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The impact of rooftop PV on distribution networks

Rooftop solar panel cells

South Africa has an excellent solar resource. Many experts agree that the wide spread proliferation of rooftop solar PV installations will take place, and is just a matter of time given the opportunities that this technology presents customers and local industry/installers.

Solar PV is a particularly attractive form of renewable generation, given that it can be relatively easily installed on existing structures (such as roofs) to generate electricity that is consumed by local loads, with any excess power being fed back into the utility network. It is a proven technology and the costs have dropped dramatically to the point that the resulting cost of electricity is comparable to tariffs charged by distributors. The technological developments and cost reductions have been driven by the large scale deployment of solar PV in several countries, with the likes of Germany having more than 35 GW of solar PV. This has been promoted with feed-in tariffs (FIT) whereby the customer is paid for energy exported into the grid. The magnitudes of FITs in Germany were reduced as PV costs decreased.

Small scale solar PV installations in South Africa have been largely limited to off-grid remote area power supplies. Such systems are typically coupled with battery energy storage and diesel generation.

Grid connected rooftop solar PV targets a different market segment – customers, which already have grid access.

Potential customers include users that want to supplement their utility energy supply with renewable energy for environmental reasons and/or to reduce electricity costs and potentially generate a financial return as may be linked to incentives and subsidies.

The consumer business case is sensitive to whether energy can be exported into the grid, and the price paid for this energy, both of which are constraining local deployment of rooftop PV in South Africa.

The power output of a solar PV module is dependent on the intensity of the incident solar irradiance (sunlight), which is a fluctuating resource. The installation of large amounts of variable solar PV hence presents a challenge to the system operator and distribution utilities.

The electrical network has historically been designed and operated to supply loads via centrally installed and dispatched generation (in the South African context large scale coal based in Mpumalanga). These networks were designed to allow the unidirectional flow of power from the transmission grid to the end use customers via the distribution network. For the purposes of this article we focus on a typical distribution network supplying domestic customers, and consider the impact that widespread rooftop PV could have on the distribution network.

The South African urban distribution network generally consists of an 11 kV medium voltage (MV) network supplying 400 V low voltage (LV) networks via MV/LV distribution transformers. The LV network consists of a three-phase backbone, with individual customers supplied with either three-phase or single-phase supplies, depending on the magnitude of load to be supplied. All power flows from the MV network to the LV network. These networks were designed to keep voltage drops within acceptable limits without overloading components.

When rooftop PV is connected to this distribution network, the generated power is consumed by the local loads in the customers’ facility. Any excess power is exported into the network where it is then consumed by other customer loads. The domestic demand typically peaks in the mornings and evenings, as linked to the time periods when consumers generally use the most appliances, such as lighting, electrical cooking, space heating and hot water heating. Solar PV only generates when the sun is shining, typically peaking at solar noon. The domestic load consumption at midday is relatively low, and may result in PV generated power flowing back into the utility network. There are a number of different commercial arrangements whereby customers can be compensated for the generated energy, such as feed-in tariffs (FITs) and the banking of energy, but these commercial aspects are not the focus of this article.

Reverse power flow in the distribution network can be problematic due to the resultant voltage rise. The distribution network has been designed and operated to curb the impacts of voltage drop, not voltage rise. The voltage control philosophy is such that during periods of low-loading the maximum voltages may already be close to the allowable limits. Local generation (causing a voltage rise) may then result in the maximum voltage limits being violated, leading to the failure or reduced efficiency of customer appliances. A South African domestic LV network is able to absorb significantly less generation as compared to the maximum amount of load that it can supply.

This is a concept that is not adequately appreciated, as the uninformed understandably assume that a customer can inject the same amount of power into the network as they can consume as a load. This is not the case in voltage limited networks, such as those that supply domestic customers.

LV customer loads vary considerably throughout the day and year, and the periods of maximum load occur when customer demand coincides. Maximum loading typically occurs on a cold winter evening due to increased cooking, hot water usage and space heating. These peak loading conditions might only occur for few hours in a year.

By comparison, most parts of South Africa have an abundance of sunny days, and it is common to have weeks of excellent sunshine whereby a solar PV installation will regularly generate maximum (or close to maximum) output during noon hours. The PV installations in a local area will peak at the same times. If the penetration of solar PV causes over-voltages (due to reverse power flow), such over-voltages may occur frequently, with serious ramifications for customers and the utility.

