05/13/14 ~ ALL TIME ELECTRICAL

Tuesday 13 May 2014

SF6 or Vacuum MV Circuit Breaker?

14:08 1 comment

SF6 or vacuum?

Approximately 35 years ago, in the mid 1960s, two new breaker technologies, one using SF6 gas and the other vacuum as its arc quenching medium, were introduced to the market.

Research and development work on both technologies has continued unabated since then, and today it can be said that, together, they have all but replaced the older types of switchgear.

Instead of an objective selection based on real-world characteristics, the choice is very much driven by the circuit-breaker manufacturer.

There is, however, not always agreement on which criteria should be used when choosing one of these two dominant technologies.

SF6 and vacuum switchgear enjoy varying market success in the different parts of the world (Figure 1) whereas Europe and most of the Middle East countries tend to favor SF6, China, Japan and the USA definitely prefer vacuum. In other regions, the two technologies are equally popular.

Bulk-oil and minimum-oil technologies are still used in China, Eastern Europe, India and Latin America, but trends clearly indicate that these technologies will disappear very soon, to be replaced by SF6 and vacuum.

ABB concentrates today almost entirely on the two dominant technologies, and is equally present in the market with both SF6 and vacuum.

Experience with more than 300,000 MV circuit-breakers of both designs installed worldwide, backed up by over 30 years of intensive involvement in research [1], has convinced ABB that the two technologies are entirely complementary, though in some cases their different designs can be seen as alternatives.

Based on this conviction that SF6 and vacuum have equally important roles to play, the company has continued to force the development of both, and hence, as the world’s largest manufacturer of MV circuit-breakers, occupies the unique position of being able to provide unprejudiced advice and assistance in the selection of switchgear for any special application.

Capacitor Application Issues

14:07 1 comment

Capacitor Ratings

Capacitors must be built to tolerate voltages and currents in excess of their ratings according to standards. The applicable standard for power capacitors is IEEE Std 18-2002, IEEE Standard for Shunt Power Capacitors.

Additional information is given in IEEE Std 1036-1992, IEEE Guide for Application of Shunt Power Capacitors. IEEE Std 18-2002 gives the following continuous overload limits.

These are “intended for contingencies and not intended to be used for a nominal design basis.”

110% of rated rms voltage
120% of rated peak voltage
135% of rated rms current (nominal current based on rated kvar and voltage)
135% of rated reactive power

Short time overload voltages were specified in IEEE Std 18-1992 (on older version of the standard) and IEEE Std 1036-1992 and are listed below. These standards state that a capacitor may be expected to see a combination of 300 such overvoltages in its service life.

Note that these overvoltages are “…without superimposed transients or harmonic content”.

2.20 per unit rms voltage for 0.1 seconds (6 cycles of rms fundamental frequency)
2.00 per unit rms voltage for 0.25 seconds (15 cycles of rms fundamental frequency)
1.70 per unit rms voltage for 1 second
1.40 per unit rms voltage for 15 seconds
1.30 per unit rms voltage for 1 minute
1.25 per unit rms voltage for 30 minutes

An even older version of the standard, IEEE Std 18-1980, also included the following permissible overvoltages.

3.00 per unit rms voltage for 0.0083 seconds (½ cycle of rms fundamental frequency)
2.70 per unit rms voltage for 0.0167 seconds (1 cycle of rms fundamental frequency)

It should be noted that some capacitor manufacturers make heavy duty capacitors particularly for industrial environments.

One manufacturer makes the following claims about its heavy duty capacitors in its literature. “… they are designed to exceed the requirements of these [ANSI/IEEE, NEMA, and IEC] standards in terms of continuous rms and peak overvoltage withstand capabilities, and in tank rupture characteristics.”

This manufacturer rates the continuous overvoltage capability at 125% (as opposed to 110%) and its continuous peak overvoltage capability at 135% (as opposed to 120%). When doing power system studies it is important to compare the measured or calculated voltages or currents against these ratings. In different study cases, different ratings will apply.

For example, harmonics are a steady-state phenomenon so the continuous limits would need to be considered. However, voltage harmonics resulting from a relatively short term event, such as transformer energization inrush, might be compared against the short time overload ratings.

