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INVESTIGATION OF THE IMPACT OF SWITCHING A HEAVILY LOADED TRANSMISSION LINE ON OPERATION OF A POWER PLANT WITH TURBINE-GENERATOR UNITS

Andrzej Kąkol / Institute of Power Engineering Research Institute, Gdańsk DivisionBogdan Sobczak / Institute of Power Engineering Research Institute, Gdańsk Division

Robert Trębski / PSE Operator SA

1. INTRODUCTION

The Polish Power Grid is a part of a large synchronised international grid of mainland Europe – ENTSO-E (formerly UCTE) – one of the largest systems in the world, with a peak demand exceeding 400 GW.

In a synchronous power grid transfers of power between individual operators influence not only the op-erators who participate in transactions but also other systems. In most cases the affected systems are direct neighbours of those directly involved in the transaction. Flows through the systems which do not participate in a power transmission transaction are known as loop flows. The loop flows are an undesirable side effect and their negative impact depends on their magnitude.

Development of the European energy market, as well as development of renewable power sources, result in a significant increase in power flow – and subsequently loop flows – within the power grids managed by main-land European operators. These are observed in eastern and western neighbours of Germany: Benelux countries, Poland, Czech Republic and Slovakia. Such a situation occurs as the excess power generated by the wind farms in the northern part of Germany and western Denmark flows to the consumers located in southern Germany, as well as Austria and Italy – areas which have considerable pump storage capacities. Loop flows caused by the wind power generation in Germany and Denmark have already reached an intensity which threatens the safe operation of power systems neighbouring Germany. Large loop flows considerably limit transmission capacities and also transfer some active and reactive power losses related to the transmission transaction to the countries which do not directly participate in it.

Except for the construction of new transmission lines (which is ruled out due to the long investment times-cale in most European countries) the loop flows can be limited by solutions such as re-dispatching conventional power generation units, changing transmission system topology and introducing phase shifters in the grid (in most cases at cross-system lines). In addition, application of solutions which limit loop flows requires coordina-tion between transmission system operators, as otherwise they might just result in transferring problems into neighbouring systems.

Large loop flows also cause some less obvious side effects. Transmission lines heavily loaded by loop flows are difficult to reconnect after an emergency trip due to the high voltage angle difference, especially in the case of lines outgoing from a power station’s switchyard. At the same time a loop flow caused by events in another system is difficult to suppress rapidly. This creates a threat of a snowball overload failure and emergency trips of an increasing number of lines – in extreme cases leading to a general system failure.

This paper presents results of a study carried out by the Gdańsk Division of the Institute of Power Engi-neering for PSE Operator SA aimed at analysing the dynamic threat for Krajnik substation and Dolna Odra Power

Abstract

Development of the European energy market, combined with development of renewable power genera-tion sources result in increased power transfers within the grid, including undesirable loop flows. If such flows occur near power stations on a single line (e.g. exchange line) they might cause some threats related to potential line trips and reconnections. Trip of a heavily loaded line considerably distorts active and reactive power balance

in the area of the power plant in question. Reconnection of a tripped line with significant voltage angle difference causes strong torque variations on the shaft of a turbine-generator unit. This paper presents results of a simulation of electromagnetic and electromechanical phenomena which occur if a heavily loaded transfer line connected to a switchyard of a large power station with turbine genera-tors is tripped and then reconnected.

Investigation of the Impact of Switching a Heavily Loaded Transmission Line on Operation of a Power Plant with Turbine-generator Units

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Plant (EDO) caused by high power transfers from Germany. Apart from the impact of tripping and reconnecting the transmission line to Vierraden, the impact of power flows from Germany on the stability of EDO’s generators was also investigated.

2. SCOPE OF RESEARCH

Investigation of potential threats for the Krajnik substation and Dolna Odra Power Station caused by high power flows coming into the Polish grid over the 220 kV line Krajnik – Vierraden included:

• Determining the relation between the power flow from Vierraden and dynamic stability of large distur-bances on the Power Station’s generators, defined by critical time of a near-to-generator three-phase short-circuit

• Determining the relation between the power flow from Vierraden and dynamic stability of small disturbances on the Power Station’s generators, defined by a damping coefficient of post-disturbance oscillations

• Determining the relation between the power flow from Vierraden on the course of electric transition phenomena following a line trip and reconnection, in particular in the case of the Vierraden line

• Determining the relation between the power flow from Vierraden on torsional vibrations of turbine-generator shafts at the Power Station following line trips and reconnections, in particular those of the Vierraden line.

