Patent Description:
A chiller is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool equipment, or another process stream (such as air or process water). As a by-product, refrigeration creates waste heat that must be exhausted to ambience, or for greater efficiency, recovered for heating purposes. In air conditioning systems, chilled water is typically distributed to heat exchangers, or coils, in air handlers or other types of terminal devices which cool the air in their respective space(s). The water is then recirculated to the chiller to be re-cooled. These cooling coils transfer sensible heat and latent heat from the air to the chilled water, thus cooling and usually dehumidifying the air stream.

A chiller utilizes an alternating current (AC) motor to drive a compressor utilized to compress and heat refrigerant utilized in the chiller and passes this through a condenser and later an evaporator to provided chilled air in an HVAC system. For high tier chiller applications, a low total harmonic current distortion (THD) is needed from converters driving an AC motor.

<NPL> discloses a fire-level bidirectional converter controlled by finite control set model predictive control.

<NPL> discloses an integrated approach with compensation process and elimination process to eliminate the adverse effect of dead-time for model predictive power control with fixed switching frequency.

<CIT> discloses a combined method for dead zone elimination and dead zone compensation of a three-level T-type inverter.

According to an aspect of the invention a system is provided as defined by claim <NUM>.

Additional technical features and benefits are realized through the techniques of the present disclosure. Embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed subject matter.

The following descriptions are provided by way of example only should not be considered limiting in any way.

Turning now to technologies relevant to the aspects of the current disclosure. High tier chiller applications typically require a low total harmonic current distortion for better operation. A Vienna rectifier can be utilized when the low total harmonic current distortion (THD) is required for these high tier chiller applications. The Vienna rectifier has the benefit of low active device count while still maintaining high current quality. However, the harmonic performance of the Vienna rectifier deteriorates when the current is lower than a certain threshold (e.g., <NUM>%). T-type rectifiers can meet the harmonic current requirement across the whole current range but has a higher total cost.

According to the one or more embodiments, aspects of the present disclosure solve the short comings of the above described issues by providing an asymmetrical semiconductor arrangement for the T-type rectifier. Specifically, an insulated gate bipolar transistor (IGBT) current rating of a half-bridge in the T-type rectifier is sized to be much smaller than a diode current rating in the same bridge structure. In the present rectifier circuit, the IGBTs in the bridge structure are enabled only when the load power is lower than a certain threshold when current distortion becomes an issue. In this case, a small current and low cost IGBT can be selected for the bridge structure with satisfactory current harmonic performance across the entire operating ranges.

Referring now to the drawings, <FIG> illustrates a basic block diagram of an exemplary chiller system <NUM> including a variable speed motor <NUM> that is coupled to a compressor <NUM> according to one or more embodiments. The compressor <NUM> includes an impeller/rotor that rotates and compresses liquid refrigerant to a superheated refrigerant vapor for delivery to a condenser <NUM>. In the condenser <NUM>, the refrigerant vapor is liquefied at high pressure and rejects heat (e.g., to the outside air via a condenser fan in an air-cooled chiller application). The liquid refrigerant exiting condenser <NUM> is delivered to an evaporator <NUM> through an expansion valve (not shown). The refrigerant passes through the expansion valve where a pressure drop causes the high-pressure liquid refrigerant to achieve a lower pressure combination of liquid and vapor. As chilled water passes through the evaporator <NUM>, the low-pressure liquid refrigerant evaporates, absorbing heat from the water, thereby further cooling the water and evaporating the refrigerant. The low-pressure refrigerant is again delivered to compressor <NUM> where it is compressed to a high-pressure, high temperature gas, and delivered to condenser <NUM> to start the refrigeration cycle again. It is to be appreciated that while a specific refrigeration system is shown in <FIG>, the present teachings are applicable to any refrigeration or HVAC system.

Also shown in <FIG>, chiller system <NUM> includes a compressor <NUM> driven by a variable speed motor <NUM> from power supplied from a grid <NUM> (or mains) through an AC-DC converter (rectifier) <NUM> and an inverter drive (sometimes referred to as an "DC-AC motor drive") <NUM>. Inverter drive <NUM> includes solid-state electronics to modulate the frequency of electrical power on line. In an embodiment, inverter drive <NUM> converts the AC electrical power, received from grid <NUM>, from AC to direct current (DC) using a rectifier <NUM>, and then converts the electrical power from DC to a pulse width modulated (PWM) voltage, using an inverter <NUM>, at a desired frequency in order to drive the motor <NUM>.

