WO2016139129A1

WO2016139129A1 - Adjustable speed drive system

Info

Publication number: WO2016139129A1
Application number: PCT/EP2016/054000
Authority: WO
Inventors: Benny J. RHEIMER; Tobias Dietz; Firuz Zare; Lucian Asiminoaei
Original assignee: Danfoss Power Electronics A/S
Priority date: 2015-03-02
Filing date: 2016-02-25
Publication date: 2016-09-09

Abstract

The invention describes a motor drive system in which a number of motor drives are controlled. A central management unit receives active power estimates and power availability data from the motor drives and receives power consumption data for the system from a meter. The power consumption data includes information regarding the active, reactive and harmonic performance of the system. The central management unit optimizes the active, reactive and harmonic power variables for each of the motor drives and instructs the motor drives accordingly.

Description

ADJUSTABLE SPEED DRIVE SYSTEM

FIELD OF THE INVENTION

The present invention relates to Adjustable Speed Drives (ASDs) and, more particularly, to a system and method for controlling a number of ASDs connected in parallel to the same electrical power supply network.

BACKGROUND OF THE INVENTION

An ASD can be used in many different industrial applications to control the power flow. Common applications includes pumps, fans,

compressors, cranes, paper mills, steel mills, rolling mills, elevators, machine tools, and the like. An advantage of using one or more ASDs is the capability of providing variable speed and/or variable torque control to an electrically driven motor or induction machine.

Figure 1 is a block diagram of a system, indicated generally by the reference numeral 1, including a known diode-based Adjustable Speed Drive (ASD) . The system 1 comprises an AC power supply 2, an ASD 4 and a motor 6. The ASD 4 includes a diode-based rectifier 8, a DC link capacitor 10 and an inverter 12. The ASD 4 may include a DC and/or an AC choke to supress harmonics.

As is well known in the art, the rectifier 8 converters AC power provided by the AC power supply 2 into a DC voltage stored at the DC link capacitor 10. The inverter 12 converts the voltage at the DC link capacitor into an AC signal provided to the motor 6. Importantly, the phase, frequency and amplitude of the AC signal provided to the motor 6 can be controlled by the inverter 12. The topology of the system 1 is technically sufficient for many

applications and is attractive in terms of cost, size and weight. However, there are a few shortcomings that arise from the natural behaviour of the diode-based rectifier that draws non-linear currents from the electrical power network. The non-linear nature of the current drawn by the diode rectifier 8 results in harmonic distortion of the power supply 2.

The generation of current harmonics into the power network causes a wide range of undesired power quality effects and voltage harmonics. One effect is the generation of additional losses in cables and power

transformer which results in higher energy bills and loss of lifetime of components. Another effect is the degradation of the Power Factor, an index used to calculate the energy consumption and thus may reflect in the electricity costs.

A further drawback of the diode-based rectifier is the lack of reverse power flow, thus it cannot provide power flow from motor to the power network.

Figure 2 is a block diagram, indicated generally by the reference numeral 20, of a known active front end (AFE) Adjustable Speed Drive system . The AFE system 20 includes the AC power supply 2 and the motor 6 of the system 1. The AFE system 20 additionally includes a bidirectional ASD converter drive 22 comprising an AFE converter 24, a DC link capacitor 26 and an inverter 28 (similar to the DC link capacitor 10 and inverter 12 described above) . The AFE converter 24 can be provided, for example, by replacing the diode-based rectifier 8 with an IGBT-based converter.

By using a bidirectional AFE converter 24, the current (power) can flow in both directions from the power supply 2 to the DC link capacitor 26 and from the DC link capacitor 26 to the power supply 2. Moreover, the shape of the current flowing to/from the power supply 2 can, to some degree, be controlled .

The bi-directional ASD converter drive 22 can operate as :

· A power converter, providing active power flow from the power supply 2 to the motor 6;

• A regenerative power unit, providing active power flow from the motor 6 to the power supply 2;

• A reactive power compensation unit, injecting inductive or capacitive reactive currents into the power supply 2;

• A harmonic current mitigation unit (like an active power filter) .

There remains a need for improved motor drive systems including active front end (AFE) converters.

