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Distributed control of a user-on-demand renewable-energy power-source system using battery and hydrogen hybrid energy-storage devices
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Distributed control of a user-on-demand renewable-energy power-source system using battery and hydrogen hybrid energy-storage devices Daiji Yamashita a,b, Katsuhiko Tsuno a, Kayo Koike a, Katsushi Fujii a,*, Satoshi Wada a, Masakazu Sugiyama b a Photonics Control Technology Team, RIKEN Center of Advanced Photonics, 2-1 Hirosawa, Wako, Saitama, 3510198, Japan b Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8904, Japan
highlights The energy storage system to keep the demand and supply balance of electric power. Modified DC-bus signaling control for battery and hydrogen storage. The battery is for fast response and hydrogen storage is for slow and large energy change. The energy compensation observed even at the step-like change of demand and supply.
article info
abstract
Article history:
A user-on-demand power source based on renewable energy requires storage devices to
Received 18 June 2019
balance power sources and power demands because of the fluctuation of power sources
Received in revised form
like solar cells or wind power generators. The role of the control system is defined as two
25 August 2019
different tasks: allowing a power-flow imbalance between demand and power sources; and
Accepted 28 August 2019
balancing the power flow inside the system. Since this control is complicated, many
Available online 21 September 2019
control methods using precise calculation of the power balance have been proposed. An analogue-like distributed control method - named “modified DC-bus signalling” - for
Keywords:
controlling a renewable-energy power source without the need for a central processing
Modified DC-bus control
unit is proposed. The modified DC bus signalling method discussed in this paper is
Renewable energy
composed of a DC-bus line connected with a battery, water-splitting electrochemical cell,
Distributed control
and a fuel cell for hydrogen-energy storage via converters. The proposed control method
Hybrid energy storage system
was demonstrated to be able to control step-like and random changes in input and output
Power flow balancing
power. The battery compensated high-frequency fluctuations in power demand, and the
Hydrogen energy storage
electrochemical cell and fuel cell handled the remaining low-frequency ones, which were matched to their response speeds. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (K. Fujii). https://doi.org/10.1016/j.ijhydene.2019.08.234 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction Power sources utilizing renewable energy are continuing to be introduced around the world. Energy production by some renewable-energy power sources, such as solar cells and wind-power generators, depends on weather conditions. In the case of grid-connected power supplies, this fluctuation of output power causes changes in the frequency of voltages distributed by the grid. In the case of standalone power supplies, lack of other sources of power generation is a critical problem; namely, blackouts occur when renewable energy cannot be obtained. An energy-storage system that can balance the power demand and the power source is therefore one of the keys for large-scale introduction of renewable-energy sources [1]. Since renewable devices like solar cells and batteries are DC devices, AC-bus systems require DC-to-AC conversion by pulse-wave-modulation (PWM) control, and the conversion causes signal delay. Although an AC system is commonly used for electric power systems, a DC-bus system connected with energy storage to balance the power demand and the source power is a major solution for renewable-power-source systems. That is, a DC-bus system is suitable to be used with DC storage devices like supercapacitors, batteries, water-splitting electrochemical cells (ECs), and fuel cells (FCs) for electricityhydrogen conversion. As for this renewable-energy power source system, energy storage-devices are required to play two major roles: rapid response and large capacity responses. These two roles are difficult to perform by one device, because electric capacitance increases with storage amount, leading to slow response. Hybrid energy storage utilizing batteries and hydrogen-based electricity storage, for example, has therefore been proposed as an appropriate means of obtaining rapid response and high capacity. Although a battery can response quickly to change in power demand, battery cost is proportional to capacity of the battery. That is, storing a large amount of energy by battery is not appropriate. In contrast, it is easier to increase the capacity of “hydrogen-energy storage” than that of batteries because a larger hydrogen tank brings larger storage capacity. However, electricity-hydrogen conversion devices of ECs and FCs have slow response because they are strongly affected by the flow of ions inside them. Batteries or supercapacitors are therefore often used to compensate the slower responses of ECs and FCs [2]. In the meantime, the control algorithms of an energystorage system should handle two different energy-power balances. One is managing different flows of energy power between the input source and the output demand outside the power-supply system. The other is balancing flows of energy power inside the power-supply system. The technology for controlling a hybrid system consisting of batteries, supercapacitors, ECs, and FCs is therefore complicated. Aiming to solve these problems, several control methods have been proposed. An example in the field of energy management is a centralized control method [3e5], which has been applied to vehicles as well [6,7]. As for this method, a controller collects all necessary information, such as voltage, current, and state of charge (SoC), for controlling the hybrid
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system. The controller then calculates the desirable power flows and controls converters according to the calculation results. This method is suitable for optimizing an entire system; however, for proper control, fast communication is required between the centralized controller and the components for actual control of the system. It is also required to unify the communication protocols in the entire system. These requirements thus become difficult to satisfy with increasing number of devices. Alternatively, in the fields of DC-bus micro- and nano-grids [8e21], distributed control systems have been widely studied. As for distributed control, each component connected to a DCbus line is responsible for stabilizing the bus voltage. That means multiple controllers must work independently; thus, in the case of that configuration, fast communication and unified protocols are less important. In previous studies, fussy logic and droop control were used for regulating the bus voltage [8e10]. Recently, adaptive droop control for fuel cell, battery, supercapacitor system [19,20], and hierarchical selfregulation control of AC/DC hybrid system with hydrogen storage [21] were proposed for the renewable energy storage. The split power based on filtering is one of the good ideas for the distributed control method to control the device performance maximum with electric power splitting [22e24]. The system, however, tends to be complicated due to the energy splitting. As another distributed control method, DC-bus signaling, which utilizes DC-bus voltage as a tool for communication between distributed controllers, was proposed [18,25,26]. DC-bus signaling therefore achieves simple and effective control for the distributed DC-bus control system. In the case of previous DC-bus signaling methods, however, DC-bus voltage is changed abruptly, since the distributed energy storages and generators are turned on or off when the bus voltage exceeds designated thresholds. These sudden changes of DC-bus voltage cause noise-related problems such as fluctuating output voltage and overshoot/undershoot of the energy power in a power-source system. The method proposed in this study - namely, “modified DC-bus signaling” - utilizes DC-bus voltage as a signal to control the output current of converters, which are connected to large-capacity energy-storage devices. DC-bus voltage is controlled according to the current to and from the battery. Since the amount of output energy is controlled, smooth DCbus voltage regulation and proper load balancing are achieved without communication lines inside a power-source system composed of batteries, FCs, and ECs. As a result, this method makes it possible to control distributed devices via DC-bus voltage as a control signal without the need for a central control unit.
Control method Modified DC-bus control method A schematic diagram of the proposed system for modified DCbus control is shown in Fig. 1. A DC-bus line connects the renewable-energy sources, the energy-storage devices, and output demands via converters. As for this control system, the energy-source devices are solar cells and wind power
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Fig. 1 e Schematic structure of DC-bus-connected hybrid (battery-hydrogen energy) storage system. MVC and SVC indicate main voltage controller and sub voltage controller, respectively. generators, and the energy-storage devices are a battery, a FC, and an EC. The detailed control method is discussed from the following sections. The proposed control system roughly works as follows. First, the DC-bus voltage is changed by detecting the current of the converter connected to the battery according to the imbalance of power flow among the sources, output demands, and storage devices. Next, the distributed controllers connected to the FC and EC detect the change in DC-bus voltage and change the currents between the DC-bus line and FC or EC via converters. This procedure continues until the current of the converters connected to the battery is zero. That is, the process stops when the power flow is finally balanced. One of the important original ideas in this study is separating the controlling process into two parts, that is, active and passive parts. Controlling DC-bus voltage to balance the power flow by a storage device with rapid response is the active part, while the passive part is measuring DC-bus voltage and controlling the current output from, or input to, other storage devices with larger capacity and slower response. These parts are discussed in detail later in the role of converters. The converters and components in Fig. 1 can be grouped into four types: input converters, output converters, a main voltage controller (MVC), and sub voltage controllers (SVCs). Thus, the high frequency fluctuations are compensated by the MVC, and the low frequency fluctuations and the continuous excesses and deficiencies are compensated by the SVCs. The limit of the high frequency fluctuation is defined by the response speed of MVC and the system capacitance. The roles of these converters and components are defined as the following sections. The superior point of this system is that the small voltage change by the MVC connected to the bus line is used as the signal to control SVC devices. Therefore, the control is simple via the DC/DC converters connected to devices compared with the direct device connection and is independent DC/DC converter control without the requirement of communications among the DC/DC converters.
