An Introduction to Surge Protection Video

Partial Discharge

Introduction for High Voltage Partial Discharge

Partial Discharge in High Voltage Cables, Switchgear, Transformers and Motors

Partial Discharge (Corona)

Corona on Insulator

High Voltage Corona Discharges

Partial Discharge Measurement

Surge Protection Devices (SPD)

Wiki, "A surge protector (or surge suppressor) is an appliance designed to protect electrical devices from voltage spikes. A surge protector attempts to limit the voltage supplied to an electric device by either blocking or by shorting to ground any unwanted voltages above a safe threshold."

Surge Protection Devices use non-linear voltage limiting (or switching) components to clamp transient voltages to a safe level

Clamping Voltage - This term is loosely used in the industry to refer to the voltage at which an SPD limits an applied surge impulse. More correctly, for MOV devices the clamping voltage is the point at which the SPD will start to draw current and is generally regarded as the knee of the VI curve at which 1mA dc current flows.

Follow-On Current - Where a “Voltage Switching” SPD after “firing” clamps below the AC supply voltage and causes a line frequency current to flow. Follow-on current is normally very large for spark gap (crow bar) type devices. It is for this reason that gas arresters are not used for AC power protection applications. “Voltage Limiting” devices such as MOVs and Silicon Avalanche Diode-based devices do not cause follow-on currents.

Figure 1 : Characteristic of SPD

Mode Of Protection

This refers to the way the SPD is connected to the circuit and can be categorized into two; Common Mode and Differential Mode. Each mode is where a dedicated direct SPD element is connected. Note that an SPD may have multiple internal elements allowing one SPD to protect multiple modes, e.g. L-N, L-G and N-G.

Common Mode 

Common Mode Voltage is a voltage between two or more conductors and ground. This is normally an interference or transient voltage between two lines such as Line and Neutral to Ground. Over voltages in common mode concern all neutral point connections. They occur between the live conductors and earth (e.g. phase/earth or neutral/earth). The neutral conductor is a live cable, as well as the phase conductors.

This overvoltage mode destroys not only earthed equipment (Class I), but also non-earthed equipment (Class II) with insufficient electrical insulation (a few kilovolts) located close to an earthed mass. Class II equipment that is not situated close to an earthed mass is theoretically protected from this type of attack.

Figure 2: Common Mode Protection

Differential mode

Overvoltages in differential mode circulate between the live phase/phase or phase/neutral conductors. They can cause considerable damage to any equipment connected to the electrical network, particularly "sensitive" equipment. These over voltages concern TT earthing systems. They also affect TN-S systems if there is a significant difference in length between the neutral cable and the protective cable (PE).

Normal Mode Voltage also referred to as Differential Mode or Transverse Mode - The voltage interference between two conductors of a circuit (Line to Line).

.

Figure 3: Differential Mode Protection

Causes of over voltages

Over voltages due to direct lightning strikes

Figure 4: Direct Lightning Strike on overhead line

Figure 5: Over voltage phenomena due to direct lightning strike

These can take two forms:

• When lightning strikes a lightning conductor or the r oof of a building which is earthed, the lightning curr ent is dissipated into the ground. The impedance of the ground and the current flowing through it create large difference of potential: this is the over voltage. This overvoltage then pr opagates throughout the building via the cables, damaging equipment along t he way.

• When lightning strikes a low voltage overhead cable, it will conduct high intensity curr ents. These will penetrate the building, also creating high voltages. The damage caused by this type of over voltage is usually considerable and can have major financial consequences. For example, a fir e in the electrical switchboard can destroy industrial equipment and even the building itself.

Figure 6: Two phenomena of Direct Lightning Strike

Over voltages due to the indirect effects of lightning strikes

Figure 7: Indirect Lightning Strike Phenomena

The overvoltages mentioned above can also occur when lighting s trikes close to a building, due to the incr ease in potential of the ground at the point of impact. The electromagnetic fields created by the lightning current will generate inductive and capacitive couplings, resulting in other overvoltages. Within a radius of several hundreds of meters, or even several kilometers, the electromagnetic field caused by lightning in the clouds can also create sudden increases in voltage. Although the consequences are less serious than in the previous case, irreparable damage is caused to sensitive pieces of equipment such as fax machines, computer power supplies and safety and communication systems.

