Modeling of Deadband Function of Governor Model and its Effect on Frequency Response Characteristics

Transcription

1 Modeling of Deadband Function of Governor Model and its Effect on Frequency Response Characteristics Saeed Mohajeryami, Aanand R. Neelakantan, Iman N. Moghaddam, Zia Salami Electrical and Computer Engineering, University of North Carolina at Charlotte Charlotte, NC 28223, USA Abstract Frequency Response has been the focus of increased attention and analysis recently due to its importance to the reliability of the network. The models used by the utilities proved to be insufficient and the inability to duplicate events using the existing models and databases has led to several initiatives to address these shortfalls. In this work, the effect of the dead-band in the governor/turbine model is investigated. For this purpose, IEEEG1 model is used to study the frequency response of the steam generators. Many of the existing industrial software used by the utilities ignore the dead-band function in their governor model. Using the incorrect parameters for the dead-band function can lead to the significantly incorrect frequency response, it is even worth in case of ignoring the dead-band function. In this paper, the effect of the dead-band setting on the frequency response is studied. The response of the generator during the frequency deviation is analyzed both with and without the dead-band. ETAP is used as a good candidate for an industrial software to study the effect of the frequency deviation on the generator response. Ignoring the dead-band is shown to depict a very optimistic picture of the frequency response. Keywords Primary frequency response; IEEEG1 governor/ turbine model; modeling; dead-band; MOD-027; ETAP I. INTRODUCTION NERC report of August 14 th, 2003 about blackout in eastern interconnection shows a huge discrepancy between what utilizes had expected based on the simulation of their models and what was observed [1]. There are many speculations about the reasons for this discrepancy in the technical literature [2-6]. The modeling of the different components of the power system is mentioned to be one of the major contributing factors [7-11]. Among others, the frequency response is one of the main issues. There are many parameters in the governor/turbine models which lack enough accuracy to model the physical governor/turbine. In response to the blackout, NERC approved 14 recommendations to address the shortcomings that contributed to the blackout. Incidentally, it reports that after-the-fact models developed to simulate the blackout indicate that the dynamic modeling assumptions, including generator parameters, were inaccurate [1]. This report initiated several standards to address the modeling issues in different components of the power system [11]. MOD-027 is a NERC standard which mandates the generator owners to test their generator components and then based on the test results verify the turbine/governor model parameters used in dynamic simulations that assess the reliability. This standard aims to assure that the parameters accurately represent the real power response of the generator unit to system frequency variation [12]. The modeling of the governor/turbine is one of the primary concerns in several NERC reliability regions, particularly in Eastern interconnection. According to Eastern Interconnection Frequency Initiative Whitepaper, the majority of the utilities that responded to the generator survey, have generator parameter values far from the standard values recommended by NERC [13]. Because of the continuous load variation, the frequency varies constantly in the power system. In the absence of a major disturbance, these variations are innocuous. But the response to the small frequency variations can cause wear and tear in steam boilers and turbines. Therefore, a dead-band controller in the governor is a necessity. The dead-band controller does not let the system to respond to the frequency deviations that do not exceed a pre-defined value. So, the speed governor is inactive within the dead-band limits [14]. Conversely, the dead-band controller can affect the stability of the power system. [15] studied the dead-band effect on the stability and showed that in South Eastern Europe, the governor dead-band can drastically change the stability of the European inter-area oscillation mode and create the limitation in the response. [16] discussed how to compensate the adverse effect of the dead-band on the stability by introducing a frequency bias parameter in the state space model of the governor. In [17-18], load dispatch concerns regarding frequency response variation is investigated. Relation between faults, transients and frequency response are addressed in [19]. Western Electricity Coordinating Council (WECC) recording of its large generation trips showed that it only observed 40% of the simulated governor response. Among other things, the dead-band is mentioned as one of the contributing factors [20-21]. So far, the reduced-form of the dead-band model is used for the study of the effect of the dead-band on the stability. The linearized reduced-form arguably cannot capture the nonlinearity of the dead-band. In this work, the full model of the dead-band is used in the newly introduced user-defined module (UDM) of ETAP. This module enables the users to create the sophisticated models that do not exist in the ETAP library.

