Frequency stability, Inertia, ROCOF, STATCOM, Supercapacitors. FACTS, Multilevel.

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1 1, rue d Artois, F PARIS C4-110 CIGRE 016 http : // Power Intensive Energy Storage and Multilevel STATCOM for frequency and voltage grid support E. SPAHIC, M. PIESCHEL, R. ALVAREZ, G. KUHN, V. HILD, G. BECK Siemens, Transmission Solutions Germany N. PLATT Siemens Ltd. United Kingdom SUMMARY The developing political and environmental reforms are driving rapid evolution of today s power systems. There is a steady increase in electrical energy being generated from renewable energy sources like wind and photovoltaic systems while decommissioning large thermal power plants. This trend has a major impact on the performance of the electrical power systems especially with respect to their dynamic behaviour. Phasing out of existing conventional synchronous generators will adversely affect auxiliary services like voltage control and frequency regulation. This paper discusses one solution to cope with these challenges: the use of a Modular Multilevel Converter based STATCOM with an additional frequency stabilizer can significantly contribute to the stable operation of a power system. The application of the frequency stabilizer has been analysed on a representation of the Ireland (All-Island) transmission grid of Ireland and the results have been presented and discussed. It has been shown that a Frequency Stabilizer of 50 MVA can reduce the frequency deviation by more than 0.1 Hz and thereby avoiding load shedding. Furthermore the ROCOF was reduced from app Hz/s to 0.4 Hz/s which could be crucial for the setting of the ROCOF protection and the grid stability. KEYWORDS Frequency stability, Inertia, ROCOF, STATCOM, Supercapacitors. FACTS, Multilevel. ervin.spahic@siemens.com

2 1. INTRODUCTION The changes in the power generation mix i.e. reducing generation from conventional power plants and the increase of decentralized power sources (particularly wind and photovoltaic) are continuously challenging the secure operation of the transmission grids [1]. The resulting decrease of system inertia further impacts the system stability during transient frequency events [], [3]. A sudden loss of a large generating plant will result in dangerously low frequencies [4] which could further escalate into cascading failures and eventually result in a blackout. Additionally, lower system inertia results in higher ROCOF (Rate of Change of Frequency). ROCOF relays in the generating stations are generally programmed to withstand a specific threshold (e.g. 0.5 Hz/s today in Ireland). With no additional measures, these values are bound to increase in the future. This reduces the reliability of the entire system which is an undesirable situation. Modern power electronic converter technology provides a vast range of applications for ac grid stabilisation. In this paper a new type of converter is presented to stabilise the ac grid s voltage and frequency. Since the application of STATCOMs for voltage support is already a well-known solution, the focus in this paper will be on the new application frequency stability.. SYSTEM INERTIA, FREQUENCY STABILITY AND ROCOF The initial response of a conventional generator to a frequency event in the grid is governed by its stored kinetic energy at the rated frequency: E 1 = J ( p ) (1) Ekin H gen = S () kin f nom where: J moment of inertia, f nom nominal frequency, H gen inertia time constant, S gen rated power of the generator. From (1) and () the maximum duration of the nominal active power infeed without any mechanically power input until it would stop rotating can be estimated. Depending on the type of the conventional generator as well as on its size different inertia constants can be applied [5]. gen On the other hand most renewable sources like wind turbines are connected with power converters to the grid, thereby decoupling the generator from the grid frequency. Exceptions are some older wind turbines and doubly fed induction generators which act very similar to asynchronous motors. Furthermore photovoltaics (PV) do not have any rotating mass. This means that the inertia contribution from PV and most wind turbines is negligible. When a frequency event occurs, a synchronous machine will automatically inject or absorb kinetic energy into or from the grid to counteract the frequency deviation. This behavior is called inertial response because the inertia dampens the frequency changes. For a drop of the frequency it has to be considered that the generator will release an amount of energy depending on the nadir (minimum) of the frequency. So until the frequency f nadir is reached the synchronous generator will only supply the available energy: E av = E 1 ( pf ) - ( pf ) ) kin, nom - Ekin, nadir = J nom nadir (3)

