Open Circuit Test and Short Circuit Test of Synchronous generator

Hello friends, I hope all of you are fine. In today’s tutorial, we are gonna have a look at the Open Circuit Test and Short Circuit Test of Synchronous generator and their relationship. There are many factors of electrical power networks that rely on the usage of the generator models for the estimation of the power system behavior, the correctness of these estimations relies on the correctness of the parameters of the generator used in the system. Some of these parameters are not easy to find so producers of the generators mentioned these parameters on the datasheet.

In today’s post, we will have a look at the different parameters of a synchronous generator that should be examined before the installation of the generator. To easily understand of the parameters of a generator, you must know the synchronous generator resultant circuit. So, let’s get started with the Open Circuit Test and Short Circuit Test of Synchronous Generator.

Open Circuit Test and Short Circuit Test of Synchronous generator

• During the measurement of the resultant circuit of the synchronous generator, we find the three parameters, that are very important to study the complete performance of the real-world synchronous generator.
• These three parameters are described here.

• • The armature resistance.
  • The relation between field current and internally generated voltage E_A
  • Synchronous reactance X_s
• To find these three parameters we will perform some tests on the synchronous generator that are described here with the detailed

Open Circuit Test of the Synchronous Generator

open circuit characteristics of synchronous generators is explained here

• To accomplish this test the generator should move at its rated speed, no-load should be at its terminals, and the value of the field winding current should be 0.
• After following these steps, we will vary the field current step by step and calculate the terminal voltage at each step.
• As there is no load at the output terminals of the generator, so there will be zero armature current.
• Hence the internal generated voltage E_A will equal to the phase voltage V_ø.
• By using this data, we can construct the curve among the internally generated voltage that equals the terminal voltage and the field current.
• This graph between E_A and the field current is known as the open-circuit characteristic (OCC) of the synchronous generator.
• By this graphical representation, we can find the value of internal generated voltage E_A any value of the field current.

• The graph of the open-circuit characteristic is shown in a given diagram,
• You can observe from the graph that at the start the curve is a straight line then it is not a straight line it due to saturation.
• The reluctance offered by the un-saturated region is lesser than the reluctance of the air gap, at the start when the iron is not saturated the field current and the terminal voltage are in direct relation with each other.
• When iron gets saturated, the reluctance offered by the iron becomes larger and the increment in the flux is not directly proportional to the field current, so the curve is not a straight line after some interval.
• The linear part of the open-circuit characteristics is known as the air gap line.

Short circuit test of Synchronous Generator

• To do a short circuit test first of set the value of field current at 0 and connect the output terminals of the generator by the ammeter.
• After that find the value of the armature current (I_A) by changing the field current.
• This graph among the field current and the armature current known as the short-circuit characteristic (SCC).
• You can see that it is a linear graph.

• To understand this straight-line behavior let’s have a look at the equivalent circuit of the synchronous generator when it is short-circuited this circuit is shown below.
• From this circuit, you can find the value of the armature current (I_A) that is given here.

I_A =E_A/(R_A+jX_s)

• The magnitude of the armature current is given here,

I_A =E_A/√(R²_A+X²_s)

• The phasor diagrams of the short circuit test of the synchronous generator are shown below.
• The fields during the short circuit test are physically shown in a given diagram.
• As the stator field B_s crosses the effect of the rotor magnetic field (B_R) so the total field B_net has low value.
• As the value of the B_net is lesser so the generator is un-saturated short circuit characteristic curve is linear.
• To recognize what data these two features have output, note from a given figure that the V_ø is 0.
• From this figure, the interior impedance of the generator is given here.

Z_s = √(R²_A+X²_s)

= E_A/I_A

• As we know that synchronous reactance Xs is greater than the armature resistance R_A so this equation becomes.

Xs= E_A/I_A

= V_ø,oc/I_A

• If the value of the internally generated voltage (E_A) and armature current (I_A) is known, we can compute the value of the synchronous reactance.
  • Measure the internal generated voltage E_A from the generator short circuit test characteristic (OCC) at the specified value of the field current.
  • Measure the short-circuited current I_A at the field current from the open-circuit characteristic (OCC)
  • Put the values of the internal generated voltage E_A and the I_A in the given below equation and find the value of the synchronous reactance.

