Ungrounded systems that need protection against earth faults usually need a grounding transformer. This is usually a delta-star transformer whose neutral can be earthed. An alternative way of grounding the system is by using a zig-zag transformer.

The Zig-zag Transformer generally has a ratio of 1:1. It consists of six windings, two for each phase – . The two windings for each phase(outer and inner windings) are wound on the same core but in opposite directions.

The windings are connected in a zig-zag fashion. For instance, the primary winding of R phase may be connected to the secondary of Y, the primary of Y phase would be connected to the secondary of B. In a perfectly balanced condition, the magnetic fluxes in the primary and the secondary are able to cancel each other; therefore, their magnetic fluxes cancel each other out. However, in a fault condition, the magnetic fluxes may not be equal and hence a fault current may flow through the neutral of the secondary winding.

Zig-zag Transformers are used to provide earthing for ungrounded systems. They are advantageous over delta-wye systems due to their low internal impedance and lower cost.

Apart from providing earthing to ungrounded systems, zig-zag transformers can be used to filter harmonic currents. Zig-zag transformers are connected close the loads that cause heavy harmonics. Since the transformer has opposing windings, the harmonic currents get cancelled.


You can tell some people about being considerate to the environment until you’re blue in the face, but they’ll still flagrantly waste power. At that point, maybe it’s time for the light switch to fight back: Peter Russo and Brendan Wypich of Stanford University have developed the SmartSwitch, a light control that gives tactile feedback as to how much energy is already being used, whenever you try to flick it.

“Equipped with a network connection and a brake pad, the switch provides its user with tactile feedback about the amount of energy being used either within their household or by the electrical grid as a whole.

SmartSwitch doesn’t restrict the user from turning on a light, but rather it passively encourages behavior change. SmartSwitches can be programmed to respond to either personal or communal electrical usage. In a home wired with SmartSwitches, lights can become harder to turn on during hours of peak demand. The switches can also be customized to reflect household-specific energy conservation goals.”

If the total energy consumption in the house (or area) is low, then the SmartSwitch is as easy to flick on as a normal light switch. However if the consumption is greater, than the SmartSwitch is physically harder to use, thanks to a brake pad inside the mechanism. The idea is that people use this tactile feedback to decide whether or not they really want to contribute to the total energy demand of their house or greater environment.

The SmartSwitch is an entry in the Greener Gadgets Design Competition, and as well as being a decent concept it’s also practical. The mechanics of the tactile switch will fit into a standard electrical box, and there’s no special wiring necessary as it uses the electricity lines themselves to communicate data.

Images and text courtesy: http://www.slashgear.com


Series reactors are used in a power system to limit the current in case of a fault. During fault conditions, extremely high currents flow through the system. If the busbars and circuit breakers are not able to withstand the fault currents, they can get damaged and the protection of the system can be seriously compromised. This is known as the fault level of a system.

While the fault level of a power system is taken into account during the design process. Over the years, the fault level can change when new sources of power such as generators and transformers are added into the system. The increased fault level cannot be handled by the existing busbars and circuit breakers. Replacing these system components will be expensive and time-consuming and almost impossible.

In this scenario, reactors provide a simpler solution. During a fault condition, the maximum current which may flow is determined by the impedance in the path of the fault circuit. Reactors work by increasing the reactance, and consequently the impedance of the system they are fitted in. This leads to reduced fault currents.

Reactors are generally fitted between adjacent busbar sections or in series with feeders. While reactors are useful in containing the fault currents, the downside is that tend to introduce a mild drop in the voltage. Hence, regulation in the form of boosters or transformer tappings may be required.

There are different kinds of reactors such as air core reactors, oil immersed reactors, etc. Series reactors are used widely in transmission and distribution systems.


Electrical enclosures such as panels,switch boxes and appliances need to be protected against the entry of foreign objects and water for healthy operation and safety of the operating personnel. The European Commitee for Electro-technical Standardization has developed IP(Ingress Protection) ratings which specify the degree of protection the panels provide.

The IP codes are defined in the IEC standard 60529. The ingress protection of each panel is in the form of a code such as IP65. The first number indicates the protection against dust and other particles while the second numbers denotes the protection against water.

The first number indicates the following.

Level Object size protected against Effective against
0 No protection against contact and ingress of objects
1 >50 mm Any large surface of the body, such as the back of a hand, but no protection against deliberate contact with a body part
2 >12.5 mm Fingers or similar objects
3 >2.5 mm Tools, thick wires, etc.
4 >1 mm Most wires, screws, etc.
5 dust protected Ingress of dust is not entirely prevented, but it must not enter in sufficient quantity to interfere with the satisfactory operation of the equipment; complete protection against contact
6 dust tight No ingress of dust; complete protection against contact


The Second letter indicates the degree of protection of the panel against the entry of water.

Level Protected against Details
0 not protected
1 dripping water Dripping water (vertically falling drops) shall have no harmful effect.
2 dripping water when tilted up to 15° Vertically dripping water shall have no harmful effect when the enclosure is tilted at an angle up to 15° from its normal position.
3 spraying water Water falling as a spray at any angle up to 60° from the vertical shall have no harmful effect.
4 splashing water Water splashing against the enclosure from any direction shall have no harmful effect.
5 water jets Water projected by a nozzle against enclosure from any direction shall have no harmful effects.
6 powerful water jets Water projected in powerful jets against the enclosure from any direction shall have no harmful effects.
7 immersion up to 1 m Ingress of water in harmful quantity shall not be possible when the enclosure is immersed in water under defined conditions of pressure and time (up to 1 m of submersion).
8 immersion beyond 1 m The equipment is suitable for continuous immersion in water under conditions which shall be specified by the manufacturer.
NOTE: Normally, this will mean that the equipment is hermetically sealed. However, with certain types of equipment, it can mean that water can enter but only in such a manner that produces no harmful effects.



NGRs or Neutral Grounding Resistors are used to limit the fault current in a generator or a transformer during earth faults.

In star connected 3 phase equipment such as a generator or a transformer, the star point is grounded. In systems where the star point is directly grounded, known as solid earthing, there is a chance of heavy currents in the windings during an earth fault as the net resistance is only the soil resistance.

This heavy current, in the order of hundreds of amperes, can damage the windings. Hence, a series resistance is introduced in the star point. This increases the net resistance in the event of an earth fault and limits the current. This resistor is known as the Neutral Grounding Resistor(NGR).

The current flowing in the Neutral Grounding resistor can be monitored. This can be used to activate the Earth Fault Relay. It is generally mounted with a Current Transformer.

Types:
NGRs can be broadly classified into two types.
Low Resistance Grounding, which limits current to more than 100A and
High Resistance Grounding, which limits current to generally 5 to 10 amperes.

The Choice of the rating of the NGR is made taking into account different factors such as the rating of the machine, overall fault level of the system and the system capacitance.

The rating of the machine should be kept in mind while deciding on the NGR value. Fault current should be limited to a value that can be safely handled by the machine. It is also essential that the fault feeds enough current which can be sensed by the earth fault protection. Too high a value of the NGR would the limit the current to a very low value which will not be able to activate the earth fault protection.

The system capacitance is another factor which should be kept in mind. A 3 phase star connected system forms capacitances with the ground. In the event of an earth fault, this capacitance can get charged with the line voltage and can cause transient overvoltages. The value NGR should be chosen in a manner which will permit a let-through current and enable the capacitances to discharge.


Related Posts:


Disadvantages of Solidly Grounded Systems

Grounding Resistances - An introduction