3. KRAJNIK SUBSTATION

Krajnik Substation is a substation of the Dolna Odra Power Station. The substation is equipped with 110 kV, 220 kV and 400 kV switchyards coupled by 220/110 kV and 400/220 kV transformers (two units). Four power generation units are connected to the 220 kV switchyard, while three others are connected to the 400 kV switchyard. A double-circuit transmission line to Vierraden substation goes out of the 220 kV switching station. Analytical research indicates that in conditions of very high wind power output in Germany, when no countermeasures are in effect, the load on this line (power flow from Germany) might even reach levels around 1000 MW (which is above the permissible capacity of the line). Fig 1 shows such a situation with 544 MW incoming flow on each circuit of the line. This makes the power flowing from Germany higher than the combined output of all EDO units operating at that time.

Each EDO unit operating at the time had a generator rated at 270.6 MVA equipped with electromechanical AC excitation systems, digital voltage regulators and system stabilisers.

Fig 1. Voltages and power flows at Krajnik substation.

Andrzej Kąkol, Bogdan Sobczak / Institute of Power Engineering Research Institute, Gdańsk DivisionRobert Trębski / PSE Operator SA

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4. MODELS OF THE GRID AND DOLNA ODRA POWER STATION

Two types of dynamic models were developed to investigate the influence of high load on the line in ques-tion:

1. System dynamics model – using PSLF software2. Model of fast-changing phenomena dynamics – using PSCAD software.The System dynamics model contains accurate models of base load (“systemic”) power stations of the

Polish, German, Czech, Slovak, Austrian and Hungarian grids. Power generation in other systems of mainland Europe was approximated (without exact specification of power generation stations, using exemplary genericmodels of generators, turbines and control systems). Wind power generation was modelled in the eastern Ger-man grid (operated by 50 Hertz Transmission). Increase of wind generation output combined with changing dispatch of conventional power plants was shown to cause increased loop flows in the Polish system. 1100 MWload on the Krajnik – Vierraden line was achieved at wind power output exceeding 10,000 MW. Such a situation should be seen as an overload of the 220 kV Krajnik – Vierraden lines and might be caused for instance by a trip of a north-south transmission line in Germany, resulting in increased flow through the Polish grid and exceedingthe permissible capacity of the 220 kV Krajnik – Vierraden lines.

The Model of fast-changing phenomena dynamics covers the Dolna Odra Power Station (generators, their voltage regulation systems and system stabilisers, and step-up transformers), 220 kV and 400 kV switch-ing stations of the Krajnik substation and transmission lines connected to that substation. Due to the fact that scope of analysis included torsional vibrations of each turbine-generator’s shaft, the shaft itself was modelled as multi-mass object. Dynamics of the Polish grid is represented by a large equivalent generator, while the German system is simulated by a fixed network to which the 220 kV Vierraden switching station is connected.

The multi-mass shaft model takes into account the shaft’s structure, discriminating several masses of high torsional rigidity, connected with elements (couplings) of lower rigidity. Rigid components represent ro-tors of individual turbine sections (high-pressure, intermediate-pressure and low-pressure), generator rotor, and optionally also exciter rotor. Typically, a multi-mass shaft model consists of four to six bodies. For example, a five-body shaft has five natural frequencies, the lowest of which is the natural frequency of the entire shaftin reference to the power grid. The remaining four values are used for analysing phenomena which potentially might cause and amplify shaft torsional vibrations such as: interaction with turbine regulation systems, interac-tion of torsional vibrations between shafts of parallel turbine units, interaction with AC/DC converters’ control systems or subsynchronous resonance with the grid compensated with series-connected capacitor banks. Multi-masses models are also used to investigate the influence of disturbances caused by grid switching operationson turbine-generator’s shaft.

The model description shows that the PSLF system model is appropriate to investigate phenomena occur-ring both in Polish and German grids, in particular those occurring in the 50 Hertz Transmission area of respon-sibility, while the latter model would only simulate phenomena occurring at the Krajnik substation and within the EDO power generation units.