Another key consideration is the impact that cloud transients will have on the network. If a cloud passes over a residential neighbourhood and shades all of the PV panels in the area, then within a few seconds the PV output will drop drastically. The distribution network suddenly has to supply all of the power that was being supplied by the PV, resulting in a rapid change in voltage. This implies that with cloud shading, the voltages supplied to customers will vary. These frequent and rapid voltage changes may exceed maximum allowable limits, and could result in voltage flicker problems.

Voltage variations are a key constraint in establishing the maximum amount of PV that can be connected to a distribution network before technical limits are violated. The phase allocation (R, W, B or A, B, C) of loads and generators has a very significant impact on voltage variations, and is an important aspect that has historically been poorly managed by South African distributors. The phase allocation of single phase customer supplies is often such that there are considerable voltage imbalances in the three phase LV network.

If phase connections are not well managed then a scenario could arise whereby all the PV installations on a particular LV network are connected to the same phase. The resultant voltage rise would be in excess of three times the balanced scenario. The management of the phase allocations in the distribution networks is hence expected to be a key issue, and this is supported by experiences in other counties, such as Australia, where the installation of rooftop PV has seen an increase in the frequency and severity of voltage imbalance problems.

The above issues are further compounded by the lack of information and visibility of the distribution network, and in particular the LV network, which has been managed in a “fit and forget” manner. This is expected to be a challenge for local distributors given that the connection of rooftop PV will necessitate adequate record keeping and associated processes. Utilities require simple “rules of thumb” to aid them in the connection of rooftop PV. Local work has culminated in NRS 097-2-3 “Simplified utility connection criteria for low-voltage connected generators”, which provides distributors with guidance on the magnitude of rooftop PV that can be connected to LV networks without having to do detailed network simulations or field assessments. Although difficult to generalise, technical issues may arise when the local generation exceeds 20% of the maximum load, and innovative solutions may be required when installed generation exceed these levels.

It is important to note that unless there is storage installed with solar PV (and batteries are expensive and environmentally sensitive), solar PV does not reduce the evening load peak, and hence does not reduce the need for the network. Domestic electricity tariffs have historically bundled generation and network charges into retail energy charges (rand/kWh). This means that a portion of the energy charge is used to offset the costs of creating and maintaining the network. A reduction in energy consumption due to rooftop PV implies there will be an under recovery of the network related costs, resulting in the need to increase tariffs to offset this shortfall, or change the design of tariffs so that grid connection and maintenance costs are recovered separately from energy charges. This requires careful consideration in tariff design to ensure that customers still pay their equitable share of network costs, but can reap the benefits of reduced energy costs due to their rooftop PV installation.

This article has focused on some of the technical challenges of rooftop PV installations in the context of the distribution network, and may create the impression that rooftop PV has a detrimental impact on the grid. Rather it should be appreciated that uncontrolled widespread rooftop PV rollout (without adequate coordination and technical checks) runs the risk of voltage and power quality problems. The likelihood and severity of problems will be dependent on historical design practices and the extent to which network operations are optimised. The safety implications for linesmen working on equipment also requires careful consideration in order to ensure that PV installations are not able to back-energise the utility network during network faults and maintenance outages. These and other safety, power quality and performance considerations are being reflected in the local standards for embedded generation and specifically addressed in the NRS 097 series.

High levels of rooftop PV penetration (as in Germany) necessitate new and innovative approaches to managing the technical issues and complexity. New solutions deployed in pilot projects include on-load tap changing MV/LV transformers, LV voltage regulators, MV electronic voltage regulators (power electronics replace the mechanical moving parts of the traditional tap changer), optimised volt/var control systems and MV static var compensators. The commercial deployment of such technologies will fundamentally change the nature of the distribution business, and will need to be supported by new business models and processes. This represents a radical departure from passive distribution networks, and is expected to drive the shift to active distribution networks with autonomous control.

Rooftop PV is expected to play an important part in the South African energy mix. As an industry we need to embrace this proven technology and find solutions that maximise its value to our customers and the economy. A national framework is required to allow rooftop PV to occur in a coordinated and sustainable manner.

This article was published in Energize, May 2015, and is republished here with permission.


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