CLNR Trial Analysis - RTTR for Power Transformers · = Mass of the transformer oil in kg . 𝑐𝑐...

Copyright Northern Powergrid (Northeast) Limited, Northern Powergrid (Yorkshire) Plc, British Gas Trading Limited, University of Durham, Newcastle University and EA Technology Ltd, 2014 Copyright Northern Powergrid (Northeast) Limited, Northern Powergrid (Yorkshire) Plc, British Gas Trading Limited and University of Durham 2014

CLNR Trial Analysis

Real-Time Thermal Rating for Power Transformers

DOCUMENT NUMBER CLNR-L130 AUTHORS Peter Davison, Newcastle University ISSUE DATE 23 December 2014

Contents Executive Summary .......................................................................................................... 2

1 Introduction ........................................................................................................... 3

2 Transformer Monitoring Sites ................................................................................. 5

3 Transformer RTTR algorithm .................................................................................. 6

3.1 Model Validation ............................................................................................................. 8

4 Transformer RTTR Sensitivity Analysis .................................................................. 10

4.1 Transformer Model Input Parameters .......................................................................... 10

4.2 Sensitivity to Input Parameters .................................................................................... 10

5 Ambient Temperature Analysis ............................................................................ 15

5.1 Cumulative Distribution Function Plots ........................................................................ 15

5.2 Box and Whisker Diagrams ........................................................................................... 18

5.3 Indoor / Outdoor Ambient Temperatures .................................................................... 19

6 Real-Time Thermal Rating Results ........................................................................ 22

6.1 Primary Substations ...................................................................................................... 22

6.2 Secondary Substations .................................................................................................. 23

6.3 Transformer Emergency Ratings ................................................................................... 27

7 On-Load Tap Changer Study ................................................................................. 29

7.1 Nameplate Parameters ................................................................................................. 29

7.2 Ratio of Load Losses to No-Load Losses ....................................................................... 30

7.3 Top Oil / Frame Temperature Study ............................................................................. 30

8 Transformer RTTR in Collaboration with EES / DSR ............................................... 32

8.1 Wooler St Mary ............................................................................................................. 32

9 Conclusions .......................................................................................................... 34

10 References ........................................................................................................... 35

11 Appendix A – Transformer Emergency Rating Results ............................................. 0

1 Copyright Northern Powergrid (Northeast) Limited, Northern Powergrid (Yorkshire) Plc, British Gas Trading Limited and University of Durham 2014

Executive Summary Real time thermal ratings of multiple secondary transformers have been derived based on the IEC 354 standard [1].

The half hourly maximum load which can be applied to the transformer has been calculated, such that the pre-determined transformer maximum temperature limits are not violated. In the CLNR project a maximum loading limit of 150% was in place, in alignment with the presently used P15 standard [2]. In this report the maximum loading limit has been removed in order to further investigate the loading limits of such transformers. The pre-determined maximum temperature limit of 140°C was reduced to 130°C in order to provide a degree of safety whilst running the Customer-Led Network Revolution (CLNR) field trials. This limit of 130°C remains in place for this analysis unless otherwise stated.

Cumulative distribution functions of the transformer Real-Time Thermal Rating (RTTR) have been derived to give an indication of the increased loading capability. With a 99.9th percentile of the distribution chosen, the typical half hourly loading values were in the range of 190-220% of nominal loading: note that this applies to a sudden increase in peak load, not the more gradual changes associated with normal circumstances.

Emergency ratings have also been calculated based on the real loading conditions and ambient conditions at site, as opposed to pre-determined maximum loading values. These suggest that for a maximum hot spot temperature limit of 130oC, the minimum increase in transformer loading capability is 18% above nominal. As risk levels increase, greater maximum loading capabilities are possible.

Considering the transformer loading as a time series, various scale factors have been applied to the load to determine the impact on transformer hot spot temperature. If additional network intervention are necessary to mitigate against potential thermal overload events, the durations and magnitudes of such responses have been quantified for a given test site.

As discussed in this report, when modelling the potential loading capability of a transformer which is located inside a building, it is important to consider the effect that the increased loading will have on the ambient air temperature surrounding the transformer. This must be properly taken into account to prevent overloading of the transformer for the resultant conditions. For those transformers located outside, this is less of an issue due to the available ambient cooling.


1 Introduction Power system components are limited in their current carrying capacity by the conditions which surround them, in the case of Power transformers, the levels of ambient temperature, solar gain and wind speed (in particular for pole mounted transformers) all affect the ability to transfer power. Higher ambient temperatures result in a reduced ability to transfer heat away from transformer components leading to reduced loading capability. Typically power system components such as transformers are rated based on a series of worst case conditions, to ensure secure power transfer capability under all possible scenarios. This however leads to high levels of redundant capacity, which, under the present rating systems are unable to be ‘unlocked’. This is primarily due to a lack of ‘real-time’ monitoring at the component sites as opposed to a lack of accurate modelling techniques.

Real time thermal ratings (RTTRs) aim to bridge this gap by using the real-time ambient conditions and necessary modelling parameters to provide increased visibility with regards to asset capability. Within the scope of the CLNR project, only ground-mounted (GM) transformers have been considered. Wind speed and solar gain have not been measured within the project.

