Electrical design and connection procedure for medium voltage solar plant

The design of a photovoltaic plant goes through several stages, from the sizing of modules, inverters, AC and DC circuits, transformers, etc., culminating in the design of the connection point. At the connection point, the physical interconnection between the distributor system and the plant is made, therefore the electrical project must be approved by the electricity concessionaire. 

This article deals with the main aspects observed by the concessionaire when evaluating the electrical project for connecting a solar plant to the medium voltage distribution system.

Connection process

The process of connecting a photovoltaic plant is governed by the Prodist module 3 (Electricity Distribution Procedures in the National Electric System), which contains the stages of the process, as shown in Table 1.

Table 1 – Connection process steps

Specifically for enterprises operating in the energy compensation system, the rules are defined by Resolution 482 of the ANEEL (National Electric Energy Agency) and are treated as consumer units, in light of the agency's resolution 414. The first stage of the process begins with the access consultation.

As a product of this stage, the concessionaire provides the “access information” document. After this step, the makes an access request, obtaining the “access opinion” document from the concessionaire. Figure 1 shows the deadlines for each stage, for the enterprise that intends to operate in the energy compensation system.

It is at the access request stage that the accessor must submit the electrical design of the connection and the studies required by the concessionaire. To do this, the needs information from the concessionaire such as short-circuit levels, protection settings and other information.

For the enterprise that will operate in the energy compensation system, the access consultation stage is optional. However, there is no way to carry out the connection project and studies without having the concessionaire's data in advance.

Therefore, some concessionaires recommend the access consultation stage, although it is optional. Considering that all information is available, the technical documents for evaluation by most dealerships are as follows:

• Electrical design of the medium voltage cabin;
• Descriptive memorial;
• Single-line diagram;
• Relay functional diagram;
• Protection study.

Other studies are requested by some concessionaires, such as electromechanical and electromagnetic transient studies, power quality studies, etc.

Installation overview

In this article we will deal with the main components of a primary cabin, medium voltage circuit, coupling transformer and protection study items commonly evaluated by concessionaires. All equipment and installations must comply with the NBR 14039 standard – Medium Voltage Electrical Installations, which is observed by electricity concessionaires in Brazil.

But as each concessionaire has its own particularities, it is important to always observe the respective standards, as projects are evaluated based on these standards. As references, Figures 2 and 3 are taken.

The connection point or delivery point is the place where the electrical connection between the concessionaire's network and the plant installation is made. It clearly defines the limit of responsibility for supplying energy by the concessionaire, as seen in Figure 4.

The connection branch is installed and maintained by the concessionaire. Up to the point of energy delivery, it is the responsibility of the concessionaire to adopt all measures to enable the supply of electrical energy, as well as its operation and maintenance. The construction, operation and maintenance of the input branch are the responsibility of the consumer and this is one of the points to be checked by the concessionaire, such as:

1. Distance between the primary cabin and the property line (measurement “a”);
2. Distance between the cabin and the dealership pole (measurement “b”);
3. Sizing of the input branch.

Each dealership adopts specific measures for installing the cabin, as well as sizing the entrance branch. These measures depend on the type of cabin, whether measurement, protection and transformation or just measurement and protection.

Furthermore, it is considered whether the entrance is aerial or underground. Therefore, it is important to consult the medium voltage electricity supply standard for each concessionaire. Table 2 shows the distances to be served by some concessionaires.

Table 2 – Distancing from the cabin

Regarding the input branch, Table 3 presents the sizing taken from Elektro's technical standard, ND.20.

Table 3 – Sizing of the input branch

lightning rod

The lightning arrester device is protective equipment required by utilities and is generally installed at the entrance to the primary cabin. The characteristics are defined by the concessionaires themselves and depend on the voltage class of the distribution system. Figure 5 shows the installation of a set of lightning rods for a 15 kV class system.

Table 4 shows the lightning rod specifications to be met by some concessionaires.

