METHOD OF VOLTAGE SITUATION ASSESSMENT IN THE...

19Method of Voltage Situation Assessment in the Transmission Grid Taking Into Account

the Regulation Technology Operation

METHOD OF VOLTAGE SITUATION ASSESSMENT IN THE TRANSMISSION GRID TAKING INTO ACCOUNT THE REGULATION TECHNOLOGY OPERATION

Jacek Jemielity / Power Engineering Department, Gdańsk Branch Ksawery Opala / Power Engineering Department, Research Institute, Gdańsk Branch

1. INTRODUCTION. OVERVIEW OF PRACTICAL METHODS OF VOLTAGE SITUATION ASSESSMENT IN A TRANSMISSION GRID

The majority of system malfunctions, regardless of their causes, develop even further due to the loss of the voltage stability reserve. Taking into account the above, in the 1990s, different voltage situation assessment methods were used and developed with a view to informing the operator about the system stability reserve.

There are several methods and indices for voltage stability testing. They differ as far as calculation me-thods and results obtained are concerned. Selection of a given method may provide results related to voltage stability of individual nodes or area stability. Some methods make it possible to determine the stability reserve and others define the stability reserve indirectly. Numerous methods involve multi-variant calculations and re-quire painstaking analyses of results obtained.

The practically used power network voltage safety assessment methods are: modal analysis, QV and PV nose curves method and continuation method. These methods are based on standard power distribution cal-culations, which means they require an up-to-date model of the transmission and distribution grid. They are time-consuming and only the continuation method being a slight simplification of the QV and PV curves method may be used for determining the system voltage stability reserve in a quasi-online mode. See publication [1] for details.

So far, operators have been practically using several indices facilitating indirect system stability reserve assessment. One of them is the reactive power reserve index (for generators, capacitor batteries, etc.) used, among others, in the BPA system in North America [2]. This index ensures ongoing monitoring of the summary range of reserves available in the system. A similar indicator called the Voltage Stability Index (VSI) has been used in the Italian ENEL system [3]. This index has been complemented with the factor taking into account the derivative of changes in the available reactive power reserves.

(1)

where:qi(t) – momentary level of reactive power generation ρ – weight coefficient

Abstract

Selected functions of the Area Voltage Regulation System (SORN) have been implemented in the Bydgoszcz Power Dispatch Centre (ODM) where the method of volt-age situation assessment described in this paper has been utilised. The method of automated control over the ARNE and ARST regulation systems operation elaborated in the SORN system aims at supporting dispatchers during the ongoing regulation status assessment, danger conditions detection and counteracting the system malfunction re-sults. This aim has been achieved by utilising the method of voltage situation assessment in the transmission grid which used normalized numerical indices describing the regulation nodes operation point. Next, the analysis of

trends related to changes in these indices was used for detection with time advance of adverse changes in the voltage profile or reactive power reserves level. The described method employs the fuzzy inference system in order to generate prompts for Power Dispatch Centre (ODM) dispatchers. The limited SORN system functions fulfilled presently are to enhance the power system safety using the existing ARNE and ARST regulation systems in-frastructure. Together with the latest modifications of the ARST system algorithms, the SORN system’s basic duty is to prevent worsening or accelerating a voltage malfunc-tion, which results from transformer regulation not being blocked on time.

ttq

tqtVSI iii

)()()( �

20

Such indices are individually assigned to each Italian grid subarea for which the so-called pilot node is determined, i.e. a reference point to the area voltage regulation. Thanks to this, the local reactive power mana-gement characteristics have been taken into account.

Another interesting indicator for the approximate stability reserve assessment is the voltage index Li [4] calculated for each grid node:

(2)

where:Vi – voltage of node i calculated for the up-to-date grid modelV0i – voltage of node i with power distribution for zero intakesIt does not inform about the value of the voltage stability reserve; however, it shows individual nodes suscep-

tible to losing stability. In order to calculate the index L, a grid model is necessary but the calculations are simplified,i.e. there is no problem related to the additional load distribution as is the case in the nose curves method.