Both Primary and Secondary substation transformers have been instrumented. All Secondary transformers are ONAN cooled, whilst the Primary transformers are both ONAN and OFAN cooled.

The interpretation of an RTTR system with regards to transformers in the CLNR project is to calculate a maximum possible loading at a particular time sampling point, such that if all other conditions remain constant, the hot-spot temperature limit of the transformer does not exceed a pre-determined maximum of 130oC after a period of 30 minutes. This period is designed to represent a potential maximum period before a form of Demand Side Response can be engaged to mitigate load increase.

The method used to calculate the RTTRs is based on the IEC 354 / IEC60076-7 standard [3] and requires transformer loading and ambient temperature as the input variables. These parameters allow calculation of the transformers’ hot-spot and top-oil temperatures, which inform the maximum loading capability of the unit.

Where transformers are located inside buildings and ambient temperature sensors are located externally, the IEC standards call for an appropriate de-rating factor to be applied to the transformer loading capability. Within the CLNR project, ambient temperature sensors have been located inside the building in order to measure the actual ambient conditions surrounding the transformer. Application of a de-rating factor was therefore non-necessary. This does however impinge upon the ability to investigate loading increases at these indoor sites, since the measured ambient temperatures contain a function of the transformer loading.

In addition to ambient temperature sensors, transformer tank temperature probes have also been installed. The hypothesis for these installations was such that if installed at the correct location, a tank temperature sensor would give an estimation of the top-oil temperature of the transformer. This will be discussed in greater detail in the report.


In total there are 23 monitoring sites: 2 Primary substations and 21 Secondary substations. Some Secondary substations have been removed from the data analysis due to data quality issues.

Data for this report has been taken from all available data sources in the period 1/10/2013 to 31/10/2014.


2 Transformer Monitoring Sites

CLNR Test Network Substation Indoor / Outdoor Transformer Rating (kVA)

Denwick PRIMARY Outdoor 20/25 (MVA) (ONAN/OFAN)

Akeld Indoor 500

Alnwick St James Indoor 350

Belford West Indoor 750

Doddington Village Outdoor 315

Stone Close Indoor 1000

Waren Mill Indoor 200

Wooler Bridge Indoor 500

Wooler Ramsey Outdoor 315

Wooler St Mary Outdoor 500

Rise Carr PRIMARY Indoor 15/18.75 (MVA) (ONAN/OFAN)

Beaumont Reservoir Indoor 750

Darlington Melrose Outdoor 500

Darlington Russell Outdoor 500

Darlington Valley Outdoor 500

Harrowgate Hill Outdoor 300

High Northgate Outdoor 750

Marwood Crescent Outdoor 500

Stooperdale Offices Outdoor 500

PV Cluster Mortimer Road Indoor 800

Tickhill Indoor 750

Heat Pump Cluster Redburn Indoor 500

Sidgate Lane Outdoor 315

Table 1 – Transformer Monitoring Site Details


3 Transformer RTTR algorithm The Siemens RDC units deployed as part of the project have been used to calculate the RTTRs of the power transformers at each of the monitoring sites. As discussed previously this algorithm is based around the IEC 354 standard. There are two main aspects to the calculated ratings. Firstly a calculated ‘Ampacity’ value which represents the maximum 30 minute loading capability of the transformer in Amps. This value is then also converted to a ‘Medium term’ and ‘Long term RTTR’. These represent real power values of maximum available capacity.

For the purposes of this report an offline model of the IEC 354 standard, following the approach used in the project, has been implemented.

In IEC 354, the hot-spot temperature is a function of the ambient temperature surrounding the transformer, the top-oil temperature rise over the ambient, and the hot-spot rise over the top-oil temperature.

𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝑇𝑇𝑇𝑇𝑇𝑇 𝑂𝑂𝐹𝐹𝐹𝐹 𝑅𝑅𝐹𝐹𝑅𝑅𝑅𝑅 𝑇𝑇𝑜𝑜𝑅𝑅𝑜𝑜 𝐴𝐴𝐴𝐴𝐴𝐴𝐹𝐹𝑅𝑅𝐹𝐹𝐴𝐴 = ��

1 + 𝑅𝑅𝐾𝐾2

1 + 𝑅𝑅�𝑥𝑥

∙ ∆𝜃𝜃𝑡𝑡𝑡𝑡𝑡𝑡−𝑡𝑡𝑜𝑜𝑙𝑙 𝑟𝑟𝑜𝑜𝑟𝑟𝑟𝑟 � (1)

𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝐻𝐻𝑇𝑇𝐴𝐴 𝑆𝑆𝑇𝑇𝑇𝑇𝐴𝐴 𝑅𝑅𝐹𝐹𝑅𝑅𝑅𝑅 𝑇𝑇𝑜𝑜𝑅𝑅𝑜𝑜 𝐴𝐴𝐴𝐴𝐴𝐴𝐹𝐹𝑅𝑅𝐹𝐹𝐴𝐴 = ∆𝜃𝜃ℎ𝑟𝑟 𝐾𝐾𝑦𝑦 (2)

