Surge Protection Device for Solar PV installation

Effects of Lightning Strike on Solar PV system:

A solar PV installation is vulnerable to the effects of lightning stroke. The potential impact of lightning stroke increases with the PV system size i.e. the exposed surface area. PV plants in areas with frequent lightning strikes can suffer repeated and significant damage to vital components, resulting in substantial downtime, repair cost and loss of revenue due non-generation. Although some reports say that the installation of Solar PV modules does not increase the risk of a lightning strike. Therefore, the need for lightning protection measures cannot be derived directly because of the mere existence of Solar PV system.

A lightning protection system with proper air terminals, down conductors, and equi-potential grounding protects the Solar PV system against direct lightning strokes. A properly designed and installed Surge Protection Device (SPD) further minimizes the potential impacts of lightning stroke. It is suggested to hire a professional Electrical Engineer to carry out the lightning risk assessment study at any Solar PV plant of significant capacity.

Importance of Surge Protection Device:

A properly designed Lightning Arrestor safely bye-pass a direct lightning stroke through the associated down conductors to earth, thus protects the solar PV modules, structures, inverters and other equipments connected to the circuit. Surge Protection Devices are used in a Solar PV system or any other electrical system to provide a discharge path to earth to save those components from high voltage transients caused by direct or indirect lightning strokes or power system abnormalies. External lightning protection system alone will not be sufficient.

Transients caused by direct or indirect lightning strokes or utility switching operations expose the electrical and electronic equipments to very high voltages of very short duration (tens to hundreds of microseconds), causing their failure. Continued exposure to transients of lower magnitude deteriorates the dielectric and insulating materials of these important equipments and eventually leading to breakdown.

Therefore, in conjunction with a appropriate Lightning protection system, the use of Surge Protection Device at key locations protects vital components such as PV modules, inverters, measuring, controlling and communication equipments.

Technical characteristics of SPD:

The commonly used Surge Protection Device in solar PV installations are Metal Oxide Varistors (MOV), which functions as a voltage clamping device. A Surge Protection Device must be able to quickly change its state (non-conducting to highly conducting) in presence of transients and to discharge the dangerous voltage and current to earth without fail. The voltage drop across the Surge Protection Device circuit should be minimum to protect the equipment it is connected to i.e. the SPD should not interfere with the normal working of the PV plant.

Surge Protection Device key operating characteristics include maximum continuous operating voltage, DC or AC application, nominal discharge current, voltage protection level (terminal voltage that is present when the Surge Protection Device is discharging a specific current) and temporary overvoltage, a continuous overvoltage that can be applied for a specific time without damaging the Surge Protection Device.

Fig.1: DC Distribution Box with DC SPD, type-2

Placing the DC Surge Protection Device at specified locations in the DC circuit mitigates the effects of these induced currents and voltages. The Surge Protection Device is placed parallel to the energised conductor and ground. It changes state from a high impedance to a low impedance when the over-voltage occurs. Surge Protection Devices are supposed to not to carry any load current.

 The Surge Protection Device must have an in-built self protecting device that disconnects it from the circuit should the device fail. To make this disconnection apparent, Surge Protection Devices come with a display flag that shows the disconnection status. The status can also be sent to a remote location opting a remote signalling feature. Some Surge Protection Devices come with a finger-safe, removable module that allows a failed module to be easily replaced without tools or the need for switching off the circuit.

AC Surge Protection Devices:

Just as DC SPDs, AC Surge Protection Devices are also used to protect the system from surges at the AC side. They should be placed as close to the AC terminals of the inverter to protect it from dangerous transients. The transients encountered at this location are of high magnitude and duration and therefore must be managed by the Surge Protection Device with appropriate high discharge current rating. The connections to the Surge Protection Device should be made through short conductor pieces of sufficient cross sectional area; so as to reduce voltage drop in the Surge Protection Device circuit during discharge and to avoid exposure of protected equipments to higher transient voltages.

During lightning strokes to nearby grounded structures, lightning protection system itself, and inter and intra cloud flashes of higher magnitude can induce transient currents into the DC cables of the PV system. These transient voltages appear at the terminals of the key components such as inverter etc leading to insulation and dielectric failures.

In large commercial and utility scale Solar PV systems operating at a maximum Open Circuit voltage of 600 V or 1000 V usually use MOV Surge Protection Device in ‘Y’ configuration. Each leg of the ‘Y’ contains an MOV module connected to each pole and the ground.

Failure of Surge Protection Device can be due to ambient heating, discharging currents that are greater than the normal rating, discharging too many times, or being exposed to continuous over-voltage condition.

Types of Surge Protection Devices:

Surge Protection Devices can be classified into three different classes –

1.      Broad Protection Type (Type-1): This class of Surge Protection Device have the highest value of admissible surge current and are designed to handle a direct lightning stroke. They are used where there is a possibility of lightning currents being conducted through the external lightning protection system and the electrical cables. For example, locations where the distance between the DC cables and the external lightning protection is too small.

