Abstract:
Upon request from a power utility to reduce the electrical demand of a building, an elevator system is automatically disconnected from power being delivered by the power utility (or remains connected but receives limited power from the power utility), and a secondary power storage device is connected to the power bus of the elevator system. The secondary power storage device provides power to the elevator system during a period in which the request to reduce electrical load of the building is in effect. When the request end, the elevator system is automatically reconnected to the power delivered by the power utility and can potentially work in a hybrid mode.

Description:
BACKGROUND 
       [0001]    The present invention relates to operation of elevator systems. In particular, the present invention relates to operation of an elevator system during a time period when a demand of electrical energy from a power utility exceeds generating capacity. 
         [0002]    Power utilities generate electrical power and distribute that power over a power distribution grid to customers. Power is supplied to residential, commercial, office, manufacturing, and other buildings of small to very large size. 
         [0003]    The demand for electrical energy from customers occasionally exceeds generating capacity of the power utilities on the grid. An example may occur in midsummer, during periods of relatively high heat, when high electrical energy consumption occurs through the operation of air conditioning equipment. Under such conditions, the power utilities that distribute power over the grid have few attractive options. A power utility can lower the voltage on the grid in an attempt to lower power consumption, but this has several problems. First, lowering the voltage can damage equipment attached to the grid. Second, for some old, and for many newer electrical devices attached to the grid, voltage is automatically compensated, so that actual power demand is not decreased. 
         [0004]    Another option is to shut off sections of the power distribution grid. This action, however, cuts off power to both essential and nonessential power equipment on those sections of grid that are shutoff. The result is a so-called “rolling blackout.” 
         [0005]    A more desirable solution is to selectively remove equipment from the grid without causing major disruption to grid power customers. The selective removal (or load shedding) can be done by public announcements requesting customers to turn off nonessential loads during periods of high electrical demand. That approach depends upon the message being received by customers, and the customers complying with the request. 
         [0006]    It is desirable for a power utility to have the ability to disconnect certain nonessential loads remotely. This feature is available for some heating ventilation and air conditioning (HVAC) systems. Power utilities can selectively and remotely change thermostat settings in buildings of selected customers in order to reduce or remove HVAC or certain internal building loads. Those customers typically benefit by reduced utility rates, or some other incentive from the power utility. 
         [0007]    Elevators can represent a significant fraction of the power demand of a building. Unlike an HVAC system, the typical elevator system cannot be simply disconnected from the utility power grid without adversely effecting building operations. If the power utility could turn off the elevator system without warning, elevator cars could be stopped between floors, and passengers trapped within those cars until power is restored. The ability for a power utility to remove elevator loads without impacting the operation of a building has not been available. 
       SUMMARY 
       [0008]    An elevator system with a secondary power storage device is capable of automatically shifting between utility grid power, a combination of utility grid power and secondary (grid-independent) power, and secondary power only upon request from a power utility. When a request to reduce electrical demand of a building is received, power to the elevator system from a power distribution system that delivers power from the power utility is reduced, and the secondary power storage device is connected to the elevator system. During the time that the request to reduce electrical demand of the building is in effect, the elevator system operates using power from the secondary power storage device alone or in conjunction with reduced grid power. When the request to reduce the electrical load ends, the elevator system receives increased power from the power utility. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The FIGURE is a block diagram illustrating a building receiving electrical power from a power utility, and having an elevator system that is capable of switching to a grid-independent source of electrical energy upon request of the power utility. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    The FIGURE is a block diagram of power distribution system  10 , which includes power utility  12 , power distribution system or grid  14 , and building  16 , which represents one of many customers of power utility  12  that receive electrical energy from power distribution grid  14 . Building  16  includes energy management system  18 , non-elevator electrical loads  20 , and elevator system  22  (which includes converter  24 , DC power bus  26 , inverter  28 , hoist motor  30 , elevator car  32 , counterweight  34 , roping  36 , controller  38 , and grid-independent power source  40 ). 
         [0011]    Building  16  receives its primary electrical power from power distribution grid  14 . Non-elevator loads  20  include various electrical systems and electrically powered systems within building  16 . Loads  20  may include, for example, HVAC systems, lighting systems, electronic equipment such as computers and other office and business equipment, manufacturing equipment, and other electrical loads depending upon the nature of the use of building  16 . For example, in a manufacturing plant, loads  20  will typically include motors and other electrically powered equipment used in manufacturing processes. 