SUM MARY OF THE INVENTION

The present invention provides a method (implemented at a management unit, e.g . the energy management unit described herein) of controlling a motor drive system comprising : receiving an active power indication (e.g . a measurement or an estimate of active power provided by a drive) and a power availability indication from one or more motor drives; receiving power consumption data regarding the active, reactive and harmonic performance of the motor drive system ; optimizing active, reactive and harmonic power variables for at least some of the one or more motor drives; and sending instructions to the motor drives accordingly (in order to optimize the active, reactive and harmonic power variables) . The power consumption data may be received from a meter (such as a so- called "smart meter") . The meter may measure current and/or voltage and use the measurements to determine power components. The present invention also provides a method comprising : determining, at each of one or more motor drives, an active power indication and a power availability indication for the motor drive and sending the active power and power availability indications to a management unit; determining (e.g. using a meter) power consumption data regarding the active, reactive and harmonic performance of a power control system;

optimizing, at the management unit, active, reactive and harmonic power variables for at least some of the one or more motor drives and sending instructions from the motor drive controller to the motor drives

accordingly (in order to optimize the active, reactive and harmonic power variables) ; and controlling the motor drive in accordance with said power instructions.

The power availability indication may be received from one of said motor drives indicating the total active, reactive and harmonic power that can be delivered by the motor drives. In some arrangements, the total power and the active power are provided, with the harmonic and reactive components being controllable up to the limits imposed by the total power availability.

The optimization of said active, reactive and harmonic power variables may be dependent on user settings. For example, a user interface may be provided . In some embodiments an Internet interface is provided (in addition to, or instead of, a user interface).

The optimization of said active, reactive and harmonic power variables may be dependent on an input from an aggregator. The aggregator may specify a reactive power requirement and/or a harmonic component provision. The aggregator may require active power scheduling or active power regeneration. The optimization of said active, reactive and harmonic power variables may include active power scheduling . Active power scheduling can, for example, be used to avoid local power peaks. Note that active power scheduling is not limited to smart motor drives.

The optimization of said active, reactive and harmonic power variables may include distribution of reactive power generation amongst at least one of the one or more motor drives. Note that some of said motor drives may not be capable for reactive power generation. Reactive power generation can, in some circumstances, be provided by a single device. However, in many circumstances, the required reactive power generation is shared between multiple devices.

The optimization of said active, reactive and harmonic power variables may include distribution of harmonic mitigation signal generation amongst at least one of the one or more motor drives. Note that some of said motor drives may not be capable for generating harmonic mitigation signals. Harmonic mitigation signal generation can, in some

circumstances, be provided by a single device. However, in many circumstances, the required harmonic mitigation signal generation is shared between multiple devices.

The present invention further provides a method of operating a motor drive (often reference to herein as a "smart motor drive") comprising : determining an active power indication and a power availability indication for the motor drive; sending the active power and power availability indications to a management unit; receiving power instructions from the management unit, the power instructions including instructions relating to active, reactive and harmonic performance requirements of the motor drive; and controlling the motor drive in accordance with said power instructions. The present invention yet further provides an apparatus comprising : a first input for receiving an active power indication and a power availability indication from one or more motor drives; a second input for receiving power consumption data regarding the active, reactive and harmonic performance of a power control system; a control module configured to optimize active, reactive and harmonic power variables for at least some of the motor drives; and a first output for sending instructions to the motor drives accordingly (in order to optimize the active, reactive and harmonic power variables). An aggregator may be provided in

communication with the control module, wherein the optimization of said active, reactive and harmonic power variables is dependent on an input from an aggregator. The present invention yet further provides a system comprising one or more motor drives and a management unit, wherein : the management unit comprises a first input, a second input, a control module and an output; each motor drive comprises an input, an output and a control module; the first input of the management unit is coupled to the output of each motor drive (directly or indirectly, for example, via a Building

Management System (BMS)), whereby each motor drive sends an active power indication and a power availability indication from said motor drives to the management unit; the second input of the management unit is coupled to a meter (directly or indirectly) in order to receive power consumption data regarding the active, reactive and harmonic

performance of the system (the meter may form part of the system) ; the control module of the management unit is configured to optimize active, reactive and harmonic power variables for at least some of the motor drives; and the output of the management unit is coupled to the input of each motor drive (directly or indirectly), whereby the management unit sends instructions to each motor drive controller in order to optimize the active, reactive and harmonic power variables. An aggregator may be provided in communication with the management unit, wherein the optimization of said active, reactive and harmonic power variables is dependent on an input from an aggregator.

