- Definition of Surge Arrester: A surge arrester, also known as a lightning arrester, is a device used to protect electrical equipment from overvoltage transients caused by lightning or switching.
- Surge Protection: Surge protection is essential to prevent damage to electrical systems from transient overvoltages, which can be many times greater than the system’s normal voltage.
- Sources of Surges: Surges can come from atmospheric lightning or from switching operations within the electrical system itself.
- ZnO Lightning Arresters: Zinc oxide lightning arresters are effective because of their non-linear current-voltage characteristics, which allow them to handle and dissipate surge energy.
- Construction and Working Principle: ZnO arresters are constructed from zinc oxide discs in a polymer or porcelain housing, and their effectiveness relies on the material’s ability to handle high surge currents through its non-linear resistance properties.
An electrical surge is a short-duration overvoltage or overcurrent. In an electrical power transmission system, lightning and switching operations are major sources. The magnitude, polarity, front time and duration depend on the event and network. A travelling voltage wave can stress electrical insulators and connected equipment, but not every surge has the same steep-front, long-tail shape shown in a standard test waveform.
Surge protection is part of insulation coordination. It combines equipment withstand levels with shielding, grounding, bonding, conductor layout and protective devices suited to the installation.
For power systems, one primary protective device is the lightning arrester or surge arrester. Surge arrester is the broader term because the device can limit lightning and switching overvoltages.
External events include direct strikes and nearby lightning. Internal events include line energisation, fault clearing and switching of transformers, capacitors or reactors.
A lightning event near an electrical transmission line can inject current directly or induce a travelling overvoltage through electromagnetic coupling.
A surge can propagate in both directions from its point of origin. Distributed line inductance and capacitance set the line’s surge impedance and wave velocity. The velocity is close to the electromagnetic propagation speed in the line dielectric, not universally the speed of light in vacuum. Part of the wave reflects wherever surge impedance changes, including at line ends, branches, cables and equipment terminals. Line resistance, dielectric loss, corona, terminations and protective devices attenuate or absorb energy. Reflections can raise or lower terminal voltage, depending on the boundary. An insulation-coordination study determines the expected stress and protection required for the electrical power system. A Lightning arrester is one component of that coordinated protection.
Switching can produce overvoltage during line or cable energisation, transformer energisation, capacitor-bank switching, shunt-reactor de-energisation and fault interruption. Premature interruption of current, called current chopping, can create a high-frequency transient in an inductive circuit. A plain electrical isolator is intended for de-energised switching, not for interrupting load current unless it has a specified load-breaking function.
The arc-quenching techniques used in an SF6 circuit breaker or a vacuum circuit breaker can be associated with current chopping or re-ignition in some circuits. The outcome depends on the breaker design, switched load and network parameters.
A rapid current change produces a voltage related to L(di/dt), where di/dt is the rate of change of current and L is the relevant circuit inductance.
The resulting transient can appear across opening contacts and propagate through the connected network. A lightning arrestor or surge arrester does not simply withstand this voltage at the end of a line. It is connected in parallel with the protected equipment, usually from phase to earth, so it carries surge current and limits the terminal voltage to its residual or discharge voltage. Short connection leads and an effective earth path reduce added inductive voltage.
The required protection cannot be assigned from equipment type alone. An electrical power transformer, electrical switchgear, cables and electrical transmission lines all have specified impulse withstand levels. Generators, an electric motor, dry type transformers and electric arc furnaces also need protection selected for their terminals and winding insulation. A lightning arrester must have a protective level below the coordinated withstand level while tolerating continuous voltage, temporary overvoltage and expected surge duty.
In an electrical sub-station, arresters may be installed at line and cable entrances and close to high-cost equipment. Placement on one or both sides of electrical power transformers depends on winding connections, line and cable exposure, switching studies and the protected transformer terminals.
Gapless ZnO metal-oxide arresters are common on AC power systems above 1 kV. Externally gapped line arresters and other surge-protective devices still serve specific applications.
Construction of Zinc Oxide Lightning Arrester
A gapless metal-oxide arrester contains cylindrical ZnO varistor discs connected in series as a stack. The dimensions and number of discs in a lightning arrester are selected for continuous operating voltage, rated voltage, protective level, charge-transfer capability and mechanical design. A porcelain or polymer housing protects the stack from the environment. Internal springs or other compression parts maintain electrical contact, while seals keep moisture out. Line and earth terminals connect the arrester in parallel with the equipment. High-voltage designs may also include grading rings, pressure-relief features and monitoring accessories.
Working Principle of Zinc Oxide Lightning Arrester
During normal operation, the applied voltage must remain within the arrester’s maximum continuous operating voltage, and only a small leakage current flows. ZnO varistors have a strongly non-linear current-voltage characteristic. Each block is made mainly from zinc oxide with smaller amounts of other metal oxides. Its microstructure contains conductive ZnO grains separated by grain-boundary junctions. The combined behaviour of those junctions produces the varistor characteristic. Block diameter, material formulation and thermal design affect the current and energy capability of the surge arrester.
Over a limited part of the characteristic, a ZnO block may be approximated by the power-law expression shown below.
In this expression, Ir and Vr are reference current and voltage values for the lightning arrester or surge arrester block. The exponent is an approximation, not one fixed value for the full operating range. At normal voltage, leakage includes a capacitive component and a smaller resistive component. During a surge, current rises steeply and the arrester limits the voltage to a specified residual value at the corresponding discharge-current waveform. Reference voltage is the voltage measured at a specified reference current, which is well below many rated discharge currents. After the surge, the varistor returns to its high-resistance state and automatically limits power-frequency follow current. Temperature affects resistive leakage and power loss, so the surge arrester must be rated for continuous voltage, temporary overvoltage, charge transfer and ambient conditions.
The resistive part of continuous leakage current produces heat. If losses generated at the applied voltage exceed the heat that the complete arrester can dissipate, rising temperature can increase losses and lead to thermal runaway. Excess surge energy, temporary overvoltage, ageing, moisture ingress or a failed block can upset this thermal balance. Housing, block and system ratings therefore work together; housing design alone does not determine thermal stability.
The operating principle of a gapless surge arrester is the non-linear V-I behaviour of its metal-oxide blocks, combined with correct voltage rating, protective level, energy capability, installation and insulation coordination.