𝑅𝑅𝐹𝐹𝑅𝑅𝑅𝑅 𝑇𝑇𝑜𝑜𝑅𝑅𝑜𝑜 𝐴𝐴𝐹𝐹𝐴𝐴𝑅𝑅 𝑇𝑇𝑅𝑅𝑜𝑜𝐹𝐹𝑇𝑇𝑝𝑝 (𝐴𝐴) = 𝐼𝐼𝐹𝐹𝐹𝐹𝐴𝐴𝐹𝐹𝐹𝐹𝐹𝐹 + �(𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝑅𝑅𝐹𝐹𝑅𝑅𝑅𝑅 − 𝐼𝐼𝐹𝐹𝐹𝐹𝐴𝐴𝐹𝐹𝐹𝐹𝐹𝐹) ∙ 1 − 𝑅𝑅−

𝑡𝑡𝜏𝜏𝑜𝑜 𝑡𝑡𝑟𝑟 𝜏𝜏𝑤𝑤�

(3)

𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝐻𝐻𝑇𝑇𝐴𝐴 𝑆𝑆𝑇𝑇𝑇𝑇𝐴𝐴 𝑇𝑇𝑅𝑅𝐴𝐴𝑇𝑇 = 𝜃𝜃𝑎𝑎 + 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝐻𝐻𝑇𝑇𝐴𝐴 𝑆𝑆𝑇𝑇𝑇𝑇𝐴𝐴 𝑅𝑅𝐹𝐹𝑅𝑅𝑅𝑅 𝑇𝑇𝑜𝑜𝑅𝑅𝑜𝑜 𝐴𝐴𝐴𝐴𝐴𝐴𝐹𝐹𝑅𝑅𝐹𝐹𝐴𝐴+ 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝑇𝑇𝑇𝑇𝑇𝑇 𝑂𝑂𝐹𝐹𝐹𝐹 𝑅𝑅𝐹𝐹𝑅𝑅𝑅𝑅 𝑇𝑇𝑜𝑜𝑅𝑅𝑜𝑜 𝐴𝐴𝐴𝐴𝐴𝐴𝐹𝐹𝑅𝑅𝐹𝐹𝐴𝐴

(4)

Where:

𝐾𝐾 = Load Factor on the transformer � 𝐿𝐿𝑡𝑡𝑎𝑎𝐿𝐿𝑅𝑅𝑎𝑎𝑡𝑡𝑟𝑟𝐿𝐿 𝐿𝐿𝑡𝑡𝑎𝑎𝐿𝐿

�

𝑅𝑅 = Ratio of load losses at rated load to no-load losses

𝑥𝑥 = Oil Exponent

𝑦𝑦 = Winding Exponent

𝜃𝜃𝑎𝑎 = Ambient Temperature

∆𝜃𝜃ℎ𝑟𝑟 = Hot-Spot to Top-Oil gradient at rated load

𝜏𝜏𝑡𝑡, 𝜏𝜏𝑤𝑤 = Oil and Winding time constants

𝜏𝜏𝑤𝑤 = 𝑀𝑀𝑤𝑤 ∙ 𝑐𝑐 ∙ 𝑔𝑔60 ∙ 𝑃𝑃𝑤𝑤

(5)

𝜏𝜏𝑡𝑡 = 𝐶𝐶 ∙ ∆𝜃𝜃𝑡𝑡𝑜𝑜 ∙ 60

𝑃𝑃 (6)

𝐶𝐶 = 0.132 ∙ 𝑀𝑀𝑎𝑎 + 0.0882 ∙ 𝑀𝑀𝑡𝑡 + 0.4 ∙ 𝑀𝑀𝑡𝑡 (7)


Where:

𝑀𝑀𝑤𝑤 = Mass of the winding in kg

𝑀𝑀𝑎𝑎 = Mass of the core and coil in kg

𝑀𝑀𝑡𝑡 = Mass of the tank and fittings in kg

𝑀𝑀𝑡𝑡 = Mass of the transformer oil in kg

𝑐𝑐 = Specific Heat of the Conductor Material (390 for Cu and 890 for Al)

𝑔𝑔 = Winding to Oil gradient at the considered load

𝑃𝑃𝑤𝑤 = Winding Loss in W at the considered load

𝐶𝐶 = Thermal Capacity

∆𝜃𝜃𝑡𝑡𝑜𝑜 = Average Oil Temperature rise above ambient temperature at the considered load

𝑃𝑃 = Total Losses (Supplied Losses) at the considered load

Whilst Equations 1 and 2 are central to calculation of the transformers temperatures, Equation 3 is the most important with regards to how the algorithm has been implemented on a half hourly basis.

At each time sampling point (t), initial values of top-oil and hot spot temperature are carried forward from the previous iteration (t-1). These values were calculated at time (t-1) based on the actual loading from the previous time step with the value of t in Equation 3 set to the length of the sampling period. The value of 𝜏𝜏 varies depending on whether the hot spot (𝜏𝜏𝑤𝑤) or top-oil (𝜏𝜏𝑡𝑡)

The values for the next iteration are therefore calculated first. The next phase involves replacing the actual observed load ratio on the transformer with values ranging up to a maximum pre-determined loading limit. The value of t is also increased to 30 minutes. The limiting value is 150% of the nominal transformer rating. The maximum load ratio value which does not result in a hot spot temperature (after 30 minutes) is then selected as the ‘Ampacity’ value.

This 150% limit has been pre-determined, and is based on the present loading criteria of ER P15. For the purposes of investigating the capability of transformer RTTR, this analysis will remove this 150% limit, but will maintain the upper hot-spot temperature limit.