The invention concerns energy optimization. When multiple drives are used, there is a "pool" of devices, most having unused capacity. The invention makes use of that capacity by power scheduling, and also by using available power resources for reactive power generation and harmonic cancellation, all controlled from a central controller. Traditional diode-based drives and bi-directional AFE drives cannot be used for reactive power generation or harmonic cancellation, but can be used efficiently by active power scheduling. A potential advantage of the present invention is the ability to harness the full potential of the installed power and using it for energy

management. This gives the customer the potential benefit of lowering the energy bill, and faster return of investment. An additional potential advantage is the provision of a communication gateway that enables a wide range of Internet based services such as remote maintenance, software update, load and application profiling, integration with customer databases, etc. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the following schematic drawings, in which :

Figure 1 is a block diagram of a known diode-based Adjustable Speed Drive; Figure 2 is a block diagram of a known active front end (AFE) Adjustable Speed Drive;

Figure 3 is a block diagram of a system in accordance with an aspect of the present invention ;

Figure 4 is a block diagram demonstrating an exemplary use of a system of Figure 3;

Figure 5 demonstrates the calculation of available power in accordance with an aspect of the present invention ;

Figure 6 is an exemplary power consumption profile of an ASD; Figure 7 shows exemplary power consumption profiles of a number of ASDs;

Figure 8 is a block diagram of a system in accordance with an aspect of the present invention ; and

Figure 9 is a block diagram demonstrating an exemplary use of a system of Figure 8.

DETAILED DESCRIPTION OF THE INVENTION

Figure 3 is a block diagram of a system, indicated generally by the reference numeral 30, in accordance with an aspect of the present invention .

The system 30 has a variety of loads, including one or more diode-based Adjustable Speed Drives 32 (similar to the ASD 4 described above) driving at least one motor 34, one or more bi-directional AFE converter drives 36 (similar to the bi-directional ASD converter drive 22) driving at least one motor 38, one or more smart motor drives 40 driving at least one motor 42 and a number of other loads, indicated generally by the reference numeral 44. The loads 44 may take many different forms, including AC-machines, capacitor banks, linear loads, non-linear loads etc. The loads 44 consume power and generate reactive and harmonic currents into the installation . The system 30 includes a power source having a transformer 46. The output of the transformer 46 provides AC power to each of the loads described above. A meter 48 (such as a so-called smart meter) is provided that provides data to an Energy Management Unit (EMU) 50. The EMU 50 typically has a number of other inputs, including, for example, a user interface 52 and an Internet interface 54. In some embodiments, a building management system 56 may provide an additional input to the EMU 50.

In use, the EMU controls the behaviour of at least the smart motor drives 40 (and typically also the bi-directional AFE converter drive(s) 36 and the diode-based ASD(s) 32) in order to achieve objectives imposed on the system 30.

In particular, the EMU 50 communicates with at least the smart motor drives 40 to determine the power resources available and also

communicates with the meter 48 to receive power measurement feedback. The EMU 50 then runs an optimization algorithm (discussed further below) to determine how to distribute the power loading to at least the smart motor drives and instructs those devices accordingly.

The smart motor drive(s) 40 have the capability to provide new features on the power network because of dedicated control algorithms that operate on the grid-side converter. The smart motor drive 40 as configured in the system 30 operates in a "local optimization" mode, including some or all of the following features :

- Power converter, providing active power flow from the power network to the motor(s) 42. - Power converter, with load scheduling to avoid local power peaks (discussed further below) .

- Regenerative power unit, providing active power flow from the

motor(s) 42 to the power network.

- Reactive power compensation unit, injecting inductive or capacitive reactive currents to compensate the power flow at the installation level .

- Harmonic current cancelation unit, to compensate harmonics currents in the system .

