7 Most Known High Voltage Insulation Methods

Homogeneous characteristics //

Before getting to the specifics of each material, there are a couple of issues that apply to all materials. First, it is important that the insulating material is homogeneous. This is to ensure that the dielectric constant is the same throughout so that the electric field strength gradient is as constant as practical.

Also, if there are any air pockets, there could be corona or localized arcing. The implications range from rapid insulation failure, to gradual, long term degradation. Thus, it’s a good idea to include a method for removing air from any encapsulant.

One implication of homogeneity is that it is generally not a good idea to use more than one insulation material. Putting two materials “in series” can do more harm than good.

Second, it is important to derate the insulation’s dielectric strength, often expressed in volts per mil. The amount of derating depends upon the shapes of the conductors (the “sharp points” issue), the insulator thickness, and the insulator homogeneity.

Generally, you derate the dielectric strength as the material gets thicker. For many materials you can find an equation that describes the relationship between thickness and dielectric strength.

Insulation materials

Here are 7 most common and known materials used in electronics assemblies as high voltage insulation, and descriptions of issues that require special attention.

1. Air

                 

It’s free and the cost is certainly a big advantage of air. However, air has the worst dielectric strength of all the insulators in this listing, so distances will need to be the greatest for a given geometry.

A second consideration is that surfaces need remain clean and free of dust over long periods, so that there is no arcing along a surface.

2. Sulphur Hexafluoride (SF6)

                 

This is a great insulator in that parts remain clean, and assemblies are easy to work on once the housing is opened and the gas is replaced by air. One difficulty with using SF6 is that the housing must be hermetically sealed. Two safety issues exist. Poisonous gasses are formed whenever there are arcs.

Also, it is extremely hazardous if someone breathes in any SF6. It is denser than air, and will settle in the bottom of one’s lungs, and suffocation can result. Since it is a colorless, odorless gas, it goes undetected. Extreme care must be taken to avoid inhalation.

Interesting reading: Are Your Circuit Breakers Healthy?

3. Epoxy

                       

While epoxy has good thermal characteristics and excellent adhesion, if not used properly during the manufacturing process, it can ruin components in a high voltage assembly. Stresses during the potting process can damage components.

Thermal stresses during equipment operation can also damage components due to unequal thermal expansion and contraction. In addition, the epoxy curing process can be highly exothermic and parts can be damaged by heat generated during curing.

Epoxy modules are unrepairable, so if there is a failure, the assembly must be discarded.

The other implication is that failure analyses are severely limited, making it difficult to learn from any problems that crop up.

4. Two-part Silicone (Two-Part)

Like epoxy, silicone encapsulation is in widespread use in the high voltage industry. Poor adhesion is perhaps the biggest shortcoming of silicones. However, preparing the surface properly can alleviate this. Some silicones are adversely affected by certain materials. In the presence of these materials, the curing process is inhibited, and the silicone never cures.

In addition, most silicones do not have good thermal conductivity, as compared to most epoxies.

5. One-part Silicone

One part silicones are available in “self leveling” and thixotropic form. Thixotropic materials do not flow and remain in whatever shape they emerge in from the tube. It is somewhat like shaving cream in that respect. Prior to curing, the thixotropic silicone can be formed into a desired shape. Air bubbles can be trapped in cured thixotropic silicones, and thus their use is limited.

One more thing to watch for is that some one-part silicones will corrode electronic assemblies.

6. Oil

Oil is in widespread use in part due to its relatively low cost and thermal characteristics.                                                                            

Prior to 1977, some oils were dangerous due to the inclusion of toxic PCB’s (polychlorinated biphenyls), but this is thankfully an issue of the distant past. Considerations include the need to have a sealed housing with a provision for expansion and contraction.

Oil can easily become contaminated by arcing and with the buildup of foreign materials. In addition, it can be messy to work with oil.

Cutaway model of an oil-filled high-voltage circuit breaker. The interior is completely filled with insulating oil during normal operation.

7. Perfluorinated Liquids

One trade name of perfluorinated liquids is FluorinertTM. They are clear, colorless, odorless and have a viscosity similar to water. Concerns include the need to have a sealed housing with a provision for expansion and contraction. It is expensive, and evaporation is an issue.

In some cases, evaporation is so fast you can almost see dollar bills disappearing into thin air!

Microprocessor and conventional secondary systems compared

Microprocessor and conventional secondary systems compared (photo credit: ghantootelectrical.com)

Reliable operation of the primary system

Secondary systems are all those facilities needed to ensure reliable operation of the primary system, e.g. a high voltage substation. They cover the functions of controlling, interlocking, signalling and monitoring, measuring, counting, recording and protecting.

With conventional secondary systems, the various functions are performed by separate devices (discrete components) which mostly work on the analogue principle and as a rule are of varying sophistication.

