application of metal oxide surge arresters df peelo

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APPLICATION OF METAL OXIDE SURGE ARRESTERS

IN HIGH VOLTAGE AC SUBSTATIONS

David F. Peelo

DF Peelo & Associates Ltd.

ABSTRACT

Applying metal oxide surge arresters is very different from the application of the silicon carbide surge arresters of

the past. This paper describes the rating basis for metal oxide arresters and then how to choose the right arrester

based on the demands of the system.

INTRODUCTION

The application of metal oxide surge arresters in high voltage AC substations is significantly different from that of

silicon carbide arresters (also known as gapped or valve type arresters). Direct substitution of a metal oxide arrester

with a certain rated voltage for a silicon carbide arrester with the same rated voltage value is NOT recommended

without due regard to the application considerations described in this paper. In contrast to silicon carbide arresters,

metal oxide arresters interact continuously with the system and must therefore be applied with this fact in mind. The

purpose of this paper is to describe the basic characteristics of metal oxide arresters and to detail the system and

substitution parameters to be considered in the arrester selection. The approach taken is that of the IEC standard [1]

on metal oxide arresters rather than the IEEE standard [2] on the grounds that the former is technically the most

valid for the purpose [3]. Where relevant the differences between the two standards will be noted or discussed.

BASIC METAL OXIDE ARRESTER CHARACTERISTICS

A metal oxide arrester is essentially a variable resistor or so-called varistor whose electrical resistance is a function

of the applied voltage. The voltage-current (VI) characteristic of a 192 kV rated arrester is shown in Figure 1. The

extreme non-linearity of the VI characteristic is evident and four points are usually defined. These points are:

FIGURE 1





1. Maximum continuous operating voltage (MCOV).

2. Rated voltage.

3. Switching impulse protective level.

4. Lightning impulse protective level.

MCOV: MCOV is the maximum allowable rms voltage that may be applied continuously across the arrester. This

voltage is equal to or greater than the maximum continuous operating voltage of the system. Note that MCOV is a

characteristic of the arrester.

Rated voltage: IEC defines rated voltage as the voltage to be withstood for 10 seconds after being heated to 60°C

and subjected to a specific injection of energy, all followed by 30 minutes at MCOV. Rated arrester voltage is thus

thermally limited being linked to energy absorption and subsequently to thermal stability at MCOV. (The IEEE

standard defines rated voltage as the “duty cycle rating” similar to that used for silicon carbide arresters. Rated

voltage is not linked to energy absorption which is highly questionable given that metal oxide arresters are thermally

limited.)

Switching impulse protective level: this is the residual (or discharge) voltage for a specified switching impulse

current. The current waveshape is 30/60 µs or longer. This voltage in kV peak is approximately two times rated

voltage in kV rms. The applicable current amplitudes are dependent on the system voltage being 1 kA at 230 kV.

Lightning impulse protective level: this is the residual (or discharge) for a specified lightning impulse current. The

current waveshape is the well-known 8/20 µs. This voltage in kV peak is approximately 2.3 times rated voltage in

kV rms. The applicable current amplitudes are 5, 10 and 20 kA dependent again on system voltage.

Superimposed on Figure 1 are the various system conditions with which the arrester has to interact. From left-to-

right, the conditions are:

• normal or continuous operating conditions;

• temporary overvoltage conditions due to faults, load rejection, etc.;

• switching surge conditions due to (for example) high-speed autoreclosing of EHV or HV lines;

• lightning surge conditions with single or multiple strikes.

In the above discussion of rated voltage, reference was made to a “specific injection of energy.” IEC defines fivelevels of rated energy as Line Class 1 through 5. The numbers are multipliers of kJ/kV rated to be applied to the

arrester rated voltage. For a Line Class 3 192 kV rated arrester, rated energy absorption is then 576 kJ. As part of

type testing on a prorated basis, the arrester is heated to 60°C, is subjected to two injections of rated energy

60 seconds apart and then must be thermally stable at rated voltage for 10 seconds followed by MCOV for

30 minutes.

METAL OXIDE ARRESTER SELECTION

The selection of metal oxide arresters is a stepwise – and sometimes iterative – process as described in the

following:

Step 1: Selecting the MCOV Value

For a system with a maximum line-to-line voltage, Vm, the MCOV of an arrester applied on that system is

determined from:

3

VMCOV m

S = kV rms

and

MCOVD ≥ MCOVS





where MCOVS is the arrester minimum MCOV demanded by the system under steady state conditions and MCOVD

is the designated arrester MCOV (i.e. designated by the manufacturer) of the applied arrester selected on the basis of

the applicable temporary overvoltages (TOVs) on the system (see Step 2).

