ERACS Power Network Model

• Report title: Computer Aided Power System Design and Analysis: Using ERACS to study the effects of various components in a power network

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Anwar Mukhtar Salim ; 10035293 EN 0712 Assignment Page i

Table of Contents LIST OF FIGURES ................................................................................................................................................ ii

INTRODUCTION ................................................................................................................................................. 1

NETWORK CONSTRUCTION AND ASSUMPTIONS MADE...................................................................................... 1

ANALYSIS AND RESULTS ..................................................................................................................................... 2

UNBALANCED FAULT CALCULATION................................................................................................................... 3

ANALYTICAL PROCEDURE FOR CALCULATING THE FAULT CURRENT AND VOLTAGE AT BUS 4 ............................. 4

COMMENTS ................................................................................................................................................... 6

EFFECT OF INSTALLING A VAR COMPENSATOR ................................................................................................... 7

EFFECT OF INSTALLING A WIND TURBINE ........................................................................................................... 8

DISCUSSION ....................................................................................................................................................... 9

i) Derivation of expression for active and reactive power flow through the line ......................................... 9

ii) Derivation of voltage drop across the line and how this voltage drop can be minimized using VAr compensator................................................................................................................................................ 10

CONCLUSION ................................................................................................................................................... 12

BIBLIOGRAPHY ................................................................................................................................................. 13




Anwar Mukhtar Salim ; 10035293 EN 0712 Assignment Page ii

LIST OF FIGURES

FIGURE 1 : LOAD FLOW ANALYSIS RESULTS .................................................................................... 2

FIGURE 2: VOLTAGE PROFILE FROM BUS1 TO BUS5 ........................................................................ 2

FIGURE 3 : FAULT LEVELS FROM BUS1 TO BUS5 ............................................................................. 2

FIGURE 4 :UNBALANCED FAULT (PHASE TO EARTH ........................................................................ 3

FIGURE 5: UNBALANCED FAULT (PHASE TO PHASE) ....................................................................... 3

FIGURE 6: LOAD FLOW ANALYSIS RESULTS WITH VAR COMPENSATOR .............................................. 7

FIGURE 7: VOLTAGE PROFILE FROM BUS1 TO BUS5 ........................................................................ 7

FIGURE 8: FAULT LEVELS FROM BUS1 TO BU5 ................................................................................ 7

FIGURE 9: LOAD FLOW ANALYSIS RESULTS WITH WIND TURBINE ....................................................... 8

FIGURE 10: VOLTAGE PROFILE FROM BUS1 TO BUS5 ...................................................................... 8

FIGURE 11: FAULT LEVELS FROM BUS1 TO BUS5 ............................................................................ 8




Anwar Mukhtar Salim ; 10035293 EN 0712 Assignment Page 1

INTRODUCTION Electrical power systems have undergone many different changes over the last century. Starting out as local generation plants feeding loads, the power systems evolved to central generators feeding a transmission grid (Jenkins, et al., 2010). However, various concerns ranging from the need to reduce losses on transmission lines to reduction in CO2 emissions and security of supply have changed the power systems dynamics. Renewable energy sources which are closer to or integrated in the loads are now increasingly being used. This has significantly affected the documented characteristics of electrical power systems. Fault levels change with changes to the make up of an electrical power system forcing a reassessment of protection equipment. This report examines an electrical power system and the impacts that VAr compensation, embedded generation and fault conditions have on an the system. AIMS OF THE ASSIGNMENT

To gain a thorough understanding of load flow analysis, balanced and unbalanced short circuit analysis, distributed generation, reactive power compensation and power & voltage control.

To enhance the student learning process, by relating the theory and analytical procedures to computer aided techniques for system analysis and problem solving.

NETWORK CONSTRUCTION AND ASSUMPTIONS MADE

Bus: Each bus is sized according to its voltage level and fault level. The default fault levels were accepted for all buses.

