Distribution system and power quality

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5_ECE5750_Feeder_Analysis_7April20203.pdf

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ECE 5750 Distribution System & Power Quality

Part 5: Distribution Feeder Analysis & Overview of distribution automation, substation design,

role of capacitors, renewable generators, and energy storage systems

Instructor: Dr. Ha Le Department of Electrical and Computer Engineering

California State Polytechnic University, Pomona

What will be presented? 1. Distribution Feeder Analysis (focus)

 Power-flow analysis

 Ladder iterative technique

2. Overview (some basic concepts)

 Overview of feeder automation and substation design

 Role of capacitors, renewable generators, and energy storage systems in distribution system

Reading: DS textbook Chapter 10 and Gonen book

Distribution Feeder Analysis

Analysis of distribution feeder

 Study of the feeder under normal steady-state operating conditions (power-flow analysis).

 Study of the feeder under short-circuit conditions (short-circuit analysis).

 In this course, we focus on power flow analysis

 Technique for power flow: An iterative technique based on modification of the “ladder” network theory of linear systems.

What does power flow determine?

1) Voltage magnitudes and angles at all nodes

2) Power flow in each line section (e.g. real and reactive power, current magnitude and angle).

3) Power loss in each section

4) Total feeder input kW and kVAR

5) Total feeder power loss

6) Load kW and kVAR

Ladder network iterative method (1)

54321 Z45Z34Z23Z12

ZL2 ZL3 ZL4 ZL5

II2 I 3I23I12 I34 I45 I4 5VS +

Solution:

Assumed that all of the line impedances and load impedances are known along with the voltage (VS) at the source.

1. Forward sweep: Calculating the voltage at node 5 (V5) under a no-load condition. With no load currents, there are no line currents and voltage drops, so V5 = VS.

Ladder network iterative method (2) 54321 Z45Z34Z23Z12

ZL2 ZL3 ZL4 ZL5

II2 I 3I23I12 I34 I45 I4 5VS +

Solution (cont.):

5 5

VI Z



4 5 45 45V V Z I 

45 5I I

34 45 4 ...I I I 

 This procedure continues until V1 is found.

V1 is compared with specified VS

sVRatio V



( )final solution currents or voltages Ratio 

2. Backward sweep: 3. Compare and scale to obtain solution

Scaling is possible as the network is linear

I4 = V4 / ZL4

Non-linear network

SV 54 I45I34I12I 23I 3I2I I

12Z 23Z 34Z 45Z1 2 3 4 5

S2 3S 4S 5S

Solution: Applying the procedure for linear network with modification

n n

SI V  

    

1. Step 1

Use backward sweep to compute V1 . If V1 differs from VS more than specified tolerance 

2. Step 2

Use (a) specified source voltage VS; (b) the line currents from the previous sweep, then

Second forward sweep: Compute V2, V3, V4, V5

3. Step 3: Using the new voltage V5, perform 2nd backward sweep to find new V1 .

The forward and backward sweep process continues until

1SV V specified  

Use voltages from the 1st forward sweep to calculate

Practice: Iterative method for non-linear network Kersting Example 10.1: Use the modified ladder method to compute the load voltage for the single‐phase lateral shown below. The source voltage at node 1 is 7200 V. Line segment 1–2 impedance:

Line segment 2-3 impedance:

Loads:

Solution: Iterative method … (1) Set initial conditions:

The first forward sweep:

Vold = V3

Solution: Iterative method … (2) The first backward sweep:

The current flowing in the line segment 2–3 is

The load current at node 2 is

Solution: Iterative method … (3) The current in line segment 1–2 is

The second forward sweep:

Solution: Iterative method … (4)  At this point, the second backward sweep is used to

compute the new line currents. This is followed by the third forward sweep.

 After 4 iterations, the voltages have converged to an error of 0.000017 with the final voltages and currents of

Typical distribution feeder (1)

Solid line: Overhead lines Dashed lines: Underground lines

Typical distribution feeder (2)

 All series elements (lines, transformers, and regulators) can be represented by their models.

 The line between nodes 3 and 4 and between 4 and 5 have “distributed” loads.

 One way to model the distributed loads is to combined them into a lump load which is then connected at the center.

Model for series components

Model for shunt components The shunt components of a distribution feeder are spot static loads.

1) Spot static loads: Induction machines, capacitor banks. An induction machine is modeled using the shunt admittance matrix (not covered). Capacitor banks are modeled as constant admittances.

2) Spot static loads can be modeled as constant complex power, constant current, constant impedance, or a combination of the three.

3) Line: Most of the time, the shunt capacitance of the line segment can be ignored. However, for long underground line segments, the shunt capacitance should be included.

 For all the loads, the “node” currents may be 3-phase, 2- phase, or 1-phase.

Ladder iterative technique 1) Set initial conditions and desired tolerance.

2) 1st forward sweep: Assuming no-load condition, calculate voltage at all nodes except for source node (VS) whose voltage is specified.

3) Compare computed source voltage to specified source voltage VS. If specified tolerance is not met, continue to Step 4.

