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Power Management for Hybrid Fuel Cell System*

Ke Jin, Xinbo Ruan, Mengxiong Yang and Min Xu

Aero-Power Sci-tech Center College of Automation Engineering

Nanjing University of Aeronautics & Astronautics Nanjing, 210016, Jiangsu Province, P.R.China

Abstrac t— This paper proposes a power managementcontrol scheme for hybrid fuel cell power system. The hybrid fuel cell system consists of a fuel cell, an isolated uni-directional converter, a bi-directional converter, an inverter, and a battery. The fuel cell powers the steady state energy and the battery compensates the dynamic energy. They need to work together to ensure the system operates with high efficiency and behaviors good dynamic performance. Therefore, the power management should be employed. The goal of the power management control scheme is to control the bi-directional converter operates under buck, boost or shut-down mode according to the condition of fuel cell and battery, so that the battery can compensate the dynamic energy. A 1kW fuel cell power system was built in the lab. Experimental results are shown to verify the theoretical analysis.

I. INTRODUCTION

The fuel cell (FC), because of its cleanness, high efficiency and high reliability, is emerging as an attractive power supply source for applications such as distributed generation power systems and electrical vehicles [1]-[3].

However, the FC has several shortcomings: 1) it cannot store energy; 2) the response is slow; 3) its output voltage fluctuates with the load; and 4) it is difficult to cold start. Therefore, an auxiliary energy source such as a battery

or an ultracapacitor must be introduced into the FC power system to improve the dynamic characteristic, enhance the peak power capacity and power the load during cold start.

If the battery or ultracapacitor is in parallel directly with the DC bus, its charge and discharge current cannot be controlled [4]. Once the load changes significantly, the rush current would destroy the battery or ultracapacitor. Therefore, a bi-directional converter needs to be inserted between the DC bus and the battery or ultracapacitor to control the charging and discharging current.

A novel hybrid FC power system was proposed in [5], as shown in Fig.1. The power system is composed of an FC, a battery, a uni-directional DC-DC converter (UDC) [6], a bi-directional DC-DC converter (BDC) [7] and an inverter [8]. The FC and battery are connected to the same DC bus through the appropriate UDC and BDC,

* This work was supported by the National Natural Science Foundation of China under Award Number 50177013, the Natural Science Foundation of Jiangsu Province, China under Award Number BK2003419, and the Program for New Century Excellent Talents in University, China.

respectively. The proposed hybrid FC power system has several advantages:

1) It can optimize power management and improve system efficiency.

2) During the start period of the system, the battery will power the system to ensure the FC cold starts easily.

3) When the load steps up or down, as the FC cannot respond quickly, the battery will provide or absorb the unbalanced energy, so the dynamic characteristic of the whole system can be improved.

4) The battery can provide peak power, thus the power rating of the FC can be decreased, reducing the total cost.

There are two power sources in the system: FC is the main power source, and the battery is the auxiliary source. Therefore, the system power flow should be managed to ensure that the whole system operates with high efficiency and high reliability. This paper proposes a power management control strategy for the hybrid FC power system. A 1kW hybrid FC power system is built in the lab to verify the theoretical analysis.

Load

220VAC

FuelCell

200-400VDC

InverterUni-directional

1# DC-DCconverter

360VDC

Bi-directional2# DC-DCconverter

Battery

220VDC

Figure 1. Hybrid fuel cell power system.

II. SYSTEM POWER FLOW

Battery is connected to DC bus through a BDC, which can operate in Buck, Boost or Shut-Down (SD) mode. The BDC should be controlled in suitable mode according to the condition of battery and FC, so that individual part of the system can cooperate well with high reliability, high efficiency, good dynamic characteristic. How to control the BDC is the key issue of the power management.

The working situation of the battery can be predicted by the voltage of the battery, vbat. Setting battery over-discharged-point and over-charged-point are VBat-min andVBat-max respectively. If vbat<VBat-min, battery should be charged. If vbat>VBat-max, battery should be discharged. If VBat-min vbat VBat-max, the battery is in normal condition and can deliver or absorb power.

Similarly, the operation condition of the FC can also be predicted by DC bus voltage, vbus. Defining FC limit-power-point is Vfc-lpp. When vbus < Vfc-lpp, it means that FC

978-1-4244-1668-4/08/$25.00 ©2008 IEEE

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cannot deliver enough power to the load, the battery needs to provide energy to the load. VUDC is UDC constant voltage point. When vbus > VUDC, it indicates that the load feeds back energy, so the surplus power must be transferred to battery. If Vfc-lpp vbus VUDC, it shows that FC is in good condition and is capable of providing enough power to the load.

Based on the analysis above, the operation conditions of the power system can be divided into nine regions, which are illustrated in Table 1. Each region corresponds to a special mode of the system. The BDC will also be controlled to operate under correct modes.

