CLEAN INPUT UPS WITH FAST RECTIFIER CONTROL AND IMPROVED BATTERY LIFE BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to uninterruptible power supplies, and particularly to the control of active rectifiers for uninterruptible power supplies. DESCRIPTION OF BACKGROUND Batteries that are implemented within uninterruptible power supply (UPS) systems typically suffer from ripple currents and fast discharge pulses that can affect the life of a system-linked battery. Under unbalanced load conditions, an output inverter draws at twice the mains frequency from a direct current (dc) link and an oscillating current that is produced by a rectifier and the battery. Normal rectifiers can only provide the mean value of this oscillating current, therefore, the ripple current must be drawn from the battery, thus affecting the life of the battery. In the event that a UPS load is unbalanced then, the associated active power is not constant. In this instance there is the possibility for two extreme operating conditions to exist. The first operating condition requires the drawing of constant active power from a utility, thus resulting in the acquisition of sinusoidal currents at the price of producing a high current ripple on the battery. The second operating condition requires the drawing of oscillating active power from the utility, and therefore acquiring non-sinusoidal currents, wherein the operation is ideally implemented without drawing any battery current. Conventionally, six or twelve pulse thyristor rectifiers, sometimes in combination with passive filters, have been used as inputs for an uninterruptible power supply (UPS). However, using these particular topologies, it was not possible to obtain the fast current control that is required to solve the above-mentioned problems. Currently, more modern power topologies are utilized to realize clean input UPSs that can draw sinusoidal currents from the mains at a high power factor. Typical front-end converters include current or voltage source rectifiers, these often being referred to as IGBT rectifiers. These converters make it possible to reach a required bandwidth for instituting a current control operation. However, additional control effort is required to take advantage of this fast current control in order to solve the mentioned problems, i.e. increase the battery life and obtain a very stiff dc link voltage control. SUMMARY OF THE INVENTION The present invention relates to the control of active rectifiers for uninterruptible power supplies. More specifically, aspects of the present invention comprise an active rectifier control system that is configured to enhance the response control performance for an uninterruptible power supply (UPS) system for servicing a load, the active rectifier control system providing rigid control of a direct current (dc) link voltage resulting from the combined linked outputs of a battery and the active rectifier, thus reducing the power drawn from the battery. The control system comprises a feed-forward voltage controller, the feed-forward voltage controller comprising a proportional-integral (PI) controller component, the PI controller component being configured to determine a voltage error reference current value in response to a dc voltage reference and a dc voltage input values, and a feed-forward current determining component, the feed-forward current determining component being configured to determine a feed-forward current value in response to an active current input value. The control system further comprises a variable limitation component, the variable limitation component being configured to determine a limited reference current value in response to a dc reference input value, and a current control component, the current control component being configured to receive the determined limited reference current value from the variable limitation component, wherein the determined limited reference current value is utilized as a reference value for a lower level current controller. Further aspects of the present invention relate to an active rectifier control system method for enhancing the response control performance of an uninterruptible power supply (UPS) system for servicing a load, the method comprising determining an active power load value, dividing the active power load value by a dc voltage value in order to determine an active current input value, determining a feed-forward current value, and determining a voltage error reference current value. The method further comprises determining a dc reference input value from a summation of the voltage error reference current value and the feed-forward current value, providing the dc reference input value to a variable limitation component, wherein the variable limitation component is configured to utilize the dc reference input value to calculate a limited reference current value, and providing the limited reference current value to a current control component. Additional features and advantages are realized through the techniques of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: Figure 1 is a diagram illustrating aspects of a current source rectifier that may be implemented within embodiments of the present invention. Figure 2 is a diagram illustrating a two-phase representation of the AC component of the current source rectifier of Figure 1. Figure 3 is a diagram detailing aspects of a feed-forward voltage controller that may be implemented within embodiments of the present invention. Figure 4 is a diagram illustrating active power distribution in the instance of balanced