4) 1st backward sweep: Calculate all line currents using the specified load power and node voltages obtained in the 1st forward sweep.

5) 2nd forward sweep: Using specified VS and the line currents obtained from the 1st backward sweep, calculate new node voltages. Compare computed source voltage to specified source voltage VS (i.e. go to Step 3).

6) Repeat forward and backward sweeps until computed and specified source voltages are within the desired tolerance.

Example: Feeder analysis Kersting Example 10.2: Consider a simple distribution feeder shown below. The “source” line segment from node 1 to node 2 is a 3-wire delta 2000-feet long line. The “load” line segment from node 3 to node 4 is 2500 ft long and is a 4-wire wye with a neutral.

Phase impedance matrices for two line segments:

Load

Infinite Bus

[ZeqS] [ZeqL]

[Iabc][IABC]

21 3 4

3-P TRF consists of three 1-phase TRF each rated:

3‐phase wye‐connected constant PQ load:

Solution for Example: Feeder analysis (1)  The forward and backward sweep matrices must be computed

for each series element. The modified ladder method use only [A], [B], and [d] matrices.

 Specified tolerance: 0.001 pu

Source line segment with shunt admittance neglected:

Solution for Example: Feeder analysis (2)

Solution for Example: Feeder analysis (3)

Load line segment with shunt admittance neglected:

Solution for Example: Feeder analysis (4) Transformer: Converting TRF per unit impedance to Ohms referenced to the low-voltage windings:

Solution for Example: Feeder analysis (5) Transformer: Sweep matrices

Solution for Example: Feeder analysis (6)

Solution for Example: Feeder analysis (7)

Node 4 loads: complex power

Infinite bus line-to-line and line-to-neutral voltages:

Solution for Example: Feeder analysis (8) Defining L-N voltages and load at Node 4: V4 lags ELNs by 30 deg.

1ST ITERATION: Backward sweep Load currents at Node 4

Voltages and currents at Node 3:

Solution for Example: Feeder analysis (9)

Voltages and currents at Node 2:

Solution for Example: Feeder analysis (10) Equivalent LN voltages and line currents at Node 1:

L-L voltages at Node 1:

Solution for Example: Feeder analysis (11)

Magnitude of the line-to-line voltage errors

 These errors are greater than the specified tolerance of 0.001 per unit.

 Forward sweep: It uses the equivalent L-N voltage at the source as the Node 1 voltage and calculates V2, V3, V4 using the line currents from the previous sweep.

Load

Infinite Bus

[ZeqS] [ZeqL]

[Iabc][IABC]

21 3 4

Solution for Example: Feeder analysis (12)

This completes the first iteration.

Solution for Example: Feeder analysis (13)

 2nd ITERATION begins by computing the new currents at the Node 4 load using the new values of the Node 4 voltages.

 The forward and backward sweeps continue until the error at the source is less than the specified tolerance of 0.001 pu.

 The Mathcad program is used to analyze the system, after 8 iterations, the load voltages at Node 4 are:

Solution for Example: Feeder analysis (14)

The flowchart of a Mathcad® program

Load voltages V4 on a 120 V base are:

The voltages at node 4 are below the desired 120 V.

Hence, three step-voltage regulators connected in wye on the secondary bus (node 3) of the substation to raise the load voltages.

Solution for Example: Feeder analysis (15) The new configuration of the feeder after the step-voltage regulator installation: The regulator is treated as an additional component between Node 3r and 3.

Potential transformer ratio = 2400 / 120 V i.e. Npt = 20

Rated current of the transformer bank:

The CT ratio is selected to be 1000 / 5 = CT = 200

Solution for Example: Feeder analysis (16) R and X settings of the compensator: The equivalent phase impedance between node 3 and node 4 is computed using the converged voltages at the two nodes:

The three regulators are to have the same R and X compensator settings.

Solution for Example: Feeder analysis (17) With the regulator in the neutral position, the input voltage to the compensator is based on the converged voltage from the previous iteration:

Compensator current:

Voltages across the voltage relays:

Solution for Example: Feeder analysis (18)

 Assume that the voltage level has been set at 121 V with a bandwidth of 2 V. The regulators will change taps until the phase voltage is at least 120V.

 The taps setting are:

Solution for Example: Feeder analysis (19) Using the taps 9, 12, 13, the sweep matrices for the regulators are:

All other matrices are zero.

Solution for Example: Feeder analysis (20)  Check the load voltages with the regulator set at the obtained

taps.

 The same modified ladder iterative technique is repeated as before. The only change is that the regulators are included as an added element between Nodes 3r and 3.

 The system converges after 4 iterations. The load voltages at Node 4 on a 120-V base are:

 The load voltages at Node 4 are within the desired limits.

Sweep equations including regulator The regulators are included as an added element between Nodes 3r and 3:

Forward sweep:

Backward sweep:

Tap changing routine

Computational flowchart

The computational sequence for the determination of the final tap settings and convergence of the system.

Load allocation  So far we compare the specified source voltage VS to the

computed source voltage to determine if the tolerance is acceptable.