Table 4 – Lightning arrester specification

It is important to highlight that these are minimum values. However, attention must be paid to systems in which the coupling transformer is in delta configuration facing the utility network. In these cases, in the event of an earth fault, the voltages in the healthy phases tend to reach the same value as the voltage between phases.

For example, in the case of FL, whose voltage class is 15 kV and whose systems are operated at voltages of 13,8 kV or 11,9 kV, the same nominal voltage is adopted for both, that is, 12 kV. If it is in the 11,9 kV system, the phase-to-ground voltage can reach up to 1,73 x Vphase, that is, 11,9 kV.

This same lightning rod, in the 13,8 kV system, may be subjected to this voltage of 13,8 kV, higher than the specified value of 12 kV. Therefore, in this case, it is recommended that a lightning arrester with a voltage greater than 13,8 kV be adopted for the 13,8 kV system.

one-line diagram

The single-line diagram must show the entire circuit between the connection point and the generation. Using this diagram, the concessionaire checks the electrical connection equipment, how they are interconnected, the various existing equipment and the current and potential transformers. The single-line diagram allows the concessionaire to model the plant to carry out feasibility studies. The single-line diagram must contain:

• Gauge of input branch conductors;
• Type of connection – whether aerial or underground;
• Identification of switches, lightning rods and circuit breaker;
• Relays and their protection functions;
• Gauges of medium voltage cables and buses;
• Identification of current and potential transformers.

Figure 6 presents a single-line diagram model suggested by Cemig in its ND 5.31 standard.

Functional diagram

The functional diagram shows the actuation of the relay in the medium voltage circuit breaker. It serves to identify the relay connection, its auxiliary supply source and the actuation of the relay on the circuit breaker opening coil. Figure 7 shows a functional diagram of the SEPAM relay.

Medium voltage cabin

The medium voltage cabinet, as shown in Figures 2 and 3, can be of two types, in relation to its constitution:

1. Measurement, protection and transformation sectors in the same environment or;
2. Measurement and protection sectors separated from the transformation sector.

The configuration to be adopted will depend on the size of the plant, the location of the generation and inverters and the distance from the connection point to the inverters, among other things. The cabins can also be classified as masonry or armored.

Both must meet the specifications of the local concessionaire, in relation to equipment, construction aspects, distances and safety items. Figure 8 shows the sectors of a masonry cabin and Figure 9 an armored measurement and protection cabin (cubicle).

When deg a medium voltage cabin, the concessionaire checks the following items:

• Area reserved for the measurement sector: physical space;
• Area reserved for the protection sector: physical space;
• Primary bus (sizing, colors);
• Switches: size and type (opening under load or not);
• Input circuit breaker: specification;
• Relay: specification and parameterization;
• Current and potential transformers: sizing;
• Coupling transformer: sizing and characteristics;
• Cabin grounding: project.

This information must be part of the project to be presented to the concessionaire. Some equipment is specified by the concessionaires themselves and must comply with their respective standards.

Medium voltage buses

Busbars can be sized depending on the demand requested by the installation. In the case of a photovoltaic plant, by its maximum generation. However, concessionaires specify these buses according to their standards, as seen in Table 5, taken from Elektro's ND.20 standard.

Table 5 – Bus sizing – Elektro

As seen in Table 5, the sizing is based on the demand in kVA and an important fact is the color that each bus must meet.

Medium Voltage Circuit Breaker

In the same way as for the busbar, the sizing of the circuit breaker must consider the installation's demand and be capable of interrupting short-circuit currents in the system (in the plant and in the concessionaire's distribution network). Table 6 presents some medium voltage circuit breaker specification items.

Table 6 – Circuit breaker specifications

In addition to the specification, the dealership also checks:

• Interruption system: vacuum – SF6 – insulating oil (< 1 liter/pole);
• Opening coil supply;
• Backup power supply autonomy of at least 2 hours in the event of a power interruption.

