Learn PV substation engineering and design automation with pvDesign

Interested in an in-depth analysis of substation and their importance? Then, keep reading. In this post, we will analyse the importance of the substation, the most used substations and their characteristics, the global substation market and its outlook. Also we will see how solar engineering companies use software to automatically design the basic engineering of the step-up substation of their PV plants.

An electrical substation acts as an interface in power networks where distribution feeders and transmission lines are connected together through switches or circuit breakers by transformers and busbars. This allows the control of power flow across the network & general switching operations for maintenance purposes.

Why do we need a substation?

To connect a solar PV plant to distribution or transmission networks, it is necessary to step-up the voltage level from medium to high voltage. The purpose of a substation is to convert low voltages from electricity generated to high voltages, or vice versa, using power transformers.

In more detail, substations have 4 key functions:

1. To satisfy load growth and transmission capacity rapidly when a location has little to no power supply infrastructure.

2. To accommodate new energy generation such as from wind or solar plants.

3. To maintain reliability requirements to address congestion in the power grids.

4. To break the power flow in scenarios of fault response.

The Power Transmission and Distribution Industry has witnessed significant upsurge due to its growing life expectancy and the rising demand for effective, safe, reliable and stable transmission & distribution networks.

The lack of an efficient electricity network and the need for infrastructure expansion of cross border networks to supply electricity with less losses across developing and developed nations has been a major industry driver and fundraiser. As an example, the European network announced an investment worth USD 400 billion by 2020 out of which two thirds of the investment will be allocated to install and upgrade the distribution grids across the region.

Additionally, the rapid expansion of transmission capacity, quick disaster response and temporary load support are few indispensable features favoring the deployment of substations.

According to the Global Market Insight Report, Global Substation Market size surpassed USD 151 billion in 2019 and annual installation is anticipated to exceed 24,500 units by 2026.

As solar projects get larger, it isn’t rare that utilities no longer build the substations but, instead leave it to solar and wind developers. For this reason, pvDesign has launched a new feature to generate the basic engineering of some of the most common substations: line to transformer substation, single busbar substations and double busbar substations which we will proceed to evaluate.

Parameters that affect the computation of substations, most used arrangements and their calculation in pvDesign.

All photovoltaic solar plants are connected to a specific point on the grid from which they supply electricity to consumers. This point is known as the Point of Interconnection or POI.

Although from a technical-economic point of view electrical substations are the most used facilities for interconnection, there are other ways of connecting a PV plant to the grid such as switching and breaking stations.

Simply put, whenever it’s necessary to raise the output voltage of the PV plant to common values for electrical transmission, a step-up substation will be designed. However, there is the possibility of connecting the PV plant to the same output voltage, that is, to medium voltage distribution networks. In these cases, a switching and breaking center may be used, as long as the characteristics of the project and POI allow it.

In a fully automatic way, pvDesign carries out the basic engineering of the substation that best suits your photovoltaic plant. As a user, you only need to select the substation card and introduce the high voltage level. Only with this data, our software is capable of generating in detail all the necessary documents of the step-up substation that will allow the connection of the photovoltaic plant to the distribution or transmission networks of the country.

Before analyzing the most used substations, in this section we will show you the most important parameters that we take into account when calculating substations and what types of electrical diagrams you will find.

Among the most important parameters are the medium and high voltage levels. The medium voltage level is the one that we obtain at the output of the transformers of the power stations of a PV plant.

The power stations will be connected by medium voltage lines to head to the step-up substation. At this point, the power transformers will raise the voltage to levels admissible by the distribution or transmission networks.

Parameters needed to generate a substation included in pvDesign (source:pvDesign)

The high voltage level depends on the POI and, therefore, this value can be modified in pvDesign. A varied range of voltages can be chosen and the substation will be calculated accordingly.

• From 45 to 66 kV the PV plant is to be connected to electrical distribution networks. As a general rule, these voltage levels are considered when the consumption points are close to the plant and its installed capacity does not exceed hundreds of MW.

• If voltages are defined between 66 and 170 kV, we would be talking about the most common values in the industry for medium-sized plants that are close to a hundred MW.

