Voltage-Current Relation for Typical MOV Elements

TLSA Selection and Specification

where: K is a function of the number of blocks, α is a non-linearity coefficient and R is an equivalent series resistance at high current.

Typically, the nonlinearity coefficient α is a function of formulation, block diameter and application, with 10 <α< 40 and TLSA tending to have lower values than station arresters. The sharing of energy among arresters is affected by α, and also by the series resistance R.

As long as the applied voltage on the arrester terminals remains lower than its operating voltage, the arrester offers a very large resistance, and the line operates as if there is nothing present. When a lightning overvoltage occurs, the arrester conducts the current needed to keep its terminal voltage below the protective level, without causing a short circuit. When the

overvoltage has decayed below the arrester operating voltage, the conducted current also returns to its initial, negligible level.

The voltage across the arrester is nearly constant for all surge currents. Thus, a good measure of arrester energy duty is obtained by multiplying operating voltage times the charge (integral of current). A TLSA with 100-kV operating voltage, absorbing 5 coulomb from a 31-kA stroke, will store 500 kJ internally in about 10 µs, with peak power dissipation of 3 GW. This compares favorably with the Flux Capacitor [Zemeckis and Gale 1985], which can only absorb 1.21 GW. Energy capability of TLSA is discussed in more detail later in this chapter.

The metal oxide characteristic over a wider range of currents is often shown graphically by a volt - ampere curve as in Figure 5-4 for a typical transmission line arrester. The arrester protective level (PL) is defined as the voltage across the arrester terminals when it is subjected to a current impulse of specified magnitude and wave shape. For instance, the PL at 9 kA 8/20 µs for the arrester represented in Figure 5-4 is 3.0 p.u. MCOVcrest, where MCOV is specified by the arrester manufacturer.

Figure 5-4

TLSA Volt-Amp Curve < substitute a modern version >

TLSA Selection and Specification Arrester Rating and MCOV

Two rating systems are used to distinguish arrester capability: • Maximum Continuous Operating Voltage (MCOV) • Duty cycle rating (sometimes referred to as "rating")

MCOV is the selection identifier of choice for most engineers. MCOV is defined as the maximum power frequency voltage that an arrester is designed to withstand continuously. It is published as an RMS line-to-ground voltage, and the published value should always be greater than the maximum line-to-line voltage/√3 for solidly grounded systems.

For instance, if a 115 kV line is controlled such that the maximum continuous voltage is 1.06 per unit nominal voltage, then the MCOV of the arrester must be greater than (115 kV x 1.06)/43 = 70.4 kV.

Arrester duty cycle rating has no link to system parameters. It is the voltage at which the arrester design will pass a duty cycle test as defined by ANSI/IEEE standard 62.11 [14]. Both MCOV and duty cycle rating are published in manufacturers' data, but duty cycle rating is less

commonly used as an arrester descriptor [16].

Temporary Overvoltages

All transmission systems are subject to fluctuations in power-frequency voltage. Any power frequency fluctuation that exceeds the normal system voltage is referred to as a temporary overvoltage (TOV). TOV may cause current to be conducted through the arrester, thereby subjecting the arrester to high levels of energy. If the arrester is to maintain its insulating properties when the system returns to normal voltages, the energy applied to the arrester by the current must be absorbed without damage to the arrester. Common sources of TOV are:

• Single line-to-ground faults • Load rejection

• Ferroresonance

• Circuit backfeeding from a higher voltage line • Ferranti voltage (lines with one end open) • Resonance

Factors that affect the magnitude and duration of TOVs on transmission lines include: • System configuration

• Operating practices • Relay and breaker settings

TLSA Selection and Specification

Arresters are designed to withstand some temporary power frequency overvoltages without damage. They can withstand TOV of lower magnitude longer than those of higher magnitude. However, the arrester TOV withstand times fall off rapidly with increasing voltage (because of the non-linear volt-ampere characteristic of the MOV valve elements). Manufacturers supply TOV time vs. magnitude data for all TLSA in the form of curves, with voltage usually given in per unit (p.u.) MCOV. These curves can be compared to system data to avoid problems

associated with overheated and failed arresters. If TOV capability is exceeded for any predicted system event, then an arrester with a higher TOV must be selected. However, gapped silicon carbide station arresters may not protect MOV TLSA. In this case, direct comparison of all expected system overvoltages and TOV curves is necessary.

