Performance of Voltage-Driven Protection Systems in Renewable-Dominated Networks: A Case Study of the Lusaka 132 kV Transmission Network

Performance of Voltage-Driven Protection Systems in Renewable-Dominated Networks: A Case Study of the Lusaka 132 kV Transmission Network

Suma Lungu

Department of Electrical and Electronic Engineering, University of Zambia

Abstract – The global transition toward renewable energy generation is significantly altering the behaviour of modern power systems. Voltage-driven protection systems, particularly distance protection, were originally designed for networks dominated by synchronous generators with high shortcircuit levels and stable voltage characteristics (Kundur, 1994; Anderson, 1999). The increasing penetration of inverterbased resources (IBRs) such as solar photovoltaic generation introduces new operational conditions including reduced system strength, limited fault current contribution, and dynamic control interactions during faults.

This paper investigates the performance of voltagedriven protection systems under increasing renewable penetration using the Lusaka 132 kV transmission network as a case study. A detailed network model was developed in DIgSILENT PowerFactory and analysed under progressive photovoltaic penetration levels ranging from 0% to 100%. Both RMS and electromagnetic transient simulations were conducted to evaluate relay performance during singlelinetoground and threephase faults.

Results demonstrate that increasing renewable penetration reduces fault current magnitude, distorts impedance trajectories, and increases the risk of relay underreach and delayed tripping (He et al., 2019; Gordon et al., 2022). Several mitigation strategies including battery energy storage systems, static VAR compensation, adaptive protection, and communicationassisted protection schemes are evaluated. The findings highlight the importance of modern protection strategies capable of maintaining reliable protection performance in renewabledominated power systems.

Keywords

Renewable Energy Integration; Distance Protection; Inverter-Based Resources; Weak Grid; EMT Simulation; Power System Protection

1. INTRODUCTION

   Power systems worldwide are undergoing a significant transformation driven by the integration of renewable energy sources such as solar photovoltaic (PV) and wind generation. This transition is motivated by the need to reduce greenhouse gas emissions, enhance energy security, and support sustainable development. However, the rapid integration of renewable generation introduces important technical challenges for power system operation and protection (CIGRE, 2019).

   Zambia is currently expanding solar photovoltaic generation to complement hydropower resources, making the evaluation of protection system performance under renewable penetration increasingly important for the national grid.

   Traditional power system protection schemes were developed for networks dominated by synchronous generators. These machines provide high fault current contributions, strong voltage support, and predictable dynamic behaviour during disturbances (Kundur, 1994). Protection devices such as distance relays and directional overcurrent relays rely on these characteristics to detect and isolate faults quickly and selectively.

   Inverterbased renewable resources behave fundamentally differently from synchronous machines. During fault conditions, gridfollowing inverters typically limit their fault current contribution to approximately 1.11.3 per unit in order to protect converter

   components (IEEE PSRC, 2022). As renewable penetration increases, synchronous generation is displaced, resulting in declining

   shortcircuit levels and reduced system strength.

   These changes can significantly affect protection system performance. Reduced fault current magnitude alters impedance measurements used by distance relays and may lead to underreach or delayed operation. In addition, fast control dynamics in inverterbased resources may distort voltage and current signals used for relay polarisation (He et al., 2019).

   Despite extensive international research on renewable integration and protection challenges, relatively limited studies have focused on African transmission networks. Zambia is currently increasing solar PV deployment to complement hydropower generation, particularly in the Lusaka region. This makes the Lusaka 132 kV transmission network a suitable case study for investigating the behaviour of protection systems in renewabledominated environments.

2. LITERATURE REVIEW

   Conventional protection philosophies were developed for systems dominated by synchronous generation. Under these conditions, fault currents are relatively high and voltage waveforms remain stable during disturbances. Distance protection schemes therefore operate reliably by calculating the apparent impedance between the relay location and the fault using voltage and current measurements (Anderson, 1999).

   The increasing penetration of inverterbased resources significantly alters these assumptions. Converterinterfaced generation typically operates as a controlled current source with strict current limiting characteristics during faults. As a result, fault current magnitudes may be substantially lower than those expected in conventional power systems (CIGRE, 2019).

   Several studies have demonstrated that declining shortcircuit levels can affect distance relay performance. He et al. (2019) showed that reduced system strength increases the likelihood of relay underreach and measurement errors in impedance calculations. Gordon et al. (2022) reported similar observations in weak grid studies conducted on renewabledominated networks.

