The need of the hour is sustainable generation of electricity and this is achieved through various renewable technologies, mainly solar PV systems. However, before indulging in solar technologies and its applications, it is crucial to understand the basic electrical concepts that form the framework for developing solar projects. Therefore, this article will be covering the power laws and common electrical measurements, the different types of circuits and phases in power supply, the types and properties of conductors that can be used in solar PV systems and some of the grounding and bonding terminologies that are vital to know in the field of electrical installations.
Power Laws and Electrical Measurements
The basics of every electrical system start with the knowledge of some laws. The primary law that deals with electricity is the Ohm’s Law. Ohm made a discovery that the amount of electric current that flows through a metal conductor is directly proportional to the voltage developed across the conductor in a given circuit for any given temperature. This is given by the formula:
- V= Voltage (in Volts)
- I=Current (in Amperes)
- R=Resistance (in Ohms)
This expression can be rearranged to 2 variations to give an equation for current (I) and resistance respectively:
Another concept to be aware of is the difference between energy and power. Energy is defined as the capacity to do work and is represented as E. The unit of energy is Joules. Joules can be represented as Watt-second or Newton-metre. Power which is integrated over time is represented as the energy of the system. Power is the rate at which the energy is transmitted in the system or the work done. It’s represented as P and its unit is Watt. Watt can be represented as Joules/second for power [1, 2].
In a circuit, electric current can either flow as a direct current (DC) or as an alternating current (AC). To understand DC and AC, one must know was current means. Current or electricity is the flow or movement of electrons through a conductor. In direct current, electrons flow only in one direction whereas for alternating current, the electrons can flow in 2 directions periodically by reversing the direction of flow of electrons. AC power can flow through longer distances and provides more power whereas DC power can flow only through short distances. AC deals with current whose magnitude varies with time whereas for DC, current is constant in magnitude with time. The different types of alternating current are sinusoidal, square, triangular and trapezoidal whereas for direct current, the different types are pure and pulsating. AC is obtained from generators and DC is obtained from batteries and cells. The power factor of AC is in the range of 0-1 whereas the power factor for DC is always 1 .
Types of Circuits and Phases in an Electrical System
Now since the concept of current has been discussed, it’s necessary to know the 2 primary circuits that electrical systems follow. They are series circuits and parallel circuits.
Series circuits are those circuits in which the resistors are placed in a chain-like structure such that current can flow only through one path. Therefore, the current that flows through each resistor is the same. The total resistance value is obtained by adding each of the individual resistor’s resistance value like: R= R1 + R2 + R3 +…
Parallel circuits on the other hand, are those circuits where the resistors are arranged such that the resistors are arranged parallel to each other. The current which flows through this circuit breaks up to flow through each parallel branch and recombines at the point where the branches conjoin gain. In parallel circuits, the voltage remains the same instead of the current. The total resistance is obtained by adding the reciprocal values of each resistance of the resistors like: 1 / R = 1 / R1 + 1 / R2 + 1 / R3 +…
Another concept in electricity is called a phase. Phase is the load distribution of an electrical system and exists either as a single-phase, three-phase or split-phase supply.
- Single-phase power systems have a two-wire alternating current power circuit. One wire is the power wire which has the current flowing through the load whereas the other wire is the neutral wire. Single-phase power supplies are most commonly used for residential homes for lighting or heating and are not used for heavy application purposes such as electric motors. They are not consistent because the voltage for single-phase experiences a lot of peaks and dips. A transformer can be used to derive a single-phase system from a three-phase system to maintain the proper voltage.
- A three-phase supply uses a 3-wire alternating current power circuit with each AC signal 120 degrees apart of the phase. These systems are capable of delivering power at a constant and steady rate. They can deliver power more efficiently which is supported by the fact that a three-phase power supply can deliver 3 times more power than a single-phase power supply. This power supply can either use 3 wires or 4 but in either case, the conductor material that’s being used to transmit the power is lesser than single-phase. The 2 main configurations of a 3-phase power supply system are known as delta (which uses 3 wires) and wye (which uses 4 wires from which the 4th wire is a neutral wire).