The smart motor drive 40 has a communication unit to exchange information with the Energy Management Unit (EMU) 50. This

communication unit may be a high performance field bus, for example complying with known protocols, such as C-bus, Profibus, Profinet, Modbus, Devicenet, etc. In this way, data and instructions can be transmitted between the EMU 50 and the smart motor drive(s) 40. The transmission of data between the EMU 50 and the smart motor drive(s) 40 can take many forms, such as wired or wireless connections. In addition to controlling the smart motor drive(s) 40, the EMU 50 may be in communication with the bi-directional AFE converter drive(s) 36. The AFE converter drive(s) 36 may have a communication unit to exchange information with the EMU 50. Typically, the AFE converter drive(s) 36 are generic AFE-ASDs, such that AFE-ASDs are able to receive references from the EMU which impose a reference to the power consumed by the application when applicable. More details regarding possible applications are discussed below.

In a similar way, the EMU 50 may communicate with the diode-based ASDs 32. The conventional diode-based power converters 32 are used as power converters, thus delivering the power from the grid to the motor(s) 34 and are typically able to receive references from the EMU which impose only a reference to the power consumed by the application when applicable. More details regarding possible applications are discussed below.

As discussed above, the communication between the EMU 50 and the smart motor drive 40 can take many different forms, such as a bus complying with C-bus, Profibus, Profinet, Modbus, or Devicenet standards. The ASDs 32 and/or 36 may be provided with the same communications interfaces so that those ASDs can also communicate with the EMU 50.

The smart motor drive 40 may be provided with an additional

communication interface, such as a high performance Field Bus that is usually available in standard ASDs. The reason for providing this

additional interface is to be able to connect any type of ASD to the EMU 50 via the smart motor drive(s) 40, even if those ASDs are not

compatible with Communication protocol used for communications between the EMU 50 and the smart motor drive 40. In this way, communications between the diode-based ASDs 32 and the EMU 50 and also between the bi-directional ASD converter drives 36 and the EMU 50 can be provided .

The Building Management System (BMS) 56 is a computer-based dedicated control unit. The BMS may, for example, control and monitor a building's mechanical and electrical equipment such as ventilation, lighting, power systems, fire systems, and security systems. The BMS 56 may consist of software and hardware; the software program, usually configured in a hierarchical manner. The BMS 56 can be implemented on both proprietary and public communication protocols. Not all ASDs installations have a BMS (this is drawn with dotted line in Figure 3) . The meter 48 is installed to read the active power component Pi, reactive power component Qi and harmonic power component H. The meter 48 is often provided on the secondary side of the transformer (i .e. the low voltage connection) . In some alternative applications, the meter 48 can be installed on the primary side of transformer (i .e. the medium voltage connection) . The result is compensation at the measuring point, thus the owner can determine this scope.

The user interface 52 may be provided for configuring the EMU 50 and to enable a user to provide input parameters. The user interface can take many different forms and can, for example, be locally installed or be provided remotely, such as via a PC, or smartphone or tablet etc.

The Internet Interface 54 may be provided for accessing different services of the EMU 50 and the connected ASDs via the Internet. The Energy

Management Unit (EMU) acts also as a portal to enable various Internet- based services such as :

• Remote maintenance and support

• Remote SW update and debugging

· Data collection for various statistics (application profiling, defect rate, lifetime, etc. )

• Integration with customer applications (web-services, centralized

databases) Figure 4 is a block diagram, indicated generally by the reference numeral 60, demonstrating an exemplary use of the system 30 in accordance with an aspect of the present invention . The block diagram 60 shows the interaction between the meter 48, the EMU 50 and a plurality of smart motor drives (40a, 40b...40k) .

The EMU 50 typically includes at least the following features : • Communication with each of the smart motor drives 40 and, if the communications are possible, with each of the diode-based ASDs 32 and/or the bi-directional AFE converter drives 36 to determine the resources available (power left not utilized in each converter) . More details of these calculations are given below with reference to Figure 5.

• Power measurements feedbacks from the meter 48. As discussed

further below, these feedbacks are typically the power measurements : Active power Pi, Reactive power Qi and Harmonic content H .

· The customer can also configure the EMU 50 by providing optimum

operation points for each AFE converter, different cost functions and load profiles of its application . Such configuration is typically achieved using the user interface 52.

• An optimization algorithm in the EMU 50 calculates how to distribute power loading to each motor drive, based on the inputs discussed above. The criterion of energy optimization is fulfilled by performing load demand (discussed below with reference to Figure 7),

compensating reactive power and mitigating harmonics in a

collaborative manner. Again, more details of these algorithms are provided below.

