Modeling 400 kV Transmission Grid with

System Protection and Disturbance Analysis

Igor Ivanković Croatian Transmission System

Operator - HOPS Zagreb, Croatia

igor.ivankovic@hops.hr

Renata Rubeša Croatian Transmission System

Operator - HOPS Zagreb, Croatia

renata.rubesa@hops.hr

Igor Kuzle University of Zagreb

FER Zagreb, Croatia

igor.kuzle@fer.hr

Abstract—Power swing and out of step conditions are two power system phenomena which are elaborated in this paper from the power system protection point of view. For that purpose a Matlab model of a part of a 400 kV transmission grid with system protection was modelled. Transmission system disturbance analysis represents a background for different types of research scenarios. Powerful tools for transmission disturbances analysis are phasor measurement data sets and transmission grid models. Recorded disturbance data is used to check and validate the transmission grid model. In this paper different types of disturbance scenarios were analyzed. The protection system response of a 400 kV transmission line is analyzed for mild and heavy power swinging scenario. The purpose of the presented research is to explore the possibility to create a central protection system for a transmission grid that is resilient to disturbances.

Index Terms—Matlab modeling of transmission grid, Matlab modeling of system protection, power swing, out of step condition.

I. INTRODUCTION

A relay protection device for transmission lines has both basic local protection functions and system protection functions. Basic protection functions practically operate only for the dedicated protected overhead or cable line. The system protection functions, built in a protection device, are power swing and out of step condition functions.

Complex power system disturbances such as power swing and out of step can occur quite often during transmission system operation. Protection function must detect the disturbance and react to it only using local measurement data which is available and collected from field measurement transformers. Nowadays, transmission companies have practice of setting and engaging the out of step function. This function is running in parallel with other protection functions in real time. The function acts locally on the circuit breakers and for a proper setting of reaction times there it is needed to have results from extensive study analyses. Thus, one can say

that the out of step function is a protection function which combines local measurement data in real time and system data from study work for its operation and setting.

Phasor Measurement Unit (PMU) measured data in the dispatch control center can be used to create real time system protection function for wide area disturbances [1], [2]. Power swing and out of step conditions on transmission level can be detected in detailed manner with the Wide Area Monitoring, Protection and Control (WAMPAC) system. In order to create such a centralized protection system extensive transmission disturbance analysis is needed to be carried out. In this paper the transmission grid model is designed and tested with the collected PMU data. The model and the study analyses were performed in Matlab for the Croatian 400 kV transmission grid shown on Fig. 1. In order to create the relay protection environment, the power swing protection function is modeled. The transmission grid and protection system model were checked for many operational situations. The results for one example of an occurrence of power swing in the 400 kV transmission grid is presented and its influence on 400 kV line protection system is analyzed.

II. TRANSMISSION GRID AND SYSTEM PROTECTION

A. Simulation environment for transmission grid

The framework for modeling the 400 kV Croatian transmission system was the Matlab environment. The focus of the transmission model design was the southern part of the 400 kV transmission grid in the vicinity of the substation Konjsko. This part of the grid is featured by long radial 400 kV lines and relatively weak grid, Fig. 1.

Each 400 kV substation in this grid is modelled by its own subsystem with equivalent Matlab blocks that represents all the characteristics of the real substation. Substation Konjsko was modelled in a more detailed manner (Konjsko extended equivalent) with part of 400 kV lines towards neighboring transmission system operators and also 220 kV and 110 kV parts, Fig. 2.

Figure 1. Transmission grid 400 kV model

Figure 2. Konjsko equivalent block

Since the purpose of the model was to investigate oscillations which are detected on the lower voltage levels, a more detailed model of the 220 kV and 110 kV transmission grids is used. The model contains 220 kV lines and substations and generations units connected to both 220 kV and 110 kV level.

B. Simulation environment for system protection function

The system protection function, alongside other protection functions in the model of the relay device, use available local voltage and current measurement, Fig. 3, collected from instrument transformers in the switchyard [3], [4].

