Field of the invention The present invention relates to a power control module and method for controlling energy flow in order to reduce the static and dynamic loads on a power system caused by an individual load object with predictable and/or unpredictable power fluctuations, according to the preambles of the independent claims.

Background of the invention

In some remote areas the power transmission system (also referred to as a grid) is not able to provide the demanded peak power to loads. This would cause voltage drops that are not acceptable as the voltage drops causes malfunctions or even shutting off the connected load/s. Also, the grid may not be able to respond to fast changes in power demand.

Alternatively, power generation is made locally using diesel generators, steam or gas turbines. This type of power generation often has insufficient capacity to deliver the peak power required for loads with varying power demands such as mine hoists (winders). In addition, such loads have frequent fast changing power demands for example when changing from continuous speed to retardation to lower speed. The power demand could also be negative i.e. the load regenerates power back to network which means that the total power swing is larger than the peak power demand. Such load demands are difficult to meet with local power generation. A solution is to install larger generating power than otherwise required which means high investment and operation cost. The peak demand must be met by the generators in operation, this generated power by the generators in operation is called spinning power.

Another alternative is to install an energy storage system that provides the power above the capacity of the generating unit(s). Known solutions use stand- alone energy storage devices connected to the plant internal network, normally on the medium voltage level such as 6, 11 or 13.6 kV. The energy storage media is a flywheel/s, batteries or similar.

Such solutions deliver power to the network when its load is high as measured by network frequency, voltage or power consumption. When the frequency, voltage and/or power consumption are normal, the energy storage unit recharges by taking energy back from the network.

US 5,712,456 A (McCarthy et al.) describes a flywheel energy storage for operating elevators. Regenerated energy is stored in the flywheel, and when the power demand is high, the energy stored in the flywheel is utilized to add energy to the DC bus to provide additional power to drive the hoist motor. An energy dissipating device is used to dump excess regenerated energy, when the flywheel has reached its limit. The system is thus energy wasting and includes numerous components to control the system.

US 5,936,375 A (Enoki) describes a method for energy storage and recovery for load hoisting equipment, in which rest power such as reversed power is regenerated to a flywheel and later stored energy in the flywheel is used when a load is lifted. This system also lacks the ability to handle excess regenerated energy, and provides insufficient control of the motors in the system.

The present invention intends to provide a system that overcomes the above stated insufficiencies.

Summary of the invention

It is an object of the present invention to provide an improved control system and method of controlling the energy flows between the devices in a network load, which uses the energy in the system in a more effective way than prior art system. A further object is to provide a system and method which allows less generating spinning power and enables improved power stability by reducing the peak load and the power swing of a load connected to a network. According to one embodiment, a Power Control Module (PCM) is arranged to control the energy flow amongst a power line, at least one Power Demanding Module (PDM) and at least one Energy Storage Module (ESM) interconnected by a DC-link, wherein the module includes:

- a sensing means adapted to sense the power demand of the PDM and to generate a power demand indicating signal in response of the sensed power demand;

- a control means adapted to generate a control signal in response of the power demand indicating signal, that is applied to the ESM to control the energy flow between the PDM and the ESM.

The power line is a power source with limited power, and may be any of a grid or a local power source.

According to one method for controlling energy flow amongst a power line and at least one Power Demanding Module (PDM) and at least one Energy Storage Module (ESM) interconnected by a DC-link, the method includes the steps of:

- sensing a power demand of the PDM and generating a power demand indicating signal in response of the sensed power demand; - controlling a control means adapted to generate a control signal in response of the power demand indicating signal, that is applied to the ESM to control the energy flow between the PDM and the ESM .

Preferably, the method includes the step of, if the power demand indicating signal is above a maximal power line threshold, generating a control signal being a generate power control signal (A), in order to transfer power from the ESM to the PDM. The PDM thus can function without disturbances caused by insufficient power supply from the power line.

Advantageously, the method further includes the steps of calculating and generating a difference signal (B) representing the difference between the power demand indicating signal and the maximal power line threshold, and further delivering the difference signal (B) to a second inverter unit (INU-2), in order to transfer the difference in power from the ESM to the PDM. The ESM thus supplies the difference in power to the PDM that the power line is not able to deliver.

According to one embodiment, the method includes the step of, if the power demand signal is equal to or below the maximal power line threshold, generating a control signal being a stop generate power control signal (C), in order to make the ESM stop generating power. Thus, if the PDM does not need more extra power from the ESM, the ESM stops generating power.

In a further embodiment, the method includes the step of, if the power demand indicating signal indicates retardation, generating a control signal being a recharge control signal (D), in order to charge the ESM and thereby reducing the fast change in power demand from the power line. Thus, to make use of regenerated power and to recharge the ESM, the ESM advantageously starts recharging as soon as the PDM starts retarding. This is advantageous for generators in use, as the instantaneous change of power is reduced and thereby more easily handled.

Preferably, the method includes the step of, if the power demand indicating signal does not indicate retardation, and if a rotational speed of the PDM is low or zero, generating a control signal being a recharge control signal (D), in order to charge the ESM. Accordingly, the ESM is charged when the power demand of the PDM is low or zero, and advantageously the power withdrawn from the power line is evened out.

