A New Method of Measuring DC Resistance of Large-sized Transformer LV Winding

Introduction

The transformer winding DC resistance measurement is one of important tests and serves as a benchmark for assessing short-circuit loss. Hence, it is necessary to conduct the test after delivery, pre-use, overhaul and tap changer adjustment. As electrical and mechanic production level has been rising in recent years, the capacity of electrical system is becoming increasingly larger, and the same goes for the capacity per transformer. The capacity of large-sized transformer coil and voltage stage are proportional to the ratio of inductance and resistance. The large-sized transformer winding DC circuit can keep steady for minutes even longer. How to measure the transformer winding DC resistance quickly and accurately is always attracting the attention of people. 

1. Basic Principle

The equivalent circuit of winding is composed of a small resistance component R with small value and a great inductance component L in series. The schematic diagram of winding resistance measurement is shown in Fig.1.

Fig.1 Diagram of winding resistance measurement

When t=0, close the switch and the equation of circuit voltage is listed below:

When t=0 i=0, see below for the winding current if adding DC voltage:

τ- time constant = L/R

The relation between time constant and current is shown in Tab.1.

Tab.1. The relation between charging time and current

As for such dynamic circuit, only when the circuit becomes steady can DC resistance be measured correctly. In other words, it takes at least 5to achieve needed accuracy in actual work. Therefore, it is very important to shorten measuring time and improve working efficiency.

According to the formula = L/R, there are two methods to shorten time: to reduce the inductance and to increase circuit resistance.

The inductance is expressed by the following formula:

μ0 - magnetic permeability = 0.4π×10-6H/m

n - number of turns

s - section area of iron core

l - length of core circuit

k - magnetic permeability coefficient

From the above equation, it can be seen that the inductance of winding is determined by number of turns, dimension of iron core and silicon steel sheet’s magnetic permeability coefficient. For to be tested transformer, n, s and l is known and only k can be changed. The relation between magnetic flux density represented by the letter B or k and magnetic field strength H is shown in Fig.2.

The relation between B/k and H is shown in Fig.2.

Fig.2 The relation between B/k and H

From the Fig.2, when magnetic field strength is great and iron core’s magnetic flux density tends to saturate, magnetic permeability coefficient declines greatly and the inductance decreases as well. An increase in steady current can reduce the inductance effectively. On the other hand, the connection of a resistor R into the circuit in series increases circuit resistance and reduces time constant.

The methods of reducing inductance and connecting circuit resistor in series are applied in the measurement of DC resistance. However, the method of reducing inductance is more effective than the other one in actual measurement. 

2. Measuring Methods

There are many ways to measure DC resistance. Basic methods include DC drop and electric bridge. Others are flux bumping, second-order oscillation, short-circuit demagnetization and identical and dynamic measurement etc. Several main ways to measure winding DC resistance are presented below.

2.1 Electrical circuit change method which increases circuit resistance

The schematic diagram of electrical circuit change is shown in Fig.3 while the relation between measuring current and time is shown in Fig.4.

When S and SB close, the circuit current equation is as follows:

When SB disconnects, the circuit current equation is shown below:

When measuring, close the button SB and add the resistor R to short circuit, all the voltage is applied to the test winding. The current rises at a high rate along the curve 1 until reaching preset value. At that time, the button SB disconnects and the current switches from curve 1 to curve 2. The charging time of winding shortens from t2 (t2 =5)tot3. If routine measuring method is adopted, the charging time is t1 (t1=5) and t3＜t2＜t1. So it is conclude that this way can shorten the measuring time.

2.2 Full voltage - constant current power supply method

The full voltage - constant current power supply method is based on forced steady state of circuit. The method applies full voltage (high voltage) in the beginning to increase the coil current rapidly and shorten the transition process. Then steady current is used to do measurement. It is a good method to study the circuit transition process with high inductance. The schematic diagram of full-voltage constant current source method is shown in Fig.5

Fig.5 The schematic diagram of full-voltage constant current source method 

When the test starts, the switch within the amplification and action unit represented by the letter S is off and the charging begins. The U value is determined by constant current value of charging time required by L and R. When the times reaches t1, transient vale of circuit current is I0 and resistance drop Un = I0Rc. At that time, the switch S is off. Hence, isolation diode VD conducts automatically and constant current source provides the current for measuring winding. The circuit current is forced to stabilize at I0 and the circuit enters steady state. Take UR readings and calculate DC resistance. The charging time of full-voltage constant current is very short, which can be neglected compared with measuring time.

