GB2519116A - Rapid on-line power reversal control device for flywheel electrical energy storage - Google Patents

Abstract

A control device enables high speed energy flow reversal for a flywheel driven electrical energy storage system, using synchronous machine technology. The flywheel storage system comprises a flywheel 16, a variable speed magnetic gearbox 15, synchronous generator/motor 13, a permanent magnet generator 12 and a connection 18 to the grid 11. The variable speed gearbox includes a variable speed drive motor 14. The rotor load angle and/or power on the synchronous generator/motor 13 is measured and the variable speed motor 14 is controlled in conjunction with an automatic voltage regulator (AVR) 24. Torque can be reversed in the gearbox by speeding up or slowing down the variable speed motor.

Description

Rapid on-line Power Reversal Control Device for Flywheel Electrical Energy Storage This invention relates to an electrical control device and on a flywheel based electrical energy storage system using a reversible torque variable speed gearbox, positioned between a flywheel and a synchronous electrical machine. The invention also petforce relates to a control system.

Background

Electrical energy storage technology has to be very flexible in operation to best satisfy all or most of the energy storage market's needs. These martets can be defined as: * Grid Frequency control with fast response e.g. short term matching of supply and demand by ramping up or down; * Diurnal Levelling; and * Storage for perhaps a few days for "Green" energy back-up, such as wind or PV.

(The latter two markets are not as critical in terms of response times as these can be predicted events.) The required flexibility is best demonstrated usually by speedof action especially in frequency control and during grid dUsturtances, to maintain grid stability.

Known storage technologies by their nature have built-in resistance to action, whether it be the opening of a large water valve, the start of a themical reaction or mechanical plant, which delay operation, perhaps by only seconds. But in grid level storage, a period of seconds is too long to be useful in stàbilising the grid. An Object Of the invention is therefore to redUce delay in release ofstored energy. This is where, for example, spinniAg reserve comesinto play. But spinthng reservein thermal plant can be limited in application by load ramping rate, which is usuaIItemperature sensitive.

An eiectrital energy storage technology control system, as well as meeting the above requirements, mitt also enable multiple machines at a single localion to run in synchronisation wfth the grid, and to load share as required from a remote control centre.

this is a normal control mode for multiple gensets iii one location: This control system also has to be capable of stable operation in "Islanded" mode and to continue to supply power to a portion or all of the downstream loads.

It is therefore proposed to use flywheels to store and release electrical energy. This technology also lends itself to operating in Spinning Reserve Capacity. This latter functionality is very closely aligned with Frequency Regulation. The mechanical inertia supplied by a large flywheel has a strong stabilising effect on the grid, particularly when large numbers Of units are installed. A control system for flywheel energy storage and retrieval also has to cope with a new function in reverse power recharging the flywheels of changing the control methodology to suit the new conditions.

Statement of invention

According to the invent ion, there is provided an overall control device enabling high speed energy flow reversal fbr a flywheeldriven, electrical energy storage system according to Is daim i. Preferred embodiments Of the invention are disdosed in the further dims dependent upon claim 1.

This inventiOn enables flywheet synchronous technology on any application of electrital energy storage, to have extremely fast response to change, in terms of electrical grid or za transmission systems, even to the reversal of the absorbed or exported power flow, within a very short t ime span and with no thermal limit on ramping tofuilload. This nleansthat if the grid or distribution system enters an operational phase where embeddèó generation is required to provide rapid grid support by either positive or negative contribution, this is possible with similar time frames to that normally associated with fauft cIeariñgactMties Using this technolbgy, 16th ramping and. load reversal is almost instantaneous; aid Therefore has a considerable Sue to thetransmission ori istrif ution system owners. Fast ramp rate can provide better Return on Investment, as some markets atread9 recognise thisteatureas a va uedattcibutefor stabil4 ty.

Similarly the invention gives multiple functionality to large flywheel energy storage systems; and this too has a value to the system owners that other storage systems cannot provide at present.

Flywheel technology has been primarily viewed as suitable mainly for grid Frequency Control. this invention enables: Multiple installations in different locations to support Transmission and Distribution networks.

o Installations can be strategically located to eliminate system bottlenecks.

