With the continuous development of urban distribution network, most of the distribution network has formed a dual power supply mode. The traditional way of distribution network load operating mainly adopts the power method, while satisfied the requirements of the safety of power grid, but will cause external short time power outages, poor user experience, realizing the distribution network outage rearrangement of load is a necessary means to improve power supply reliability. This paper presents mathematical model of load transfer in distribution network. The differences of voltage of amplitude and phase angle on both sides are calculated by the power flow. According to the differential pressure to determine whether the loop can be closed at the loop operation, thereby improving the success rate of operation.
Along with the progress of the construction of the power grid, the grid of 35 kV and 10 kV distribution network structure gradually perfect, the most of the region has achieved annular space truss structure. Under normal operation, the ring network of distribution network are somewhere open loop operation, distribution network load by different main power supply, when the main power supply and distribution network components need power outage overhaul or adjust the operation mode, often need to pour the distribution network load to another power supply. Distribution network before electricity use “cold” means more, namely power brake way, this way will interrupt the power supply to users, depending on the operating mode and the complexity of interruption of power supply in the time range from a few minutes to half an hour Power short stop will cause greater damage to the user, before the power failure also need a lot of user related communication and coordination, seriously affecting the maintenance work schedule and efficiency.
Literature [
According to the wiring diagram can induce distribution network, 10 kV distribution network closed loop operation ways mainly have the following kinds.
The current load distribution model of the distribution network has a variety of ways, such as the load evenly distributed, the load at the beginning or end of the line, etc. In this paper, when the load model is established, the load comb structure of the distribution network is fully considered, and the load is evenly distributed on the distribution network. Then, the load transfer is carried out by mathematical method.
Assuming a uniform line of length L, the load S is evenly distributed over the line. If the load is moved to the line on both sides, computation formula is as follows.
S i = S j = 1 2 S (1)
In the Formula (1), S i and S j are the load values equivalent to S at both ends of the line.
At present, the level of distribution automation is limited; most of the distribution load cannot be collected by SCADA system. SCADA system can only collect substation 10 kV outlet load. Therefore, the distribution load model is processed by the following method.
In
SCADA system can get the power of the switch, but cannot collect the load of the line. In this paper, the load is distributed by the coefficient method. The calculation formula of the load Si is as follows:
S i = S n i ∑ i = 1 n − 1 S n i ( S 0 + S n ) (2)
According to the (State Grid Corporation of the power system voltage quality and reactive power management regulations [
If the breaker in
{ | V 1 − V 2 | V n < 20 % | θ 1 − θ 2 | < 25 ∘ (3)
Equation (3) is the criterion for the loop operation.
There are two main reasons for the distribution of loops [
The excitation in the calculation model is the phase voltage of the L1 phase.
E ˙ = Δ u ˙ ∠ 30 ° 3 (4)
Δ u ˙ is the voltage difference on both sides of the contact switch.
Δ u ˙ = U ˙ 1 − U ˙ 2 (5)
The instantaneous value of the loop current should satisfy the following differential equation.
E max sin ( ω t + α ) = R i + L d i d t (6)
α is the initial phase angle of E ˙ . It is determined by the phase angle difference between the voltages on both sides of the initial time.
The calculation of the surge current is as follows.
i ( t ) = − M e − ( R / L ) t sin ( α − φ ) + M sin ( ω t + α − φ ) (7)
M = E max R 2 + ω 2 L 2 is the amplitude of the loop current component. φ = a r c t g ( ω L R ) is the phase angle between the periodic component and φ .
The maximum surge current I M is calculated as follows.
I M = 1.62 ( I m 2 )
I m is the amplitude of the closed loop steady current
Power flow calculation is one of the most basic and important calculations in power system analysis. Typical computational methods include Gauss-Seidel method, Newton-Raphson method and Van Amidogen’s BX universal fast decoupling method. The fast decoupling method overcomes the problem that the R > x network cannot converge. In this paper, the fast decoupling method is used to calculate the power flow [
Fast decoupling method is the main method of power flow calculation. The basic idea is that the node power is expressed as the polar coordinate formal of the voltage vector, and the active power error is used as the basis for correcting the voltage vector angle, the reactive power error is used as the basis for the correction voltage amplitude. The iterations of active power and reactive power are carried out separately. The characteristics of fast decoupling method are to improve the computing speed, lower demand for computer storage capacity.
Active power imbalance equation is
Δ P i = P i s − P i = P i s − V i ∑ j = 1 n V j ( G i j cos δ i j + B i j sin δ i j ) = 0 ( i = 1 , 2 , ⋯ , n − 1 ) (8)
Reactive power imbalance equation is
Δ Q i = Q i s − Q i = Q i s − V i ∑ j = 1 n V j ( G i j sin δ i j − B i j cos δ i j ) = 0 ( i = 1 , 2 , ⋯ , m ) (9)
In this paper, two 10 kV lines in Deyang power grid are used to verify the correctness of the load model. In the loop mode, all the equipment in the loop is not
overloaded. During the whole operation, the 10 kV bus voltage of the substation within the ring does not allow the limit.
Surge current is transient variable. It does not last more than five cycles, so most of the non-periodic component decays to zero within 0.1 s.
Through the study of the modeling and simulation of power grid connection diagram in
From
Using the distribution network load model to do power flow calculations put forward in this article, and calculate the difference between the phase angle and the voltage difference on both sides of the loop. This difference compared with closed loop conditions, and then determines whether can be closed loop operation. The simulation of typical grid, to validate the distribution network load of the simplified model is reasonable.
Yin, Q., Ding, R., Zhao, Y.C., Liu, Y.F. and Liu, X.L. (2017) The Feasibility Research on Distribution Network Closed Loop Based on the Load Transfer Model. World Journal of Engineering and Technology, 5, 12-20. https://doi.org/10.4236/wjet.2017.54B002