Step 2: Selecting the Rated Voltage Value

The arrester rated voltage is determined from the TOVs that occur on the system. The TOVs that need to be

considered are those due to ground faults, load rejection, Ferranti effect, resonance, etc. or a combination thereof.

TOVs are often (but not always) decisive ranging from 1.4 pu for effectively grounded systems to 1.73 pu for

ungrounded systems. The duration of these TOVs is dependent on the fault clearing time, which is generally less

than 1 second for effectively grounded systems but can be several hours on ungrounded systems.

Arrester manufacturers publish curves of TOV capability such as shown in Figure 2 (this is an example only; refer to

the manufacturer catalogues for actual TOV capabilities).

FIGURE 2

The capability is expressed in pu of rated voltage and both curves assume an initial block temperature of 60°C and

imply thermal stability at MCOVD after the occurrence of the TOV. The conservative and recommended approach is

to assume that the arrester has previously absorbed maximum rated energy and thus the operating points should be

to the left of curve C2. The general procedure is:

1. For each TOV, establish the arrester rated voltage based on the manufacturer’s published TOV capability

curves.

2. Choose the highest rated arrester as determined under 1 above.

3. Confirm that MCOVD ≥ MCOVS.





Step 3: Check the Protective Levels

The protective level is the residual voltage at the nominal discharge current. For example, at 230 kV this current

would be 10 kA. The setting of the protective level is the prerogative of the user but a recommended approach

would be to consider protection over the life of the protected equipment. Note also that the distance between the

arrester and the protected equipment is also a consideration.

Having selected the arrester rated voltage and MCOVD under Steps 1 and 2, check the associated lightning and

switching surge protective levels provided by such an arrester by reference to manufacturer published data. If the

protective levels are higher than the required values, then it may be necessary to reconsider Step 2 or to consider

adding arresters at the line entrance.

Step 4: Determine the Nominal Discharge Current

The nominal discharge current is the lightning discharge current through the arrester for which protection of

equipment is required. Standards values are usually used [4]:

• Range 1: above 1 kV up to 230 kV: 5 kA or 10 kA. 5 kA is common for distribution systems and in some

cases 72.5 kV (low lightning density). Above 72.5 kV, 10 kA is the recommended value.

• Range 2: above 245 kV: 10 kA or 20 kA. For 420 kV and below, 10 kA is generally sufficient. Above

420 kV, 20 kA arresters may be required.

Step 5: Determine the Line Class

The line class is determined by the required energy absorption capability. This capability is dependent on the

transient overvoltages involved – lightning, closing or reclosing long lines or capacitor bank (or cable) switching –

and is calculated accordingly. The next high line class to the calculated requirement should be selected.

Step 6: Determine the Pressure Relief Class

In the event of internal failure, the fault current through the arrester should not cause violent shattering of the

housing. For this reason the fault current withstand of the arrester should be equal to or greater than the maximum

fault current level at the installation point of the arrester.

Step 7: Determine the External Insulation Values

Because surge arresters are self-protecting, the housings do not require the same BIL or SIL as other equipment in

the substation. Various multipliers are applied to the respective protective levels to determine the housing external

insulation withstand values. With respect to creepage distance, the same requirement as for other substation

equipment (e.g. bushings, post insulator, instrument transformers, etc.) should be used.

DISCUSSION

Metal oxide arresters are very application dependent. However, if proper consideration is given to the IEC approach,

misapplication can be avoided and it should also be possible for users to standardize their arrester requirement to a

large degree.

REFERENCES

1. IEC 60099-4: Surge arresters – Part 4: Metal-Oxide Surge Arresters Without Gaps For A.C. Systems.

Edition 1.2, 2000-12.

2. IEEE Std C62.11-1993: IEEE Standard for Metal-Oxide Surge Arresters for Alternating Current Power

Circuits.





3. Peelo, D. F., “Metal Oxide Surge Arrester Standards: A Utility Perspective.” Canadian Electrical Association

Meeting, Toronto, 1989.

4. IEC 60099-5: Surge Arresters – Part 5: Selection and Application Recommendations.

application of metal oxide surge arresters df peelo

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