Industrial Power-System Protection--Fuses (part 2)

Application of selection of fuses

The fuses blow in case the currents flowing through them last for more than its withstand time. This property limits the use of fuses in circuits where the inrush currents are quite high and flow for considerable time like motors, etc., which draw more than six times their full load current for a short time ranging from milliseconds to few seconds depending on the capacity. Hence, it’s not possible to use fuses as overload protection in such circuits, since it may be necessary to select higher-rated fuse to withstand inrush currents. Accordingly, the fuses are mostly used as short-circuit protection rather than as overload protection in such circuits.

The fuses can be used as either for overload and short-circuit protection or for short circuit protection as noted below:

• Circuits where the load does not vary much above normal value during switching on and operating conditions. Resistive circuits like lamps show such characteristics. Hence, it’s possible to use fuses as overload protection in such circuits. They also protect against short circuits.

• Circuits where loads vary considerably compared to the normal rating e.g.:

- Direct-on-line motors

- Cranes

- Rolling mills

- Welding set, etc.

In these cases, fuses are used to provide short-circuit protection only as it’s not possible to select a size meeting both overload and inrush conditions.

Fuse selection depends on a number of factors:

• • Maximum fault kVA of circuit to be protected
• • Voltage of circuit.

The above factors help to calculate the prospective current of circuit to be protected.

The full prospective current is usually never reached due to rapid operation of the fuse and hence the following factors need to be considered.

1. Full load current of circuit: Short-circuit tests show that the cut-off current increases as the rating increases. Hence if a higher-rated fuse is used it may take longer time to blow under short circuits which may affect the system depending upon the value and duration. Hence, greater benefit is derived from use of correct or nearest rating cartridge fuses compared to the circuit rating.

2. Degree of overcurrent protection required: It’s necessary to consider slightly higher rating for the fuses compared to maximum normal current expected in a system. This factor is called the fusing factor and can be anywhere between 1.25 and 1.6 times the normal rating.

3. Level of overcurrent required to be carried for a short time without blowing or deteriorating e.g. motor starting currents. This point is important for motor circuits. Fuses must be able to carry starting surge without blowing or deteriorating.

4. Whether fuses are required to operate or grade in conjunction with other protective apparatus. This factor is necessary to ensure that only faulty circuits are isolated during fault conditions without disturbing the healthy circuits. Depending on the configuration used, discrimination must be achieved between:

• Fuses and fuses

• Fuses and relays, etc.

There is no general rule laid down for the application of fuses and each problem must be considered on its own merits.

General rules of thumb

Short-circuit protection

Transformers, fluorescent lighting circuits Transient switching surges - take next highest rating above full-load current.

Capacitor circuits Select fuse rating of 25% or greater than the full-load rating of the circuit to allow for the extra heating by capacitance effect.

Motor circuits:

Starting current surge normally lasts for 20 s. Squirrel cage induction motors:

• Direct-on-line takes about 7 times full-load current

• 75% tap auto-transformer takes about 4 times full-load current

• 60% tap auto-transformer takes about 2.5 times full-load current

• Star/delta starting takes about 2.5 times full-load current.

Overload protection

Recommend 2:1 ratio to give satisfactory discrimination.

Special types

Striker pin

This type is most commonly used on medium- and low-voltage circuits. When the fuse blows, a striker pin protrudes out of one end of the cartridge. This is used to hit a tripping mechanism on a three-phase switch-fuse unit, so tripping all three phases. This prevents single phasing on three-phase motors.

Note: On switch-fuse LV distribution gear, it’s a good policy to have an isolator on the incoming side of the fuse to facilitate changing.

Drop-out type

Used mainly on rural distribution systems. Drops out when fuse blows, isolating the circuit and giving line patrolman easy indication of fault location.

General

The fuse acts as both fault detector and interrupter. It’s satisfactory and adequate for both of these functions in many applications. Its main virtue is speed.

