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| Influence of Action Delay and Amount on the Control Effectiveness of Low Inertia Systems in Frequency Emergency Control |
| Li Zhaowei1,2, Fang Yongjie1,2, Wu Xuelian2, Li Jianhua3, Lai Yening2 |
1. School of Electrical and Electronic Engineering North China Electric Power University Beijing 102206 China;
2. Nari Group Corporation State Grid Electric Power Research Institute Nanjing 211106 China;
3. East China Branch of State Grid Corporation of China Shanghai 200120 China |
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Abstract Frequency emergency control (FEC) is the second defense line to ensure frequency safety after power grid failures, usually triggered by event of fault, and has become a standard configuration of ultra-high voltage DC (UHVDC). With the rapid development of high proportion of new energy (NE), the frequency regulation ability and system inertia are continuously decreasing. The frequency response characteristics after power grid faults have some new characteristics of large drop depth and fast speed. The effectiveness of emergency control is a key factor related to the frequency safety.
This article first establishes frequency response model and transfer function of a high proportion NE power system considering FEC. To analyze the effectiveness of emergency control, this article focuses on two types of disturbances: permanent power disturbances (PPD) and short-term power disturbances (STPD). PPD are represented by faults such as DC blocking, unit tripping, and NE disconnection; STPD are represented by low voltage ride through (LVRT, without disconnection) of NE or flexible DC, DC commutation failure.
According to the derivation, the time-domain expression of the system frequency response considering FEC under PPD can be obtained. FEC may not always solve the transient frequency insecurity risk depending on whether the system deviation reaches the control threshold of under-frequency load shedding (UFLS) or over-frequency generator tripping (OFGT) after FEC. When FEC action delay τ0 is less the time when the frequency reaches its lowest point tm, the maximum frequency deviation will be improved. But excessive emergency control amount can cause the risk of transient high-frequency in the system. When the delay of FEC is greater than the moment when the system's minimum frequency occurs, FEC cannot improve the maximum frequency deviation. For the current technical level, the emergency control delay is considered as 300ms.If a PPD of 30% load capacity occurs, the limit value for NE access is 65% of the load.
Under STPD the steady-state frequency deviation is zero. With the continuous increase of NE, DC and other power electronic power sources, the amount of STPD will continue to increase. In severe cases, it can also cause the system's transient frequency to cross unsafe boundaries, which may lead to the current frequency corrective control line action and lead to power outages. As the proportion of synchronous machines decreases, the released kinetic energy decreases due to the rotational inertia. So that under the same STPD, the lowest frequency of the system gradually decreases to the threshold value of the third defense such as UFLS, and the demand for FEC is becoming increasingly high. Although timely FEC can improve the minimum frequency and avoid triggering UFLS, excessive FEC may lead to risks of OFGT. As the proportion of synchronous machines gradually decreases, the range of reasonable amounts of FEC that can be taken gradually decreases. When the proportion of synchronous machines is less than 34% of load, under a given STPD boundary, FEC cannot solely rely on traditional one-time actions to avoid triggering UFLS and OFGT.
The following conclusions can be drawn. (1) The large-scale replacement of traditional synchronous power sources with NE has led to a decrease in the inertia and frequency regulation ability of the power grid, and the increased ROCOF is an important reason for affecting the adaptability of FEC. (2) In a high proportion of NE grids, large-scale LVRT of NE and flexible DC, conventional DC commutation failure caused by AC short circuit faults can generate significant STPD. Compared to traditional DC blocking faults, it may cause system frequency "flicker" and easily trigger UFLS. When responding to FEC, it is necessary to not only meet the low-frequency control requirements, but also avoid interlocking high-frequency after control implementation, which becomes more and more difficult to adapt to.
STPD such as large-scale LVRT of NE pose significant challenges to the frequency security defense system. Further research is needed on how to improve the grid performance of NE, build grid inertia capacity, optimize FEC and corrective control strategies based on the actual needs of the power grid.
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Received: 23 July 2023
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2024, Vol. 39 
