Rooftop Solar PV Systems | Distribution Impact Study
Rooftop solar PV systems connected to the electric distribution system are examples of what is generally called “Distributed Generation”. The term “Embedded Generation” is sometimes used. Installation of rooftop solar PV systems has dramatically increased in the past few years as environmental concerns, technology advances, and with the help of government programs aimed to promote the use of renewable energy sources. However, the increased penetration of these systems has caused electric distribution utilities (DUs) to implement measures in order to maintain the safety and reliability of the grid.
As part of the technical evaluation, DUs have the option to conduct a Distribution Impact Study (DIS) to evaluate the capability of the distribution system to safely and reliably accommodate a proposed interconnection of a distributed generation or DG.
The changing landscape of the electrical grid is brought about by the increasing penetration of DGs. A significant percentage of this comes from rooftop solar PV systems.
Drivers of Distributed Generation
Conventional power flows starts with the generating plant that is usually controlled by a centralized system. Power flows through the transmission lines, distribution lines, and finally to the end-users of electricity. This, however, is gradually changing with the generating plants directly connecting to the distribution system. So what drives this change? The key factors that influence the change are the following:
Environmental Concerns
The concern over climate change is one of the major concerns causing the shift to renewable energy resources. With conventional power plants mostly fossil fuels, concerns over greenhouse gas emissions affecting the environment have risen.
Technological Advancements
With the shift into renewables, advances in technology made it possible to efficiently connect generating plants to the distribution system.
Government Policies
These policies are aimed to make the cost renewables competitive to the existing fossil fuel prices making the shift more attractive. Others include subsidies like feed-in tariff and net-metering programs.
The Net-Metering Program
This program allows electricity consumers to install a small-scale renewable energy facility (usually a maximum of 100kW) in their premises to generate electricity for their own use. Furthermore, any excess energy produced can be exported to the distribution system and is paid by the distribution utility at the DU’s blended generation cost. This program allows consumers to become prosumers (producer and consumer at the same time) of energy.
Scope of the Distribution Impact Study for Rooftop Solar PV Systems
The increased penetration of rooftop solar PV systems has caused electric distribution utilities to implement measures in order to maintain the safety and reliability of the grid. As part of the technical evaluation, DUs have the option to conduct a Distribution Impact Study (DIS) to evaluate the capability of the distribution system to safely and reliably accommodate a proposed interconnection. The scope of the DIS includes the following:
Impact of short-circuit infeed on the distribution equipment
Short-circuit infeed is evaluated to determine the impact of the DG installation on the fault level at the connection point. This is very much important since existing equipment connected at that point were sized according to the short-circuit duty prior to the DG installation.
Coordination of the protection system
The short-circuit infeed is also assessed to determine its impact on the coordination of the protective devices in the distribution system. The assessment will determine if changes to settings of protection relays, LV-circuit breakers, and trip levels of fuses are necessary.
Impact on thermal component ratings and voltage variation
Load flow analysis is conducted to determine the power flows especially during the times when the DG is exporting power to the grid.
Impact of user development on power quality
Power quality parameters such as DC current injection, flicker severity, and harmonics are assessed based on energy regulatory standards.
Steps in Conducting a Distribution Impact Study
Step 1: Data Gathering
The first step of the DIS is to gather information about the existing customer demand and the RE facility to be installed. Generally, the following data are required:
- Technical specifications of the RE system specifying the kVA capacity and voltage rating. A typical example is shown in the figure below.
- Customer load profile.
- Type verification test results and/or
certifications or equivalent; should include the following:
- Power Quality:
- Harmonics with THD and TDD summary
- Voltage fluctuations and flicker
- DC injection
- Power Factor
- Protection:
- Frequency tests
- Voltage tests
- Current tests
- Anti-Islanding functionality tests
- Power Quality:
Step 2: Assessment of RE System Compatibility to the Existing Utility Connection
This is a basic step to ensure that the RE system is compatible with the nominal system voltage at the connection point. There is no easy way of connecting a 120V RE system to a 230V utilization voltage, right?
Step 3: Impact of Short-circuit Infeed on the Distribution Equipment
The next step of the study to assess the impact of the short-circuit contribution of the RE system to the existing fault duty at the distribution system and compare it against the short-circuit rating of existing equipment. This short-circuit information can be found on the RE system type verification test report. The following figures show a typical short-circuit information from a sample type verification test report.
You may want to read this to understand the parameters from this report.
If no such information is available, short-circuit contribution may be assumed to be
It should be noted that for PV-inverters, the short-circuit contribution is a result of the current limiting function of the controller and hence not affected by any impedance between the PV-inverter and the fault location.
This short-circuit contribution is added to the existing distribution system fault duty.
Step 4: Coordination of the protection system
The new fault duty arising from the short-circuit contribution of the RE facility will be used to assess the coordination of protection relays, LV-circuit breakers, and trip levels of fuses are necessary. Time-current coordination study is required in this step. Figure 4 shows a sample TCC curve in many coordination studies.
Step 5: Impact on thermal component ratings and voltage variation
Plot the customer load profile and typical generation profile of a RE system with the applied capacity. This is available in many computer software such as ETAP. You can also do an online search of typical generation profiles and scale it to the applied capacity. In this step, we are interested in two extreme cases.
Case 1: Minimum voltage at the load center, low voltage side; maximum load, and no generation.
This is to establish the baseline data at the utility connection point. Thermal component ratings and voltage variations are based only on the customer loads.
Case 2: Maximum voltage at load center, low voltage side; minimum load (during day-time), and maximum generation.
This is the case where the RE facility produces considerably more power than the customer load. In this case, increased thermal loading of transformer, lines or cable, and voltage rise is likely.
The plots should look similar to the figures below.
From the plots, get the data with the widest margin between the customer load and generation. In this example, the widest margin for case 1 is 77.25kW (77.25 – 0), while for case 2, it’s -55.36kW (6.72 – 62.08).
Do a load flow study using these two sets of values. Our interest here is to determine the maximum thermal loading and voltage rise on the LV feeder.
Step 6: Impact of user development on power quality
From the RE system type test report, verify the results for DC current injection, flicker severity, and harmonics as shown in the following figures.
Bonus Step: Harmonic Performance
In most cases, the harmonic performance of the RE system is further evaluated based on the utility harmonic planning levels. In the absence of these, other methods are used. A combination of the following evaluation methods is usually done to assess the harmonic performance of the RE system.
- Voltage Droop Concept (Based on Electricity Engineer’s Association Power Quality Guidelines)
- IEC 61000
- IEEE 519
Recommendations
The results of the Distribution Impact Study will determine if the existing distribution system infrastructure can safely and reliably accommodate the proposed connection. In cases where a distribution network upgrade is required, a Distribution Asset Study is conducted in order to define the necessary upgrades.
References
Moeller & Poeller Engineering, “Distribution Impact Study Guideline: Guideline for Studying the Impact of Rooftop PV-Systems on Distribution Networks in the Philippines”, December 2013
Operations Directorate, “Distributed Generation Connection Guide”, Energy Networks Association, October 2010