* Synchronous technology for best response to grid excursions.

o Due to stable rotation on flywheel, IVRT (Low Voltage Ride Through) is not an issue.

* Detection of rotor load angle and rate of change of rotor load angle, enables corrective action on pole slipping, before it takes place, furthering grid stability and unique in its application.

* Grid Frequency Control.

o Flywheel technology is recommended *for this duty cycle, due to its capability for very high number of power reversals.

This technology is disruptive as the unit size and control technology gives this product the edge in terms of versatility.

o Afew large units in place of multiple small units.

* VArs injection into grid for voltage control. (Sync COmp mode.) o Minimising future damage on transformer tap changing apparatus.

* Spinning reserve.

o Minimise the use of thermal plants.

o Maximise the life of thermal plants by minimising the damaging start/stop temperature cydes.

* Standby.

* Diurnal levelling.

* Grid controlled interruptible load, when in recharge cycle. (No customer impact.) * Promotes grid stability by means of the large flywheel's inherent mechanical to electrical inertia.

* Minimising peak load demand and hence Maximum Demand Charges for large industrial plant.

S * Bridging Power to fill gap between stopping one generating plant and starting.

another.

* Back-up to Wind,PV,.Wave and Tidal Power Systems.

* Peak shaving.

* Black Start.

* Running in "lsland"mode.

The control system described herein addresses.the.issue.of flywheel power flow control, in either storage or discharge operational mode and instant, power reversal using.the drive means and synchronous electrical machine components already described in Patent Applications PcTJGB2O12/000495 & PcT/G82012f000496 and appropriate control measures by algorithms, ladder lpgic or other control processes/logic known to those skilled in the art.

Apart from the common measurements of voltage, frequency and phase alignment angle used for synchronising with the grid, the following, are also measured: rotor load angle within electrical machine, i.e. between rotor and stator rotating magnetic fields, either lagging or leading, positive or negative, depending on whether it is acting as a generator or motor; and main stator current power flow direction and magnitude, thereby being aware of the "Null" point between the motoring and generating quadrants of operation value.

Running in either mode of charging or discharging, machines must be capable of control of' load sharing to equalize loading on all the machines on line.

The electrical machine response is governed by the time constants of the main field and excitation components by the typical means of a sophisticated Automatic Voltage Regulator (AVR) and associated excitation system. The latter usually consisting of a brushless design comprising a shaft mounted Permanent Magnet Generator (PMG), 30' powering the AVR, which supplies the frame mounted stationary exciter fIeId inside which the shaft mounted exciter stator rotates. This shaft mounted stator powers the DC electrical machine main field winding by generating AC which is converted to DC by means of the usual shaft mounted rotating rectifiers and associated surge suppression components or similar technology.

The toad ramping rate to the maximum operating power level is also determihed by the S time constants of the variable speed asynchronous or synchronous motor on the magnetic gearbox and its associated power electronics, to load ramp the main electrical machine, in conjunction vith the machine operational level of excitation. This is the primary impediment to ramping as thermal effects are non-existent. The secondary impediment is that of maximum torque design capability.

The rotor load angle measurement and/Or power flow together with the control function and the ability to reverse torque easily by means of the variable speed drive on the variabFe speed gearbox are the key to this invention. Torque can be reversed in the gearbox by speeding up or slowing down the variable speed drive motor.

Torque reversal is sufficient to change mode of operation, charging or discharging. This can Is be achieved using only the variable speed motor on the variable speed geatbox,buttatk of control* could iCad to unpredictable results. The control system herein described i less stressful -than prior art controls to the components of the system and to the grid or distribution system.

The controt cycle can be described as follOws: Status F14.wtieel Gearbox Electrical Machine Offilne. Storage Running at or-above Stationary Stationary Mode minimum spee& cange Engage clutch Energy from Gearbox up to NiOw running no kadç 30% between fly*hesl stafts to minimum speed* speed.

flywheel output feed gearbox rotary::cft,ch fully extenial shaft mOtion engaged.

ándgearbox Ratio changes to Now running at 100% full speed speed, can be excited to match with the grid voltage, frequency and phaser -alignment regulated by gearbox, ready to synchronise. Synthronised.