In most cases the TOV capability of the TLSA will exceed that of MOV station arresters. For a quick assessment of the vulnerability of a TLSA to a TOV, it is necessary to compare the published TOV data for the TLSA with that of the MOV station arresters applied at the

substation at the end of the line. If the TOV durations are longer for the TLSA, then the station arresters will protect the TLSA from damage from TOV.

< an example of this graph here would be helpful >

Arrester Protective Levels and Insulation Coordination

To apply TLSA properly, engineers must balance the protective level of arresters with the withstand capability of the line insulation. This means that engineers must select arresters with an MCOV that is:

1. High enough to prevent arrester failure on the system 2. Low enough to prevent external flashovers

It is usually not difficult to satisfy these requirements in transmission applications. In fact, the task of deciding which TLSA to apply is more straightforward than determining whether or where to apply them. Because the principal insulation on transmission towers is external and self-restoring, engineers need only to consider the statistical flashover performance of a given transmission line when determining the appropriate arrester protective levels (PL). Arrester PL that are lower than necessary to prevent flashovers of the tower insulation offer no additional benefits such as increased equipment life. On the other hand, transmission line flashovers due to extreme, statistically rare events can be tolerated since permanent damage to the equipment is generally minimal.

It is not necessary to choose the minimum TLSA MCOV that would be permitted by

consideration of system voltage control and TOV application rules. However, it is important that the protective level of the TLSA be sufficiently below the CFO of the insulator in service at the maximum surge current expected through the arrester. For example, a 230 kV line might have 14 standard 5.75 x 10-inch insulators per phase, with an insulator impulse CFO of 1100 kV (derated 10% for poor atmospheric conditions) for a particular make of insulator. If the maximum

continuous power frequency voltage to ground per phase is 140 kV, then one might select an arrester with the same 140 kV MCOV rating. If such an arrester has a maximum expected surge

TLSA Selection and Specification

current of 40 kA, its protective level might have a corresponding expected value of 740 kV (depending on the manufacturer). This is 67% of the flashover level and far below what would still provide good protection. When the MCOV of a gapless arrester is reduced, its cost will lower, but there will be a greater risk of failure. Hence, in order to improve reliability, and where tower-head space and funding permit, it may sometimes be advantageous to select

arresters with MCOV ratings somewhat higher than the minimum value permitted, as long as the protective level at the maximum expected surge current is at least 15% below the insulator CFO.

One philosophy for dealing with excessive energy dissipation in TLSA application on unshielded lines takes this one step further. The TLSA protective level for an unshielded line can be set very close to the insulator CFO, so that large-amplitude flashes on long spans will cause a few tripouts rather than arrester failures.

It is also practical to reduce the MCOV and TOV requirements in TLSA applications by

inserting a series gap. This practice is well established for older silicon carbide (SiC) nonlinear elements. The major difference in gapped SiC and MOV arrester design is that:

• Gaps for SiC arresters need to interrupt tens or hundreds of amps of power-frequency follow current at line voltage

• Gaps for MOV arresters need to interrupt tens or hundreds of milliamps of power-frequency follow current at line voltage

In this respect, the MOV arrester functions nearly identically to a polluted insulator. The

conditions for arc re-ignition on polluted insulators in the current range of 10 mA to 1 A are well understood [Rizk Elektra 1969]. The peak voltage necessary for re-ignition, Ûcx, is given by:

526 . 0 ˆ 6 . 71 ˆ m cx i x U =