2. ASSUMPTIONS FOR THE VOLTAGE ASSESSMENT METHOD IN THE TRANSMISSION GRID INTEGRATED WITH REGULATION TECHNOLOGY

The method described in this paper has been implemented in the SORN system taking into account the specific character of the northern National Power System (KSE) area subordinated to the Bydgoszcz Power Di-spatch Centre (ODM). The transmission grid in this area has nominal voltage equal to 400 kV and 220 kV. There are two pumped storage power plants (Żarnowiec and Żydowo) operating in the grid, the HVDC circuit and 11 high-voltage power stations. Both power plants are equipped with ARNE regulation systems. 10 high-voltage stations are equipped with ARST systems and 4 stations are equipped with manually operated capacitor bat-teries. From the point of view of the operator’s practice, the characteristic feature of the area is the intensive utilisation of regulation systems resulting in frequent daily corrections of setpoints set for regulators.

Lack of ongoing availability of the full grid model for the Bydgoszcz Power Dispatch Centre (ODM) (lack of measurements from the 110 kV grid) limits the practical usage of known methods of calculating the voltage stability reserve in the direct online mode. Bearing in mind the fact that only one Żarnowiec power plant may practically influence the ODM area voltage level, the type index (1), taking into account the reactive power re-gulation reserve for this power plant, is a natural method of the ODM area voltage situation assessment. While designing the SORN system for the Bydgoszcz Power Dispatch Centre (ODM), the main emphasis was put on the automatic supervision of the voltage regulation systems operation in order to enhance their operating safety in the case of a risk of a voltage type malfunction.

3. VOLTAGE SAFETY INDICES

In order to facilitate an objective assessment of values monitored within the SORN system, three non--dimensional voltage safety indices have been suggested:

1. WQ – index of reactive power regulation range utilisation in power plants2. WU – voltage index for generating and transmitting nodes defining the deviation from the desirable

voltage profile3. WZ – index of the transformer voltage transformation ratio regulation range utilisation.The operating secondary regulation systems, according to assumed regulation criteria and settings, auto-

matically react to load variations and disturbances in the power system operation. Indices WQ , WZ and WU should facilitate the assessment both of the ongoing value of monitored quantities and the predicted changes in these values on a time horizon from a few to over a dozen minutes. Thus, the currently registered index changes and the resulting change trend are important. Safety indices, which are calculated taking into account the change trend reach, in advance, the W = 1 or W = -1 criterion values thus allowing time for a proper reaction of the dispatcher.

Treating the index run, determined on the basis of momentary values of monitored quantities, as a random process, the index W value, taking into account the change trend, is calculated as a sum of two components:

i

ii V

VL

0


21

W = w(t) + Δw(t) (3) where:w(t) – estimated index value on the basis of the run from moment tΔw(t) – value increase resulting from the continuation of the change trend to moment t, as long as this

tend exists, i.e. has a sufficiently high determination coefficient.Since component w(t) in formula (3) is an estimated value, it causes averaging and smoothing of the index

W run and makes it resistant to momentary disturbances. On the other hand, the incremental component Δw(t) represents the dynamics of changes, including that caused by the regulation system operation.

The experiences resulting from the SORN system operation show that the trend must be determined with a variable time horizon adapted to the shape of the run. We assume that in (3) the predicted increment Δw(t) will be present for the time interval for which the most reliable trend has been determined. Fig. 1 shows two runs: voltage test run (Voltage U) and predicted value of this voltage determined on the basis of analysis of the last 20 minutes of the test run (U + trend). The runs shown in the figure illustrate the operation of the trendanalysis algorithm used.

In sections A-B and C -D in the figure, this algorithm uses an increasing time interval as a basis for cal-culating the change trend. Thus, the voltage change increases. It guarantees detection of slow signal changes lasting a dozen or so or several dozen minutes. In the B-C and D-E sections, following the incremental change of the test run derivative, a quick correction is noticed, i.e. the algorithm finds a more reliable trend determinedon the basis of only the last test signal samples.