The rectifier <NUM> is further depicted in <FIG> depicts a circuit topology of a rectifier <NUM> according to one or more embodiments. The rectifier <NUM> is configured with three bridge structures, each bridge structure including a set of bypass switches <NUM> and a set of diodes <NUM>. In the illustrated example, the bridge structures are in a half bridge configuration and each include two active switches <NUM> and two diodes <NUM>. However, in one or more embodiments, a full bridge structure can be utilized. Also, in one or more embodiments, the active switches <NUM> can be any type of switch including, but not limited to, an insulated gate bipolar transistor (IGBT) or a metal-oxide-semiconductor filed effect transistor (MOSFET). The three bridge structures are coupled to a filter circuit <NUM>. The filter circuit <NUM> is coupled to a three-phase alternating current (AC) power supply <NUM>. The combination of the three-phase power supply <NUM> and filter <NUM> supplies power in three different phases (typically offset by <NUM> degrees) to the bridge structures. The bridge structures each receive power in a different phase and then rectifies the AC power signal to a DC power signal. The rectifier <NUM> also includes a capacitor bridge made up of two capacitor <NUM>, <NUM> which are utilized to smooth the DC output power signal. The rectifier <NUM> also includes a set of bi-direction switches <NUM>. These bi-direction switches <NUM> allow the creation of a kind of active filter, which operates in parallel to half bridge diode rectifiers. Through the active control of the middle branch bidirectional switches <NUM>, it provides the additional paths for the input current. Therefore the current at the grid side <NUM> becomes sinusoidal with low harmonic contents. Note that the three bidirectional switches <NUM> are interconnected at the mid-point of a capacitor bridge, made up of two capacitors <NUM>, <NUM> needed to ensure voltage balancing. RLoad is the load on the rectifier <NUM>.

In one or more embodiments, the rectifier <NUM> also includes a controller <NUM> that is utilized to operate the active switches <NUM> and the bi-directional switches <NUM>. According to the invention, the rectifier <NUM> is operated as a standard Vienna type rectifier when the active switches <NUM> are operated in an off-state. The rectifier <NUM> can be operated as a T-type rectifier when the active switches <NUM> are controlled in a pulse width modulation (PWM) fashion. In one or more embodiments, the controller <NUM> controls the operation of both the active switches <NUM> and the bi-direction switches <NUM>. The controller <NUM> determines a threshold current for utilization in operation of the rectifier <NUM>. The threshold current can be determined based on where the lower current of the AC power supply begins to cause current distortion. When the input current to the bridge structures is less than or below this threshold current, the rectifier <NUM> operates as a T-type rectifier and the active switches <NUM> are operated in parallel with the diodes <NUM>. When the input current to the bridge structures is greater than the threshold current, the rectifier <NUM> operates as a Vienna rectifier and the current can only flow through the diodes <NUM> and active switches <NUM> are in OFF state. In one or more embodiments, the current rating of the active switches <NUM> is much smaller than the current rating of the diodes <NUM> in these bridge structures of the rectifier <NUM>.

According to the invention, the current rating of the active switches <NUM> is an order of magnitude smaller than the current rating of the diodes <NUM>. When operate in Vienna rectifier mode, the harmonic distortion becomes large at light load condition as the input current becomes discontinuous due to unidirectional power flow feature. This harmonic distortion can be corrected if the active switches are enabled in PWM mode. In this case, the active switches provide additional current path, making the harmonics of input current much reduced.

<FIG> depicts a rectifier topology with a single phase power supply according to one or more embodiments. The rectifier <NUM> includes a single bridge structure that includes a set of two active switches <NUM> and a set of two diodes <NUM>. In the illustrated example, the bridge structure is in a half bridge configuration and each include two active switches <NUM> and two diodes <NUM>. However, in one or more embodiments, a full bridge structure can be utilized. Also, in one or more embodiments, the active switches <NUM> can be any type of switch including, but not limited to, an insulated gate bipolar transistor (IGBT) or a metal-oxide-semiconductor filed effect transistor (MOSFET). The bridge structure is coupled to a filter circuit <NUM>. The filter circuit <NUM> is coupled to a single-phase alternating current (AC) power supply <NUM>. The bridge structure receives power through the filter <NUM> from the power supply <NUM> and then rectifies the AC power signal to a DC power signal. The rectifier <NUM> also includes a capacitor bridge made up of two capacitor <NUM>, <NUM> which are utilized to smooth the DC output power signal. The rectifier <NUM> also includes a bi-direction switch <NUM>. The bi-direction switch <NUM> allows the creation of a kind of active filter, which operates in parallel to half bridge diode rectifiers. Through the active control of the middle branch bidirectional switch <NUM>, it provides the additional path for the current. Therefore the current at the grid side <NUM> becomes sinusoidal with low harmonic contents. Note that the bi-directional switch <NUM> is interconnected at the mid-point of a capacitor bridge, made up of two capacitors <NUM>, <NUM> needed to ensure voltage balancing. The rectifier <NUM> drives a DC load (RLoad).