         [0012]    Power converter  24  and power inverter  28  are connected by power bus  26  to form a regenerative elevator drive. Power distribution grid  14  provides electrical power to power converter  24  under normal operating conditions. Power converter  24  is a three-phase power inverter that is operable to convert three-phase AC power from grid  14  to DC power. In one embodiment, power converter  24  comprises a plurality of power circuits that are connected to controller  38  to receive pulse width modulation (PWM) gating pulses. Controller  38  controls the power circuits to convert the three-phase AC power from grid  14  to DC output power. The DC output power is provided by power converter  24  on power bus  26 . It is important to note that while grid  14  has been described as delivering three-phase AC power, in some cases the power available from grid  14  may be a single phase AC power or even DC power. 
         [0013]    The power circuits of power converter  24  also allow DC power on power bus  26  to be inverted and provided to grid  14 . In one embodiment, controller  38  employs pulse width modulation (PWM) to produce gating pulses so as to periodically switch the transistors of power converter  24  to provide a three-phase AC power signal to grid  14 . This regenerative configuration reduces the power demand by up to building  16  on grid  14 . 
         [0014]    Power inverter  28  is a three-phase power inverter that is operable to invert DC power from power bus  26  to three-phase AC power. Power inverter  28  comprises a plurality of power circuits. Controller block  38  controls the power circuits to invert the DC power on power bus  26  to three-phase AC output power. The three-phase AC power at the outputs of power inverter  28  is provided to hoist motor  30 . In one embodiment, controller  38  employs PWM to produce gating pulses to periodically switch transistors of power inverter  28  to provide three-phase AC power to hoist motor  30 . Controller  38  may vary the speed and direction of movement of elevator car  32  by adjusting the phase, frequency and magnitude of the gating pulses to power inverter  28 . 
         [0015]    In addition, the power circuits of power inverter  28  are operable to rectify power that is regenerated when movement of elevator car  32  and counterweight  34  drives hoist motor  30 . For example, if hoist motor  30  is generating power, controller  38  controls the transistors in power inverter  28  to allow the regenerated power to be converted from AC to DC and provided to DC power bus  26 . 
         [0016]    Hoist motor  30  controls the speed and direction of movement between elevator car  32  and counterweight  34 . The power required to drive hoist motor  30  varies with the acceleration and direction of elevator car  32 , as well as the load in elevator car  32 . For example, if elevator car  32  is being accelerated, run up with a load greater than the weight of counterweight  34  (i.e., heavy load), or run down with a load less than the weight of counterweight  34  (i.e., light load), power is required to drive hoist motor  30 . If elevator car  32  is leveling or running at a fixed speed with a balanced load, it may be using a lesser amount of power. If elevator car  32  is being decelerated, running down with a heavy load, or running up with a light load, elevator car  32  and counterweight  34  drive hoist motor  30 . In this case, hoist motor  30  regenerates three-phase AC power that is converted to DC power by power inverter  28  under the control of controller  38 . The converted DC power may be returned to grid  14 , supplied to grid-independent power source  40  for storage, and/or dissipated in a dynamic brake resistor (not shown) connected to power bus  26 . 
         [0017]    It should be noted that while a single hoist motor  30  is shown in  FIG. 1 , elevator system  22  can include multiple hoist motors  30 , cars  32 , and counterweights  34 . For example, a plurality of power inverters  28  may be connected in parallel to power bus  26  to provide power to a plurality of hoist motors  30 . In addition, it should be noted that while grid-independent power source  40  is shown connected to DC power bus  26 , power source  40  could alternatively be connected to one phase of the three phase input of power converter  24 . 
         [0018]    Grid-independent power source  40  may be an energy storage system, or may be a backup power source such as a generator. Grid-independent power source  40  may include one or more devices capable of storing electrical energy that are connected in series or parallel. In some embodiments, power source  40  includes at least one supercapacitor, which may include symmetric or asymmetric supercapacitors. In other embodiments, power source  40  includes at least one secondary or rechargeable battery, which may include any of nickel-cadmium (NiCd), lead acid, nickel-metal hydride (NiMH), lithium ion (Li-ion), lithium ion polymer (Li-Poly), iron electrode, nickel-zinc, zinc/alkaline/manganese dioxide, zinc-bromine flow, vanadium flow, and sodium-sulfur batteries, or other commercially available batteries. In other embodiments, other types of electrical or mechanical devices, such as flywheels, can be used to store energy. Power source  40  may include one type of storage device or may include combinations of storage devices. 