Figure 3. Metering for line protection

Power swing and out of step functions operate by only using local data from one side of the protected line. All calculations for protection, monitoring and control function in the relay device were based on line terminal voltage (1) and current (2) measurements.

)sin()cos()( UULL tjtUtU

)sin()cos()( IILL tjtItI

For their internal calculation s relay devices use complex voltage (3) and current (4) values, defined as:

LLL UjUU ImRe

LLL IjII ImRe

Protection relay measured line voltage UL at the line end is defined as, (5) and (6):

LLLLL IjXIRU

LLLLLL IjIjXRUjU ImRe)(ImRe

The impedance measurement calculation for the line protection function is running in millisecond cycle and is constantly compared to the set values. The impedance is also calculated in the complex and used for system protection functions (7) and (8):

22ImRe

ImReReIm

LL

LLLLL

II

IUIUX

22ImRe

ImImReRe

LL

LLLLL

II

IUIUR

Nowadays, the most commonly used impedance characteristic for distance protection function in a relay device is polygon shaped characteristic, shown in Fig. 4. Distance protection uses specifically tailored characteristics for line protection. For remote back up protection more than one distance zone was used.

Power swing and out of step functions use different, usually much wider characteristic than distance protection. The characteristic, envelopes all impedance zones of the distance protection and it is the most outer placed characteristic in the R/X plane.

Figure 4. Line distance protection characteristic

The protection model was created in Matlab to simulate line impedance protection functions, as shown in Fig.5. In this particular analysis the focus was on the power swing phenomenon and its influence on the transmission grid. Protection functions were checked on many disturbance scenarios in order to be securely implemented as central protection function using PMU data flow.

Figure 5. Matlab protection equivalent block

With this protection model it is possible to trace different strengths of power swings in the transmission grid. This is shown as a stable swing trajectory number 1 (Fig. 4) and out of step condition, trajectory number 3 (Fig. 4). It is also possible to trace the short circuit line fault, trajectory number 2 (Fig. 4).

III. ELECTROMECHANICAL OSCILLATION IN POWER PLANT

Local electromechanical oscillations on the lower voltage level of one generating unit in some cases can spread through the network even to higher transmission levels. The WAM system [5], [6] installed in the Croatian Transmission System Operator (HOPS) has enough sensitivity to precisely detect such events. Additionally, the SCADA system records such events and correctly alarms the dispatcher in the control center.

Modern turbine control and voltage control of generation unit smoothly run the unit in normal operation conditions. A failure or error in these control systems or testing activities can have unpredictable consequences on the transmission grid and the generation unit. Such case happened in the Croatian power system on the 14th November 2014. An oscillation of local character was detected in the southeast continental Europe interconnected power system that caused problems in the 400 kV transmission system. The source of the oscillation was located on the 110kV voltage level in the south region of the Croatian power system.

This severe disturbance was used to evaluate the Matlab model and to analyze the impact of these oscillations on 400 kV line protection system in substation Konjsko.

A. Disturbance on generation units

The oscillations originated from the hydro power plant Zakučac in the south Croatian region. The oscillation occurred during testing procedures of the new generation unit in the power plant installed on the 110 kV voltage level. During the testing procedures a fault caused the 110 kV busbar splitting, Fig. 6. At that time, the generating units G1 and G4 were disconnected from one 110 kV busbar and remained connected only with power grid transformer 220/110 kV. A sudden topology change in the power plant caused a high increase in the network impedance seen by units G1 and G4 connected to the 110 kV level. This was a trigger which induced oscillations on the generating units. Both 110 kV units operated at nominal rated power at the moment the disturbance occurred.

Figure 6. SCADA image for HPP Zakučac

The generation unit installed on the 220 kV level G2 was also running at nominal power but it did not start oscillating. The unit G3 was not in operation at that moment. The SCADA system trend functions recorded generation power output oscillation on all units, Fig. 7. The SCADA sampling data has enough quality to record this disturbance on 220 kV and 110 kV level. The oscillations continued and lasted within a longer period until the generating unit G1 was disconnected from the network by dispatch action.