 However, usually the input complex power (kW and kVAR, total 3- phase or per phase) to a feeder is known via metering at the substation. Then, it is desirable to force the computed input complex power to the feeder matching the metered input. The steps are:

1) Compute Ratio = metered input / computed input

2) Scale phase loads by multiplying the loads by Ratio

3) Use ladder iterative method to determine a new computed input to the feeder.

4) Repeat Step 1-3 until the computed input is within a specified tolerance of the metered input.

Overview of distribution automation,

substation design, capacitors role, renewable generators

& energy storage systems

Distribution automation

2) The supervisory control and data acquisition (SCADA) system involves generation and transmission systems using significant automatic monitoring and control features.

3) The distribution automation and control (DAC) system oversees the distribution system, including connected load.

The power grid operations involve geographically dispersed and functionally complex monitoring and control systems (shown in the diagram).

1) The EMS exercises overall control over the total system.

Motivation for DS automation The need for gathering substation and power plant data and motivation for DS automation are increasing because of:

1) Increased reporting requirements of reliability councils and government agencies.

2) Operation of the electric system closer to design limits.

3) Increased efficiency requirements because of much higher fuel prices.

4) The tendency of utilities to monitor lower voltages than previously.

 Overall: Development of Smart Grid

DAC automation & control functions Utilities generally do not agree on types of functions that should be handled by a DAC. Some of the automated distribution functions correlated with locations are shown in the table below.

Vision of future grid

 Network of microgrids.

 Distributed generation (conventional, renewable such as wind, solar, geothermal etc.).

 Distributed energy storage systems (ESS): pumped- storage, compressed-air ES, battery, electric vehicles etc.

 Sensors, IT, communication systems everywhere.

 Real-time monitoring, pervasive control and automation.

Substation design Some important issues are:

1) Substation costs: Site costs, transmission cost, transformer cost, feeder bus-work/getaway costs:

2) Substation diagram: The electrical and physical arrangements of the switching and busing which are selected based on safety, reliability, economy, simplicity, and other considerations.

3) Substation location: Dictated by the voltage levels, voltage regulation, subtransmission costs, substation costs, and the costs of primary feeders, mains, and distribution TRF, and other factors.

4) Rating: Substation capacity determined based on pre-determined substation service area, system load changes in short-term and long-term planning.

Examples of substation diagram

A typical double bus–double breaker scheme A typical main-and-transfer bus scheme

Role & benefits of capacitors Role / applications:

1) Power factor correction

2) Reactive power compensation and voltage regulation

3) Application in power quality: Harmonic filters

Question: Name an application of series capacitors?

Economic benefits from capacitor installation:

1) Released generation and transmission capacity

2) Released distribution substation capacity

3) Reduced energy (copper) losses

4) Reduced voltage drop and improved voltage regulation

5) Saving on capital expenditure by delaying system expansions

6) Revenue increase due to voltage improvements etc.

Role of renewable generators & energy storage systems (1)

They are called Dispersed Storage and Generation (DSG) devices.

Examples of DSG technologies: Hydroelectric and diesel generators, wind turbines, photovoltaics (PVs), fuel cells, batteries, electric vehicles, hydroelectric pumped storage, compressed-air energy storage (CAES) etc.

 If properly planned and operated, DSG may provide benefits to distribution systems by reducing capacity requirements, improving reliability, and reducing losses.

 But, integrating them also creates lots of problems to power grid operation.

Role of renewable generators & energy storage systems (2)

Some technical, economic, and environmental benefits:

1) Reduction of environmental pollution and global warming concerns.

2) Diversify power supply sources: Renewable sources in addition to conventional (coal, oil, gas).

3) Overall grid power quality and reliability improvement.

4) Making use of local power supply, which may reduce transmission losses.

5) Enabling the development and usage of Microgrids  can increase grid reliability and customer energy independence.

Role of renewable generators & energy storage systems (3)

Some technical problems:

1) Islanding: Islanding is an undesirable condition where a Distributed Energy Resource (DER) system continues to supply power to the utility grid during a utility outage.

2) Power quality: Harmonics from converter systems used by PV and wind turbines impact power quality.

3) Power fluctuation: Intermittent flow, causing power deficit or power surplus many times per day in distribution networks, leading to increased losses, overload, voltage instability etc.

4) Reverse power flow: Many utility substations cannot handle reverse power flow. Protection setting is more difficult.

 Overall, grid operation is much harder.

References

1. S. Santoso, Fundamentals of Electric Power Quality, 2012.

2. R. C. Dugan, M. F. McGranaghan, S. Santoso, W. Beaty, Electrical Power Systems Quality, McGraw Hill 2012.

3. T. A. Short, Electric Power Distribution Handbook, 2003.

4. J. D. Glover, M. S. Sarma, T. J Overbye, Power System Analysis and Design, 5th Ed., CENGAGE Learning, 2012.

5. T. Gonen, Electric Power Distribution Engineering, 3rd ed., 2014, CRC Press, ISBN 9781482207002.

6. W. H. Kersting, Distribution System Modeling and Analysis, 3rd ed., CRC Press, 2012.

7. IEEE Std. 519-1992.

8. Other sources