3.1 Model Validation

Figure 1 – Measured, Modelled RTTR Values for the Rise Carr Primary Transformer

Figure 1 shows the results of a validation test period for the offline transformer model. As can be seen there is good agreement between the values generated by the offline model against those calculated in the project.

The significant step changes are due to modifications in the transformer nominal rating carried out as part of additional field trials in the wider project. The square wave nature of the observed values is further enhanced by RTTR values consistently being limited by the 150% nominal rating limit.

Figure 2 – Measured and Modelled with increased maximum load ratio RTTR Values for the Rise Carr Primary

Transformer


Figure 2 shows RTTR values for the same time period after the 150% maximum rating limit has been removed. Where the nominal rating of the transformer has been significantly reduced, the potential RTTRs are also reduced.


4 Transformer RTTR Sensitivity Analysis

4.1 Transformer Model Input Parameters

As shown in equations 1-3, the IEC 354 model requires various input parameters which characterise the transformers response to changes in load / ambient conditions. The majority of the parameters used in the CLNR modelling approach are those found in Table 2 of IEC 354, others are from P15. Deviations from these parameters are shown in red.

Transformer Input Parameter Value

Oil Exponent 0.8

Winding Exponent 1.6

Hot Spot Rise over top-oil 22 (K)

Top-of-winding oil rise 56 (K)

Winding Time Constant 4 (min)

Transformer Losses at Rated Load 10

Transformer No Load Losses 1

∴ Loss Ratio 10

Oil Time Constant 180 (min)

Hot Spot Temperature Limit 130oC

Table 2 – Transformer Model Input Parameters

P15 specifies a hot-spot temperature limit of 140oC which cannot be exceeded. In the approach adopted by the CLNR project this hot-spot limit has been reduced to 130oC in order to provide a margin of safety.

As a caveat to all results shown in this report, all transformers, at both Primary and Secondary substations use the above data parameters as inputs. Parameters only differ for the detailed study of the Wooler Bridge OLTC transformer.

4.2 Sensitivity to Input Parameters

Since the implemented transformer models at all sites contain the same parameters it is useful to consider the sensitivity of the calculated values to these input parameters. For the following analysis, the various input parameters have been varied singularly to determine the sensitivity of each to the calculated half hourly RTTR values. For reference, the nominal conditions used to calculate the baseline half hourly rating are as follows:


• Initial Transformer Loading Varies from 0.1 to 1 • Ambient Temperature = 10 • R = 10 • Oil Time Constant = 180 min • Winding Time Constant = 4 min • Top Oil Rise = 56 K • Hot Spot Rise = 22 K

Each of the parameters (other than ambient temperature) is then modified across a hypothetical range. For reference the nominal rating for these given conditions is a maximum loading of 2.13.

Figure 3 – Effect of Load loss ratio on half hourly rating

Figure 3 shows the effect of the load loss ratio (load loss / no load loss) on the half hourly ratings. As will be shown in later sections of the report, the load loss ratio is most affected by the tap setting of the transformer. As can be seen, the effect of the load loss ratio on 30 minute ratings is relatively small for the same initial loading conditions, although there is larger variation across the range of starting conditions.


Figure 4 – Effect of Winding Time Constant on half hourly rating

Again as per the influence of load loss ratio, the largest variation shown from the effect of winding time constant is from changes in the initial loading conditions. This is potentially due to the short length of the winding time constant in relation to the half hourly rating.

Figure 5 – Effect of Oil Time Constant on half hourly rating

The percentage change in nominal rating based on the oil time constant is relatively large for smaller values of time constant, decreasing as oil time constant increases. The question is therefore what is the likely variation in values of transformer oil time constant?

Table 3 shows estimated mass of windings; oil etc. for a small subsection of the CLNR test transformers in order to estimate the transformer oil time constant. As can be seen there a relatively small changes in transformer oil time constant for variations in transformer oil mass etc.


Total Transformer

Mass (kg) Ma (kg) Winding Mass

(kg) Oil Mass

(kg)

Mass of Tank and fittings (kg)

Oil Time Constant

(min)

1688 730 365 599 359 245

1788 857 428.5 523 408 239

2320 1340 670 685 1425 258

2320 1340 670 685 1425 258

2520 810 405 1095 615 268

Table 3 – Estimated Transformer Parameters

Figure 6 – Effect of Hot Spot Rise on half hourly rating


Figure 7 – Effect of Top Oil Rise on half hourly rating

The hot-spot and top-oil rises show the greatest impact on half hourly rating relative to the variation of input values. It would therefore be suggested that for future work on transformer real-time thermal ratings, that transformer temperature tests are carried out to determine these parameters correctly.

The results of transformer temperature tests would also be used in the calculation of parameters such as winding and oil constants, leading to greater accuracy in the results.


5 Ambient Temperature Analysis Since all parameters related to the transformer ratings other than ambient temperature and load factor are equal across all the monitoring sites, we will examine these variables independently.

