means, that inverters provide decoupling between the voltages
across the terminals of the ECSs from one side and the grid voltage
from the other side. It also provides a decoupling between the
frequency of the ECSs from one side and the grid frequency from the
other side. The invert control philosophy including types and
functions is shown in Fig. 2. The power flow from an ECS into the
grid may be driven by the grid or by the ECS itself [7-8].
In a grid driven feeding mode, the power flow from the ECS is
controlled regarding the power requirements of the grid while in an
ECS driven feeding mode, the power flow is controlled according to
the requirement of the ECS itself. A grid feeding mode can be
realized through two different cases which are grid forming and
grid supporting modes. An ECS driven feeding mode may be realized
through a grid parallel inverter. Grid feeding mode inverters are
discussed in more detail in [2-4]. With the management and control
topologies in [2, 7], a grid can be designed via several inverters
with different operating functions (grid forming, grid supporting
and grid parallel modes) and power ratings as discussed in [2].
This ensures that the system is expandable and flexible.
3 Control strategy of conventional power systems
Conventional power systems, including large power plants, are
widely interconnected and are operated not only to handle the
continuously increasing electricity demand, but also to increase
the reliability and quality of power systems. Examples are the
interconnected grids in Germany and Europe, and The Seven Countries
Interconnection Project (SCIP). The state variables such as voltage
and frequency of the power system are generally actively controlled
in the high voltage levels.
There are generally three main control levels to manage the
entire power system which are unit level, local level and
supervisory level. These three control levels contain primary,
secondary and tertiary controls respectively. The primary control
is related to the unit level, which is responsible for control of
the state variables (frequency and voltage) and for the control of
the power values of the unit to the grid (active power and reactive
power). This also includes the sharing power to prevent any
generator from taking the entire load in their local grid.
Fig. 2. Feeding modes related to the grid side.
The secondary control of the local level is responsible for
bringing the frequency back to the nominal value. This can be
achieved by means of power frequency control. Moreover, the desired
power exchange for both active and reactive powers between grids
can also be managed from the secondary control in the local level.
The controlled area of the local level will be limited at the
borders. At the borders, there are measurement units, which measure
all data for control and communicate it to the secondary control.
The set points of the secondary control are sent by the higher
level to get an optimum operation.
The tertiary control is related to the supervisory level which
organizes the energy management of the overall power system. (Le.
system optimization, dispatch control strategy, load flow
management, meteorological forecasting, network management and
communication management). The tertiary control collects
information of the interconnected grids such as forecasting data,
power profile, load data and etc. The optimization is processed in
this level to get the reference values to feed in to the local and
unit levels. This also includes the optimized power dispatch
between grids.
4 Control strategy of distributed power system As mentioned
above, future power distribution requires advanced expandability
and flexibility in the integration of DG. The inverters, which are
used for interfacing DERs to the grids, are an important part of DG
systems. Therefore, the control strategy in the interconnected
grids should be combined with the control methodology of inverters
(grid forming, grid supporting and grid parallel modes). Inverter
feeding modes are discussed in more detail in [3-5]. Load
management, synchronization and load sharing with respect to
generation rating, meteorological forecasting and user settings,
are all required in order to implement a control methodology of
inverters into an interconnected system. Moreover, due to the
flexibility and expandability of the inverters' control strategy,
inverters in different feeding modes can be implemented into
interconnected grids. This paper introduces an example control
structure for interconnected grids including the combination of
grid forming and grid supporting inverters.
A sample layout for a control structure of interconnected grids
including the combination of grid forming mode and grid supporting
mode inverters is shown in Fig. 4. This example system is
controlled by the control strategy adapted from the conventional
power system. Therefore, the control
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hierarchy will not be described again in this section. This
section focuses directly on the flexibility of the inverter control
structure that can be implemented along with the control strategy
for grid interconnection.
The role of the grid forming mode inverter is to establish and
maintain the state variables (voltage and frequency) of the grid.
Therefore, in the control scheme of the grid forming mode inverter,
the voltage is controlled by the d-component, while the frequency
is controlled by the q-component. The power injection in the
connection point of the inverter is related to active and reactive
power controllers.
Synchronization and load sharing are also required in the
control systems. Load sharing is handled, using the voltage and
frequency droop control functions, in the primary control at unit
level. The voltage droop is related to the reactive power variation
of the grid and the frequency droop is related to active power. The
secondary control at local level is included to bring the frequency
back to the nominal value. Moreover, it can be used to exchange
active and reactive power between the grids. To optimize the
system, the tertiary control will calculate and manage the
reference values for the controllers.
Inverter
Primary Control
L,.
c,.
Primary Control
The grid supporting mode inverter is used for power balancing
and produces predefined amounts of power. These predefined amounts
of power can be adjusted according to the system requirements and
user settings via the secondary and tertiary controls. The grid
supporting mode inverter feeds the grid with a specified amount of
power, which is active or reactive power, or a combination of both.
The control strategy for the grid supporting mode inverter using
active and reactive power consists of four controllers, two for the
real part of the grid current ill and imaginary part of the grid
current i", and two for the active power P and reactive power Q. P
is controlled by id, while Q is controlled by i". The offset power
from the secondary control will be fed into the summation points of
the active and reactive power control as shown in Fig. 3.
For the grid parallel mode inverter, there is no need to have a
secondary control, since it is a power production unit that is not
controlled according to the requirements of the electrical system.
As the control topology of the inverters, which is introduced in
[I], is expandable and flexible, this proposed control strategy can
be implemented into DG power systems, with different types of DERs,
as well as into conventional power systems.
V1
lV.