Figure 8: Three phenomena of Indirect Lightning Strike

How to get Effective Electronic Systems Protection

In order to provide effective protection, a transient overvoltage protector/SPD must:

• be compatible with the system it is protecting
• survive repeated transients
• have a low `let-through’ voltage, for all combinations of conductors
• not leave the user unprotected, at the end of its life, and
• be properly installed 

Compatibility

The protector must not interfere with the system’s normal operation:

• mains power supply SPDs should not disrupt the normal power supply such as creating follow current that could blow supply fuses, or cause high leakage currents to earth
• SPDs for data communication, signal and telephone lines should not impair or restrict the systems’ data or signal transmission 

Survival

It is vital that the protector is capable of surviving the worst case transients expected at its installation point/LPZ boundary. More importantly, since lightning is a multiple event, the protector must be able to withstand repeated transients.

The highest surge currents occur at the service entrance. For buildings with a structural LPS, the lightning current SPD could be subject to as high as 25kA 10/350μs surge currents per mode on a 3-phase TNS mains system (up to 2.5kA 10/350μs per mode on a signal or telecom line) for a worst-case lightning strike of 200,000A.

However, this 200kA level of lightning current itself is extremely rare (approx. 1% probability of occurring) and the peak current the SPD would be subject to further assumes that a structure is only fed with one metallic service. Almost all structures have several metallic services connected to them such as gas, water mains, data & telecoms. Each service shares a portion of the lightning current when the protected building receives a strike, greatly reducing the overall current seen by any single service, and as such any SPD fitted to the electric service lines.

Transient overvoltages caused by the secondary effects of lightning are considerably more common (lightning flash near a connected service up to 1km away from the structure) and therefore are unlikely to have currents exceeding 10kA 8/20μs.

Let-through voltage

The larger the transient overvoltage, the greater the risk of flashover, equipment interference, physical damage and hence system downtime. Therefore, the transient overvoltage let through the protector (also known as the protection level Up of the SPD) should be as low as possible and certainly lower than the level at which flashover, interference or component degradation may occur.

Transient overvoltages can exist between any pair of conductors:

• phase to neutral, phase to earth and neutral to earth on mains power supplies
• line to line and line(s) to earth on data communication, signal and telephone lines

Thus, a good protector (enhanced SPDs to BS EN 62305) must have a low let-through voltage between every pair of conductors.

End of life

When an SPD comes to the end of its working life it should not leave equipment unprotected. Thus in-line protectors should take the line out of commission, preventing subsequent transients from damaging equipment. SPDs for data communication, signal and telephone lines and protectors for low current mains power supplies are usually in-line devices.

Where SPDs are installed at mains power distribution boards it is usually unacceptable for these to suddenly fail, cutting the power supply. Consequently, to prevent equipment being left unprotected, the SPD should have a clear pre end-of-life warning, which allows plenty of time for it to be replaced.

Installation

The performance of SPDs is heavily dependent upon their correct installation. Thus, it is vital that SPDs are supplied with clear installation instructions. The following is intended to supplement the detailed guidance given with each product in order to give a general overview of installation. This should not be viewed as a substitute for the Installation Instructions supplied with the SPD. 

Installing parallel connected SPDs for mains power supplies:

• SPDs should be installed very close to the power supply to be protected, either within the distribution panel or directly alongside of it (in an enclosure to the required IP rating)
• Connections between the SPD and phase(s), neutral and earth of the supply should be kept very short (ideally 25cm or less, but no more than 50cm)
• SPD performance is further enhanced by tightly binding connecting leads together (simply using cable ties or similar), over their entire length
• For safety and convenient means of isolation, the phase/live connecting leads should be suitably fused using HRC fuses or switchfuse, MCB or MCCB

Installing in-line SPDs for data, signal, telephone or power:

• SPDs are usually installed between where cabling enters or leaves buildings and the equipment being protected (or actually within its control panel)
• The installation position should be close to the system’s earth star point (usually the mains power earth) to enable a short and direct connection to earth
• In-line, or series, connected SPDs generally have connections marked line and clean. The line end of the SPD should be connected to the incoming or “dirty” line (from where the transient is expected). The clean end of the SPD should be connected to the line or cable feeding the equipment
• Cables connected to the SPD’s clean end should never be routed next to dirty line cables or the SPD’s earth bond

Types of SPD:

BS 62305 deals with provision of SPDs to protect both effects of indirect lightning strike and high energy direct lightning strikes.

