## Theory and Procedure of the Experiment - 3

A. Determination of Positive Sequence reactance (X

_{1})A system component operating under balanced condition of voltage and current is in effect in a positive sequence mode. The positive sequence reactance X_{1} of a synchronous machine under steady state condition is the direct axis synchronous
reactance X_{d} of the machine.
The positive sequence impedance can also be defined as the impedance offered by the machine to the flow of
positive sequence currents in the armature windings create a rotating magnetic field. The positive sequence reactance can be caluclated by the expression

X_{1}=E / I_{sc}

where E and Isc are No load armature voltage and short circuit armature current respectively. _{1}=E / I

_{sc}

1. Open Circuit Test

- Run the machine at rated speed.
- Connect a voltmeter and ammeter according to the open circuit diagram
- Note the readings for different exciting currents.

2. Short Circuit Test

- Make the connections as shown in Short circuit diagram
- Run the machine at rated speed.
- Apply low voltage to the field circuit so that exciting current is small. Alternately connect a high resistance in the field circuit with full applied voltage.
- Apply three-phase short circuit at the synchronous machine terminal with an ammeter connected in any phase.
- Measure the short circuit current corresponding to the field current.

B. Determination of Negative Sequence reactance (X

_{2})- The negative sequence reactance X

_{2}can be obtained by running the machine at rated speed with a low excitation and with a sustained two phase short circuit between the open phase and any short circuited phase.Let open circuit voltage be Vos and the short circuit current I

_{sc}. The negative sequence impedance Z

_{2}and reactance X

_{2}can be calculated using the following expressions:

**Z**

_{2}=V_{os}/3.I_{sc}**X**

_{2}=Z_{2}sin Ø**where Ø = cos**

^{-1}(P/V_{sc}I_{sc})- Make the connections as shown in figure for determining X
_{2} - Run the machine at rated speed.
- Short circuit two phases of the alternator through an ammeter and the current coil of the wattmeter.
- Connect the voltage coil of the wattmeter and the voltmeter between the open phase and any short circuited phase.
- Gradually increase the excitation such that the short circuit current does not exceed its rated value.
- Note the reading of voltage, current and power.

C. Determination of Zero Sequence reactance (X

_{0})- The machine is driven at rated speed. Connect all three phases in parallel and the voltmeter and ammeter according
to the figure for determining X

_{0}.

- Connect the armature winding in parallel according to the circuit diagram
- Run the machine at rated speed.
- Apply low voltage from a variac and measure both voltage V
_{0}and current I_{0}taken by the armature windings and zero sequence reactance can be caluclated using the following expression:

**X**

_{0}=3.Z_{0}/I_{0}