- Split-phase power supply is the third type of phase power supply. This system is classified as a single-phase 3-wire system for power distribution. This system saves the conductor material used as compared to single-phase systems and is placed in such a way that two AC line voltages of 120V are out of phase by 180 degrees to each other. The neutral wire is connected to the ground and an AC power supply of 240 volts can be obtained by connecting a load between two 120 V AC lines. Split-phase power supply systems are commonly used for applications like reduction of electromagnetic noise and reduction of electrical shock hazards [4, 5].
Types and Properties of Conductors and Conduits Used in Solar PV Systems
Wires are used for transferring electricity through the system and vary as per the materials used and the insulation used. These factors can greatly affect the applications for which they’re used. The 2 most common materials used for wires are Aluminium and Copper. Aluminium wires are prone to weakening during bending for installations but they’re cost-efficient. They’re usually used for commercial applications and not used for interior residential wiring designs. Copper wires are more conductive and are able to carry larger current as compared to Aluminium.
These wires can either be solid or stranded. Stranded wires allows for greater flexibility which makes it suitable for large-sized applications. They have better conductivity due to larger wire surface area.
The colour coordination of these wires depends on their application. For un-grounded, hot applications for DC and AC, the wire colour is usually red and even black for AC. For grounded conductors, the wire colour for both DC and AC is white and for equipment grounding, the wire colour is either green or bare for both DC and AC .
There is a type of wire known as multi-conductor or multi-core wires which use multiple conductors. These wires are mostly used in residential buildings and are very well-insulated. Multi-conductor wires have 5 conductors which have 5 wires protected by a jacket which offers extra security and insulation to the wires from potential degradation such as moisture, animals, chemicals, heat and light.
For solar PV systems, the wire types used for wet and indoor conditions are:
TW, THW and THWN = They can be used for indoor or wet conditions. They are moisture and heat resistant, flame retardant and made from thermoplastics. Their maximum temperature ratings are 75oC.
PV wires, USE-2 and RHW-2= They can be used for outdoor or indoor applications for both dry and wet conditions. Their maximum temperature ratings are 90oC-150oC. Such wires are sunlight resistant, UV resistant and moisture resistant.
These jackets for wire cables are termed as a cable tray system which is made from different materials. The purpose of these cable trays is to protect, support and route the cables for efficient transmission of power. They are designed to withstand extreme harsh conditions and are rated according to the different conditions such as flammability testing, corrosion, mechanical resistances, temperature conditions, etc. The different types of cable trays that can be used for solar PV systems are mentioned below:
- NMC: These stand for non-metallic sheathed cables which comprise of 2 or more conductors which are insulated in a non-metallic sheath. These cables are moisture-resistant, flame-resistant and can be used in damp conditions.
- Romex Cables: These cables are trade-named for electrical conductors sheathed with non-metallic jackets. They’re used for residential wiring systems and can be classified as a non-metallic sheathed cable (NMC).
- Tray Cables: These are insulated conductors which have a non-metallic jacket that may or may not have bare or covered grounded conductors. They are used in industrial power circuits with a voltage rating of 600V. These cables are tested for crush and impact testing so that they can be supported in the industrial environment.
- Metal Clad Cables: These cables consist of conductors insulated with a metal sheath that’s interlocked. It can also be covered overall with a plastic jacket. They can also be used for industrial applications with voltage ratings of 600V are suitable for both indoor and outdoor use.
- Conduits: These are tubing or enclosures that are provided for safe current conduction. They also protect the wires and provide them with a route to conduct electricity. Conduits are typically used when wires are exposed to the environment where they’ll be subject to disfigurement. Conduits are usually made from metal or plastic and they can be flexible or rigid. Conduits are often paired with electrical fittings made from a similar material and they’re installed according to the local codes and regulations of the region. Electrical wirings which are provided with an enclosed physical pathway are known as raceways. There are several types of conduits and they are:
- RMC and IMC- These are rigid metallic conduits (RMC) which is fabricated with galvanized steel. They’re commonly used for outdoor applications and even provide structural support for panels, cables and other electrical equipment. IMC stands for Intermediate metal conduit which is a lighter and thinner version of RMC and can be used for the same applications as the RMC.