• It is becoming more and more common in the industry to build mega photovoltaic plants whose capacities are close to or exceed the GW. In many countries, these plants are not located near the points of consumption but rather hundreds of kilometers away. These are the main reasons that would make it possible to raise the voltage of the transmission over long distances to values such as 220 kV and 400 kV.

However, although the user can only modify the voltage levels, more parameters are necessary for the correct sizing of the substation. We can highlight the following:

• The medium voltage lines of the PV plant and their capacity. These lines play a major role in the sizing of power transformers.

• Environmental conditions such as temperatures and elevation relative to sea level. These variables will help us to compute in detail the insulation coordination of the substation, the busbars and the different electrical components.

3D Single busbar substation (Source: Hitachi ABB)

With these and other factors, the goal is to offer substations that are common to players in the photovoltaic industry. For this, pvDesign evaluates between generating line-transformer substations or substations with a single or double busbars.

Line to transformer arrangement

The line-transformer substation is the simplest and least expensive solution. This type of configuration is usually common for small photovoltaic plants.

As seen in the image, the transformer is connected directly to the power line without busbars in between. As a result, only elements that protect the transformer and the line output are necessary.

According to the Substation Green Book of Cigré, an offset to its greatest advantage, low cost, is that this substation compared to other types of circuit arrangements would be the worst valued in:

• Service security. If there is a primary fault, the consequence is the loss of the whole substation

• Availability during maintenance. During maintenance work, the whole substation is lost.

• Operational flexibility. There is no possibility to split the outgoing circuits.

However, as the energy production of PV plants is intermittent; hence, maintenance works can be done in not electricity generation hours, and primary faults are almost negligible, both requirements, availability during maintenance and service security are less significant than capital cost.  

Profile view of a line to transformer substation generated by pvDesign (Source: pvDesign)
Single busbar arrangements

We could say that single busbar arrangements are somewhat more expensive than line to transformer configurations, because in these cases, a busbar is installed between the output line and the transformer bay. That is, in addition to this new coupling element, the number of protection elements such as disconnectors and circuit breakers as well as measurement elements, essentially current and voltage transformers, increase. This also implies an increase in the substation area with its consequent increase in civil engineering costs.

Even so, the International Electrotechnical Organization Cigré, points out that these configurations compared to others are also less valued in terms of flexibility and level of security.

It is true that adding a busbar allows us to increase the number of output circuits that we can connect to the network but, in the event of a failure or maintenance on the busbar, we continue to lose our entire substation and we could not inject the energy generated into the network.

Profile view of a single busbar arrangement generated by pvDesign (Source: pvDesign)

pvDesign will automatically choose a single busbar arrangement when, due to design criteria, the power of the plant cannot be supplied to the grid only by a power transformer. To make this decision, factors such as the installed capacity, the medium voltage level, the admissible current through the cables or the short-circuit studies play a main role.

Moreover, pvDesign will choose on the behalf of the user the type of the power transformer to step-up the voltage. A two-winding or three-winding transformer will be chosen based on the total current of the substation.

Double busbar arrangements

Double busbar arrangements are recommended for large substations in which security of supply is, besides from costs, the most important criteria to take into account.

Double busbar substations are indicated for plants whose installed capacity is significant. Additionally, these configurations provide flexibility by allowing circuits to be connected to either of the two busbars, while the cost is not as high as other arrangements such as one and a half circuit breaker, triple busbars or two circuit breaker configurations. Furthermore, the bus-coupler bay makes it also possible to move circuits from one busbar to the other while they are energized.

This gives double bus substations a very interesting cost-safety balance when considering them in large-scale photovoltaic projects.

Profile view of a double busbar arrangement generated by pvDesign (Source: pvDesign)

pvDesign automatically evaluates the relationship between the installed capacity of the plant and the medium voltage level so that from a determined value, the double busbar arrangements are generated as a solution to the substation.

Similarly to the single busbar substation, the choice of the power transformer type will be automatic to opt for a cost-effective substation.