Ambient temperatures have been represented in two ways, both as a cumulative distribution function (CDF) and as box and whisker diagrams. The CDF plots allow investigation of the means and outliers of the ambient temperatures at the substations, whilst the CDF’s show the percentage time for which ambient temperatures are exceeded during the data observation period. The CDF’s will inform decisions if a ‘fixed’ ambient temperature were to be chosen to replace the current transformer rating approach, whilst the box plots show the potential for deviation away from such a value. For this section, CDF’s for all sites will be shown, with a selected sample of box plots. All further box plots can be found in the appendices.

When examining the data over the sampling period, the ambient temperature values at the Darlington Russell and Harrowgate Hill were found to have systematic measurement errors and have therefore been excluded from this analysis.

Ambient temperature measurements have been taken at points which are representative of the ambient air in which the transformer is located. I.e. for those transformers which are enclosed in buildings, corrective values for ambient temperature are not required.

5.1 Cumulative Distribution Function Plots

Figure 7 – Ambient Temperature CDF plots for Secondary Substations on the Denwick Test Network


Figure 8 – Ambient Temperature CDF plots for Secondary Substations on the Rise Carr Test Network

Figure 9 - Ambient Temperature CDF plots for Secondary Substations on the PV Cluster Test Network


Figure 10 - Ambient Temperature CDF plots for Secondary Substations on Heat Pump Test Network

As can be seen in Figures 8 and 9, the spread of observed temperatures on the Denwick network is more so than on the Rise Carr network. This is not to be unexpected as the area covered by the rural Denwick network is far larger than the urban Rise Carr primary network.

The temperature distribution at the Stone Close substation appears significantly skewed towards higher values than at all the other measurement sites (considering only secondary substations).

This is potentially due to the high penetration of customers with an E7 tariff on this substation and the subsequent large overnight storage heater consumption. A similar penetration of customers is found on the Waren Mill substation, again, where high ambient temperatures at the transformer are observed.

Elexon Customer Classification

Substation 0 1 2 3 4 5 6 7 8

Stone Close 0 22 122 2 2 0 0 0 0

Percentage 0 14.9 82.4 1.4 1.4 0 0 0 0

Waren Mill 1 8 37 0 0 0 0 0 0

Percentage 2.2 17.4 80.4 0 0 0 0 0 0

Table 4 – Customer Breakdown on Denwick Secondary Substations


5.2 Box and Whisker Diagrams

Figure 11 - Ambient Temperature Box and Whisker Diagrams for Secondary Substations on the Denwick Test

Network

Figure 12 - Ambient Temperature Box and Whisker Diagrams for Secondary Substations on the Rise Carr Test

Network


Figure 13 - Ambient Temperature Box and Whisker Diagrams for Secondary Substations on the PV Cluster Test

Network

Figure 14 - Ambient Temperature Box and Whisker Diagrams for Secondary Substations on the Heat Pump Test

Network

5.3 Indoor / Outdoor Ambient Temperatures

As noted in Table 1 some transformers are located indoors, some outdoors. This will affect the ambient temperature which surrounds the transformer. Those located within buildings will experience a temperature which is a function of both the ambient temperature outside of the building, and the thermal capacity of the construction.


Each of the IEC standards noted earlier contains modifications to the ambient temperature appropriate to the site of installation, however in the CLNR project, the ambient temperature has been measured to be representative of the actual ambient air which surrounds the transformer. I.e. no modification of the values is required.

In order to investigate this phenomenon average weekday and weekend load profiles and ambient temperatures have been calculated for all sites. The results of this analysis for a selection of sites are shown in Figures 10-13.

Figures 15- 16 - Stone Close Secondary Substation (Indoor Ambient Temperature)


Figure 17 - 18 - Wooler St Mary Secondary Substation (Outdoor Ambient Temperature)

As can be clearly seen, the ambient temperature at the Wooler St Mary substation appears to follow typical observations with a maximum temperature between 12 and 3 pm. The temperature at the Stone Close substation is much more a function of the loading on the transformer, with a typical peak between 3 and 6 am.

Results of this analysis suggest that as expected, the ambient temperature of an indoor located transformer is informed far more by the loading on the transformer than those located outdoors.

It is therefore suggested that for further monitoring of transformers, those located in buildings should be instrumented ahead of those located outdoors, where potentially more typical ambient temperature values can be used.


6 Real-Time Thermal Rating Results Within the CLNR project, the maximum possible increase in transformer loading is fixed at 150% of the rated capacity. Since the loading on the transformers is typically far below the nameplate rating, the potential increase is often simply 150% at each time period.

In order to further investigate the loading capability of the transformer, this upper limit of 150% has been removed, whilst the 130oC limit for each rating period of 30 minutes or 3 hours remains in place. This is thought to be a suitable way to demonstrate the potential loading capability of the transformer, in a way which would be acceptable for DNO operation.

Each of the following plots shows the cumulative distribution function of 30 minute transformer RTTRs at each of the sites. Results are shown for values of ‘Max Load Ratio’ where a figure of 1 represents the nominal capacity of the transformer (500kVA, 315kVA etc.)

Percentiles of each sites’ distribution function are shown in Table 5.

