Gradual reduction in cost of solar photovoltaic (PV), threat of climate change and energy security are mainly responsible for promoting growth of global solar PV. In view of this, the National Solar Mission, initiated by Govt. of India, has a target of 100 GW of solar PV by 2022. As the share of distributed and centralized solar PV is gradually increasing in total generation capacity, the challenge of its integration into the power grid becomes more important. This challenge limits the solar PV capacity addition into the power grid without alterations in power system planning. It means, continuous growth of solar PV, to meet national targets of capacity share, needs systematic power system planning in all three areas. The three areas of power system planning include generation, transmission and distribution. In view of this, we shall hereby discuss, how the planning practices are to be modified to add a high penetration level of solar PV. We shall also discuss how to use other technologies such as generation control, load control and energy storage. As the solar PV generation is going to be cheaper in future in addition to other aforesaid benefits, it shall become highly competitive to thermal power generation or fossil fuel generation in general. The large share of solar PV is in the form of distributed generation and hence it is developed in accordance with codes and standards that govern distributed generation, such as IEEE 15471. This code of practice provides guidelines for generation of distributed solar PV with low penetration. However, this article is based on the assumption of high penetration of distributed solar PV and its impacts on the power system.
High penetration of solar PV imposes challenges of grid integration for the following reasons.
• Solar PV is a variable source of generation. Its power output is affected by solar radiation intensity and ambient temperature.
• Solar PV is considered as a distributed source of generation. As the capacity addition of solar PV increases, there are two possibilities. First is the use of energy storage, so that no power is returned to the system. The other possibility is that power is sent to other loads in the system. The second possibility avoids capital investment for energy storage.
• The codes and standards that provide guidelines for the integration of solar PV into power grid are based on simplified power systems that require minimum interaction of solar PV with power grid. When solar PV becomes a significant source of generation in the power system, the aforesaid code and standards are not applicable. These challenges are very well faced by power utilities and solar PV developers. Hence, there is continual efforts by solar PV utilities and government in developing relevant codes and standards for high penetration of solar PV. In the field of power systems, planning is the activity of utmost importance especially for integration of high penetration of solar PV into the grid.
Impact Of High-penetration Of Solar Pv On Power System Planning
High penetration can be defined in two ways. One is in terms of energy and other is in terms of peak power. Definition of solar PV penetration is the volume of energy supplied by solar PV to the system resulting in reduction in fossil fuel generation and carbon emissions. The energy based definition is highly useful in analyzing large systems. The negative aspect of this definition is that it depends on environmental factors and location of energy source i.e. solar PV. It means, with different environmental conditions of solar radiation & temperature as well as location of solar PV, we get different power output. A power based definition provides better understanding of power system parameters. It is expressed as the nameplate capacity of solar PV divided by the peak load of the power system. In this article, definition based on peak load shall be preferred over energy based definition.
1) Impact Of Variable Renewable Energy Generation
The intermittent behavior of solar PV generation is the prime challenge of its integration into the power grid. Generation utilities think in terms of peak load and available generation capacity. They want that at any particular time they must have enough available generation capacity to feed the peak load. Let us compare a 200-MW thermal power plant with a 200-MW solar PV plant. Assuming a 6% outage rate, a thermal power plant shall provide 94 % capacity factor, whereas a 200- MW solar PV plant shall provide capacity factor between zero and 200 MW depending upon solar radiation intensity and temperature. The variability related to solar PV generation adds complexity to the power system planning process. Fossil fuel generators now have to adjust their operation cycle in accordance with the variable nature of solar PV. This increases the per unit operating cost from fossil fuel generation, lower fossil fuel generation, lower fuel usage and lower carbon emissions. These effects are useful for national welfare but not for owners of fossil fuel generators because of the increase in per unit cost. This increase in per unit cost of fossil fuel is termed as “integration cost”.
2) Modifications In Generation Planning
The correct way to effectively include intermittent nature of solar PV in the capacity planning process is to consider the system energy, not the peak load. This is discussed as below.
Process flows of traditional and emerging capacity planning practices for penetration of solar PV are compared in Figure 1.