Old technology

The resulting situation is as follows:

1. Each task is performed by devices employing different technologies (electromechanical, electronic, solid-state or microprocessor-based).
2. These discrete devices may require many different auxiliary voltages and power supply concepts.
3. The links between the devices and with the switchgear require a great deal of wiring or cabling and means of matching.
4. The information from the switching apparatus has to be applied separately to numerous inputs for protection, control, interlocks etc., so monitoring the interfaces is complicated.
5. Checking the performance of the individual devices is accompanied by more difficult verification of overall performance.

With the new control technology for switching installations, the emphasis is on the system and its function as a whole. Digital methods are employed for the respective functions, using programmable modules based on microprocessors.

9 new technology features

The distinguishing features of the new technology are:

1. Use of identical device components or combined devices based on microprocessors for the various tasks or functions.
2. Standardized power supply and supply concept.
3. Serial data transfer minimizes wiring (bus technique).
4. Fibre optic cables are used near the process to reduce the cost of established adequate electromagnetic compatibility.
5. Composite use made of data from the switchgear.
6. Self-diagnosis with continuous function check-up, hence simpler testing of overall system and subsystem.
7. Simple correct-sequence signal acquisition with a resolution of about 1 ms.
8. Reduced space requirements.
9. Records of station functions.

Another major innovation of the new approach is the man-machine interface (MMI).

While the access interface to conventional secondary technology is switch panel- or mimic control panel-oriented with the elements of switches, buttons, lamps and analogue instrumentation, access to the new control systems is usually through a display at bay level andthrough monitors and keyboards at substation and system control centre level.

Operation is mostly menu-guided, so no programming or computer skills are necessary.

Practices for grounding and bonding of cable trays

Grounding and bonding of cable trays (on photo: Ground wire connected to cable tray;

Metallic Cable Trays

Cable tray may be used as the Equipment Grounding Conductor (EGC) in any installation where qualified persons will service the installed cable tray system. There is no restriction as to where the cable tray system is installed. The metal in cable trays may be used as the EGC as per the limitations of table 392.60(A).

All metallic cable trays shall be grounded as required in Article 250.96 regardless of whether or not the cable tray is being used as an equipment grounding conductor (EGC).

The EGC is the most important conductor in an electrical system as its function is electrical safety.

Grounding and bonding of cable trays

There are three wiring options for providing an EGC in a cable tray wiring system:

1. An EGC conductor in or on the cable tray.
2. Each multi-conductor cable with its individual EGC conductor.
3. The cable tray itself is used as the EGC in qualifying facilities.

Correct Bonding Practices To Assure That The Cable Tray System Is Properly Grounded

If an EGC cable is installed in or on a cable tray, it should be bonded to each or alternate cable tray sections via grounding clamps (this is not required by the NEC® but it is a desirable practice). In addition to providing an electrical connection between the cable tray sections and the EGC, the grounding clamp mechanically anchors the EGC to the cable tray so that under fault current conditions the magnetic forces do not throw the EGC out of the cable tray.

A bare copper equipment grounding conductor should not be placed in an aluminum cable tray due to the potential for electrolytic corrosion of the aluminum cable tray in a moist environment. For such installations, it is best to use an insulated conductor and to remove the insulation where bonding connections are made to the cable tray, raceways, equipment enclosures, etc. with tin or zinc plated connectors.

NEC Table 250.122 – Minimum size equipment grounding conductors for grounding raceway and equipment

Table 2 – Minimum size equipment grounding conductors for grounding raceway and equipment

Aluminum Cable Tray Systems

Table 392.60(A) – Metal area requirements for cable trays used as equipment grounding conductors

Metal area requirements for cable trays used as equipment grounding conductors

For Sl units: 1 square inch = 645
* Total cross-sectional area of both side rails for ladder or trough cable trays or the minimum cross-sectional area of metal in channel cable trays or cable trays of one-piece construction.
** Steel cable trays shall not be used as equipment grounding conductors for circuits with ground-fault protection above 600 amperes. Aluminum cable trays shall not be used as equipment grounding conductors for circuits with ground-fault protection above 2000 amperes.

Table 392.60(A) “Metal Area Requirements for Cable Trays used as Equipment Grounding Conductors” shows the minimum cross-sectional area of cable tray side rails (total of both side rails) required for the cable tray to be used as the Equipment Grounding Conductor (EGC) for a specific Fuse Rating, Circuit Breaker Ampere Trip Rating or Circuit Breaker Ground Fault Protective Relay Trip Setting.

These are the actual trip settings for the circuit breakers and not the maximum permissible trip settings which in many cases are the same as the circuit breaker frame size.

If the maximum ampere rating of the cable tray is not sufficient for the protective device to be used, the cable tray cannot be used as the EGC and a separate EGC must be included within each cable assembly or a separate EGC has to be installed in or attached to the cable tray.