• The EMU 50 communicates to the Building Management System 56 (if provided in the system 30) to pass on requests for low energy consumptions request for an entire building .

• The EMU 50 sends the references back to the smart motor drives 40 and also (if possible) to the bi-directional AFE converter drives 36 and/or the diode-based ASDs 32 in order to achieve the global goals at installation level . This provides low feed in tariffs and low energy consumption of the entire plant. As described above with reference to Figure 4, the EMU 50 can

communicate with the smart motor drives 40 (for example, directly, or via the Building Management System 56, if provided) and, if possible, with the diode-based ASDs 32 and/or the bi-direction AFE converter drives 36 to determine the power resources available from those drives. In general the current consumed by loads from the power supply is characterized by 3 components (assuming the non-fundamental active power is negligible P_n=0) :

- Fundamental Active power P_{l r} consumed by the loads to produce a desired effect (e.g . turning a motor),

- Fundamental Reactive power Q_{l r} consumed by inductors,

transformers, motors, capacitors to energize

- Harmonic Distortion power H, generated by non-linear loads (such as diode-rectifiers and inverters) due to their intrinsic behaviour. Each of the power components creates losses (see the table below), determines overheating and overrating of the power system .

Furthermore, harmonic currents decrease the utilization factor of the network and cause harmonic voltage distortion, undesirable for all other equipment connected to the power system, such as capacitors, ac- machines, control and protection equipment, measuring instruments and electronic power converters.

Table : Effects created by the circulation of different types of powers power system.

Power components Losses created Other effects on the in the power power system

system P_Losssum

Active power Pi PLOSSP Peak load

Grid frequency variation

Reactive power Qi PLOSSQ Decrease of Power Factor

PF Grid voltage variation Distortion power H PLOSSH Decrease of Power Factor

PF Harmonic effects

Assuming that the grid voltage is sinusoidal with no distortion, the non- fundamental active power is negligible P_n=0. The total apparent power S consumed by an ASD is determined by the active power Pi, reactive power Qi and the harmonics H:

Where Si is the total apparent power.

Figure 5 shows a graph, indicated generally by the reference numeral 70, demonstrating the calculation of available power for the smart motor drives 40 in accordance with an aspect of the present invention . The graph 70 includes an indication 72 of the total power that can be provided by the relevant motor drive. The total power indication 72 is a sphere since, as reactive power and/or harmonic power increase, the maximum amount of active power that can be provided decreases. Similarly, if the active power requirement P is low, then the power available for reactive power generation and/or harmonic power cancellation is high .

In Figure 4, the first step described above was the communication from each of the relevant motor ASDs to the EMU 50 of the available power. For the smart motor drives 40, the available power is dependent on the active power required Pi (see the vector Pi in Figure 5) and the total available power (see the indication 72 in Figure 5) . The graph 70 plots the total apparent power S by plotting the variables

If harmonic currents are removed then the total apparent power becomes lower, i .e. Si, thus lowering also the cost of electricity. If reactive power Qi is also removed, then the cost of electricity only depends on the Pi, which is even lower.

P_x < S_x < S

Power Factor (PF) index is another indicator used to determine quality of the electrical power consumed by loads, often used to as a weighted factor to determine the energy paid by consumers. The Power Factor (PF) is defined (assuming the non-fundamental active power is negligible

S JsfTJ S^(\ + THD^)

If the line current is purely sinusoidal, the PF can be simplified as a displacement factor (PFdisp) .

P P

P

Thus, the PF for a distorted grid current can be calculated as follows :

Where the distortion factor is defined as below :

Thus, PF < PF_disp due to the distortion factor.

Therefore, harmonic currents produced by diode-based ASDs are not desired first because of their hazardous effects on the power system, second because of the enforced existing Power Quality standards, and third due to the increased energy consumption finally paid by the owner. The smart motor drive(s) 40 can provide a good quality of the output current, thus only Pi exists. When the smart motor drive(s) operate at partial power, this leaves the smart motor drive (s) with the capability to use the rest for other purposes. The equation 5^* = + Q\ + H² is simplified as :

N is the Non-Active power vector that consists of Reactive power Qi and Distortion power H.