6.1 Primary Substations

Figure 19 – RTTR CDF for the Denwick Primary Transformer Site


Figure 20 - RTTR CDF for the Rise Carr Primary Transformer Site

6.2 Secondary Substations

Figure 21 - RTTR CDF for the Denwick Secondary Transformer Sites


Figure 22 - RTTR CDF for the Rise Carr Secondary Transformer Sites

Figure 23 - RTTR CDF for the PV Cluster Sites


Figure 24 - RTTR CDF for the Heat Pump Cluster Sites

CLNR Test Network Substation Transformer

Rating (kVA)

99.9th Percentile

Load Ratio

90th Percentile

Load Ratio

97.5th Percentile Load Ratio

80th Percentile

Load Ratio

Primaries

Denwick 20/25 (MVA) (ONAN/OFAN)

2.00 2.06 2.08 2.17

Rise Carr 15/18.75 (MVA) (ONAN/OFAN)

1.93 1.99 2.06 2.17

Secondaries

Denwick Akeld 500 2.16 2.18 2.19 2.25

Alnwick St James 350 2.02 2.06 2.08 2.15

Belford West 750 2.12 2.15 2.17 2.22

Doddington Village (though loading values

are only indicative of

one LV feeder)

315

2.20 2.21 2.23 2.27

Stone Close 1000 2.01 2.05 2.06 2.12


Waren Mill 200 1.77 1.90 1.94 2.07

Wooler Bridge 500 2.12 2.14 2.16 2.20

Wooler Ramsey 315 2.08 2.12 2.14 2.22

Wooler St Mary 500 2.11 2.14 2.16 2.22

Rise Carr Beaumont Reservoir 750 2.11 2.15 2.17 2.22

Darlington Melrose 500 2.03 2.09 2.11 2.18

Darlington Valley 500 1.92 1.95 1.97 2.08

High Northgate 750 2.09 2.14 2.16 2.21

Marwood Crescent 500 1.84 1.90 1.92 2.02

Stooperdale Offices 500 2.11 2.15 2.17 2.23

PV Cluster Mortimer Road 800 2.11 2.15 2.17 2.22

Tickhill 750 2.03 2.09 2.11 2.18

Heat Pump Cluster Redburn 500 2.11 2.15 2.17 2.22

Sidgate Lane 315 2.03 2.09 2.11 2.18

Table 5 – Percentiles of 30 minute Transformer RTTRs

As discussed previously, the limitation of 150% loading from the CLNR project is not present in this analysis, therefore the minimum value of 30 minute RTTR is greater than 150% at all sites.

These high values of RTTR (often RTTRs greater than twice the nominal rating of the transformer are possible) are perhaps not unlikely in this particular interpretation of a transformer RTTR. The 30 minute RTTR values consider a maximum possible loading for a singular half hour period, and are unconcerned with any continuous temperature rises either side of the period. The hot spot temperature is certain to exceed the 98oC rise which is necessary to maintain a unity ageing rate of the transformer, a factor which would need to be taken into consideration if such a method were to be routinely employed.


6.3 Transformer Emergency Ratings

Since the 30 minute rating calculated in the CLNR project is highly specific, rating values which are more in-line with those shown in P15 have been derived. The initial step is to convert the measurements from each substation into half hourly values. The half hourly loading values are then used to populate a set of daily load profiles. These are then normalised to the peak loading value and passed to the transformer rating algorithm. The normalised profile is then scaled by successive multiplier values until the increased scaling results in a violation of the hot-spot temperature limit.

These calculated ratings are intended to represent a similar number to that of the emergency rating given in P15. Again, as per the approach to RTTR adopted by CLNR, the reduced hot-spot temperature limit of 130oC has been maintained. When calculating the rating over the course of the day, three rating conditions have been considered:

1. The actual observed load profile against the maximum recorded temperature for that particular day with a hot-spot limit of 130oC (in line with the CLNR project)

2. As per (1) but with the hot-spot limit now increased to 140oC 3. As per (1) and (2) but now the hot-spot limit is reduced to 98oC. This is intended to provide

comparison with the maintenance of the unity ageing rate commented upon in IEC 354 and IEC 60076-7.

It is important to note that this analysis has been carried out solely for transformers which are located outdoors. In calculating the half hourly RTTR values the effect of loading on ambient temperature for indoor transformers was not taken into account, however for this analysis this assumption is deemed to be invalid.

It is also important to note that P15 considers that not all load profiles observed at substations are particularly similar, and that a potential reduction in the 150% limit may be necessary based on observed conditions as well as profiles. These ratings are simply presented to explore rating capabilities given that on-site measurements have been taken.

Figures 26 and 27 show the emergency ratings for analysis method (1) for the Secondary transformers.

The complete tabulated results of the Emergency Transformer Rating Analysis are shown in Appendix A.


Figure 25 – Secondary Transformer Emergency Ratings (2013) for 130oC Hot Spot Temperature

Figure 26 – Secondary Transformer Emergency Ratings (2014) for 130oC Hot Spot Temperature

As shown in the Appendix A results, the 140oC hot spot limit leads to the highest ‘emergency’ ratings as expected. Many of these are greater than the 150% limit previously discussed, however this is not unlikely as the load profiles on each of the transformers can be favourable, as well as the measured air temperature conditions at site.


7 On-Load Tap Changer Study As part of the CLNR project, On-Load Tap Changing Transformers have been installed at three secondary substations. From the tests carried out on these transformers we can update a number of the tabulated pre-determined parameters used in the transformer models. For this section we will focus on the 500 kVA, 20000/400V Wooler Bridge Secondary transformer.

