Electric Power Systems and Smart-Grid Evolution

Introduction

Electric power systems convert primary energy into electrical energy, transport that energy over long distances, distribute it to customers, and enable its safe and useful consumption. For more than a century, these systems were designed around a largely one-way model: relatively few large generators supplied predictable demand through transmission and distribution networks. This model produced substantial economies of scale and high reliability, but it also created technical, economic, environmental, and institutional limitations.

The smart grid represents an evolution of the power system rather than a completely separate technology. It combines conventional electrical infrastructure with sensors, communications, automation, advanced protection, data platforms, distributed energy resources, and new market and operating practices. Its purpose is to make electricity systems more observable, controllable, flexible, resilient, efficient, and participatory.

A smart grid is therefore not defined by a single device or communication standard. It is a coordinated cyber-physical system in which electrical flows, information flows, control actions, market signals, and customer decisions interact.

Learning Objectives

After studying this chapter, you should be able to:

1. Describe the functions of generation, transmission, distribution, and utilization.
2. Explain why electric power systems are interconnected and operated as networks.
3. Identify limitations of conventional grids and the drivers of modernization.
4. Distinguish important smart-grid terms and technologies.
5. Explain smart-grid characteristics, stakeholders, and development pathways.
6. Analyze how distributed energy resources and digital control alter grid operation.
7. Evaluate technical, economic, regulatory, cybersecurity, and social issues in smart-grid deployment.

1. The Electric Power System as an Interconnected Chain

The power system is commonly divided into four functional stages:

• Generation: production of electrical energy from primary energy sources.
• Transmission: bulk transfer of power, usually at high voltage, across regions.
• Distribution: delivery from substations to homes, commercial premises, and smaller industrial customers.
• Utilization: conversion of electrical energy into useful services such as lighting, motion, heating, cooling, communication, and computation.

These stages are physically interdependent. A generator cannot deliver useful power without a network, and a distribution network cannot serve customers without adequate generation and transmission capacity. Protection, control, communications, forecasting, maintenance, and market arrangements support all four stages.

1.1 Generation

Generators convert mechanical, chemical, nuclear, solar, wind, hydroelectric, or other forms of energy into electricity. Conventional synchronous generators use electromagnetic induction: a rotating magnetic field induces alternating voltage in stationary windings. Inverter-based resources, including photovoltaic systems and battery energy-storage systems, connect through power-electronic converters rather than directly through rotating machines.

Generation technologies differ in controllability, response speed, operating cost, emissions, geographic constraints, and availability. A hydroelectric unit may respond rapidly, while a nuclear unit is often operated for sustained output. Solar and wind power have low fuel cost but variable output linked to weather. Gas turbines can provide flexible output, although fuel price and emissions affect their economics.

Power-system planners distinguish between energy capacity and power capacity. Power capacity is the maximum instantaneous output, measured in watts. Energy is the accumulated production or consumption over time, measured in watt-hours. A 10 MW battery with 40 MWh of stored energy can theoretically deliver 10 MW for four hours, subject to efficiency, operating limits, and state of charge.

1.2 Transmission

Transmission networks operate at high voltages to transfer large amounts of power efficiently. For a balanced three-phase system, apparent power is related to line voltage and current by:

where S is apparent power in volt-amperes (VA), V_L is line-to-line RMS voltage in volts (V), and I_L is line current in amperes (A). This relationship assumes a balanced three-phase system and uses RMS quantities.

For a three-phase load, real power is:

where P is real power in watts and cosφ is the power factor. If the same real power is transmitted at a higher voltage, the current is lower. Since resistive losses in a conductor are approximately:

where R is the resistance of one phase conductor in ohms, high-voltage transmission reduces current-dependent losses. The expression assumes three identical conductors and neglects other losses such as corona and dielectric loss.

Transmission systems are interconnected to share reserves, improve reliability, access diverse resources, and reduce the need to build generation for only a local peak. Interconnection, however, also introduces risks: disturbances can propagate across boundaries, and power flows follow network physics rather than contractual paths alone.

1.3 Distribution

Distribution systems begin at substations that reduce transmission voltage to medium-voltage levels. Feeders then deliver power through overhead or underground conductors to local transformers. Distribution transformers reduce voltage again for customer service.

