The Compensation Mothed for Temperature Rise Test of 750kV Magnetic Controllable Reactor

. The article introduces the principle of Magnetic Controllable Reactor(MCR) in power system applications, the temperature rise test is the important type test in the factory,but it will lead to errors of the temperature rise test results in the factory because the test conditions in the factory is different from the actual operation circuit. In this paper, a compensation method for temperature rise test is introduced, which can make up for the shortcomings of temperature rise test in factory,and the results of simulation by PSCAD and measurement show that the method presented in this pape is effective.


Introduction
Magnetic Controllable Reactor(The paper referred to as "MCR") is the power equipment, which is suitable for super / ultra-high voltage power system suppression over-voltage and reactive power compensation.MCR can also solute frequency overvoltage under the longterm no-load operation mode and transmission network losses under overload issues. The MCR is not only as reliable as the transformers maintenance, but also has low grid losses, low cost, small harmonics.MCR can be directly linked to the high voltage line and bus [1] .
Temperature rise test of transformer/reactor is one of the important test items in type test to identify product quality. The purpose of temperature rise test is to determine whether the temperature rise of each component of transformer/reactor meets the requirements of relevant standards, so as to provide a reliable basis for long-term safe operation of transformer or reactor [2][3] .
Due to the limitation of test equipment, the capacity of the megnetic controllable reactor is far more than the the power supply capacity. In addition, there is no FACTS(Flexible AC Transmission Systems) in the factory test system, and the reactive power balance of the system can not be adjusted in real time. If the selected capacity of the fixed capacitor is inappropriate, the phenomenon of over-voltage for the power windings will occurs during the test process. This paper introduces a compensation method for temperature rise test, which makes up for the shortcomings in the process of factory temperature rise test.

The Principle of MCR
The magnetic controllable reactor automatically adjusts the trigger angle αaccording to the voltage variation of the power grid (the deviation of the voltage of the power grid from the standard value), and changes the core saturation of the reactor by changing the DC current of the DC control winding, thus changing the equivalent impedance of the reactor, so as to achieve the purpose of regulating the reactor capacity. The principle diagram of the magnetic controllable reactor is shown in Figure 1. The three-phase magnetically controlled reactor is composed of the power windings, the compensation windings and the control windings. Its wiring diagram is shown in figure 2. (1)MCR power winding, Wye connection, singlephase has two windings in parallel with the grid, the other end to the earth, rated voltage is 800kV.
(2)MCR compensation winding, delta connection, supply energy to the rectifier, rated voltage is 40.5kV.
(3)MCR control winding, which has two windings .The two windings has dotted terminal, rated voltage is 41.85kv.
(4)The excitation transformer, as viewed from the compensation winding ,is a step-down transformer winding end, the capacity is 1MVA, a voltage is 40.5kv, secondary voltage 0.2kv, Y / △ connection MCR is based on the three-winding transformer structure design, which is using the direct current to control core saturation, therefore the inductive impedance will change. As follows, after the power winding is energized, the compensation winding used for suppression of current third harmonic will induce a corresponding voltage and supply the rectifier device power. When the current flows out from the three-phase bridge rectifier, a DC current flow into the control winding, by changing the saturation of the core thereby changing the controllable reactor side of the net impedance.

Introduction of Temperature Rise Test And Test Problem
Testing of MCR is performed in two stages. First stage testing of MCR at the factory of origin, Second stage testing of MCR at the substation after mounting and connection to the grid.
Temperature rise test is a kind of type test at the factory of origin. The traditional temperature rise test examines the average temperature rise of winding and the temperature rise of oil top layer. If the two measured values of temperature rise do not exceed the allowable temperature rise limit specified in the standard, the transformer is considered to have passed the type test of temperature rise test. (1) Plant generators with a capacity of 60MVA, because then single-phase controlled reactor temperature rise test conducted, so the generator needs to change the singlephase method.

Introduction of the test circuit
(2) Capacitor towers, uncharged switching mode, in accordance with the demand of reactive power capacity of linear compensation, but not supply real-time compensation. (3) Step-up transformer capacity of 63MVA,the ratio is: (1700/ 3 )kV/132kV.
(5) DC control and protection system, which controls the DC output and protect the control circuit.

3.2The Impact of MCR's Characteristics On the Temperature-Rise Test
Through the introduction 3.1 shown the test circuit can be equivalent to a single large load of small power supply systems. Because there is no FACTS devices, the real time reactive power balance could not be adjusted, can only rely on a fixed capacitor tower linear compensation, but when MCR temperature-rise test carry out, the impedance of MCR will change nonlinear, when MCR high voltage winding current from no load to rated current state changes, the reactive power which capacitor tower supply would not accurately follow the nonlinear changes impedance of MCR, if we rise generator export voltage to MCR rated voltage may cause capacitive reactive power to the generator, the resulting over-voltage may damage insulation of MCR. Fig 4 shows the over-voltage curve on the grid side caused by the variable impedance between the capacity tower capacitor and magnetic controllable reactor. Also, because when the control winding together with the rated excitation current, it will lead to singlephase controllable reactor two core columns saturation inconsistent, because there is no control of the threephase windings connected in parallel with the high voltage winding will result in both sides of the control winding inducing overvoltage.
So if we want to complete temperature-rise test, we need to solve the following problems: (1)Make sure the high voltage winding current from the load current to the rated state, capacitor impedance changes do not cause capacitive reactive send down cause overvoltage.
( It should be introduced that MCR temperature -rise test on single-phase power-up sequence: power winding load, applied control winding to excitation current ,when the rated excitation current reached, the power winding applied to voltage, when the rated current reach ,stop the high voltage winding voltage and start the temperaturerise test timing.
However, as described in Article 3.2, with the power winding voltage of the application process, the impedance is changing, and this has led to a fixedcapacity capacitor could not follow MCR impedance change process.
The MCR nonlinear impedance limit the temperature-rise test. If we can guarantee MCR impedance range is limited, the inductive reactive power should be always higher than the capacitive reactive power, can make the temperature-rise test carried out smoothly. Therefore, the mothed is feasible to increase in the compensation side suitable impedance, this method can not only protect the MCR's core saturation impedance will not change dramatically, but also can ensure the compensation winding be assessment by the temperature-rise test.  Based on this approach, the author uses the PSCAD simulation, simulation results are as follows:  Figure 9 is the data, which the author use the sofware of PSCAD to simulate. The high voltage reaches the rated current work status, the status satisfied to the temperature-rise test. The control winding voltage amplitude and harmonics of voltage wouldn't be hazards to the rectifier, meet the test requirements. According to the above compensation method, the temperature rise test of 750 kV megntic controllable reactor was carried out in the factory, and the temperature rise test was carried out successfully. The test curves are as follows:

Conclusion
The results of simulation prove the compensation method is effective for solving the risks of temperaturerise test, and ensuring the smooth progress of the test. The simulations also verified by experiments megnetic controllable reactor characteristic impedance variations and harmonic levels.