For specific areas requiring bonding for electrical continuity, refer to Figures 1-4.

Figure 1 left: Expansion splice plates; Figure 2 right: Horizontal adjustable plates

Figure 3 left: Discontinuos segments; Figure 4 right: Cable tray sections vertical adjustable splice plate

Non-metallic cable trays do not serve as a conductor. It is also recommended that wire mesh cable trays not be used as an equipment grounding conductor.

Although permitted by the NEC, it is recommended due to the unique nature of the wire mesh, fittings are manufactured in the field from straight sections by cutting away the current carrying structural wires, reducing the current-carrying capability of the system. As such, the use of wire mesh cable trays as an equipment grounding conductor is not recommended.

If the wire mesh cable tray is to be used as an equipment grounding conductor, then theinstallation of a ground wire is recommended.

If a wire mesh cable tray is supporting cable with a built-in equipment grounding conductor or control or signal cables, then the tray should have a low impedance path to a non-system ground to reduce noise and remove induced or stray currents. A separate grounding cable attached to the wire mesh cable tray is not usually required.

Anti Pumping And Lockout Relays

Anti-pumping relay - Powell (a - position of AP relay inside circuit breaker)

What is ANSI code for Antipumping relay?

ANSI codes 94 or even 52 can be used.

How relays are classified based on functional categories?

Relays can be divided into six functional categories shown below:

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Protective relays

Detect defective lines, defective apparatus, or other dangerous or intolerable conditions. These relays generally trip one or more circuit breaker, but may also be used to sound an alarm.

Monitoring relays

Verify conditions on the power system or in the protection system. These relays include fault detectors, alarm units, channel monitoring relays, synchronism verification, and network phasing. Power system conditions that do not involve opening circuit breakers during faults can be monitored by verification relays.

Reclosing relays

Establish a closing sequence for a circuit breaker following tripping by protective relays. Regulating relays are activated when an operating parameter deviates from predetermined limits. Regulating relays function through supplementary equipment to restore the quantity to the prescribed limits.

Auxiliary relays

Operate in response to the opening or closing of the operating circuit to supplement another relay or device. These include timers, contact-multiplier relays, sealing units, isolating relays, lockout relays, closing relays, and trip relays.

Synchronizing (or synchronism check) relays

Assure that proper conditions exist for interconnecting two sections of a power system.

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Many modern relays contain several varieties of these functions. In addition to these functional categories, relays may be classified by input, operating principle or structure, and performance characteristic.

The following are some of the classifications and definitions described in ANSI/IEEE Standard C37.90 (also ANSI/IEEE C37.100 “Definitions for Power Switchgear”):

Inputs
Current
Voltage
Power
Pressure
Frequency
Temperature
Flow
Vibration
Operating Principle or Structures
Current balance
Percentage
Multirestraint
Product
Solid state
Static
Microprocessor
Electromechanical
Thermal

Performance Characteristics
Differential
Distance
Directional over current
Inverse time
Definite time
Under voltage
Over voltage
Ground or phase
High or low speed
Pilot
Phase comparison
Directional comparison
Current differential

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Solid-state (and static) relays are further categorized under one of the following designations:

Analog

Analog relays are those in which the measured quantities are converted into lower voltage but similar signals, which are then combined or compared directly to reference values in level detectors to produce the desired output (e.g., SA-1 SOQ, SI-T, LCB, circuit shield relays).

Digital

Digital relays are those in which the measured ac quantities are manipulated in analog form and subsequently converted into square-wave (binary) voltages. Logic circuits or microprocessors compare the phase relationships of the square waves to make a trip decision.

Numerical

Numerical relays are those in which the measured ac quantities are sequentially sampled and converted into numeric data form. A microprocessor performs mathematical and/or logical operations on the data to make trip decisions.

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How do we classify an Antipumping relay?

It is classified as an auxiliary relay.

Why Antipumping relay is used?

Anti pumping relaying is done primarily to indicate the spring failure of TNC switch and a person need to attend the problem. The function of this relay is to cut off the supply to 52C (CB closing coil) in case of TNC switch spring failure and prevent CB hunting effect (i.e. continuous closing, opening operation).

Anti pumping is also called trip free mechanism of any circuit breaker. Suppose the breaker has been instructed to close by manual instruction by pressing the TNC (trip neutral close push button) switch. The operating mechanism will start operating for closing operation.

Meanwhile a fault has taken place and relay closes the trip circuit of breaker. The trip free mechanism/ Anti pumping feature permits the circuit breaker to be tripped by protective relay even if it is under process of closing. Thus auxiliary relay (or sometimes the inbuilt feature of circuit breaker) which prevent alternate tripping and closing of the circuit breaker if closing push button is held close during fault is called the trip free mechanism or anti pumping mechanism.

What will happen if Antipumping relay circuit is not present?