Traditional distribution feeders were designed mainly for one-way power flow from the substation toward customers. Photovoltaic generation, batteries, electric vehicles, and flexible loads can produce reverse or bidirectional flows. This affects voltage regulation, protection coordination, thermal loading, fault current, and power-quality management.

Distribution networks may be radial, looped, or meshed. Radial systems are relatively simple and economical but have fewer alternative supply paths. Automated switches and tie points can reconfigure a feeder after a fault, isolating the damaged section and restoring unaffected customers.

1.4 Utilization

Utilization includes all customer-side equipment and services. Residential loads include lighting, refrigeration, heating, cooling, appliances, and electronics. Industrial loads may include motors, furnaces, drives, pumps, compressors, and process-control systems. Commercial buildings often have substantial heating, ventilation, air-conditioning, lighting, and computing demand.

The electrical system must maintain acceptable voltage and frequency while meeting changing demand. Loads are not equally flexible. A water heater may be shifted by several minutes or hours, whereas a hospital life-support system cannot be interrupted. This distinction is central to demand response and smart-grid planning.

2. Fundamental Operating Principles

2.1 Balance of supply and demand

At every moment, generation and consumption must be balanced, allowing for network losses. In a simplified system-wide form:

The terms represent real power in watts. The equation is a balance relationship, not a complete dynamic model. It assumes a common time interval and a consistent sign convention. If generation temporarily falls below demand, stored kinetic energy in rotating machines is released and frequency declines. Control systems respond through generator governors, automatic generation control, storage, demand response, or emergency measures.

2.2 Frequency and voltage

Frequency reflects the balance between active power supplied and consumed in an interconnected synchronous area. Voltage is strongly influenced by reactive power, network impedance, and local operating conditions. Active and reactive power are coupled in actual networks, although engineering analyses often examine them separately under suitable approximations.

A system operator must maintain frequency within operating limits and voltage within acceptable ranges. Protection systems must detect abnormal conditions rapidly, while control systems must act selectively so that a local fault does not become a widespread outage.

2.3 Reliability and resilience

Reliability generally concerns the ability to provide electricity continuously and within specified quality limits under expected conditions. Resilience concerns the ability to withstand, adapt to, recover from, and learn from disruptive events such as storms, wildfires, cyberattacks, equipment failures, fuel interruptions, or extreme heat.

Reliability planning commonly uses criteria such as reserve margins, contingency analysis, and interruption indices. Two frequently used distribution indices are:

Here, U_i is the annual interruption duration for customer group or event i, N_i is the number of customers interrupted in that event, and N_T is the total number of customers served. SAIDI is typically expressed in customer-hours per customer, often reported as minutes per customer per year; SAIFI is interruptions per customer per year. Definitions and exclusions vary by jurisdiction, so comparisons require care.

3. Limitations of the Conventional Grid

The conventional grid has achieved high performance, but several characteristics constrain it.

Limited observability

Many transmission substations are highly instrumented, whereas large parts of distribution networks historically had limited real-time measurement. Operators may not know the exact voltage, loading, or status of every feeder segment. This makes fault location, congestion management, and distributed-resource coordination more difficult.

One-way design assumptions

Traditional infrastructure assumed centralized generation and passive customers. High penetration of rooftop solar, electric vehicles, batteries, and flexible loads challenges these assumptions and creates bidirectional power and information flows.

Aging infrastructure

Transformers, conductors, breakers, poles, underground cables, and control systems require replacement or refurbishment. Aging assets may have increased failure probability and limited compatibility with modern monitoring and automation.

Variable renewable generation

Wind and solar generation vary with weather and time. Their variability is not inherently unmanageable, but it increases the value of forecasting, flexible generation, storage, stronger interconnection, transmission expansion, and demand response.

Peak demand and low utilization

Networks must often be sized for a small number of high-demand hours. Air-conditioning peaks, electric-vehicle charging, and electrification of heating can increase local and system peaks. Underused capacity raises costs unless planning and tariffs encourage efficient utilization.

Incomplete coordination

Utilities, independent generators, aggregators, customers, regulators, equipment manufacturers, and technology providers may possess different objectives and data. Poor coordination can produce technically sound devices that do not deliver system-level value.