One of the interesting implications of these overvoltage allowances is that capacitors can be applied at voltages in excess of their ratings for very short periods of time.

Why would one do this? The main reason is because the kvar produced by a capacitor is related to the square of the voltage ratio. For example, a capacitor applied at a voltage 40% higher than its nameplate will produce double its nameplate kvar.

High Voltage Techniques Course

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Introduction

The potential benefits of electrical energy supplied to a number of consumers from a common generating system were recognized shortly after the development of the “dynamo” commonly known as the generator.

Power transfer for large systems, whether in the context of interconnection of large systems or bulk transfers, led engineers invariably to think in terms of high voltages.

The rapidly increasing transmission voltage level in recent decades is a result of the growing demand for electrical energy, coupled with the development of large hydroelectric power stations at sites far remote from centres of industrial activity and the need to transmit the energy over long distances to the centres.

In order to meet the growing demand, more and more power stations, substations and transmission lines are being built and the transmission voltages are being raised for efficient transmission. Increase in transmission voltage by 20 times results in 400 times reduction in transmission losses. This illustrates the main reason for the need of “High Voltage”.

It is desirable to increase the transmission voltage to obtain higher efficiency, but “the insulation of high voltage system“ limits this desire. The insulation of all parts of high voltage power system (generators, transformers, cables, insulators, circuit breakers, etc.) should be preserved in order to provide an “uninterruptable energy supply”or continuous energy flow.

Gas, liquid and solid insulating materials are utilized for the insulation of high voltage systems. The loss of insulation is technically called “breakdown”. Mechanisms of electrical breakdown of insulation is one of the subjects of this course.

These voltage ranges are also valid for IEC (International Electrotechnical Commission)Definition of some important standardized rated insulation levels for high voltage equipment according to IEC 62271-1 is given above.

Definitions

Rated voltage – Upper limit of the highest voltage of the network for which a switching device is rated.

Rated short duration power frequency withstand voltage – RMS value of the sinusoidal a.c voltage at operating frequency that the insulation of a device must withstand under the specified test conditions for 1 minute.

Rated lightning impulse withstand voltage – Peak value of the standard voltage surge 1.2/50us that the insulation of a device must withstand

Rated switching impulse withstand voltage – Peak value of the unipolar standard voltage surge 250/2500us which the insulation of a device with a rated voltage of 300 kV and above must withstand.

Electric-Power-Application-Sizing

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Protection of Electrical Networks

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General structure of the private distribution network

Generally, with an HV power supply, a private distribution network comprises (see Figure 1-1):

Figure 1-1: Structure of a private distribution network

An HV consumer substation fed by one or more sources and made up of one or more busbars and circuit-breakers;
An internal generation source;
One or more HV/MV transformers;
A main MV switchboard made up of one or more busbars;
An internal MV network feeding secondary switchboards or MV/LV substations;
MV loads;
MV/LV transformers;
Low voltage switchboards and networks;

The Application of Switchgear and Switchboards

13:56 1 comment

The Application of Switchgear and Switchboards

One important element of good power system design is the proper selection of the distribution equipment. The choice of using either switchgear or switchboard must be based on many different criteria and the design of the power system requires thought be given to each one.

Following are insights into just a few of the differences to help in making those decisions.

Standards and Testing

Switchgear and switchboard structures are built and tested to different standards:

• Switchgear to ANSI standard C37.20.1, UL standard 1558, and NEMA standard SG-5,
• Switchboards to NEMA PB-2, and U L -891.

Switchgear incorporates only low-voltage power circuit breakers (LVPCB) which conform with ANSI C37.13 , NEMA SG-3 and are listed per UL-1066, whereas switchboards may include any combination of protective devices including insulated case (ICCB), molded-case circuit breakers (MCCB) listed per UL-489, fusible switches listed per U L-508 and 977 and power circuit breakers listed to UL-106 6.

Unfused switchgear is short circuit tested at 15% power factor for a full 30 cycles, while switchboards are tested at 20% power factor for only 3 cycles .

MV-LV transformer substations – ABB

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General information on MV/LV transformer substations

An electrical transformer substation consists of a whole set of devices (conductors, measuring and control apparatus and electric machines) dedicated to transforming the voltage supplied by the medium voltage distribution grid (e.g. 15kV or 20kV), into voltage values suitable for supplying low voltage lines with power (400V – 690V).

