Effect of rooftop PV on distribution transformers



Rooftop-mounted photovoltaic systems (RTPV) are expected to have a dramatic impact on distribution networks, and concern has been expressed on the effect on distribution transformers (DT), as increased thermal cycling is expected. However, several diverse studies have shown that a reasonable penetration of RTPV can extend the lifetime of a DT. Nonetheless, if the penetration increases above an optimum point, the effects can be negative. 

Residential rooftop solar has shown a tremendous growth in recent years, and although penetration in residential areas in South Africa is fairly low, uptake is expected to increase. The pattern in other countries has been supported by subsidies, tax breaks and other methods which have made in economically sound for householders to install their own systems. Although PV systems present advantages to consumers and providers, high penetration develops power quality problems such as current and voltage unbalance [6]. One of the disadvantage of load unbalance on a distribution transformer is the reduction of its useful lifetime [6]. On the other hand, the PV generation during peak time can reduce the load, and consequently, extend the lifetime of distribution transformers.

The impact of a high penetration of PV on the distribution network components is an unknown as little experience has been gained in this area, and it remains  a source of concern to network managers. Primary concerns with high penetration of rooftop PV are:

  • Voltage levels
  • Phase imbalance
  • Effect on the lifetime of distribution transformers (DT) and tap changers
  • Effect on the sizing requirements of distribution transformers
  • The impact of reverse feeding on the operation protection equipment

Several studies have been undertaken on the impact of PV introduction on DT lifetime. [1 – 6]. DTs are designed and sized to handle a particular load profile, and the introduction of PV into distribution networks will alter the load profile, which could affect the lifetime of the transformer. Varying results were obtained from the different studies, but the methodology used proves useful. Studies covered both own-use only and feed-in scenarios.

Factors affecting DT Lifetime.

The lifetime of a distribution transformer is mainly determined by insulation life which itself is affected by the transformer loading including magnitude and quality, ambient temperature, and the moisture and the oxygen content of the oil. The most important factor is the hot spot temperature. It has been established that the rate of ageing of paper insulation roughly doubles for every 6 °C increase in temperature [2].

The most important parameter that influence the transformer useful life is the hottest-spot temperature (θH). The relationship between the hot spot temperature and the transformer useful life is obtained by using experimental results and the Arrhenius equation given in the IEEE standard. Practical versions are given in eqns. (1) and (2 ) which describe, respectively, the relative ageing rate, or ageing acceleration factor, VT , for a thermally upgraded paper (reference temperature of 110°C) and non-thermally upgraded paper (reference temperature of 98°C) [3].

(1)

 

 (2)

 

The effects of hotspot temperature on aging are shown in Fig. 1.

Fig. 1: Variation of the per unit life with the hottest spot temperature [4].

Prediction models have been developed to estimate the winding hot-spot temperature (HST) and top-oil temperature. Methodologies to calculate the HST are presented in IEEE standard  C57.91-1995 and The IEC standard 60076-7. These are fairly complicated and will not be discussed here, except to mention that the main factor affecting HST is the loading of the transformer.  As loading varies with time, a knowledge of the time based load profile of the DT is necessary in any investigation.

Methodology

All studies mentioned were based on actual networks, so existing load profiles were available. The methods used in the different studies differ in complexity, but all follow the same basic steps. The first step is to establish a base case using the existing load profile. The loss of life or deterioration is estimated for the base case over a period of time, at east a full year and usually several years. the second step is to simulate transformer load profiles  incorporating different amounts of rooftop PV, and the loss of life  or deterioration is recalculated for each profile over the same time period. The effect of PV usually incorporates information on the irradiation pattern (timewise) and the seasonal variations in radiation.  To be accurate, the new profile must incorporate seasonal load variations and seasonal radiation variations. Deterioration is estimated for each new profile and comparison made with the base case profile.

The process incorporates  the following steps:

  • Estimate the HST profile based on the load profile. ( stepwise or continuous)
  • Estimate the deterioration associated  with each HST time segment
  • Aggregate the LOLs  over the study period.

This is a very simplified version of a very complex process. the process is decribed mathematically by Behi [6] which defines a loss of life (LOL) parameter as follows:

(3)

 

where LOL is loss of life of transformer in days, and VEQA is the equivalent ageing factor over the time period of t, which is formulated as:

(4)

 

Where:

n = an index for the time interval t

N = the total number of time intervals being considered

Δtn = the time interval

Vn = the ageing acceleration factor for the time interval Δtn

The variation in hot spot temperature will depend on the load on the transformer, and the time that the transformer spends at each hot spot temperature, ie the temperature cycle profile of the transformer. Studies all attempted to estimate the hotspot temperature based on conditions existing without PV and with PV. In all cases existing networks were used as the model. Studies were based on calculated values of HST using measured loads using the models and not on actual measurements of temperature.

Results

All studies showed a reduction of hot spot temperatures during hours of solar power production, and concomitant reduction in deterioration, and increase in expected lifetime of the DTs. The amount of expected increase depends on the composition of the network. A purely residential network has high morning and evening peaks with  a moderate midday demand, so the effect of PV is limited. Commercial and industrial networks have high midday peaks and do not have early morning and evening peaks, so the effect of PV on DT lifetime is more pronounced. Per unit loading of the transformer has a major impact, and DTs with lower loading will benefit less from the inclusion of PV.

Results from some of the studies are given below:

Table 1 shows the results of study in [6].

Table 1: Increase in DT lifetime due to PV [6].
Year  Trans loading (pu) LOL base case (days) LOL 38% solar (days) Improvement (days)
1 1,0 20 11 9
2 1,1 45 22 23
3 1,2 184 85 99
4 1,3 855 375 480
5 1,4 4486 1868 2618
Total 5590 2361 3229

Fig.2 shows the results of study [2]

Fig. 2 (a): Demand profile for the month of July.

Fig. 2 (b): Cumulative aging for the month of July.

Other impacts covered by the studies

Some of the studies considered the effects of harmonics and current and voltage imbalance due to PV on the transformer aging. The most significant effect was found to be current imbalance due to uneven distribution of PV installations between phases, resulting in increased neutral current and increased losses. This also resulted in voltage unbalance. In some cases where a significant penetration of PV had been achieved it was found to be necessary to rebalance the loads between the phases [3]. High PV penetration has been found to increase the tap changer operation which could negatively affect the life of the tap changer.

Fig. 3: Rate of paper insulation ageing for the transformers with highest and lowest PV penetration [2].

References

[1] A Manito: “Evaluation of utility transformers’ lifespan with different levels of grid-connected photovoltaic systems penetration”, Renewable energy Vol. 96, October 2016.

[2] D Martin, et al: “Effect of Rooftop-PV on power transformer insulation and on-load tap changer operation”, APPEEC, 2015.

[3] H Pezeshki: “Impact of High PV Penetration on Distribution transformer lifetime”, IEEE transactions on power delivery, 0885-8977, 2013.

[4] Banu Uçar: “Influence of PV Penetration on Distribution Transformer Aging”, Journal of Clean Energy Technologies, Vol. 5, No. 2, March 2017.

[5] R Gadu: “Effect of Loads and Other Key Factors on Oil-Transformer Ageing: Sustainability Benefits and Challenges”, Energies, 2015.

[6] B Behi: “Distribution transformer lifetime analysis in the presence of demand response and rooftop PV integration”, Renew. Energy Environ. Sustain. 2, 27, 2017.

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