Input converters The input converters (DC-DC and AC-DC types) are interfacing between the power sources and DC-bus line. They are
responsible for maximum-power-point tracking (MPPT) for renewable energy generators such as solar cells and wind turbines. Since the DC-bus voltage is defined by the other converter, the input converters operate to maximize the input power.
Output converters The output converters are DC-DC converters and/or DC-AC inverters interfacing DC and/or AC loads and DC-bus line, respectively. They are responsible for supplying constant DC and/or AC voltage required by the loads connected to the power source system. As explained in the next, the DC-bus voltage is slightly changed depending on the operating conditions of the power sources and demands. Therefore, output converters should regulate output voltage properly under the DC-bus voltage change. This is not a difficult operation because the fluctuation of the bus voltage is relatively small and smoothly controlled by two types of voltage controllers.
Main voltage controller (MVC) The MVC is responsible for the active part of the energy control procedure, namely, controlling bus voltage. The control flow of the MVC is shown in Fig. 2. Since the MVC is the active part of the controller, one MVC should be connected to the DCbus under the proposed modified DC-bus control. The input/ output current of the MVC (IMVC ) is integrated to obtain the charge change in the main voltage control device. This change is used to calculate the next target voltage of the DC-bus line (Vbus target ). The center of the bus voltage (Vbus center ) is defined as the reference standard when the power source and demand are balanced. The output of the MVC is the voltage controlled by the DC-DC converter to maintain the DC-bus voltage (Vbus ) at Vbus target . In Fig. 2, KIV ðsÞ is controller gain, and GC ðsÞ; GP ðsÞ; and HðsÞ are the controller, power stage, and feedback transfer functions of the converter in the frequency domain, respectively. The value of Vbus target is determined autonomously in a directly proportional manner to the integral of IMVC , or SoC of the energy storage attached to the MVC, as also shown in Fig. 2. The slope of Vbus as a function of SoC corresponds to
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Fig. 2 e Control flow of the main voltage controller (MVC) and the relationship between SoC- (state of charge) indication value and the target voltage of the DC-bus. See text for the abbreviation explanations.
IMVC to Vbus gain (KIV ) in Fig. 2. When KIV is larger, bus voltage changes more rapidly under the same power flow imbalance. Vbus target is increased (decreased) corresponding to the increase (decrease) in SoC. Following the determination of Vbus target , the other voltage controllers start to absorb (desorb) electrical energy from the DC-bus line as discussed later, which in turn decreases (increases) IMVC . This control leads to a finite value of integrated IMVC , or SoC. Vbus target is therefore converged to a value that brings a steady state in which the DC bus is balanced in terms of its input and output. This MVC unit firstly responds to the imbalance and absorbs rapid changes in power fluctuation. It then ignites the other voltage controllers to stabilize the power balance. The MVC should therefore consist of energy-storage devices that allow rapid charging and discharging, but its energy capacity might not necessarily be large. In consideration of this requirement, batteries and supercapacitors are suitable for the MVC devices.
Sub voltage controller (SVC) The SVCs sense the DC-bus voltage and determine the current charge to/discharge from the energy-storage devices. The control flow of each SVC is shown in Fig. 3. As shown in the graph in Fig. 3, SVC target current (Itarget ) is proportional to Vbus Vbus ref , and the proportionality constant is the gain (KVI ). This KVI defines the response speed of SVC, thus it can limit the fast response also. Different values of the reference voltage to turn on the SVCs, Vbus ref , can be selected for each SVC, and this possibility allows flexible system operation. Output current of each SVC (ISVC ) is controlled to the target current (Itarget ) controlled by the DC-DC converter. In Fig. 3, KVI ðsÞ is controller gain, and GC ðsÞ; GP ðsÞ; and HðsÞ are the controller, power stage, and feedback transfer functions of the converter in the frequency domain, respectively. These values are similar to the ones in the case of the MVC. The SVCs do not control the DC-bus voltage directly; therefore, they are the passive part of the two types of busvoltage control procedures. It is possible to use a component with one-directional energy flow, such as a FC for energy supply to DC-bus or an EC for energy consumption. For proper system control, either one bidirectional SVC or a combination of unidirectional SVCs for charging and discharging are
necessary. For unidirectional SVCs, converter output should be properly limited in one direction by using the limit function shown in Fig. 3 in order to protect the device from reverse voltage application. The SVCs work after the MVC responds to the change of bus voltage, and they absorb or desorb slow changes of power-flow balance. It is therefore possible to use energy storage devices with relatively slow response, such as FCs and ECs. In detail, since the energy consumption from the bus line to FCs is impossible, the system control is set the output enable and the input disenable for FCs. Similar to the FCs, since the energy supply to the bus line from ECs is impossible, the system control is set the input enable and the output disenable for ECs.
Analysis of the behavior of the entire renewableenergy power-source system As mentioned earlier, the MVC senses IMVC to control Vbus , and the SVCs sense Vbus to control ISVC . These two functions regulate Vbus in a specific range. Under the assumption that the converters respond to external disturbances quickly enough, converter transfer functions in Figs. 2 and 3 can be neglected. This assumption gives a simple mathematical evaluation of transitional and static behaviors of the MVC and the SVCs. In this condition, the difference between the input and output currents to the DC-bus system is equal to the sum of the currents of the MVC and the SVCs, namely, Iin Iout ¼ DI ¼ IMVC þ
X ISVCk k
(1)
where Iin is the input current to the DC bus through the input converters, Iout is the output current from the bus through the output converters, IMVC is the current of the MVC, and ISVCk is the current of the k-th SVC. For convenience, DI is used instead of Iin Iout in the following equations. In the control flow shown in Fig. 2, DC-bus voltage in the frequency domain is given by Vbus ¼ Vbus center þ
KIV IMVC s
(2)
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Fig. 3 e Control flow of a sub voltage controller (SVC) and the relationship between bus voltage (Vbus ) and output current of sub voltage controller (ISVC ). See text for the abbreviation explanations.