According to one embodiment, the method includes the step of calculating the power demand signal from a torque reference value in a first inverter unit (INU-I) and a rotational speed of the PDM. A more direct control of motors in the system is thus possible, and the dynamic performance is improved.

Advantageously, the method includes the step of charging the ESM to nominal power before the PDM is started. This to ensure that the ESM is charged with enough power to be able to transfer power to the PDM if needed from the very beginning. In applications with cyclic loads, this is done without loss of time.

Preferred embodiments are set forth in the dependent claims.

Short description of the appended drawings

Figure 1 shows a schematic block diagram of the power control module (PCM) according to the invention connected to at least one PDM, one ESM and a power line.

Figure 2 shows the signal flow to and from the control means.

Figure 3 shows a flowchart of a starting procedure of a PDM, e.g. a motor that drives a hoist/lift.

Figure 4 shows a flowchart with steps carried out when the PDM requires more power than the power line can provide.

Figure 5 shows a flowchart with steps carried out to charge the ESM.

Figure 6 shows a schematic block diagram of the power control module (PCM) connected to a PDM, an ESM and a power line in greater detail.

Figure 7 shows a graph of the power demand of a double drum hoist without power swing reduction.

Figure 8 shows a graph of the resulting withdrawn power from the power line when using power swing reduction.

Detailed description of preferred embodiments of the invention

According to the invention, a power control module (PCM) is arranged to control the energy flow amongst a power line, at least one power demanding module (PDM) and at least one energy storage module (ESM). The parts are interconnected by a DC-link, which makes it possible to reduce resulting peak power and power swing of the PDM as seen by the power line compared to prior art systems. A general block diagram of the structure can be seen from figure 1. It is to be understood that several ESM: s and PDM: s may be connected to the same PCM.

The PCM further includes a sensing means adapted to sense the power demand of the PDM and to generate a power demand indicating signal in response of the sensed power demand. A control means is also included in the PCM, adapted to generate a control signal in response of the power demand indicating signal, which is applied to a second inverter unit (INU-2, see figure 6, denotation 11) to control the energy flow between the PDM and the ESM. By controlling the energy flow in response of the sensed power demand of the load, it is possible to control the PDM and the ESM in a more direct way than in previously known systems.

Figure 2 shows the signal flow to and from the control means. A start signal signals to the control means to initiate a start procedure, as can be seen from figure 3. A maximal power line threshold, indicating the maximal power output from the power line, is set manually or automatically to a desired level. Numerous signals may be sent from the control means to the second inverter unit (INU-2) to control the energy flow, and these are: Generate power control signal (A), Difference signal (B), Stop generate power control signal (C) and Recharge control signal (D). These signals are further explained below.

In one embodiment and with reference to figure 6, the power control module includes a Voltage Source Inverter (VSI) 1 including; a first inverter unit (INU-I) 9 connected between the DC-link 8 and the PDM 2, a second inverter unit (INU-2) 11 connected between the DC-link and the ESM 3 and an Active Front End (AFE) 7 connected between the DC-link and the power line 17. The inverter units are double acting, i.e. they convert from both AC to DC and reversed.

The AFE converts the AC power line input into a controlled DC-link voltage. When needed, the AFE converts DC-link voltage to AC voltage to the power line. The performance of the PDM is thus improved, as the AFE reacts directly during e.g. load changes between motoring and regeneration and no delay time is needed.

The PDM includes a first motor connected to a load, e.g. a hoist or winder. The ESM may be a flywheel energy storage including a second motor/generator and a flywheel adapted for charging and generating power. The motors in the system are SM/IM, (synchronous/induction motor). The network load caused by the peak power and power swing of the driven load is reduced by the flywheel energy storage. It is thus to be understood that several other storage/generating solutions are possible, e.g. capacitors or batteries. The first inverter unit (INU-I) controls the first motor connected to the load, and the second inverter unit (INU-2) controls the second motor/generator connected to the flywheel.

In one embodiment, the power demand signal is calculated from a torque reference value in the first inverter unit (INU-I) and a rotational speed of the PDM. In a further embodiment, the rotational speed is estimated from the torque reference signal.

In case of high power demand With reference to the flow chart in figure 4, the case when a high power demand of the PDM above what the power line can supply, is explained. The maximal output from the power line is referred to as the maximal power line threshold.

If the sensed power demand indicating signal is above the maximal power line threshold, the control means generates a generate power control signal (A). This signal is transferred to the second inverter unit (INU-2) which controls the motor/generator in the ESM to start generating power, in order to transfer power from the ESM to the PDM.

In one embodiment, in order to transfer the correct amount of power from the ESM to the PDM, the control means is adapted to calculate and generate a difference signal (B) representing the difference between the power demand indicating signal and the maximal power line threshold. This difference signal (B) is delivered to the second inverter unit (INU-2), which controls the generator in the ESM to generate the difference in power, in order to transfer the generated power from the ESM to the PDM.