   In addition to reduced fault current levels, inverter control systems introduce complex dynamic behaviour during disturbances. Phaselocked loop (PLL) instability, harmonic distortion, and converter current limiting may distort relay measurement signals and affect directional polarisation (IEEE PSRC, 2022).

   These challenges have led researchers to investigate alternative protection strategies including adaptive relaying, communicationassisted protection schemes, and dynamic voltage support technologies such as synchronous condensers and battery energy storage systems (Nikmehr et al., 2021). These approaches aim to improve protection reliability under varying system operating conditions.

3. METHODOLOGY

   This study adopted a simulationbased approach to evaluate the behaviour of voltagedriven protection systems in the Lusaka 132 kV transmission network. A detailed model of the transmission network was developed using DIgSILENT PowerFactory. The model includes transmission lines, transformers, load models, and photovoltaic generation sources connected at selected network nodes. The simplified single line diagram of the Lusaka 132 kV transmission network used in this study is shown in Figure 1.

   DIgSILENT

   External .. External ..

   Line(12) 132kV_Lin..

   LH_132 LSMFEZ_132

   Line 132kV_Lin..

   Line(13) 132kV_Lin..

   KKI_132

   LHILL Load

   Line(7) 132kV_Lin..

   Line(9) 132kV_Lin..

   LSFMFEZ Load

   Line(8) 132kV_Lin..

   Line(10) 132kV_Lin..

   CHG Load

   WW_132

   Line(1) 132kV_Lin..

   CHG_132

   Line(6) 132kV_Lin..

   AVO_132 COV_132

   Line(5) 132kV_Lin..

   Line(11) 132kV_Lin..

   Line(4) 132kV_Lin..

   Line(2) 132kV_Lin..

   Line(3) 132kV_Lin..

   RMA_132

   LSW_132

   IND_132

   Liine((14)) 132kV_Liin….

   Lusaka West load

   Roma load

   LSW_132(1)

   PowerFactory 15.1.7

   p>Project: Graphic: Grid

   Date: 8/21/2025 Annex:

   Figure 1: Single Line Diagram of the Lusaka 132kV Network

   Renewable penetration levels were increased progressively to represent future grid scenarios. Simulation cases were developed for photovoltaic penetration levels of 0%, 25%, 50%, 75%, and 100% of the generation contribution to the Lusaka network.

   Fault studies were performed for both singlelinetoground faults and threephase faults at selected locations along protected transmission lines. Relay performance was evaluated based on trip time, zone operation, and impedance trajectory behaviour.

   Two simulation domains were used. RMS simulations were conducted to analyse steadystate behaviour and general protection performance trends. Electromagnetic transient simulations were then performed to capture fast dynamic phenomena associated with inverter control behaviour including current limiting, harmonic distortion, and PLL response.

4. RESULTS AND ANALYSIS

   Simulation results show a progressive degradation in distance relay performance as photovoltaic penetration increases. Under the base case scenario dominated by synchronous generation, the network exhibits high fault current levels and stable voltage behaviour. Distance relays operate correctly within their intended protection zones and trip times remain consistent with coordination requirements.

   Table 1: Distance relay response to PV Penetration sweeps for both single line to ground and three phase line fault.

   PV Penetration (%)	Fault Type	Fault Location (Line %)	Fault Resistance ()	Relay Zone Triggered	Relay Trip Time (s)
   100%	L-G	90%	15.688	No zone operated	No trip operated
   			15.896

   			With LH infeed at remote end
   100%	3 Phase	90%	8.110	No zone operated	No trip operated
   			8.196 With LH infeed at remote end
   75%	L-G	90%	16.336	Zone 2	0.420s
   75%	3 Phase	90%	8.174	Zone 2	0.420s
   50%	L-G	90%	15.811	Zone 2	0.420s
   50%	3 Phase	90%	8.141	Zone 2	0.420s
   25%	L-G	90%	15.434	Zone 2	0.420s
   25%	3 Phase	90%	8.122	Zone 2	0.420s
   0%	L-G	90%	15.154	Zone 2	0.420s
   0%	3 Phase	90%	8.185	Zone 2	0.420s

   As photovoltaic penetration increases, the displacement of synchronous generation results in declining shortcircuit levels. The apparent impedance measured by the relay increases and impedance trajectories move closer to the protection boundaries. In Figures 2 and 3 below, the impedance plots for the extreme cases of the trip / no trip thresholds are presented in the X-R plane.