where Vbus center is the center voltage of the DC-bus determined by the MVC. ISVCk is given by Vbus from the following equation: ISVCk ¼ Vbus Vbus refk , KVIk ¼ KIV IMVC Vbus refk ,KVIk Vbus center þ s
IMVC ¼
s þ KIV
k
( KVIk
DI
X
(3)
Vbus center Vbus refk KVIk
) (4)
k
Equation (4) is a high-pass filtered DI with time constant P 1=KIV KVIk . The second term of Equation (4) is a constant k value and does not affect to the relation between IMVC and DI. As a result, the MVC responds to the power imbalance firstly and absorbs the earlier (rapid) part of the imbalance. The current of the i-th SVC (ISVCi ), is derived as follows. For simplification, Vbus refk is assumed to be the same for all of the SVCs, so Vbus refk ¼ Vbus ref for all k
(5)
Under this condition, the specific ISVCk is proportional to KVIk . Thus, ISVCi is written as
k
X KVI ISVCk ¼ P i ðDI IMVC Þ KVIk k
In Equation (7), ISVCi is a low-pass filtered DI with time P constant 1=KIV KVIk . The time constant is the same as the one k for the MVC. As a result, the SVCs respond to the later (slower) part of power imbalance. The steady-state relationship between the DC-bus voltage and power-flow imbalance is explained by the equations below. IMVC becomes zero after enough time has elapsed from the change in DI because IMVC is high-pass filtered DI. IMVC is shown from the final-value theorem. Hereafter, a step change of DI, namely, 1=s in the frequency domain, is assumed. The final value of IMVC can be calculated from s2 P
lims,IMVC ¼lim
s/0 sþKIV
s/0
k
X 1 þ Vbus center Vbus refk KVIk ¼0 KVIk s k (8)
Equation (1) in this condition is therefore given as DI ¼ 0 þ
X X KVIk Vbus Vbus refk ISVGk ¼ k
k
(9)
Equation (9) can be simplified by using of Equation (5), so the following equation is obtained: DI Vbus ¼ Vbus ref þ P KVIk
X KVI Vbus Vbus ref ISVC ISVCi ¼ P i , ISVCk ¼ P i ISVCk k KVIk Vbus Vbus ref k
(7)
k
In Equation (3), subscript k means the variables corresponding to the k-th SVC. ISVCk can be eliminated from Equation (1) by using Equation (3). The relation between DI and IMVC is written as s P
KIV DI þ Vbus center Vbus ref s ISVCi ¼ KVIi , X KVIk s þ KIV
(10)
k
(6)
k
In Equation (6), IMVC can be eliminated by using Equation (4) to give
According to Equation (10), a larger sum of SVC gains (KVIk ) results in a smaller change of target bus voltage (Vbus ) against the power-flow imbalance. In practical situations, the gains in the system are determined according to the response speed of the device components and allowable fluctuation of DC-bus voltage.
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It should be noted that these analyses are not available in the case that any voltage, current or SoC are out of limits. Additional control is therefore required to handle such situations.
System simulation and real system operation System simulation scheme To evaluate the energy control properties, the power-supply system structure used for computer simulation of operations is shown inFig.4. One battery for the MVC and oneFCand oneECfor the SVCs are connected to the DC-bus line. To simulate converter responses with realistic delays, delays of 50 ms at the output charge changing from 0% to 100% were introduced. The sign of IMVC is positive when the MVC is charging and negative when the MVC is discharging. The sign of ISVC1 ð¼ IEC Þ is positive when the SVC consumes energy from the DC-bus. The sign of ISVC2 ð¼ IFC Þ is positive when the SVC supplies energy to the DC-bus. MatlabSimulink and PLECS were used to simulate the control flows and electrical characteristics of the energy control system. The integrator element in the MVC control flow is regarded as ideal in the following simulations. The model for DC/DC converters is average value model in order to simplify the modelling. However, the simulation employed a simple numerical integration using staircase approximation with 0.1-s sampling time considering with actual implementations. The MVC senses the current (IMVC ) every 0.1 s and integrates current values multiplied by MVC gain KIV to calculate the target voltage of the DC-bus (Vbus ). The conditions used in the simulation are listed in Table 1. The numbers in Table 1 were selected in consideration of the time constants of realistic devices. Cbus is the capacitance of the DC-bus line. It is equal to the sum of the output capacitances of the converters. The simulations were conducted in three patterns: steplike input and output changes, fluctuating input power, and additional installation of SVC device. In the first step-like change pattern, the behaviours of the main- and sub-voltage controllers can be seen very clearly.
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Table 1 e Assumed conditions in system simulations. Parameters MVC MVC gain (KIV ) Step response speed Vbus center SVC1 (EC) SVC gain ðKVI1 Þ Step response speed Vbus ref 1 SVC2 (FC) SVC gain ðKVI2 Þ Step response speed Vbus ref 2 Cbus DC-bus input current DC-bus output current
2.5 kW, 7 A to 7 A 0.25 50 ms for 0%e100% change 300 V 2.5 kW Max. 0 A to 7 A 4.0 50 ms for 0%e100% change 300 þ 0.5 V 2.5 kW Max. 0 Ae7 A 4.0 50 ms for 0%e100% change 300e0.5 V 5 mF 50 ms for 0%e100% change 50 ms for 0%e100% change
Iload was set from 0 to 7 A at t ¼ 10 [s] and then turned off at t ¼ 30 [s]. The results of the simulation are shown in Fig. 5. How the MVC and SVC stabilize the voltage of the DC-bus line is discussed in relation to three important events in the simulation as follows.
Input enabled Iin was set from 0 to 7 A at t ¼ 1.0 [s]. Firstly, the MVC began to absorb excess power of the DC-bus line. This behavior is observed as a positive spike of IMVC in Fig. 5. Secondly, Vbus was increased due to the increase of SoC of the battery connected as the MVC. Thirdly, the SVC started to absorb excess power corresponding to the positively changing Vbus . Finally, IMVC converged to almost zero when Vbus was stabilized.
Load enabled Iload was set from 0 to 7 A at t ¼ 10 [s]. Firstly, the MVC absorbed the sudden change of the power balance at the bus, which can be observed as a negative spike of IMVC in Fig. 5. Secondly, Vbus was decreased due to the decrease of SoC of the battery connected as the MVC. Finally, Vbus settled at 300 V, which is the center voltage (Vbus center ), since Iin and Iload were balanced.
Pattern1: step-like changes of input and output
Input disabled
Step-like changes of input and output were simulated first. Iin was set from 0 to 7 A at t ¼ 1 [s] and then turned off at t ¼ 20 [s].
Iin was set from 7 to 0 A at t ¼ 20 [s]. Firstly, the MVC began to release some power into the DC-bus line in order to maintain the target bus voltage, which is observed as a negative spike of
Fig. 4 e System structure for simulating energy control system. Power sources and loads are assumed as ideal current sources. MVC, SVC1, and SVC2 indicate main voltage controller, sub voltage controller (current only from the bus), and sub voltage controller (current only to the bus), respectively.
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Fig. 5 e Results of simulation of system behavior under changes in input and load power for pattern 1: step-like changes in input and output. The system shown in Fig. 4 was used for the simulation. Given input and output currents (Iin and Iload ), targeted and simulated bus voltages (Vbus and Vbus target ), and simulated currents for MVC, EC, and FC (IMVC , IEC , and IFC ) are shown here.
Fig. 6 e Result of simulation of system behavior under changes in input and load power for pattern 2: fluctuated input power. The system shown in Fig. 4 was used for the simulation. Given input and output currents (Iin and Iload ), targeted and simulated bus voltages (Vbus and Vbus target ), and simulated currents for MVC, EC, and FC (IMVC , IEC , and IFC ) are shown here.
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Table 2 e Parameters of the additional SVC used in simulation pattern 3. Parameters SVC3 (Battery) SVC gain ðKVI3 Þ Step response speed Vbus ref 3
2.5 kW, 7 A to þ 7 A 2.0 50 ms for 0%e100% change 300 V
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due to the input power fluctuation. The MVC set Vbus to higher values when input energy was in excess and lower values when input energy was in shortage. The SVCs detected this change of Vbus and controlled ISVC to compensate the powerload imbalance. Since the high-frequency part of the disturbance was absorbed by the MVC, the current changes of the EC and FC were smoothed.
