Abstract:
A DC power plant system is provided utilizing battery back up for emergency power for use with telephone switching equipment or other loads, whereby the system provides for a more efficient use of the DC rectifiers and allows a standby emergency power source to be sized to fit only the actual load rectifiers which will reduce the cost of the emergency standby power system.

Description:
PRIOR PATENT APPLICATION 
     This application claims the benefit of prior Provisional patent application Serial No. 60/171,193 filed on Dec. 16, 1999 all of which is incorporated herein by reference thereto. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to a method and apparatus for providing an improved DC power plant system capable of delivering emergency DC power to telephone equipment or other loads. More specifically this invention relates to a DC power back-up system using AC driven rectifiers connected to charge back-up batteries and power DC telecom equipment. 
     BACKGROUND 
     In telephone switching equipment, communication and computer equipment, and in many other applications, the need for an uninterrupted source of DC power is critical. Rectified commercial AC power is typically used as the primary source of DC power for such equipment. The DC power from the rectifiers is delivered to the back-up batteries and the load through a load bus and returned to the rectifiers along a ground bus. 
     To avoid any interruption or outage in power service, it is common practice to employ a battery back-up system for the primary DC source. Back-up battery systems typically include strings of batteries or cells connected in parallel with the primary DC source and the load. Some systems are also equipped with AC generators to further back up the AC line power. In the event of a drop in the DC load bus voltage below a predetermined threshold, the back-up battery supplants or supplements the primary source of DC power. The battery back-up continues until either AC line power is resumed or the AC emergency generator is activated to supply AC power to the rectifiers. Back-up battery systems are designed to replace the primary DC power source for a predetermined period of time. Within that period of time, the resumption of primary AC line power is expected to occur. 
     In conventional back-up battery systems, the nominal system load bus voltage is typically dictated by battery characteristics. For example, in a telephone switching plant, back-up batteries are commonly employed which each have a design float charge cell voltage of 2.17 volts, for optimum health of the battery cell. Twenty-four cells are typically connected in series to form a string resulting in a nominal load bus voltage of approximately −52.10 volts. A bank of strings supplies the necessary back-up DC power. 
     When AC line power is initially turned on or AC emergency power is activated in place of the AC line power, the back-up batteries tend to draw an excessive amount of current since these are placed in parallel with the DC load. This design architecture of a typical back-up battery system presents a number of disadvantages. Partially or fully discharged batteries, due to their electrochemical constitution, will draw an excessive amount of current in order to recharge themselves as quickly as possible. All batteries, no matter the time spent on discharge, will initially demand a high amount of recharge current from the rectifiers or the primary DC power source. Consequently, upon the return of AC power to the rectifiers the initial current to recharge the batteries is must be counted as connected load. Emergency AC generators for use with central office DC power plants are thus routinely sized to provide for this one-time power drain, which occurs at most but occasionally. 
     In addition, the paralleling of all available rectifiers or rectified DC power sources forces them all to share and satisfy the initially high connected load leading to inefficient operation of the AC to DC conversion by the rectifiers. Therefore, the two main disadvantages of the current systems are: 
     1. Over subscribed kilowatt sizing for installed AC emergency generator power. 
     2. Less efficient operation of all rectified DC power sources. 
     SUMMARY OF THE INVENTION 
     This invention provides a system to improve the efficiency of the utilized DC rectifiers while still providing standby spare rectifiers that automatically come online in the event a load rectifier fails. In addition, because of the specific rectifier arrangement of the invention, the standby emergency power source can be reduced in size to satisfy the initial connected current flow from only the rectifiers connected to the load and back-up batteries rather than all the available rectifiers. 
     The invention achieves the improved system by segregating the available rectifiers into two groups. A first group of load rectifiers for delivering and satisfying normal charging operation of the back-up batteries and the load and a second group of spare rectifiers that serves as spares for selective connection to the system when one of the load rectifiers fails. 
     Both disadvantages are addressed by the invention in that the segregation of the load rectifiers from the spare rectifiers provides alternative options that allow the DC power plant to operate more efficiently by intentionally failing to provide sufficient DC power to satisfy the start-up current demands imposed by the parallel connected back-up batteries and the load and allowing an initial small amount of DC voltage drop to occur on the load bus. This condition lasts only briefly, but permits one to reduce the startup current to thus reduce the power capacity or sizing of the emergency generator, while enabling a spare rectifier to be activated to supply DC power when a load rectifier fails. 
