ervin.spahic@siemens.com

Power Intensive Energy Storage and Multilevel STATCOM for frequency andvoltage grid support

E. SPAHIC, M. PIESCHEL, R. ALVAREZ,G. KUHN, V. HILD, G. BECK

N. PLATT

Siemens, Transmission SolutionsGermany

Siemens Ltd.United Kingdom

SUMMARY

The developing political and environmental reforms are driving rapid evolution of today’spower systems. There is a steady increase in electrical energy being generated from renewableenergy sources like wind and photovoltaic systems while decommissioning large thermalpower plants. This trend has a major impact on the performance of the electrical powersystems especially with respect to their dynamic behaviour. Phasing out of existingconventional synchronous generators will adversely affect auxiliary services like voltagecontrol and frequency regulation. This paper discusses one solution to cope with thesechallenges: the use of a Modular Multilevel Converter based STATCOM with an additionalfrequency stabilizer can significantly contribute to the stable operation of a power system.The application of the frequency stabilizer has been analysed on a representation of theIreland (All-Island) transmission grid of Ireland and the results have been presented anddiscussed. It has been shown that a Frequency Stabilizer of 50 MVA can reduce the frequencydeviation by more than 0.1 Hz and thereby avoiding load shedding. Furthermore the ROCOFwas reduced from app. 0.45 Hz/s to 0.4 Hz/s which could be crucial for the setting of theROCOF protection and the grid stability.

KEYWORDS

Frequency stability, Inertia, ROCOF, STATCOM, Supercapacitors. FACTS, Multilevel.

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1. INTRODUCTION

The changes in the power generation mix i.e. reducing generation from conventional powerplants and the increase of decentralized power sources (particularly wind and photovoltaic)are continuously challenging the secure operation of the transmission grids [1].The resulting decrease of system inertia further impacts the system stability during transientfrequency events [2], [3]. A sudden loss of a large generating plant will result in dangerouslylow frequencies [4] which could further escalate into cascading failures and eventually resultin a blackout. Additionally, lower system inertia results in higher ROCOF (Rate of Change ofFrequency). ROCOF relays in the generating stations are generally programmed to withstanda specific threshold (e.g. 0.5 Hz/s today in Ireland). With no additional measures, these valuesare bound to increase in the future. This reduces the reliability of the entire system which is anundesirable situation.Modern power electronic converter technology provides a vast range of applications for acgrid stabilisation. In this paper a new type of converter is presented to stabilise the ac grid’svoltage and frequency. Since the application of STATCOMs for voltage support is already awell-known solution, the focus in this paper will be on the new application – frequencystability.

2. SYSTEM INERTIA, FREQUENCY STABILITY AND ROCOF

The initial response of a conventional generator to a frequency event in the grid is governedby its stored kinetic energy at the rated frequency:

( )2221

nomkin fJE p××= (1)

gen

kingen S

EH = (2)

where: J moment of inertia, fnom nominal frequency, Hgen inertia time constant, Sgen ratedpower of the generator.

From (1) and (2) the maximum duration of the nominal active power infeed without anymechanically power input until it would stop rotating can be estimated. Depending on thetype of the conventional generator as well as on its size different inertia constants can beapplied [5].On the other hand most renewable sources like wind turbines are connected with powerconverters to the grid, thereby decoupling the generator from the grid frequency. Exceptionsare some older wind turbines and doubly fed induction generators which act very similar toasynchronous motors. Furthermore photovoltaics (PV) do not have any rotating mass. Thismeans that the inertia contribution from PV and most wind turbines is negligible.

