Scott-T Connection of Transformer:

       Transforming 3 Phase to 2 Phase:

§  There are two main reasons for the need to transform from three phases to two phases,

1.      To give a supply to an existing two phase system from a three phase supply.

2.      To supply two phase furnace transformers from a three phase source.

§  Two-phase systems can have 3-wire, 4-wire, or 5-wire circuits. It is needed to be considering that a two-phase system is not 2/3 of a three-phase system. Balanced three-wire, two-phase circuits have two phase wires, both carrying approximately the same amount of current, with a neutral wire carrying 1.414 times the currents in the phase wires. The phase-to-neutral voltages are 90° out of phase with each other.

§  Two phase 4-wire circuits are essentially just two ungrounded single-phase circuits that are electrically 90° out of phase with each other. Two phase 5-wire circuits have four phase wires plus a neutral; the four phase wires are 90° out of phase with each other.

§  The easiest way to transform three-phase voltages into two-phase voltages is with two conventional single-phase transformers. The first transformer is connected phase-to-neutral on the primary (three-phase) side and the second transformer is connected between the other two phases on the primary side.

§  The secondary windings of the two transformers are then connected to the two-phase circuit. The phase-to-neutral primary voltage is 90° out of phase with the phase-to-phase primary voltage, producing a two-phase voltage across the secondary windings. This simple connection, called the T connection, is shown in Figure

§  The main advantage of the T connection is that it uses transformers with standard primary and secondary voltages. The disadvantage of the T connection is that a balanced two-phase load still produces unbalanced three-phase currents; i.e., the phase currents in the three-phase system do not have equal magnitudes, their phase angles are not 120° apart, and there is a considerable amount of neutral current that must be returned to the source.

 The Scott Connection of Transformer:

§  A Scott-T transformer (also called a Scott connection) is a type of circuit used to derive two-phase power from a three-phase source or vice-versa. The Scott connection evenly distributes a balanced load between the phases of the source.

§  Scott T Transformers require a three phase power input and provide two equal single phase outputs called Main and Teaser. The MAIN and Teaser outputs are 90 degrees out of phase. The MAIN and the Teaser outputs must not be connected in parallel or in series as it creates a vector current imbalance on the primary side.

§  MAIN and Teaser outputs are on separate cores. An external jumper is also required to connect the primary side of the MAIN and Teaser sections.

§  Scott T Transformer is built with two single phase transformers of equal power rating. The MAIN and Teaser sections can be enclosed in a floor mount enclosure with MAIN on the bottom and Teaser on top with a connecting jumper cable. They can also be placed side by side in separate enclosures.

§  Assuming the desired voltage is the same on the two and three phase sides, the Scott-T transformer connection consists of a center-tapped 1:1 ratio main transformer, T1, and an 86.6% (0.5√3) ratio teaser transformer, T2. The center-tapped side of T1 is connected between two of the phases on the three-phase side. Its center tap then connects to one end of the lower turn count side of T2, the other end connects to the remaining phase. The other side of the transformers then connects directly to the two pairs of a two-phase four-wire system.

§  The Scott-T transformer connection may be also used in a back to back T to T arrangement for a three-phase to 3 phase connection. This is a cost saving in the smaller kVA transformers due to the 2 coil T connected to a secondary 2 coil T in-lieu of the traditional three-coil primary to three-coil secondary transformer. In this arrangement the Neutral tap is part way up on the secondary teaser transformer . The voltage stability of this T to T arrangement as compared to the traditional 3 coil primary to three-coil secondary transformer is questioned

Palm oil insulation of transformers

University of Leicester. "Palm oil insulation could transform transformers." 

This research by a University of Leicester student has identified an environmentally alternative to a major industrial use of oil. Abdelghaffar Abdelmalik has

discovered a way to treat palm kernel oil so that it can be used to insulate electrical transformers.

Transformers use petroleum derived oil as insulation between electrical components but this

makes them reliant on fossil fuels and also causes environmental problems if there is a leak.