     Segregating the load and spare DC power sources of the DC power plant thus makes more efficient use of the load rectifiers, and generates capital cost savings. The cost savings are realized by decreasing the size of the emergency power generator so as to cover only the current demanded by the load rectifiers, and not the spare rectifiers as are common in conventional systems. 
     Accordingly, it is the object of the invention to provide a system and method, which improves the operating efficiency of standby emergency DC power plant systems. 
     Another object of the invention is to provide a system and method, which lowers the costs associated with standby emergency AC generators used to provide power to DC power plant systems. 
     Another aspect of the invention comprises a method for more efficiently operating a DC power plant system utilizing battery back up and a standby emergency power source for use with telephone switching equipment or other loads. 
     The invention and its particular features and advantages will become more apparent from the following detailed description considered with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating an emergency DC power system in accordance with the invention. 
     FIG. 2 is a more detailed schematic diagram of the emergency DC power system of FIG.  1 . 
     FIG. 3 is a timing diagram illustrating the output voltage of each spare rectifier, as adjusted by a controller used in the system of FIG. 1, in response to a change in the load bus voltage. 
     FIG. 4 is a block diagram illustrating a standard spare DC rectifier. 
     FIG. 5A is a flow diagram illustrating the operating sequence of the emergency DC power system of FIG.  1 . 
     FIG. 5B is a continuation of the flow diagram of FIG.  5 A. 
     FIG. 6 is a block diagram illustrating an alternate embodiment of the emergency DC power system. 
     FIG. 7 is a schematic diagram of the power system illustrating the alternate embodiment of the emergency DC power system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1 a system  2  for providing DC power to a system load  4  is shown. A primary DC source  6 , using variable load rectifiers as illustrated in FIG. 2, is segregated from a spare DC source  8 . The primary DC source  6  supplies DC power to system load  4  onto a load bus  10  and has a return path through ground bus  12 . The spare DC source  8  delivers DC power onto a spare bus  14  through which load  4  supplemental current is provided in the event of a partial or complete failure in the primary DC source  6 . A control  16  is provided to monitor the voltage on load bus  10 , the AC line power  22 , and the AC power on line  24  from an emergency source  26 . A remotely controlled automatic transfer switch  20  enables automatic switching of AC power between the line source  22  and the emergency AC generator power on line  24 . 
     The DC load bus  10  is also connected to a battery system  28 , which provides DC power to the load  4  at least for such time as the emergency standby AC power generator  26  requires for start up and the automatic transfer switch  20  can switch to the emergency AC power on line  24 . Normally, the batteries are sized to provide DC power for a much longer interval. 
     The spare bus  14  is connected to the load bus  10  through the control  16  in the event of a failure within the primary DC source  6 . The primary DC source  6  is sized so as to provide the required DC voltage for the load bus throughout the normal current loads imposed by the load  4  and a charging of batteries  28 . As a result when an emergency arises, such as when there is partial failure within the DC source  6 , and an insufficient amount of DC power is available for the load, the DC source becomes more heavily loaded and its output voltage on load bus  10  drops. When the load bus  10  voltage drops approximately 0.7 volts, the diode  38  allows the power on the spare bus  14  to be instantly available to the load bus  10 . This 0.7-volt drop is sensed by control  16 , which then energizes relay or contactor  44 . This directly connects the spare bus  14  and the load bus  10  together and eliminates the 0.7 volt drop caused by the diode  38 . 
     When control  16  senses a failure in the available line voltage on line  22 , DC power from the batteries  28  drives the load  4 . The loss of AC line power causes the emergency generator  26  to be activated either by control  26  or manually. The initial current surge demanded by the batteries  28  when the generator  26  comes on line is limited by the available current from the load source  6 . Since this current capacity has a predetermined limit the emergency standby AC power generator  22  can be sized to feed this limit and thus satisfy only the primary DC source  6 , rather that both the primary DC source  6  and the spare DC source  8 . 
     FIG. 2 depicts interconnections of the primary DC source  6 , the spare DC source  8 , and the control  16 . The primary DC source  6  and the spare DC source  8  are each formed of a plurality of parallel connected conventional rectifiers  32  and  30  respectively. FIG. 4 is illustrative of one such rectifier  30 . 1  and includes a full wave bridge circuit  35 , a filter  37  and a regulator  39 . As shown in FIG. 2 the outputs  31 . 1 - 31 . 4  of spare rectifiers  30 . 1 - 30 . 4  are all connected to common spare bus  14 . Once a spare rectifier  30  is activated, such as by raising the magnitude of its output voltage, the spare rectifier  30  will supply power to the load  4  as needed. 
