Evaluation of Control Methods to Prevent Prime-mover Stalling in a Mixed Source Microgrid ·...
Transcript of Evaluation of Control Methods to Prevent Prime-mover Stalling in a Mixed Source Microgrid ·...
Evaluation of Control Methods to Prevent Prime-mover
Stalling in a Mixed Source Microgrid
Mariana Pulcherio Student Member, IEEE
The Ohio State University
Columbus, OH 43210, USA
Email: [email protected]
Ajit A. Renjit Student Member, IEEE
The Ohio State University
Columbus, OH 43210, USA
Email: [email protected]
Mahesh S. Illindala Senior Member, IEEE
The Ohio State University
Columbus, OH 43210, USA
Email: [email protected]
Amrit S. Khalsa Member, IEEE
American Electric Power
Groveport, OH 43125, USA
Email: [email protected]
Joseph H. Eto Member, IEEE
Lawrence Berkeley National Laboratory
Berkeley, CA 94720, USA
Email: [email protected]
Abstract—For a microgrid with a mix of distributed energy
resources (DERs), major challenges on its survivability are
found in the islanded condition. In particular, a sudden loss of
generation or a large and fluctuating load could force the
microgrid to operate near its capacity limits. Such a situation
can cause a cascading collapse of the system, even when the
load demand is within the DER’s kW rating as observed
during several tests at the Consortium for Electric Reliability
Technology Solutions (CERTS) Microgrid Test Bed. This
paper analyzes the prime-mover stalling phenomena behind
the system collapse. It highlights how the reserve margin of the
system is lowered during transient conditions. Furthermore,
two control methods are evaluated to resolve the microgrid
collapse problem.
Index Terms—Energy resources, governors, industrial power
systems, inverters, power system modeling, synchronous
generators, control systems, internal combustion engines.
I. INTRODUCTION
Distributed energy resources (DERs) are small rated
energy generation and storage technologies installed within
the electric distribution system [1]. They include wind
turbines, photovoltaics (PV), fuel cells, microturbines,
reciprocating engines, combustion turbines, cogeneration,
and energy storage systems [2]−[6]. A practical scenario
involves integration of various kinds of DERs, known as
mixed source microgrid, to supply the load demand.
Many papers were published on the dynamic behavior of
a microgrid comprising a mix of synchronous generator-
based and inverter-based DERs [7]−[11]. In [7], the load
demand is met by the microgrid where inverter-based DER is
programmed like a virtual synchronous generator with droop
controls. An investigation was carried out in [8] to find the
cause for poor transient load sharing in an islanded
microgrid. In [9] and [10], a modified droop control
technique and virtual impedance was proposed to limit the
inverter’s current during overloads. In summary, all these
papers analyzed the microgrid performance assuming normal
operating conditions.
However, major challenges are found when the mixed
source microgrid operates in the islanded mode of operation
near its capacity limits. In particular, the survivability of
microgrid is at risk when it experiences a s u d d e n loss
of generation from even a single large DER. Similar
situation can result when a large and fluctuating load
condition happens in an industrial power system. These
could lead the entire system to a cascading collapse [12].
Recently, several tests were carried out at the Consortium
for Electric Reliability Technology Solutions (CERTS)
Microgrid test bed at American Electric Power. During the
experimental investigation, it was observed that a large
electrical load demand, sometimes even within the DER’s
kW-rating, could result in a frequency/voltage collapse due
to prime-mover stalling. In a reciprocating engine driven
synchronous generator-based DER (i.e., genset), the prime-
mover speed is proportional to the DER’s frequency. Hence,
the stalling in genset causes a frequency collapse in the
microgrid. By contrast, the prime-mover stalling in an
inverter-based DER results in a voltage collapse [13]. This is
because the inverter-based DER has an additional power
conditioning stage after the permanent magnet synchronous
generator (PMSG). This work was supported by the Office of Electricity Delivery and
Energy Reliability, Transmission Reliability Program of the U.S. Department of Energy under subcontract 7004227 with The Ohio State
University administered by the Lawrence Berkeley National Laboratory.