If the circuit breaker is closed using TNC switch, and at the same instant let spring of TNC switch has failed and it did not came to its neutral state, and primary protection relay had been operated at this instant and opened the CB, since supply is being extended to 52C (CB closing coil) because of TNC spring failure, then CB will close again, since fault is still persisting the following cycle occurs.

CB trip, because of protection. Operation -> CB close, because of TNC switch spring failure-> CB trip, because of protection. Operation-> CB close, because of TNC switch spring failure-> CB trip, because of protection. Operation -> CB close, because of TNC switch spring.

The above cycle is termed as circuit breaker hunting. Circuit breaker hunting will cause damage to breaker and it can be avoided by using 52 (Circuit breaker Antipumping relay).The function of this relay is to cut off the supply to 52C (CB closing coil)  coil in case of TNC switch spring failure and prevent CB hunting effect (i.e. continuous closing, opening operation).

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Let us analyse the behavior of circuit in case anti pumping is present and in case it is not present, one by one:

CASE 1: Closing Coil Circuit Without 52 Relay

The path for CB close with out 52 relay is shown in Figure 1. In this case if spring of TNC switch fails, then supply will be get extended to 52C (CB closing coil)  coil as shown in Figure 2.

Figure 1 - Circuit breaker closing coil without Antipumping relay

If the TNC switch spring has failed then supply will be extended as shown below.

Figure 2 - showing how supply will be extended to closing coil because of TNC spring failure

CASE 2: Closing Coil Circuit With 52 Relay

Closing coil circuit with 52 present is as shown in Figure 3. Path 1 in Figure 3 shows how 52C (CB closing coil)  will be energized when we press TNC switch CS1.if CS1 comes to its original state once we release it then 52 will not come in picture. if CS1 does not comes to its original state then 52 will be energized as shown in path 2.then 52-NC contact present in path 1 will be got opened up and prevents extension of supply.

We have the concept that initially for the energization of 52 80% to 90% of 110V is required. But once after energization 60-70% of 110V is sufficient to maintain its energization state. This logic is achieved by introducing a resistance in 52 energization maintenance path. This path is shown inFigure 4.

Figure 3 - With Antipumping relay present

Figure 4 - Antipumping relay with a resistor

Once 52 is energized it will give an alarm and a maintenance person in the substation need to attend that problem. This is how 52 is useful in avoiding CB hunting effect.

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What is ANSI code for Master trip lockout relay?

ANSI code 86 is used.

What is function & Utility of lockout relay?

When a ‘‘drive-to-lockout’’ input is energized, the reclosing relay will go into lockout from any point in the sequence. The relay will stay in lockout until the input is removed and the breaker is closed manually or by supervisory control. Upon removal, the recloser will go through its reset sequence and return to ‘‘ready’ state.

All the different trip signals from fault detection devices are wired to the lockout relay, which is the master trip relay for the circuit breaker. It is usually a manually reset relay with an indicating flag or lamp. In case of DG a similar philosophy of tripping is used in the engine control panel in which all the mechanical failure signals are received at the engine lockout relay (86-T, T for turbine). This lockout relay also trips the generator circuit breaker.

When either the 86-G or the 86-T relay sends its signal to the circuit-breaker trip coil, time is taken before the circuit breaker begins to move and finally reach its fully open state. The generator switchgear is recommended to have two separate lockout relays. One will receive all the electrical protection relay trip signals, and denoted as 86–1. The other will receive a master trip signal (or several trip signals) from the turbine unit control panel, and denoted as 86–2.

It operates whenever any of the protection relay operates and would prevent any circuit breaker operation until the lock out relay is manually rest.

Can we manage without anti pumping since operation of lock out relay would prevent any further operations of CB even continuous opening-closing?

Figure 5 - Antipumping relay & Lockout relay contacts in closing coil circuit

Let us assume that we have closed the circuit breaker using the TNC switch CS1.At this instant TNC switch spring failed and fault is there which is sensed by primary protection relay and sends tripping command to circuit breaker. This would energize the trip coil of breaker; this will result in operation of lock out relay.

The lock out relay will go into lock out state as shown in Figure 6 below:

Figure 6 - The lock out relay goes into lock out state

Now let a person had attended that fault and resetted 86 lockout contacts. After resetting its status will be NC and a closed command will be extended to closing coil without any human interface which is a mal function.

So an 86 lockout relay cannot fulfill the purpose of anti pumping relay.

The function of anti pumping relay is to cut off the supply to closing coil in case of TNC switch spring failure and prevent CB hunting effect (i.e. continuous closing, opening operation) and a person needs to attend the problem. Where as 86 lock out relay contact in closing circuit is meant to ensure that a person had attended the fault occurred and we are closing the circuit after clearing the circuit.

It is extra mechanical protection meant for safety of equipments considering the cost and sophistication involved; it is always advisable to go for extra human interface.