* Synchronised More energy from Slowly changes** On line generating power.

load ramping to*** flywheel ratio to* Rotor load angle stable and did level. compensate loss increasing to 100% rated of flywheel RPM power and desired MVA r.

POWER!-FREQUENCY CONTROL REVERSAL 1 -External remote Changes ratio to Reduce excitation to a signal Of * reverse power minimum to support rated frequency * * flow in parallel voltage and use gearbox to: excursion with * de-fatilitate the opposite excitation, quadrant of operation by going to zero torque.

Now acting in Speed slowly Kept in control Now acting as synchronous reverse mode * increases or via load angle* motor I generator with an decreases ** and power in appropriate power flow, * MVA being charging or discharging: -absorbed or flywheel energy storage. * --distharged Control system automatically set -accordingly. * 6

Synchronous Condenser mode Declutched Declutched Small power draw from grid to produce MVAr's for.

voltage and power factor control.

Engaged Engaged Small power draw from flywheel to produce VArs: for voltage and power* * . factor control. Can now: * ... function as true grid -: support to supply inertia, spinning reserve, storage * . and frequency control in * .. .--this mode as well.

These capabilities give this flywheel technology multiple functionality -Frequency Control, Synchronous Condenser for voltage control, SpinninE Reserve, longer time frame Grid LeveiStorage, Fast Ramping, etc. S. Starting system when in storage mode is as follows: There must be some useful energy in storage, say >10% of minimum and this energy revel is readily monitored by flywheel RPM measurement, which is easily done and hihiy accurate All can be executed remotely from a control centre, automaticalh, or manually or --at site iTi a similar manner.

The clutch on the exterior shaft-to the gearbox is activated to bringthe gearbox and generator up to speed quickly in circa two-seconds. The-generator is excited and on matching volts, frequency and phase angle of AC generatiOn, is synchronised to thegrid:via -synchrdnising-breaker, by the usual-means.-fuH-load can be achieved -in approximatelyone seconcL Complete cycle from-activation is predicted to be around fOur seconth. This mode -has--least parasiticlosses. (Onlythose due to the flywheel.) 7_--In standby mode, the clutch is engaged, the gearbox controls the generator spinning at synchronous speed. Upon signal electrical machine is synchronised, on-line and up to full load in around two seconds. This mode has intermediate parasitic losses. (Flywheel, drive system complete windage and friction losses in gearbox and variable speed drive, control and motor.) In spinning reserve mode the clutch is already engaged, the machine is synchronised with zero load and rated voltagt Full toad can be achieved in about one second: This mode has most parasitic losses. (Flywheel, drive system, total system windagé and friction, losses iii gearbox and variable speed drive motor, excitation system and iron losses on synchronous machine.) This mode of operation can also be used as spinning Sync. Camp (Synchronous condenser mode), pumping VArs into a grid when needed, taking little real power from the flywheel energy store, whist being capable to* **i*nstn change to frequency control, emergency power generation or power absorption. Frequency control can also be executed in this mode of operation as well.

Is. It can of course be runin Sync. Compmode on the-grid-aloneanddeclutched fromthe rest of the drive train; but this maytake several seconds to support the grid, whilstthe rest of the drive train is engaged, either by dutch or automatic self'synthronising coupling between the generator! motor and gearbox, ROtorload angle and/or watts on the main generator/motorcanbe usedto control loading in conjunctionwith the AVR and a variable speed drive on the magnetic gearbox,which will bethanging/increasingthe torquefed to the gearbox speed decreases, using ratio change to maintain a fixed speed: output driveto the generator.

To reverse the powerfiow and hencetorgue: L Excitation system is decreased.

:25 2. Simultaneouslythe torque delivery decreased tothe generator! motor: (Simply changing the gearbox ratio wilF have théalmost instantaneous reversaF of torque, but thismethod isless:stressful on thesystem) 3. Load ängli and main stator amps decrease.