Pattern 3: additional installation of SVC
IMVC in Fig. 5. Secondly, Vbus was decreased due to the decrease of SoC of the battery connected as the MVC. Thirdly, the SVC started to compensate the power balance. Finally, IMVC converged to almost zero when Vbus was stabilized. In this simulation, power imbalance of about ± 2:1 kW was converted to around ±2 V change in Vbus . As shown here, the step-like power-flow change was absorbed by the MVC and SVCs.
Pattern 2: fluctuated input power In the second pattern, input power was fluctuated randomly. The load was enabled at t ¼ 10 [s], and Iload was set from 0 to 5 [A]. The result of the simulation is shown in Fig. 6. After the load was enabled, the power-flow balance changed alternately
In the third pattern, the additional SVC of the battery installed in pattern 2 was evaluated. The detailed parameters of the additional SVC are summarized in Table 2. The current of the additional SVC (ISVC3 ¼ Ibattery ) is positive when the current is charging and is negative when the current is discharging. The simulation results for pattern 3 are summarized in Fig. 7. The additional SVC cooperated with the existing MVC and SVC in stabilizing bus voltage. Compared with the former case with pattern 2, fluctuation of Vbus and IMVC became small because the same bus-voltage shift enhances the total SVC current in the case of pattern 3.
Simulated system response In the three simulation cases, output current of MVC (IMVC ) was nearly zero when DC-bus voltage (Vbus ) was stabilized
Fig. 7 e Results of simulation of system behavior under changes in input and load power for pattern 3: additional installation of SVC. The system shown in Fig. 4 plus an additional battery of SVC was used for the simulation. Given input and output currents (Iin and Iload ), targeted and simulated bus voltages (Vbus and Vbus target ), and simulated currents for MVC, battery for SVC, EC, and FC (IMVC , Ibattery , IEC , and IFC ) are shown here.
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Fig. 8 e Whole view of the real system.
even when the direction of IMVC was frequently changing. This result means that the MVC can use a battery or a supercapacitor with small capacity. The analysis of the entire-system behaviour discussed in an earlier section explains the simulation results (i.e., a steplike power-change) well. At the step transient, the expected time constant of the MVC and SVCs from Equations (4) and (7) P is 1=KIV KVIk ¼ 1=ð0:25 4Þ ¼ 1 ½s, which matches the simuk P lation results. In this time constant calculation, the sum KVIk k
was calculated from working SVCs. There are two unidirectional SVCs in the simulation model, but they do not work at the same time. The sum was therefore simplified to a single SVC. As a result, change in DC-bus voltage can be calculated P from Equation (10) as DI= KVIk ¼ ±7=4 ¼ ±1:75 ½V. The simuk
lated change in DC-bus voltage was slightly larger, probably due to the non-ideal integration method used in the simulation. As for the additional SVC (pattern 3), the range of DC-bus
Table 3 e Devises used in the real operation. Devices: Experimental Energy System Variable DC Source (instead of solar cells) Variable DC Sink (instead of use points) DC/DC Converter Electrochemical Water Splitting Cell (EC) Fuel Cell (FC) Hydrogen Tank Rechargeable Battery
Takasago ZX-S-1600HA (2 series) Kikusui PLZ1004WH þ PLZ2004WHB TDK Lambda EZA-2500 Enoah EHC-750 (7 series) Enoah EOS-1000 (2 parallel: 1 device used) 2 m3, <0.7 MPa Pb 12 V (4 series)
voltage was narrowed by the additional SVC, and that result is also predicted by Equation (10). If the voltage fluctuation of the DC-bus line is large, the output voltage of the system may be affected, resulting in output noise. In the simulations, the changes in DC-bus voltage were smooth despite of the rapid changes in the power balance. This smooth voltage change is useful information for the stable output-voltage for the output converters. Long-term energy management is not considered in this study; however, it is possible to manage the SoC of each energystorage device by controlling Vbus ref externally. For example, if Vbus ref of the FC is set to a lower value, the amount of hydrogen stored would be larger. It is possible to manage the SoC of multiple energy storage devices using with the same manner.
Real system operation The proposed control method was applied to a real energy control system with a 340-V DC-bus center connected to a battery and a part of EC and FC with almost 1 kW power for each via a DC-DC converter. The DC bus voltage for the real system was selected 340 V to reduce the current below 10 A at a few kW system. The whole view of the system is shown in Fig. 8. The devices used in the real system are shown in Table 3. The control parameters of the MVC and SVCs were set to KIV ¼ 0.915 and KVI ¼ 2.4 for both the EC and FC. The system simulation results were used as the references of the real gain parameters. The values of KIV and KVI were selected the gain factors the devices being able to respond and the values were not optimized. The value of KVI can be selected much larger to be slower the device reactions. The input and demand were power source and electric load, respectively, in order to control the amount of power artificially. The results of real system operation under abrupt
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Fig. 9 e Result of real-system operation under abrupt changes in input and load power. The system shown in Fig. 4 with devices listed in Table 3 was used. Bus voltage (Vbus ), and currents for battery (MVC), EC, and FC (Ibattery , IEC , and IFC ) are shown here.
changes in input and load are shown in Fig. 9. The swing width of DC-bus voltage was set larger than the case of simulation for clear observation. Although some noise is observed in the real operation, the controlled current and voltage follow basically the same trends as the simulated ones.
Conclusion A distributed control method, called “modified DC-bus signaling”, for renewable energy sources with different energy-storage devices was proposed. This method enables smart cooperation among the devices connected to the bus without the need for a high-speed communication line. In the controlling method, it uses two kinds of storage devices: a main-voltage controller (MVC) and sub-voltages controllers (SVCs). The MVC is an energy-storage device with rapid response connected to a DC-bus line via a DC-DC converter, and it initially set DC bus voltage according to the power balance in the bus line. The MVC senses its input or output current and controls the target DC-bus voltage. The SVCs control their input or output current according to the DC bus voltage. The MVC and SVCs are controlled individually but interact with each other through DC-bus voltage to balance the energy flow. Since the MVC responds to the earlier (rapid) power-flow imbalance, the capacity of energy storage in the MVC is relatively small. On the other hand, the capacities of energy storage in the SVCs are relatively large because the SVCs deal with the later (slow) change or constant power-flow imbalance. Instead, the response speed of the SVCs is relatively slow because a rapid power change is absorbed by the MVC and the capacitance of the bus. These results are important when different types of energy storages of SVCs are used effectively. In the computer simulations of power control, all three evaluated patterns (step-like changes in input and output power, randomly fluctuating input, and an additional SVC) showed good stabilization of the bus voltage. About step-like
power-flow change into the DC-bus of ±2:1 kW was converted to a small fluctuation in the bus voltage, approximately ±2 V. The control method was further applied to a real system, which is composed of a battery as an MVC and a pair of EC and FC (each 1 kW in power capacity) as SVCs. The voltage of this example of real DC bus system was stabilized at 340 ± 12 V upon a step-like input power fluctuation of about 0.6 kW. This swing width was set to be larger than the simulation case for clear observation. The method can manage the SoC of SVCs if long-term prediction of weather and energy consumption is available. By adjusting the parameter of each SVC externally, the energy stored in the device can be altered: e.g., a low Vbus ref results in a larger energy stored in an SVC. With the method proposed here, it is possible to handle a massive number of connected SVCs. Harmonized control of power flow and voltage in a DC bus can be realized by an appropriate set of parameters, KIV for an MVC and KVI’s for SVCs, corresponding to both the response speed and the energy capacity for each device.