3.1. Index of reactive power regulation range WQ utilisationThe phenomenon of voltage instability is often, although not always, accompanied by the earlier depletion

of the generating units reactive power regulation range. Reactive power reserves level monitoring is a basic function of voltage stability monitoring systems. The suggested index of reactive power regulation range utilisa-tion is a standard value of reactive power generation relating to the minimum reactive power Qmin or maximum reactive power Qmax of generating units operating on connected systems of switchgear busbars – 400, 220 or 110 kV. The regulation range centre is always suggested as a reference level. The value of index WQ is definedby the formula (4):

WQ = q(t) + Δq(t) (4)

Fig. 1 llustration of the tend analysis algorithm operation

Method of Voltage Situation Assessment in the Transmission Grid Taking Into Account the Regulation Technology Operation

Voltage U U + trend

Time [s]

Volta

ge [

kW]

22

where:q(t) – estimated ongoing value of the standard reactive power generation, i.e. value Q(t) in the generating

node, related to the difference between the power limit value Qmin. or Qmax. and half of the regulation rangeΔq(t) – predicted index increase provided the existing trend is continued:

(5)

Tab. 1. Criterion values of index W

Value WQ -1.0 0.0 1.0

Node status Maximum utilisation of the regulated capacitive power range Q = Qmin.

Centre of the reactive power regulation range(Qmin. , Qmax)

Maximum utilisation of the regulated inductive power range Q = Q

3.2. Voltage index WUFor the transmission grid, in given conditions an optimum voltage profile is assumed – the voltage set for

the secondary regulation system in connected busbar systems. The suggested voltage index WU is a standard node voltage deviation from the current set voltage related to the voltage of the overvoltage or undervoltage interlock of the regulation system. The advantage of this solution consists in connecting the calculated value of WU with current settings (set voltage) and parameters of the regulation system (interlock).

WU = u(t) + Δu(t) (6)

where:u(t) – estimated ongoing value of the standard voltage deviation related to the value of the closest voltage

interlockΔu(t) – predicted index increase provided the existing trend is continued:

(7)

(8)

(9)

U – voltage of connected busbar systems Uset – set voltageU BLP – undervoltage interlock voltage UBLN – overvoltage interlock voltage

Tab. 2. Criterion values of index WU

Value WU -1.0 0.0 1.0

Node statusVoltage deviation on the undervoltage interlock level

Lack of voltage deviation from the regu-lation system set value

Voltage deviation on the overvoltage interlock level

)()(for

)( )()()( .minmax2

1

.minmax21

minmax21

QQtQQQQQtQtq

.

set

setBLN

set )(for)()( UtUUUUtUtu

set

BLPset

set )(for)()( UtUUUtUUtu

set)(for0)( UtUtu


23

For the transmission grid, admissible regulated voltage limits for the switchgear are assumed. The ma-jority of ARST transformer regulation systems control the voltage in the 110 kV grid. The voltage index for the 220 kV or 400 kV side of a transformer must be determined by calculating the set value of the 110 kV side pro-portionally in relation to the admissible set voltage values (fig. 2).

If the regulation system controls the transformer reactive power flow or the automatic regulation is bloc-ked and done manually, in order to calculate index WU, for the 110 kV side the voltage equal to the measured voltage is assumed as if the voltage regulation has just finished. In this case index WU = 0 (unless the possible change trend is taken into account).

3.3. Transformer regulation range utilisation index WZThe suggested transformer regulation range utilisation index WZ is an auxiliary index which consists in

facilitating the objective assessment of the transformer automatic regulation influence on the current voltagestatus. Index WZ is a normalized deviation of the transformer tap switch location from the “middle” tap by which the nominal voltage transformation ratio is defined. Its criterion values WZ = 1 and WZ = -1 are reached in the extreme transformer switch taps for the minimum and maximum voltage transformation ratio, regardless of the transformer construction1.

WZ = z(t) + Δz(t) (10)

where:z(t) – estimated value of the standard deviation of the tap from the middle location related to the extreme

tap positionΔz(t) – predicted index increase provided the existing trend is continued:

Fig. 2. Method of calculating the 110, 220 and 400 kV set voltage to determine index Wu

1 Two kinds of tap switches are used: the voltage ratio increases or decreases together with the increasing tap number.

Uset. min.Uset. max.


Uset.

Uset.

Uset.

Uset.

24

(11)

(12)

z(t) = 0 for Z(t) = Z0 (13)

where:Z0 – middle tapZmin., Zmax – extreme taps

Tab. 3. Criterion values of index Wr

Value WZ -1.0 0.0 1.0

Tap status Extreme tap of the minimum trans-former 110kV side voltage

Middle tap number, i.e. neutral posi-tion of the transformer tap switch for the nominal voltage transformation ratio

Extreme tap of the maximum transformer 110kV side voltage

Index WZ must be determined excluding shorted taps for which the transformer voltage transformation ratio does not change.