   14.4

   [pri.Ohm]

   13.5

   12.6

   11.7

   10.8

   9.90

   9.00

   8.10

   7.20

   6.30

   5.40

   4.50

   3.60

   2.70

   1.80

   0.90

   DIgSILENT

   LSMFEZ – WW_Relay Zone (All): Polarizing

   Zl A 25.341 pri.Ohm 101.36°

   Zl B 11698.102 pri.Ohm 57.47°

   Zl C 25.819 pri.Ohm 37.35°

   Z A 15.12 pri.Ohm 69.4°

   Z B 201.452 pri.Ohm 119.86°

   Z C 202.476 pri.Ohm 0.72°

   Fault Ty pe: A (50PP1) Fault Ty pe: A (50PP2) Fault Ty pe: A (50PP3) Fault Ty pe: A (50PP4) Fault Ty pe: A (50G1/50L1) Fault Ty pe: A (50G2/50L2) Fault Ty pe: A (50G3/50L3) Fault Ty pe: A (50G4/50L4) Tripping Time: 0.42 s

   -8.10

   -7.20

   -6.30

   -5.40

   -4.50

   -3.60

   -2.70

   -1.80

   -0.90

   0.90

   1.80

   2.70

   3.60

   4.50

   5.40

   6.30

   7.20

   8.10

   9.00

   9.90

   10.8

   11.7

   12.6

   13.5

   [pri.Ohm]

   -0.90

   -1.80

   LSMFEZ_132\Cub_2\LSMFEZ – WW_Relay

   R-X Plot(1) Date: 8/22/2025 Annex:

   Figure 2: Impedance plot of applied fault, with relay characteristic and fault impedance plotted on the X/R plane. Notable is the correct trip operation.

   14.4

   [pri.Ohm] 13.5

   12.6

   11.7

   10.8

   9.90

   9.00

   8.10

   7.20

   6.30

   5.40

   4.50

   3.60

   2.70

   1.80

   0.90

   DIgSILENT

   LSMFEZ – WW_Relay Zone (All): Polarizing

   Zl A 190.191 pri.Ohm 98.02°

   Zl B 281.368 pri.Ohm 80.59°

   Zl C 191.119 pri.Ohm 62.91°

   Z A 8.909 pri.Ohm 65.72°

   Z B 325.378 pri.Ohm 69.41°

   Z C 323.578 pri.Ohm 92.06°

   Fault Ty pe: – (50PP1) Fault Ty pe: – (50PP2) Fault Ty pe: – (50PP3) Fault Ty pe: – (50PP4) Fault Ty pe: – (50G1/50L1) Fault Ty pe: – (50G2/50L2) Fault Ty pe: – (50G3/50L3) Fault Ty pe: – (50G4/50L4) Tripping Time: 9999.999 s

   -8.10

   -7.20

   -6.30

   -5.40

   -4.50

   -3.60

   -2.70

   -1.80

   -0.90

   0.90

   1.80

   2.70

   3.60

   4.50

   5.40

   6.30

   7.20

   8.10

   9.00

   9.90

   10.8

   11.7

   12.6

   13.5

   [pri.Ohm]

   -0.90

   -1.80

   LSMFEZ_132\Cub_2\LSMFEZ – WW_Relay

   R-X Plot(1) Date: 8/22/2025 Annex:

   Figure 3: Impedance plot of applied fault, with relay characteristic and fault impedance plotted on the X/R plane. Notable is the no trip operation.

   At high renewable penetration levels, the system begins to exhibit weak grid characteristics. Voltage depressions during faults become deeper and voltage recovery becomes slower. These conditions distort the voltage and current signals used by distance relays to calculate apparent impedance.

   Electromagnetic transient simulations reveal additional phenomena not visible in RMS analysis. Inverter current limiting behaviour

   DIgSILENT

   and phaselocked loop dynamics introduce waveform distortion during fault events.

   200.00 100.00 0.00 -100.00 -200.00 -300.00 0.1763 0.2056 0.2348 0.2641 0.2934 [s] 0.3227 LSMFEZ_132: Phase Voltage A in kV LSMFEZ_132: Phase Voltage B in kV LSMFEZ_132: Phase Voltage C in kV
   3.00 2.00 1.00 0.00 -1.00 -2.00 0.1763 0.2056 0.2348 0.2641 0.2934 [s] 0.3227 LSMFEZ – PV Line: Phase Current A/Terminal i in kA LSMFEZ – PV Line: Phase Current B/Terminal i in kA LSMFEZ – PV Line: Phase Current C/Terminal i in kA
   10.00 5.00 0.00 -5.00 -10.00 -15.00 0.1763 0.2056 0.2348 0.2641 0.2934 [s] 0.3227 LSMFEZ – WW line: Phase Current A/Terminal i in kA LSMFEZ – WW line: Phase Current B/Terminal i in kA LSMFEZ – WW line: Phase Current C/Terminal i in kA
   	Virtual Instrument Panel	Date: 10/3/2025 Annex: /1

   Figure 4: EMT results for the three phase to ground fault with 0% PV penetration. Notable in the third trace of the LSMFEZ WW line current is the susained and undistorted fault current.

