Ratings of power circuits vary according to a number of factors, most noticeably ambient temperature for transformers and overhead lines and burial conditions for underground cables. Traditionally, planners and designers have assumed worst case conditions when determining the capacity of the network to set protection, judge when upgrades are required, curtail generation and generally operate the network. However, the worst case conditions statistically rarely occur, and so for the majority of the time the operating and design limits of the network are less than the actual limit.
The purpose of dynamic line rating is to operate the network at its actual, real time thermal rating and thus use the full capacity of an overhead line rather than an artificially
constrained capacity and thereby avoid capital investment to uprate the circuit or unnecessary constraints on DG. This is particularly relevant to wind generation, as maximum power output from a wind generator is naturally coincident with maximum wind speed, and hence with potential cooling effects of the wind on nearby line conductors.
The components of a dynamic rating system can include:
Some form of direct measurement of line parameters
Examples include measurement of the conductor temperature by embedded thermocouples, line sag using cameras, theodolites or inclinometers, conductor tension using load cells
Measurement of local weather conditions using a standard weather station.
Communication of these measurements to a controller
This might be via fibre optic cable run with the line or as OPGW or fibre wrap (which can be retrofitted to existing earth or power conductors), GPRS, UHF/VHF radio, microwave.
Logic to determine what action should be taken given the measured value
There are two weather related calculation methods35, the Weather Model and the Temperature Model, provided in IEE 738-1993 “Standard for Calculating the Current-Temperature Relationship of Bare Overhead Wires”. Therefore, the logic sequence could use these methods to determine the real time maximum current rating of the conductor, and then take action to ensure that current flow is not exceeded.
Alternatively, the logic or control loop could compare the measured value such as conductor sag or tension against a setpoint representing maximum allowable sag or tension and take action on that basis.
Communication of an action request to a controlled variable, most likely to DG in the form of an instruction to curtail or relief of a constraint
Verification that the action has kept the conductor within its determined limits
Possible dynamic modification of protection systems to ensure grading with the dynamic rating system.
Although several other systems using thermocouple and load cell measurement have been in use for some time, including the Kema36 Dynamic Current Rating Optimisation, the Power Donut™ is being used by E.ON in the Skegness RPZ, and also by EDF. It has been the subject of numerous papers and studies and does appear to offer a unique solution to some of the problems with dynamic ratings raised in this report, and so shall be used here as a specific case to represent the generic class of dynamic rating devices. The Power Donut™ also provides a useful study base as it incorporates conductor tension, angle of inclination (which can be fed into a catenary calculation to give the mid span height), temperature and current measurements. In addition, dynamic rating systems based on weather measurements were considered. Scottish Power in collaboration with others is currently investigating thermal modelling with the goal of producing an active thermal controller using weather data combined with direct measurement of equipment parameters.
6.4.2 Technical
Safety and Environment
One of the key concerns with the dynamic rating principle is that if the process used to calculate the actual rating errs or fails in some way, then the actual rating could be exceeded causing possible equipment failure risks to safety and the environment.
Failure or miscalculation will be a concern in cases where overcurrent protection, which is normally set to detect faults rather than overloads, will not activate before ground safety clearances of an overhead line are breached through excessive sag. In this case, some changes to the protection scheme might be required, or duplicated dynamic line conductor
35 “Prospects for Dynamic Transmission Circuit Ratings”, K. E. Holbert, G. T. Heydt, Arizona State University
36 KEMA, http://www.kema.com
rating systems based on different principles could be used to ensure increased confidence. Multiple measurement points will also decrease the chance of incorrect measurements or measurements remote from the sagging line leading to a breach of ground safety clearances.
Failure or error could also be of significant (safety, rather than operational) concern when protection settings are being changed to achieve differentiation between the dynamically calculated maximum current and the overcurrent alarm and trip levels.
For example, to achieve this grading it might be necessary to use summer and winter ratings on overcurrent protection settings (any form of unit protection scheme should be unaffected). Most modern protection relays in operation by DNOs have this capability5, and these relays will already be communicating with the main SCADA systems in some fashion, although possibly only through aggregated alarms.
Functionality
The potential advantages of dynamic ratings for underground cables are limited, as burial conditions are static. Therefore, to gain advantage from dynamic rating on a particular circuit, any portion of the circuit that is constructed overhead must be the limiting factor on that circuit. Similarly, to gain maximum benefit on an individual circuit, the circuit must currently or be predicted to shortly exceed its conservative rating. When these conditions are met, then use of dynamic ratings to increase thermal capacity on a line might
reasonably be considered. Whilst a significant proportion of the Great Britain distribution network is underground, the planning and cost barriers to erecting duplicate lines for those circuits that are overhead are considerable.