5. RESULTS ACHIEVED WITH THE SYSTEM DYNAMICS MODEL

The full scope of investigation as used for determining the stability of a base load power plant of the Polish power grid was carried out using the systemic model.

Angle stability of major disturbances was investigated by comparing critical times of near-to-genera-tor three-phase short-circuits during trips of EDO’s outgoing transmission lines. Additionally, for a short-circuit cleared after 150 ms, the number and duration of voltage losses below 80% of nominal value was recorded. As-sumed criteria did not reveal any significant impact of the load on the Krajnik – Vierraden line on the stability ofEDO operation for major disturbances.

Angle stability of minor disturbances was determined with Prony’s method, by determining frequencies and damping coefficients for primary modes in post-disturbance variations of active power generated by EDOgenerators. No impact of the load on the Krajnik – Vierraden line on the angle stability of EDO generations for minor disturbances was identified.

Investigation of the Impact of Switching a Heavily Loaded Transmission Line on Operation of a Power Plant with Turbine-generator Units

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The influence of switching operations on the Krajnik – Vierraden line was investigated by simulating the following sequence:

• 1 sec – trip of the first circuit of the line• 5 sec – trip of the second circuit of the line• 20 sec – reconnection of the first circuit of the line• 24 sec – reconnection of the second circuit of the linefor various values of the line load (power incoming from Germany).Fig. 2 presents variation of the active power of the G5 generator feeding power to the 220 kV switching

station. Fig. 3 shows corresponding variations of 220 kV voltage in Krajnik and Vierraden substations when the power flow from Germany is 1100 MW. At this load on the line a trip of both circuits of the Krajnik – Vierraden line causes a serious disturbance in both Polish and east German grids. At the 220 kV Krajnik station there is a sudden voltage surge and considerable oscillations of output of EDO generators connected to the 220 kV station (with a magnitude exceeding 100 MW). Voltage surge might be even higher if other outgoing lines of the 220 kV Krajnik station are disconnected. It is possible that the maximum voltage for the power plant’s 220 kV switching station – which is 245 kV – will be exceeded.

The simulations have revealed that tripping the Krajnik line at high power flow from Germany causes more serious disturbance within the east German grid. Voltage surge at Vierraden is almost twice as high as in Krajnik. Local excess of reactive power would occur in the area of Vierraden, with a simultaneous reactive power deficit in the remaining area (losses due to suddenly increased load on transmission lines). The final effect would depend on the availability of quickly dispatchable sources of reactive power in Germany. In a model where the wind farms do have such properties, the situation is kept under control. In a model with restricted wind power control capabilities, voltage losses occur and lead to tripping of some generation units. Increased demand for reactive power in Germany affects operation of the Polish Turów Power Plant, which experiences an increased reactive power load on its generators, following a trip of the Krajnik line.

Fig 2. Variations of active power of the G5 generator for simulated trip-reconnection sequence of the 220 kV line Krajnik – Vierraden loaded at 1100 MW.

Andrzej Kąkol, Bogdan Sobczak / Institute of Power Engineering Research Institute, Gdańsk DivisionRobert Trębski / PSE Operator SA

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Fig 3. Variations of the 220 kV voltage at Krajnik and Vierraden switching stations (the latter showing larger surge) for simulated trip-recon-nection sequence of the 220 kV line Krajnik – Vierraden loaded at 1100 MW.

After the line trip, the voltage angle difference between 220 kV switchyards in Krajnik and Vierraden exceeds 60°. At this value it is not possible to switch the line back into operation. The presented active power variations indicate that reconnection of the first circuit is just as major a disturbance for EDO generators feeding power into a 220 kV switching station as a near to generator three-phase short circuit.

The identified threat for the power plant and Krajnik station, as well as possible serious disturbances in the grid of eastern Germany after a trip of the second circuit of the Krajnik – Vierraden line show that the load on that line should be limited to a value which – in the case of a single circuit trip – does not automatically result in an immediate trip of the other circuit due to overload.

