LOAD I LOAD I
CHAPTER 10 CHAPTER 10 CONCLUSION
CHAPTER 10
the ac side, a true four-quadrant power converter.
This four-quadrant capability is the basis for an alternative to conventional ac uninterruptible power supplieso Having a bank of batteries as the energy storage elements calls for interfaces between the de batteries and the ac line and the ac load. Conventionally, a battery is charged from the ac line by a battery charger and is used to power the ac load through an ac inverter. The flow of power is always from the line to the battery and to the load. The alternative way of interfacing the de battery to the ac line and load is the use of a
switched~mode amplifier. Now, there is only one power stage with de (battery) on one side and ac (line and load) on the other side. With the power up, the line powers the load directly, and the amplifier behaves as a unity power factor battery charger (ac-to-dc). With the power disrupted, the amplifier is converted to an inverter to power the load (dc-to-ac). Therefore, the amplifier can be referred to as a complete dc-ac interface since it works in either direction.
The dc-to-ac inverter was analyzed in detail from the large signal de transfer ratio to small-signal ac characteristics.
performed by use of the state-space averaging method.
The analysis was The method was shown to be valid for cases where the operating frequency (line
frequency) is much smaller than the power bandwidth of the amplifier.
This constraint is very realistic since high power bandwidth means smaller sizes of storage elements of the amplifier. To meet this criterion simplifies the calculations, because one can assume large-
signal variation of the states of the system to be a series of steady-state
solutions for the circuit. De behavior of the inverter can now be easily characterized by interaction of the two constituent converters of the amplifier with the load. Nonlinear transfer characteristics of converters are transferred to the amplifier comprising those converters.
It was shown that a small amount of parasitics can reduce this
nonlinearity to a reasonable value. Assumption of quasi- steady-state at each operating point, simplifies the ac analysis by consideration of each operating point and its associated small-signal dynamics. Therefore, the analysis of large-signal variation of the system is approximated by a series of small-signal analyses for which powerful analytical
techniques already exist. Very often it is required t6 regulate the output of the inverter to desensitize the output to variations of various system parameters, reduce harmonics, remove de, etc. The loop design was performed around the quiescent operating point, and then checked at various operating conditions for possible degradation of stability. It was shown that owing to the complete symmetry of the system, dynamics of the system at the quiescent operating point can be analyzed by examination of only one converter, provided the correct steady-state and other conditions are met. However, at any other point, this symmetry is not valid and the complete amplifier must be
analyzed. An experimental amplifier comprising two coupled-inductor
~
Cuk converters was then carefully examined. A feedback loop was designed, and was checked against operating points to verify stability. The
design may be refined later to meet the requirement of transition between the modes.
The battery charger mode of operation is different for the inverter mode in the sense that it is forced upon the amplifier by a mandatory feedback around the system. In this case, the amplifier has an ac voltage source on its ac port (instead of a dissipative load). In this case, with the amplifier being a nonideal voltage source, the outputs of two voltage sources are paralleled, which can cause large currents. The output impedance of the amplifier is very low at low frequencies and a current feedback is required to convert this open-loop impedance to the desired form. For example, for unity power factor application, one prefers to have resistive impedance for the amplifier ac port to draw a current proportional to the voltage. Also one wants to vary the value of this effective resistance to be able to control the rate of power flow. Again, the feedback is designed around the quiescent operating point, and then checked against various operating points encountered.
The control parameter is output current, and the dynamics of the system experience a large change owing to the replacement of the load by a voltage source. It was shown that with some precautions, at the quiescent operating point, the dynamics of the amplifier can be
~
approximated from that of a single converter. Experimentally, the Cuk amplifier was analyzed and a two-loop feedback was designed and checked for various operating conditions.
Finally, the complete UPS system was checked that has a single power amplifier and two control circuits for each mode of operation.
The transition from inverter to charger mode is the controllable one where a smooth change of mode is possible. The reason is that the
requirement of equality of the final state conditions of one mode to the initial state condition of another mode can be exactly fulfilled. This includes incorporation of additional circuitry to preset the
compensation network to the correct value too. The charge to inverter transition, however, can happen randomly and thus, there is no guarantee for a smooth transition. Still, the requirements on equality of initial and final states, suggest a circuit similar to the charger, to preset the correct voltages in the compensation network of the inverter. The worst case of this transition was found and some remedies to improve the
situation were given. The experimental system was also checked for the worst case transition.
Therefore, the design of our UPS alternative starts with an
appropriate power amplifier for which two separate control circuits are designed, one for the inverter, and one for the charger. At last these designs are refined to be able to improve the transition capabilities.