     The load rectifiers  32 . 1 - 32 . 4  have their outputs  33 . 1 - 33 . 4  connected to load bus  10 . Current from the spare rectifiers  30  is supplied to load bus  10  through a blocking circuit  36  composed of a diode  38  in parallel with a shorting switch  44  actuated by a relay  42 . The relay  42  is controlled with signals on line  46  by a programmable controller  34 . 
     The controller  34  is connected, via lines  40  to voltage control inputs  41 . 1 - 41 . 4  of rectifiers  30 . 1 - 30 . 4  respectively to set certain output voltages in a manner as will be explained. The controller  34  senses the load bus voltage along line  45 . 1 , the spare bus voltage along line  45 . 2 , the line voltage along line  45 . 3  and the emergency AC power along line  45 . 4 . 
     System  2  employs two basic modes of operation. They are: 
     Normal mode of operation—During normal operation the blocking circuit  36  blocks current from the spare rectifiers  30  from passing onto the load bus  10 . This requires that the output voltages of the spare rectifiers  30  are set sufficiently low in magnitude to assure that the diode  38  remains reverse biased. 
     Fail mode of operation—In case of a failure of a load rectifier  32 , the voltage on the load bus  10  drops and one of the spare rectifiers  30 , such as  30 . 1 , has its output voltage set to provide supplemental current through the diode  38  to make up for the loss of the failed load rectifier  32 . The controller  34  then allows the normally closed relay contacts  44  to close and thus provide a short across the diode  38  and enable the full voltage from the spare rectifier to be made available on the load bus  10  without the voltage drop across diode  38 . 
     The timing diagram of FIG. 3 illustrates how controller  34  responds to successive rectifier failures. Initially controller  34  sets the output voltages of the spare rectifiers as illustrated at time t 0 , the normal load bus voltage for spare rectifier  30 . 1  or equal to −52.1V, −51.4 for rectifier  30 . 2 , −42 volts for rectifiers  30 . 3  and  30 . 4 . These voltages are illustrative and can vary. This renders the spare rectifiers  30 . 2 ,  3  and  4  essentially out of the circuit until they are needed, with the level for spare rectifier  30 . 2  set to avoid dropping the load bus below −51.4V. The load bus  10  voltages are shown on the top of the diagram. 
     At time t 1  the output voltage from the load rectifiers  32  is noted by controller  34  to drop from a normal set level due to a first rectifier failure. The voltage can drop to a level that is determined by the output voltage of spare rectifier  30 . 1  less the voltage drop across diode  38 . The output voltage of rectifier  30 . 1  is set by controller  34  via line  40 . 1  at the same voltage as normally is set for the load bus or in this case −52.1V. When the voltage drop across diode  38  is taken into account, the minimum voltage the load bus  10  can thus drop to at time t 1  is −51.4 because of the small 0.7V drop across diode  38 . 
     This level remains for load bus  10  until at time t2 controller  34  releases relay  42  and allows its normally closed state to shunt diode  38  with a short and thus directly connect the spare bus  14  to the load bus  10 . This in effect enables the load bus voltage to rise to its normal voltage of −52.1V. The spare rectifier  30 . 2  with its −51.4 output voltage does not affect this. Spare rectifier  30 . 1  now in effect has become a load rectifier. 
     When at time t3 another load rectifier  32  fails, the load bus  10  is not permitted to drop below a minimum level because spare rectifier  30 . 2  was initially set at this minimum level of −51.4V. Hence, at time t3 the load bus drops to a level that is limited to −51.4. At time t4, when controller  34  reacts to the rectifier failure by raising the magnitude of the spare rectifier  30 . 2 &#39;s output voltage to −52.1V the load bus  10  resumes its normal output voltage. Spare rectifier  30 . 2  now has become a load rectifier. 
     Since, the load bus  10  needs to be protected from dropping below the −51.4V level, the output voltage from spare rectifier  30 . 3  is also changed at time t4 to −51.4V by controller  34 . Then when still another load rectifier fails at time t5 the controller  34  raises the magnitude of the output voltage of spare rectifier  30 . 3  at time t6 to −52.1V and that of spare rectifier  30 . 4  to 51.4V. 