4. System vóItä andsynthroñisation are mákitained throughout all sequences.

5. WhennuWpoint isreachedorvthegeneratorf motor; thegererator [motor crosses to:theopposite quadrant inthepowerthart:and nowisfunctioning inthe opposite mode to that in #1 and can be excited accordingly up to rated power. A full load reversal cycle.

6. Control is back again with reversed load angle and/or power / amps flow, variable speed drive and AVR combination.

S N.B. Load angle monitoring and control function has the added potential for maintaining stability of the complete system by measurement of lOad änglé and rate ofchange of load angle with respect to time, to predict possible loss Of synchronisation (Pole slipping) and prevention theteof. This will be a functionof the control system. Maximum load angle is 90 Degrees and is not a function of the number of poles on the generator/motor and should not be confused with phase angle used in synchronisingor power factor angle, which is the cosine of the angle of displacement between volts and amps sine waves.

All of this is possible due to the stability of the energy storage capability of the flywheel as it interacts to sudden changes in energy flow, either positively or negatively, unlike prior art systems which have built-in responses to change such as control vah,es in pumped storage (slow and can get to overspeedquiëkIi)fuel inertia in cAES(slowandprime mover runs away on griddisturbance), chemical reaction in flow batteries, etc; The rateof changeof load aneparticu1adyas itapproaches themaxirnum values, plus or minus, during grid excursions, can provide a form of prediction for the possibility of pole slipping and is used to prevent these destabilising events whenever possibleby rapid torque control ofthe gearbox. This is in complete contrast to normal generators with prime movers which due t fuel Inertia, especially gas fuelled,findit some*hat more diffltult to achieve Low Voltage Ride Through. (LVRT) * Under extremegid disturbances during recharging of flywheel energy byincreasingRPMs and thus inertia, the load tan be considered to be interruptible to instantly decrease system demand and bring gild stability sooner. The flywheel energy Storage system can be put back on line when power is available for system charge completion. There are no. impedimentsTh this respect as noili effectsare anticipated.

By maintaining dynamic voltage control using Synchronous Condenser mode, thE use of :tap changing on transMission aMLdistributiOn transformers can be miiimised. Tap changing controls-the votts by means-cxi changingthe turns ri: in the transfOrmer by increasing cidecreasing turns in the:drcuit,.therebyalterhtg.systenivOltage. The cunent and future penetration of wind and 1W generation makes voltage fluctuations more likely than was the case when the distribution and transmission systems were originally designed. The net result with this new distributed generation paradigm is that the frequency of tap changing to control volts will tesult in very quickly exceeding the normal S life expectancy of tap changing devices and Mean Time Between Failures (MTBF) reduction will result, leading to a less reliable system.

This tethnology is able to function as a synchronous condenserfor voltage and power factor control, spinning reserve, standby mode unsynchronised. grid level energy storage for renewable energy sources, frequency regulation and diurnal levelling. io

Description of drawings

The invention will now be described by way of example only, with reference to the accompanying Figures, wherein: Figure 1 hows a load curve; Figures 2 & 3 show pf curves for Unity pf and current / volts offset for pf>1; figure4 shows the interactivity between components and control system; Figure5 ilkistratesa typical Perfbrmance Chart of a synchrbnous machine; and figureS showsthe relative-positions Of the-main components.

Figure 1:hows a typicalload curve-Of "C axis 3,iffustratingfdll load at IPU and "y" axis 1, with a normal running at IPUvalue of circa 25 Degrees, shown by 1 The PU load line 4 shows the approximate relationship between toad Angle andThioad. No matter-the number of poles-on the rotor of the machine, the steady state stability limit is 90 Degrees.

U dec fault conditions (Transient Stability Limit) oscillations beyond 90 Degrees can occur, If these-oscillations diminish, the machine is stiff considered stable. Load angle Sn also be opposite, when switched over froni either running as a motor or generator, with consequent torque value change + or-.

* Figures 2 & 3 dañiy the difference between -Power Pactor Angle and Rotor Load Angle.

3(-The V axis-shows-+ and -signs to represent asinusoidal AC voltage 5 with sinusoidät AC current 6 on a time line Of -x axisS. line 7 Showsthat volts and:arnps 5 &-6sinusoithl curves peak at the same time. The power factor is expressed as the Cosine of angIe 9, which is 0 Degrees and so = 1. This is the desired value as it is the most efficient.