The modelling done for the Skegness RPZ dynamic line rating scheme suggests that that scheme might see a potential doubling of thermal capacity at the highest wind speeds, allowing an increase of DG output on the line of up to 600 A onto the line in question without capital upgrades. Assuming that the modelling of the relationship between the wind cooling effect, ambient temperature and line capacity is correct, then the main variable in this system with regards to quantified benefit will relate to the coincidence of high wind speeds at the wind farm and high wind speeds perpendicular to the line. If the line is physically removed from the wind farm, or in a valley (and modern planning
requirements often dictate least visible line routes) or behind some other obstacle to the cooling effect of the wind, then this benefit might be diminished.
Location of Measurement
Temperature and wind effects can be very localised, and the weather experienced by one span can vary to that experienced the next. A further consideration will be whether the same conductor is used over the entire circuit length. Each type of conductor will have different properties and will respond differently to changing weather conditions. Therefore, not only might a single span be the effective bottleneck on the rating of the entire line, but the particular span might change within a period of time. Therefore, one technical risk to be carefully managed with dynamic ratings relates to the position of the measurement devices.
Communications
One of the key components regarding dynamic rating systems will be the communications from the measurement system to the controller and from the controller to the controlled
device (and then verification that the controlled device has taken the correct action).
Considering the variables that the dynamic rating system is based on (that is weather variables such as wind speed, direction and ambient temperature), a complete cycle time of a few minutes would seem a reasonable expectation. Specifying a faster time might be unnecessarily onerous. The speed of response of conductor sag to changes in weather conditions is discussed further below.
The Power Donut™ system uses standard GPRS mobile phone technology. A central controller is fitted with a standard SIM card and given a fixed IP address and each unit in the network can be set to transmit its information to that IP address.
Clearly the reliability of this system is reliant not just on the components in the measuring device and the server but also on the mobile network provider. The manufacturer quotes reliability of levels of over 99.9% during the longest trial of this version of the
communications platform at an Italian site since 2004. One of the advantages of this system is that no additional infrastructure is required, particularly in Great Britain which has extensive GPRS coverage. This coverage also means that for many sites, the line being measured will be within range of more than one base station tower, providing redundant paths to the internet and hence to the server. This would have to be confirmed on a case by case basis.
Data transfer by GPRS is a well proven technology and should be very simple to install and operate. Although not as fast as, for example, a direct fibre optic or microwave link, for this application update times should be perfectly acceptable. The available bandwidth should be more than sufficient given that the current GPRS network in Great Britain has been designed with significant data transfer capability.
Communications will also be required from the controller to the DG. It is likely that two levels of communication will be required, one to send a curtailment (or relieve a
curtailment) instruction and verify that action has been taken and the second to send a trip signal if the DG has failed to act as expected, thus pre-empting any protection trips to avoid disruption to other customers. The specification for the first application will be low speed, low bandwidth but preferably good reliability to avoid unnecessary curtailment (this could be decided by commercial arrangements with the DG developer).
For the second application, moderate speed (assuming the remainder of the line measurement, decision making, communication and verification steps have been completed within the time for the line to respond to changing conditions) and excellent reliability will be required. Previous reports into existing DNO communication systems suggest that both specifications should be relatively easily achieved using existing equipment.
Calculation of Dynamic Rating
The relationship between conductor temperature, line sag and current rating is not straightforward. Systems based on weather station data seem particularly vulnerable to error. The IEEE 738-1993 standard states that wind blowing perpendicular to a line provides a 60% greater convection cooling effect than wind blowing parallel.
Perpendicular wind speed has a greater impact on conductor temperature than any other parameter (eg, solar radiance). That such a large range of cooling can occur due to something as variable as wind direction implies that the calculations must either be very accurate, and based on accurate and comprehensive weather data measurements, or else
incorporate considerable safety margins. Indeed, studies35 have shown that the accuracy of the calculated capacity is particularly sensitive to the accuracy of the measured wind speed and direction. Measurements have shown up to a 30 °C temperature rise within several minutes of a sudden wind speed drop or change of wind direction37.
It might be possible to use just local weather data if dynamic rating is being used for a region of interconnected lines and wind generators. However, the individual line capacity benefit would be reduced through having to incorporate safety margins to allow for local wind speed and direction variability.
The key parameter with regards to line rating is generally line sag rather than temperature, as ground clearances are likely to be breached before the conductor fails through excess temperature. Therefore, a system based on measured sag could bypass most forms of mathematical modelling of the interaction between line rating and weather and simply ensure that the sag does not exceed a setpoint based on clearance to ground. The Power Donut™ derives sag from measured inclination and fixed parameters such as conductor mass. Trials performed have also shown good correlation between conductor temperature and sag, so that temperature could also be used as the measured parameter with minimal indirect calculation required.
The logical sequences and communications installed for one dynamic rating system should be relatively simple to replicate so that increasing numbers of lines with these systems would not cause increasing complexity by themselves. That complexity is more likely to arise from the interactions with the DG.