When a frequency event occurs, a synchronous machine will automatically inject or absorbkinetic energy into or from the grid to counteract the frequency deviation. This behavior iscalled inertial response because the inertia dampens the frequency changes. For a drop of thefrequency it has to be considered that the generator will release an amount of energydepending on the nadir (minimum) of the frequency. So until the frequency fnadir is reachedthe synchronous generator will only supply the available energy:

( ) ( )( )22,, 22

21

nadirnomnadirkinnomkinav ffJEEE pp -××=-= (3)

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Taking this into account the available inertia time constant of each synchronous generator Havcan be defined as a function of the nadir frequency:

gennom

nadirnom

gen

avav H

fff

SEH ×

-== 2

22

(4)

Hence the generator injects more kinetic energy if the nadir of the frequency is lower. Amaximum deviation of 1 Hz in a 50 Hz system for example will lead to an available inertiaand thus energy of about 4 % of the nominal value.The system inertia is the combined inertia of the rotating masses of all synchronousgenerators and turbines. The system inertia time constant (Hsys) is given by the weightedaverage of the generating units:

å×

=i sys

igenigensys S

SHH ,, (5)

System inertia can have large variations during a day or season, depending on the operatingpower plants. For example in the night only a few generators are operating and the inertia ofthe system reduces. The lower the system inertia of a power system, the higher is thesensitivity of the grid frequency to abrupt changes in generation and load patterns. The systeminertia determines the ROCOF (rate of change of frequency):

fHS

PdtdfROCOF

syssys

××D

==2

1(6)

The maximum active power infeed of an individual generator can be calculated with (7) for agiven ROCOF.

maxmax

2dtdf

fH

SPnom

genn ××= (7)

It can also be seen that when the inertia constants are based on the apparent power of eachindividual generator the equation for Hsys can be simplified to an addition of the inertiaconstants of the generators.

The simulation results of the impact of different Hsys on a test grid are as shown in Figure 1.Various inertia constants were applied and the behaviour of the system was recorded, whereasthe frequency event was consistent with the loss of the largest generator in the grid.

49,0

49,2

49,4

49,6

49,8

50,0

50,2

0 5 10 15 20

H = 11 sH = 16 sH = 21 sH = 36 s

Frequency (Hz)

Time (s)

Figure 1: Impact of different system inertia (H) on the frequency

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With lower inertia of the system the transient frequency drop is much lower and faster (higherROCOF) - see blue solid line. On the contrary with higher system inertia (synonymous withmore conventional generators in operation) the frequency drop and ROCOF are much lower -purple line.It can thus be inferred that an increased renewable energy penetration, or in other words areduction of the overall system inertia in the grid, produces a significant impact on theROCOF in the case of network events such as loss of generation, load loss, sudden large loadchanges etc.In the UCTE/ENTSO-E operation handbook [6] there are stepwise security measures definedin case of frequency events. Firstly at 49.8 Hz (deviation of 200 mHz) the quick-start powerplants should be connected to the grid. Secondly when the frequency reaches 49 Hz, therecommendation is to perform the 10-20 % load shedding. Thirdly at 48.7 Hz, further 10-15 % load shedding shall be performed. Additional 10-15% load shedding is performed at48.4 Hz. If all these measures do not regain stable operation of the system and the frequencyreaches 47.5 Hz, then all power plants have to be disconnected from the grid without any timedelay to safeguard their auxiliary power supplies.The primary task of the Frequency Stabilizer is to reduce the initial ROCOF in the system andthe frequency drop in order to avoid any of these security measures (e.g. load shedding) oreven to avoid blackouts by fast injecting of active power into the grid (providevirtual/synthetic inertia) during transient events in the system. An example of one suchapplication is shown in the following section.

Additionally the Frequency Stabilizer is also capable of absorbing active power in scenariosof sudden fluctuations of wind/PV generation or load rejection which may cause the system totend to dangerous over frequency scenarios which are not desired.

3. SVC PLUS FREQUENCY STABILIZER

The SVC PLUS Frequency Stabilizer comprises a STATCOM and supercapacitors for energystorage. The advanced STATCOM is a power electronics based modular multilevel converter(MMC), also known as SVC PLUS® [7], for reactive power generation. A typical single linediagram is shown in Figure 2 – grey part.

B CA

Short time powerintensive storage

Frequency Stabilizer (FS)STATCOM (SVC PLUS)

Figure 2: Single line diagram of a typical configuration of SVC PLUS Frequency Stabilizer

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Despite its main application as a flexible ac transmission system (FACTS) device, it is alsoemployed for voltage balancing for grid fed railroad lines and flicker mitigation of electric arcfurnaces.