The traditional planning process is not designed to consider intermittent generation, therefore it was excluded from capacity planning. The process starts with indication of the energy growth followed by indication of the peak load. The generation and transmission capacity are planned to match the mentioned peak load. Renewable generation is considered to be “as available”. The output of this process is substandard system operation, lower capacity factor resulting in lower efficiency, higher emissions and higher per unit costs. The emerging practice is to consider renewable energy generation in the beginning of the planning process after the energy growth factor. This enables full integration of renewable generation into the planning process. The key is in considering variable renewable generation as a part of the load. The planning process is now based on the net load. The system load is reduced to take into account the renewable generation. The amount of reduction in system load is estimated based on renewable generation available at present and that planned in future.
2.2) Characterizing The Impact On Fuel Mix
Net load curves are used to understand which type of generation is replaced by the intermittent renewable generation like solar PV. Each system has a complete generation portfolio. This portfolio is scheduled on a daily basis to feed the load. Total generated electrical energy can be segregated into various types of fuels used to produce it. This is the base of understanding the phenomenon of fuel mix of the system. Each type of generation has different per unit cost. A typical fuel mix diagram is shown in Figure 2. X axis indicates hours and Y axis indicates MW.
Here the net load duration curve with 30% solar PV penetration is shown. Bottom of the curve starts with nuclear power then hydro power followed by solar PV then coal and petroleum and finally natural gas.
2.3) Generation Flexibility
The need for generation flexibility evolves from the requirement to control the system frequency within designed range. Frequency is a global phenomenon. It means, whole power grid operates at the same frequency. The frequency is directly proportional to rotating speed of generators. It is also equal to the output frequency of solar PV inverter. If the load increases, the frequency goes down. If the load reduces, the frequency increases. Consider the sketch shown in Figure 3.
A frequency control area is represented by a closed curve and it includes the total generation “G” and the total load “L.”. Two importing interfaces to represent generation intake are shown on the top and an exporting one to indicate power output is shown at the bottom left. The speed of the enclosed area’s “equivalent generator” is determined by the following differential equation:
Jω * (dω/dt) = PGen – PLoad + PImports – PExports
Where J stands for the equivalent moment of inertia of all generators, ω is the equivalent angular velocity (proportional to frequency), and the power terms have self-explanatory meanings.
3) Modifications In Transmission Planning
Integration of solar PV does not require specific changes to the process of transmission planning. Solar PV generation must be calculated accurately. This accuracy becomes highly significant as the solar PV penetration increases. When creating power system models, it is not recommended to represent each solar PV source individually. Hence, they are combined together to make a single model for several solar PV generators. Equivalent impedances should represent the parameters of the distribution line. Also, indicate at least two levels of voltage transformation that exist between transmission level voltage and distributed PV. Furthermore, distributed PV generation is connected to the distribution system through the inverters, and modeling the performance characteristics of the inverters must be considered. The general characteristics of PV inverters and the specific modeling requirements are discussed as below.
3.1) Common Characteristics Of Pv Inverters
PV inverters typically consist of two separate stages, the PV module interface, called the boost converter, and the grid interface, called the grid converter. An illustrative PV inverter diagram indicating the boost and grid converters is shown in Figure 4.
The DC capacitor CDC shown at the boundary of two converters is shared by the converters and it provides energy transfer between the two converters. The work of the boost converter is to continuously intake energy from the solar array and transfer it to CDC. The boost converter modulates the switch S1, it continuously adjusts its duty cycle to control VPV relative to VDC. Adjusting VPV determines the current from the solar array (labeled IPV). Solar arrays have nonlinear VI characteristics and adjusting VPV is important to achieve maximum power point tracking (MPPT). VI characteristic is shown in Figure 5. By selecting VPV corresponding to point A in Figure 5, power intake from the solar PV panel is maximized, represented as maximum power point. The role of the boost converter is to track this maximum power point for changing insolation.
At the same time, the grid converter takes the energy from CDC and supplies it to the grid, represented here as an AC source of voltage labeled VAC. The voltage across CDC (VDC) must always be greater than or equal to the peak of VAC to enable operation without significant AC current non-linearity (AC current is labeled IAC in Figure 4). The grid converter controls the magnitude of IAC to supply the desired amount of power to VAC. At steady state, power supplied to the grid matches the power intake from the solar array, and the voltage VDC across CDC is maintained at the constant value. The grid converter also can control the phase angle of IAC relative to VAC to exchange reactive power with the grid. Limit on the reactive power supplied by the inverter depends on current capacity of the switches Q1 through Q4.