2 Fig. 1. IEEEG1 model The paper will continue with the description of the governor and dead-band model in section II. Section III describes the case study and the results for the test case and discusses the results. Section IV closes the paper with drawing conclusion from the provided discussion and results. II. MODELING A. Governor model The IEEEG1 model is the IEEE recommended model for steam turbine. There are many other legacy models recommended by IEEE, but IEEEG1 is more detailed and also it entails more parameters that can accurately model the governor/turbine. The model is shown in Fig. 1. The model is used in ETAP UDM to model the governor/turbine. It s illustrated in Fig. 2. IEEEG1 is a general model of the steam turbine system with both the speed governor and the turbine stages. In this model, the dead-band block is included. The dead-band block represents the intentional dead-band that the plant control system incorporates into its working to prevent the governor from responding to the tolerable changes in the frequency [22]. Table 1 contains the parameter values used in the developed IEEEG1 model. These values are the real parameter values of one of the steam power plants in the eastern interconnection. B. Implementation of Droop characteristics The droop characteristics is used to control the magnitude of governor response for a given change in frequency. With the dead-band involved, there are two ways of implementing the droop characteristics: step implementation and non-step implementation. In step implementation of the droop characteristics, the regulation curve will step into the droop characteristics outside the dead-band. Through this setting, the governor response would step into the actual response that one would expect without the dead-band in place. Mathematically the step implementation can be represented as follows: Fig. 2. Developed Governor/Turbine model in ETAP UDM TABLE 1: IEEEG1 PARAMETERS PARAMETER DESCRIPTION VALUES DB Deadband block [Hz] K Governor Gain (1/droop) [pu] 20.0 T1 Lag Time Constant [s] 0.82 T2 Lead Time Constant [s] 0 T3 Valve Position Time Constant [s] 0.97 Uo Maximum Valve Opening Rate [pu/s] 1.38 Uc Maximum Valve Closing Rate [pu/s] Pmax The maximum output of the Unit [pu] 1.03 Pmin The Minimum output of the Unit [pu] 0 T4 Time Constant for Steam Inlet [s] 0.80 T5 Time Constant for Second Boiler Pass [s] 6.36 T6 Time Constant for Third Boiler Pass [s] T7 Time Constant for Fourth Boiler Pass [s] 0.86 K1 HP Fraction 0.19 K3 HP Fraction 0.22 K5 HP Fraction 0.21 K7 HP Fraction 0.38

3 y = 0 D x + D y = kx x + D y =+ kx x D Where k is the slope of the characteristics = 1/R (R = Droop Setting. Usually 5%) and D is a dead-band. NERC standards require the plant owners to have a dead-band not more than ±0.036 Hz. The step implementation is represented in Fig. 3a. In the non-step implementation of the droop, the characteristic curve starts from the dead-band and does not step into the standard curve. Through this implementation, the starting point of the curve will be at the dead-band and the slope will be at the regulation value (usually at 5%). (1) Mathematically the non-step implementation can be represented as follows: y = 0 D x + D y = k( x D) x + D (2) y =+ k( x+ D) x D Where k is the slope of the characteristics = 1/R (R = Droop Setting. Usually 5%) and D is a dead-band. The nonstep implementation of the droop characteristics is shown in Fig. 3b. in the main grid. The test case is inspired by a real system in an eastern interconnection To analyze the effect of deadband on the frequency response, the generator was equipped with two governor models; one without the deadband model and the other one with the deadband model using the non-step implementation of the droop characteristics. The reason for choosing the non-step implementation was that the inspiring real model used as a Fig. 3a. Step implementation of droop characteristics A. Case study III. RESAULTS AND DISCUSSION In this section, a small microgrid consisting of a generator and 4 loads is selected as a test case. Microgrids are smallscale form of an electric network and typically include combination of distributed generators and interconnected loads. Standby diesel generators, fuel cells, micro turbines, wind generators, and solar panels are typical types of generators in microgrids [9]. When a microgrid is connected to the main electrical network, it can work either in a grid-connected mode or an island mode. Grid-connected is a normal operation mode for a microgrid. However, under some circumstances, microgrids must function as an island or off-grid mode. The switching of grid connection can either be done automatically (because of a fault) or manually. They will be disconnected from traditional network to operate separately for grid disturbance mitigations and grid resilience improvement. Microgrids are able to work continuously to mitigate grid disturbances and increase flexibility of power systems. Consequently, they increase penetration of renewable energy resources in the grid. To observe the effect of the deadband in modeling of governor, a microgrid in an islanding mode has been picked. In the main grid, the effect of deadband under a small fault is not big enough. In case of a blackout, there would be a cascading faults one after another and in that case, the effect of deadbands are pronounced enough to be observed. Since it is hard to model the whole grid, a microgrid has been picked for this study. The effect of a small fault in a microgrid is as big as combined faults in the moments before a blackout Fig. 3b. non-step implementation of droop characteristics Fig. 4. Test Case Single Line Diagram

