2.3. Load (Power) Flow Analysis
2.3.2. Load Flow Studies in Distribution Systems
28 . 2 ( 0 V
0
22
11
J J Q P
The Jacobian matrix is only calculated at the start of the computation unlike being calculated during each iteration for the generic Newton Raphson method.
The FDLF method thus performs better than the generic Newton Raphson in terms of speed and storage space requirements [2.5, 2.9]. However, since the Jacobian matrix is calculated once, this introduces some errors, which requires more iteration to reach the same level of accuracy as the generic method. The quadratic convergence property is also lost at this point. For lines with high resistance to reactance ratio, errors due to this assumption become overly large thus making the technique unreliable [2.10]. In summary, it is seen that the FDLF method is useful in making approximate fast estimations, the full Newton Raphson method is better suited [2.7, 2.10].
2.3.2. Load Flow Studies in Distribution Systems
The modern day distribution system in contrast to a traditional one comprises both generation and load in the system. This makes the concept of load flow in distribution systems even more challenging since they now have to cope with bidirectional power flow and a higher level of uncertainties. Distribution systems have unique features which distinguish them from transmission systems. Some of these features include [2.11-2.14];
i. Radial or a weakly meshed nature
ii. High resistance to reactive impedance ratio (R/X)
16 iii. Unbalanced operation state iv. Multiphase
v. Dispersed generation
vi. Large number of branches and nodes
Considering some of the assumptions, the traditional Newton-Raphson method and the Fast decoupled method see the distribution system as “ill-conditioned”
hence they result in poor convergence [2.11]. In fact, it was observed to diverge in [2.11] in most of the cases considered. In the same vein, the conventional Gauss-Seidel method is extremely inefficient in solving large power systems, which is a typical feature of distribution systems [2.11]. In general, load flow techniques employed in transmission systems cannot be directly applied to distribution systems.
In view of the shortcomings of the classical methods (for transmission systems), other methods have been proposed to efficiently solve the distribution load problem while fully considering its peculiarities. Several methods [2.11-2.30] have been proposed for carrying out load flow studies in distribution systems. Generally, most of these methods are based on backward-forward substitution, modified Newton Raphson, and Gauss approach.
However, most of the methods require some form of backward-forward substitution in attaining convergence.
The Gauss Zbus approach is one of the earliest techniques used for the distribution load flow problem [2.15]. The approach uses the Ybus matrix (which is sparse) and current injections to solve the network equations. The ZBR
approach was introduced in [2.16]. Unlike the previous Ybus technique, it utilizes the branch impedance matrix, the current injection alongside a branch-path incidence matrix in solving the network equations [2.16].
Improved Newton Raphson methods have also been employed for load flow studies [2.17-2.20]. In [2.17] and [2.18], a Newton Raphson method with a reduction in the Jacobian matrix (to save time) was proposed. A current injection based on Newton Raphson method is employed in [2.19] for the load
17
flow studies. In [2.21] a sequence based load flow using Newton Raphson was proposed for solving load flow in unbalanced distribution systems. One drawback of the Newton Raphson based technique is the need to evaluate the Jacobian matrix during each iteration process [2.9]. In addition, its inherent convergence problem still occurs for systems with high unbalance and loading [2.11].
With the Backward-Forward sweep (BFS), the current or power injected into a branch is computed relative to the end voltage while the voltage drop is evaluated in the forward sweep using the calculated current and or power. In [2.11, 2.12], the Backward-Forward sweep method with a layered structured branch numbering is employed. However, because the method can only cater for radial distributions, it was extended to the weakly meshed network in [2.11]
using the breaking point mechanism. The ladder network, as employed in [2.13] is based on BFS. The models for representing various transformers, voltage regulators, capacitors, etc. are detailed in [2.13]. A technique called the fast and flexible radial power flow (FFRPF), which is a variant of BFS, is used in [2.14]. Here, bus and branch related matrices are formed and used in updating the voltages and currents. BFS methods with improvements (e.g. in data handling, bus numbering etc.) are also proposed in [2.22-2.24]. All these techniques are derivative free unlike the Newton Raphson based methods;
rather, they employ simple circuit analysis which makes them attractive.
Other methods such as the direct approach [2.25-2.26] have also been previously introduced. With the direct approach, two main matrices are required; the first relates the branch current to the load current while the other matrix reflects the relationship between the branch current and the bus voltage.