N _{= A}/ ² + H²

The quantity of Ν can be written as :

N = ² - P²

Figure 5 shows how the use of Active power P± leaves the inverter with a certain quantity of available power N (that consists of Reactive power Qi and Distortion power Η) that can be used for other purposes. The available power is a quadratic function as explained by the equation above and indicated in Figure 5.

The following table provides numerical values suggesting how much power is available for N.

Thus, an algorithm is provided that receives power information from each ASD at the EMU 50. Once the AFE-ASD reports how much power Pi, the EMU 50 calculates how much power N (comprising both reactive power Q and harmonic power H) is left available for each AFE-ASD.

A total budget of available power can be calculated by the EMU 50. The EMU 50 then decides how to redistribute the power amongst the ASDs. The algorithm could be based on a non-linear optimization problem with boundary functions and with respect to electricity tariff.

The EMU 50 that ma kes the decision of how to distribute the references for each ASD in a collaborative manner splitting the effort as needed to achieve the goals. For example :

- One of the smart motor drive(s) 40 with higher available power can compensate the Harmonic power H because it is known that harmonic compensation is more demanding .

- If harmonics are too high to be compensated by a single smart motor drive, then a joint effort is done by two or more smart motor drives.

- The effort of compensating harmonics can be separated in frequency, such that some smart motor drives can compensate for some

harmonic orders (such as the fifth harmonic), and other smart motor drives can compensate for other harmonic order (such as the seventh harmonic) .

- One of the smart motor drive(s) can compensate Reactive power Qj . If a quantity of Qi is too high to be compensated by a single smart motor drive, then a join effort is done by two or more smart motor drives.

- If a smart motor drive has enough available power then it can take both compensations of reactive power and Harmonics at the same time. - An objective and optimization can be to increase Power Factor (PF) of the system to reduce electricity cost (a penalty given by a utility company due to poor Power Factor) . Regardless of the type of front-end converter, unidirectional or

bidirectional, any ASD is needed due to the very intrinsic nature of the application, i .e. the motor has to be controlled at a variable speed/power /torque/position, etc. , to serve the application needs. If an application needs constant operation of the motor then one does not need to install an ASD, but connect the motor directly to the power network. Thus, the ASD energizes the motor when needed and to the amount required by application .

As described above with reference to Figure 4, a feature of the present invention is active power scheduling under the control of the EMU 50.

Active power scheduling is not only relevant to the smart motor drives 40 but can also be provided by the diode-based ASDs 32 and/or the bidirectional AFE converter drives 36. In the prior art, the power size of a converter is selected based on the peak power of the application to be able to cover its maximum load .

Additionally, the designer of the installation could take conservative decisions to provision more margins to the power size of the converter for various reasons such as : increased reliability and lifetime, retrofitting future increased in demands, off the shelf availability, cost driven decisions, etc. Thus, there may be additional margin reserved for an ASD besides the fact that it is not fully loaded . This means that the power size of the converter remains not utilized at its full capability all the time, even though the customer pays for the total installed power. Figure 6 shows a load profile, indicated generally by the reference numeral 80, showing different operation points of an ASD, including full loading for a certain period (operation point N). In addition, the user may provision additional margin. The entire shaded area shows the unused capacity of the converter.

A typical industrial installation is composed of a plurality of ASDs, of various power sizes, connected in parallel and serving different scopes in the installation, which may or may not be in relation with the others. But again each ASD operates at some degree of partial loading.

Since ASDs may not all operate at the same time, the likelihood of having an overlap of unused capacity increases. Thus, the cumulated unused capacity of multiple ASDs in the entire plant becomes even higher, at the expense of the owner investment on components.

Figure 7 shows a load profile scenario, indicated generally by the reference numeral 90, where the ASDs are fully loaded at the same time due to the nature of the application. Depending on the design capacity of the plant (transformer, cables, agreement with electrical company, etc.), there can be two possibilities :

- Case A: the peak load is higher than the installed capacity. This may result in a surcharge of the electrical bill as a penalty imposed by network operator for consuming higher power than agreed.