To other .... units
Secondary Control Fig. 3 Example of control strategy in
interconnected grids including inverters as sources.
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5 Case study To verify the proposed control strategy, it is
tested by the simulation of two grids including grid forming mode
and grid supporting mode inverters as shown in Fig. 4. The first
grid is supplied by the first grid forming mode and the first grid
supporting mode inverters (OFI and OS I). The Second grid is
supplied by the second grid forming mode inverter and the second
grid supporting mode inverter (OF2 and OS2). The power system
operates at the rated frequency!r(l/(t/ = 50 Hz and the reference
voltage line to line VL.L = 400 V nn.' Rated apparent power of grid
forming mode and grid supporting mode inverters are Sr = 1 25 kV A
and Sr = 80 kV A respectively. Both grid supporting mode inverters
are set to supply active and reactive power of 6 kW and 3.3 kvar
respectively. The cable line used in the simulation is NA YY 4x50
SE: RI = 0.772 Mm and XI = 0.083 Qlkm. The droop factors of these
inverters are set using the same percentages to clearly see the
load sharing between the inverters. The secondary control is
included in the simulation to control the power in each grid as
well as the power exchange between the grids. This also leads to
the system frequency that can be brought back to the nominal
value.
In Fig. 4, the active power and reactive power loads of the
first grid are the same as those of the second grid. They both
start at 1 6 kW and 7.3 kvar. The total active power and reactive
power of the system are 32 kW and 1 4.6 kvar respectively. At t = 1
5 s, in the first grid, the active power steps up to 20.2 kW and
reactive power load steps up to 7.37 kvar. Therefore, the total
active and reactive powers after the load step are 36.2 kW and 1
4.67 kvar respectively. The simulation results including the active
power, reactive power, frequency, three phase of voltages and
currents are shown in Figs. 5 to 8 respectively.
Active power of the inverters is shown in Fig. 5.a. At the
beginning, the inverters supply active power of approximately 32
kW; around 20 kW is supplied by each grid forming mode inverter
equally and the rest 1 2 kW is supplied by each grid supporting
mode inverter equally. At t = 1 5 s, the step load of 4.2 kW is
added to the first grid. All inverters of the system directly react
to compensate the additional load. After the load step, the
secondary control manages the generating units
Fig. 4. Two mini-grids including two grid forming mode and two
grid supporting mode inverters.
at the first grid to compensate the disturbance of the grid by
itself. Therefore, the active power of the first grid inverters,
which are located close to the additional load, is increased, while
the active power of the inverters at the second grid remains at its
normal state.
Reactive powers of all inverters are shown in Fig. 5.b. At the
beginning, all inverters supply reactive power of approximately 1
4.6 kvar. At t = IS s, the step load of 70 var is added to the
first grid. As the secondary control of reactive power is not
implemented into this test simulation, the grid forming mode
inverters of both grids supply to compensate the additional load
step while the grid supporting mode inverters supply the same
amount.
The frequency of the system is shown in Fig. 6. The primary
control and the secondary control have direct impact on the
frequency behavior. First, due to the droop control function of the
primary control, at the load step t = IS s the frequency drops from
the nominal frequency as shown in the small figure in Fig. 6. This
frequency drop can be brought back to the nominal value by the
secondary control.
The first grid forming mode inverter (OFI) outputs are shown in
Fig. 7. It can be recognized that it is supplying fixed voltage
output and responds by adapting its current to the load
13 12 11 t\.. 10 Q; 9 (l. 8 II> 7 > 'D U 6 49.6 ::J 49.5 V
U. 49.4 15'
49.310 15 20 25 30 t[s)
Fig. 6. Frequency of the system
r' ; , c: -' I
'5:4 15. 35 40 45 50
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steps. At t = 1 5 s, the step load is applied. The three phase
voltages have been kept constant by the controllers all the time as
shown in Fig. 7.a, and the three phase currents as shown in Fig.
7.b supplied by the first grid forming mode inverter (GFI) are
increased.
Figs. 8.a and 8.b show the three phase voltages and currents of
the load at the first grid respectively. At t = 1 5 s, the step
load is applied. The load voltage has been kept nearly constant
while the current is increased due to the step load at the first
grid.
400 r--r--r--'--r--r-'--'---'--'
Cl c:
1 :g
300 200 100
o CJ -100 '0 -200 ., Cl Jl! g
-300 -400
20 15 10
5 o
-5 '0 -10 C -15 ::> -20
(J
Fig. 7. (a) Three phase voltages of GFI at step t = 15 s and (b)
Three phase currents of GF I at step t = 1 5 s.
;; ...J '0 ., CI)
>
400 300 200 100
0 -100 -200 -300 -400
50 40 30 20 10
o -10 -20 -30 -40 -50 14.96 14.98 15
(b) t[s]
Fig. 8. (a) Three phase voltages of loadl at step t = 1 5 s and
(b) Three phase current of load I at step t = 1 5 s.
6 Conclusion Future power supply infrastructure will have DG
integrated into conventional power supply systems. The challenge of
combining with large numbers of DERs to the power systems has to be
carefully planned and managed. The control strategy and management
concept of the interconnected systems should be flexible and
reliable to handle the various DG. The paper introduces a control
strategy for DG interconnected grids based on the control strategy
of conventional power systems. This proposed strategy is
integrating DERs and managing interconnected grids to operate in
parallel. The power dispatch, exchanged power, frequency control
and voltage control can be automatically managed by the proposed
strategy. The simulation results illustrate that the strategy can
be implemented into the power systems due to its adaptability,
flexibility and efficiency. With the proposed strategy, mini-grids
can be widely interconnected to each other and existing
conventional systems to form huge power systems.
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