In addition to the ambient temperature sensors installed at the substations, frame (tank) temperature sensors were also installed at all of the secondary substations. One area of interest is that in using the frame temperature measurements as a proxy measurement for the top-oil temperature of the transformer. Clearly there are a number of variables which are unable to be commented on, such as accuracy of placement of the tank temperature sensors. However this study is intended to inform further work in this area.

Due to the sensitivity of the transformer top-oil temperature to the ratio of load losses to no-load losses and also to the oil time constant, it seems sensible to carry out validation of the frame/top-oil proxy hypothesis for transformers where increased information is available.

In addition to the frame and ambient temperature measurements, the tap position of the transformer has also been recorded.

7.1 Nameplate Parameters

Transformer Parameters (from nameplate) Value

Connection Dyn11

Mass of Core and Coil 880 (kg)

Total Mass 2610 (kg)

Oil Litres 1120

Oil Assumed Density 0.87 kg/l

Oil Mass 974.4 (kg)

Table 6 – Nameplate data from the Wooler Bridge OLTC Transformer


7.2 Ratio of Load Losses to No-Load Losses

Short Circuit and Open Circuit tests have been carried out at the Wooler Bridge transformer at nominal, minimum and maximum tap values. From these we can determine the Ratio of load losses to no-load losses at each of the tap settings, assuming a linear relationship (as in IEC 60076-7).

Tap Setting Load Losses at Rated Load (W)

No-Load Losses at Rated Load (W) Ratio of Load Losses to No-Load Losses

1 6965 871 8.0

5 (nominal) 7230 871 8.30

9 7555 871 8.67

Table 7

Figure 27 – Ratio of Load Losses to No-Load Losses for the Wooler Bridge OLTC Transformer at all possible Tap

Settings

7.3 Top Oil / Frame Temperature Study

Due to the various parameters which have been calculated as a result of the tests carried out on the transformer, the tabulated value of the oil time constant can be replaced by that of the actual transformer. This value has been used in the calculation of the following top oil temperatures. The


ratio of load losses to no-load losses for the appropriate tap positions seen in the trials has also been used, based on the analysis of the previous section.

Figure 28 – Calculated and Measured Frame and Top Oil Temperatures

Figure 29 shows the measured frame temperatures and calculated top oil temperatures for a small sub-section of the observed data. Whilst open and short circuit tests have been carried out on the transformer, a temperature test was not carried out. Therefore only a limited number of the required parameters can be calculated. The blue line of Figure 29 uses the nominal value of top oil rise from the IEC standard, whilst the red line shows where this value has been increased by 10oC. Clearly the values of frame temperature and top-oil temperature should not realistically be exactly equal. However, in order to better quantify the accuracy of any potential offset, further transformer information is required; in particular a study to determine the various temperature gradients.


8 Transformer RTTR in Collaboration with EES / DSR The ‘emergency ratings’ which have been previously calculated, consider a normalised profile in order to quantify the maximum potential transformer loading over the period of one day. Whilst useful in quantifying the ultimate capability of the transformer, these ratings do not give insight as to the potentially limiting factors based on the actual loading of the transformer.

In addition to the ‘emergency ratings’, a study has been carried out to investigate the effect of scaling the real loading at the transformer site, to determine the size and frequency of loading conditions which result in exceedance of the transformer hot-spot temperature limit. Again as per the majority of this report, a reduced limit of 130oC has been used.

The load on the transformer is scaled using simple multipliers, and for each scaling value in turn is then passed to the transformer rating algorithm as a time-series. The hot-spot and top-oil temperatures are then calculated for each time step.

As a preliminary step, no load reduction interventions are considered. This is in order to baseline the potential requirement of load reduction services. In the second iteration of the method, if the hot-spot temperature for the time step under consideration exceeds the pre-determined hot-spot limit, the present loading value is reduced until the hot-spot temperature is reduced to below the limit. The necessary reduction in load is then calculated.

In the course of the algorithm 100% of the necessary load reduction is assumed to be both available and to have occurred. Therefore the initial hot-spot temperature for the next time step is the result of the simulated reduced loading conditions. This is to prevent the hot-spot temperature returning to an unrealistically large value after a possible intervention. This is felt to be more in-line with a real-life scenario.

If the real-loading at the next time step is sufficiently high then the hot-spot limit is likely to be exceeded and therefore an additional load reduction will be required.

As for the transformer emergency ratings this study has been carried out at the outdoor substations though has been limited to the secondary transformers.

8.1 Wooler St Mary

Wooler St Mary

Response Magnitude (kVA) 500 kVA

Max New Load Ratio Max New Load Max Necessary Response 5% 10% 80% 90% 1.7388 869.4 75 68.25 60 10 5 1.7871 893.55 115 100 90 25 10 1.8354 917.7 150 120 109 24.5 15 1.8837 941.85 170 140 125 30 15 1.932 966 195 151.25 130 30 15

1.9803 990.15 215 165 140 25 15 2.0286 1014.3 240 170 155 30 15 2.0769 1038.45 260 185 160 25 15

Table 8 – Maximum and percentiles of required response to mitigate hot-spot temperature limit violations


Table 8 shows the maximum necessary response required by an additional source in order to reduce the hot-spot of the transformer below the pre-determined limit. As an example, if the transformer load were to be increased such that the new peak load is 869.4kVA, the maximum response required would be 75kVA. However, this is the response required to mitigate against the peak event. For 90% of the observed excursions, the required response was limited to 5kVA. If a combined RTTR and DSR/EES system were to be installed, this 5kVA value represents a significant decrease over the maximum required response, thus informing the nature of the response(s) required to mitigate against the variety of observed excursions.