The reactive power output of the MMC Scon in (8) is limited by the current carrying capabilityIrated of the employed power electronics which is an insulated gate bipolar transistor (IGBT)type. In order to increase the output power capability, the connection voltage Ubus needs toincrease which can achieved by increasing the number of series connected sub-modules. TheSVC PLUS Frequency Stabilizer has two main applications: supporting reactive and activepower, whereas providing active power during frequency events is prioritized [8].

ratedbuscon IUS 3= (8)

The Frequency Stabilizer (blue part) embedded into the sub-modules in the construction ofthe device is as shown in Figure 2. It is based on a very fast short time power intensivestorage which supports the grid at frequency events with full rated converter power. Based onthe demands for the frequency support (short time, high power), supercapacitors have beenchosen as the storage medium [8] and [9].

In Figure 3 the layout of a SVC PLUS FS Level/tower has been shown. On the right side thesetup in the testing facility is shown. Figure 4 shows an example for the layout of an entireSVC PLUS FS station rated at 50 MVA. The grid connection has been assumed to be at highvoltage level (132/220 kV).

DC inductances

SCM‘s

PM1

PM2

DC capacitor

Figure 3: Layout of SVC PLUS FS Level/tower (SCM – supercap module, PM – powermodule) and two towers in a testing facility

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Figure 4: Example for the layout of a 50 MVA SVC PLUS FS station

The main electrical parameters of the SVC PLUS Frequency Stabilizer are given in Table 1.

Table 1: Main Electrical Parameters of SVC PLUS FSRated power 50 MVARated active power 50 MWStored energy 400 MJInertia of SVC PLUS FS (H) ≈8 s

Currently there are also other storage technologies in discussion and being tested in pilotprojects around the world, mainly focussing on battery storages with high energy densities. Acomparison of the SVC PLUS Frequency Stabilizer and typical battery storage applicationsfor fast frequency response (full power for duration up to several seconds) is given in Table 2.

Table 2: Investment and Footprint of SVC PLUS Frequency Stabilizer and Typical BatteryStorage for Applications of Frequency Response

50 MVA solution SVC PLUSFrequency Stabilizer

Typical battery storage

Investment (p.u.) 1 3-7Footprint (p.u.) 1 7-10

As can be seen from the table, investment costs and footprint requirements are much lower forthe Frequency Stabilizer utilizing supercapacitor technology compared with battery storage.Coupled with advantages of high power density and modularity, the supercapacitortechnology was chosen for the analysed specific grid applications.

4. APPLICATION IN THE TRANSMISSION GRID OF ALL-ISLAND

The All-Island power transmission system represents the synchronous operation of Republicof Ireland (EirGrid) and of Northern Ireland (SONI). The All-Island transmission grid is avery good example of future developing grids and changes in the transmission grids withspecial focus on transient frequency behaviour. The governments have set an ambitious target

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of 40 % of renewable power integration for the Ireland (All-Island) power system by 2020.The primary target however would continue to be to enable secure, efficient and mostimportantly reliable operation of the power grid. In order to maintain these targets, detailedtechnical analyses are already in progress to propose new solutions. These solutions areoriented to both new products as well as additional system services.

A representation of the planned Ireland (All-Island) transmission grid for the year 2022 wasmodelled based on [10]. The modelled grid comprises of over 400 lines, 300 substations and150 loads – see Figure 5. The modelled transmission system of Ireland (All-Island) consists of4 different transmission voltages (400 kV, 275 kV, 220 kV and 110 kV).

Figure 5: Model of the transmission grid of Ireland (All-Island)

To highlight the performance of the SVC PLUS FS in the grid, a test case involving bothtransient frequency as well as the transient voltage events are considered as shown inFigure 6. The operation point of the system is taken to be summer 2022 – low load case. Thetransient voltage event is modelled as a three phase short circuit fault in the grid at time t = 2 sand is cleared successfully after a further 150 ms. The frequency event follows at time t = 10 swhich is defined as a loss of a 500 MW generation unit in the grid.

