Dry Vs Oil Transformers

dry type transformers do not have oil in them for insulation. oil type transformers have oil in them to increase the insulation as well as to help with keeping the transformer cool. dry types air is used to cool the transformers. is that what you are asking?

THANK U THIS WAS ONE PART OF MY QUESTION, BUT THE SECOND PART IS HOW DOES DG START AS SOON AS THE SUPPLY CURRENT GETS SHUT OFF, WHAT IS THE SYSTEM WHICH MAKES THIS

What is "DG" ?

DG MEANS DIESEL GENERATORS WHICH WE USE WHEN THE SUPPLY CURRENT GETS SHUT OFF

"What is "DG" ?"

WAG:

Misspelling of DOG?

Distributed generation is a new trend in the generation of heat and electrical power. The Distributed Energy Resources (DER) concept permits "consumers" who are generating heat or electricity for their own needs (like in hydrogen stations and microgeneration) to send surplus electrical power back into the power grid - a process also known as net metering - or share excess heat via a distributed heating grid. Distributed generation systems with Combined Heat and Power (CHP) systems can be very efficient, using up to 90% of the potential energy in the fuel they consume. CHP can also save a lot of money and fuel. Estimates are that CHP has the potential to reduce the energy usage of the USA by up to 40%. A cluster of distributed generation installations is viewed as a Virtual power plant.

Even if the term "distributed generation" is quite well established, terms like distributed power, distributed energy, distributed energy resources, embedded generation, decentralized power, decentralized energy, dispersed generation, and onsite generation can also be found in the literature. Although some of those terms may be used with a different meaning, typically they de facto refer to distributed generation.

In many parts of the United States, a significant amount of new generation capacity is being installed through the
installation of Distributed Generation (DG) facilities. The 2003 Northeast Blackout will undoubtedly accelerate DG
installation in the US as more businesses awaken to the need for reliable on-site generation. Interconnect protection
allows DG to operate in parallel with the utility power system and is the single most important technical issue in
most DG projects. Typically, protection requirements to connect distributed generators to the utility grid had been
established by each utility. These guidelines generally cover smaller distributed generators (10 MW or less), which
are usually connected to the utility system at the subtransmission and distribution level. These utility circuits are
designed to supply radial loads. Thus, the introduction of generation provides a source for redistribution of the fault
current on the feeder circuit, which can cause the loss of relay coordination and potential overvoltages. Within the
past few years, there have been efforts by the IEEE, as well as individual states, to develop standards and guidelines
for the interconnection of DG. The stated goal of these standards/guidelines is to have a single document of standard
technical requirements for DG interconnection rather than having to conform to local utility practices and
guidelines. This paper examines how well this objective has been met.
The paper also updates the author's previous papers (Ref. 3, 9, 10) on DG. It outlines the specific protection
challenges to interconnect distributed generators into utility systems, as well as methods to reconnect these
generators after interconnect protection tripping. It also highlights the importance of choosing the proper
interconnection transformer-winding configuration and discusses potential overvoltage problems that can be caused
by DGs. It is the author's opinion that IEEE 1547, the recently completed IEEE standard for the interconnection of
DG with the utility system, does not adequately address these topics. Also, P1547 does not address protection
methods other than over/under voltage and frequency.
DGs need to be protected not only from short circuits, but also from abnormal operating conditions. Many of these
abnormal conditions can be imposed on the dispersed generator by the utility system. Examples of such abnormal
conditions are: overexcitation, overvoltage, unbalanced currents, abnormal frequency and shaft torque stress due to
utility automatic reclosing. When subjected to these conditions, damage or complete generator failure can occur
within seconds. Machine damage due to these causes is a major concern of DG owners. Utilities, on the other hand,
are generally concerned that the installation of a dispersed generator will result in damage to their equipment or to
the equipment of their customers.
Most DGs are typically connected to the utility system at the distribution and subtransmission level. These utility
circuits are designed to supply radial loads. Islanded operation of dispersed generation with utility loads external to
the DG site on these circuits is not allowed for two major reasons:
1. The utility needs to restore the outaged circuits and this effort is greatly complicated by having
islanded generators with utility loads. Automatic reclosing is universally the first method attempted to
restore power to customers. Having islanded generators complicates both automatic reclosing, as well as
manual switching which requires synchronizing the generator/load islanded to the utility system.
2. Power quality (voltage and frequency levels, as well as harmonics) generally cannot be maintained
by the islanded dispersed generators within acceptable level provided by the utility and could result in
damage to the customer equipment.
Properly-designed interconnection protection should address the concerns of both the dispersed generator owner, as
well as the utility, at the lowest possible cost. The major functions of interconnect protection is to prevent system
islanding by detecting asynchronous dispersed generator operation—in other words, determining when the generator is no
longer operating in parallel with the utility system. This detection and tripping must be rapid enough to allow automatic
reclosing by the utility.