     FIGS. 5A and 5B illustrate a flow diagram  60  for controller  34  to achieve the operational connection of spare rectifiers  30  as described with reference to FIG.  3 . At  62  controller  34  monitors the load bus  10  to recognize when the load bus voltage changes indicative of a failure of a load rectifier. This is accomplished by entering values of the voltages sensed on the load bus  10 , VDC L , a DC reference voltage VDC REF , AC line power  22  VAC U , and emergency AC power VAC E  to the voltage level occurrence and what such changes indicate as actions for the controller  34  to initiate. 
     A test is then entered at  64  whether the magnitude of the load bus voltage is greater than a predetermined amount set at a level to assure that small load bus voltage variations are not erroneously interpreted as a rectifier failure. If not, there is no failure and the controller operation returns to step  62 . If a failure is detected a test is made at  66  whether utility power was lost. If so, then the controller notes at  68  that the batteries are driving the load  4  and turns “off” remaining spare bus rectifiers  30 . 1 - 30 . 4 . A message or visual indication on a suitable display is made at  70  and  72  to alert the operator that AC power has failed and that the emergency AC generator will be activated and store the information. 
     At  74  a test is made whether emergency AC power is available and if so, a return is made to step  62 . When AC power from the AC generator  26  turns “on” the load rectifier  32 . 1 - 32 . 4 , the controller  34  monitors the load bus voltage and waits until the voltage level is above 51.5 volts. At that time it restores the remaining spare rectifiers to normal operation. If not, an appropriate message to that effect is sent or displayed for the operator at  76 . 
     If the test at  66  indicated that AC line power is available a test is entered at  78  to determine whether this was the first rectifier failure as tested for at  64 . If so, the relay contacts  44  are closed at  80  and after a short time delay at  82  a test is made at  84  whether the relay contacts indeed did close. Checking for a voltage difference across the contacts  44  can make such test. If the relay did not close an alert to that effect is sent at  86  together with the display at  88  and storage at  90  that a first rectifier had failed. 
     In the event the test at  78  indicates that there had been an earlier failure of a load rectifier, then a test is made at  96  whether there had been a third rectifier failure. If so, the voltage of spare rectifier  30 . 4  is adjusted at  98 , a message that there has been a fourth failure of a rectifier is sent at  100  and stored at  102 . 
     Similarly failures of other rectifiers  30 . 3  and  30 . 2  are handled as shown in with the steps  104 - 112  and  114 - 120  respectively. 
     With reference to FIG. 6 a system  200  for providing DC power to a system load  204  is shown. A primary DC source  206 , using variable load rectifiers as illustrated in FIG. 7, is paralleled with a spare DC source  208 . These two DC sources  206  and  208  are segregated from each other via their respective reference voltages. The primary DC source  206  has a fixed reference voltage of −52.1 volts DC and supplies DC power to system load  204  onto a load bus  210  and has a return path through ground bus  212 . The spare DC source  208  has a controllable variable reference voltage. The controls  216  adjusts the reference voltage of the spare DC source  208  rectifiers to −51.4 volts DC until a partial or complete failure in the primary DC source  206  occurs. The control  216  is provided to monitor the voltage on load bus  210 , the AC line power  222 , and the AC power on line  224  from an emergency source  226 . A remotely controlled automatic transfer switch  220  enables automatic switching of AC power between the line source  222  and the emergency AC generator power on line  224 . 
     The DC load bus  210  is also connected to a battery system  228 , which provides DC power to the load  204  at least for such time as the emergency standby AC power generator  226  requires for start up and the automatic transfer switch  220  can switch to the emergency AC power on line  224 . Normally, the battery system  228  is sized to provide DC power for a much longer interval. 
     The spare DC source  208  is directly connected to the load bus  210  and has it&#39;s reference voltages adjusted by the controls  216  to come online in the event of a failure within the primary DC source  206 . The primary DC source  206  is sized so as to provide the required DC voltage on load bus  210  throughout the normal current loads imposed by the load  204  and a charging of batteries  228 . As a result when an emergency arises, such as when there is partial failure within the DC source  206 , and an insufficient amount of DC power is available, the primary DC source  206  becomes more heavily loaded and its output voltage on load bus  210  drops. When this voltage drops to a value of −51.4 volts DC, all spare rectifiers help to share the load  204 . This voltage drop is sensed by controls  216 , which then adjusts one or more spare rectifiers to an output voltage of −52.1 volts DC until the load bus  210  is restored to −52.1 volts. DC power from the spare DC source  208  is now directly connected to the load bus  210 , and the rectifier or rectifiers adjusted to the higher voltage of −52.1 volts DC are considered load rectifiers. 