4. Drivers of Smart-Grid Modernization

Smart-grid development is driven by several interacting forces:

1. Decarbonization: electrification of transport, heating, and industrial processes can reduce emissions when electricity production becomes cleaner.
2. Distributed energy resources: rooftop photovoltaics, batteries, microturbines, fuel cells, and controllable loads are increasingly connected at distribution level.
3. Reliability and resilience: severe weather and other hazards motivate self-healing networks, sectionalizing, microgrids, and improved situational awareness.
4. Customer participation: advanced meters and digital platforms can support time-varying tariffs, demand response, and energy-service choices.
5. Operational efficiency: automation can reduce losses, truck rolls, restoration time, and unnecessary reserve requirements.
6. Technology progress: sensors, wireless networks, cloud computing, edge computing, power electronics, and artificial intelligence have become more capable and affordable.
7. Policy and regulation: renewable targets, reliability standards, data-protection rules, market reforms, and public funding influence investment pathways.

These drivers do not imply that every grid requires the same solution. A rural radial network, an urban underground system, and an islanded microgrid have different priorities and constraints.

5. Smart-Grid Terminology and Architecture

A smart grid is an electric power system enhanced by two-way communication, sensing, computation, automation, and coordinated control across generation, transmission, distribution, and customer domains.

Important related terms include:

• Advanced metering infrastructure (AMI): smart meters, communication networks, and data-management systems supporting measurement and two-way exchange.
• Distributed energy resource (DER): a small-scale generation, storage, or controllable demand resource connected near customers or within distribution networks.
• Demand response: deliberate change in customer electricity consumption in response to prices, incentives, reliability needs, or control signals.
• Microgrid: a group of interconnected loads and DERs with clearly defined electrical boundaries that can operate connected to, or independently from, the main grid.
• Virtual power plant (VPP): coordinated aggregation of distributed resources that behaves operationally or commercially like a larger power plant.
• Distribution management system (DMS): software supporting monitoring, analysis, switching, voltage control, outage management, and feeder operations.
• Energy management system (EMS): control and optimization functions for generation, transmission, or customer facilities.
• Supervisory control and data acquisition (SCADA): systems for remote monitoring and supervisory control of field equipment.
• Phasor measurement unit (PMU): a device that measures synchronized voltage and current phasors using a common time reference.
• Interoperability: the ability of diverse devices, systems, and organizations to exchange and correctly use information.
• Prosumers: customers who both consume and produce, or store, electricity.
• Self-healing grid: a network able to detect, isolate, and restore service after faults with limited human intervention.

A useful conceptual architecture contains five interacting layers:

1. Physical layer: generators, lines, transformers, switches, meters, loads, and storage.
2. Measurement and sensing layer: sensors, intelligent electronic devices, meters, and event recorders.
3. Communication layer: fiber, radio, cellular, mesh, utility private networks, and relevant protocols.
4. Application and control layer: state estimation, forecasting, protection, optimization, outage management, and automation.
5. Market, policy, and governance layer: tariffs, settlements, regulation, privacy, cybersecurity, and institutional responsibilities.

A failure in any layer can reduce overall performance. For example, an accurate algorithm is not useful if sensors are poorly calibrated, communications are unavailable, or regulations prevent the required control action.

6. Characteristics of a Smart Grid

A mature smart grid commonly exhibits the following characteristics.

Observability

Sensors and communication provide timely information about topology, voltage, current, frequency, asset condition, and customer demand. Observability supports better decisions but requires data quality, time synchronization, cybersecurity, and appropriate analytics.

Controllability and automation

Remote switching, voltage regulation, inverter controls, and automated restoration allow operators to respond more quickly. Automation must be coordinated with protection settings and human oversight.

Flexibility

Flexible generation, storage, interconnection, and demand can respond to uncertainty and changing conditions. Flexibility is especially valuable when renewable output and demand are variable.

Interoperability

Devices from different manufacturers should exchange information through recognized interfaces and standards. Interoperability reduces vendor lock-in and supports phased modernization.

Resilience and security

The smart grid must tolerate equipment faults, communication failures, physical attacks, cyber incidents, and extreme events. Cybersecurity should use defense in depth, least privilege, network segmentation, secure authentication, logging, patch management, and recovery planning.

Customer empowerment

Customers may access consumption information, participate in demand response, install generation and storage, or select energy services. Participation must be voluntary or transparently governed, accessible, and supported by understandable information.