In accordance with NBR 14039, and concessionaire standards, auxiliary sources must be provided, appropriately sized, to supply the opening coil of the general MV circuit breaker, in order to guarantee its performance. These sources can be:

• Battery bank and its charger, powered by the auxiliary transformer;
• Capacitive source (capacitive trip).

Coupling Transformer

Coupling transformer is the element that makes the voltage of the utility's distribution system compatible with that of the plant's generation. For example: 13,8 kV/690 V. The configuration to be adopted during installation will depend on the concessionaire. This is an item to be evaluated by the concessionaire when presenting the project, as depending on the configuration, one or another protection function is adopted.

For example, if the configuration is delta facing the utility network, the earth fault protection function to be implemented in the relay is neutral overvoltage (59N). If it is a grounded star, the protection function to be used is the neutral directional overcurrent function (67N). Table 7 shows the configurations for some utilities' coupling transformers.

Table 7 – Transformer configuration

Another important aspect in the project is that the transformer must contain a minimum number of primary taps (derivations), in accordance with the concessionaire's standards. Table 8 shows FL requirements.

Table 8 – Transformer TAPs according to FL requirements

Potential current transformers

Current transformers (CTs) and potential transformers (PTs) are the sensors that translate the currents and voltages from the medium voltage system to the relay that acts on the medium voltage circuit breaker. For the sizing of CTs, standard NBR 6856 – Current transformer is adopted, while for VTs, standard NBR 6855 – Potential transformer.

The methodology for sizing these transformers is part of the protection study and must follow the guidelines of each concessionaire. When sizing CTs, the secondary load estimate may vary between utilities. For example, the relay impedance to be considered may vary by 2 to 4 times.

Protection relay

The relay is the equipment that will act on the medium voltage circuit breaker depending on the electrical signals observed in the system. The protection functions required by each dealership and by Prodist module 3 must be verified, as shown in Table 9.

Table 9 – Protection relay protections

Some of these functions are required in the relays themselves or in the inverters. The protection study must indicate the performance of these protections. In the case of photovoltaic generation, where the connection is made through inverters, some functions must be present in these equipment, such as function 25 and the anti-islanding function.

Medium Voltage Cabin Grounding Design

The grounding design of a cabin must meet the personal safety conditions and the installation's equipment. A grounding design must ensure that step and touch potentials do not exceed safety limits. But it is common, when it comes to grounding an installation, to immediately think about the value of the grounding resistance.

The NBR 14039 standard itself does not define a value to be met, but recommends a value of 10 ohms. This practice is followed by most concessionaires, which require grounding resistance values ​​between 10 and 25 ohms, without worrying about the occurrence of potentials in the installation, especially touch potentials. When inspecting a cabin, a grounding report is required to check whether the project meets the recommended value.

Protection study

Concessionaires request a protection study from s, a document that serves to prove the selectivity between the concessionaire's protections and that of the medium voltage cabin, as well as presenting the adjustments to the protections requested by the concessionaire. In general, in the protection study, concessionaires evaluate:

• Data used to model the distributor's system;
• Short circuit values;
• Feeder protection adjustments;
• Photovoltaic solar plant system data;
• Input cubicle circuit breaker data – general protection;
• Specification of coupling transformer and auxiliaries;
• TC and TP calculation memorial;
• Generation Transformers Point Ansi;
• Relay adjustment proposal;
• Protection curves (coordination chart).

Once the adjustments have been proposed, time x current graphs of the plant's and utility's overcurrent protections must be constructed, that is, coordinate charts, as shown in Figure 10.

Through the coordination chart, the concessionaire verifies the construction of the protection curves and can the selectivity between the protections.

References

• NBR 14039 – Medium voltage electrical installations
• Prodist – Module 3
• Resolution 482/2012 – ANEEL
• SEPAM, relay catalog
• Cemig and FL concessionaire standards