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When multiple DERs are integrated in the microgrid,
they are expected to share the reserve margins with each
other. However, the system collapsed in few test cases
conducted at the CERTS Microgrid. This paper highlights
that the reserve margin is lowered under transient
conditions. Furthermore, a root cause analysis is carried out
and two control methods are evaluated for resolving the
problem.
II. MIXED SOURCE MICROGRID
A. System under Test
A mixed source microgrid has a diversity of distributed
energy resources (DERs) to support the load demand. For
example, as shown in Fig. 1, it can include reciprocating
engine driven synchronous generator-based DER1 (i.e.,
genset) and inverter-based DER2. These dispatchable energy
resources act as voltage sources and have frequency droop
controls [1]. In addition, a renewable resource like
solar/photovoltaic (PV) system is also indicated in Fig. 1.
However, this PV system is run as a grid following current
source, and hence is regarded as a negative load. Under
islanded operation of the microgrid (cf. Fig. 1), the power
balance between generation and load can be mathematically
expressed as
������� � ������ � � ∙ � ����� � ����� � �� � � ∙ � (1)
where the net load demand on DER1 and DER2 is
�� � � ∙ � � ������ � ��� � � ∙ ���� (2)
In this study, the load reactive power is considered to be
zero (i.e., QL = Qload = 0). Because of the intermittent power
from the renewables (���), the net load (��) can change all
of a sudden.
∴ ������ � ������ � �� (3)
where �� � ����� � ��� (4)
and ����� � ����� � 0 (5)
B. Problem Description Microgrid System Collapse
A particular cause of concern is the survivability of the
microgrid under sudden loss of generation from one of the
energy sources (DERs). For instance, if the PV system
suddenly stopped supplying power (PPV), it produces a
substantial change in net load demand (PL) on the remaining
two DERs (viz., DER1 and DER2). At the Consortium for
Electric Reliability Technology Solutions (CERTS)
Microgrid Test Bed [14], an experimental investigation was
carried out to study the dynamic behavior of the mixed
source microgrid. The specifications of the two reciprocating
engine driven DERs, i.e., synchronous generator-based
DER1 (i.e., genset) and inverter-based DER2 are provided in
[12], [13]. It should be noted that each DER is rated to
deliver 100 kW continuous load. Two test cases are
presented below showing different results for a large step
load change (PL) from 75 kW to 150 kW. The genset
(DER1) is run with an isochronous governor and the
inverter-based DER2 is controlled with 1% active power-
frequency (P−ω) droop.
Fig. 1. Simplified schematic of the mixed source microgrid comprising synchronous generator-based and inverter-based DERs
V���e�����
V!��e��"
������� ∙ ����
X$ � X�!%�
�������� ∙ �����
Engine-2 PMSG-2
Tlim2
DER-2
&'
V!�e��(
�������� ∙ ����� Engine-1 SG-1
Tlim1
X�!%�
DER-1
DSP Controls
Load Bank
���
Solar/PV
System
)'
������ � ���� � ∙ ����
Net load change, �� � � ∙ � �
Power Conditioning System (PCS)
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Test Case 1: Fig. 2 displays the dynamic response
observed during experimental testing of the mixed source
microgrid when the two DERs were given equal power
allocations at first (with Pref2 = 37.5 kW). This test case
ended in the collapse of the microgrid system upon a step
load change (PL) of 75 kW to 150 kW [12]. The inverter-
based DER2 experienced a voltage collapse at first upon the
load change. It is manifested in Fig. 2 by the large negative
reactive power (Qelec2) of DER2 that is fed by the positive
reactive power (Qelec1) of DER1 in line with (5). The
failure of DER2 caused an overload in DER1 (i.e., Pelec1) as
the net load demand of 150 kW is beyond its rated
capability of 100 kW. Therefore, the DER1 also collapsed
resulting in a cascading failure of the microgrid system.