         [0019]    Controller  38  controls operation of converter  24 , inverter  28 , and grid-independent power source  40 . It may also receive feedback signals from converter  24 , inverter  28 , and power source  40 , or from sensors associated with those devices. Controller  38  also controls when elevator system  22  will operate using power from power distribution grid  14 , and when elevator  22  will operate using power supplied by grid-independent power source  40 . 
         [0020]    In system  10  shown in the FIGURE, power utility  12  has the ability to request that building  16  reduce power demand on grid  14 . Communication link  42  allows power utility  12  to communicate with energy management system  18 . Communication link  42  may be, for example, an internet connection, a phone line connection, or wireless network connection such as a cell phone or pager type connection. 
         [0021]    Under conditions when the overall demand for electrical energy exceeds the generating capacity of power utility  12 , a request may be sent over communication link  42  to building  16  to turn off certain loads, or to modify operation so that electrical power demand is reduced. For example, in response to a request from power utility  12 , energy management system  18  may provide different control set points to HVAC systems operating within building  16 , so that energy consumption is reduced. In some cases, nonessential devices or systems among loads  20  may be turned off entirely until the request from power utility  12  is no longer in effect. 
         [0022]    In the case of elevator system  22 , any reduction in consumption of grid power should not interrupt elevator service so that passengers are stranded on floors or trapped in an elevator car that is stopped between floors. When energy management system  18  receives a request from power utility  12  to reduce power demand on grid  14 , it provides a signal to controller  38  of elevator system  22 . Controller  38  will then determine whether it is possible for elevator system  22  to switch to grid-independent operation using power from power source  40 . In addition, controller  38  may estimate, based upon signals from grid-independent power source  40 , how long elevator system  22  may operate in a grid-independent mode. Controller  38  monitors the state of any energy storage or independent energy supply used by grid-independent power source  40  in order to determine whether to accept a request from energy management system  18  to switch from grid-dependent to grid-independent operation. For example, when grid-independent power source  40  is an electrical energy storage system, controller  38  may monitor voltage, current, and temperature to determine a state-of-charge of the storage system, from which a determination of available energy (and estimated operating time) can be made. 
         [0023]    During grid-independent operation, controller  38  may disable converter  24 , or may open a switch, so that converter  24  cannot receive power from grid  14 . Controller  38  also connects grid-independent power source  40  to DC power bus  26  or to an input converter  24 . 
         [0024]    During grid-independent operation, controller  38  continues to monitor energy available from grid-independent power source  40 . When available energy drops below a predefined or estimated limit, controller  38  signals energy management system  18  that it is no longer capable of grid-independent operation, and will transition elevator system  22  back to grid-dependent operation. Depending on the request received by energy management system  18  over communication link  42 , and the state of power available on grid  14 , controller  38  may cause elevator system  22  to safely shut down, rather than operating with insufficient power from grid  14 . Alternatively, elevator system  22  may continue to operate in a regenerative mode until people within the building are at a safe level. 
         [0025]    If the request by power utility  12  to energy management system  18  is withdrawn or ended while elevator system  22  is still in a grid-independent operation mode, energy management system  18  will notify controller  38 . Elevator system  22  will then return, under the control of controller  38 , to operation using power from grid  14 . Grid-independent power source  40  may remain connected to power bus  26  so that it can be recharged to a target state-of-charge level in anticipation of the next time that grid-independent operation will be needed or to reduce the demand of electricity from the grid (i.e. hybrid mode of operation) under normal condition. 
         [0026]    With system  10  shown in the FIGURE, the ability of the power utility to automatically cause customers to reduce demand during periods of high power consumption has been enhanced. Grid-independent operation of elevator systems, in response to a request from the power utility, reduces power demand on the grid, in a seamless fashion. Essential elevator services are maintained during an automatic change between grid-dependent and grid-independent operation. 
         [0027]    In other embodiments, a request from the power utility may cause energy management system  18  to select a hybrid mode of operation of elevator system  22 , in which power is supplied jointly from grid  14  and from grid-independent power source  40 . In this hybrid mode, power source  40  can be used to provide more power than usual, or to reduce peaks from the utility. This allows elevator system  22  to be connected to the grid but using power from the grid at a reduced level. 
         [0028]    In addition, to the extent sufficient power is available from elevator system  22  during grid-independent operation, certain non-elevator loads may be powered from elevator system  22  rather than from grid  14 . 
         [0029]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, although the FIGURE shows a regenerative drive formed by converter  24 , DC bus  26 , and inverter  28 , other types of hoist motor drives may also be used with the invention.