3 Taking this into account the available inertia time constant of each synchronous generator H av can be defined as a function of the nadir frequency: E f - f H H av nom nadir av = = S gen fnom gen (4) Hence the generator injects more kinetic energy if the nadir of the frequency is lower. A maximum deviation of 1 Hz in a 50 Hz system for example will lead to an available inertia and thus energy of about 4 % of the nominal value. The system inertia is the combined inertia of the rotating masses of all synchronous generators and turbines. The system inertia time constant (H sys ) is given by the weighted average of the generating units: H sys = å i H gen, i S S sys System inertia can have large variations during a day or season, depending on the operating power plants. For example in the night only a few generators are operating and the inertia of the system reduces. The lower the system inertia of a power system, the higher is the sensitivity of the grid frequency to abrupt changes in generation and load patterns. The system inertia determines the ROCOF (rate of change of frequency): ROCOF = df dt gen, i DP 1 = S H sys The maximum active power infeed of an individual generator can be calculated with (7) for a given ROCOF. P max sys max f H gen df = Sn (7) f dt nom It can also be seen that when the inertia constants are based on the apparent power of each individual generator the equation for H sys can be simplified to an addition of the inertia constants of the generators. The simulation results of the impact of different H sys on a test grid are as shown in Figure 1. Various inertia constants were applied and the behaviour of the system was recorded, whereas the frequency event was consistent with the loss of the largest generator in the grid. Frequency (Hz) 50, (5) (6) 50,0 49,8 49,6 H = 11 s H = 16 s H = 1 s H = 36 s 49,4 49, 49, Time (s) 15 0 Figure 1: Impact of different system inertia (H) on the frequency 3

4 With lower inertia of the system the transient frequency drop is much lower and faster (higher ROCOF) - see blue solid line. On the contrary with higher system inertia (synonymous with more conventional generators in operation) the frequency drop and ROCOF are much lower - purple line. It can thus be inferred that an increased renewable energy penetration, or in other words a reduction of the overall system inertia in the grid, produces a significant impact on the ROCOF in the case of network events such as loss of generation, load loss, sudden large load changes etc. In the UCTE/ENTSO-E operation handbook [6] there are stepwise security measures defined in case of frequency events. Firstly at 49.8 Hz (deviation of 00 mhz) the quick-start power plants should be connected to the grid. Secondly when the frequency reaches 49 Hz, the recommendation is to perform the 10-0 % load shedding. Thirdly at 48.7 Hz, further % load shedding shall be performed. Additional 10-15% load shedding is performed at 48.4 Hz. If all these measures do not regain stable operation of the system and the frequency reaches 47.5 Hz, then all power plants have to be disconnected from the grid without any time delay to safeguard their auxiliary power supplies. The primary task of the Frequency Stabilizer is to reduce the initial ROCOF in the system and the frequency drop in order to avoid any of these security measures (e.g. load shedding) or even to avoid blackouts by fast injecting of active power into the grid (provide virtual/synthetic inertia) during transient events in the system. An example of one such application is shown in the following section. Additionally the Frequency Stabilizer is also capable of absorbing active power in scenarios of sudden fluctuations of wind/pv generation or load rejection which may cause the system to tend to dangerous over frequency scenarios which are not desired. 3. SVC PLUS FREQUENCY STABILIZER The SVC PLUS Frequency Stabilizer comprises a STATCOM and supercapacitors for energy storage. The advanced STATCOM is a power electronics based modular multilevel converter (MMC), also known as SVC PLUS [7], for reactive power generation. A typical single line diagram is shown in Figure grey part. STATCOM (SVC PLUS) Frequency Stabilizer (FS) Short time power intensive storage A B C Figure : Single line diagram of a typical configuration of SVC PLUS Frequency Stabilizer 4