Test Case 2: In comparison to the earlier test case, this
time the inverter-based DER2 was given zero initial power
allocation of Pref2 = 0 kW. The same 75 kW to 150 kW step
load change was tested. Fig. 3 shows selected waveforms
illustrating the dynamic response observed during
experimental testing at the CERTS Microgrid [12]. As seen
in this figure, the microgrid system survived in this test
without collapsing.
III. ROOT CAUSE ANALYSIS
A. Reserve Margin of the Microgrid System
The contrasting outcomes (for the same step load change
of 75 kW to 150 kW) realized in the islanded microgrid
system for the two test cases pose the question:
o What is the reserve margin of the microgrid system?
Reserve margin is the value of generation capacity
available to satisfy the expected load demand [15]−[18]. For
the microgrid system comprising two DERs, if the rated
capacity is denoted by PRi (i = 1, 2), the reserve margin Ri (i
= 1, 2) is derived as
*��+ � �,� � �������+ (6)
*��+ � �,� � �������+ (7)
∑* �+ � *��+ � *��+ (8)
where ∑* is the total reserve margin of the two DERs in
the microgrid system.
For the two test cases presented earlier, the reserve
margins before the 75 kW to 150 kW load change event are
determined as displayed in Table 1 and Table 2,
respectively. It should be remarked that the DER kW-ratings
�.. 0., �,� � 100+3, and�,� � 100+3 were used in arriving at these estimates.
However, the two test cases produced different outcomes
when the step load change (from 75 kW to 150 kW) took
place. Whereas the microgrid system crashed in Test Case 1,
Fig. 2. Test Case 1: Experimental results for a step load change from 75
kW to 150 kW. DER1 (genset in red): isochronous governor, DER2 (inverter-based DER in blue): Pref2 = 37.5 kW, Pmax2 = 100 kW.
Fig. 3. Test Case 2: Experimental results for a step load change from 75 kW to 150 kW. DER1 (genset in blue): isochronous governor, DER2
(inverter-based DER in red): Pref2 = 0 kW, Pmax2 = 100 kW.
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it survived in Test Case 2. These contrasting results are
despite having the same total reserve margin �∑* at initial 75 kW load for both test cases. Therefore, further
examination is carried out into the DER kW-rating (i.e., PRi
= 100 kW) assumed earlier in estimating the reserve margin.
Table 1: Reserve margin estimation for Test Case 1 at initial 75 kW load
78*9 �,9 �����9 *9 Σ R
78*� 100 kW 37.5 kW 62.5 kW 125 kW
78*� 100 kW 37.5 kW 62.5 kW
Table 2: Reserve margin estimation for Test Case 2 at initial 75 kW load
78*9 �,9 �����9 *9 Σ R
78*� 100 kW 75 kW 25 kW 125 kW
78*� 100 kW 0 100 kW
Fig. 4 illustrates the fuel map limit of a GM 8.1L natural
gas engine [19] adopted as the prime-mover in the genset
(i.e., DER1). As seen in this figure, the power limit of the
engine prime-mover is derated at lower speeds. Hence, the
engine’s mechanical power input to the generator is
constrained to the value
:�;�<� � =�9;� ∙ >� (9)
where =�9;� is the torque limit of engine fuel map.
