Safety Considerations and Protection Practices in Grid Connected Home Energy Storage System (HESS)
By Md Rukonuzzaman
Thanks to the introduction of feed-in-tariff (FIT) and net-metering system, prosumers have the options either to store the extra power generated by distributed generators to the battery or deliver the extra power to the utility grid when load demand is less than the generating capacity. The sharp fall of the price of Li-ion batteries and long lifetime compared to other energy storage technologies have further increased the popularity of battery storage based distributed generators among the household and small-scale industrial consumers. HESS based distributed generators ensure reliable and uninterruptible power supply during the power outage and even when PV based distributed generators are not available. Distributed power generation and storage in household consumers involve bidirectional battery inverter and PV inverter in two separate units or hybrid inverter (PV inverter plus bi-directional battery inverter) in a single unit, and sophisticated white goods like refrigerator, washing machine, and microwave oven. Stringent measures need to be taken into consideration while designing the energy storage system as integrated with distributed generators to protect household electrical and electronic equipment from damage, and prosumers and maintenance personnel from hazardous electrical shock.
This article focuses on safety functions and protection features of home energy storage system (HESS), which are considered in distributed generators to make the system reliable, safe and robust.
Islanding occurs when grid power is unavailable, and grid connected distributed generators continue generating power. Unintentional islanding in a smart grid with various distributed power generation systems is a serious safety concern for personnel and utility grid connected equipment. Therefore, it is critical to quickly detect a utility grid power outage and isolate equipment.
Islanding may be detected using passive methods such as UVR (Under Voltage Relay), OVR (Over Voltage Relay), UFR (Under Frequency Relay), and OFR (Over Frequency Relay). These methods are enough in most of the cases to detect islanding except in non-detection zones (NDZ) when a variation of real and reactive powers either received from or supplied to the grid at the point of common coupling (PCC) is very low and consequently when both active and passive detection techniques are required. The worst case occurs when the load only receives power supplied by the distributed generator. In this situation and during blackouts, the frequency or the voltage at the point of common coupling is unaffected and the OVR/UVR/OFR/UFR protection functions do not work. In practice, implementation of active methods such as injecting reactive power continuously based on the change in fundamental or other harmonic components can detect islanding in all load conditions including NDZ.
Low Voltage Ride Through (LVRT) Strategy
The grid-code regulation, which is known as the Low-Voltage Ride-Through (LVRT) strategy, requires HESS to remain connected to the grid when the voltage drops for a specific period to avoid grid blackouts. LVRT strategy requires distributed power generators to remain in operation and support the grid with reactive current. On the contrary, anti-islanding detection techniques require distributed generators to detect islanding and immediately isolate the utility grid during a grid blackout. While LVRT strategy and anti-islanding detection appear to conflict, distributed generators must have the ability to implement LVRT capability and anti-islanding detection simultaneously. This requirement ensures that the system connected to the grid operates properly when the grid voltage drops. Additionally, LVRT requirement contributes to the recovery of the grid voltage by supplying the reactive current in the designated voltage range.
Grid Voltage Rise Control
Distributed generators inject active power into the grid when load demand is lower than its generation capacity. This may cause a voltage rise over the line and lead to high voltages at the PCC. In case the voltage at PCC exceeds the permissible voltage level, additional measures must be provided. Modern distributed generators can control their reactive power output, a functionality that can be used to influence the voltage at their PCC.
Relay Coordination Strategies
Protective relays at the HESS based distributed generator must be coordinated with other distributed generator relays connected to the utility grid to minimize disruption when a fault occurs. The key principle is that protective relay systems are designed to protect against faults within a specific zone. For HESS with PV inverters and bidirectional battery inverters, battery charging must be coordinated such that it is not simultaneously charged from PV inverter output and utility grid. Similarly, during normal load operation when grid power is available, relays should be connected in a way such that critical loads are powered by either the utility grid or distributed generator. Moreover, the protective relays should be turned on and off at or near the zero-crossing point of the utility grid voltage or distributed generator’s voltage to avoid excessive current flow and relay damage.
Fault Monitoring and Diagnosis
Remote fault monitoring is a useful troubleshooting tool for HESSs manufacturers and consumers. In inverters, monitoring different parameters before, during and after a fault will enable accurate diagnosis and corrective actions. Moreover, analyzing data acquired from monitoring distributed inverters operated by other consumers and connected to the central cloud server, allows implementation of effective preventative maintenance plans and early fault detection.
Reverse Power Flow Prevention
In certain regions of the world where the price of utility power is variable, HESS based distributed power is not allowed to flow in the reverse direction to the utility grid. When allowed, it can deliver power to the utility grid only for a limited period. For compliance, the HESS power conditioner should have the capability to detect reverse power flow within a specified time and disconnect the energy storage system from the utility grid to prohibit excess reverse power flow into the utility grid.
During long utility power outages, the HESS, especially the battery inverter should have the capability to black start. In the pre-charging mode, the utility grid is used to wake up the battery of the battery inverter from sleep mode requiring a very small current draw from the battery cells. During power outages, the independent mode of PV inverter can be utilized instead to pre-charge the battery. After waking the battery up from sleep mode, the independent output of the PV inverter can be synchronized with that of the battery inverter to charge the battery as well as to deliver power to critical loads.
Flicker and Other Power Quality Problems
HESS should incorporate power quality requirements such as withstanding voltage sag, voltage flicker, harmonics, voltage unbalance and frequency variation to ensure high quality of distribution power and maintain stable and reliable utility grid operation.
Safety of the Battery and Additional Protective Functions
Consumers should be protected from physical injuries caused by an abnormal HESS battery operation. To ensure consumer safety, battery management systems in HESS must include overcharging and over-discharging protection, temperature rise suppression, overcurrent protection, and unbalance in cell voltage protection. Other protective functions of the grid connected inverter include AC over-current, DC overvoltage and under-voltage, DC-component detection, AC overvoltage and under-voltage detection, instantaneous (unbalanced) overvoltage, suppression function for temperature rise, suppression function for voltage rise, soft-start function feature. The inverter shall not have reactive malfunctions because of distribution line voltage distortions, surge voltage, or other external noises or create adverse effects such as radio noise, conducted disturbance etc. to outside.
Integration of HESS can result in robust, and safe power systems and provide consumers with same or better quality of uninterrupted power supply to various loads. When connecting HESS, stringent safety precautions, protection practices, and diagnostic capabilities need to be satisfied to ensure reliable power supply from distributed generators.
This article edited by Hossam Gabber