VSD--Low-voltage networks--Application + selective coordination; Earth/ground leakage protection

Application + selective coordination

The basic theory of selective co-ordination is applicable for all values of the electrical fault current.

• Milli-amperes: Earth (ground) leakage protection
• Hundreds of amps: Overload protection
• Thousands of amps: Short circuit protection.

Earth (ground) leakage protection will be discussed later. In the short circuit situation, it’s generally accepted that most short circuit currents that occur in practice fall below the calculated theoretical value for a three-phase bolted fault. This is so, because not all faults occur close to the MCCB (except when the supply cable is connected to the bottom of the MCCB). The resistance of the cable between the MCCB and the fault reduces the fault current. Most faults are not bolted faults - the arc resistance helps to reduce the fault current even further. For economic and practical reasons, it’s not feasible to apply the same sophisticated relay technology, as used on the medium-voltage to low-voltage networks, as this would result in a very complicated and expensive system. The present system of using air and MCCBs is a successful compromise developed over many years. These devices, however, are current operated as described previously, so it’s possible to achieve varying degrees of coordination by the use of:

• Current grading
• Time grading
• Current and time grading.

Air-circuit breaker: Let us now consider the protection on the ACB on the LV side of the main in-feed transformer.

++++11 LV ACB on transformer.

Transformer overload condition: The thermal element on the ACB can be set, to protect the transformer against excessive overloading, as the same current that flows through the transformer flows through the ACB. Tripping this breaker removes the overload and allows the transformer to cool down. The transformer has not faulted - it’s only being driven above its continuous design rating, which if allowed to persist for some time, will cook the insulation leading to an eventual failure. Checking the temperature indicators on the transformer allows the operator to have a clear indication of the problem. The transformer is still alive from the HV side so it has not faulted. It’s purely on an overload condition.

It has often been a common practice to trip the transformer from the HV IDMT overcurrent relay, for an overload condition. With this approach, the operator does not know if the transformer has faulted or if it was just an overheating condition. When faced with such a decision, deciding to test the unit before switching it again could lead to an excessive downtime. In addition, the HV IDMT overcurrent relay (normal inverse) does not have the appropriate characteristic for an overload protection.

Short-circuit protection Short circuits at points A, B, and C must be considered. The fault currents will be the same, as there is virtually no impedance between them. The short circuit protection on the ACB should therefore be set with a short time delay, to allow the downstream MCCB to clear fault C. However, if the fault is on the busbar, the time delay should be short enough, to ensure a relatively fast clearance to minimize damage and downtime.

Time; Current; Thermal overload; Short circuit.

++++12 ACB-adjustable protection tripping characteristics.

Fault A will have to be cleared by the HV overcurrent relay in order to protect the cable, from the transformer to the LV switchboard. This in turn, should have a longer time delay, to co-ordinate with the LV ACB and also to provide, the discrimination for faults B and C. These requirements show the value, of specifying adjustable current pickups and time delays, for the protection devices on ACBs, most of which are available in an electronic form.

In addition, they also come equipped, with a very high set instantaneous overcurrent feature, having a fast fixed time setting of 20 ms, to cover 'closing-onto-fault' conditions.

Molded case circuit breakers

A reasonable degree of current grading can be achieved, between the two series-connected MCCBs by simply applying, a higher rated breaker, up-stream of a given unit. The extent of the co-ordination is shown on the following time-current characteristic curves. It will be noted that selectivity is obtained in the thermal overload and partial high current region, co-ordination being lost, above the short-circuit pickup current level, of the up-stream breaker.

++++13 Current coordination.

For large consumers, the integrity of the supply is important. The ability of the up-stream breaker to hold in, under such fault conditions is enhanced, when it’s equipped with an additional short time delay facility, provided by the modern electronic trip elements.

Note: It’s important to note that an MCCB does not have a short time fault current withstand rating unlike ACBs (which can usually withstand rated short circuit current for a duration of 1s). A time difference of tripping of at least 0.3s is necessary for proper coordination between an upstream and downstream breaker which is not provided in MCCBs as an adjustable delay for short-circuit trip. Because of this, accurate time co-ordination cannot be achieved between an upstream and downstream circuit when both of them use MCCBs.

Also, the setting for short circuit cannot provide the required current discrimination as the fault levels usually are much higher compared to the breaker current settings and also the point of fault does not significantly affect the fault current magnitude in industrial systems where usually LV feeder lengths are kept short.

++++14 Current-time coordination – Time, Current, Fault current, Selective coordination, Short delay.

MCCB un-latching times:

Once triggered, MCCBs have an un-latching time which is dictated by the physical size and inertia of the mechanism. It stands to reason that the physically smaller, lower-rated breakers will have a shorter un-latching time than the higher-rated, larger up-stream breakers, thereby enhancing their clearing time.

Experience in practical installations of the fully rated breakers, has shown that unexpected degrees of discrimination have been achieved because of this. For current-limiting circuit breakers, where contact parting occurs, independent of the mechanism, the un-latching times don’t have such an impact on their clearance times.