Sustainability and efficiency

Digital control can reduce technical losses, integrate clean energy, improve asset utilization, and coordinate electrification. Digital technologies themselves consume energy and require materials, so sustainability assessment should consider life-cycle effects.

7. Stakeholders and Governance

Smart-grid decisions involve multiple stakeholders:

Governance questions are as important as engineering questions. Who may access meter data? Who is responsible if an automated control action causes damage? How should modernization costs be allocated between participants and nonparticipants? How can low-income or digitally excluded customers benefit? Such questions require transparent rules, impact assessment, and stakeholder engagement.

8. Development Pathways

Smart-grid transformation usually occurs incrementally rather than through a single replacement project.

Stage 1: Foundation

Organizations establish asset inventories, communication connectivity, data governance, cybersecurity procedures, and baseline reliability measures. Conventional protection and safety remain essential.

Stage 2: Visibility

Utilities deploy AMI, feeder sensors, substation automation, outage-management improvements, and improved geographic information systems. The objective is to know what is happening.

Stage 3: Automation

Remote-controlled switches, fault location, isolation and service restoration, volt/volt-ampere-reactive optimization, and automated demand response are introduced. The objective is to act faster and more consistently.

Stage 4: Coordination

DER management systems, flexible interconnection procedures, storage dispatch, aggregators, and distribution-transmission coordination become important. The objective is to treat distributed resources as system assets while respecting local constraints.

Stage 5: Integrated transactive operation

Advanced tariffs, local flexibility markets, peer or community energy arrangements, and automated customer participation may be used where appropriate. Such arrangements require rigorous settlement, consumer protection, cybersecurity, and market-power safeguards.

Progress should be measured by outcomes rather than the number of devices installed. Relevant measures include interruption duration, restoration time, peak reduction, renewable hosting capacity, energy losses, customer participation, emissions, affordability, cyber-incident performance, and total life-cycle cost.

9. Applied Case: Distribution Automation and Rooftop Solar

The following is an illustrative case, not a report of a particular utility.

Background

A suburban distribution feeder serves 2,400 customers through a radial 12.47 kV network. Rooftop photovoltaic systems have grown rapidly, and midday generation occasionally exceeds local demand. The feeder also experiences storm-related interruptions.

Problem

The utility has limited real-time visibility beyond the substation. During faults, crews patrol long feeder sections to locate damage. At high solar output, voltage near the end of the feeder approaches the upper operating limit. Traditional voltage regulators and fixed protection settings were not designed for frequent reverse power flow.

Analysis

The utility first collects feeder measurements, smart-meter voltage data, inverter operating records, and outage histories. A feeder model is validated against field measurements. The analysis identifies three needs: improved fault location, adaptive voltage control, and coordinated DER settings.

Approach

The utility installs line sensors and remotely controlled sectionalizing switches. It integrates these devices with an outage-management and distribution-management platform. Inverters are configured to provide approved voltage-support functions, and a battery at a community facility is dispatched to absorb excess midday energy and support evening demand. Customer data are protected through role-based access and aggregation for planning studies.

When a fault occurs, sensors identify abnormal current and voltage patterns. The system opens switches around the faulted section and closes a normally open tie where capacity permits. Operators review the proposed switching sequence before execution during the initial deployment phase. Voltage-control settings are coordinated with inverter capability and protection requirements.

Outcome

The illustrative project reduces patrol time and restores unaffected customers more quickly. Better voltage visibility permits additional photovoltaic connections without immediately rebuilding the entire feeder. The battery reduces the evening peak and provides backup service to the community facility. However, the project requires continuing communications maintenance, staff training, inverter interoperability testing, and customer consent procedures.

Learning points

1. Digitalization produces value only when measurements are accurate and connected to operational decisions.
2. DER integration requires joint consideration of voltage, protection, thermal limits, and market rules.
3. Automation should be introduced with staged testing, human oversight, and safe fallback modes.
4. Reliability benefits should be evaluated together with cybersecurity, privacy, equity, and life-cycle cost.

10. Challenges and Limitations

Smart-grid projects face technical, financial, institutional, and social barriers.

Technical limitations include communication latency, inaccurate models, interoperability gaps, inverter behavior during faults, limited battery duration, and uncertain performance during extreme conditions.

Economic limitations include high initial investment, uncertain benefit allocation, stranded assets, and difficulty valuing resilience or avoided emissions. A project may be beneficial to the system but unattractive to the organization required to fund it.