Transmission lines: These were sized according to their positive, negative and zero sequence reactances. Resistance is negligible.

Distribution lines: These were sized according to their positive, negative and zero sequence resistances and reactances.

Transformer: A 30MVA transformer rating was selected based on the loads supplied and then sized according to the winding reactances given. The resistances were calculated from the X/R ratio given. Neutral earthing data was selected with zero neutral earthing resistance and reactance.

Tap changer: A tap changer with 1.0p.u. target voltage at Bus 4 was selected and turned on.

Loads: Loads were configured from the MW/p.f selection and sized appropriately. Grid: The grid was sized based on the grid fault in feed of 2000MVA and X/R = 10





ANALYSIS AND RESULTS Active and Reactive Power Flow: The grid supplies 76.084MW and 67.957MVAr to the

network. The loads absorb 76MW and 61.9MVAr of the power supplied. The transmission lines and the transformer absorb the balance. The transmission lines have no resistance and thus consume no real power.

Voltage Profile: The voltage starts from 1.0p.u at bus 1 and drops to 0.934p.u at Bus 3. The effect of the transformer and tap changer raises the voltage to 1.005p.u at bus 4 and it decreases to 0.989p.u at bus 5 due to the loads and line resistance a shown in the figures below.

Fault Levels: The fault levels decrease as we move down the network from 2000MVA at Bus 1 to 156.83MVA at Bus 5 as shown below. This is because the fault level at high transmission voltages is higher than that at low voltages (Weedy, 1987).

Figure 1 : Load flow analysis results

Figure 2: Voltage Profile from bus1 to bus5 Figure 3 : Fault levels from bus1 to bus5

Load34MW 0.8pf

Load30MW 0.7pf

132/11KV

Load8MW 0.9pf

0.88

0.9

0.92

0.94

0.96

0.98

1

1.02

pV (pu)

0

500

1000

1500

2000

2500

3F (MVA)





UNBALANCED FAULT CALCULATION Phase to ground fault: An unbalanced fault calculation was conducted using ERACS.

The simulation at Bus 4 between phase and earth caused a current surge at that point from 0.464KA to 17.203KA on the phase with the fault. The current at the other phases dropped to zero. The voltage of the affected phase VL1 dropped to zero.

Phase to phase fault: This caused the current on the red phase IR to drop to zero and for the yellow and blue phases IY and IB to surge to 11.87KA. The voltage at VR was double the voltage at VY and VB. The result is shown below

Figure 4 :Unbalanced Fault (Phase to Earth

Figure 5: Unbalanced Fault (Phase to Phase)

Load Load30MW 0.7pf

132/11KV

Load8MW 0.9pf

Load Load30MW 0.7pf

132/11KV

Load8MW 0.9pf





ANALYTICAL PROCEDURE FOR CALCULATING THE FAULT CURRENT AND VOLTAGE AT BUS 4 Assumptions made; Resistance = 0 and the network is on no load before occurrence of the fault. The Thevenin equivalent circuit of the system at point of fault is given below;

a) MVA

base = 100MVA

Gridfault = 2000MVA Gridfault (p.u) = Grid

fault / MVAbase

= 2000/100 = 20p.u. X

grid (P.U.) = v2 (p.u) / Gridfault (p.u) = 1/20 = j0.05p.u.

Ibase = MVABase/√3.11 = 100/√3.11 = 5.249KA The positive, negative and Zero sequence networks are given below: Positive sequence:

Z1 = j0.05 + j0.08 + j0.08 + j0.15 = j0.36

Z1





Negative sequence:

Z2 = j0.05 + j0.08 + j0.08 + j0.15 = j0.36

Zero sequence:

Z0 = j0.15

The sequence network is given below:

I1 = I2 = I0 = E/ (Z1 + Z2 + Z0) = 1/( j0.36 + j0.36 + j0.15) = -j1.149

Ifault = 3I1 = 3 x (-j1.149) = -j3.45p.u

IK.A = Ifault p.u x Ibase = -j3.45 x 5.249 x 103 = -j18.1KA = 18.1∟-90 KA b) The voltages of the healthy phases at bus 4, VY and VB are calculated below; -V1 = E – (Z1 x I1); V2 = -I2Z2 ; V0 = -I0Z0 V1 = 1 – (-j1.149 x J0.36) = 1 – 041364 = 0.5864p.u V2 = - (-J1.149 x j0.36) = -0.414p.u V0 = -(-J1.149 X J0.15) = -0.172p.u





VY = a2V1 + aV2 + V0 VY = (-0.5 –j0.866) (0.5864) + (-0.5 + j0.866) (-0.414) – 0.712 VY = 0.904∟-106.6 VY (KV) = VY x Vbase = (0.904∟-106.6) x 11/√3 = 5.74∟-106.6 KV IVYI = 5.74KV VB = aV1 + a2V2 + V0 VB = (-.5 + j0.866) x (0.5864) + (0.5 – j0.866) x (-0.414) – 0.712 VB = 0.904∟106.6 VB (KV) = VB x Vbase = (0.904∟-106.6) x (11/√3) = 5.74∟106.6 IVBI = 5.74KV COMMENTS The fault current from analytical result of 18.1KA is close to 17.203KA from ERACS. The difference is because the network used in the analytical calculation is assumed to be an ideal model with no resistances. Losses are also ignored. The voltage magnitude in both the phases VY and VB are 5.74KV in the analytical method but 5.812KV and 5.67KV in ERACS. This is because an ideal model was assumed in the calculation and losses were ignored.





EFFECT OF INSTALLING A VAr COMPENSATOR A synchronous motor of 20MVAr output was used to represent a VAr compensator. This changed the voltage profile, reactive power demand and fault levels of the network. The voltage received a boost across the network and reactive power demand from the grid was reduced as compared to the first model. The fault levels of the network were significantly affected. This is shown in the figures below; Figure 6: Load flow analysis results with VAr compensator

Figure 7: Voltage Profile from bus1 to bus5 Figure 8: Fault Levels from bus1 to bu5

Load

VAr Compensator

Load30MW 0.7pf

132/11KV

Load8MW 0.9pf

0.88

0.9

0.92

0.94

0.96

0.98

1

1.02

pV (pu)

pV (pu) withVarCompensator

0

500

1000

1500

2000

2500

3F (MVA)

3F (MVA) withVarCompensator





EFFECT OF INSTALLING A WIND TURBINE The wind turbine injects active power and absorbs reactive power from the network. This results in a lower demand for active power and higher demand for reactive power. The voltage levels are also significantly affected with a voltage rise in the 11KV side because of the boost provided by the wind turbine and a drop in the 132KV side because of the increased reactive power consumption of the network. There is also a slight rise in fault levels. This is shown as compared to the original network in the figures below;

Figure 9: Load flow analysis results with wind turbine

Figure 10: Voltage Profile from bus1 to bus5 Figure 11: Fault Levels from bus1 to bus5

Load34MW 0.8pf

Load30MW 0.7pf

132/11KV

Load8MW 0.9pf

Wind Turbine

0.88

0.9

0.92

0.94

0.96

0.98

1

1.02

pV (pu)

pV (pu) withWind Turbine

0

500

1000

1500

2000

2500

3F (MVA)

3F (MVA)WindTurbine





DISCUSSION The thevenin equivalent of a transmission system and the phasor diagram are given below:

Fig. 12: Power Transmission phasor diagram:

i) Derivation of expression for active and reactive power flow through the line

From the phasor diagram above, it can be seen that:

V2 2 = (V1 + ΔV)2 + δV2

V2 2 = (V1 + IRCos ϴ + IXSin ϴ)2 + (IXCos ϴ - IRSin ϴ)2

But S = P + jQ = IVCos ϴ + jIVSin ϴ Thus, P1 = IV1 Cos ϴ ; Q1 = IV1Sin ϴ ICos ϴ = P/V1 ; ISin ϴ = Q/V1 Substituting: V2