3. 4. Node status monitoring by means of indicesUsing standard indices related to regulation systems facilitates the node status analysis. It is noticeable

that:If the generating units regulated power Q reserve is present, i.e. -1 < WQ < 1, voltage index WU for

a switchgear will be equal to zero or will be close to zero (voltage deviation within the regulation dead band limits). If power Q reaches the limit value Q min. or Qmax, voltage index WU of the switchgear with generation may start changing.

If the regulation system receives a new setpoint, a given index WU for the switchgear will be temporarily different from zero. If, at the same time, other indices, i.e. WQ WU or Wz do not reach criterion values, it means that the regulation system is during regulation (activation delay).

In the case of a station with a 220/110 or 400/110 kV/kV transformer, one may notice that:• if the regulation system regulates the 110 kV voltage, index WU/110 for the 110 kV switchgear will be

equal to zero (deviation within the dead band zone limits) until the extreme tap is reached if regulation is blocked due to, e.g. too low 220 kV or 400 kV voltage. In such a case WZ reaches the criterion value 1 or WU/220 or WU/400 value -1;

• assuming the criterion values by indices WU/220 or WU/400 and Wz for a transformer station may constitute the signal to block the ARST system automatic regulation, change a criterion or correct the setpoint.

The above conclusions enabled the authors to create the Fuzzy Inference System (FIS) generating prompts for Power Dispatch Centre dispatchers. A system built on the basis of fuzzy logic and fuzzy implications tables is often used to create expert systems and knowledge databases. See below for necessary theoretical bases of the method and its practical application in the SORN system.

0

0

0 )(for)()( ZtZZZZtZtz

max

0

min.0

0 )(for)()( ZtZZZtZZtz


25

4. FUZZY INFERENCE SYSTEM BASED ON INDICES WU, WQ AND WZ

The bases of inference used by the authors in relation to the Bydgoszcz Power Dispatch Centre (ODM) area are safety indices WU and WZ values calculated on the basis of data from ARST systems and the value of 400 kV voltage measurement in the Słupsk power station. Input signals (prompts) relate to the set voltage values U110 set (for transformers operating in the 110 kV voltage regulation criterion) or 110 kV side voltage (for transformers in manual regulation) or activation/deactivation of a battery section.

For power plants in the Power Dispatch Centre area and in its direct vicinity, WQ indices monitoring has been used. In the Żarnowiec power plant the value of the index has been used to generate prompts related to utilisation of the HVDC circuit capacitor battery in the Słupsk power system.

4.1. Theoretical bases for the fuzzy sets method in control and expert systemsFuzzy systems are automatons using fuzzy logic laws in order to take decisions in uncertain conditions,

e.g. in the case of lack of a precise mathematical model for the examined phenomenon or if the model is too complex [5]. Such automatons have a certain knowledge database in the form of a set of inference rules which come from an expert who creates the system. The effectiveness of the fuzzy inference system depends chiefly on the quality of the expert’s knowledge and secondly on the correctness of its modelling by means of fuzzy logic.

A typical fuzzy inference process takes place in three stages:1. Making the input values fuzzy, i.e. conversion of real values of input signals into linguistic variables.2. Using fuzzy implications, i.e. analysis of the pre-defined set of relations between fuzzy linguistic terms of

input signals and the output signal.3. Specifying (defuzzification) the output value, i.e. conversion of the blurred value of an output signal into

a given value. Fig. 3 shows a substantially simplified schematic diagram of the fuzzy inference system used in the SORN

system. We have here three input linguistic variables WQ, WU and WZ which are processed into fuzzy sets consti-tuting premises for inference rules included in the database of rules. In the output, we obtain output linguistic variables, i.e. system prompts:

• analogue (suggested 110 kV voltage);• two-stage (warning and alarm signalling and discrete prompts for capacitor batteries control and swit-

ching ARST systems operation modes).

The fuzzy inference system generates a single number in its output, i.e. the result of the input signals specifying block operation. The authors have attempted to obtain the effect of reconstructing the dispatcher’s intuitive actions. The system operation has been tested on real runs registered during one week before the well known malfunction on 26.06.2006 and during the day of its occurrence.