According to one embodiment, if the power demand signal is equal to or below the maximal power line threshold, the control means is adapted to generate a stop generate control signal (C). This in order to make the ESM stop generating power when the power demand of the PDM is lowered to or below what the power line can give. In case of retardation

When the PDM is retarding, the power demand indicating signal indicates retardation. Retardation is indicated by differention of the rotational speed, and if negative the PDM is retarding. A flowchart explaining the steps can be seen from figure 5. The control means is then adapted to generate a recharge control signal (D), in order to charge the ESM and thereby reducing the fast change in power demand from the power line. If at retardation the PDM is regenerating power, the power has the opposite direction as when accelerating and power is transferred to the DC-link. The ESM is then charged from the PDM. If no or insufficient power is regenerated during retardation, then the ESM is charged from the power line during retardation, low speed and standstill.

In one embodiment, if said power demand indicating signal does not indicate retardation, and if a rotational speed of the PDM is low or zero, the generated control signal is a recharge control signal (D), in order to charge the ESM. Thus, the ESM is charged also after retardation in case the PDM has a low power demand or stands still. This ensures that the flywheel is charged and evens out the withdrawn power from the power line.

According to one embodiment and with reference to figure 6, the VSI 1 is connected to the power line via a circuit breaker 5 and a transformer 4. The circuit breaker is used for normal switching in and out as well as in case of a fault situation to trip and isolate the network load 16 and the transformer transforms the voltage from the power line to a suitable level, if required.

The ESM is preferable initially charged to nominal power before the PDM is started, to avoid taking power from the DC-link that is needed by the PDM and to have a loaded ESM to be able to supply the PDM in case of high power demand.

With reference to figure 6, a power system 15 for a mine hoist (mine winder) as the PDM 2 is described below. Before the hoist 2 starts, the flywheel 3 is started at controlled acceleration charging the flywheel 3 to nominal power. Then the hoist 2 starts at set acceleration. As the speed increases, the power demand increases. Power is delivered by the power line 6 via a circuit breaker 5, a transformer 4, or other electrical equipment with similar characteristics, and the active front end 7, a DC-link 8 and a first inverter unit (ESfU-I) 9 connected to the first motor, a hoist motor 10. When the power demand of the hoist 2 reaches the threshold for what the power line 6 can supply, the flywheel 14 delivers the power above that threshold. The power is transferred via the second inverter unit (INU-2) 11 via the DC-link 8 and the first inverter unit (INU-I) 9 to the motor 10 of the hoist machinery 12.

After reaching desired full constant speed and when the power demand of the PDM 2 falls to a level that the power line 6 can fully deliver, the flywheel 14 stops delivering power. When the hoist 2 starts retarding, the power demand is normally reduced sharply. At that point, it is normally optimal to start recharging the flywheel 14. If at retardation the hoist 2 is regenerating, the power for recharging has the opposite direction as at acceleration. The hoist motor 10 then delivers power via the first inverter unit (INU-I) 9, the DC-link 8 and the second inverter unit (INU-2) 11 to the flywheel motor 13. If during retardation the hoist motor 10 is not regenerating, the flywheel 14 is charged by the power line 6 via the circuit breaker 5, the transformer 4, the active front end 7 and the second inverter unit (ESf U-2) 11. The power demand of the motor 10 is measured by the first inverter unit (INU-2) 9. Since the power through the active front end 7 and the transformer 4 is now limited to what the network 6 can supply, their sizes and power losses can be reduced.

With reference to the figures 7 and 8, examples of the effect achieved by the present invention are illustrated. Fig. 7 shows a graph representing the power demand of a PDM, in this case including a double drum hoist, during a phase where the PDM is first constantly accelerating and subsequently lowers its power demand until it starts regenerating power (power demand is negative) and finally has a power demand corresponding to zero. In this case no ESM is connected. The total power swing as seen from the power line (or the PDM) is: 10.7 + 2.5 = 13.2 MW.

With reference to fig. 8, a graph representing the power withdrawn from the power line is shown, when the ESM is connected according to the present invention. The ESM supplies power to the PDM when the power demand of the PDM is above the power line threshold (in this case 6 MW). When retardation starts, the ESM starts recharging (here at about 95 sec). The amplitude of fast changes in power demand from the power line is reduced, and the total power swing as seen from the power line is: 6.0 + 0.0 = 6.0 MW. Accordingly, the power swing is here less than half the power swing compared to the case when no ESM is used (13.2 MW) and is thus greatly reduced. The power step at start of retardation is also reduced, in this case from approximately 4.2 MW to 1.7 MW.

The present invention is not limited to the above-described preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims.

List of references:

I. Voltage Source Inverter (VSI) 2. Driven object such as a hoist/winder or other load with similar characteristics

3. Flywheel energy storage

4. Transformer (if required)

5. Circuit breaker

6. Supply network (MV or LV) 7. Active Front End (AFE)

8. DC-link

9. First inverter unit (INU-I), for the driven object

10. Hoist motor (or other motor for the driven object)

I I. Second inverter (INU-2), for the flywheel or similar 12. Machinery (hoist, winder or other)

13. Motor/generator

14. Flywheel

15. Power system

16. Network load 17. Grid power line or local power generation, with limited power