Fully rated systems:

When time-delayed MCCBs are used to achieve an extended co-ordination, all the down-stream circuit breakers must be rated, to withstand and clear, the full prospective short circuit current, at the load side terminals. The installation of an MCCB and making cable connections to its terminals require caution.

Cautions for installation:

Change ratio of the rated current value according to the installation angle; Don’t remove the rear cover; Don’t remove the compound inserted into the screw part of the base rear or the rear cover.

++++15 Cautions for installation.

Cautions for connection:

1. Take sufficient insulation distance

2. Don’t apply oil to the threaded parts

1. Parallel conductors for all poles

Take care, as the insulation distance may be insufficient according to the installation position of the connection conductor.

As some type are provided with insulation barriers, they should be used under reference top.

Don’t apply lubrication oil to the threaded parts.

Application of lubrication oil reduces the friction of the threaded part, so that loosening and overheat can be caused. In case of lubrication, even the standard tightening torque can produce excessive stress in the threaded part and thus breaking of the screw

Install the connection conductors in parallel for all poles

++++16 Cautions for connections

Cascading systems:

This approach can be used, if saving on the initial capital cost is the overriding factor.

This necessitates using a current-limiting breaker, to contain the let-through energy, thereby allowing lower-rated (hence less costly) breakers, to be used downstream. To achieve a successful co-ordination, careful engineering is required. This is so with regard to clearance and un-latching times, additionally to, the size and length of the interconnecting cables, together with an accurate calculation of the fault levels.

If the let-through energy is sufficient to cause the downstream breaker to un-latch, then the faulty circuit will be identified. The upstream current-limiting MCCB will also have tripped, to drop the whole portion of the network being served, by this main breaker.

However, if the down-stream breaker does not un-latch then an extended outage time is inevitable (to trace the fault location). It’s vital that the complete system be tested and approved to ensure that the delicate balance of the system is not disturbed. There are a number of factors that need careful consideration.

Sluggish mechanisms:

It’s well known that any electromechanical assembly of links, levers, springs, pivots, etc. which remain under tension or compression for a long period of time, tend to 'bed in'. Dust and corrosion also contribute further, to retarding the operation, after long periods of inactivity. The combined effect could add a delay of 1-3 ms when eventually called into operation.

This additional delay has little effect on fully rated breakers which generally operate after one cycle (20 ms), but on the current-limiting MCCBs, which are required to operate in 5 ms, the additional 1-3 ms will have a significant impact on their performance. The increased energy let-through could have disastrous results, both for itself and the downstream breaker.

Point-on-wave switching

Most specifications and literature show current/energy-limitation, based on fault initiation occurring at a point-on-wave corresponding to the current zero. Should the fault occur at some other point on the wave, the di/dt of the fault current would be much greater than that shown, resulting in a higher energy let-through.

++++17 Effect of point-on-wave fault occurrence.

Service deterioration:

'Qualification-type tests' in most international specifications require the MCCB to successfully perform one breaking operation and one or two make-break operations. In practice, it’s rare that the number of operations by a breaker under short-circuit conditions is monitored. This shortcoming is not critical on fully rated systems, as the protection of the downstream breakers is of no consequence.

However, in a series-connected cascading system, where the downstream breakers rely for their survival on the energy-limiting capabilities of the upstream current-limiting breaker, there is always the danger that replacement of the upstream device is overlooked.

Therefore, there is a strong case for monitoring the number of operations.

Maintenance:

For reasons stated above, any up-stream or downstream breakers in a cascade system must be replaced with identical breakers from the same manufacturer, in accordance with the original test approvals. This also applies to any system extensions. Any deviation could prove disastrous.

Incorrect replacement of the up-stream breaker could result in a higher energy let through and longer operating times. Incorrect replacement of the downstream breakers may lead to a lower energy-handling capability, coupled with shorter operating times.

These conflicting requirements are such that, even experienced or well-trained technicians may be confused, unless they are fully conversant with the principle of the cascade system.

There could be an even greater problem for the maintenance electrician and his artisan, in selecting a replacement device, which may often be dictated by availability.

Identification:

In view of the problems of staff turnover and the possibility of decreasing skills, it becomes a stringent requirement that all switchboards carry a prominent identifying label, together with all relevant technical information, to ensure the satisfactory operation and maintainability of the cascaded or the series-connected systems.

General:

Cascaded systems offer attractive savings in the initial capital expenditure. They, however, require a higher level of engineering for the initial design and extensions.

Maintenance can be difficult, as the total knowledge and understanding of the system and all its components is required by all operating personnel. The consultant, contractor, or user is thus faced with the decision of choosing between two very different systems:

• A fully rated properly coordinated system

• A system based on cascaded ratings.

The first choice may have a slightly higher initial cost. The alternative offers some initial cost savings whilst sacrificing some system integrity, selectivity, and flexibility.

Earth (ground) leakage protection

In the industrial and mining environment, the possibility of operators making direct contact with live conductors is very remote. This is because the conductors are housed in specially designed enclosures, which are lockable and where only trained and qualified electricians are allowed access. Danger due to indirect contact (by touching the enclosure of an equipment having an eart/ground fault) is addressed mainly by clearing the fault as quickly as possible. Since LV circuits are provided a metallic return path to the neutral point of source by proper earth-ground bonds, it’s ensured that the fault current magnitudes are high enough to be sensed and cleared by protection devices meant for isolating short-circuit faults such as fuses or circuit breakers.