4 model of this test case has implemented its droop in the same way. The frequency response from the generator with and without the deadband is then compared. A.1. Initial Condition The generator of the system is rated at 711MW (790MVA at 0.9p.f) and is set to supply a total grid load of 370MVA represented by four lumped loads rated at 150MVA, 140MVA, 50MVA and 30 MVA respectively. Initially the system is set to supply 351MW of power (working with %50 of nominal power) to the system with the grid loads taking up 342MW (370 MVA with power factor of 0.95). The grid loads are split among two buses with each bus connected to the point of interconnection with transmission lines represented by impedances. A.2. Test Condition To test the generator for its frequency response, a 3 phase fault is simulated on one of the buses in the grid side and is cleared in 0.1 seconds. This leads to an increase in grid frequency, for which the governor responds by changing its output to support the frequency. The response from the generator in terms of its mechanical power and corresponding speed was recorded for analysis. B. Results and Discussion B.1. Case 1 The generator is equipped with a governor that has no deadband setting in its system. The response from the generator is shown in Fig. 5. If observed, since the governor is not reflecting the deadband setting, the generator responds to even the slightest change in frequency. The response continues till the speed (frequency) settles down at 3600 rpm (60Hz). B.2. Case 2 The generator is equipped with the governor that has the deadband setting incorporated into its model. The deadband is set at ±2 rpm (±0.033Hz) with non-step implementation of droop characteristics. The response of the generator is shown Fig. 5. Governor response without dead-band setting Fig. 7. Governor response with 20 rpm dead-band setting Fig. 8. Comparison of mechanical power Fig. 6. Governor response with 2 rpm dead-band setting Fig. 9. Comparison of generator speed

5 in Fig. 6. It can be observed that the response is much smaller than the response of the case without deadband and the speed settles down at a value outside the deadband value of 2rpm. Having 2rpm deadband helps the generator not change its mechanical power frequently. It saves a lot of unnecessary wear and tear. But the downside is that the speed does not settle where it was before the fault. But arguably there are secondary systems that helps the governor speed to settle to 3600 rpm eventually after a few minutes. So, as far as the system is capable to bring the speed into an acceptable range, the system is safe. B.3. Case 3 Without the dead-band in place, the speed deviation was around 10 rpm due to the fault condition. To analyze the response from the generator during conditions where the deviation is within the dead-band, the setting in the governor was changed to a ±20 rpm (±0.33Hz) dead-band and the system was tested for the same fault condition. The response is shown in Fig. 7. Comparing the mechanical power with case 1 and 2, there is hardly any response from the generator and the speed does not settle down at the rated value of 3600 rpm. Arguably it can be dangerous under some circumstance. So, there is always a tradeoff between the sensitivity of the governor and the amount of wear and tear that the mechanical system can tolerate in the long run. Figs 8 and 9 compare the results of all three cases. As it s shown, if the deadband is not modeled properly, the system model might show a response while in practice, there is not any response. The effect of missing a deadband model in the governor model is very small, but if the model of all the generators in a big region miss the deadband model, then the effect is noticeable. IV. CONCLUSION The deadband setting in a governor/turbine model can either be implemented as a step implementation or a non-step implementation, along with the droop characteristics. The effect of the deadband setting on the frequency response characteristics of a generator is analyzed by observing the response of the generator during a fault in the system. The governor without the deadband setting is more responsive to frequency deviations when compared to its response with the deadband. Also, it is observed that as long as the frequency deviation is within the deadband, there is hardly any response from the generator. These observations have led to the suggestion that it is important that the deadband setting is modelled as part of the steam unit governor models to obtain a response that is closer to the actual response. Without modeling of deadband, the simulated response from the system may predict a more responsive state compared to the real observations. As discussed, NERC report showed that the utilities in eastern interconnection had expected their system to handle the fault in the system. But the system had not responded as they expected. 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