Xs= E_A/I_A

• There is some issues for this technique to the measurement of the synchronous reactance.
• As the internal generated voltage E_A is measured from the OCC at that point where the generator is partly saturated for a higher value of the field current and the armature current (I_A) is taken from the SCC where the generator is un-saturated for all values of the field current.
• So, at the larger field current, the internal generated voltage calculated from the OCC at a specified value of the I_F is not similar to the internal generated voltage measured from the SCC at the same value of the field current I_F.

• This difference affects the value of the synchronous reactance X
• So, the result by this method can be applied to the saturation point, the unsaturated X_S can be measured by this equation at any value of the I_F in the straight-line part of the open-circuit characteristic (OCC).

Xs= E_A/I_A

• The estimated value of X_S fluctuates with the gradation of saturation of the open-circuit characteristic curve, therefore the value of the X_S to be used in a specified problem must be measured at the estimated load on the generator.
• A graph of estimated (approximated) Xs as a function of I_F is in a given figure.

• It is significant to identify the resistance of winding as well as its Xs, resistance can be measured by by providing dc voltage to the windings when the generator is not working
• During the dc application, the value of the reactance of the winding will be 0.
• This method does not give proper results because the alternating current resistance is somewhat higher than the value of the direct current resistance.

Short-Circuit Ratio Synchronous Generator

• It is the ratio of the field current needed for the rated voltage at open-circuited conditions to the field current needed to produce the rated armature current at short-circuited conditions.
• Though the short-circuit ratio tells nothing new that is not we know about synchronous generator reactance. But in industries, this fact is used so we should know of it.

Short circuit test of alternator

• Short circuit tests give details about the current features of synchronous generators. It is done by operating a generator at rated speed when terminals of armature windings are short-circuited.
• The ammeter is connected in series combination with any one of the short lines of a 3-phase alternator. The field current is increased and the resultant value of the armature current is measured. Per phase short circuit is drawn as a function of field current, the resultant graphical representations are called short circuit characteristics of a generator.

 Open circuit test of an alternator

Open circuit test used to measure the value of synchronous impedance. The alternator is operating at rated synchronous speed and load terminals are open.

Excitation current can increase to get more thatn 25 percent higher thatn the rated voltage, about 125 percent of the rated voltage.

The graph is made between open circuit phase voltage Eg and field current Ig and the resultant curve called open circuit characteristic.

Faqs

• What is the difference between OC and SC test?
• OC test is used to find core or no load features and SC test is used to measure copper or full load parameters. OC test is done at rated voltage and frequency and SC uses just five to ten percent rated voltage.
• What is a short circuit test of a generator?
• Short circuit tests give details about the current features of synchronous generators. it is done with operating a generator at a rated speed when terminals of armature winding are short-circuited. Ammer is configured in a series combination of shorted lines.
• What is the short circuit test of a synchronous motor?
• The machine is operated at a rated synchronous speed and armature terminals are short circuits with ammeter. The field current increases from zero until the short circuit armature current gets its safe maximum value, which is about 125 to 150% of the rated current.
• What is another name for OC test?
• Open circuit test also called no load test is used for measuring no load impedance in the excitation branch of the transformer. No load is denoted with an open circuit.
• What are the advantages of OC test?
• It draws a large load current for easy reading.
• It has fewer core losses
• It needed less input power.
• It needed less voltage for the test performed
• What is SC test also known as?
• The short circuit test is done on the transformer to find the impedance and losses of the transformer through application of low voltage to the transformer’s secondary winding and the short circuit at the primary winding. This test also called  impedance or load loss test

Read also:

• Introduction to Synchronous Generator
• Synchronous Generator Equivalent Circuit  
• Synchronous Generator Phasor Diagram
• Synchronous Generator Power and Torque
• Synchronous Generator Operating Alone
• Synchronous Generator Parallel Operation
• Synchronous Generator parallel with Large Power system
• Synchronous Generator Parallel with same Size Generator
• Synchronous Generator Ratings
• Synchronous Generator Capability Curves
• Synchronous Generator Transients

That is the detailed article on the Synchronous Generator Parameter we did different tests for finding different parameters if you have any queries ask in the comments. See you in the next tutorial Synchronous Generator Operating Alone.