Fig. 5 depicts the original operating point of Test Case 1
(i.e., ������ = 37.5 kW) on the speed vs. power
characteristics of the engine driven DER1 generator. In this
figure, the :�;�<� line delineates the safe zone and stalling
zone for the electrical power output from DER1 [20]. Due to
the lower inertia, the speed of rotating generator undergoes
huge swings upon a large load disturbance until the
governor controls restore the speed close to the synchronous
speed (>?@A�). Whereas the rated power is 100 kW at the
synchronous speed, the DER1 is derated at lower speeds as
observed in Fig. 5. Hence, the rated capacity of DER1 is
found to be
�,�B>� � >?@A�C � 100+3 (10)
and �,�B>� D >?@A�C D 100+3 (11)
Reserve margin is the amount of generation capacity
available to satisfy the forecast load demand. From (6), the
reserve margin for Test Case 1 can be estimated as
*�B>� � >?@A�C � 62.5+3 (12)
and *�B>� D >?@A�C D 62.5+3 (13)
Similar conclusions could be established for inverter-
based DER2 based on the engine fuel map characteristics of
its prime-mover. The rated capacity and reserve margin of
DER2 for Test Case 1 are
�,�B>� � >?@A�C � 100+3 (14)
and �,�B>� D >?@A�C D 100+3 (15)
*�B>� � >?@A�C � 62.5+3 (16)
and *�B>� D >?@A�C D 62.5+3 (17)
Thus, the reserve margin of each DER is lowered under
transient conditions. It should be mentioned that the speed
dynamics of the prime-movers in synchronous generator-
based DER1 and inverter-based DER2 vary against each
other. Hence, under load transient conditions, the total
reserve margin of the mixed source microgrid is
∑*B>� D >?@A�, >� D >?@A�C
� *�B>� D >?@A�C � *�B>� D >?@A�C
Fig. 4. Fuel map limit of GM 8.1L natural gas engine prime-mover of
genset (DER1) [19]
������ *�B>� � >?@A�C
�,�B>� D >?@A�C
Speed (rpm)
Power (kW)
Fig. 5. Effect of prime-mover speed on reserve margin
Power limit �:�;�<�)
Torque limit �=�9;�)
37.5
*�B>� D >?@A�C
�,�B>� � >?@A�C
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∴ ∑ * B>� D >?@A�, >� D >?@A�C D 125+3 (18)
The total reserve margin for Test Case 2 also complies
with the inequality in (18). However, the transient response
in this case (cf. Fig. 3) is not the same as Test Case 1 (cf.
Fig. 2). This indicates that the two test cases have different
values of total reserve margins.
B. Prime-mover Stalling Behavior
To investigate further the root cause of microgrid
collapse, modeling and analysis was carried out. Earlier, the
co-authors developed detailed computer models that were
validated with experimental testing at CERTS Microgrid
Test Bed [12], [13], [20]−[22]. The modeling of
synchronous generator-based DER and inverter-based DER
was published in [13]. Graphical and analytical methods for
prime-mover stalling were presented in [20].
Referring back to the mixed source microgrid Test Case 1
(cf. Fig. 2), the inverter-based DER2 took the majority of
load change from 75 kW to 150 kW. This is because of its
relatively faster frequency regulation controls than the genset
speed governor controls. When the load change occurred,
the prime-mover speed varies during transient conditions.
The rate of change of kinetic energy in permanent magnet
synchronous generator (PMSG) in DER2 is governed by
∆IJ
∆K� �;��L � ����� � ���??
(19)
where �;��L is the mechanical power input, ����� is the
electrical power output, and ���?? covers all the losses in the
PMSG and inverter. When a large load change occurs, the
stored kinetic energy of prime-mover supplies the increased
demand initially until the mechanical power from engine
increases to match the electrical load. However, if the
electrical power output remains above mechanical power
input for a long duration, the stored kinetic energy is drained
thereby causing the stalling of prime-mover.
Fig. 6 illustrates the locus of electrical power and
mechanical power (minus losses) on the speed vs. power
characteristics of permanent magnet synchronous generator
(PMSG) in DER2 [12]. In this figure, the :�;�< line
delineates the safe zone and stalling zone. Since the
electrical power trajectory crossed the :�;�< line, which is
the maximum mechanical power provided by engine, the
prime-mover stalling took place. Since the PMSG speed is
proportional to its voltage, the prime-mover stalling gets
reflected as a voltage collapse in the inverter-based DER2.
