16-06-2012, 11:33 AM
Constant Power Loads and Negative Impedance
Instability in Automotive Systems
Constant Power Loads and Negative Impedance.pdf (Size: 367.61 KB / Downloads: 81)
Abstract
Power electronic converters and electricmotor drives
are being put into use at an increasingly rapid rate in advanced
automobiles. However, the new advanced automotive electrical
systems employ multivoltage level hybrid ac and dc as well as
electromechanical systems that have unique characteristics, dynamics,
and stability problems that are not well understood due to
the nonlinearity and time dependency of converters and because
of their constant power characteristics. The purpose of this paper
is to present an assessment of the negative impedance instability
concept of the constant power loads (CPLs) in automotive power
systems.
INTRODUCTION
IN ADVANCED automotive systems such as more electric
vehicles (MEVs), hybrid electric vehicles (HEVs), electric
vehicles (EVs), and fuel cell vehicles (FCVs), electrical power
is utilized to drive automotive subsystems that, in conventional
cars, have been driven by a combination of mechanical, electrical,
hydraulic, and pneumatic systems [1], [2]. Recent advances
in the areas of power electronics, electric motor drives, control
electronics.
AUTOMOTIVE ELECTRICAL POWER SYSTEMS
Advanced automotive systems are the most popular examples
of multiconverter power systems [1], [3]. In this section,
descriptions of the system architectures and the role of power
electronic converters and motor drives are presented.
CPLS
Power electronic converters and electric motor drives in
advanced automotive systems, when they are tightly regulated,
behave as CPLs at the input terminals. In fact, there is a
tendency of the loads of multiconverter systems to be constant
power. On the other hand, several loads such as electric
motors, actuators, and power electronic converters have to be
controlled such that constant output power is maintained for
them. Output power is equal to the input power, if we neglect
the losses, which is assuming 100% efficiency for the drive
system. Furthermore, the output power is constant; therefore,
the input power is constant. As a result, these loads present
constant power characteristics to the system [1], [11], [12].
NEGATIVE IMPEDANCE INSTABILITY
In a CPL, power, which is the product of voltage and current
of the load (V · I), is constant. Therefore, as shown in Fig. 7,
if the voltage across a CPL increases/decreases, the current
through it decreases/increases. This has a destabilizing effect
[1], [9]–[16] on the system that the CPL is connected to.
Fig. 8 depicts a simple circuit in which a CPL with power P
is connected in series to an inductor L and voltage source v.
The equilibrium operating point of the circuit is obtained when
source voltage equals the CPL voltage. The circuit will operate
in steady state at this point. The stability of a circuit at equilibrium
may be assessed from the steady-state voltage–current
curves of the source and load.
CONCLUSION
In this paper, the concept of the negative impedance instability
of the CPLs in advanced automotive systems was
described. Advanced multiconverter power electronic based
architectures for HEVs, EVs, and FCVs associated with more
electric environment were reviewed.