Acknowledgment The authors are grateful to all the members of the Sugiyama Laboratory at RCAST of The University of Tokyo and the Photonics Control Technology Team at the RIKEN Center for Advanced Photonics (RAP) for supporting this study. This study was supported by a grant from RIKEN.
references
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Distributed control of a user-on-demand renewable-energy power-source system using battery and hydrogen hybrid energy-storage devices Daiji Yamashita a,b, Katsuhiko Tsuno a, Kayo Koike a, Katsushi Fujii a,*, Satoshi Wada a, Masakazu Sugiyama b a Photonics Control Technology Team, RIKEN Center of Advanced Photonics, 2-1 Hirosawa, Wako, Saitama, 3510198, Japan b Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8904, Japan
highlights The energy storage system to keep the demand and supply balance of electric power. Modified DC-bus signaling control for battery and hydrogen storage. The battery is for fast response and hydrogen storage is for slow and large energy change. The energy compensation observed even at the step-like change of demand and supply.
article info
abstract
Article history:
A user-on-demand power source based on renewable energy requires storage devices to
Received 18 June 2019
balance power sources and power demands because of the fluctuation of power sources
Received in revised form
like solar cells or wind power generators. The role of the control system is defined as two
25 August 2019
different tasks: allowing a power-flow imbalance between demand and power sources; and
Accepted 28 August 2019
balancing the power flow inside the system. Since this control is complicated, many
Available online 21 September 2019
control methods using precise calculation of the power balance have been proposed. An analogue-like distributed control method - named “modified DC-bus signalling” - for
Keywords:
controlling a renewable-energy power source without the need for a central processing
Modified DC-bus control
unit is proposed. The modified DC bus signalling method discussed in this paper is
Renewable energy
composed of a DC-bus line connected with a battery, water-splitting electrochemical cell,
Distributed control
and a fuel cell for hydrogen-energy storage via converters. The proposed control method
Hybrid energy storage system
was demonstrated to be able to control step-like and random changes in input and output
Power flow balancing
power. The battery compensated high-frequency fluctuations in power demand, and the
Hydrogen energy storage
electrochemical cell and fuel cell handled the remaining low-frequency ones, which were matched to their response speeds. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (K. Fujii). https://doi.org/10.1016/j.ijhydene.2019.08.234 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction Power sources utilizing renewable energy are continuing to be introduced around the world. Energy production by some renewable-energy power sources, such as solar cells and wind-power generators, depends on weather conditions. In the case of grid-connected power supplies, this fluctuation of output power causes changes in the frequency of voltages distributed by the grid. In the case of standalone power supplies, lack of other sources of power generation is a critical problem; namely, blackouts occur when renewable energy cannot be obtained. An energy-storage system that can balance the power demand and the power source is therefore one of the keys for large-scale introduction of renewable-energy sources [1]. Since renewable devices like solar cells and batteries are DC devices, AC-bus systems require DC-to-AC conversion by pulse-wave-modulation (PWM) control, and the conversion causes signal delay. Although an AC system is commonly used for electric power systems, a DC-bus system connected with energy storage to balance the power demand and the source power is a major solution for renewable-power-source systems. That is, a DC-bus system is suitable to be used with DC storage devices like supercapacitors, batteries, water-splitting electrochemical cells (ECs), and fuel cells (FCs) for electricityhydrogen conversion. As for this renewable-energy power source system, energy storage-devices are required to play two major roles: rapid response and large capacity responses. These two roles are difficult to perform by one device, because electric capacitance increases with storage amount, leading to slow response. Hybrid energy storage utilizing batteries and hydrogen-based electricity storage, for example, has therefore been proposed as an appropriate means of obtaining rapid response and high capacity. Although a battery can response quickly to change in power demand, battery cost is proportional to capacity of the battery. That is, storing a large amount of energy by battery is not appropriate. In contrast, it is easier to increase the capacity of “hydrogen-energy storage” than that of batteries because a larger hydrogen tank brings larger storage capacity. However, electricity-hydrogen conversion devices of ECs and FCs have slow response because they are strongly affected by the flow of ions inside them. Batteries or supercapacitors are therefore often used to compensate the slower responses of ECs and FCs [2]. In the meantime, the control algorithms of an energystorage system should handle two different energy-power balances. One is managing different flows of energy power between the input source and the output demand outside the power-supply system. The other is balancing flows of energy power inside the power-supply system. The technology for controlling a hybrid system consisting of batteries, supercapacitors, ECs, and FCs is therefore complicated. Aiming to solve these problems, several control methods have been proposed. An example in the field of energy management is a centralized control method [3e5], which has been applied to vehicles as well [6,7]. As for this method, a controller collects all necessary information, such as voltage, current, and state of charge (SoC), for controlling the hybrid
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system. The controller then calculates the desirable power flows and controls converters according to the calculation results. This method is suitable for optimizing an entire system; however, for proper control, fast communication is required between the centralized controller and the components for actual control of the system. It is also required to unify the communication protocols in the entire system. These requirements thus become difficult to satisfy with increasing number of devices. Alternatively, in the fields of DC-bus micro- and nano-grids [8e21], distributed control systems have been widely studied. As for distributed control, each component connected to a DCbus line is responsible for stabilizing the bus voltage. That means multiple controllers must work independently; thus, in the case of that configuration, fast communication and unified protocols are less important. In previous studies, fussy logic and droop control were used for regulating the bus voltage [8e10]. Recently, adaptive droop control for fuel cell, battery, supercapacitor system [19,20], and hierarchical selfregulation control of AC/DC hybrid system with hydrogen storage [21] were proposed for the renewable energy storage. The split power based on filtering is one of the good ideas for the distributed control method to control the device performance maximum with electric power splitting [22e24]. The system, however, tends to be complicated due to the energy splitting. As another distributed control method, DC-bus signaling, which utilizes DC-bus voltage as a tool for communication between distributed controllers, was proposed [18,25,26]. DC-bus signaling therefore achieves simple and effective control for the distributed DC-bus control system. In the case of previous DC-bus signaling methods, however, DC-bus voltage is changed abruptly, since the distributed energy storages and generators are turned on or off when the bus voltage exceeds designated thresholds. These sudden changes of DC-bus voltage cause noise-related problems such as fluctuating output voltage and overshoot/undershoot of the energy power in a power-source system. The method proposed in this study - namely, “modified DC-bus signaling” - utilizes DC-bus voltage as a signal to control the output current of converters, which are connected to large-capacity energy-storage devices. DC-bus voltage is controlled according to the current to and from the battery. Since the amount of output energy is controlled, smooth DCbus voltage regulation and proper load balancing are achieved without communication lines inside a power-source system composed of batteries, FCs, and ECs. As a result, this method makes it possible to control distributed devices via DC-bus voltage as a control signal without the need for a central control unit.
Control method Modified DC-bus control method A schematic diagram of the proposed system for modified DCbus control is shown in Fig. 1. A DC-bus line connects the renewable-energy sources, the energy-storage devices, and output demands via converters. As for this control system, the energy-source devices are solar cells and wind power
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Fig. 1 e Schematic structure of DC-bus-connected hybrid (battery-hydrogen energy) storage system. MVC and SVC indicate main voltage controller and sub voltage controller, respectively. generators, and the energy-storage devices are a battery, a FC, and an EC. The detailed control method is discussed from the following sections. The proposed control system roughly works as follows. First, the DC-bus voltage is changed by detecting the current of the converter connected to the battery according to the imbalance of power flow among the sources, output demands, and storage devices. Next, the distributed controllers connected to the FC and EC detect the change in DC-bus voltage and change the currents between the DC-bus line and FC or EC via converters. This procedure continues until the current of the converters connected to the battery is zero. That is, the process stops when the power flow is finally balanced. One of the important original ideas in this study is separating the controlling process into two parts, that is, active and passive parts. Controlling DC-bus voltage to balance the power flow by a storage device with rapid response is the active part, while the passive part is measuring DC-bus voltage and controlling the current output from, or input to, other storage devices with larger capacity and slower response. These parts are discussed in detail later in the role of converters. The converters and components in Fig. 1 can be grouped into four types: input converters, output converters, a main voltage controller (MVC), and sub voltage controllers (SVCs). Thus, the high frequency fluctuations are compensated by the MVC, and the low frequency fluctuations and the continuous excesses and deficiencies are compensated by the SVCs. The limit of the high frequency fluctuation is defined by the response speed of MVC and the system capacitance. The roles of these converters and components are defined as the following sections. The superior point of this system is that the small voltage change by the MVC connected to the bus line is used as the signal to control SVC devices. Therefore, the control is simple via the DC/DC converters connected to devices compared with the direct device connection and is independent DC/DC converter control without the requirement of communications among the DC/DC converters.