In one or more embodiments, the rectifier <NUM> also includes a controller <NUM> that is utilized to operate the active switches <NUM> and the bi-directional switch <NUM>. In one or more embodiments, the controller <NUM> controls the operation of both the active switches <NUM> and the bi-direction switch <NUM>. The controller <NUM> determines a threshold current for utilization in operation of the rectifier <NUM>. The threshold current can be determined based on where the lower current of the AC power supply begins to cause current distortion. When the input current to the bridge structures is less than or below this threshold current, the rectifier <NUM> operates as a T-type rectifier and one or more of the active switches <NUM> are operated to create a bypass path around the diodes <NUM>. When the input current to the bridge structures is greater than the threshold current, the rectifier <NUM> operates the active switches <NUM> in an off state and the current can flow through the diodes <NUM> through forward bias. In one or more embodiments, the current rating of the active switches <NUM> is much smaller than the current rating of the diodes <NUM> in these bridge structures of the rectifier <NUM>. According to the invention, the current rating of the active switches <NUM> is an order of magnitude smaller than the current rating of the diodes <NUM>.

<FIG> depict circuit topologies of the bi-directional switches in the rectifier circuit according to one or more embodiments. The circuit topology 400a includes a bi-direction switch <NUM>, in which the IGBTs are connected in a common emitter way. The circuit topology 400b includes a bi-direction switch <NUM>, in which the IGBTs are connected in a common collector way. The benefit of the <NUM> over <NUM> is that it simplifies the gate driver power supply design for the two IGBTs in the bi-direction switch.

<FIG> depicts a flow diagram of a method for operating a rectifier according to one or more embodiments. The method <NUM> includes providing a rectifier comprising a set of bridge structures coupled to an input current of a power supply, wherein each bridge structure in the set of bridge structures comprises a set of diodes and a set of active switches, wherein each active switch in the set of active switches is configured to provide a parallel path around each diode in the set of diodes when in an on state, as shown at block <NUM>. At block <NUM>, the method <NUM> includes determining, by a controller, a threshold current for the rectifier. And at block <NUM>, the method <NUM> includes operating, by the controller, one or more active switches in the rectifier in a PWM stage based on the input current being less than the threshold current.

Additional processes may also be included. It should be understood that the processes depicted in <FIG> represent illustrations, and that other processes may be added or existing processes may be removed, modified, or rearranged without departing from the scope of the present disclosure.

Claim 1:
A system comprising
a rectifier (<NUM>; <NUM>) comprising:
a plurality of bridge structures configured to receive an input current of a power supply (<NUM>; <NUM>), wherein each bridge structure in the plurality of bridge structures comprises a plurality of diodes (<NUM>; <NUM>) and a plurality of active switches (<NUM>; <NUM>), wherein each active switch in the plurality of active switches is configured to provide a parallel path around each diode in the plurality of diodes when in a PWM state; and wherein the power supply (<NUM>) comprises a three-phase power supply configured to supply three input currents to the rectifier (<NUM>);
a plurality of bi-directional switches (<NUM>; <NUM>) coupled to the plurality of bridge structures and a midpoint of a capacitor bridge; and
a controller (<NUM>; <NUM>) configured to:
determine a threshold current for the rectifier (<NUM>; <NUM>); and
operate one or more active switches in the rectifier in a PWM state based on the input current being less than the threshold current, wherein the plurality of active switches (<NUM>; <NUM>) comprise insulated gate bipolar transistors (IGBT), and wherein a current rating of the plurality of active switches (<NUM>; <NUM>) is an order of magnitude less than a current rating of the plurality of diodes (<NUM>; <NUM>),
wherein each bi-directional switch (<NUM>; <NUM>) comprises two IGBTs and two diodes, and wherein the capacitor bridge comprises two capacitors (<NUM>, <NUM>) configured to smooth a DC output power signal; and
wherein the controller (<NUM>; <NUM>) is further configured to:
operate the plurality of active switches (<NUM>; <NUM>) in the rectifier in an off state based on the input current exceeding the threshold current in order to operate the rectifier as a Vienna type rectifier.