2.3 Saturated magnetic method

The saturated magnetic method is to force iron core to saturate rapidly, lessening self-inductance effect and reducing circuit inductance. High voltage windings are connected with low voltage ones in series. The same phase and polarity high voltage windings are used to saturate. The number of high voltage winding turns is much more than that of low voltage winding. Hence, low current can make iron core saturate and reduce winding inductance greatly with the help of HV winding turns.

Fig.6 presents the schematic diagram of saturated magnetic circuit method.

In the wiring diagram shown in Fig.6 (a), a and c terminal line are connected with a voltmeter to measure the voltage Uac; accurate constant current source I0 provides measuring power source. Under the influence of high voltage side saturated magnet, low voltage side flux saturates and inductance value becomes smaller. The transition process shortens so that we can take readings soon. According to the formula Rac=Uac/I0, the value of Rac is calculated. In a similar way, Rbc and Rab can also be calculated.

Rac =Ra × (Rb + Rc) /(Ra+ Rb + Rc)

Rbc =Rc (Ra+ Rb)/(Ra+ Rb + Rc)

Rab =Rb(Ra + Rc)/(Ra+ Rb + Rc)

Ra,Rband Rcare calculated and measurement error falls within the range.

3. Analysis and Comparison of Test Results through Current Methods

The traditional method is used to measure the DC resistance value of large-sized LV winding. The measuring value is represented by Rbc and Rac. It takes 30min for the current to become stable. When measuring Rab, it takes 1-2h to become stable. Sometimes the current does not become stable, resulting in overheated wire and equipment. The data precision also fails to meet the requirements. To include, long-time measurement and huge measurement error are not acceptable in the work.

According to the schematic diagram of saturated magnetic method introduced in the section 2.3, the DC resistance of SFP-720000/50 transformer LV winding is measured. Test results are shown in Tab.2.

Tab.2 Test results

R - DC resistance

tat - Charging stable time

I0- constant power charging current

From the above data, it can be found that measuring time shortens and the speed of measuring increases by one times. Meanwhile, three phase circuit becomes stable. The measuring error is much less than ±2%. It is concluded that the saturated magnetic method is a good method.

In terms of measuring speed and precision, the full-voltage constant current power supply method is better than a common method. However, its measuring time is longer than that of saturated magnetic method.

The current methods have four disadvantages.

a. It still takes at least 40min to measure the three phase winding, which needs to be further shortened.  

b. When using saturated magnetic method to measure the resistance value of different phases, please pay attention to the offset effect of residual magnet on measuring power source. The opposite direction will have a severe impact on measuring speed and precision.

c. The large-sized transformer LV windings are connected in a triangle method and there exists mutual inductance for windings in parallel. The charging time extends and test data is lack of credibility.

d. One problem always occur when measuring the DC resistance of transformer LV side with triangle connection. In other words, measurement time of one winding is obviously longer than that of other two windings. The charging current cannot reach the stability that the instrument requires. Therefore, if the wiring is in a triangle manner, its measuring method of DC resistance to be further studied.

4. Study of A New Measuring Method

In order to measure DC resistance of LV side winding rapidly and accurately and reduce measurement error, a new method, which combines full-voltage constant current power supply method with saturated magnetic method, is put forward. The schematic diagram of Rac measurement via this new method is shown in Fig.7

Fig.7 The schematic diagram of Rac measurement

From the Fig.7, it is found that the new method introduces full-voltage constant current power supply as measurement power source. The advantage is to use higher voltage to increase the current more rapidly and shortens the target time. The test object is not one winding coil but two winding coils in series, so the voltage applied to low voltage a and c sides should be lower than DC voltage. The general trend is falling. However, the change of current is opposite, from rising to stabilization. The waveforms of voltage and current are shown in Fig.8.

Fig.8 Waveforms of voltage and current

The power supply - U starts to charge at zero and the current rises; the current rises to I0 at t0; the DC voltage source disconnects automatically and the constant current source provides the electricity through diode; the current stabilizes to I0 at t1. At t2, the voltage also becomes stable. Hence, the resistance value Rac can be obtained according to the formula R=U/I. Similarly, resistance values of other phases can be calculated.

5. Conclusions

a. The high voltage small current constant current power source can measure the winding resistance of large-sized transformer rapidly.

b. During the measuring process, the voltage source disconnects automatically and constant current power connects automatically. 

c. It has high accuracy and adjustment of Rc value can measure the broad range.

d. The solution to the problem of residual magnet and demagnetization is to ensure that the direction of flux in the iron core is the same as residual magnet during the measuring process.

If the mutual inductance exists in the parallel windings, the new method applies full voltage to increase the current of coils quickly instead of eliminating mutual inductance effect.