   200.00 100.00 0.00 -100.00 -200.00 -300.00 0.1492 0.1837 0.2182 0.2527 0.2872 [s] 0.3217 LSMFEZ_132: Phase Voltage A in kV LSMFEZ_132: Phase Voltage B in kV LSMFEZ_132: Phase Voltage C in kV
   4.00 2.00 0.00 -2.00 -4.00 -6.00 0.1492 0.1837 0.2182 0.2527 0.2872 [s] 0.3217 LSMFEZ – PV Line: Phase Current A/Terminal i in kA LSMFEZ – PV Line: Phase Current B/Terminal i in kA LSMFEZ – PV Line: Phase Current C/Terminal i in kA
   4.00 2.00 0.00 -2.00 -4.00 -6.00 0.1492 0.1837 0.2182 0.2527 0.2872 [s] 0.3217 LSMFEZ – WW line: Phase Current A/Terminal i in kA LSMFEZ – WW line: Phase Current B/Terminal i in kA LSMFEZ – WW line: Phase Current C/Terminal i in kA
   	Virtual Instrument Panel	Date: 10/3/2025 Annex: /1

   DIgSILENT

   Figure 5: EMT results for the three-phase fault with 100% PV penetration. Notable in the first trace of phase voltage is the collapse of the 132kV LSMFEZ busbar voltage at fault inception, the highly distorted fault current contribution from the LSMFEZ PV line in trace two, an effect evident on the LSMFEZ WW line current in trace three upon which the distance relay performance can be analysed.

5. MITIGATION STRATEGIES

   Several mitigation strategies were investigated to improve protection performance in renewabledominated networks. Dynamic voltage support technologies such as battery energy storage systems and static VAR compensators can improve voltage stability during fault conditions, thereby improving relay measurement accuracy.

   Adaptive protection schemes that modify relay settings based on system operating conditions also provide improved protection performance under varying renewable penetration levels.

   Communicationassisted protection schemes such as line differential protection were identified as robust alternatives to conventional voltagedriven protection systems. These schemes rely on current comparison between line terminals rather than local voltage measurements and are therefore less sensitive to changes in system strength.

   Table 2: Mitigation Strategy Comparison

   Strategy	Single-Phase Faults	3-Phase Faults	Overall Effectiveness
   BESS	Partial	Good	Moderate
   Capacitor	Poor	Poor	Low
   SVC	Excellent	Excellent	Best
   FRT tuning	Limited	Limited	Limited
   Adaptive Distance	Poor	Poor	Low
   Line Differential	Excellent	Excellent	Best

6. CONCLUSION

The increasing penetration of inverterbased renewable generation fundamentally alters power system behaviour and introduces new challenges for conventional protection schemes. This study demonstrates that declining shortcircuit levels and inverter control dynamics significantly affect the performance of voltagedriven protection systems.

Simulation results show that increasing photovoltaic penetration leads to reduced fault current levels, distorted impedance trajectories, and increased risk of relay underreach. EMT analysis further highlights dynamic phenomena such as inverter current limiting and waveform distortion that influence relay measurement accuracy.

The results highlight the need for modern protection approaches including adaptive protection schemes, communicationassisted protection, and dynamic voltage support technologies to ensure reliable protection performance in renewabledominated power systems.

REFERENCES

1. Kundur, P. (1994). Power System Stability and Control.

2. Anderson, P. (1999). Power System Protection.

3. CIGRE WG B5.50 (2019). Protection Challenges in Systems with Distributed Energy Resources.

4. IEEE PSRC (2022). Protection Challenges for Interconnecting InverterBased Resources.

5. He, J., et al. (2019). Impact of Reduced ShortCircuit Ratio on Distance Protection.

6. Gordon, R., et al. (2022). Reduced Fault Levels and System Strength: Scotland Case Study.

7. Nikmehr, N., et al. (2021). Protection Challenges in Renewable Dominated Networks.