However, from RatedPower we are aware that the type of circuit arrangement may be a requirement of the Transmission System Operators (TSO) / Distribution Network Operators (DSO) and may vary depending on the project and location. Therefore, we give the user the possibility to manually choose the arrangement they prefer for their substation. Thus, this decision will not be the responsibility of the software but rather the user can customize its result.

Option to manually choose the arrangements of the substation (source:pvDesign)

Switching and breaking station

The switching and breaking stations are modular cubicles that allow the connection of an electricity generation plant to the medium voltage networks. Properly said, they are not substations.

These facilities are made up of cubicles with different necessary functionalities to guarantee a secure connection from the plant to the grid. These functionalities include:

1. Feeder cubicles protect the input and output lines with instruments such as switch-disconnectors and current transformers.

2. Protection cubicles equipped with breakers to open or close the flow of current in case of failure or maintenance.

3. Metering cubicles that quantify the voltage and current values that flow through the line.

4. Auxiliary cubicles that allow the connection of an auxiliary services transformer.

Single Line Diagram of the switching and breaking station generated in pvDesign (source:pvDesign)

The breaking stations are usually considered when you want to connect the PV plant directly to a medium voltage line, whose voltage is between 11 and 36 kV. These cases usually occur when PV plants are designed to supply electro-intensive industries or residential points located very close to the plant.

Cost analysis

According to a study conducted by the Agency of the Cooperation of Energy Regulators (ACER), the number of transformer bays and the number of busbars are two of the most important factors when pricing a substation.

Taking into account that the cost of a substation depends on the characteristics of a certain project, in its study, ACER published the following prices to estimate the cost of AC substations.

Based on their publication, we could estimate the cost of our substation either by the amount of installed capacity (38,000  € / MVA) or by the voltage level (42,500 € / kV). Also, depending on the number of transformer bays and voltages, we would obtain the following indicators:

1. From 1-4 transformer bays: 33,000 € / kV

2. From 5-8 transformer bays: 35,500 € / kV

3. For more than 9 transformer bays: 44,000 € / kV

Another interesting information that the study provides is the average price of a power transformer: 10,000 € / MVA.

According to the 2019 PV Status Report from the Joint Research Center (JRC), the European Commission’s science and knowledge service, we can approximate the cost of a photovoltaic plant to be 1 € / Wp. By combining it to 38,000 € / MVA, we obtain that the cost of the substation could represent 5% of the total project and in some cases up to 10%.

Added Value with pvDesign

The calculation model of the step-up substation introduced in pvDesign allows in a very simple and fast way to obtain the basic engineering of a cost-effective interconnection facility of the PV plant. This calculation is outlined in several documents necessary for a complete and detailed design. .

The step-up substation or the breaking station generated by the software are customized for each project. In other words, the plant's medium voltage lines, the plant's capacity and the environmental conditions are taken into account.

Moreover, pvDesign automatically selects and calculates a three-winding power transformer to connect big PV plants to the grid; hence, reduce the number of transformer bays to generate a more cost-effective substation. Likewise, when the PV plant is small, the software will select a two-winding power transformer instead to retain a lower cost of the facility.

Among the most important documents that pvDesign generates are:

1. Single line diagrams (SLDs) as a graphic representation of the electrical installation. Giving a deep look into the substation’s bays starting from the incoming MV power lines till the output bay along with their corresponding electrical devices and their ratings.

2. A design report that includes the descriptive memory of all the necessary equipment in the substation, such as: power transformers, switches, disconnectors, voltage and current transformers and surge arresters. Additionally, we also include information on safety distances, insulation coordination, values of short-circuit currents and characteristics of the busbar. Moreover, the design report includes the descriptive memory of all the necessary equipment in the breaking station, such as feeder, auxiliary, protection and metering cubicles.

3. Profile and top views and general sections of the substation. In these blueprints the position of the equipment is visualized as well as the office buildings and access roads.

4. A design methodology in which it is explained in detail step by step how the generated results are obtained.

5. A bill of quantities that includes the interconnection facility’s components and electrical equipment along with their quantities.