However, as a protective device it does have a number of limitations, the more important of which are as follows:

• It can only detect faults that are associated with excess current.

• Its operating characteristic (i.e. current/time relationship) cannot be adjusted or set.

• It requires replacement after each operation.

• It can be used only at low and medium voltages.

Because of these limitations, fuses are normally used only on relatively unimportant, small power, low- and/or medium-voltage circuits.

Series overcurrent AC trip coils

These are based on the principle of working of fuses where the coils are connected to carry the normal current and operated as noted below:

• A coil (instead of a fuse) is connected into the primary circuit and magnetism is used to lift a plunger to trip a circuit breaker.

• Refinement on this arrangement is the dashpot, which gives a time/current characteristic like a fuse.

Time (s) Current (A) Low-voltage fuse link or protective device maximum total operating characteristic (referred to HV side); Protective device characteristic on source side; High-voltage fuse link minimum pre-arcing time; Permissible overload; Transformer full-load current; Secondary terminal ground fault; Inrush Maximum fault current at location of LV protective device (referred to HV side)

++++ Characteristic of transformer HV/LV

• Amps-turns are a measurement of magnetism; therefore for the same flux (i.e. lines of magnetism necessary to lift the tripping plunger) say 50 Amp turns, a 50 A coil would have 1 turn, whereas a 10 A coil would have 5 turns.

++++ Series overcurrent AC trip coil characteristic

Limitation of this type of arrangement is:

Thermal rating

This coil must carry the full fault current and if this is high then the heating effect may be so great as to burn out the insulation. The design must therefore be very conservative.

Magnetic stresses: High fault currents induce tremendous magnetic forces inside the trip coil tending to force the windings apart. Here again the design must display a large margin of support and clamping to contain such stresses.

IS-limiter

A very 'special' type of fuse is the IS-limiter, originally developed by the company ABB. The device consists of two main current conducting parts, namely the main conductor and the fuse.

++++ Construction of IS-limiter

The device functions as an 'intelligent fuse'. The functional parts are the following:

1. Current transformer (detects the short-circuit current)

2. Measuring and tripping device (measures the current and provides the triggering energy)

3. Pulse transformer (converts the tripping pulse to busbar potential)

4. Insert holder with insert (conducts the operating current and limits the short circuit current).

The IS-limiter is intended to interrupt very high short-circuit currents very quickly, in order to protect the system against these high currents. Currents of values up to 210 kA (11 kV) can be interrupted in 1 ms. This means that the fault current is interrupted very early in the first cycle.

When a fault current is detected, the main conductor is opened very swiftly. The current then flows through the fuse, which interrupts the fault current. The overvoltage occurring due to the interruption of current is relatively low due to the fact that the magnitude of current on the instant of interruption is still relatively low. The main conductor and parallel fuse have to be replaced after each operation.

++++ -- Functional diagram of IS-limiter: Short-circuit; current limited by the fuse element

++++ Fault current cycle.

++++ A practical use of the IS-limiter, where the combined fault current supplied by two transformers in parallel would be too high for the switchgear panel to withstand.

…without IS-limiter…

Current I = I1 + I2 at the short-circuit point

Single line diagram of a bus tie for a system with I k =25 kA

and with an I s-limiter

T1 T2 I =25 kA I = I1 + I2 with Is-limiter

2) (25 kA ? x? I k =25 kA

++++ Practical use of IS-limiter

Here I2 is interrupted first thereby decreasing the fault current to the value of I1, and I1 is interrupted subsequently. The net resultant fault current follows the path of I1 once the limiter functions thereby limiting the overall fault current. Rate of current rise

The IS-limiter is only intended to interrupt high fault currents, leaving the lower fault currents to be interrupted by the circuit breakers in the system. This is achieved by the measuring device detecting the instantaneous current level and the rate of current rise.

The rate of current rise (di/dt), is high with high fault currents, and lower with lower fault currents. The IS-limiter only trips when both set response values are reached.