Research on inverse time overcurrent protection of digital generator

Based on the accurate analysis of the generator overload thermal model, according to the thermal balance process of the stator winding, the heat dissipation effect of the winding is fully considered, and a new digital inverse time overcurrent protection criterion is proposed, which overcomes the shortcomings of the traditional inverse time overcurrent protection. Improved protection.

The material utilization rate of large generators is high, the ratio of heat capacity to copper loss and thermal time constant are relatively small, and the relative overload capacity is low. It is easy to cause excessive temperature rise of winding due to overload, which affects the normal life of the unit. The generator is faulty, so overload protection must be installed.

According to the relevant regulations, the generator whose stator winding is directly cooled and has low overload capacity must be equipped with overcurrent protection with inverse time characteristic suitable for its overload capacity, and can react to the heat accumulation process of the generator when the current changes. Commonly used inverse time overcurrent protection relays are electromagnetic, static and digital. In the electromagnetic relay, the heat accumulation process is completed by the rotation of the disk (the rotational speed depends on the value of the stator current), and the turntable starts to rotate when the current reaches the starting current value, and the position of the rotating disk corresponds to the heat. The accumulated output, due to the influence of the rotational inertia, has a large operational error and is prone to malfunction. Static relays simulate the heat accumulation and heat dissipation of the stator windings by charging and discharging the capacitors. However, when the current is large, it is difficult to achieve a certain accuracy of the operating time. With the continuous maturity of generator transformer microcomputer protection technology, digital generator inverse time overcurrent protection has also been widely used. However, the current digital generator inverse time overcurrent protection still uses the principle of traditional protection. By analyzing its principle and its limitations, a more precise protection criterion is obtained.

1 Physical description of the conductor heating model According to the principle of heat balance, part of the heat generated by the conductor will be stored in the object and the temperature of the object will rise in a certain period of time, and the other part will be lost in the cooling medium. Let Q be the heat generated by the conductor in every second. The unit is Wc is the specific heat of the conductor, and the number of joules of heat required to increase the temperature of 1 kg by 1 °C, the unit is J/(kg?°C)S For the conductor cooling surface, the unit is the m heat dissipation coefficient, that is, the unit area, the unit temperature difference, and the number of joules of heat dissipated per unit time. The unit is W/(m constant θ is the temperature rise, that is, the difference between the conductor temperature and the temperature of the cooling medium, The unit is °CG is the weight of the object, the unit is kg. In the dt time, the following formula holds: the first term on the right side of equation (1) is the heat absorbed by the object temperature rise dθ, the second term is Heat dissipation of the cooling medium during dt.

Let the temperature rise of the conductor at t = 0 be θ0, then the solution of the differential equation (1) is where τ is the conductor heating time constant (s), τ = the conductor stable temperature rise (°C), θ can be seen from above When the conductor heats up, the temperature rises exponentially τ, the conductor begins to cool, and its temperature drops according to the same law until it is the same as the temperature of the cooling medium.

For stator windings, the heat generated per unit time is Q = R (R is the resistance of the stator winding and I is the stator current).

2 Traditional inverse time-over-current protection principle and limitations Domestic generator stator winding overload protection is often used as a criterion. I is the inverse time start value A is the overheating capability of the generator.

If the generator is running at rated state, the temperature rise of the winding will reach a stable value, and the heat generated by the winding will be dissipated to the outside through the cooling surface. If a short-term overload occurs again under the rated condition, it is assumed that the heat relative to the rated loss is still radiated outward through the cooling medium. The heat generated by the loss exceeding the rated state is all used for the temperature rise of the winding without external heat dissipation, that is, the winding is in an adiabatic state when the power automation device is overloaded. Then, the following formula holds: (rated temperature rise), then the solution of the differential equation (5) is that when the winding temperature rise θ exceeds the maximum temperature rise θ protection action allowed, that is, the criterion is θ ≥ θ max, the protection action time In order to compare the action criterion (3) of the traditional inverse time protection, it can be obtained in the microcomputer protection, A represents the heat accumulation value I of the winding at the Kth sampling interval, and the current standard value T is the sampling interval), and the action criterion is There is a layer of insulation wrapped around the A stator winding. The larger the generator capacity, the higher the voltage level and the thicker the corresponding insulation layer. For non-direct cooling generators, due to the thermal resistance effect of the winding insulation and the high quality of the core, the heat is slowly generated, so that there is thermal insulation between the winding and the core when the winding is initially heated. However, for a directly cooled generator (generally a large generator), although there is thermal insulation between the stator winding and the iron core in a short time, the cooling medium (such as water, hydrogen) is directly in contact with the winding, and there is no short-term thermal insulation. Phenomenon, so the traditional criteria are more conservative. Under certain conditions, there are obvious irrationalities. For example, when the generator is running at low load and rated load, for the same overload current, according to the traditional criterion, the action time is the same as the actual load. When the temperature rise of the former is significantly smaller than that of the latter, the time rises to the operating temperature rise of the winding under the same overload current, the former should be larger than the latter.