The failure of inverter-based DER2 caused an overload in
genset (i.e., DER1), which led to a collapse of DER1. Fig. 7
presents the locus of electrical power and mechanical power
for the DER1. As seen in this figure, the prime-mover
stalling gets manifested as a frequency collapse because
the frequency of a generator is proportional to its speed.
It should be noted that the frequency of inverter is
decoupled from the prime-mover speed by the power
conditioning system (PCS). This is indicated in the inverter
Fig. 6. Locus of the prime-mover speed vs. power characteristics for
inverter-based DER2 in Test Case 1
Fig. 8. Locus of the inverter frequency vs. power characteristics for DER2
in Test Case 1
Fig. 7. Locus of the prime-mover speed vs. power characteristics for synchronous generator-based DER
1 in Test Case 1
EPm
ax2 lin
e
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frequency vs. power characteristics shown in Fig. 8.
However, a collapse in the PMSG voltage cannot be
prevented from propagating through the PCS due to lack of
sufficient energy storage [13].
IV. CONTROL METHODS TO PREVENT PRIME-MOVER
STALLING
The prime-mover stalling can be prevented by restricting
the electrical power output from each DER to stay within
the safe zone of its prime-mover speed vs. power
characteristics. The load shared by each unit should be
within the available reserve margin. It was shown in the
previous section that the transient reserve margin is reduced
due of derating of DER kW-rating when its prime-mover
runs at a lower speed.
For limiting the electrical power output from a DER
������ to stay within the safe zone, the CERTS �;�<
controls [23], [24] are used in commercial products. This
section evaluates the CERTS �;�< controls and a new
control method based on limiting the maximum torque (i.e.,
=;�< controls). Such controls can be integrated with the
conventional frequency droop controllers commonly
employed in the industrial products. Therefore, these
controls cannot be implemented into the isochronous
governor controlled genset (DER1). However, these controls
are applied in the frequency controller of the inverter-based
DER2.
A. CERTS �;�< Controls
The CERTS �;�< controls offer flexibility to limit the
DER’s electrical power output ����� to within the �;�<
value programmed [23], [24]. A block diagram of the
CERTS �;�< controller is shown in Fig. 9. It consists of an
integral gain controller that is inactive during normal
operation for ����� D �;�< . This is implemented through a
hard limiter that clamps the integrator output to zero on the
higher side. When the electrical power output ����� exceeds
the programmed �;�< value, this controller forces a
decrease in the DER’s frequency. In an interconnected
microgrid system, a temporary decrease in frequency of a
DER helps in lowering its relative phase angle and thereby
limits the unit’s electrical power generation. Fig.9. Block diagram of the CERTS �;�< controller. Frequency droop is
1% and K = 300.
Pmax
+
+
−−−−
Pelec
∑∑∑∑ MN
∆ω'*
ωnom
+
+
+
−−−−
Pelec
∑∑∑∑ ∑∑∑∑ Frequency
Droop ∆ω*
ω*
0
Pref
Fig. 10. Locus of the prime-mover speed vs. power characteristics for
inverter-based DER2 in Test Case 1 with Pmax2 = 70 kW
Fig. 12. Locus of the inverter frequency vs. power characteristics for
DER2 in Test Case 1 with Pmax2 = 70 kW
Fig. 11. Locus of the prime-mover speed vs. power characteristics for
synchronous generator-based DER1 in Test Case 1 with Pmax2 = 70 kW
EPm
ax2 lin
e
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In general, the value of �;�< is programmed to match
with the maximum power that can be provided by the
engine prime-mover, which is 100 kW in the inverter-based
DER2 installed at the CERTS Microgrid. However, it was
found that �;�< � 100+3 could not prevent prime-mover
stalling for a few test cases. In fact, the plots shown for Test
Case 1 shown in Fig. 2 and Figs. 6−8 for a 75 kW to 150
kW step load change with isochronous governor controlled
DER1 and Pref2 = 37.5 kW for DER2 had been obtained
with �;�< � 100+3. This is indicated in Fig. 8 by the
constant power EPmax2 line at 100 kW.