It is little confusing as to what is desired to be discussed: Whether DG "Diesel Generator" or "Distributed Generation"? Originator of the question may like to clarify.

SIR,

IT IS DIESEL GENERATORS

THANKS

THE RELAY YOU ARE LOOKING FOR IS "UNDERVOLTAGE RELAY"IT SENSES LOSS OF VOLTAGE ON UTILITY.IT HAS DRY CONTACTS TO USE WITH INTERLOCKING.USUALLY ASSOCIATED WITH TIMER RELAYS,LOCKOUT RELAYS ETC.IT IS THIS UNDERVOLTAGE RELAY,WHICH INITIATES START SIGNAL TO GENERATOR AND PROTECTIVE DEVICES ON YOUR SWITCHGEAR.LOOK INTO YOUR GEN.AND PLANT SYSTEM DIAGRAMS/SINGLE LINES ETC. YOU WILL SEE CONTACTS REFERING TO #(27) DEVICE=UNDERVOLTAGE RELAY.HOPE THIS HELPS.....

Dear Nagesh

Dry type transformers are basically without oil.The insulation medium is the resin.The windings are resin cast or impregnated.In oil filled transformers the insulating medium is oil.

Dry type trafos are almost double the cost of oil filled trafos.Dry type trafos can be installed inside the building while the oil filled trafos need to be located outside and are required to follow certain safety stipulations with respect to fire hazard.

100% DG back up means :

Ths DG set is able to take the entire load of the apartment or theatre or the plant.Otherwise, normally, the DG sets (DG power being costlier) are designated as emergency DG sets where in certain emegency loads are fed from the DG set.Thereby the rating of the DG set is optimised.For example AC emergency lighting,source to UPS,certain Lubricating oil pumps etc are designated as emergency loads ,which need to be live even when there is normal AC blackout.

DG set starts automatically only if u opt for such a feature.This is called AMF feature ie Auto Mains Failure.

On failure of normal AC supply,the dead bus (consistent undervoltage) signal is conveyed by the undervoltage relay.This gives a starting pulse to the excitation system of the DG set,which kick starts the DG set.It takes about 30 seconds to come to FSNL (full speed NO load )and build up the required terminal voltage.After which the loads are able to get a healthy voltage and the same pick up.

The above is a broad caricature of the phenomenon involved.Further details of how ,when and why may be furnished on knowing the purpose for which this information is required.

regards

Ravipra

Mr Ravi answers clearly and completely,the questions raised by Mr Nagesh.

THIS WAS PERFECT ANSWER WHAT I GOT. AND ALSO I WANTED TO KNOW WHAT DO YOU MEAN BY DEDICATED EARTHING

THANKS

ANSWER OF YOUR FIRST PART

Oil filled transformers

In oil filled transformer oil is used as insulation for winding as well as for cooling.In general, oil filled transformers are hermetically sealed with integral filling.These transformers are particularly suited to:hermetically sealed with integral filling.These transformers are particularly suited to:unsupervised substations (zero maintenance),severe environments if the tank is suitably protected (active parts protected),cyclic consumption applications (with goodthermal inertia).On the other hand, the liquid dielectric has some inherent risks:ground water pollution (in case of leaks of thedielectric), from which results the obligation, incertain cases, to provide for a back-up retentiontank,fire which is why they are prohibited in certain buildings.These risks are taken into account in the variousregulatory texts and standards concerning theconditions of installation and limits of use.