Abdelghaffar, who is studying for a PhD in the University's Department of Engineering, is

exploring the possibility of using a derivative of palm kernel oil which is environmentally friendly,

non-toxic and has suitable properties such as low viscosity and low conductivity. This would

extend the life of electrical transformers and greatly reduce the effects of leakage.

Abdelghaffar's research has already been acknowledged as significant by the Dielectrics and

Electrical Insulation Society, part of the Institute of Electrical and Electronics Engineers, which last year awarded him a $5,000 research grant to support his

innovative work.

"The results of the work done so far are encouraging," said Abdelghaffar. "There are indications that this research may produce a sustainable and all-purpose electrical insulating fluid that would serve as an effective alternative to mineral-based insulating oil."

Professor John Fothergill, Head of the Department of Engineering, added:

"The currently used silicone oils are recognised as having excellent characteristics but they are environmentally unfriendly. The new oil that has been synthesised from Palm Kernel Oil is surprisingly good and in many respects appears to be better that the silicone oil. It is also environmentally friendly."

This research is being presented at the Festival of Postgraduate Research on 16 June.

Power system stability

POWER SYSTEM STABILITY

 Introduction

The stability of an interconnected power system is its ability to return to normal or stable
operation after having been subjected to some form of disturbance.
The study of stability is one of the main concern of the control engineer whose methods
may be applied to electrical power system.

Power system stability is classified into three, namely
(a) Steady state stability
This is primarily concerned with the ability of the system generators to remain in
synchronism after minor disturbances such as gradual load changes, changes in
excitation, line switching and so on.
(b)Transient stability
This is concerned with generators synchronism after a large or sudden disturbance such
as large load drop or addition, line switching, short circuit, sudden loss of big generating
(c)Long term stability
This forms the transition between transient and steady state stability.
(d)Swing curves
The swing curve is a plot of the rotor angle against time, and is used to determine
transient stability of a power system. A figure is used to illustrate the characteristics of a
system subject to disturbance.
(e)Equal area criterion
The equal area criterion is used to predict transient stability without solving the swing
equation for simple system such as one machine supplying an infinite bus-bar or two
machine systems.

Electrical Protection

Electrical Energy is

Generated at few kV and stepped up.

Transmitted through AC and HVDC lines.

Stepped down and distributed at load centers.

Its natural mode of synchronous operation knits the system together.

Why do we need protection?

Electrical apparatus operates at various voltage levels and

may be enclosed or placed in open. Under abnormal

operating conditions protection is necessary for

Safety of electrical equipments.

Safety of human personnel.

Types of Protection

Apparatus Protection:-

Transmission Line Protection
Transformer Protection
Generator Protection
Motor Protection
Busbar Protection

System Protection:-

Out-of-Step Protection

Under-frequency Relays

Islanding Systems

Rate of Change of Frequency Relays

A transformer works on the principle of electromagnetic induction between two coils or coupled circuits. According to this principle, an emf is induced in a coil if it links a changing flux. In core type transformers,half of the LV(and HV) winding is on one limb and the other half is on the second limb. In shell type transformer, the LV and HV windings are sandwiched.however for simplifying the drawing and analysis of both these types of transformers, the primary winding is connected to an alternating voltage source,therefore an alternatting current starts flowing through turns. the alternating mmf(NI) sets up alternating flux which is confined to the high permeability iron path. the alternating flux induces voltage in the primary and the secondary. If the load is connected across the secondary,a load current starts flowing.

In addition of the secondary winding,there may be a third (or tertiary) winding on the same iron core. The emf induced in the secondary or tertiary winding is usually referred to as the emf due to transformer action. Thus the transformer action requires the existence of alternating flux linking the various winding on a common magnetic core.

A transformer having primary and secondary winding is called a two winding transformer whereas a transformer having primary, secondary and tertiary winding is known as a three winding transformer. As stated before, primary is connected to source whereas the secondary and tertiary feed the load.