B. Active power oscillations on transmission level

WAM system recorded the oscillations generated by the generation units on the 400 kV transmission lines, Fig. 8, with much higher resolution than SCADA system (Fig. 7).

Figure 7. Unit active power oscillations

The WAM system data resolution is 20 milliseconds (synchrophasor data sampling time). The described oscillations were detected and were present in the entire Croatian transmission grid and broader. Highest oscillation were detected on the lines which are closest to the source of the oscillations, in this case the 400 kV lines Konjsko-Velebit (yellow line on Fig. 8) and Melina-Velebit (blue line on Fig. 8). The starting moment of the oscillations can also be seen on Fig. 8.

110kV GEN4 P(MW) GEN1 P(MW)

220kV GEN2 P(MW) GEN3 P(MW)

Figure 8. Active power oscillation on 400kV lines

From the relay protection perspective, the power swing lasted long enough to endanger normal operation conditions. Because of that an additional effort was made to analyze the behavior of the system protection functions in the installed line protection devices.

The frequency of the active power oscillations on the overhead lines corresponds to the oscillation frequency of the generation unit, which range around 1 Hz. From the active power readings of the 400 kV line Melina-Velebit, measured in the substation Melina, with the Prony analysis the frequency and damping component of the signal was calculated and is shown on Fig. 9. The sampling period for the PMU recordings used in HOPS system is 20 milliseconds. Prony’s method calculated the main parameters of the dominant mode for the electromechanical oscillations: frequency f0 = 0.96 Hz and damping ξ=0.057. The damping factor has a positive value which indicates an undamped oscillation.

0 50 100 150 200 250 300 350 400-180

-170

-160

-150

-140

-130

-120

-110

-100

-90

Samples

Sig

nal P

[MW

]

Prony Analysis of Model Order 18 in Time Domain

Prony Approximate

Measured P[MW] Melina Velebit

Figure 9. Prony analysis of the active power measurement

All of abovementioned events can have influence on the 400 kV line relay protection system. Line protection system must remain stable during active power oscillations and trip

the line only in very severe situations. 3 protection systems must operate in parallel and run coordinately. These are the distance protection, power swing protection and out of step function. The detected oscillations from 50 to 70 MW on the 400 kV transmission line are a challenging disturbance for line protection system.

The out of step function operates only using local data from one side of the protected line while distance protection and power swing functions used wider area data. This fact makes it very challenging to fulfill the demands for local and wider reactions of the out of step function because the disturbances (such as one described earlier) can occur anywhere in the transmission system and trigger any of the mentioned functions.

IV. IMPACT OF POWER SYSTEM SWING ON TRANSMISSION

LINE PROTECTION

A. Analyses of disturbance on HPP Zakučac generation units

As mentioned earlier, in the Matlab simulation environment the power system behavior and line protection system were modeled. The model was tuned to be able to simulate this particular disturbance which started in the power plant Zakučac. The analyses were focused on the 400 kV line Konjsko – Velebit, particularly on the Konjsko line terminal where protection actions were expected. Simulation environment was set to last 10 seconds and the oscillations started at t=3.0 seconds and lasted till t=9.0 seconds. The goal was to check if the impedance trajectory during this disturbance enters in any of the relay protection activation characteristics. The impedance characteristic with the highest impedance setting is used for the power swing function. This particular polygon is the first one which can be reached by the impedance trajectory (Fig. 10).

Figure 10. Impedance trajectory trace during simulation

The result of these analyses is presented on Fig. 10 in the R/X plane. Power swing polygon has the following setting R=65 ohm and X=65 ohm (black line). Line impedance

resistance and reactance have the following values Z=3.4+j34.1 ohm (red line).

During oscillations, the impedance trajectory swings on a safe distance from the power swing polygon. The swing occurs around normal position of the load impedance. The trajectory of just one swing is shown in Fig. 11.

