Energy storage device – reduce the cost of distribution transformer loss and reduce the cost of power failure

Energy storage device - reduce the cost of distribution transformer loss and reduce the cost of power failure

  1. Reduce the cost of distribution transformer loss

The loss of the transformer mainly includes no-load loss and short-circuit loss (load loss). No-load loss, also known as iron loss, is the loss generated when the magnetic flux changes periodically due to the excitation current in the iron core under the rated voltage of the transformer, so it is called iron core. Loss, as long as the transformer is running, no matter what its power is, this core loss exists and is constant, no-load loss includes hysteresis loss, eddy current loss and additional loss. The short-circuit loss is also called the load loss. When the transformer is in the rated operation state, the primary and secondary windings both flow the rated current, and the loss generated in the winding is the short-circuit loss. Short-circuit loss can also be measured by short-circuit test, that is, short-circuit the secondary winding of the transformer, add the power supply voltage to the primary side, and when the rated current passes through the primary winding, the power consumed by the transformer is the short-circuit loss. The basic short-circuit loss is mainly proportional to the square of the rated current. In addition, the circulating current loss between the winding wires and the skin effect caused by the leakage magnetic field increase the copper loss of the effective resistance of the wire, which can be called the additional loss in the short-circuit loss.

The user’s distribution transformer loss is an important part of its total power loss, and the loss affected by the load size is its copper loss, which is proportional to the square of the load rate. The charging of the energy storage system during the valley load is equivalent to increasing the load rate of the distribution transformer, which increases the loss of the distribution transformer when the user is under the valley load. Variable losses are reduced. Since the transformer loss is included in the user’s electric energy meter, due to the existence of the peak-to-valley electricity price difference, the role of the energy storage device in reducing the cost of distribution and transformer loss is also more obvious. Therefore, its corresponding annual income E can be expressed as Figure 1:

where P, – the load power in the i-th hour;
P.–short-circuit loss of distribution transformer; Sy-distribution transformer capacity (MVA);
cosq–The power factor on the load side of the transformer.

  1. Reduce the cost of power outages

Power outage losses refer to all the economic losses borne by the society when the power supply is not completely reliable or expected to be not completely reliable (that is, when a power outage or blackout occurs due to interruption or insufficient power supply). Including the economic losses of power companies. For the user’s power outage loss, it is generally divided into direct outage loss and indirect outage loss. The former refers to the loss borne by the actual power outage and for a period of time afterward, while the latter refers to the user’s adjustment of its activities in order to reduce the impact of the power outage. The additional cost paid, or the cost of employing backup power, is usually determined by the short-term effects of unforeseen outages for direct outages, and the longer-term considerations in anticipation of outages that are indirect. The power outage loss of users is mainly affected by the following factors:

1) The category of the user. Different types of users have different power consumption methods and power outage characteristics. When researching power outage losses, it should be micro-directed for specific users. That is, for the same type of users, the power outage loss caused by each kWh of power shortage may also be different.

2) The time when the outage occurred. Because the user’s power consumption is temporal, the power outage loss is also related to the specific time of the power outage. For example, for residential users, because the main power consumption time of the day is in the first half of the night, the loss caused by the power outage during the day or the second half of the night to the residents The inconvenience is significantly lower than the impact caused by the power outage in the first half of the night. For industrial users whose production has seasonal characteristics, the power outage loss will also change with the change of seasons.

3) Frequency of power outages. Obviously, the more power outages, the more serious the damage.
4) The duration of the power outage. In general, the power outage loss increases with the extension of the outage time.

Furthermore, the consequences of a sudden or unprepared power outage are far more severe for users than a planned or pre-announced outage. Because if you are informed in advance that there will be a power outage, the user can reduce the possible direct outage losses to indirect outage losses by adjusting activities, strengthening precautions, or using other energy sources.

One of the main methods to evaluate the power outage loss of users is to use the power outage loss evaluation rate (IEAR) to solve the problem. IEAR is defined as the economic loss caused by the interruption of power supply to the user due to the lack of unit electricity, and is an index to evaluate the reliability level of the system. IEAR indicator, the power outage loss OC of the system during the study period can be calculated according to OC=IEAR XEENS, and EENS (Expected Energy Not Supplied) is the expected value of the system’s power shortage during the study period.

The IEAR index can be determined for the load point or the system. This project mainly takes important user points as the research object, and evaluates the user’s power outage loss by estimating its IEAR index. figure 2:

In the formula, A, – the average failure rate of the user point;
rp – the average failure duration of the user point;
-ay–The average missing load of the user at the time of failure is the average load of the user point.
The denominator of formula (7-5) represents the power shortage of the user under the fault, the numerator represents the corresponding expected value of the power outage loss, and c, (r, ) represents the unit power outage loss of the user point under the fault duration r, .
For large and medium-sized users who have high requirements for power supply reliability, the losses caused by power outages include two aspects: the operating income caused by the power outage during the outage (which can be calculated by the power outage loss evaluation rate) and the product scrap caused by the power supply interruption ( This is related to the frequency of power outages in the regional power grid). As a UPS, the battery energy storage system can realize the switch from the mains to the UPS in milliseconds, preventing the product from being scrapped due to short-term power interruption. At the same time, during the mains power failure, the power supply can be continued to reduce the loss of power shortage. The resulting annual return E can be expressed as Figure 3:

Among them Figure 4:

In the formula, REA is the evaluation rate of the user’s power outage loss;
ENs – the expected value of the user’s power shortage caused by each power outage;
T, – the user’s production hours per year;
A, – the reliability of power supply of the distribution network;
Po-user guarantees the minimum power supply required for normal production;
w, – the amount of electricity remaining in the energy storage device in the ith hour;
a, – The power failure rate of the power supply on the bus side of the user when the energy storage device is not put into use;
n’, – the power failure rate of the system after the energy storage device is put into the distribution bus (1/a); A – the failure rate of the energy storage device (1/a);
n, – the repair time of the distribution network and the repair time of the energy storage device (a);
E- Expected value of economic loss to users caused by each power interruption.

piW, <EENs) is the probability that a power outage accident occurs when the remaining power is less than Es after the energy storage device is put into operation (that is, it is considered that the energy storage device cannot provide enough power to support continued production at this time), w, and the operation of the energy storage device The strategy is related, and its operating strategy is determined according to the peak/valley/normal period, so the calculation of p(W, <EENs) is related to the division of the peak/valley/normal period. Figure 5 shows the time-of-use electricity price of the user in each time period. In order to facilitate the establishment of the expression of w, it is taken at 22 o’clock in the valley period (the stored electricity is completely released at this time, and a new round of charging and discharging starts)
From 21:00 to 21:00 the next day, i take respectively, then W,=(P-P).

Figure 5 - Time-of-use electricity prices for users in different time periods
Figure 5 – Time-of-use electricity prices for users in different time periods

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