6. RESULTS OBTAINED WITH A MODEL OF FAST-CHANGING PHENOMENA

Simulated sequences of trip/reconnection of the Krajnik – Vierraden line reveal that considerable tor-sional vibrations of the turbine units’ shafts occur upon the trip of the second circuit of the line (ultimate line trip) and after reconnection of the first circuit. The magnitude of the vibrations increases along with the load on the line (power flow incoming from Germany). The highest torque occurs between the generator rotor and the low-pressure turbine shaft. When a transmission line load exceeds 1000 MW, the oscillations are comparable to the torque values obtained during simulations of near-to-generator short-circuits. Fig. 4 presents torsional oscillations between the low-pressure turbine shaft and generator shaft of the G5 unit recorded for a simulated trip/reconnection sequence of the 220 kV Krajnik – Vierraden line.

Fig. 4. Torsional vibrations (expressed in relative units of the generator’s nominal power) between the LP turbine rotor and rotor of the G5 generator for simulated trip-reconnection sequence of the 220 kV lines Krajnik – Vierraden loaded at 1100 MW.

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REFERENCES

7. IMPACT OF GRID SWITCHING OPERATIONS ON EDO TURBINE-GENERATOR’S SHAFTS

Each line switching operation – either scheduled or unscheduled – (e.g. trip due to short-circuit, fol-lowed by an autoor manual reconnect attempt) occurring near the power station results in a sudden change of the generator’s electric moment and induces torsional vibrations on the turbine-generator unit’s shaft. Vi-bration amplitude depends on the angle between voltage vectors on the circuit breaker and on the electrical distance between the breaker and the generator. Torsional oscillations cumulatively influence material fatigue and shorten shaft lifetime – the magnitude of those effects strongly depends on the amplitude values. The shaft of a turbine-generator unit is designed to withstand up to several dozen strong shocks caused, for example, by near-to-generator short-circuits or unsuccessful synchronisation attempts and a very high number of lighter shocks caused by remote or minor disturbances, e.g. line switching at small angle differences. According to the available sources a disturbance which results in a change of a electrical generator output lower than 50% of its nominal active power does not affect the actual lifetime of a shaft [1, 2, 3, 4].

Torsional oscillations have one more significant feature – they have low natural damping. Therefore, two disturbances occurring over a short period could result in much greater oscillations than any of those distur-bances would cause separately. For that reason, if switching operations are carried near a power station, it is required to separate them in time (according to the previously quoted references – by at least 10 seconds).

In the case of the Krajnik station and Dolna Odra Power Plant, a loop flow over the 220 kV Krajnik – Vier-raden line results in a high load not only on this particular line, but also on other transmission lines connected to the Krajnik substation. Potential scheduled or unscheduled switching of the lines outgoing from that station will thus be carried out at larger voltage angle differences, resulting in increased torsional oscillations. The simula-tions have revealed that if the transmission from Vierraden reaches 1100 MW, the 50% active power criterion may be kept also when attempting to switch on the lines outgoing from the 400 kV switchyard.

8. CONCLUSION

The investigation carried out with a system dynamics model and a model for dynamics of fast-changing phenomena has shown that dynamic threats for the Dolna Odra Power Plant related with power transmission from Vierraden to Krajnik exceeding 700-800 MW result mainly from a possible loss of that connection – a trip of both circuits following potential emergency trip of a single circuit. In such a case trip of the second circuit causes a serious disturbance on the EDO generators and voltage surges at Krajnik and Vierraden substations. After a trip of both circuits of the Krajnik – Vierraden line reconnection may only be possible after decreasing the voltage angle difference which would most probably require considerably decreasing power generation output in north-eastern Germany. High power flows from Vierraden increase load on other lines going out from the Krajnik station which impedes switching operations and increases the risk of inducing excessive torsional vibra-tions on EDO turbine-generator units’ shafts. Such risks, however, only occur when the load on the cross-border line exceeds permissible values.

1. Kundur P, Power System Stability and Control, McGraw-Hill Professional Publishing, 1994.2. IEEE Working Group Interim Report: Effects of switching network disturbances on turbine – generator shaft sys-

tems, IEEE Transaction on Power Apparatus and Systems, vol. PAS-101, issue 9, September 1982.3. Opera L, Popescu V., Sattinger, Coordinated synchronism check settings for optimal use of critical transmission

network corridors, IEEE 20074. Lamrecht D.R., Problems of torsional stresses in shaft lines of turbine generators, CIGRE WG 11.01, Section 3, Rec-

ommendations, Electra no. 143, August 1992.

Andrzej Kąkol, Bogdan Sobczak / Institute of Power Engineering Research Institute, Gdańsk DivisionRobert Trębski / PSE Operator SA