The system has four kinds of power flow as shown in Fig.2. As seen in Fig.2(a), the FC powers the load and charge the battery via the BDC, which operates in Buck mode. This power flow occurs in the following situations: 1) vbat < VBat-max and vbus > VUDC, which means the load feeds back energy, and the surplus power needs to be stored in the battery; 2) vbat < VBat-min and Vfc-lpp vbus

VUDC, which means that FC is in good condition and the battery is over discharged and needs to be charged. 3) VBat-

min vbat VBat-max and Vfc-lpp vbus VUDC, which means that FC and battery are in good conditions, so FC powers the load, and floating charges battery to compensate battery self-loss.

In Fig.2(b), the FC powers the load and the BDC is shut down. This power flow occurs when vbat <VBat-min and vbus

< Vfc-lpp, battery is over discharged and FC cannot provide enough power.

In Fig.2(c), the FC and the battery power the load through the UDC and BDC respectively, The BDC

operates in Boost mode. This power flow occurs when vbat>VBat-min and vbus<Vfc-lpp, the system is in overload state.

In Fig.2(d), the battery powers the load through the BDC, which operates in Boost mode. This power flow occurs when vbat > VBat-min and vbus < Vfc-lpp, which occurs when the system is in cold start.

If vbat > VBat-max and vbus > VUDC, the power system works abnormally, BDC should be shut down, and a energy release path should be introduced to prevent the collapse of the UDC, BDC and inverter.

III. IMPLEMENTATION OF THE CONTROLCIRCUIT

In this section, the UDC, BDC and power management control circuits will be presented. Fig.3 shows the block diagram of the system control circuit.

A. Uni-directional Converter Control The UDC employs voltage loop and current loop

control, as shown in Fig.3. The voltage loop and current loop are paralleled. The traditional phase-shift control is used.

During transients, the FC dynamic response time is very slow. From the experimental result, it can be seen that the response time of the FC is tens of seconds during the cold start state. Once it warms up and operates normally, the FC response time is hundreds of ms when the load steps up from 0 to full load. But the UDC response time is just tens of us. Compared to the FC, the UDC dynamic performance is much better, but the FC response time is dominant. Therefore, UDC transient

Table 1 Bi-directional converter operation mode.

vbus < Vfc-lpp Vfc-lpp vbus VUDC vbus>VUDC

vbat < VBat-min SD Buck Buck

VBat-min vbat VBat-max Boost Buck Buck

vbat > VBat-max Boost SD SD

Fuel CellUni-directional

DC-DCConverter

Bi-directionalDC-DC

Converter

Inverter Load

Battery

Fuel CellUni-directional

DC-DCConverter

Bi-directionalDC-DC

Converter

Inverter Load

Battery

(a) (b)

Bi-directionalDC-DC

Converter

Fuel CellUni-directional

DC-DCConverter

Inverter Load

Battery

Fuel CellUni-directional

DC-DCConverter

Bi-directionalDC-DC

Converter

Inverter Load

Battery

(c) (d)

Fig.2 System operation mode.

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performance is not an extreme issue in the hybrid FC system.

B. Bi-directional Converter Control

vCfly

Opti-Isolated

VBDC-Buck

VUDC

IUDC

Comp1+

_

VRAMP1

Uni-directionalconverter

vbus

iU

Voltagecompensator

Currentcompensator

VRAMP 2

Bi-directionalconverter

vbus

iU

Demulti-plexer

EN

vbatDemultiplexer

EN

Demultiplexer

EN

iL Demulti

plexerEN

VBDC-Boost

IBDC-Buck

VEN

IBDC-Boost Flyingcapacitorcontrolcircuit

Currentcompensator

Voltagecompensator

vH/2

Phase-shiftedcontroller

Comp2+

_PWM

controller+

+_

+

_

+

_

+

_

+

Comp3+

_

Comp4+

_

Comp5+

_

Comp6+

_

vbus

vbat

Vbat-min

Vbat-max

Logiccircuit

VUDC

Vfc-lpp

VEN

UDC control circuit

BDC control circuit

Power management control circuit

VSD

-K

+

+

+

Fig.3 Block diagram of system control circuit.

When VEN is at a low level, the BDC operates in buck mode. Channel 0 of the demultiplexer is chosen, so (VBDC-

Buck v

The BDC also employs voltage loop and current loop control. The control circuit of BDC should ensure that the BDC operates freely in constant voltage mode and limited current mode. The BDC can also work under buck mode, boost mode, or SD mode. The BDC operation mode is determined by the enable signal VEN and shut down signal VSD, which comes from the system power management control.

bat) and (I iBDC-Buck L) are sent to the voltage compensator and current compensator, respectively.

When VEN is at a high level, BDC operates in Boost mode, and Channel 1 of the demultiplexer is chosen. So (V vBDC-Boost bus) and (I (-iBDC-Boost L)) are sent to the voltage compensator and current compensator, respectively.

In addition, in order to ensure the flying capacitor voltage, V , be keep at vCfly bus/2, and a flying capacitor voltage control circuit is employed. The operation principle is detailed in [9].

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C. Power Management Control

Uni-directionalconveter

Battery

Bi-directionalconverter

Inverter

Fuel cell As analyzed in the last section, the system can be divided into nine regions according to the conditions of the FC and the battery. The function of power management control is to determine which region system is in, then send the appropriate signal to the BDC and ensure the BDC operates in the correct mode.