- Rigid PVC Conduits: These are made from polyvinyl chloride (PVC) which is a plastic. They are fabricated to be watertight and therefore, can be used for underground wiring and keep them protected from corrosion.
- EMT: These stand for electrical metallic tubing which is made from aluminium or galvanized steel. They are very lightweight compared to RMCs and can be bent easily for various applications.
- ENT: These stand for electrical non-metallic tubing which is made from grooved plastic tubes that are flame and moisture resistant. These conduits cannot be installed outdoors or in exposed locations so they’re commonly installed within walls.
- FMC and LFMC: FMC stands for flexible metal conduit and LMFC stands for liquid-tight flexible metal conduit. FMC, as the name suggests, is made to be flexible so that it can be adopted on different walls and surfaces. They’re usually used in dry environments for short connections. LMFCs are made from a flexible PVC inner layer and reinforced with a PVC jacket [9-11].
Grounding and Bonding Terminologies
To understand some of the electrical terminologies, one must be aware of what grounding and bonding means. Bonding is the joining of metal parts together permanently. These metal parts which are connected form an electrically conductive path that can safely transmit current without any faults. Grounding or grounded is the connection established to the earth or to some other body which substitutes the earth. Other terminologies are based on these definitions of bonding and grounding and they’re listed below:
- Ground: An intentional or accidental conducting connection developed between any equipment or electrical circuit and the earth or a body that used in replacement of the earth.
- Effectively Grounded: These are connections that are intentionally connected to the earth which has low impedance and which have a high enough capacity to carry current without building up the voltages. If voltage build-up increases, it may lead to hazards that can affect both the equipment and the person.
- Grounded Conductor: These are circuits which are deliberately grounded.
- Grounding Conductor: These are conductors of a wiring system which connect the grounding electrodes to the grounded circuit.
- Solidly Grounded: These are connections made to the ground without implementing any impedance device or resistor.
- Equipment Grounding Conductor: This is a conductor that connects a system grounded conductor, grounding electrode conductor, or both, to non-current carrying metal parts, enclosures or raceways of the system. The connection is established at the source of a separately derived system or at the service equipment.
- Main Bonding Jumper: This is the connection formed at the service between the equipment grounding conductor and the grounded circuit conductor.
- Grounding electrode conductor: This conductor connects the equipment grounding conductor to the grounding electrode at each building’s service.
- System Bonding Jumper: This is a connection developed at a separately derived system where the equipment grounding conductor is connected to the grounded circuit conductor.
- Ground Fault: This is an electrical connection that is unintentionally formed between an ungrounded conductor and a non-current carrying conductor, metallic components or the earth.
- Ground Fault Current Path: This is an electrically conductive path that is formed from the point of the ground fault to the electrical supply source.
- Effective Ground Fault Current Path: This is a low impedance, permanently constructed electrical path which have the ability to carry current from the ground fault point to an overcurrent protection device or ground fault detectors during a ground fault condition.
- Ground Fault Circuit Interrupter: This device protects personnel working on de-energizing the circuit when the current flowing to the ground exceeds the limits set for that circuit.
- Ground Fault Protection of Equipment: This system prevents line-to-line ground fault currents from affecting the equipment by opening or disconnecting all the ungrounded conducts associated with the faulty circuit.
- Qualified Person: Any person who has he necessary skills and expertise in the field of construction, operation and installation of electrical equipment with certified trainings on electrical systems and hazards are called as a qualified person. Only such people are legally allowed to install, operate and maintain electrical systems for any applications .
In conclusion, this article aimed to provide a holistic overview on some of the key electrical concepts that are necessary for understanding solar PV systems. The base of any solar project lies in the electrical design and wiring and this article aimed to provide an understanding on the power laws, circuits, phase power supplies, wires and some common electrical terminology that’s used in installations.
Image References and Bibliography