- Case B: the owner increases the rating of the installation and

renegotiates the contract with the network operator to allow higher peak load . This yields higher costs but also larger amount of unused capacity (shaded area increases) . The EMU 50 seeks to prevent the situation shown in Figure 7 from occurring by power scheduling that prevents all relevant ASDs (not just the smart motor drives 40) from consuming pea k active power at the same time. This can reduce the peak power consumption which has a big impact on electricity cost reduction .

Figure 8 is a block diagram of a system, indicated generally by the reference numeral 100, in accordance with an aspect of the present invention .

The system 100 includes the one or more diode-based Adjustable Speed Drives 32, one or more motors 34, one or more bi-directional AFE converter drives 36, one or more motors 38, one or more smart motor drives 40, one or more motors 42, other loads 44, power source having a transformer 46, meter 48, Energy Management Unit (EMU) 50, user interface 52, Internet interface 54 and building management system 56 of the system 30 described above.

The system 100 additionally includes an aggregator 102 and a power network comprising one or more power generators 104, power

transmission 106 and power distribution 108.

The aggregator 102 is in communication with the power generator(s) 104 and with companies providing power transmission 106 and power distribution 108. The aggregator is also in communication with the EMU 50.

The aggregator 102 may, for example, request that the EMU 50 makes reactive power available to the power network. If the system 100 is able to provide such reactive power, the EMU 50 requests that one or more of the smart motor drives 40 provide the requested reactive power. Similarly, the aggregator 102 may, for example, request that the EMU 50 provides harmonic compensation for the power network. If the system 100 is able to provide such harmonic compensation, the EMU 50 requests that one or more of the smart motor drives 40 provide the requested harmonic compensation .

The aggregator 102 could also request that active power consumption be curtailed or even that active power be provided to the power network. If the system 100 is able to meet such active power requests, the EMU 50 instructs one or more of the relevant ASDs according .

A 'Power network services' contract may be provided between the aggregator and the provided of the EMU 50 such that the provider of the EMU is compensated for providing reactive power and/or harmonic power to the power network at the request of the aggregator and/or active power curtailment.

The smart motor drive 40 as configured in the system 100 may operate in the "local optimization" mode described above. Alternatively, the smart motor drive(s) 40 as configured in the system 100 may operate in a "power network services" mode, including some or all of the following features : - Power converter, providing active power flow from the power network to the motor(s) 42.

- Power converter, with load scheduling to avoid local power peaks.

- Regenerative power unit, providing active power flow from the

motor(s) 42 to the power network. - Reactive power compensation unit, thus exporting inductive or capacitive reactive currents on demand under the control of the EMU 50 (at least partly on the request of the aggregator 102) .

- Harmonic current cancelation unit, from other non-linear loads as well thus exporting on demand under the control of the EMU 50 (at least partly on the request of the aggregator 102) .

Both operation modes ('Local optimization' and 'Power network services') are set by the Energy Management Unit (50) by sending the appropriate references to each of the smart motor drive(s) 40 and, where

appropriate, the diode-based ASD(s) 32 and/or AFE converter drive(s) 36.

Figure 9 is a block diagram, indicated generally by the reference numeral 110, demonstrating an exemplary use of the system 100 in accordance with an aspect of the present invention. The block diagram 110 shows the interaction between the aggregator 102, the meter 48, the EMU 50, a plurality of smart motor drives (40a, 40b...40k) .

The EMU 50 typically includes at least the following features :

• Communication with each of the smart motor drive(s) 40 and, if the communications are possible, with each of the diode-based ASDs 32 and/or the bi-directional AFE converter drives 36 to determine the resources available (power left not utilized in each converter) .

· Power measurements feedbacks from the meter 48. As discussed

above, these feedbacks are the power measurements : Active power Pi, Reactive power Qi and Harmonic content H .

• The EMU 50 communicates with the Aggregator 102 to receive various information regarding grid codes and power references, including electricity tariff information. • The customer can also configure the EMU by providing optimum operation points for each AFE converter, different cost functions and load profiles of its application . Such configuration is typically achieved using the user interface 52.

· An optimization algorithm in the EMU 50 calculates how to distribute the loading to each AFE, based on all these inputs. The criterion of energy optimization is fulfilled by performing load demand,

compensating reactive power and mitigating harmonics in a

collaborative manner as described above.

· The EMU communicates to the Building Management System (if

provided) to pass on requests for low energy consumptions request for the entire building .