5 Despite its main application as a flexible ac transmission system (FACTS) device, it is also employed for voltage balancing for grid fed railroad lines and flicker mitigation of electric arc furnaces. The reactive power output of the MMC Scon in (8) is limited by the current carrying capability Irated of the employed power electronics which is an insulated gate bipolar transistor (IGBT) type. In order to increase the output power capability, the connection voltage Ubus needs to increase which can achieved by increasing the number of series connected sub-modules. The SVC PLUS Frequency Stabilizer has two main applications: supporting reactive and active power, whereas providing active power during frequency events is prioritized [8]. S con = 3U bus I rated (8) The Frequency Stabilizer (blue part) embedded into the sub-modules in the construction of the device is as shown in Figure. It is based on a very fast short time power intensive storage which supports the grid at frequency events with full rated converter power. Based on the demands for the frequency support (short time, high power), supercapacitors have been chosen as the storage medium [8] and [9]. In Figure 3 the layout of a SVC PLUS FS Level/tower has been shown. On the right side the setup in the testing facility is shown. Figure 4 shows an example for the layout of an entire SVC PLUS FS station rated at 50 MVA. The grid connection has been assumed to be at high voltage level (13/0 kv). DC inductances DC capacitor PM1 PM SCM s Figure 3: Layout of SVC PLUS FS Level/tower (SCM supercap module, PM power module) and two towers in a testing facility 5

6 Figure 4: Example for the layout of a 50 MVA SVC PLUS FS station The main electrical parameters of the SVC PLUS Frequency Stabilizer are given in Table 1. Table 1: Main Electrical Parameters of SVC PLUS FS Rated power 50 MVA Rated active power 50 MW Stored energy 400 MJ Inertia of SVC PLUS FS (H) 8 s Currently there are also other storage technologies in discussion and being tested in pilot projects around the world, mainly focussing on battery storages with high energy densities. A comparison of the SVC PLUS Frequency Stabilizer and typical battery storage applications for fast frequency response (full power for duration up to several seconds) is given in Table. Table : Investment and Footprint of SVC PLUS Frequency Stabilizer and Typical Battery Storage for Applications of Frequency Response 50 MVA solution SVC PLUS Typical battery Frequency Stabilizer storage Investment (p.u.) Footprint (p.u.) As can be seen from the table, investment costs and footprint requirements are much lower for the Frequency Stabilizer utilizing supercapacitor technology compared with battery storage. Coupled with advantages of high power density and modularity, the supercapacitor technology was chosen for the analysed specific grid applications. 4. APPLICATION IN THE TRANSMISSION GRID OF ALL-ISLAND The All-Island power transmission system represents the synchronous operation of Republic of Ireland (EirGrid) and of Northern Ireland (SONI). The All-Island transmission grid is a very good example of future developing grids and changes in the transmission grids with special focus on transient frequency behaviour. The governments have set an ambitious target 6

7 of 40 % of renewable power integration for the Ireland (All-Island) power system by 00. The primary target however would continue to be to enable secure, efficient and most importantly reliable operation of the power grid. In order to maintain these targets, detailed technical analyses are already in progress to propose new solutions. These solutions are oriented to both new products as well as additional system services. A representation of the planned Ireland (All-Island) transmission grid for the year 0 was modelled based on [10]. The modelled grid comprises of over 400 lines, 300 substations and 150 loads see Figure 5. The modelled transmission system of Ireland (All-Island) consists of 4 different transmission voltages (400 kv, 75 kv, 0 kv and 110 kv). Figure 5: Model of the transmission grid of Ireland (All-Island) To highlight the performance of the SVC PLUS FS in the grid, a test case involving both transient frequency as well as the transient voltage events are considered as shown in Figure 6. The operation point of the system is taken to be summer 0 low load case. The transient voltage event is modelled as a three phase short circuit fault in the grid at time t = s and is cleared successfully after a further 150 ms. The frequency event follows at time t = 10 s which is defined as a loss of a 500 MW generation unit in the grid. Frequency (Hz) 50,4 Voltage (pu) 1, 50, 50,0 49,8 49,6 49,4 49, System Frequency Voltage at the PCC 1 0,8 0,6 0,4 0, 49, Time (s) Figure 6: Frequency of the system and voltage at the PCC with SVC PLUS FS 0 7