Figure 3 shows the effect of a reduced power factor by the shift of line 7 to line 10 on the X (time) axis 8, representing time andthé phase shift of curves Sand 6. Cosine of the angle 9 now represents the new power factor. This shift in power factor *for example on *a generator can be positive or negative depending on whether load is inductive (Motors) or capacitive in characteristic.

Figure 4 illustrates the connection and drive relationship of flywheel 16,variabte speed magnetic gearbox 15, generatOr / motor 13 and electrical connection 18, to grid 11:The permanentmagnet generator (PMG) is shown by 12, variable speed gearbox drive motor 14 and torsional power flow by rotational arrows17.

The variable speed drive motor on gearbox 14; its Power Electronics Control Box 19 and control circuit 27 from motor 14 and control box 19 are shown.

The connections from the main controller 20 topowerelectronics control boriS are shown by 23 and 26.One of these connections 23 & 26 is for control signal from:main controller 20 to drive motor controlbox 19; andtheother is forfeedback oftorquevaiues-on main shafting 30, derived from loadingon variablespeed controller 19for motor 14.

Line 21iliustrates the control signal of power measurement on electrical connection IS to thegrid to the main contrOller 20.

Lines 22 (2 off)are measuring the rotor loadanglC measured by stator output/input volts and comparing this to a base reference measurement taken from PMG 12. Rotor:load angle can be leading or lagging, dependent on whether the main electrical machine is performing as a mOtor or generator.

Line 25 shows the suppht volts to AVR24 from PMG 12; and line 2 shows the contS of the excitation syttem31, from AVR24.

Figure 5: shows a typical Performance Chart of a 6MW synchronous* generator and Sn * example of load-angle 5. The X axis 49 represents Reactive Power and 1 canbe tagging to the right ofthe Vertical or V axis 50. Vertical axis 50 shows-the actual power in watts. to * the left dl theY axis it is operatirg inthe Leading sectorJt isdesirEblè to staywithinthe havy dotted iine34 enclOsed area, forthe runningvaluesT of PowerFactor (pf)ard MW.

The synthronous machine is rated byconventionatthe váiue:iüdicated by the line 33. On a prime mover drive, this is determined by the power rating of the prime mover; and on a flywheel drive system, by the maximum torque combined with lowest speed rating of the total drive system, including the gearbox. When operating as a synchronous condenser, it will be operating generally within the area marked 39, generating mostly MVAr (X axis) and S consuming little load in MW. (Y axis) Load aver the high lagging power factor range as indicated by 31, is governed by the 2.Spu excitation curve shown by 37. Rotor heating and the maximum excitation current allowable is the limiting factor. Line ff6shows the 2Opu excitation line and line 35 shows thetOpu excitation line forillustration purposes.

Load indicatedin area 32 is limited by the MVAratihg of the generator and the maximum permitted stator temperature rise.

Load indicated by 33 is limited by the maximum power from the drive system of the flywheel (Torque & RPM) and extends from lagging 0.8 pf to a value in the leading sector range. Operating in this leading pf range can lead to machine instability and the final section of thePerformance chart 38,typicallyhasasafetyfactorbuilt intominimise this possibility toad angle for operation atfuil load; O.8pf is iliustrateubyangle band dotted line 51.

Figure 6 illustrates the alternate and preferred árientation of the equipment.

Synchronous electricatmathine 40 is on top of a veftical drive system and is coupledwith a synchronising clutch / coupling 41 to a variable speed gearbox 42. The variable speed capability isderived ton, a variable speed outer ringgeardriven Wa variable speed motor 43 The gearbox may be magnetic or mechanical. The former is the preferred embodiment.

The gearbox input is driven via a magnetic geattox/ hermetical seal / coupling 1 dUtth arrangement 44, thtough the flywheel containment 1 vacuum vessel 45 to the flywheel 46.

The magnetic gearboxf hermetical seal I coupling / dutch arrangement 44; is designed: sud that the outer half 48can be axially engaged or diseogagedto give a ckitthing facility 4.7.