Figure 11. Impedance trajectory during one swing

These swings were stable and did not start any of the protection functions inside 400 kV line protection devices. During this time the transmission line was lightly loaded. Hence, the additionally power swing did not push impedance inside any of the relay protection activation regions. Oscillation originated from the 110 kV grid and with some damping effect propagated to the 400 kV grids [7]. Electromechanical oscillations produced by generation unit on the lover voltage level propagated to the highest transmission level but in this case did not endanger the normal power system operation.

B. Analyses for disturbance on 400 kV transmission grid

Further analyses were done for two different, hypothetical cases. The first one is a heavy power swing with origin on the 400 kV level and the second one is out of step disturbance also originating from 400 kV level.

The simulations were carried out in the southern part of the Croatian transmission grid. The first disturbance was simulated on the 400 kV Konjsko – Mostar and the protection performance was checked on the 400 kV line Konjsko – Velebit at Konjsko line terminal.

The results of this simulations shows that the impedance trajectory reaches the vicinity of the power swing polygon, Fig. 12. The simulation time had the same duration as in the previous scenarios (10 seconds).

Figure 12. Impedance trajectory during power swing on 400 kV line

Konjsko – Velebit

This particular simulation scenario shows how close to the power swing polygon the impedance trajectory can reach. The normal load impedance was nearly half of value from the scenario with oscillations triggered by 110 kV generation units. The load region was near 1000 ohms, much closer to the relay characteristic. Oscillations start at t=3.0 seconds. The additional power swing pushed the impedance trajectory near the power swing characteristic. Resistance strongly decreases near to zero ohms and reactance value reaches only hundred ohms, [8]. Although the 400 kV line was lightly loaded with 200 MW active power, with the power swing source on the 400 kV level, the normal operation was endangered.

Simulations for the out of step disturbance were also performed on the 400 kV line Konjsko – Mostar. During the simulation time the out of step conditions occurred three times, Fig. 13. During this serious disturbance the impedance trajectory passes several times through the power swing polygon. The disturbance also spread to other 400 kV lines in the transmission grid.

Figure 13. Impedance trajectory during the multiple out of step

disturbance

When the impedance trajectory enters the polygonal characteristic of the power swing protection function it will enable circuit breaker tripping or will interact with distance protection. The settings of the distance protection can issue blocking commands to distance zones to block all or to block only some zones.

If the impedance trajectory also passes across the line impedance (red line) inside the polygon, Fig. 14, this line must be disconnected from the grid or otherwise consecutive out of step conditions will occur.

Figure 14. Impedance trajectory during the first out of step

Out of step disturbance phenomena happens in a short period of time. In the simulation process the disturbance occurs in a time frame of couple seconds, Fig 14. Also this new state for the transmission grid must be treated by protection otherwise it can repeat itself and continually spread across the grid.

Severe power swings in case of out of step condition definitely push the transmission system towards alert state or even emergency state. The operation of generating unit can also be endangered. Proper reaction to this kind of disturbance must be carried out.

V. CONCLUSION

Modeling of transmission grid and system protection in Matlab gives a powerful simulation environment to research the transmission system behavior during transmission disturbances. Simulations were done for real recorded disturbance originating from a power plant Zakučac in Croatian power system. With the measured real disturbance the model was validated and then the behavior of the system during other disturbances was simulated.

The system protection function in-built in the relay protection devices installed on chosen 400 kV lines was tested for particular disturbance from power plant Zakučac and also for some other possible disturbance scenarios and the results were described.

Future work can be directed towards the establishment of the central system protection function based on PMU data flow. The new central system will be simulated in Matlab with

the design of the enhanced higher level module of the protection system.

VI. ACKNOWLEDGEMENT

The work of the authors is a part of the FENISG- Flexible Energy Nodes in Low Carbon Smart Grid funded by Croatian Science Foundation under project grant No. 7766.

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