linear load. a Figure 5 is a diagram illustrating active power distribution in the instance of an unbalanced linear load. Figure 6 is a screenshot illustrating an unbalanced linear load with a variable parameter of K=l. Figure 7 is a screenshot illustrating an unbalanced linear load with a variable parameter of K=0. The detailed description explains the preferred embodiments of the invention together with advantages and features, by way of example, with reference to the drawings. DETAILED DESCRIPTION OF THE INVENTION One or more exemplary embodiments of the invention are described below in detail. The disclosed embodiments are intended to be illustrative only since numerous modifications and variations therein will be apparent to those of ordinary skill in the art. The present invention relates to UPS systems, in particular with the control of active rectifiers for UPS systems. In operation, active rectifiers are used to realize clean input UPS systems that can draw sinusoidal currents from mains at a high power factor. Aspects of the present invention relate to a control system, wherein the control system is implemented to realize fast rectifier control operations that result in the improved life of a battery that is linked to the active rectifier of the UPS system. Within aspects of the present invention under unbalanced load conditions, it is possible to select the desired behavior between the two possible extreme conditions— that is the implementation of a clean power input that results in the reduced life of the battery or the implementation a non-clean power input that results in the improved life of the battery. Additionally, the present invention utilizes fast rectifier control to make it possible to obtain the very rigid control of a dc link voltage, even under extreme step load variations. This aspect also results in an improved battery life (no energy is drawn from the battery during step load variations), an increased dynamic stiffness of the output inverter, as well as the increased reliability of the power converters; especially in the instances that the battery is not connected or an* alternative energy storage system is used. As mentioned above, the present control system makes it possible to minimize the ripple current drawn from the battery under unbalanced conditions. Additionally, the fast rectifier control that is implemented within the control system allows for the possibility to obtain very rigid control of the dc link voltage even under extreme step load variations. This functional aspect is accomplished by first minimizing the amount of energy drawn from the battery during important step load variations, and secondly, obtaining an increased dynamic stiffness of the output inverter (this aspect is very useful in the instances where pulsating loads exist, such as medical imaging systems). Finally, by avoiding high dc link voltages under sudden load release, an increased reliability of the power converters is obtained, especially if the battery is not connected or in case an alternative energy storage system is used. Figure 1 illustrates aspects of the topology of a current source rectifier that can be implemented within embodiments of the present invention. As shown in Figure 1, the converter 100 is composed of a bridge with six IGBTs 105 (TR1 to TR6) and six diodes 110 (Dl to D6). The AC side of the bridge comprises a three-phase LC filter 115 (LI, L2, and L3, and C12, C23, and C31) and is used to avoid injecting high frequency switching harmonics into the grid 120 (voltage sources VI, V2. and V3). The DC side of the bridge comprises a free wheeling diode DO 125, a filter inductance Ldc 130, and a dc link capacitor Cac 135. The current source iinv 140 represents the current drawn by an inverter. The basic operation of the rectifier 100 is explained as follows. Assuming a constant dc link current iac 145 and a proper modulation of the six switches 105, it is possible to create three sinusoidally pulse width-modulated currents 155 (iri, iT2, and ir3). As seen from the utility and through the three-phase LC filter 115, these currents are the three-line currents 150 isi, iS2, and isa, and are controlled to be sinusoidal. This exemplarily configuration presents two systematic control problems. The first problem relates to current control, that is it is desirable to draw three sinusoidal currents 150 isi, iS2, and iS3 while concurrently compensating the resonance of the LC filter 115. This problem is dealt with in a two-phase reference frame that rotates at the mains frequency. By performing the required transformations of the AC side of the converter, one obtains the two circuits (205, 210) represented in Figure 2. In reality the two circuits would be coupled, but for the purpose of this control example the coupling will be ignored. In steady state, all the sinusoidal variables of the AC side of the converter of Figure 1 become constant variables in the rotating two-phase representation of Figure 2. This allows a state space control with disturbance feed-forward to be designed around the two circuits of Figure 2. Specifically, the state variables are the line currents ISA, and iSB (206, 211), and the filter capacitor voltages ucA, and UCB (208, 213). The disturbance variables are the utility voltages umA, and umB (209, 214), and the command variables are the currents irA, and irB (207, 212). These are the rotating two-phase reference frame transformation of the three currents 155 iri, ir2, and ira of Figure 1. These currents are obtained through a pulse width modulation of the dc link current i