0

0,2

0,4

0,6

0,8

1

1,2

49,0

49,2

49,4

49,6

49,8

50,0

50,2

50,4

0 5 10 15 20

System Frequency

Voltage at the PCC

Frequency (Hz) Voltage (pu)

Time (s)

Figure 6: Frequency of the system and voltage at the PCC with SVC PLUS FS

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The response from the SVC PLUS FS device to the transient events is as shown in Figure 7.The device is capable of providing reactive power support in order to aid the AC systemduring the transient voltage event. Clearly the device reacts very quickly to voltage deviationsand provides its rated reactive power (limited by the residual voltage) to the system. Thereaction time post fault is also quick, thus the voltage is brought back to its set-point.

-10

0

10

20

30

40

50

60

0 5 10 15 20

Active powerinjection (MW)Reactive powerinjection (Mvar)

Power (MW/Mvar)

Time (s)

Figure 7: Response of the SVC PLUS FS for both voltage and frequency event

Continuing on to the transient frequency event, a loss of 500 MW of power causes thefrequency to cross its defined deadband and thus triggers the active power injection from theSVC PLUS FS device as shown in Figure 7. Since the reactive power controller is a very fastcontroller, the small deviation in voltage during the transient frequency event is also actedupon and there is a corresponding increase of reactive power injection of approx. 20 Mvar.The corresponding increase in active power injection is countered with a steady decrease inthe reactive power. However, immediately the active power injection reaches its rated valueof 50 MW, the priority limiter limits reactive power to zero. The duration of the active powerinjection continues as long as possible until the available energy falls below its lower set-point.

Figure 8 shows the activation of the ∆f loop in the active power control, and hence the activepower injection follows the frequency after 15 seconds. However this is independent in caseof a ROCOF detection, where the df/dt loop as shown in Figure 3 takes over and rated activepower is injected into the grid within 200 ms of the detection of the fault and continues untilthe available energy is consumed. There is a clear improvement in the frequency with theSVC PLUS FS device as shown in Figure 8, where the frequency nadir is improved by app.100 mHz.

48,9

49,1

49,3

49,5

49,7

49,9

50,1

0 2 4 6 8 10 12 14

w/o SVC PLUS Frequency Stabilizer

with SVC PLUS Frequency Stabilizer

Time (s)

Frequency (Hz)

Figure 8: Comparison of the frequency with and without SVC PLUS FS in the Ireland (All-Island) transmission grid

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The average of the ROCOFs measured is used to represent the average ROCOF of the entiresystem. These values are measured and averaged over a 500 ms window (moving averagefilter). A reduction in the ROCOF from app. 0.45 Hz/s to 0.4 Hz/s has also been achievedwith the application of only one 50 MVA SVC PLUS FS - see Figure 8. This can play acrucial role when setting the ROCOF protection.To elaborate on the impact of higher ROCOF values in the future from a machine perspective,in terms of mechanical stress evaluation, higher torque values result from increasing ROCOFvalues. For example the current ROCOF setting in Ireland is 0.5 Hz/s, for which thecalculated maximum torque is 140 % [4]. This also remains a crucial factor to be consideredespecially when possible solutions such as increasing the threshold values in the existingROCOF relays are presented and discussed.In the above cases discussed the SVC PLUS FS aids the initial inertial response provided bythe synchronous generators in the grid. Since the system inertia of the grid in the future isexpected to be lower, the risk of ROCOF exceeding the current threshold looms large forsmall and medium islanded systems.

5. CONCLUSION

The SVC PLUS Frequency Stabilizer presents a solution for the future grid with morerenewable and less conventional generation and improves both voltage and frequencystability. The device emulates synthetic inertia, utilizing the energy stored in thesupercapacitors, releasing it in case of transient events when the frequency thresholds arereached, thus ensuring a continued and safe operation of the grid. The initial ROCOF can bereduced owing to the fast control response to deviations in the frequency, thus avoidingpreventive measures such as load shedding or even possible blackouts.In comparison to other solutions feasible for frequency support, the application of SVC PLUSFrequency Stabilizer with features such as modularity and high power intensive supportparticularly in case of fast frequency response, offers economic as well as technical benefits.

ACKNOWLEDGMENT

This work was supported in part by the German Federal Ministry for the Environment, NatureConservation, Building and Nuclear Safety Nr. 0325663.

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