1. Direct Lightning Strikes are protected by lightning current or equipotential bonding SPDs class 1.
2. Indirect Lightning Strikes and switching transients are protected by transient overvoltages SPDs class 2 and 3.

Figure 9 : Selection Class of Protection for SPD

Class of Protection for SPD

1. Class I test - SPD tested with maximum impulse current (Iimp) and nominal discharge current (In).
2. Class II test - SPD tested with maximum discharge current (Imax) and nominal discharge current (In).
3. Class III test - SPD tested with combination wave

Terminology :

• 2/50 μs wave: Standardized overvoltage waveform created on networks and which adds to the network’s voltage.
• 8/20 μs wave: Current waveform which passes through equipment when subjected to an over voltage (low energy).
• 10/350μs wave: Current waveform which passes through equipment when subjected to an overvoltage due to a direct lightning strike.
• Type 1 surge protective device: Surge protective device designed to run-off energy caused by an over voltage comparable to that of a direct lightning strike. It has successfully passed testing to the standard with the 10/350 μs wave (class I test).
• Type 2 surge protective device: Surge protective device designed to run-off energy caused by an over voltage comparable to that of an indirect lightning strike or an operating over voltage. It has successfully passed testing to the standard with the 8/20 μs wave (class II test).
• Up: Voltage protection level. Parameter characterizing surge protective device operation by the level of voltage limitation between its terminals and which is selected from the list of preferred values in the standard. This value is greater than the highest value obtained during voltage limitation measurements (at In for class I and II tests).
• In: Nominal discharge current. Peak current value of an 8/20 μs waveform (15 times) fl owing in the surge protective device. It is used to determine the Up value of the surge protective device.
• Iscwpv: Short-circuit photo voltaic DC current withstand.
• Imax: Maximum discharge current for class II testing. Peak current value of an 8/20 μs waveform fl owing in the surge protective device with an amplitude complying with the class II operating test sequence. Imax is greater than In.
• Iimp: Impulse current for class I testing. The impulse current Iimp is defined by a peak current Ipeak and a charge Q, and tested in compliance with the operating test sequence. It is used to classify surge protective devices for class I testing (the 10/350 μs wave corresponds to this definition).
• Un: Nominal AC voltage of the network : nominal voltage between phase and neutral (AC rms value). Nominal Discharge Current (In) The peak value of the current flowing through the SPD during the application an 8/20μs waveshape. Note: IEC 61643-1requires SPDs tested to Test Class II, to withstand 15 impulses at In followed by 0.1, 0.25, 0.5, 0.75 and 1.0 times Imax. 
• Impulse Current (Iimp)- Peak impulse current withstand with a 10/350μs current wave shape. This is often used for the classification of SPDs tested to Test Class I, but is not the only acceptable wave shape.
• Maximum Discharge Current (Imax)- The maximum single shot current, having an 8/20μs wave shape, which the SPD can safely divert 
• Uc: Maximum continuous operating voltage (IEC 61643-1).The maximum r.m.s. or d.c. voltage which may be continuously applied to the SPDs mode of protection. This is equal to the rated voltage.
• Ucpv: Maximum continuous operating voltage on specific photovoltaic DC networks.
• Ng: Lightning strike density expressed as the number of ground lightning strikes per km2 and per year.
• UT: Temporary overvoltage withstand. Behaviour of an SPD when subjected to a temporary overvoltage UT for specific time duration tT 
• Ifi: Follow current interrupting rating Ifi (kArms). It is a parameter for spark-gaps and gas discharge tubes (Type 1 SPDs) and does not concern varistors. Ifi is the rms-value of the follow current, which can be interrupted by the SPD under Uc. It is the prospective short-circuit current that a SPD is able to interrupt by itself. Ifi of the SPD should be equal to or higher than the prospective short-circuit current at the point of installation (Ip). If not, the upstream fuse will melt each time the spark-gap ignites.
• Ip: Prospective short-circuit current of a power supply (Ip) (kArms). Ip is the current which would flow at a given location in case of short-circuit at this location.
• Surge Current Rating - Maximum current withstand of an SPD for a single current impulse waveform (with MCOV voltage applied) of defined waveshape. The clamping voltage after this test should not differ by more than 10% of the value prior to the test. Most commonly surge ratings are quoted for an 8/20μs current waveform, but 10/350μs and 10/700μs are others used.