Input converters The input converters (DC-DC and AC-DC types) are interfacing between the power sources and DC-bus line. They are
responsible for maximum-power-point tracking (MPPT) for renewable energy generators such as solar cells and wind turbines. Since the DC-bus voltage is defined by the other converter, the input converters operate to maximize the input power.
Output converters The output converters are DC-DC converters and/or DC-AC inverters interfacing DC and/or AC loads and DC-bus line, respectively. They are responsible for supplying constant DC and/or AC voltage required by the loads connected to the power source system. As explained in the next, the DC-bus voltage is slightly changed depending on the operating conditions of the power sources and demands. Therefore, output converters should regulate output voltage properly under the DC-bus voltage change. This is not a difficult operation because the fluctuation of the bus voltage is relatively small and smoothly controlled by two types of voltage controllers.
Main voltage controller (MVC) The MVC is responsible for the active part of the energy control procedure, namely, controlling bus voltage. The control flow of the MVC is shown in Fig. 2. Since the MVC is the active part of the controller, one MVC should be connected to the DCbus under the proposed modified DC-bus control. The input/ output current of the MVC (IMVC ) is integrated to obtain the charge change in the main voltage control device. This change is used to calculate the next target voltage of the DC-bus line (Vbus target ). The center of the bus voltage (Vbus center ) is defined as the reference standard when the power source and demand are balanced. The output of the MVC is the voltage controlled by the DC-DC converter to maintain the DC-bus voltage (Vbus ) at Vbus target . In Fig. 2, KIV ðsÞ is controller gain, and GC ðsÞ; GP ðsÞ; and HðsÞ are the controller, power stage, and feedback transfer functions of the converter in the frequency domain, respectively. The value of Vbus target is determined autonomously in a directly proportional manner to the integral of IMVC , or SoC of the energy storage attached to the MVC, as also shown in Fig. 2. The slope of Vbus as a function of SoC corresponds to
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Fig. 2 e Control flow of the main voltage controller (MVC) and the relationship between SoC- (state of charge) indication value and the target voltage of the DC-bus. See text for the abbreviation explanations.
IMVC to Vbus gain (KIV ) in Fig. 2. When KIV is larger, bus voltage changes more rapidly under the same power flow imbalance. Vbus target is increased (decreased) corresponding to the increase (decrease) in SoC. Following the determination of Vbus target , the other voltage controllers start to absorb (desorb) electrical energy from the DC-bus line as discussed later, which in turn decreases (increases) IMVC . This control leads to a finite value of integrated IMVC , or SoC. Vbus target is therefore converged to a value that brings a steady state in which the DC bus is balanced in terms of its input and output. This MVC unit firstly responds to the imbalance and absorbs rapid changes in power fluctuation. It then ignites the other voltage controllers to stabilize the power balance. The MVC should therefore consist of energy-storage devices that allow rapid charging and discharging, but its energy capacity might not necessarily be large. In consideration of this requirement, batteries and supercapacitors are suitable for the MVC devices.
Sub voltage controller (SVC) The SVCs sense the DC-bus voltage and determine the current charge to/discharge from the energy-storage devices. The control flow of each SVC is shown in Fig. 3. As shown in the graph in Fig. 3, SVC target current (Itarget ) is proportional to Vbus Vbus ref , and the proportionality constant is the gain (KVI ). This KVI defines the response speed of SVC, thus it can limit the fast response also. Different values of the reference voltage to turn on the SVCs, Vbus ref , can be selected for each SVC, and this possibility allows flexible system operation. Output current of each SVC (ISVC ) is controlled to the target current (Itarget ) controlled by the DC-DC converter. In Fig. 3, KVI ðsÞ is controller gain, and GC ðsÞ; GP ðsÞ; and HðsÞ are the controller, power stage, and feedback transfer functions of the converter in the frequency domain, respectively. These values are similar to the ones in the case of the MVC. The SVCs do not control the DC-bus voltage directly; therefore, they are the passive part of the two types of busvoltage control procedures. It is possible to use a component with one-directional energy flow, such as a FC for energy supply to DC-bus or an EC for energy consumption. For proper system control, either one bidirectional SVC or a combination of unidirectional SVCs for charging and discharging are
necessary. For unidirectional SVCs, converter output should be properly limited in one direction by using the limit function shown in Fig. 3 in order to protect the device from reverse voltage application. The SVCs work after the MVC responds to the change of bus voltage, and they absorb or desorb slow changes of power-flow balance. It is therefore possible to use energy storage devices with relatively slow response, such as FCs and ECs. In detail, since the energy consumption from the bus line to FCs is impossible, the system control is set the output enable and the input disenable for FCs. Similar to the FCs, since the energy supply to the bus line from ECs is impossible, the system control is set the input enable and the output disenable for ECs.
Analysis of the behavior of the entire renewableenergy power-source system As mentioned earlier, the MVC senses IMVC to control Vbus , and the SVCs sense Vbus to control ISVC . These two functions regulate Vbus in a specific range. Under the assumption that the converters respond to external disturbances quickly enough, converter transfer functions in Figs. 2 and 3 can be neglected. This assumption gives a simple mathematical evaluation of transitional and static behaviors of the MVC and the SVCs. In this condition, the difference between the input and output currents to the DC-bus system is equal to the sum of the currents of the MVC and the SVCs, namely, Iin Iout ¼ DI ¼ IMVC þ
X ISVCk k
(1)
where Iin is the input current to the DC bus through the input converters, Iout is the output current from the bus through the output converters, IMVC is the current of the MVC, and ISVCk is the current of the k-th SVC. For convenience, DI is used instead of Iin Iout in the following equations. In the control flow shown in Fig. 2, DC-bus voltage in the frequency domain is given by Vbus ¼ Vbus center þ
KIV IMVC s
(2)
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Fig. 3 e Control flow of a sub voltage controller (SVC) and the relationship between bus voltage (Vbus ) and output current of sub voltage controller (ISVC ). See text for the abbreviation explanations.