Fig. 3. Schematic diagram of the fuzzy inference system

If..., then...If..., then...If..., then...

...

...

...If..., then...


Making inputs fuzzy

Inference rules database

Specifying outputs

Warning Signal

Capacitor Banks Operation

26

4.2. Making input signals fuzzyIn the case of the Wu index values of the lower and upper transformer voltage (W110 and W220/400 respecti-

vely) five linguistic expressions have been assumed which define the index value ranges: very low, low, medium, high and very high. Fig. 4 below presents the described expressions and their respective WU index ranges as functions of fuzzy sets affinity corresponding to the linguistic variable describing index WU.

In the case of safety index WQ of the power plant, three linguistic expressions describing the index value ranges have been assumed (fig. 5): low, medium and high.

One must pay attention to the fact that, thanks to assuming a proper form of the safety indices related with the interlocks and regulation systems setpoints, the inference system input signals become permanently fuzzy for changeable system operation conditions. For example, assuming a new voltage profile for an incomple-te grid configuration naturally causes the WU indices re-scaling and the prepared inference rules remain valid.

Fig. 4. Example of the index WU affinity function Fig. 5. Example of the power plant WQ index value becoming fuzzy

2 Criterion D – voltage regulation of the transformer lower voltage side 110 kV.

4.3. Inference rules for a transformer stationFor transformers working in the transmission grid, when the ARST regulation system works with criterion D2,

a rule has been assumed stating that the lower set voltage 110 kV – Uset is the control value. If the control value regulation system is deactivated, it is simply the 110 kV – U110 voltage. Following the process of defining the output signal a prompt adequate to the ongoing ARST system operation status is generated, as long as realisation of this prompt requires changing the location of the tap switch. Tab. 4 shows the set of rules depending on the W110 and W220/400 input variables.

Tab. 4. Inference rules used for defining prompts related to UDzad. for a typical transformer station

W220/400W110

very low low medium high very high

very low 0 0 2 2 2

low 0 0 1 1 2

medium -2 -1 0 1 2

high -2 -1 -1 0 0

very high -2 -2 -2 0 0

In the process of defining the output signal, values {-2, -1, 0, 1, 2} are converted into the set voltage from the <Uset min., Uset max > range (fig. 2).


very low low medium high very high low medium high

WU

27

4.3.1. Transformer station – recognising emergency conditionsThe simplest emergency condition detection in a power grid may be achieved by means of tab. 5. See

below for W110 and W220/400 coefficient combinations explicitly predicting voltage problems in a given transformerstation and its immediate vicinity.

Tab. 5. Inference rules used for defining prompts related to emergency conditions for a typical transformer station

W220/400

W110


very low-2 0

low

medium -1 0 1

high0 2

very high

Meaning of output signals:

– 2 area of voltages which are too low showing the power deficit in a given supply point

2 area of voltages which are too high showing the power surplus in a given supply point

– 1 area of voltages which are too low showing a possible approaching system malfunction

(voltage collapse)

1 area of voltages which are too high showing a possible approaching system malfunction

0 area of ARST control engineering standard operation in a given station

0 area where ARST condition is ambiguous (e.g. following a setpoint change).

4.3.2. Transformer station with a capacitor bankIn the case of a station with a capacitor bank installed, the set of input values included in tab. 4 is still

valid. In tab. 6 additional inference for capacitor banttery activation or deactivation has been introduced.

Tab. 6. Inference rules for a transformer station with a capacitor battery

W220/400

W110


very low 2 2 2 0 0

low 2 1 1 0 0

medium 2 1 0 -1 -2

high 0 0 -1 -1 -2

very high 0 0 -2 -2 -2

Meaning of output signals:2 – activate all sections1 – activate a section0 – no changes– 1 – deactivate a section– 2 – deactivate all sectionsThe prompt related to the capacitor bank control is valid as long as the input signal values do not change.

In this case, the regulation effect is immediate and should influence the inference rule input signals.


28

4.3.3. Słupsk 400 kV station (SLK – a special case of a station equipped with a capacitor batteryThe SLK station differs from other stations mostly in the fact that the HVDC circuit is connected to its

400 kV switchgear and is not equipped with the ARST regulation system. Indices W110, W220/400 and Wz are not calculated for this station. Taking the above into account, a separate fuzzy inference table has been developed. The capacitor battery operation in SLK (2 x 95 Mvar and 95 Mvar filter) is strongly correlated with the Żarnowiecpower plant operation (reactive power generation). Due to the above, the variable input voltage has been defi-ned as the voltage in the 400 kV switchgear in the SLK station and index WQ for the Żarnowiec power plant.

