Three-phase load imbalance hazard - Solutions - Huaqiang Electronic Network

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1. Impact on distribution transformers
(1) Three-phase load imbalance will increase the loss of the transformer:
Transformer losses include no-load losses and load losses. Under normal circumstances, the operating voltage of the transformer is basically constant, that is, the no-load loss is a constant. The load loss varies with the transformer operating load and is proportional to the square of the load current. When the three-phase load is unbalanced, the load loss of the transformer can be regarded as the sum of the load losses of the three single-phase transformers.
From the mathematical theorem we know that assuming that a, b, and c are all greater than or equal to zero, then a+b+c≥33√abc.
When a=b=c, the algebra and a+b+c take the minimum value: a+b+c=33√abc.
Therefore, we can assume that the three-phase losses of the transformer are: Qa=Ia2 R, Qb= Ib2 R , Qc =Ic2 R, where Ia, Ib, Ic are the secondary load phase current of the transformer, and R is the phase resistance of the transformer. Then the loss expression of the transformer is as follows:
Qa+Qb+Qc≥33√[(Ia2 R)(Ib2 R)(Ic2 R)]
It can be seen that when the load of the transformer is constant, when Ia=Ib=Ic, that is, when the three-phase load reaches equilibrium, the loss of the transformer is minimized.
Then transformer loss:
When the transformer three-phase balanced operation, that is, Ia=Ib=Ic=I, Qa+Qb+Qc=3I2R;
When the transformer is operating at maximum imbalance, ie Ia=3I, Ib=Ic=0, Qa=(3I)2R=9I2R=3(3I2R);
That is, the maximum loss is equal to three times the balance.
(2) Three-phase load imbalance may cause serious consequences of burning the transformer:
In the above imbalance, the heavy-load phase current is too large (increased by 3 times), and the overload is excessive, which may cause overheating of the winding and the transformer oil. The winding is overheated and the insulation aging is accelerated; the transformer oil is overheated, causing oil quality deterioration, rapidly reducing the insulation performance of the transformer, reducing the life of the transformer (the temperature is increased by 8 ° C, the service life will be reduced by half), and even the winding is burned.
(3) Unbalanced operation of the three-phase load will cause the zero-sequence current of the transformer to be too large, and the temperature rise of the local metal parts will increase:
In a three-phase load unbalanced operation, the transformer will inevitably produce zero-sequence current, and the existence of zero-sequence current inside the transformer will produce zero-sequence flux in the core. These zero-sequence flux will be in the tank wall of the transformer. Or a loop formed in other metal components. However, the design of the distribution transformer does not consider these metal components as the magnetic conductive components, and the hysteresis and eddy current loss caused by these causes the components to generate heat, causing the temperature of the local metal parts of the transformer to rise abnormally, which may cause a transformer operation accident.
2. Impact on high voltage lines
1) Increase the high voltage line loss:
When the low-voltage side three-phase load is balanced, the high-voltage side of 6~10k V is also balanced. The current of each phase of the high-voltage line is I, and the power loss is: ΔP1 = 3I2R
The three-phase load imbalance of the low-voltage grid will be reflected to the high-voltage side. In the maximum unbalance, the high-voltage corresponding phase is 1.5I, and the other two phases are 0.75 I. The power loss is:
ΔP2 = 2(0.75I)2R+(1.5I)2R = 3.375I2R =1.125(3I2R);
That is, the power loss on the high voltage line is increased by 12.5%.
(2) Increase the number of trips on high-voltage lines and reduce the service life of switchgear:
We know that high-voltage line overcurrent faults account for a considerable proportion, because the current is too large. The unbalanced three-phase load of the low-voltage power grid may cause the current of a certain phase of the high voltage to be too large, which may cause the over-current trip of the high-voltage line to be cut off, causing a large-scale power outage accident, and the frequent tripping of the switching equipment of the substation will reduce the service life.
3. Impact on distribution panels and low voltage lines
(1) Three-phase load imbalance will increase line loss:
Three-phase four-wire power supply line, the load is evenly distributed to the three phases, the current of each phase is I, the neutral current is zero, and the power loss is: ΔP1 = 3I2R
In the maximum imbalance, that is, a phase is 3I, the other two phases are zero, the neutral current is also 3I, and the power loss is:
ΔP2 = 2(3I)2R = 18I2R = 6(3I2R);
That is, the power loss at the time of maximum unbalance is six times that of the balance. In other words, if the maximum imbalance is 1200 kWh per month, only 200 kWh is lost during the balance, so that the potential for the loss of the three-phase load can be adjusted.
(2) Unbalanced three-phase load may cause serious consequences of burning the line and burning the switchgear:
In the above imbalance, the heavy-load phase current is too large (increased by 3 times) and the overload is excessive. Since the calorific value Q=0.24I2Rt, the current is increased by 3 times, and the calorific value is increased by 9 times, which may cause the temperature of the phase wire to rise linearly, so that it is blown. And since the neutral wire cross-section should generally be 50% of the phase line cross section, when it is selected, some tend to be small, and the joint quality is not good, so that the wire resistance increases. The probability of a neutral wire being blown is higher.
In the same way, on the power distribution panel, the switch heavy load phase burnout, the contactor heavy load phase burnout, and the whole machine damage and other serious consequences.
4. Impact on power supply enterprises The power supply enterprises will directly manage the power supply enterprises, and the loss of the low-voltage power grid will reduce the economic benefits of the power supply enterprises and even cause the power supply enterprises to lose money. The loss of the farmer's electric contractor area, the line loss of the high-agricultural electrician bonus was deducted, and even the wages were not available, which will inevitably affect the mood of the agricultural electrician, but the work is negative, but in order to get money and crime.
The transformer is burned, the line is blown, and the switchgear is burnt. On the one hand, the power supply cost of the power supply enterprise is increased. On the other hand, power outage maintenance and purchase replacement cause long-term power outage and less power supply, which not only reduces the economic benefits of the power supply enterprise, but also affects The reputation of the power supply company.
5. Impact on users Three-phase load imbalance, one-phase or two-phase distortion, will increase the voltage drop in the line, reduce the power quality, and affect the user's electrical use.
If the transformer is burned, the line is blown, and the switchgear is burned out, it will affect the user's power supply. It will cause inconvenience, but it will cause greater economic losses. For example, if the power outage causes the death of the animals and plants, or the contract cannot be punished according to the contract. Neutral line blows can also cause accidents in which a large number of low-voltage electrical appliances are burned.