by Francis Luces
Introduction
For developers of power plants, one of the important factors to consider is where and how to interconnect a plant to an existing transmission network in order to reliably deliver its full output. For conventional power plants (i.e. coal, oil, natural gas, etc.), the availability of fuel supply and environmental permitting are the main considerations for siting. In the case of solar photovoltaic (PV) projects, given the availability of land area for mounting solar panels and sufficient solar irradiance, the point of interconnection (POI) to the grid can be the determining factor for siting. An assessment of the thermal capacity at potential POIs provides an effective screen for potential sites. Using transmission capacity injection analysis, developers can swiftly determine the capability of the existing network to support additional power from a new source such as a PV project. With this type of analysis, solar power project developers can know fairly early in the development process if the selected site and POI can support the plant’s output.
Why thermal capacity assessment?
When a power project developer has selected a prospective site for their power plant based on results of a feasibility study, the traditional process is to evaluate its interconnection to the transmission network by conducting a system impact study. In a system impact study, full technical assessment such as thermal capacity, fault duties, and transient stability, among others, are investigated. After this study, the developer may find out that there are issues in the interconnection that may require grid re-configuration in order to support the plant capacity.
In a transmission capacity injection analysis, only thermal capacity is investigated. This is done before the power project developer has selected their final site where the power plant is to be located. A number of proposed project sites possibly near or with an easy access to an existing transmission facility (transmission line or substation) are evaluated for thermal capacity. This type of study is sufficient as initial basis for selecting POIs for the solar power plant as well as its feasibility to be connected to the grid due to the following reasons:
1. Thermal violation or overload tends to require additional capacity which in most cases is costly to resolve. A thermal violation is the result of the application of reliability criteria to the planned system model. Aside from the basic requirement that the electrical grid facilities be able to support the thermal load of the interconnecting plant under normal, all-in conditions, transmission security criteria further anticipate outages. The typical form of the criteria is the n-1 which requires that all system facilities remain within their emergency ratings following the loss or outage of any single facility. An emerging, more restrictive, form of constraint is the n-1-1 which states that all facilities remain within the emergency rating or 1-hour rating of equipment after loss of any single element, followed by a 30-minute period of recovery, followed by another outage of a single element. To relieve the overload, the worst-case solution is the construction of new transmission lines and/or transformers which could reduce the economic viability of a project. A new transmission line to be built will require right-of-way (ROW) acquisition, site assessment/survey, and line design/study, while a new transformer will require additional space inside the substation, additional switching devices (circuit breakers, disconnecting switches), metering and protection requirements (instrument transformers, lightning arresters, relays, batteries, etc), and support structures (firewalls, gantries, supports, etc.), the sum of which could turn out to be high ticket add-on costs.
2. Competition among other power project developers. The deregulation of the electricity market and the joint efforts of countries worldwide for the reduction of CO2 emissions have driven the power project proponents to build more power plants utilizing renewable energy resources. The number of project proponents has increased in the recent years which can at times result in competition for transmission capacity. The competition adds additional need to ensure there is sufficient margin for capacity to handle competing projects at proposed interconnection points into an existing transmission network.
Methodology
Transmission capacity screening analysis aims to determine the amount of allowable capacity of a power project from a thermal capacity standpoint. The analysis may be required to address several potential points of interconnections (POIs) in the vicinity of a proposed project. These sites are “screened” or tested for a certain amount of power injection in the transmission network during normal and contingency conditions. To be able to determine whether the project will have an significant impact in the existing network, a distribution factor (DFAX) methodology may be used. The DFAX represents the sensitivity of transmission facilities to changes whenever there is a change on power flow from one part of the system into another. The following distribution factors are commonly used in assessing thermal transfer capability:
1. Power Transfer Distribution Factor (PTDF) – A measure of the responsiveness or change in electrical loading on system facilities due to a change in electric power transfer from one area to another, expressed in percent (up to 100%) of the change in power transfer. It is defined as
2. Line Outage Distribution Factor (LODF) – is a measure of the redistribution of electric power on remaining system facilities caused by an outage (or removal from service) of another system facility, expressed in percent (up to 100%) of the pre-contingency electrical loading on the outaged facility. It is expressed as
The use of distribution factors can be realized from the illustration in Figure 1.