Dry Type Transformer

In Dry type Transformer,the isnulation of winding is provided by either Epoxy cast resin or by method of Vaccumm Impregnation system.It is completely dry.

Dry"-type transformers are more appropriatefor:v locations with controlled environments: dust- humidity - temperature, etc. and must beperiodically cleaned and dusted,v buildings, in particular high-rise buildings; sincethey can have good fire behaviour (e.g. class F1according to NF C 52-726) and meet non-toxicityof fumes criteria.CharacteristicsThe various rated values are defined by IEC 76

ANSWER OF YOUR SECOND PART

DG means Diesel Generator . when the supply goes off the diesel generator starts automatically. For this UVR is used on the line and bus side of the system. For more clarification let us assume that a system is supplied by two source , one Main incomer(supply is coming from Utility ) and the other DG as back up. UVR is connected on the line side of Main Supply and on DG as well as on the bus supply .once the main supply goes off UVR will send close command to DG in order to start automatically ,provided that there is no volt condition on Bus side. This is more explained below:

When the normal supply fails, induction motors that remain connected to the busbar slow down and the trapped rotor flux generates a residual voltage that decays exponentially. All motors connected to a busbarwill tend to decelerate at the same rate when the supply is lost if they remain connected to the busbar. This is
because the motors will exchange energy between themselves, so that they tend to stay 'synchronised' to each other. As a result, the residual voltages of all the motors decay at nearly the same rate. The magnitude of this voltage and its phase displacement with respect to the healthy alternative supply voltage is a function of
time and the speed of the motors. The angular displacement between the residual motor voltage and the incoming voltage will be 180° at some instant. If the
healthy alternative supply is switched on to motors which are running down under these conditions, very high inrush currents may result, producing stresses
which could be of sufficient magnitude to cause mechanical damage, as well as a severe dip in the alternative supply voltage.
Two methods of automatic transfer are used:
a. in-phase transfer system
b. residual voltage system
The in-phase transfer method
Normal and standby feeders from the same power source are used.Phase angle measurement is used to sense the relative phase angle between the standby feeder voltage and the motor busbar voltage. When the voltages are approximately in phase, or just prior to this condition through prediction, a high-speed circuit breaker is used
to complete the transfer. This method is restricted to large high inertia drives where the gradual run down characteristic upon loss of normal feeder supply can be
predicted accurately.

The residual voltage method,

which is more common, especially in the petrochemical
industry.Two feeders are used, supplying two busbar sections connected by a normally open bus section breaker. Each feeder is capable of carrying the total busbar load. Each bus section voltage is monitored and loss of supply oneither section causes the relevant incomer CB to open.Provided there are no protection operations to indicate
the presence of a busbar fault, the bus section breaker is closed automatically to restore the supply to the unpowered section of busbar after the residual voltage
generated by the motors running down on that section has fallen to a an acceptable level. This is between 25% and 40%, of nominal voltage, dependent on the
characteristics of the power system. The choice of residual voltage setting will influence the reacceleration current after the bus section breaker closes.
For example, a setting of 25% may be expected to result in an inrush current of around 125% of the starting current at full voltage. Alternatively, a time delay could
be used as a substitute for residual voltage measurement, which would be set with knowledge of the plant to ensure that the residual voltage would have
decayed sufficiently before transfer is initiated.The protection relay settings for the switchboard must take account of the total load current and the voltage
dip during the re-acceleration period in order to avoid spurious tripping during this time. This time can beseveral seconds where large inertia HV drives are EVOLVED.

Thanks