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ANSI Codes of Frequency Protection Relay&

Significance of Leakage Reactance of Transformer

ANSI codes of frequency relay are 81H, 81L, 81 R. The 81H, 81L, 81 R are frequency relay acronyms for over-frequency, under-frequency, and rate of change of frequency.

The protection relay is a very important device for the protection of the electrical system. The various types of protection relays like over-current, overload, distance protection, frequency protection, reverse power, etc, have the ANSI protection code. The protection relay ensures that the faulty section isolates immediately at the time of the fault. Thus, protection relays are the backbone of the electrical [power network.

Every protection relay has ANSI standard device number. The ANSI code shows the type of relay & its function. We will confine our discussion to the frequency relay in this post.

ANSI Standards Code of Frequency Protection Functions

The followings are the ANSI standard codes of frequency relay.

  • Frequency Relay ANSI Code 81H – Overfrequency
  • Standard ANSI Code 81 L – Underfrequency
  • ANSI Code 81 R – Rate of change of frequency( df/dt)

ANSI Code 81H – Overfrequency

The electrical equipment has its design frequency for which equipment intend to run. Any deviation in the frequency from its rated frequency deteriorates the efficiency of the equipment. As a result of variation in the frequency, the problem of overfluxing or underfluxing beget.

Generally equipment is designed for 50 Hz operation in India and 60Hz in the USA. The frequency tolerance is in the range of 50 Hz +/- 5 %. The 50 Hz rated capacity equipment work in the range of 48.5 to 51.5 Hz. The frequency if rises above 48.5 Hz, the under-frequency element actuates. The relay continuously measures the system frequency and gives a trip signal to the breaker when the frequency is above the set point.

ANSI Code of Frequency Relay 81L – Underfrequency

The frequency protection relay measures the frequency and outputs contact when the frequency drops below the lower setpoint of frequency. The relay contact can be used for load shedding of non-critical loads or for total disruption of the supply.

We use Under frequency relays to shed a certain portion of load automatically when the system frequently drops below the lower set frequency.

ANSI Code 81R – Rate of Change of Frequency( ROCOF Relay)

The relay monitors the rate of change of frequency. The relay outputs a tripping contact when the rate of change of frequency is not within the specified range.

The frequency and load have a close relationship. The frequency decrease when the load increase and, the frequency increase when the load decreases.

The generator frequency increase or decrease according to the loading on the generator. The turbine governor controls the steam to correct the frequency.

When load increase on the grid

The sequence of operation is ;

  • Frequency decrease
  • One generator trips
  • More generator connected to bus trips
  • Total blackouts

The solution to the above problem is;

  • Frequency decrease
  • df/dt relay senses the rate of change of frequency
  • Partially load shedding

Significance of Leakage Reactance of Transformer

The leakage reactance of the transformer is caused by flux leakage from the magnetic core. The entire flux does not link to both the primary & secondary transformer winding and thus some part of the flux leaks from the magnetic path.

On application of AC voltage to primary of the transformer, the flux generates in the primary winding which travels through the core and links to the secondary.

The flux generated in the primary must link to both the primary and secondary winding. Does it is practically possible that all the flux generated will link to both of the winding?

In an ideal transformer, all the flux links with the both primary and secondary winding. However, In reality, all the flux do not link with both winding in a practical transformer, all the flux may link either winding but not both.

Leakage Flux in Transformer

The part of the flux diverts its path from the core, and it passes through winding insulation to transformer oil. The leakage of flux from the main core is the leakage flux. The magnetic leakage takes place in every transformer which causes leakage reactance.

The leakage flux begets leakage reactance in the transformer. The transformer draws more current to produce the same amount of flux to nullify the effect of leakage flux. The current drawn by the transformer causes an additional voltage drop in the primary and secondary winding. Thus, the voltage regulation of the transformer deteriorates with an increase in leakage flux or leakage reactance. Moreover, the percentage impedance of the transformer also increases with an increase in the leakage flux.

We can calculate the total voltage drop in the primary and secondary winding by measuring the voltage drop caused by the resistance and reactance of the transformer. The primary and secondary winding of the transformer has copper as a winding material that has a very low resistance. The combined resistance of the transformer is known as the resistance of the transformer.

How do leakage reactance and resistance cause voltage drop?

The combination of the resistance and reactance is called the impedance of the transformer.If R1 and R2 and X1 and X2 are primary and secondary resistance and leakage reactance respectively, then the impedance of primary(Z1) and secondary(Z2) windings are as follows;

If we apply AC voltage V1 across the primary of the transformer, the voltage drop in the primary will take place due to a voltage drop in the leakage reactance and resistance. Let the primary current be I1. The total voltage drop in primary on account of leakage reactance and resistance is ;

The voltage equation of the transformer on the primary side is as given below.

Similarly, the transformer secondary supplies current to load. Let the secondary current be I2. The secondary resistance and reactance are R2 and X2 respectively. EMF induced in the secondary is E2.

The total voltage drop in secondary on account of resistance and reactance is ;

The voltage equation of the transformer on the secondary side is as given below.

The secondary voltage is ;

Effect of Leakage Flux on Transformer Performance

  1. The leakage flux flow opposite in the main flux which reduce the net flux in the core. The reduction in the net flux cause reduction in secondary voltage of the transformer.
  2. The transformer primary current increase with an increase in the leakage flux, the increased current is inductive in nature and therefore the power factor gets poor with increased reactance.
  3. The leakage reactance causes voltage drop in primary and secondary of the transformer, hence it deteriorates the transformer voltage regulation and increases leakage reactance.
  4. Increased transformer current with high leakage reactance cause more copper losses in the transformer. The efficiency of transformer lowers with increased leakage reactance.
  5. The short circuit capacity improves with increase in the leakage reactance. This is positive point with increased leakage reactance.

How to Control Leakage Flux of Transformer?

  1. Placing the primary and secondary winding as close as possible
  2. Keeping space between primary and secondary winding small
  3. By taking the proper dimensions of winding