Although the technique is simple, fast and straightforward, it does not accommodate unbalanced or distributed load [2.25].
Of the techniques mentioned, back-forward schemes are mostly used due to their simplicity and non-requirement of a Jacobian matrix, amongst other factors. This informs its use as the benchmark for other distribution system load flow methods [2.16]. An algorithm based on the BFS scheme is discussed further in the succeeding section.
18 2.3.2.1. The Backward Forward Sweep
The procedure for the distribution load flow described below is adapted from the back-forward sweep and the ladder network as presented in [2.13] and [2.27]. The current is evaluated in the backward sweep while the voltage is updated in the forward sweep. The method is based on child-parent branch identification fully described in this section. The procedures involved are explained in the steps below.
Step 1: Formulate a parent child matrix to identify the connected nodes before and after a node using the network information. For instance, considering a simple 8 bus test system [2.18] shown in Fig. 2.2, the parent child matrix Mpc is given by Table 2.2. The Mpc matrix is very easy to form as it does not involve node renumbering, the only clear regulation is that the branch number should be one less than the value of the receiving node number. The Mpc matrix is very important for the forward and backward sweep since it gives the sending and receiving nodes.
1 2
3
4
5
7 8
6 1
3 2
5
6 7
4
1 1
Branch Number Node Number
Fig 2.2: A Simple 8 Bus Distribution Test System
Table 2.2: The MPC Matrix for the 8 Bus Test System Node Parent Children
1 - 2
2 1 3,4,7
3 2 -
4 2 5,6
19 distributed generation etc. These currents are calculated depending on the type of load (constant impedance, constant current and constant power) and the load connection type (Delta or Wye). Detailed explanations are found in [2.13] and [2.27].
Step 3: The backward sweep is carried out to update the current in the branches working from the end node towards the source node. A single line diagram of the distribution line segment is shown in Fig. 2.3. The update formula used is dependent on the connector (distribution line, transformer or switch) between the nodes. The update formulae for nodes connected with distribution lines are given in (2.29)-(2.32), while (2.33) and (2.34) are respectively for nodes connected by switches or transformers [2.13]. The values for the coefficients ct
and dt in (2.3.4) vary depending on the transformer configuration. These values are given in Appendix A for wye-wye and delta-wye transformers, values for other configuration can be found in [2.13].
Distribution Lines:
Switches and Transformers:
(2.33) generators, capacitors and loads on node k. Ikp gives the nodal current on bus k
20
plus currents from children branches, Ik represents the branch current for line k, Vk is the voltage on node k, Zk and Yk are respectively the impedance and admittance matrix for line k, ct and dt are coefficients dependent on the transformer configuration and U is a unit matrix.
Zk
1/2Yk 1/2Yk
IGCL Vk-1
Ik
Vk Ikp
ChildrenBranches
Fig 2.3: Single Line Diagram of the Distribution Line Segment
Step 4: In the forward sweep, the main aim is to evaluate the voltage drop across the lines in order to estimate the voltage at each of the nodes. The voltage is evaluated starting from the source node and gradually moving towards the end node. This is evaluated using (2.35) [2.13].
(2.35) ] ][
[ ] ][
[ k 1 k
k A V B I
V
The parameters A and B are determined based on the type of connection between the nodes. The values of A and B for grounded wye-grounded wye and delta-grounded wye transformers are presented in Appendix A while those for connection types can be found in [2.13]. For nodes connected by switches, a zero drop is assumed across the switch, henceVkVk1.
Step 5: Once the backward and forward sweeps are completed, the convergence test is carried out to ensure the errors are within the limits. The iteration is terminated once the errors between the voltage (on all phases) from the previous iteration and the current iteration are within the prescribed limits.
21 Backward Sweep Calculate branch currents using
(2.29)-(2.34)
Forward Sweep
Evaluate nodal voltages using (2.35) Calculate total nodal currents from all
components such as loads, shunt capacitor, distributed generators etc
Set iteration counter k=0 Formulate the parent
child matrix Mpc
Check if maximum mismatches for all phases
and nodes
are less than the tolerance;
Vmax<
Calculate line flows, power losses
Output results for voltage angle and magnitude, power
flow and losses Read and input load flow data
k=k+1
Yes
No
Fig 2.4: Backward-Forward Sweep for Distribution Load Flow Study
A summary of the load flow process using the backward-forward sweep method is presented in Fig. 2.4.