2.      Medium Protection Type (Type-2): This class of Surge Protection Device have a lower value of admissible surge current and are used for protection against indirect lightning effects. When a local lightning strike occurs on the external lightning protection system it can induce dangerous voltages into the electric circuit because of the associated electromagnetic fields. However, the value of lightning current, its duration and inherent energy is low in this case. Type-2 class of Surge Protection Device are used to protect against these over-voltages. The DC SPD shown in fig.1 is a type-2 class.

3.      Fine Protection Type (Type-3): These Surge Protection Devices have the lowest value of admissible surge current and are used to protect sensitive electronic end devices from the effects of lightning stroke far away from the location.

Surge Protection Device check list:

Effective Surge Protection Device installation should consider –

1.      Placement in correct location,

2.      Proper rating and type,

3.      Proper grounding,

4.      Local or remote indication facility,

5.      Easily replaceable modules.

Faults in Power System

"A fault in an electrical circuit is defined as the defect in the electrical circuit because of which the current in the circuit is diverted from the intended path."

For example suppose a circuit has two parallel paths, opening up of a path will divert the current to the other path and in the process it may damage the path or the conductor. Thus, faults can damage or disrupt the power system in many ways.

Causes:

In a power system, the faults occur because of insulation failure which may be because of a system over-voltage such as switching surges or lightning stroke. Faults may also be due to a broken insulator or a conductor. Various other reasons such as improper operating habits may also lead to a fault; for example, loading a distribution transformer beyond its normal rated capacity. 

Nearly one half of the faults occur on power lines which are widely branched, have greater length, operate under variable weather conditions and are more exposed to atmospheric disturbances.  

Also read:

• Classification of  Electrical Faults

Effects of fault

Faults give rise to abnormal operating conditions. When a fault occurs at any point in the power system large currents, large forces and or abnormal voltages are developed. The excessive current because of the fault is determined by the internal e.m.f.s of the machines in the network, their impedances, and the impedance in the network between the machines and the fault.

Faults currents, also called short circuit currents, are many times greater than the normal currents. Large voltage stresses the insulation of the various equipments, which are on the way, beyond their breakdown value causing the failure. 

Similarly large currents overheat the equipment or the element of the power system. Sometimes faults lower the system voltage below the permissible voltage limit causing unwanted and teasing interruption of various equipments and components. Faults can also cause a three-phase system to become unbalance.

Action to be performed during a fault

It is necessary that the faults or the faulty section should be removed immediately so that the normal operation of the rest of the system is maintained. The protective relays employed in the power system or network should immediately detect the faults or the faulty section without fail and send trip signal for the operation of circuit breakers.

To obtain proper setting of the protective relays and the interrupting capacities of circuit breakers, the values of these fault currents and voltages should be known with great accuracy. Short circuit studies and calculations provide currents and voltages on a power system during fault conditions.          

Shielding method Protection of Transmission Lines against Lightning

Last updated: January 20, 2017

Surges due to lightning are mostly injected into the power system through the long transmission lines. Substation apparatus is always well protected against direct lightning strokes. The commonly adopted and effective method of protecting transmission lines against direct strokes is by the use of overhead ground wires. This method of protection of transmission lines is known as shielding method which does not allow an arc path to form between the line conductor and the ground.

Ground wire is a conductor run parallel to the main conductors of the line. It is supported on the same towers, is placed higher than the main conductors and is adequately grounded at every tower. For horizontal arrangement of conductors, there are two ground wires to provide effective shielding to power conductors from direct lightning strokes whereas in vertical configuration of conductors there is only one ground wire.

Fig 1: A 765 kV transmission line with ground wire

The ground wire is made up of galvanized steel or ACSR conductors. Modern Extra High Voltage (EHV) transmission lines have ground wires of ACSR conductors of the same size as the power conductors. In case of a direct lightning stroke, the ground wire intercepts the stroke and by providing multiple paths for conducting the stroke to the ground they reduce the induced voltage. It also helps to increase the effective capacitance between the conductor and the ground which in turn reduces the voltage induced on the conductors from nearby strokes.

The ratio of the induced voltage on a conductor of a line provided with ground wire protection to the induced voltage which would exist on the conductor in the absence of ground wire is known as protective ratio. Each ground wire has a protective angle which is defined as the-

" Angle between the vertical line passing through the ground wire and the line passing through the outermost power conductor is called the protective angle."

 The protective angle is in the region of 20^o to 45^o.

The voltage to which a transmission line tower is raised when a lightning strikes the tower is independent of the operating voltage of the system. 

Basic design requirements for protection against direct lightning strokes:

The basic requirements for the design of a line to safeguard it against direct lightning strokes are-

1.      The ground wire used should be mechanically strong and should be so located that they provide sufficient shielding.

2.      There should be sufficient clearance between the power conductors and the tower structure.

3.      There should be an adequate clearance between the line conductors and the ground wires, particularly at the mid-span, so as to avoid flashover to the power conductor upto the protective voltage level used for the line design.

4.      The tower footing resistance should be as low as permissible.