In general, it would appear technically preferable to base the dynamic line rating on direct measurement of the conductor rather than on estimation performed from weather
measurements. Alternatively, if a large region or a long line was being dynamically rated, weather measurements could be used with a few strategically placed direct measurement devices providing data backup and verification. The Skegness RPZ is using the latter model, with line rating data supplied to the ENMAC™ control system which then dynamically adjusts alarm setpoints and other relevant parameters. The results of this arrangement, including how well the measurements taken of conductor temperature and tension and inclination correlate with ratings based on weather station data will be of interest.
Other Advantages
A device such as the Power Donut™ provides a large amount of data, including current and voltage waveforms. It is conceivable that this information could be used for event analysis and in other ways.
6.4.3 Operational
The primary maintenance related impact of dynamic line ratings will be related to the measurement systems. The Power Donut™ can be fitted or removed live using supplied hot sticks38, and weather stations local to the line can also be installed or removed without impacting network operations. If a DNO used dynamic line ratings on a number of lines, it might be appropriate to hold spare units, in which case replacement on failure of the
37 From the USi, the manufacturers of the Power Donut ™
38 A video demonstrating the fairly simple installation procedure is available from the manufacturer’s website.
measuring device should take no more than an hour plus travel time. Otherwise, these are off the shelf items and hence readily available.
The Power Donut™ is powered from the electromagnetic field of line current, however that current must be greater than 50 A (it should be noted that this is a limitation on the use of this device although with typical conductor ratings measured in hundreds of amperes this should not be a problem). If a line is out of service for more than 12 hours, then this device will require a minimum 120 A before charging commences. Therefore, should the normal operating current range be between 50 A and 120 A, it might be necessary to manually charge the unit following an extended outage. Otherwise, maintenance would appear to be minimal.
Maintenance of a weather station would depend on the particular system chosen. Solar radiation sensors might require cleaning, and any dirt or other material in anemometer bearings might affect wind speed measurement. Overall, the weather station is likely to require approximately annual maintenance and calibration. Some form of power supply will be required for the weather station, preferably solar.
Operations staff are likely to require some training on the first dynamic line rating system to understand changing power flows on the network and what impact they might have on alarm and control setpoints. However the principles involved are intuitive and directly repeated for each installation.
Designing failsafe mechanisms to curtail DG should the dynamic rating system be compromised in anyway should ensure there is no negative effect on CML, CI etc.
Customers on the same line as DG that is allowed to connect because thermal ratings are increased should see no impact (unless there other, voltage related issues). Rather, access to greater information about network parameters such as current and voltage might allow faster identification of problems.
6.4.4 Planning
The interaction between dynamically increasing the capacity of a line and P2/6 security requirements will need to be considered on a case by case basis. In general the line capacity increase will be related to curtailable DG rather than load increases, and so it is unlikely to require other changes to the network to maintain P2/6compliance. Dynamically increasing the capacity of a line might however provide a significant increase in transfer capability, thus allowing the line to assist in maintaining P2/6 compliance for other parts of the network.
The life expectancy of the Power Donut™ is greater than 20 years according to the
manufacturer’s information regarding the first units that were in service, although the latest units have considerably more features and capabilities, and so its life expectancy could not be judged. Other components of the dynamic line rating system, namely the controller and the communications to the DG likely to be standard components and so cost of end of life replacement will be spread over a broader range of technologies.
The overall cost of dynamic line rating would appear to be low, particularly compared to the cost of increasing line capacity by new line reinforcements. The Power Donut™
package including software to communicate with the DNO control system has been quoted at approximately £8,000, and weather stations are similarly priced. The most complex part of the upfront engineering work to set up the scheme will be determining where to
measure the weather or line conditions to ensure that the true bottlenecks on the line are being captured. If the dynamic rating system has been triggered by a wind generation connection application, it might be possible to coordinate wind data collation (something in the best interest of the developer who would otherwise be likely to pay for the DNO costs).
The communications to the DG should be able to be achieved cost effectively if integrated at the time the DG connects and using the capabilities of standard protection relays.
Where the dynamic line rating system is being used in conjunction with wind generation, the cost of curtailment to the DG developer is likely to be minimal given the
aforementioned correlation between conductor capacity and wind generation. This might not be the case for other types of generation, in which case load factors could be analysed against the expected capacity increases to run a last on, first off scheme similar to that used in the Orkney RPZ.
6.4.5 Other
Once again, the use of dynamic ratings by one UK DNO, particularly in the context of an RPZ, might prohibit similar projects from qualifying from IFI or RPZ funding. However, incremental variations might include using dynamic rating on lower voltages, using it
across a broader area and also using it in conjunction with DG other than wind generation.
Dynamic rating based on different technologies or algorithms to that used by Central Networks should also qualify.