source to diesel generator sets for shipboard electrical power plant. Load sharing
has to be controlled, especially when the battery system is operating in parallel ...
Batteries installation and load sharing in hybrid power plants Frank Wendt – Facing a growing demand for higher power plant efficiency, reduced fuel consumption and lower emission levels, the marine market is evaluating concepts based on the use of hybrid power plant with energy storage systems. Given the availability of high power and energy dense batteries, such systems are now being considered as a
1 Cell performance of lithium Ion battery (Corvus Energy)
SLPB 125255255H 1.0C = 70.0A Charge : CC-CV. 0.5C, 4.2V, 3.5A Cut-off @ 23ºC ± 3ºC Discharge : CC. 0.5C, ~ 10.0C, 2.7V Cut-off @ 23ºC ± 3ºC 4.4
0,33C@90%5oC
4.2
0,33C 1,0C 3,0C 8,0C
4.0
Voltage / V
3.8
0,5C 2,0C 5,0C 10,0C
3.6 3.4 3.2 3.0
10C@15%5oC
2.8 2.6 2.4
0
10
20
30
40
50
60
Discharge capacity / Ah
70
80
possible additional and/or alternative power source to diesel generator sets for shipboard electrical power plant. Load sharing has to be controlled, especially when the battery system is operating in parallel with other power sources, and this article describes a load sharing method which allows a direct connection of the battery with a DC-link system.
W
hen using a battery-based energy storage system in a diesel-electric power plant, load sharing between the battery system and diesel generator(s) has to be controllable. The battery system can be connected either to the common DC bus in a multidrive variable speed drive system or directly into a DC grid power distribution system. The voltage at the batteries’ terminals varies with their state of charge (SoC) and the charge or discharge current. The variation in voltage depends on the battery chemistry and, for a lithium ion cell, can be up to 20-25 percent between a typical operation point 0.33C@90 percent SoC and 10C@15 percent SoC, C being the rated discharge current (Figure 1). Furthermore, unlike other power sources such as diesel generators, there is no way of controlling a battery that enables direct power sharing.
Battery system
Battery
2 Battery system connected through a DC/DC converter to DC-link)
DC DC DC/DC converter
MCC Need for power backup of safety essential equipment; eg, steering, cooling pumps
DC DC-link/ DC-distribution
AC Island Inverter
DC AC Inverter
Distribution transformer
Variable speed drives Need for power backup of safety and operation essential equipment; Energy storage of regenerative braking energy; Smoothening of power consumption in case of cyclic load variation; e.g. drilling drawwork
AC DC Diesel generator
Rectifier
DC AC
Thruster/propulsion Need for power backup to keep positioning and heading; Smoothening of power consumption in case of fast cyclic load variation
Inverter
Battery system
Battery
3 Battery system directly connected to DC-link
MCC Need for power backup of safety essential equipment; eg, steeting, cooling pumps
DC-link/ DC-distribution
DC AC Island inverter
DC AC Inverter
Distribution transformer
Variable speed drives Need for power backup of safety and operation essential equipment; Energy storage of regenerative braking energy; Smoothening of power consumption in case of cyclic load variation; e.g. drilling drawwork
AC DC Diesel generator
Controlled rectifier
DC AC Inverter
Thruster/Propulsion Need for power backup to keep positioning and heading; Smoothening of power consumption in case of fast cyclic load variation
4 Battery voltage droop
Battery voltage droop
4,2
980
3,8
940 920
rv
cu e
900
ry t te Ba
p oo dr
880
cu
0
2
4
6
C rated discharge
8
10
Cell voltage @ 10Ah Cell voltage @ 50Ah
The marine market is evaluating concepts based on the use of hybrid power plant with energy storage systems.
e rv
3
op ro
3,2
rd
3,4
e ifi
3,6
ct re
DC-voltage [V]
d
960
Example: Battery nominal voltage@SoC = 960V Battery natural voltage droop@SoC = 10%
lle tro
4
on C
Cell voltage [V]
5 Voltage droop based load sharing
860
20
40
60
80
100
Load [%]
The main power consumers in a diesel-electric power plant are usually variable speed drive (VSD) systems, including, for example, propulsion thrusters and cargo and drilling drives. Modern VSDs are based on voltage source converter technology which uses a relatively constant DC voltage intermediate circuit. To guarantee full performance of the VSD, the DC link voltage has to stay above certain defined levels. When using batteries as part of the power source for VSD systems, the voltage variation of the battery can be compensated for through the use of DC/DC converters, which boost the changing battery voltage level up to the required DC link voltage. The DC/DC converter also enables the control of direct power load sharing between the battery system and diesel generator (Figure 2). In high power battery energy storage systems, however, the DC/DC converter contributes significantly to the size and cost of the overall battery energy storage system and can cause additional losses. An alternative configuration is to connect the battery directly to the DC link (Figure 3). In such a system the battery voltage determines the DC-link voltage and all power consumers have to be rated according to the variation of the DC link voltage. This mainly affects the current and voltage rating of the power components in the system, as these must be able to convert or produce the required power at both maximum and minimum voltage levels. Load sharing between the battery system and the diesel generator(s), as well
6 Controlled rectifier voltage droop control
DC Voltage Feedback
Voltage Reference
PI
+
Rectifier Control
Rectifier
Droop
Load Sharing Control
Power Feedback
Battery SoC&Current Feedback
Voltage reference and droop can be adjusted by Load Sharing Control System
as battery charging/discharging, has to be controlled by the AC/DC rectifier unit(s), which feed power into the DC link system. In applications with a high C-rate discharge current, the natural droop of the battery voltage can be used for load sharing between a diesel generator set and the battery. The voltage droop (cell voltage versus discharge current) is relatively linear, but changes with the SoC of the battery (Figure 4). Voltage droop based load sharing is an effective and robust method for parallel operating power sources. With the DC voltage common for both the battery and rectifier, each unit supplies the amount of power, which corresponds to its applied droop curve. As the load on the DC link increases, the DC-voltage drops. By implementing a voltage droop control algorithm in the rectifier control system, the output voltage of the rectifier can be adjusted to control both the power flow in the battery and the AC/DC rectifier and consequently between the battery and diesel-generator (Figure 5). The natural droop curve of the battery is only quasi-static and changes with the SoC. With current and SoC feedback from the battery management controller, a load sharing controller can adjust the voltage reference and droop curve settings of the droop controller in the controlled rectifier. By adjusting the voltage reference and droop curve setting in the droop controller, not only can the load sharing between the battery and diesel generator be controlled, but also the battery charging.
Frank Wendt ABB AS, Marine CoE O&G Vessels
Battery Management