The values of VBat-min, V , VBat-max fc-lpp and VUDC are given in the last section. vbat is sensed and sent to comparators to be compared with V and VBat-min Bat-max, respectively. Similarly, vbus is sensed and sent to comparators to compare with Vfc-lpp and VUDC, respectively. Then the outputs of the four comparators are sent to a logic circuit to judge which region the system is in.

According to Table 1, each region corresponds to a mode of the BDC. If the BDC should operate in buck mode, VEN is at a low level and VSD is at a low level. If the BDC should operate in boost mode, V

Fig.4 Photograph of the fuel cell power system. EN is at a high level

and VSD is at a low level. If the BDC should be shut down, V Table 2 The specifications and parameters for the whole system is at a high level. Then both VSD EN and VSD are sent to the BDC control circuit, so that the BDC will operate in the appropriate mode according to the situation of FC and battery.

Rated power 1kW Fuelcell Output voltage 200-400VDC

Capacity 10AhIV. EXPERIMENTAL RESULTS AND DISCUSSION BatteryRated voltage 220VDC

In order to verify the theoretical analysis, three modular H-FB TL LLC UDC, TL buck/boost BDC and FB inverter are built in the lab. Fig.4 shows the photograph of the experiment environment, and the specifications and parameters of the system are listed in Table 2.

Input voltage 200-400VDC

Output voltage 360VDC

Output current 3AFig.5 shows the waveforms of the FC output voltage vfc,

DC bus voltage vbus, output voltage vo and output current io

under full load in steady state. In this situation, the UDC operates under constant voltage mode.

Fig.6 gives the waveforms of the system’s cold start state. It shows the battery will power all the load at the beginning of the cold start. Then, the fuel cell slips into the system and delivery the power.

Fig.7 shows the waveforms of the transient response. Fig.7(a) shows waveforms of vfc, vbus, vo and io when the load steps up from zero to full load. It can be seen that the UDC operates under constant voltage mode before the load changes. When the load steps up, the UDC enters into limit current mode and vbus decreases due to the FC’s low response capability. The FC begins to produces more power, and vbus increases. When vbus reaches 360V, the UDC returns to constant voltage mode. Fig.7(b) shows waveforms when the load steps down from full load to zero. The system also works well under this condition.

Fig.8 shows the waveforms from half load to 150% overload. Fig.9 shows the waveforms from 150% overload to half load. The FC and the battery power the load simultaneously during overload. The UDC works under limited current mode and the BDC operates under boost constant voltage mode, so that the FC output power is limited to 1kW and the overload energy is provided by the battery. When the load steps down to half load, the battery slip out of the system smoothly.

Fig.5 to 9 illustrate that the system can operate well in steady mode, load step-up and -down modes, cold start mode, and overload mode. This is well in agreement with

the theoretical analysis, and verifies the effectiveness of the power management control scheme.

V. CONCLUSIONS

The power management control scheme for hybrid fuel cell power system proposed in the paper. It ensures the system operates with high efficiency. The key point of the control scheme is to control the BDC operates under buck,

Transformer turns ratio 10:16 UDC

Resonant inductor L 21.7uHr

Magnetizing inductor L 120.6uHm

Resonant capacitor C 94.4nFr

Boost mode output voltage 340VDC

Boost mode inductor current 5.8A

Buck modeoutput voltage 220VDC BDC

Buck mode inductor current 1A

Inductor L 400uHf2

Output voltage 220VAC

Rating output power 1kVA

Output frequency 50HzInverter

Filter inductor L 600uHf

Filter capacitor C 30uFf

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Ch1: vfc[100V/div]

Time [10ms/div] Ch2: io[5A/div]

Ch4: vo[400V/div]

Ch3: vbus

[40V/div]360V

Time [200ms/div]

Ch1: vbus

[100V/div]

Ch2: io[5A/div]

Ch4: vo[200V/div]

Fig.5 System waveforms in steady state Fig.6 The waveforms of the cold start state.

Time [50ms/div]

Ch1: vfc[100V/div]

Ch2: io[5A/div]

Ch4: vo[400V/div]

Ch3: vbus[40V/div]

360V

Time [50ms/div]

Ch1: vfc[100V/div]

Ch2: io[5A/div]

Ch4: vo[400V/div]

Ch3: vbus[40V/div]360V

(a) Load steps up form zero to full load (b) Load steps down from full load to zero Fig.7 The waveforms of transient response.

Time [100ms/div]

Ch1: vfc[100V/div]

Ch2: io[10A/div]

Ch4: vo[400V/div]

Ch3: vbus[40V/div]340V

Time [100ms/div]

Ch1: vfc[100V/div]

Ch2: io[10A/div]

Ch4: vo[400V/div]

Ch3: vbus [40V/div]360V

Fig.8 The waveforms of load steps up from half load to 150% overload. Fig.9 The waveforms of load steps down from 150% overload to half load.

boost or shut-down modes according to the condition of the FC and battery, so that the battery can be charged or

discharged when appropriate. The experimental results are shown to verify the theoretical analysis.

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