3.2) Pv Inverters’ Behavior During Grid Faults
Currently, PV inverters are required to disconnect from the grid during grid faults. Experience gained from the wind industry suggests that staying connected during the fault and helping to restore the voltage after the fault is cleared, aids system stability. It therefore is reasonable to expect that the PV industry will face similar requirements as the penetration levels increase. This feature is known as low-voltage ride through (LVRT) or zero-voltage ride through (ZVRT). Suppose, for example, that the grid experiences a fault and due to this fault VAC falls to some low value (it is “shorted” by the fault). During the fault, the grid inverter no longer can supply power to VAC (power is the product of voltage and current, and if the voltage is brought to zero due to a fault, the power delivered to the AC circuit also becomes zero). This makes it impossible to remove energy from CDC, and if the boost converter continues to transfer energy from the solar array to CDC, VDC will start to rise. The rise of VDC is a signal to the boost converter that something is wrong and so it moves the operating point from point A to point B as shown in Figure 5. (Point B is shown at a power higher than zero to indicate that some active power is used internal to the inverter, and that some power can be fed to the grid even during faults; generally, fault current travels through the conductors to reach the short circuit and this has associated power losses.) Operating point B can be assumed in a matter of milliseconds and it then is maintained for the duration of the fault. After the fault is cleared (many are cleared within 100 ms), the boost inverter has to come back to point A; generally no information is provided by the manufacturers on the speed of this reverse transition.
4) Modification In Distribution Planning
Distributed solar PV generation affects distribution system planning in three essential ways.
• It affects feeder voltage regulation.
• It makes contributions to fault currents.
• It can provide an ungrounded source of voltage.
Each of these effects is discussed in the following subsections.
4.1) Feeder Voltage Regulation
If the penetration level of the solar PV inverters is adequately high, reverse power flow through the distribution system towards sub-station might occur during some intervals of the day, when load demand is sufficiently low. This can create undesirable voltage levels at sub-station and cause miss operation of the substation voltage control mechanism. This reverse power flow causes a voltage gradient from the distributed solar PV towards the substation. This results in voltage rise beyond the limits suggested by appropriate codes and standards. In many situations, this voltage rise can be brought to within limits by adjusting the on load tap changer (OLTC) in the substation. For the control of this voltage rise sophisticated control schemes based on communications of remote voltage points are also possible.
4.2) Contributions To Fault Currents
Modern PV inverters employ self-commutated inverters that operate in current control mode. This results in the extremely fast short-circuit protection and limiting of fault currents to less than 2-pu peak value that is removed within 1ms. Compared to fault currents fed by conventional generators, inverter fault currents are negligible and unlikely to cause significant damage. Inverter-coupled solar PV sources have a more significant impact on protective relaying. Transformer connected PV inverters can provide a ground path and affect the magnitude of zero-sequence currents.
4.3) Ungrounded Source Of Voltage
PV inverters can be coupled to the distribution system via transformers and based on the transformer connection they can provide an ungrounded source of voltage to the distribution system island that is formed after the substation breaker is opened. This can cause high line-to-ground voltages on the ungrounded phases during single line-to-ground fault. These problems can be avoided by selecting a star connected transformer at the solar PV side.
• As the penetration level of solar PV rises, it might be required to provide performance characteristics similar to those of traditional generators. Such requirements could include frequency droop characteristics, reactive power injection and the ability to control active power. Present inverter technology is able to support these grid-friendly features, at the cost of inverter efficiency and slightly higher capital cost. However, many inverters being sold at present are highly suitable for higher efficiency but have no ability to provide grid-friendly features. To overcome this, a careful evolution of codes and standards is required.
• In addition, designing inverters to be grid friendly accomplishes only part of the job. The other necessary part is the communication link between the system operators to every installed inverter. This communication link can take many forms. Most sophisticated design for this purpose is high-speed Internet infrastructure.