• The EMU sends the references back to the smart motor drive(s) 40 and, if possible, to the bi-directional ADE converter drive(s) 36 and/or the diode-based ASD(s) 32 in order to achieve the global goals at installation level . This provides low feed in tariffs and low energy consumption of the entire plant.

The embodiments of the invention described above are provided by way of example only. The skilled person will be aware of many modifications, changes and substitutions that could be made without departing from the scope of the present invention . For example, although the system described herein include diode-based AFEs, bi-directional ASD converter drives and smart motor drives, this is not essential to all embodiments of the invention . Indeed, the principles of the invention are applicable if only smart motor drives are provided . The claims of the present invention are intended to cover all such modifications, changes and substitutions as fall within the spirit and scope of the invention .

Claims

CLAIMS:

1. A method of controlling a motor drive system comprising :

receiving an active power indication and a power availability indication from one or more motor drives;

receiving power consumption data regarding the active, reactive and harmonic performance of the motor drive system;

optimizing active, reactive and harmonic power variables for at least some of the one or more motor drives; and

sending instructions to the motor drives accordingly.

2. A method as claimed in claim 1, wherein the power consumption data is received from a meter.

3. A method comprising :

determining, at each of one or more motor drives, an active power indication and a power availability indication for the motor drive and sending the active power and power availability indications to a

management unit;

determining power consumption data regarding the active, reactive and harmonic performance of a power control system;

optimizing, at the management unit, active, reactive and harmonic power variables for at least some of the one or more motor drives and sending instructions from the motor drive controller to the motor drives accordingly; and

controlling the motor drive in accordance with said power

instructions.

4. A method as claimed in any one of claims 1 to 3, wherein the power availability indication received from one of said motor drives indicates the total active, reactive and harmonic power that can be delivered by the motor drives.

5. A method as claimed in any preceding claim, wherein the

optimization of said active, reactive and harmonic power variables is dependent on user settings.

6. A method as claimed in any preceding claim, wherein the

optimization of said active, reactive and harmonic power variables is dependent on an input from an aggregator.

7. A method as claimed in any preceding claim, wherein the

optimization of said active, reactive and harmonic power variables includes active power scheduling.

8. A method as claimed in any preceding claim, wherein the

optimization of said active, reactive and harmonic power variables includes distribution of reactive power generation amongst at least one of the one or more motor drives.

9. A method as claimed in any preceding claim, wherein the

optimization of said active, reactive and harmonic power variables including distribution of harmonic mitigation signal generation amongst at least one of the one or more motor drives.

10. A method of operating a motor drive comprising :

determining an active power indication and a power availability indication for the motor drive;

sending the active power and power availability indications to a management unit; receiving power instructions from the management unit, the power instructions including instructions relating to active, reactive and harmonic performance requirements of the motor drive; and

controlling the motor drive in accordance with said power instructions.

11. An apparatus comprising :

a first input for receiving an active power indication and a power availability indication from one or more motor drives;

a second input for receiving power consumption data regarding the active, reactive and harmonic performance of a motor drive system;

a control module configured to optimize active, reactive and harmonic power variables for at least some of the motor drives; and

a first output for sending instructions to the motor drives

accordingly.

12. An apparatus as claimed in claim 11, further comprising an aggregator in communication with the control module, wherein the optimization of said active, reactive and harmonic power variables is dependent on an input from an aggregator.

13. A system comprising one or more motor drives and a management unit, wherein :

the management unit comprises a first input, a second input, a control module and an output;

each motor drive comprises an input, an output and a control module;

the first input of the management unit is coupled to the output of each motor drive, whereby each motor drive sends an active power indication and a power availability indication from said motor drives to the management unit; the second input of the management unit is coupled to a meter in order to receive power consumption data regarding the active, reactive and harmonic performance of the system;

the control module of the management unit is configured to optimize active, reactive and harmonic power variables for at least some of the motor drives; and

the output of the management unit is coupled to the input of each motor drive, whereby the management unit sends instructions to each motor drive controller in order to optimize the active, reactive and harmonic power variables.

14. An apparatus as claimed in claim 13, further comprising an aggregator in communication with the management unit, wherein the optimization of said active, reactive and harmonic power variables is dependent on an input from an aggregator.