     When controls  216  sense a failure in the available line voltage on line  222 , DC power from the batteries  228  drives the load  204 . The controls  216  effectively turn “off” all spare rectifiers not being used as load rectifiers. The loss of AC line power causes the emergency generator  226  to be activated either by control  226  or manually. The available current from the primary DC source  206  limits the initial current surge demanded by the batteries  228  when the generator  226  comes on line. Since this current capacity has a predetermined limit, the emergency standby AC power generator  222  can be sized to feed this limit and thus satisfy only the primary DC source  206 , rather that both the primary DC source  206  and the spare DC source  208 . 
     FIG. 7 depicts interconnections of the primary DC source  206 , the spare DC source  208 , and the controls  216 . The primary DC source  206  and the spare DC source  208  are each formed of a plurality of parallel-connected conventional rectifiers  232  and  230  respectively. FIG. 4 is illustrative of one such rectifier and includes a full wave bridge circuit  35 , a filter  37  and a regulator  39 . As shown in FIG. 7 the outputs  231 . 1 - 231 . 3  of spare rectifiers  230 . 1 - 230 . 3  are all directly connected to the load bus  210 . Once a spare rectifier  230  is activated, such as by raising the magnitude of its output voltage, the spare rectifier  230  will supply power to the load  204  as needed. 
     The load rectifiers  232 . 1 - 232 . 4  have their outputs  233 . 1 - 233 . 4  connected to load bus  210 . Current from the spare rectifiers  230  is supplied to load bus  210  when either a) the controller  234  adjusts the spare rectifier outputs  231 . 1 - 231 . 3  up to the voltage level of the load bus  210 , or b) the failure of a load rectifier drops the load bus  210  voltage to the output voltage level of the spare rectifiers. 
     The controller  234  is connected, via lines  240  to relays  242 . 1 - 242 . 6 , each with corresponding sets of contacts  244 . 1 - 244 . 6 . The controller operates the relays  242 . 1 - 242 . 6  to control the magnitude of the voltage applied to the voltage control inputs  241 . 1 - 241 . 3  of rectifiers  230 . 1 - 230 . 3 . The voltage control inputs  241 . 1 - 241 . 3  of rectifiers  230 . 1 - 230 . 3  set the respective output voltages of the rectifiers. The controller  234  senses the load bus voltage along line  245 . 1 , the line voltage along line  245 . 2  and the emergency AC power along line  245 . 3 . 
     System  200  employs two basic modes of operation. They are: 
     Normal mode of operation—During normal operation the output voltages of the spare rectifiers  230  are set at a −51.4 volt DC level to assure that they are not supplying any current to the load bus  210 . The predetermined DC voltage level of the load bus  210  is entirely supported by the load rectifiers  232 . 
     Fail mode of operation—In case of a failure of a load rectifier  232 , the voltage on the load bus  210  drops and one or more of the spare rectifiers  230 , has its output voltage set at −51.4 volts DC to provide supplemental current to the load bus  210  to make up for the loss of the failed load rectifier  232 . The controller  234  then sequentially activates the relays K 1 -K 6 , according to the logic programming, to adjust the reference voltage inputs  231 . 1 - 231 . 3  over control lines  241 . 1 - 241 . 3 . Closing relay contacts  244 . 1  effectively shorts out the diode  238 . 1  and resistor R 1  enabling the output voltage from the spare rectifier of −52.1 volts DC to be made available on the load bus  210 . This control sequence from the controller  234  is repeated for each spare rectifier  230  each time a load rectifier fails and the load bus  210  voltage drops to the predetermined output level of the next spare rectifier  230 . 
     The forgoing specification described the emergency DC power plant system as utilized with a telecommunications system, however, the invention may be used in any power plant that uses battery back up with a positive or negative ground. The invention has been described with reference to a particular arrangement of parts, features and the like, and are not intended to exhaust all possible arrangements or features, and indeed many other modifications and variations will be ascertainable to those of skill in the art.