The capacitor battery activation or deactivation moment must take into account the HVDC circuit opera-tion status. While importing or exporting power, per each 200 MW of power flowing though the HVDC circuit,approximately 80 Mvar of reactive power is reserved. If power is not transmitted through the HVDC circuit, a battery operation may be controlled as in other typical stations. Taking the above into account, following the filter activation, practical voltage regulation with both batteries is possible only by power transmission in the-200 MW < PHVDC < 200 MW range. Prompts are not generated for other transmission values. On the basis of operational experiences, it has been assumed that if the SLK400 > 419 kV level is achieved, one battery is deactivated, or if none of them is operational, a choke is activated. However, when voltage SLK400 < 391 kV, one battery is activated. Additionally, before a battery is activated, the set voltage in the Żarnowiec power plant is usually decreased by a few kV.

See tab. 7 for inference rules assumed for capacitor battery operation control in the SLK station.

Tab. 7. Inference rules for capacitor battery operation in SE SLK

Item. SLK – HVDC SLK400 ZRC – WQ Resuly

1

-200 MW < PHVDC <

200 MW

low

low

activate battery

2 medium no changes

3 high deactivate battery

4 low

medium

activate battery

5 medium no changes

6 high deactivate battery

7 low

high

activate battery

8 medium activate battery

9 high no changes

4.3. 4. Stations adjacent to power plantsTwo stations this type, ZRC and ZYD, operate in the Bydgoszcz Power Dispatch Centre (ODM) area. Both

stations have all generating units connected to one switchgear. In ARNE systems, the reactive power control is achieved by setting the voltage in the switchgear to which the generation is connected. In the ZRC station, the ARNE system maintains the UG - 400 kV voltage and in the ZYD station the voltage is UD - 110 kV. Set voltage for both ARNE systems is not maintained only after the generators reach the reactive power generation limits. Taking the above into account, monitoring of WQ indices of both power plants has been suggested. These indices define the predicted regulation range and provide sufficient information for the power plant operation asses-sment from the point of view of the reactive power generation. In the SORN system, the indices are presented on a display with two pointers (fig. 6). The black pointer shows the current index value and the red one showsthe WQ. value resulting from the trend calculated on a current basis.


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REFERENCES

5. SUMMARY

The presented method of voltage situation assessment in a transmission grid makes it possible to identify emergency conditions and generate prompts for dispatchers in order to minimize the malfunction results. The unquestionable advantage of this method is the possibility to use it on the basis of measurements from the transmission grid without the need for creating an up-to-date grid model. Another advantage is the fact of using non-dimensional safety indices taking into account, firstly, the status of the ARNE and ARST regulation systems operation and settings and, secondly, the trend of changes taking place in the power system. The method has been used in the SORN system operating in the Bydgoszcz Power Dispatch Centre (ODM). Analysis of the me-thod results, one year after its implementation, has enabled the authors to confirm its effectiveness. Results of precise analyses of registered data relating to the SORN system operation over a longer period of time will be presented later.

1. IEEE/PES Power System Stability Subcommittee Special Publication, Voltage stability assessment, procedures and guides, January 2001.

2. Taylor CW., Ramanathan R., BPA Reactive Power Monitoring and Control following the August 10, 1996 Power Failu-re, Invited paper, VI Symposium of Specialists in Electric Operational and Expansion Planning, Salvador, Brazil, 24-29 May 1998.

3. Corsi S., Pozzi M., Bazzi U., Moncenigo M., Marannino P, A simple Real-time and on-line voltage stability index under test in Italian Secondary Voltage Regulation, Paris, Cigre session 2000.

4. Hatziargyrion N.D., Cutsem T. van (editor), Indices predicting voltage collapse including dynamic phenomena, tech-nical report TF 38-02011, Cigre session 1994.

5. National Instruments, LabView – PID and Fuzzy Logic Toolkit User Manual, June 2009.

Fig. 6. WQ values display in the ZRC station


Q power regulation range utilisation: Value Trend

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