Application and selective coordination; Ground leakage protection [Industrial Electrical Power Systems]

Application and selective coordination

The basic theory of selective coordination is applicable for all values of electrical fault current.

• • Milli-amperes: Ground leakage protection
• • Hundreds of amps: Overload protection
• • Thousands of amps: Short circuit protection

Ground leakage protection will be discussed later under Section 5. Considering 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 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; also, most faults are not bolted faults - the arc resistance helping 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 therefore of using air and molded case circuit breakers 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 provided by the air circuit breaker on the LV side of the main in-feed transformer.

Transformer overload condition

The thermal element on the air circuit breaker can be set to protect the transformer against excessive overloading, as the same current that flows through the transformer flows through the air circuit breaker. 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 eventual failure. By checking the temperature indicators on the transformer, the operator then has clear indication of the problem. The transformer is still alive from the HV side so it has not faulted. It’s purely 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. He is now faced with a decision and if he is conscientious he may decide to test the unit before switching in again. This could lead to excessive downtime. In addition, the HV IDMT overcurrent relay (normal inverse) does not have the correct characteristic for overload protection as pointed out.

++++ LV air circuit breaker on transformer

Short circuit protection

Short circuits at points A, B and C must now be considered. The fault currents will be the same as there is virtually no impedance between them. The short circuit protection on the air circuit breaker (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 effect relatively fast clearance to minimize damage and downtime.

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 coordinate with the LV ACB and provide discrimination for faults B and C. These requirements show the value of specifying adjustable current pick-ups and time delays for the protection devices on air circuit breakers, most of which are available in 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 two series-connected MCCBs by simply applying a higher-rated breaker upstream of a given unit. The extent of the coordination 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 coordination being lost above the short circuit pick-up current level of the upstream breaker. For large consumers, the integrity of the supply is important. The ability of the upstream 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.

Current --Thermal overload-- Short circuit

++++ ACB adjustable protection tripping characteristics Current

++++ Current coordination in MCCB Current Fault current Selective coordination Short delay

++++ Current-time coordination

MCCB unlatching times

Once triggered, MCCBs have an unlatching 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 unlatching time than the higher-rated, larger upstream breakers, thereby enhancing their clearing time.

Experience in practical installations of 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 unlatching times don’t have such an impact on their clearance times.

Fully rated systems:

When time delayed MCCBs are used to achieve extended coordination, all downstream circuit breakers must be rated to withstand and clear the full prospective short-circuit current at the load side terminals.

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 so allowing lower-rated (hence less costly) breakers to be used downstream. To achieve successful coordination it will be appreciated that careful engineering is required especially with regard to clearance and unlatching times, in addition to size and length of interconnecting cables together with accurate calculation of fault levels.

If the let-through energy is sufficient to cause the downstream breaker to unlatch, then the faulty circuit will be identified, although 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 downstream breaker does not unlatch then extended outage time is inevitable to trace the fault location. It’s vital that the complete system be tested and approved to ensure the delicate balance of the system is not disturbed. There are 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, tend to 'bed in'. Also, dust and corrosion 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 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 for both itself and in particular 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 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 higher-energy let-through.

++++ 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.

There is therefore a strong case for monitoring the number of operations.

Maintenance:

For reasons stated above, any upstream 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 upstream breaker could result in higher energy let-through and longer operating times, whilst incorrect replacement of downstream breakers may lead to 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 therefore 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 cascaded or series-connected systems.

General:

Although cascaded systems may offer an attractive saving in initial capital expenditure, it requires a higher level of engineering for the initial design and extensions. Maintenance can be difficult, as 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 quite different systems:

1. A fully rated properly coordinated system

2. 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.

Ground leakage protection

In the industrial and mining environment the possibility of persons, 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 qualified electricians are allowed access.

The danger lies, however, when an ground fault occurs on a machine and because of poor ground bonding, the frame of the machine becomes elevated to an unsafe touch potential as illustrated in ++++ 11.18 below. This is an 'indirect' contact situation, which must be protected.

Motor Ground fault VIF xRb Contactor Core-balance relay Circuit-breaker; Transformer; Grounding bond IF; Bond resistance (Rb)

++++ Protection against indirect contact

Safety codes specify that in mining and industrial installations any voltage above the range of 25-40 V is considered to be unsafe. These figures are derived from the current level that causes ventricular fibrillation - 80 mA times the minimum resistance of the human body which can be in the range of 300 ohm (3 × safety factor) 500 ohm (2 × safety factor). Please refer to Section 4.

It would not be possible to utilize the sensitive domestic ground leakage devices (30 mA, 30 ms) in these 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 coalmines to determine the maximum resistance that could occur on an open ground bond. This was measured as 100 ?. With 25 V specified as the safe voltage, a current of 250 mA can be regarded as the minimum sensitivity level (derived from dividing 25 V by 100 ?). This level was found to be stable for motor starting. It’s however above the 80 mA fibrillation level of the heart, so speed is now of the essence if we are to save human life.

The ground leakage relays used in industrial applications should therefore operate in 30 ms.

Modern ground leakage relays can achieve this and one such method is to use a unique sensitive polarized release as illustrated.

N S RY B N S Flux paths

++++ I.E.S. 4 polarized release

Construction

The device consists of an 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 current transformer. When an ground fault occurs, an output is generated by the core balance CT into the coil which 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. 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.

Description of operation

When an ground fault or 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 diode D1, resistor R and charges up capacitor C. On the negative half cycle, the current flows through diode D2, resistor R and charges up capacitor C even further.

D2 D - silicon diodes R - resistors C - capacitor I- IES release scr - silicon-controlled rectifier

++++ Internal circuitry

The voltage across the capacitor C is monitored by the resistor divider and once it reaches a pre-set 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 coordination of ground leakage relays

A family of relays has been designed to provide coordinated ground fault protection for low-voltage distribution systems. Using the above-mentioned technology, the following time/current inverse curves have been developed. This allows coordinated sensitive ground fault protection to be applied to a typical distribution system. They afford 'back-up' protection to the end relay, which provides instantaneous protection to the apparatus where operators are most likely to be working.

0.1 0.2 0.3 0.4 1 2 3 4 20 10 30 40 100

Current - amperes 1A 2.5A

++++ Time/current response curves

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

++++ Typical LV distribution system

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, current pick-up levels and time grading intervals, which in turn will be dictated by the system configuration.

All are interdependent and in the following example, it will be seen that 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.

M T3 T2 T1 X R4 R3 R2 R1 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 1.0s 660ms 1.5s 460ms 360ms 750ms 60ms

++++ Optimum philosophy.