where Vbus center is the center voltage of the DC-bus determined by the MVC. ISVCk is given by Vbus from the following equation: ISVCk ¼ Vbus Vbus refk , KVIk ¼ KIV IMVC Vbus refk ,KVIk Vbus center þ s
IMVC ¼
s þ KIV
k
( KVIk
DI
X
(3)
Vbus center Vbus refk KVIk
) (4)
k
Equation (4) is a high-pass filtered DI with time constant P 1=KIV KVIk . The second term of Equation (4) is a constant k value and does not affect to the relation between IMVC and DI. As a result, the MVC responds to the power imbalance firstly and absorbs the earlier (rapid) part of the imbalance. The current of the i-th SVC (ISVCi ), is derived as follows. For simplification, Vbus refk is assumed to be the same for all of the SVCs, so Vbus refk ¼ Vbus ref for all k
(5)
Under this condition, the specific ISVCk is proportional to KVIk . Thus, ISVCi is written as
k
X KVI ISVCk ¼ P i ðDI IMVC Þ KVIk k
In Equation (7), ISVCi is a low-pass filtered DI with time P constant 1=KIV KVIk . The time constant is the same as the one k for the MVC. As a result, the SVCs respond to the later (slower) part of power imbalance. The steady-state relationship between the DC-bus voltage and power-flow imbalance is explained by the equations below. IMVC becomes zero after enough time has elapsed from the change in DI because IMVC is high-pass filtered DI. IMVC is shown from the final-value theorem. Hereafter, a step change of DI, namely, 1=s in the frequency domain, is assumed. The final value of IMVC can be calculated from s2 P
lims,IMVC ¼lim
s/0 sþKIV
s/0
k
X 1 þ Vbus center Vbus refk KVIk ¼0 KVIk s k (8)
Equation (1) in this condition is therefore given as DI ¼ 0 þ
X X KVIk Vbus Vbus refk ISVGk ¼ k
k
(9)
Equation (9) can be simplified by using of Equation (5), so the following equation is obtained: DI Vbus ¼ Vbus ref þ P KVIk
X KVI Vbus Vbus ref ISVC ISVCi ¼ P i , ISVCk ¼ P i ISVCk k KVIk Vbus Vbus ref k
(7)
k
In Equation (3), subscript k means the variables corresponding to the k-th SVC. ISVCk can be eliminated from Equation (1) by using Equation (3). The relation between DI and IMVC is written as s P
KIV DI þ Vbus center Vbus ref s ISVCi ¼ KVIi , X KVIk s þ KIV
(10)
k
(6)
k
In Equation (6), IMVC can be eliminated by using Equation (4) to give
According to Equation (10), a larger sum of SVC gains (KVIk ) results in a smaller change of target bus voltage (Vbus ) against the power-flow imbalance. In practical situations, the gains in the system are determined according to the response speed of the device components and allowable fluctuation of DC-bus voltage.
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It should be noted that these analyses are not available in the case that any voltage, current or SoC are out of limits. Additional control is therefore required to handle such situations.
System simulation and real system operation System simulation scheme To evaluate the energy control properties, the power-supply system structure used for computer simulation of operations is shown inFig.4. One battery for the MVC and oneFCand oneECfor the SVCs are connected to the DC-bus line. To simulate converter responses with realistic delays, delays of 50 ms at the output charge changing from 0% to 100% were introduced. The sign of IMVC is positive when the MVC is charging and negative when the MVC is discharging. The sign of ISVC1 ð¼ IEC Þ is positive when the SVC consumes energy from the DC-bus. The sign of ISVC2 ð¼ IFC Þ is positive when the SVC supplies energy to the DC-bus. MatlabSimulink and PLECS were used to simulate the control flows and electrical characteristics of the energy control system. The integrator element in the MVC control flow is regarded as ideal in the following simulations. The model for DC/DC converters is average value model in order to simplify the modelling. However, the simulation employed a simple numerical integration using staircase approximation with 0.1-s sampling time considering with actual implementations. The MVC senses the current (IMVC ) every 0.1 s and integrates current values multiplied by MVC gain KIV to calculate the target voltage of the DC-bus (Vbus ). The conditions used in the simulation are listed in Table 1. The numbers in Table 1 were selected in consideration of the time constants of realistic devices. Cbus is the capacitance of the DC-bus line. It is equal to the sum of the output capacitances of the converters. The simulations were conducted in three patterns: steplike input and output changes, fluctuating input power, and additional installation of SVC device. In the first step-like change pattern, the behaviours of the main- and sub-voltage controllers can be seen very clearly.
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Table 1 e Assumed conditions in system simulations. Parameters MVC MVC gain (KIV ) Step response speed Vbus center SVC1 (EC) SVC gain ðKVI1 Þ Step response speed Vbus ref 1 SVC2 (FC) SVC gain ðKVI2 Þ Step response speed Vbus ref 2 Cbus DC-bus input current DC-bus output current
2.5 kW, 7 A to 7 A 0.25 50 ms for 0%e100% change 300 V 2.5 kW Max. 0 A to 7 A 4.0 50 ms for 0%e100% change 300 þ 0.5 V 2.5 kW Max. 0 Ae7 A 4.0 50 ms for 0%e100% change 300e0.5 V 5 mF 50 ms for 0%e100% change 50 ms for 0%e100% change
Iload was set from 0 to 7 A at t ¼ 10 [s] and then turned off at t ¼ 30 [s]. The results of the simulation are shown in Fig. 5. How the MVC and SVC stabilize the voltage of the DC-bus line is discussed in relation to three important events in the simulation as follows.
Input enabled Iin was set from 0 to 7 A at t ¼ 1.0 [s]. Firstly, the MVC began to absorb excess power of the DC-bus line. This behavior is observed as a positive spike of IMVC in Fig. 5. Secondly, Vbus was increased due to the increase of SoC of the battery connected as the MVC. Thirdly, the SVC started to absorb excess power corresponding to the positively changing Vbus . Finally, IMVC converged to almost zero when Vbus was stabilized.
Load enabled Iload was set from 0 to 7 A at t ¼ 10 [s]. Firstly, the MVC absorbed the sudden change of the power balance at the bus, which can be observed as a negative spike of IMVC in Fig. 5. Secondly, Vbus was decreased due to the decrease of SoC of the battery connected as the MVC. Finally, Vbus settled at 300 V, which is the center voltage (Vbus center ), since Iin and Iload were balanced.
Pattern1: step-like changes of input and output
Input disabled
Step-like changes of input and output were simulated first. Iin was set from 0 to 7 A at t ¼ 1 [s] and then turned off at t ¼ 20 [s].
Iin was set from 7 to 0 A at t ¼ 20 [s]. Firstly, the MVC began to release some power into the DC-bus line in order to maintain the target bus voltage, which is observed as a negative spike of
Fig. 4 e System structure for simulating energy control system. Power sources and loads are assumed as ideal current sources. MVC, SVC1, and SVC2 indicate main voltage controller, sub voltage controller (current only from the bus), and sub voltage controller (current only to the bus), respectively.
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Fig. 5 e Results of simulation of system behavior under changes in input and load power for pattern 1: step-like changes in input and output. The system shown in Fig. 4 was used for the simulation. Given input and output currents (Iin and Iload ), targeted and simulated bus voltages (Vbus and Vbus target ), and simulated currents for MVC, EC, and FC (IMVC , IEC , and IFC ) are shown here.
Fig. 6 e Result of simulation of system behavior under changes in input and load power for pattern 2: fluctuated input power. The system shown in Fig. 4 was used for the simulation. Given input and output currents (Iin and Iload ), targeted and simulated bus voltages (Vbus and Vbus target ), and simulated currents for MVC, EC, and FC (IMVC , IEC , and IFC ) are shown here.
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Table 2 e Parameters of the additional SVC used in simulation pattern 3. Parameters SVC3 (Battery) SVC gain ðKVI3 Þ Step response speed Vbus ref 3
2.5 kW, 7 A to þ 7 A 2.0 50 ms for 0%e100% change 300 V
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due to the input power fluctuation. The MVC set Vbus to higher values when input energy was in excess and lower values when input energy was in shortage. The SVCs detected this change of Vbus and controlled ISVC to compensate the powerload imbalance. Since the high-frequency part of the disturbance was absorbed by the MVC, the current changes of the EC and FC were smoothed.