2 = (V1 + RP/V1 + XQ/V1)2 +(XP/V1 – RQ/V1)2

Therefore: ΔV = (RP + XQ)/V1 ; δV = (XP – RQ)/V1

In a typical power system, line resistance is negligible compared to reactance (Weedy, 1996). Thus,





ΔV = XQ/V1 ; δV = XP/V1 But δ = Sin -1 (δV/V2) ; δV/V2 = Sinδ Therefore, δV= V2 Sinδ ; V2 Sinδ = XP/V1

V1V2Sinδ = XP ; P = (V1V2 Sinδ)/X Similarly, V2 – V1 = ΔV = XQ/V Therefore: V2V1 – V1

2 = XQ Q = (V2V1 – V2)/X δQ/ δV1 = (V2 – 2V1)/X This is the change in reactive power necessary to change the voltage. It can be seen from the expressions for both P and Q (active and reactive power) that the main factors are the voltages and line reactance. Thus the smaller the reactance, the more the reactive power needed and the more active power supplied.

ii) Derivation of voltage drop across the line and how this voltage drop can be minimized using VAr compensator.

From above, if δV<< V1 + ΔV

V22 = (V1 + RP/V1 + XQ/V1)2 + (XP/V1 – RQ/V1)2

V2 = V1 + RP/V1 + XQ/V1

V2 - V1 = (RP + XQ)/V1 = ΔV

ΔV = Voltage difference between sending end and receiving end = Voltage drop

This can be minimized by using reactive power compensation as detailed below.

Considering a case where the 3 phases of the line are short circuited,

Isc = V1/X where Isc = Short circuit current, assuming R<<X

Let SCL = V1Isc where SCL = Short circuit level

Thus SCL = V12/X

Using our voltage drop equation, it may be represented as

ΔV/V1 = (RP + XQ)/V12

ΔV/V1 = ((RP + XQ)/V12) x (X/X)

ΔV/V1 = (X/V12) (RP/X + Q)

ΔV/V1 = 1/SCL (RP/X + Q)





When R = 0, ΔV/V1 = Q/SCL

This gives an appropriate value of voltage change due to change in reactive power. When a VAr compensator is used for voltage control, the equation becomes

ΔV/V1 = 1/SCL (RP/X + (Q-QC)) where QC = Power supplied by the VAr compensator

This equation shows that voltage drop due to both active and reactive power can be controlled by controlling the output of the VAr compensator.





CONCLUSION The effects of VAr compensation, distributed generation and fault conditions have been examined in this report. A study on reactive power flow and VAr compensation has also been undertaken. It has been found that distributed generation and VAr compensation while beneficial in some aspects, increase the fault levels of the network. The operation of relays will have to be adjusted to compensate for that. In the case of distributed generation, directional current settings will have to be adjusted. This will in effect decrease the sensitivity of the relays (Jenkins, et al., 2010).

The main methods used by electricity suppliers to control voltage levels are tap changing transformers. These offer variable control to keep the voltage within statutory limits.

To control active and reactive power flow, suppliers mostly use VAr compensators. This compensates for reactive power needs of the network and reduces losses.





BIBLIOGRAPHY CHAPMAN, J. S., 2002. Electric Machinery and Power System Fundamentals. Boston: McGraw Hill.

Era Technology Works, n.d. ERACS “ User Guide” and “Technical Manual”, Surrey: s.n.

HINGORANI, N. G. & GYUGYI, L., 2000. Understanding FACTS: Concepts and Technology. New York N.Y: IEEE Press.

Jenkins, N., Ekanayake, J. & Strbac, G., 2010. Distributed Generation. London: The Institution of Engineering and Technology.

Weedy, M. B., 1987. Electric power systems. Chichester: Wiley.




ERACS Power Network Model

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