Later, when a lower value of �;�< � 70+3 was tested
in the simulation model, the microgrid system did not
collapse. As shown in Figs. 10−12, the programming of
controller with �;�< � 70+3 has prevented prime-mover
stalling in DER2. Here, the constant power EPmax2 line is at
70 kW as shown in Fig. 12. However, it has resulted in the
reduction of generation capacity of DER2 at the
synchronous speed of prime-mover from 100 kW to 70 kW.
This has led to the proposed =;�< controls explained below.
B. Proposed =;�< Controls
As explained earlier, the transient reserve margin of the
microgrid system is lesser at lower prime-mover speeds due
to the derating of DER’s kW-rating. This finding calls for
lowering the �;�< value at the same rate as the prime-mover
speed. The proposed =;�< controller can achieve this aim as
it was designed to regulate the electrical load torque
=���� � �����/>∗. As seen in Fig. 13, this controller is only
active when the electrical load torque =���� is higher than the
programmed maximum torque value of =;�< .
Figs. 14−16 display the results of the mixed source
microgrid for Test Case 1 with =;�<� � 265RS that
corresponds to the full 100 kW rated capacity at the nominal
60 Hz frequency of inverter-based DER2. However, at lower
frequencies that are encountered under transient conditions,
the electrical power generation capacity is derated. This is
indicated in Fig. 16 by a constant torque 8�;�<� line that
varies according to the available reserve margin in the DER.
The results illustrating the performance of the proposed
=;�< controller are also shown as time-domain plots in Fig. Fig. 13. Block diagram of the proposed =;�< controller. Frequency droop
is 1% and K = 300.
Tmax
+
+
−−−−
=����
� ����� >∗T
∑∑∑∑ MN
∆ω'*
ωnom
+
+
+
−−−−
Pelec
∑∑∑∑ ∑∑∑∑ Frequency
Droop ∆ω*
ω*
0
Pref
Fig. 14. Locus of the prime-mover speed vs. power characteristics for
inverter-based DER2 in Test Case 1 with Tmax2 = 265 Nm
Fig. 16. Locus of the inverter frequency vs. power characteristics for
DER2 in Test Case 1 with Tmax2 = 265 Nm
Fig. 15. Locus of the prime-mover speed vs. power characteristics for
synchronous generator-based DER1 in Test Case 1 with Tmax2 = 265 Nm
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17. As seen in this figure, the microgrid system has operated
safely without collapsing for =;�<� � 265RS. Thus,
programming of DERs with the =;�< controller enables
improved coordination among interconnected DERs in a
microgrid system that is operated near its capacity limits.
V. CONCLUSION
This paper analyzed the operation of a mixed source
microgrid comprising a synchronous generator-based DER
and an inverter-based DER in an islanded condition. When a
sudden load increase or loss of generation happens, the
survivability of the microgrid is challenged when the DERs
are forced to operate near their capacity limits. It was found
that the microgrid is susceptible to a collapse due to DER
prime-mover stalling. In this paper, it was shown that the
transient reserve margin of the microgrid is lower than the
estimated value based on DER kW-ratings. To prevent the
microgrid system collapse, two control methods were
evaluated. The first method was �;�< controls, which is
used in the commercial DER equipment at the CERTS
Microgrid Test Bed. An alternative control method, namely
=;�< controls, gave a better performance. As compared to
the CERTS �;�< controls, the proposed =;�< controls could
prevent prime-mover stalling (and system collapse) at the
full capacity and nominal frequency.
ACKNOWLEDGMENTS
The work described in this paper was coordinated by the
Consortium for Electric Reliability Technology Solutions
(CERTS). The authors would like to thank R. H. Lasseter of
University of Wisconsin-Madison and David A. Klapp of
Advanced Microgrid Systems for their support during the
course of this project.
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