Excursion Duration (min)

Max New Load Ratio Max New Load Max Duration 5% 10% 80% 90% 1.7388 869.4 17 17 15 1 1 1.7871 893.55 50 50 46 1 1 1.8354 917.7 106 88 25 2 1 1.8837 941.85 108 106 92 2 1 1.932 966 115 106 40 2 1

1.9803 990.15 182 76 16 2 1 2.0286 1014.3 184 33 20 2 1 2.0769 1038.45 191 35 18 2 1

Table 9 – Maximum and percentiles of durations of hot-spot temperature limit violations

Whilst the magnitude of the excursions is important, the duration is also crucial if cost-effective sizing of a potential response is to be carried out. A high magnitude, short duration response differs greatly from a small magnitude long duration event. Table 9 shows similar results to Table 8, however we now consider the duration of excursion events.


9 Conclusions This report has shown findings from analysis of the RTTR trials carried out as part of the CLNR project. If limitations in transformer ancillary components can be addressed then there is significant potential to increase the ratings of power transformers.

Based on a half hourly maximum loading approach there is evidence to suggest that loading values in the region of 200% of nominal capacity are possible. Clearly there is a need to greater understand the impacts of loading transformers to such capacities on a regular basis, such as the impact of ageing and the effect on components.

Emergency ratings have been calculated for transformers, based on their real measured load profiles and ambient conditions. This increased visibility shows that these ratings can often be increased, again providing additional loading capacity with relatively minimal monitoring.

Where a form of interventional response is available, significant step increases in loading have been shown to be possible, not limited to singular half hourly periods, but for the entire data monitoring period. The duration of required responses is small even in order to mitigate against a peak load 1.7 times the original. Such a small response is perhaps viewed as a negative, since installation of devices such as energy storage represent high capital cost investments. It is however more likely that such an intervention has been installed to provide responses to multiple power system problems, of which only one could be to mitigate against thermal rating events. In this scenario, it can be seen that a relatively small quantity of response is necessary, to give significant increases over present loading states.


10 References

[1] "IEC 60354 - Loading guide for oil-immersed power transformers " 1991.

[2] "Engineering Recommendation P15: Transformer Loading Guide," 1971.

[3] "IEC 60076-7 - Loading Guide for Oil-Immersed Power Transformers," 2008.


11 Appendix A – Transformer Emergency Rating Results Transformer Emergency Rating (% of nominal) 2013 2014 Percentile Percentile

Hot Spot Limit = 98 Transformer Rating

(kVA) 99.9 99 97.5 80 50 99.9 99 97.5 80 50 Primary

Denwick 20/25 (MVA)

(ONAN/OFAN) 95 96.87 99 102 107 Secondary High Northgate 315 103 103 104 110 113 93 96 98 104 109 Marwood Crescent 500 106 106 108 116 120 89 90 96 104 112 Sidgate Lane 750 139 141 147 154 160 107 112 118 144 156 Wooler Ramsey 500 105 105 107 119 124 96 102 105 113 119 Wooler St Mary 315 108 109 110 114 118 98 99 102 111 117

Table 10 – Transformer Emergency Ratings – 98oC Hot Spot Temperature Limit

Transformer Emergency Rating (% of nominal) 2013 2014 Percentile Percentile

Hot Spot Limit = 130 Transformer Rating (kVA)

100 99 98 80 50 100 99 98 80 50 Primary


122 125 126 129 134 Secondary High Northgate 315 130 130 131 136 140 120 124 126 131 137 Marwood Crescent 500 132 132 135 143 148 118 120 126 133 142 Sidgate Lane 750 172 175 181 191 198 136 146 154 183 196 Wooler Ramsey 500 130 130 135 149 154 122 129 133 142 150 Wooler St Mary 315 135 136 138 142 146 128 129 132 140 146

Table 11 - Transformer Emergency Ratings – 130oC Hot Spot Temperature Limit


Transformer Emergency Rating (% of nominal) 2013 2014 Percentile Percentile

Hot Spot Limit = 140 Transformer Rating (kVA)

100 99 98 80 50 100 99 98 80 50 Primary


130 133 133 137 142 Secondary High Northgate 315 138 138 139 144 147 128 131 134 139 144 Marwood Crescent 500 140 140 142 151 156 127 128 134 141 150 Sidgate Lane 750 181 184 191 202 208 143 155 164 194 207 Wooler Ramsey 500 137 137 143 157 162 130 137 141 151 158 Wooler St Mary 315 143 144 146 149 154 137 137 140 149 155

Table 12 - Transformer Emergency Ratings – 140oC Hot Spot Temperature Limit


For enquires about the project Contact [email protected]

www.networkrevolution.co.uk

Copyright Northern Powergrid (Northeast) Limited, Northern Powergrid (Yorkshire) Plc, British Gas Trading Limited and the University of Durham 2014

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