VSD--Low-voltage networks: Intro and Molded-case circuit

Introduction

The low-voltage network is a very important component of a power system as it’s at this level that maximum power is distributed and utilized by the end-user. Essential loads such as lighting, heating, ventilation, refrigeration, air-conditioning, etc. are generally fed at voltages such as 380V, 400V, 415 V, 500V, 525V, three-phase three-wire, and three phase four-wire. In the mining industry, heavy motor loads often require voltages as high as 1000V. Due to the diverse nature of the loads, coupled with the large number of items requiring power, it’s usual to find a bulk in-feed to an LV switchboard. This is followed by numerous outgoing circuits, of varying current ratings, in contrast to the limited number of circuits, at the medium-voltage level.

Large-frame (high current rated) air-circuit breakers (ACBs) are therefore referred to as 'incomers' from the supply transformer and MCCBs for all outgoing feeders. The down- stream network generally consists of MCCBs of varying current ratings, and as the current levels drop, MCBs are used for compactness and cost saving.

Air-circuit breakers

These breakers are available in frame sizes ranging from 630 to 5000A in 3- and 4-pole versions and are generally insulated for 1000V. Rated breaking capacities of up to 100 kA rms symmetric to IEC947-2 are claimed at a rated voltage of 660 V. Fixed and draw-out models are available and each unit invariably comes complete with a protection device, which in keeping with modern trends, is generally of the electronic type. This will be discussed in more detail later.

Typical total breaking times are of the order of 40-50 ms for short-circuit faults. Their operating speed is important, as ACBs are applied as the main incoming devices, to the low-voltage network, where they are subject to the highest fault levels determined by the supply transformer.

A typical construction of an ACB.

++++1 Typical internal construction of an-ACB.

Molded-case circuit breakers

MCCBs are power switches with built-in protective functions used on circuits that require low current ratings. They include the following features:

• • Normal load current - open and close switching functions
• • Protection functions to automatically disconnect excessive overloads and to interrupt short circuit currents as quickly as possible
• • Provide indication status of the MCCB either open, closed, or tripped.

Although many different types are manufactured, they all consist of five main parts:

1. Molded case (Frame)

2. Operating mechanism

3. Contacts and extinguishers

4. Tripping elements

5. Terminal connections.

Molded case

This is molded from resin/glass fiber materials, which combine ruggedness with high dielectric strength.

The enclosure provides a frame on which to mount the components, but more importantly, it provides insulation between the live components and the operator.

Different physical sizes of case are required by the maximum-rated voltage/current and interrupting capacity, and these are assigned a 'frame size'.

Operating mechanism

This provides a means to open and close the MCCB via the handle. In passing from ON to OFF (or vice versa) the handle tension spring passes through alignment with the toggle link, and in doing so, a positive rapid contact-operating action is produced to give a 'quick break' or a 'quick make' action. This makes it independent of the human element, i.e., the force and speed of operating the handle.

The mechanism also has a 'trip free' feature, which means it cannot be prevented from tripping by holding the operating handle in the ON position. In other words, the protective contact-opening function cannot be defeated. Another important feature in addition to indicating when the breaker is ON (in the up position) or OFF (in the down position), the handle indicates when the breaker has tripped by moving midway between the extremes.

To restore the service after the breaker has tripped, the handle must first be moved to the OFF position to reset the mechanism before being moved to the ON position.

Handle White line indicates ON Handle centered: ON indication line not visible ON indication line ON OFF (a) On (b) Tripped (c) Off

++++2 Handle positions.

Contacts and extinguishers

A pair of contacts comprises of a moving contact and a fixed contact. The instance of opening and closing impose the most severe duty. The contact materials must therefore be selected with consideration to the following three criteria:

1. Minimum contact resistance

2. Maximum resistance to wear

3. Maximum resistance to welding.

Silver or silver alloy contacts are low in resistance but wear rather easily. Tungsten or tungsten alloys are strong against wear due to arcing but rather high in contact resistance.

Thus, the contacts are designed to have a rolling action, containing mostly silver at the closing current-carrying points, and mostly tungsten at the opening (arcing) point.

Majority of tungsten, Majority of silver

++++ Dual function contacts: (a) Closing; (b) Opening.

In order to interrupt high short-circuit currents, large amounts of energy must be dissipated. This is achieved by using an arc shute that comprises of a set of specially shaped steel grids, isolated from each other, and supported by an insulated housing. When the contacts are opened and an arc is drawn, a magnetic field is induced in the grids, which draws the arc into the grids.

The arc is thus lengthened and chopped up into a series of smaller arcs which are cooled by the grids' heat conduction. Being longer, it requires far more voltage to sustain it and being cooler tends to lose ionization and extinguishes at the first current zero.

Grid; Induced flux; Arc; Supporting frame; Grids; Attraction force

++++ The Arc shute.

Tripping elements

The function of the trip elements is to detect the overload or the short-circuit condition and trip the operating mechanism.

Thermal overload The thermal trip characteristic must be in close proximity to the thermal characteristics of cables, transformers, etc. To cover this overload condition, two types of tripping methods are available, namely Bimetallic and Hydraulic.

Bimetallic method:

The thermal trip action is achieved by using a bimetallic element heated by the load current.

The bimetallic element consists of two strips of dissimilar metals bonded together. Heat due to an excessive current will cause the bimetallic element to bend because of the difference in the rate of expansion of the two metals. The bimetallic element must deflect far enough to physically operate the trip bar. These thermal elements are factory-adjusted and are not adjustable in the field. A specific thermal element must be provided for each current rating.

A number of different variations on this theme are available.

The bimetal is temperature-sensitive and automatically re-rates itself with variations in the ambient temperature.