8 The response from the SVC PLUS FS device to the transient events is as shown in Figure 7. The device is capable of providing reactive power support in order to aid the AC system during the transient voltage event. Clearly the device reacts very quickly to voltage deviations and provides its rated reactive power (limited by the residual voltage) to the system. The reaction time post fault is also quick, thus the voltage is brought back to its set-point. Power (MW/Mvar) 60 Active power 50 injection (MW) Reactive power 40 injection (Mvar) Time (s) Figure 7: Response of the SVC PLUS FS for both voltage and frequency event Continuing on to the transient frequency event, a loss of 500 MW of power causes the frequency to cross its defined deadband and thus triggers the active power injection from the SVC PLUS FS device as shown in Figure 7. Since the reactive power controller is a very fast controller, the small deviation in voltage during the transient frequency event is also acted upon and there is a corresponding increase of reactive power injection of approx. 0 Mvar. The corresponding increase in active power injection is countered with a steady decrease in the reactive power. However, immediately the active power injection reaches its rated value of 50 MW, the priority limiter limits reactive power to zero. The duration of the active power injection continues as long as possible until the available energy falls below its lower setpoint. Figure 8 shows the activation of the f loop in the active power control, and hence the active power injection follows the frequency after 15 seconds. However this is independent in case of a ROCOF detection, where the df/dt loop as shown in Figure 3 takes over and rated active power is injected into the grid within 00 ms of the detection of the fault and continues until the available energy is consumed. There is a clear improvement in the frequency with the SVC PLUS FS device as shown in Figure 8, where the frequency nadir is improved by app. 100 mhz. Frequency (Hz) 50,1 49,9 49,7 w/o SVC PLUS Frequency Stabilizer with SVC PLUS Frequency Stabilizer 49,5 49,3 49,1 48, Time (s) Figure 8: Comparison of the frequency with and without SVC PLUS FS in the Ireland (All- Island) transmission grid 8

9 The average of the ROCOFs measured is used to represent the average ROCOF of the entire system. These values are measured and averaged over a 500 ms window (moving average filter). A reduction in the ROCOF from app Hz/s to 0.4 Hz/s has also been achieved with the application of only one 50 MVA SVC PLUS FS - see Figure 8. This can play a crucial role when setting the ROCOF protection. To elaborate on the impact of higher ROCOF values in the future from a machine perspective, in terms of mechanical stress evaluation, higher torque values result from increasing ROCOF values. For example the current ROCOF setting in Ireland is 0.5 Hz/s, for which the calculated maximum torque is 140 % [4]. This also remains a crucial factor to be considered especially when possible solutions such as increasing the threshold values in the existing ROCOF relays are presented and discussed. In the above cases discussed the SVC PLUS FS aids the initial inertial response provided by the synchronous generators in the grid. Since the system inertia of the grid in the future is expected to be lower, the risk of ROCOF exceeding the current threshold looms large for small and medium islanded systems. 5. CONCLUSION The SVC PLUS Frequency Stabilizer presents a solution for the future grid with more renewable and less conventional generation and improves both voltage and frequency stability. The device emulates synthetic inertia, utilizing the energy stored in the supercapacitors, releasing it in case of transient events when the frequency thresholds are reached, thus ensuring a continued and safe operation of the grid. The initial ROCOF can be reduced owing to the fast control response to deviations in the frequency, thus avoiding preventive measures such as load shedding or even possible blackouts. In comparison to other solutions feasible for frequency support, the application of SVC PLUS Frequency Stabilizer with features such as modularity and high power intensive support particularly in case of fast frequency response, offers economic as well as technical benefits. ACKNOWLEDGMENT This work was supported in part by the German Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety Nr

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