The danger lies, however, when an earth/ground fault occurs on a machine and because of poor earth-/ground-bonding, the frame of the machine becomes elevated to an unsafe touch potential. This is an 'indirect' contact situation, which must be protected.

Motor Earth (ground) fault:

VIf ×Rb Contactor; Core-balance relay; Circuit-breaker; Transformer; Earthing bond; If Bond resistance (Rb)

++++18 Protection against indirect contact.

Safety codes specify that in mining and industrial installations any voltage above the range of 25-40 V is considered unsafe. These figures are derived from the current level that causes ventricular fibrillation - 80 mA times the minimum resistance of the human body, which is in the range of 300 ohm (3 × safety factor) to 500 ohm (2 × safety factor).

It would not be possible to utilize the sensitive domestic earth/ground leakage devices (30 mA, 30 ms) in such applications because of the transient spill currents that occur during motor starting. Instantaneous tripping would occur and the machine would never get started. Tests have been carried out in coal mines to determine the maximum resistance that could occur on an open earth/ground bond. The results were measured as 100 ohm. With 25 V specified as the safe voltage, then a current of 250 mA can be regarded as the minimum sensitivity level (derived from dividing 25 V by 100 ohm). This level was found to be stable for motor starting. It is, however, above the 80 mA fibrillation level of the heart, so speed is now of the essence, if we are to save a human life. The earth/ground leakage relays used in industrial applications should therefore operate in 30 ms. Modern earth/ground leakage relays can achieve this and one such method is to use a unique sensitive polarized release.

Construction:

The device consists of a U-shaped stator on top of which sits an armature. The magnet mounted adjacent to one limb sets up a flux strong enough to hold the armature closed against the action of the spring. There is a multi-turn coil on the other limb, which is connected to the core balance CT. When an earth/ground fault occurs, an output is generated by the core balance CT into the coil. This reduces the standing flux to the extent that the spring takes over, to flip the armature onto the tripping bar, to open the breaker. The calibration grub screw is a magnetic shunt.

Flux paths

++++19 IES 4 polarized release.

Screwing it in bleeds off magnetism from the main loop making the release more sensitive. Screwing it out allows more magnetism around the main loop, making the armature attraction stronger, hence less sensitive. The burden of the release is only 400 micro VA (10 mV, 40 mA) which allows extremely high sensitivities to be achieved.

The release can be complimented by the addition of some electronics in order to produce a series of inverse-time/current tripping curves.

d- silicon diodes

r –resistors

C – capacitor

i - IES release

scr – silicon-controlled rectifier

++++20 Internal circuitry

Description of operation

When an earth/ground fault or an earth/ground leakage condition occurs on the system, the core balance CT mc generates an output. On the positive half cycle, the secondary current flows through the diode d1, resistor r, and charges up capacitor c. On the negative half cycle, the current flows through the diode d2, resistor r and charges up capacitor c even further.

The resistor divider monitors the voltage across the capacitor c and once it reaches a preset voltage level the gate of the scr is triggered. All of the energy stored in the capacitor now flows through the release i to cause operation of the relay.

The capacitor is now fully discharged enabling the relay to be reset immediately.

By varying the values of r and c, the charge-up time can be varied.

Application and co-ordination of earth/ground leakage relays

A family of relays has been designed to provide a coordinated earth/ground fault protection for low-voltage distribution systems. Using the above-mentioned technology, the time/current inverse curves have been developed.

Current-(amperes) 1A 2.5A

++++21 Time/current response curves

This allows coordinated sensitive earth/ground fault protection to be applied to a typical distribution system. They afford 'backup' protection to the end relay, which provides instantaneous protection to the apparatus where operators are most likely to be working.

Optimum philosophy:

It’s important to note that the choice of relay settings cannot be considered in isolation. They are influenced by the manner of neutral grounding/earthing, current pickup levels, and time grading intervals, which will in turn be dictated by the system configuration.

MS2 - curve 3 MS2 - curve 2 MS2 - curve 1, Circuit breakers, Core-balance units, Contactors Motors MS1- ins HV LV, Neutral earthed/grounded solidly or via current limiting resistance

++++22 Typical LV distribution system M T3 T2 T1 X R4 R3 R2 R1 Ditto Curve X Curve T1 Curve T2 Curve T3 Relay characteristic Clearance times Neutral restriction Remarks IDMTL DTL R4 R3 R2 R1, R2, R3, R4 could all be 250mA R1= 250mA R2= 375mA R3= 500mA R4= 1000mA 1A 2.5A 1 or 2.5A 5.0s 2.0s 1.0s 660ms 1.5s 460ms 360ms 750ms 60ms

++++23 Optimum philosophy.

All are inter-dependent it will be seen that an optimum philosophy for the system would be a 'definite time lag' philosophy (DTL) as opposed to an 'inverse definite time lag' philosophy (IDMTL) as faster clearance times can be achieved.