A change in flow Δfl on line i-j is observed whenever there is a change in power injection at bus I by ΔP. When this change increases, the line reaches its normal limit at a certain power injection level. The PTDF tells how fast a transmission element approaches its normal limit as it is graphically illustrated by the slope of the blue line. When a contingency is to occur, there is a certain power injection level where the flow of the line hits its emergency limit. This could be used as a tool for thermal analysis in transmission systems. In this case, the LODF tells how fast a transmission element approaches its emergency limit as it is graphically illustrated by the slope of the red line.
The above observations can be best applied in a simple three bus power system shown in Figure 2.
From Figure 2, generation at Bus 1 and 3 supplies a load located at Bus 2. The PTDF for a power flow from Bus 1 to Bus 2 is 60/65 or 0.92307. This means that about 92.307% (60 MW) of power injection from Bus 1 flows along Line 1-2. The remaining 7.693% (5 MW) flows along Line 1-3. If the power injection at Bus 1 and 3 is increased following an increase of load at Bus 2, the flows on the Lines 1-2, 1-3, and 2-3 will also increase, as well as their respective PTDFs. The tendency is that, the flows on each line will eventually hit their normal ratings. This will be significant if we consider also outages in this example. From the same figure, an outage of Line 1-3 will result in an additional flow in Line 1-2 by 5 MW. This means that 100% [(65-60)/5] of the power flow along Line 1-3 is transferred to Line 1-2. When this happens, a thermal violation is observed along with an increasing power transfer from Bus 1 exceeds the contingency rating of Line 1-2.
For large transmission networks with several thousands of lines and buses, the DFAX method works with an aid of a computer. For a specific monitored transmission element, there exists a number of LODF with respect to different contingencies. These LODFs are used to determine the worst contingency that limits the flow of power in a transmission element. These results can be obtained easily since the method is based on a linear solution i.e. the resistance on the line is so small that it can be neglected and only active power flow is considered. The influence of voltage and reactive power flow on the branch loadings are also ignored to achieve a linear solution.
Case Study
This is an actual study wherein a developer intends to put up a 100-MW solar plant and wanted to know the capability of existing system found in the prospective sites.
Site A for the solar plant has an existing 69-kV looped transmission serving several adjacent towns. Two point of interconnections (POIs) were identified here and are illustrated in Figure 3 and 4. Meanwhile, Site B is a 230-kV portion of a large system where several loads and nearby generators were identified. Two POIs (each on the parallel lines) were selected and are illustrated in Figures 5. For most POIs, the plant is connected along a transmission line via a tap while the other POIs are directly interconnected in an existing substation.
For each POI, an injection of 100 MW is used based the plant’s capacity. This injection is increased up to 200 MW until an overload of any transmission element is observed for both normal and outaged conditions. The results obtained from a power flow program are illustrated in Table 1.
The results show that the transmission system located in Site B for both POIs 1 and 2 (either Bus 1 – Bus 2 line ckts1 & 2) has a sufficient thermal capacity to support the entry of a 100-MW solar power plant at the desired location. The system has a thermal capacity to support up to 200 MW which allows for future growth in case of competing generation projects on the specific site. For Site A, the existing system shows that it can handle only 91.5 MW of injection upon the entry of a 100-MW solar power plant. It is selected as the maximum injection for both POI 1 and 2 since a contingency Bus 1 – POI and POI – Bus 2 transmission lines constraints the existing system to support additional power from the solar plant. It could be also observed here that most DFAX (especially LODFs) were near or equal to 1.0 which is evident because the local transmission elements are the most impacted by the contingencies located also in the vicinity of the POI. The sample case study concludes that POIs at Site B would be one of the best options for a location to site a solar power plant.
Conclusions
The method of thermal capacity assessment can be used to identify best location for siting a solar power plant. The linear nature of this assessment finds its best application to analyze thermal capacity of existing transmission systems in a competitive environment. However, care should be taken on applying the method since voltage and reactive power influence the thermal capacity of the system for longer-term interconnection studies.
References
On Using Linear Approximation and Distribution Factors. Pterra Techblog. https://www.pterra.com/on-using-linear-approximation-and-distribution-factors/
Wood A.J., Wollenberg B.F., and Sheble G.B., Power Generation, Operation, and Control 3rd Edition 2014. John Wiley & Sons Inc., Hoboken, NJ