How It Works Real Time Ampacity

Mar 2011

Mar 2011 2

RT Ampacity Determination Two equations :

1. Sag – temperature The State Change Equation

detailed f.e. in Kiessling et al Overhead Power lines,

Springer 2003, page 546, section 14.5

2. Conductor temperature – line loading & environment conditions

The IEEE thermal model IEEE Std 738 - 2006

Dynamic Rating (Ampacity)

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1. State Change Equation (8f2 / 3L2) - (pL2 / 8f ES) - aT = Cte (*)

with variables: f = sag, T = conductor mean temperature

and constants related to physical properties of the line :   thermal expansion coefficient (a)   Young modulus (E)   conductor cross section (S)   conductor weight per meter (p)   ruling span length (L)

These constants can be known with relative good accuracy and do not tend to change in time.

(*) SI Units

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1. State Change Equation - Known State

To obtain a useful formula the constant must be determined from a known state (f,T). This “Known State” is obtained as follows:

•  The sag is obtained directly from the fundamental span frequency, with very good accuracy not needing any other parameters or any calibration. We can therefore consider that the sag is measured “directly”. (See HIW Sag)

•  The Conductor Mean Temperature is obtained using the IEEE dynamic thermal model. This model suffers from possible inaccuracies from its parameters, mainly the environmental data (wind speed, wind direction, solar radiation, ambient temperature).

•  Good accuracy of the “Known State” is obtained by observing during best, stable conditions. This is during night when there is no direct solar radiation, with stable wind (over minimum level). Several nights are observed.

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1. State Change Equation – Known State

IEEE Thermal Model

Ambient (*) Conditions

Line Loading

Conductor Constants

Conductor Mean Temperature

Weather Station TSO

Constant (Known State)

Ruling Span Sag

Ampacimon Monitor

(*) Stable observation conditions

State Change Equation

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2. IEEE dynamic thermal model The IEEE model is a differential equation due to the thermal inertia of

the conductor.

Joule losses + Solar Radiation = Convection + Radiation

The parameters of the IEEE model are: •  Environmental parameters

–  Ambient temperature –  Solar radiation –  Wind speed & direction

•  Conductor constants –  Thermal conductivity –  Specific Heat –  Emissivity and absorbtivity

•  Line loading –  Line current

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Solar radiation: •  Full / Cloudy •  Evolution curve in function of localisation, moment of year and

orientation of the span Absorptivity: = 0.9 (Cigre TB299 - conservative) Emissivity: = 0.7 (Cigre TB299 – conservative) Rain:

not directly considered, compensated by effective ambient conditions

2. IEEE model – IEEE parameters

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The accurately measured sag is converted to the mean conductor temperature using the State Change Equation (once the Constant has been determined from a Known State).

The conductor temperature is also simulated using the IEEE model with the actual current (from TSO) and environmental conditions (from web/weather station).

Deviations may be due to limitations of the thermal model or inaccuracies of weather data (local variations). To match with the actual behaviour of the span, the environmental parameters (wind speed and solar radiation) are adapted to effective values. This is done in a conservative way, avoiding overestimation of windspeed and thus ampacity.

2. IEEE model - Effective Ambient Conditions

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IEEE Thermal Model

Effective Ambient

Conditions (*)

RT Line Loading

Conductor Constants

TSO

State Change Equation

Ruling Span Sag

Mean Conductor Temperature

Ampacimon Monitor

(*) Effective wind & solar radiation

2. IEEE model - Effective Ambient Conditions

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The Effective ambient conditions are conservative:

1.  Measured Sag > Simulated Sag + tol (*)

2. IEEE model - Effective Ambient Conditions

Conductor Temp: Real > Simulated

Dissipation / Ampacity : Real < Simulated

The simulated sag is matched onto the measured sag:

•  The wind speed is reduced (perpendicular wind)

•  At wind speed = 0 the solar radiation is increased

•  At solar radiation = 1000 W/m^2 : Warning Log

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2.  Measured Sag < Simulated Sag – tol (*)

2. IEEE model - Effective Ambient Conditions

Conductor Temp: Real < Simulated

Dissipation / Ampacity : Real > Simulated

The simulated sag is matched onto the measured sag + tol :

•  The solar radiation is reduced •  At solar radiation = 0 the wind

speed is increased •  At wind speed increase = 5 m/s

(max) : Warning Log

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3. IEEE dynamic model - RT Ampacity

The RT Ampacity of each potentially critical span is determined for the maximum sag or the maximum conductor temperature whichever comes first.

To determine the Ampacity the IEEE dynamic model is used with the effective wind speed and solar radiation.

Note that as the sag increases, the system becomes obviously more and more accurate. For a limit situation where the measured sag equals the maximum sag, the Ampacity is then exactly reached ! In such safety situation, Ampacimon only relies on its measurement of the span fundamental frequency without any other parameter or calibration !

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3. IEEE dynamic model – RT Span Ampacity

IEEE Thermal Model

Effective Ambient

Conditions

RT Span Ampacity

Conductor Constants

State Change Equation

Max Design (*) Conductor

Temperature

Maximum Conductor

Temperature MIN

Maximum Span Sag →

Ruling Span Sag

TSO TSO TSO

(*) < Annealing temperature

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3. RT Ampacity – Static Rating Safety

In some cases the RT Ampacity cannot be determined :

•  Missing Line Current value •  Missing or faulty weather data •  Missing Sag measurement (low line current, analysis not concluding)

Then the Ampacity is determined using the IEEE thermal model with Safe Static Rating conditions:

•  Wind speed : 0.5 m/s •  Solar radiation: 1000 W/m^2 •  Ambient temperature: defined per localisation & season

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3. IEEE dynamic model - RT Line Ampacity

The RT Line Ampacity finally is the minimum Ampacity of all the observed spans. Note that there is absolutely no need that these spans belong to the same section.

The Ampacity is determined dynamically taking into account the thermal inertia (thermal time constants). This allows to calculate in RT the Dynamic Ampacity for a given duration. The information is shown to the Dispatching Operator in a simple graph to assist him for Congestion Management or Dynamic Line Rating (DLR)

Ampacity is indicated as total intensity (A) or as transit (MVA).

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3. RT Line Ampacity

RT Span Ampacity

Span #4

RT Line Ampacity

MIN

TSO

RT Span Ampacity

Span #3

RT Span Ampacity

Span #2

RT Span Ampacity

Span #1

Monitors may be installed in same or/and in different sections !

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3. RT Ampacity - Dispatching Operator Screen

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3. RT Sag & RT Ampacity / Time

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Ampacimon RT Ampacity - Conclusions

•  Ampacimon determines RT Ampacity using the IEEE thermal model with adjusted effective ambient parameters (wind speed), making full profit of the intrinsic accuracy of the Ampacimon vibration monitors to measure the Sag.

•  The sag - conductor temperature equation is set by recent measured values instead of installation values that may have become inaccurate.

•  Ampacimon integrates the Ruling Span concept while measuring locally on potentially critical spans for optimum accuracy.

•  Ampacimon RT Ampacity is dynamic, assisting the dispatching operator safely and efficiently in Congestion Management & DLR.

•  Commissioning is fast: < 4 weeks with no calibration needs towards transit intensity.