++++ Thermal tripping methods: (a) Direct heating; (b) Indirect heating; (c) Direct-indirect heating; (d) CT heating -- Deflection Moving core Secondary CT core Fixed core Fixed core Fixed core Fixed core Heating resistor Heating resistor Bimetal; Moving core.

Hydraulic method:

For its operation, this device depends on the electromagnetic force, produced by the current flowing in a solenoid, wound around a sealed non-magnetic tube. The tube, filled with a retarding fluid, contains an iron core, which is free to move against a carefully tensioned spring.

For a normal load current, the magnetic force is in equilibrium with the pressure of the spring. When an overload occurs, the magnetic force exceeds that of the spring and the iron core begins to move, reducing the air gap in the tripping armature. Once the magnetic field is large enough, the armature closes to trip the mechanism. The time delay characteristic is controlled by the retarding action of the fluid.

++++ Hydraulic tripping method: (a) Normal operation; (b) Long-delay tripping; (c) Instantaneous tripping.

Short circuits:

During short circuit conditions, response time of the thermal element is slow, therefore, a faster type of protection is required to reduce any damage. For this reason, a magnetic trip action is used in addition to the thermal element.

When a fault occurs, the short circuit current causes the electromagnet to attract an armature that unlatches the trip mechanism. This is a fast action and the only delay is the time it takes for the contacts to physically open and extinguish the arc. This is normally of the order of 20 ms - typically 1 cycle. In the hydraulic method, the current through the solenoid will be large enough to attract the armature instantaneously, irrespective of the position of the iron core. The interruption speeds for this type of breaker for short circuit currents are also less than 1 cycle (20 ms) and similar to the bimetallic type.

In both of the above methods, the thermal-magnetic and the hydraulic-magnetic, the tripping characteristics generated follow the same format.

Electronic protection MCCBs:

MCCBs of the conventional types mentioned above are increasingly being replaced by electronic trip units and CTs which are an integral part of the breaker frame.

This modern trend in technology results in an increased accuracy, reliability, and repeatability. The main advantage is the adjustability of the tripping characteristics, compared to the above-mentioned electromechanical devices, which are generally factory preset and fixed for each current rating. Discrimination can then be improved. Further- more, semiconductor controlled power equipment can be a source of harmonics which may cause mal-operations.

Electronic protective devices detect the true RMS value of the current, thereby remaining unaffected by the harmonics.

Long delay (bimetal) operation area; Instantaneous (electromagnet) operation area; Operating time Operating time; Overcurrent; Thermal/hydraulic-magnetic; Electronic 100%; 100% Ramp

++++ Typical tripping characteristics.

A comparison of the thermal-magnetic and hydraulic-magnetic types.

Terminal connections

These connect the MCCB to a power source and a load. There are several methods of connection such as bus bars, straps, studs, plug-in adaptors, etc.

Up to 250-300A whenever cables are used, compression type terminals are used to connect the conductor to the breaker. Above 300 A, stubs, bus bars, or straps are recommended to ensure reliable connections, particularly when using aluminum cables.

• Operating time
• Current
• Low temperature
• Low frequency
• Low frequency
• Horizontal
• High frequency
• High temperature
• Ceiling

Operating current is affected by ambient temperature (bimetal responds to absolute temperature not temperature rise) Negligible effect up to several hundred Hz; above that the instantaneous trip is affected due to increased iron losses Bimetal must provide adequate deflection force and desired temperature characteristic. Operating time range is Negligible effect Affected only to the extent that the damping-oil viscosity is affected Trip current increases with frequency, due to increased iron losses Oil viscosity, cylinder, core and spring design, etc., allow a wide choice of operating times Mounting attitude changes the effective weight of the magnetic core Ambient temperature Frequency Flexibility of operating characteristics Mounting attitude Item Thermal-Magnetic Type Hydraulic-Magnetic Type On-Off-Off

++++ Comparison of thermal-magnetic and hydraulic-magnetic types.

Current-limiting MCCBs:

Current-limiting MCCBs are essentially extremely fast-acting breakers, that interrupt the short circuit fault current, before it reaches the first peak, thereby reducing the current or energy let-through, in the same manner as a fuse. They are therefore required to operate in the first quarter of a cycle, i.e., 5 ms or less and limit the peak short circuit current to a much lower value, after which they can be switched on again, if necessary, without the replacement of any parts or elements. Prospective short-circuit current Let-through current: Current t i

0 Inductive (magnetic stress) energy=

1/ 2 Li 0 2 Thermal (resistive heating) energy=

R i 2 dt =Ri 2t

0 t Time

++++9 Limited short circuit let-through current

This high contact speed of separation is achieved by using a reverse loop stationary contact. When a fault develops, the current flowing in the specially designed contacts causes an electrodynamic repulsion between them. The forces between the contact arms increases exponentially rather than linearly. As the contact gap widens, the arc is quickly extinguished by a high-performance arc shute.

By limiting the let-through current, the thermal, and the magnetic stresses on protected equipment such as cables and bus bars is reduced in the event of a short circuit. Provided combination series tests have been done and certified, this also permits the use of MCCBs with a lower short circuit capacity to be used at downstream locations from the current-limiting MCCB. This is known as 'cascading' and results in a more economical system. Additional care must be taken, to preserve the discrimination between the breakers.

Accessories

The following accessories are available with all different makes of MCCB:

• • Shunt trip coils
• • Under-voltage release coils
• • Auxiliary switches
• • Mechanical interlocks
• • Electrical closing mechanism (in higher ratings). A typical MCCB.

When selecting an MCCB for an application it’s important to ensure that the following ratings are correct:

• 1. Voltage rating.
• 2. Current rating.
• 3. Breaking capacity rating.

++++ Typical MCCB.