Pattern 3: additional installation of SVC
IMVC in Fig. 5. Secondly, Vbus was decreased due to the decrease of SoC of the battery connected as the MVC. Thirdly, the SVC started to compensate the power balance. Finally, IMVC converged to almost zero when Vbus was stabilized. In this simulation, power imbalance of about ± 2:1 kW was converted to around ±2 V change in Vbus . As shown here, the step-like power-flow change was absorbed by the MVC and SVCs.
Pattern 2: fluctuated input power In the second pattern, input power was fluctuated randomly. The load was enabled at t ¼ 10 [s], and Iload was set from 0 to 5 [A]. The result of the simulation is shown in Fig. 6. After the load was enabled, the power-flow balance changed alternately
In the third pattern, the additional SVC of the battery installed in pattern 2 was evaluated. The detailed parameters of the additional SVC are summarized in Table 2. The current of the additional SVC (ISVC3 ¼ Ibattery ) is positive when the current is charging and is negative when the current is discharging. The simulation results for pattern 3 are summarized in Fig. 7. The additional SVC cooperated with the existing MVC and SVC in stabilizing bus voltage. Compared with the former case with pattern 2, fluctuation of Vbus and IMVC became small because the same bus-voltage shift enhances the total SVC current in the case of pattern 3.
Simulated system response In the three simulation cases, output current of MVC (IMVC ) was nearly zero when DC-bus voltage (Vbus ) was stabilized
Fig. 7 e Results of simulation of system behavior under changes in input and load power for pattern 3: additional installation of SVC. The system shown in Fig. 4 plus an additional battery of SVC was used for the simulation. Given input and output currents (Iin and Iload ), targeted and simulated bus voltages (Vbus and Vbus target ), and simulated currents for MVC, battery for SVC, EC, and FC (IMVC , Ibattery , IEC , and IFC ) are shown here.
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Fig. 8 e Whole view of the real system.
even when the direction of IMVC was frequently changing. This result means that the MVC can use a battery or a supercapacitor with small capacity. The analysis of the entire-system behaviour discussed in an earlier section explains the simulation results (i.e., a steplike power-change) well. At the step transient, the expected time constant of the MVC and SVCs from Equations (4) and (7) P is 1=KIV KVIk ¼ 1=ð0:25 4Þ ¼ 1 ½s, which matches the simuk P lation results. In this time constant calculation, the sum KVIk k
was calculated from working SVCs. There are two unidirectional SVCs in the simulation model, but they do not work at the same time. The sum was therefore simplified to a single SVC. As a result, change in DC-bus voltage can be calculated P from Equation (10) as DI= KVIk ¼ ±7=4 ¼ ±1:75 ½V. The simuk
lated change in DC-bus voltage was slightly larger, probably due to the non-ideal integration method used in the simulation. As for the additional SVC (pattern 3), the range of DC-bus
Table 3 e Devises used in the real operation. Devices: Experimental Energy System Variable DC Source (instead of solar cells) Variable DC Sink (instead of use points) DC/DC Converter Electrochemical Water Splitting Cell (EC) Fuel Cell (FC) Hydrogen Tank Rechargeable Battery
Takasago ZX-S-1600HA (2 series) Kikusui PLZ1004WH þ PLZ2004WHB TDK Lambda EZA-2500 Enoah EHC-750 (7 series) Enoah EOS-1000 (2 parallel: 1 device used) 2 m3, <0.7 MPa Pb 12 V (4 series)
voltage was narrowed by the additional SVC, and that result is also predicted by Equation (10). If the voltage fluctuation of the DC-bus line is large, the output voltage of the system may be affected, resulting in output noise. In the simulations, the changes in DC-bus voltage were smooth despite of the rapid changes in the power balance. This smooth voltage change is useful information for the stable output-voltage for the output converters. Long-term energy management is not considered in this study; however, it is possible to manage the SoC of each energystorage device by controlling Vbus ref externally. For example, if Vbus ref of the FC is set to a lower value, the amount of hydrogen stored would be larger. It is possible to manage the SoC of multiple energy storage devices using with the same manner.
Real system operation The proposed control method was applied to a real energy control system with a 340-V DC-bus center connected to a battery and a part of EC and FC with almost 1 kW power for each via a DC-DC converter. The DC bus voltage for the real system was selected 340 V to reduce the current below 10 A at a few kW system. The whole view of the system is shown in Fig. 8. The devices used in the real system are shown in Table 3. The control parameters of the MVC and SVCs were set to KIV ¼ 0.915 and KVI ¼ 2.4 for both the EC and FC. The system simulation results were used as the references of the real gain parameters. The values of KIV and KVI were selected the gain factors the devices being able to respond and the values were not optimized. The value of KVI can be selected much larger to be slower the device reactions. The input and demand were power source and electric load, respectively, in order to control the amount of power artificially. The results of real system operation under abrupt
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Fig. 9 e Result of real-system operation under abrupt changes in input and load power. The system shown in Fig. 4 with devices listed in Table 3 was used. Bus voltage (Vbus ), and currents for battery (MVC), EC, and FC (Ibattery , IEC , and IFC ) are shown here.
changes in input and load are shown in Fig. 9. The swing width of DC-bus voltage was set larger than the case of simulation for clear observation. Although some noise is observed in the real operation, the controlled current and voltage follow basically the same trends as the simulated ones.
Conclusion A distributed control method, called “modified DC-bus signaling”, for renewable energy sources with different energy-storage devices was proposed. This method enables smart cooperation among the devices connected to the bus without the need for a high-speed communication line. In the controlling method, it uses two kinds of storage devices: a main-voltage controller (MVC) and sub-voltages controllers (SVCs). The MVC is an energy-storage device with rapid response connected to a DC-bus line via a DC-DC converter, and it initially set DC bus voltage according to the power balance in the bus line. The MVC senses its input or output current and controls the target DC-bus voltage. The SVCs control their input or output current according to the DC bus voltage. The MVC and SVCs are controlled individually but interact with each other through DC-bus voltage to balance the energy flow. Since the MVC responds to the earlier (rapid) power-flow imbalance, the capacity of energy storage in the MVC is relatively small. On the other hand, the capacities of energy storage in the SVCs are relatively large because the SVCs deal with the later (slow) change or constant power-flow imbalance. Instead, the response speed of the SVCs is relatively slow because a rapid power change is absorbed by the MVC and the capacitance of the bus. These results are important when different types of energy storages of SVCs are used effectively. In the computer simulations of power control, all three evaluated patterns (step-like changes in input and output power, randomly fluctuating input, and an additional SVC) showed good stabilization of the bus voltage. About step-like
power-flow change into the DC-bus of ±2:1 kW was converted to a small fluctuation in the bus voltage, approximately ±2 V. The control method was further applied to a real system, which is composed of a battery as an MVC and a pair of EC and FC (each 1 kW in power capacity) as SVCs. The voltage of this example of real DC bus system was stabilized at 340 ± 12 V upon a step-like input power fluctuation of about 0.6 kW. This swing width was set to be larger than the simulation case for clear observation. The method can manage the SoC of SVCs if long-term prediction of weather and energy consumption is available. By adjusting the parameter of each SVC externally, the energy stored in the device can be altered: e.g., a low Vbus ref results in a larger energy stored in an SVC. With the method proposed here, it is possible to handle a massive number of connected SVCs. Harmonized control of power flow and voltage in a DC bus can be realized by an appropriate set of parameters, KIV for an MVC and KVI’s for SVCs, corresponding to both the response speed and the energy capacity for each device.
Acknowledgment The authors are grateful to all the members of the Sugiyama Laboratory at RCAST of The University of Tokyo and the Photonics Control Technology Team at the RIKEN Center for Advanced Photonics (RAP) for supporting this study. This study was supported by a grant from RIKEN.
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