3 New criteria Because the generator inverse time overcurrent protection is mainly used for direct cooling type large generators, if the heat dissipation of the winding conductor is not considered. This protection cannot fully exert the overload capability of the generator in terms of operational characteristics.

The generator is composed of different parts with different heat conduction, and the heating process is extremely complicated, and there are often several different heat sources inside the motor, so that the parts are simultaneously heated. Therefore, it is very difficult to accurately calculate the heat exchange inside the motor. To simplify the problem, assume that there is no temperature difference between each point of the generator and that the heat dissipation along the surface is the same.

When the generator is rated for operation, the winding reaches a stable temperature rise. At this time, the copper loss of the generator is equal to the external heat dissipation of the winding. The heat dissipation is mainly composed of two parts: the heat generated by the winding through the cooling medium (such as water, hydrogen, etc.) and the passing of iron. The core's external heat dissipation. In the overload process, the stator core and the temperature of the cooling medium remain unchanged. In the formula, a is the heat dissipation coefficient of the winding to the iron core and the cooling medium, respectively. The heat dissipation area θ of the winding to the iron core and the cooling medium is respectively the iron core. The temperature of the cooling medium θ, θ is the temperature and rated temperature of the winding, respectively.

The first term on the left side of (7) is the heat dissipation of the winding to the iron core, and the second term is the heat dissipation of the winding to the cooling medium.

In the rated state, the stator copper loss is equal to the external heat dissipation of the winding. The following formula holds: 2. According to the principle of heat balance, the following equation is established: where c is the specific heat of the stator winding G is the weight of the stator winding a is the stator winding The heat dissipation coefficient S is the equivalent contact area between the stator winding and the medium, and θ is the winding temperature.

Write it as a difference equation and set Δt =T to the sampling period. Then, θ is the difference between the temperature of the K and K 1 sampling interval windings and the rated temperature.

If both sides of the equation are divided by I to order BR, then the above formula becomes B =0 in the rated state, assuming that the initial state is the rated load. If I 1 , the value of B can be seen from equation (13) and gradually Rise, and stabilize (I negative, and gradually decrease, and finally stabilized at (I 1) τ. Compared with the definition of the A value of the traditional generator inverse time overcurrent protection, it can be seen that A K. is stable when it corresponds to the stable load. The heat accumulation value, the root power automation formula (13), the load heat change value also changes, and when the action criterion B ≥ A is satisfied, the protection action.

According to the physical meaning of I: the current value of the generator that allows long-term operation, that is, the steady heat accumulation value at this current is equal to the A value specified by the generator. When this current value is exceeded, the protection will act.

From this, it can be obtained that the heat accumulation value at different times can be obtained by obtaining the formula (13) of the Ï„ generation.

The above derivation is based on the premise that the temperature of the cooling medium remains unchanged. In fact, the temperature of the stator core changes during the entire overload process. As the winding temperature increases, the temperature difference between the winding and the core increases, and the core temperature begins to rise. Since the temperature change of the winding is a dynamic process, the heat of the winding felt by the iron core is also a variable amount, so it is difficult to calculate the temperature of the iron core. It can be seen from equation (6) that the heat generated by the calculated winding through the iron core is larger than the actual value regardless of the rise of the core temperature, but since the heating time constant of the iron core is relatively large, the temperature rises slowly, and The external heat dissipation of the direct-cooling generator mainly depends on the heat conduction of the cooling medium. In addition, regardless of the temperature effect of the stator winding resistance and the specific heat, the influence of the temperature rise of the iron core is compensated to some extent.

When the initial state is different, the operating time is different under the same overload current. From the recursion formula (14), it can be concluded that the initial values ​​are different, and the time for the heat accumulation to reach a certain action value is different, which is more in line with the actual situation.

4 Comparison of the action characteristics of the two criteria = 1.2, the calculation results of the criterion 1 (traditional criterion) and the criterion 2 (new criterion) are plotted as shown in the curve of Fig. 1, from which it can be seen that I At 2 o'clock, when the action characteristics of the two criteria are not much different, I 2, the action time of criterion 2 is significantly larger than the criterion 1.

5 Conclusions By studying the principle of traditional inverse time overcurrent protection and the stator winding heating model, according to the principle of motor thermal balance, the heat dissipation characteristics of the winding are considered to some extent, and a new and reasonable judgment is given for the realization of microcomputer protection. According to the data, there is a certain improvement than the traditional protection principle. This criterion can be further developed and applied to generator negative sequence inverse time protection and field winding overload protection.

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