The electrical substations can be divided into public substations and private substations:

Public substations

These belong to the electricity utility and supply private users in alternating single-phase or three-phase current (typical values of the voltage for the two types of power supply can be 230V and 400V). In turn, these are divided into urban or rural type substations, consisting of a single reduced-size power transformer.

Urban substations are usually built using bricks, whereas rural ones are often installed externally directly on the MV pylon.

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Private substations

These can often be considered as terminal type substations, i.e. substations where the MV line ends at the point of installation of the substation itself.

They belong to the user and can supply both civil users (schools, hospitals, etc.) with power and industrial users with supply from the public MV grid. These substations are mostly located in the same rooms of the factory they supply and basically consist of three distinct rooms:

Delivery room

where the switching apparatus of the utility is installed. This room must be of a size to allow any construction of the in-feed/output system which the utility has the right to realise even at a later time to satisfy its new requirements. The take-up point is found in the delivery room, which represents the border and connection between the public grid and the user plant.

Figure 1 - Conceptual diagram of the substation

Instrument room

where the measuring units are located. Both these rooms must have public road access to allow intervention by authorised personnel whether the user is present or not.

User room

destined to contain the transformer and the MV and LV switching apparatus which are the concern of the user. This room must normally be adjacent to the other two rooms.

Selecting Energy Efficient Distribution Transformers

13:48 2 comments

Definition of transformer losses

Transformer losses can be divided into two main components: no-load losses and load losses. These types of losses are common to all types of transformers, regardless of transformer application or power rating.

There are, however, two other types of losses; extra losses created by harmonics and losses which may apply particularly to larger transformers – cooling or auxiliary losses, caused by the use of cooling equipment like fans and pumps.

No-Load losses

These losses occur in the transformer core whenever the transformer is energised (even when the secondary circuit is open). They are also called iron losses or core losses and are constant.

They are composed of:

Hysteresis losses

Caused by the frictional movement of magnetic domains in the core laminations being magnetized and demagnetized by alternation of the magnetic field. These losses depend on thetype of material used to build a core.

Silicon steel has much lower hysteresis than normal steel but amorphous metal has much better performance than silicon steel. Nowadays hysteresis losses can be reduced by material processing such as cold rolling, laser treatment or grain orientation.

Hysteresis losses are usually responsible for more than a half of total no-load losses (~50% to ~70%).

This ratio was smaller in the past (due to the higher contribution of eddy current losses particularly in relatively thick and not laser treated sheets).

Eddy current losses

Caused by varying magnetic fields inducing eddy currents in the laminations and thus generating heat.

These losses can be reduced by building the core from thin laminated sheets insulated from each other by a thin varnish layer to reduce eddy currents. Eddy current losses nowadays usually account for 30% to 50% of total no-load losses. When assessing efforts in improving distribution transformer efficiency, the biggest progress has been achieved in reduction of these losses.

There are also marginal stray and dielectric losses which occur in the transformer core, accounting usually for no more than 1% of total no-load losses.

Load losses

These losses are commonly called copper losses or short circuit losses. Load losses vary according to the transformer loading.

They are composed of:

Ohmic heat loss

Sometimes referred to as copper loss, since this resistive component of load loss dominates. This loss occurs in transformer windings and is caused by the resistance of the conductor.

The magnitude of these losses increases with the square of the load current and is proportional to the resistance of the winding. It can be reduced by increasing the cross sectional area of conductor or by reducing the winding length. Using copper as the conductor maintains the balance between weight, size, cost and resistance; adding an additional amount to increase conductor diameter, consistent with other design constraints, reduces losses.

Conductor eddy current losses.

Eddy currents, due to magnetic fields caused by alternating current, also occur in the windings. Reducing the cross-section of the conductor reduces eddy currents, so stranded conductors are used to achieve the required low resistance while controlling eddy current loss.

Effectively, this means that the ‘winding’ is made up of a number of parallel windings. Since each of these windings would experience a slightly different flux, the voltage developed by each would be slightly different and connecting the ends would result in circulating currents which would contribute to loss.

This is avoided by the use of continuously transposed conductor (CTC), in which the strands are frequently transposed to average the flux differences and equalise the voltage.