Cybersecurity risks increase as more devices become connected. Threats include unauthorized access, malware, false data injection, denial of service, and compromised distributed devices. Cybersecurity cannot be treated as a one-time procurement feature; it requires continuous governance.

Privacy risks arise because high-resolution consumption data can reveal occupancy patterns, business activity, or appliance use. Data minimization, purpose limitation, access control, retention rules, anonymization where suitable, and transparent consent are important safeguards.

Equity concerns arise if only affluent customers can afford solar, batteries, electric vehicles, or automated controls, while all customers pay for infrastructure. Programs should consider low-income households, renters, rural communities, accessibility, and digital inclusion.

Human factors remain important. Operators need training in automated systems, customers need comprehensible interfaces, and organizations need clear responsibility when systems interact across institutional boundaries.

11. Contemporary Developments and Professional Practice

Current development emphasizes grid-forming inverters, long-duration storage, flexible electric-vehicle charging, advanced distribution management, digital twins, edge analytics, and artificial intelligence. AI can assist load forecasting, fault classification, asset-health assessment, and anomaly detection. It should not be assumed to be infallible: models may inherit biased data, fail under unusual conditions, or produce decisions that are difficult to explain. Safety-critical applications require validation, monitoring, fail-safe operation, and human accountability.

Electrification increases the coupling between power systems and other infrastructures. Electric vehicles connect transport and electricity; heat pumps connect buildings and electricity; hydrogen production may connect industrial energy demand and renewable generation. Planning therefore increasingly requires sector coupling, scenario analysis, and coordination across institutions.

Good professional practice includes:

• begin with a defined operational problem rather than a technology trend;
• establish baseline performance before deployment;
• use validated network models and representative scenarios;
• design cybersecurity and privacy from the outset;
• test interoperability and failure modes;
• involve customers and affected communities;
• evaluate total life-cycle costs and benefits;
• provide manual overrides and recovery procedures;
• document assumptions, data quality, and accountability;
• measure outcomes continuously and revise the design.

Chapter Summary

Electric power systems comprise generation, transmission, distribution, and utilization, supported by protection, control, communications, markets, and governance. Their operation requires continuous balance between supply and demand, acceptable frequency and voltage, and coordinated management of faults and uncertainty. High-voltage transmission reduces current and therefore resistive losses for a given power transfer, while distribution systems deliver energy locally and increasingly accommodate bidirectional flows.

The conventional grid is limited by incomplete observability, aging infrastructure, one-way design assumptions, variable renewable generation, peak demand, and fragmented coordination. Smart grids address these pressures through sensing, communications, automation, distributed energy resources, storage, demand response, advanced analytics, and customer participation. Their defining characteristics include observability, controllability, flexibility, interoperability, resilience, security, sustainability, and empowerment.

Smart-grid development is a staged pathway from foundational data and communications to visibility, automation, coordination, and potentially integrated market operation. Successful modernization is not merely a hardware exercise. It requires sound engineering, standards, regulation, cybersecurity, privacy protection, equitable participation, trained professionals, and outcome-based evaluation.

Key Terms

Generation: Conversion of primary energy into electrical energy.

Transmission: High-voltage bulk transfer of electrical power.

Distribution: Local delivery of electricity from substations to customers.

Utilization: Customer-side conversion of electricity into useful services.

Distributed energy resource: Small-scale generation, storage, or controllable demand connected near loads.

Smart grid: A digitally enabled, coordinated electric power system with enhanced sensing, communication, and control.

Advanced metering infrastructure: Smart meters and associated communications and data systems.

Demand response: Intentional modification of electricity consumption in response to signals or incentives.

Microgrid: Bounded group of loads and resources able to operate with or without the main grid.

Virtual power plant: Coordinated aggregation of distributed resources.

SCADA: Supervisory control and data acquisition system.

Phasor measurement unit: Synchronized device measuring voltage and current phasors.

Interoperability: Ability of diverse systems to exchange and correctly use information.

Prosumer: Customer that consumes and produces or stores electricity.

Reliability: Ability to provide electricity adequately and continuously under specified conditions.

Resilience: Ability to withstand, recover from, and adapt to disruptive events.

Demand response: Change in customer demand stimulated by system conditions, prices, or control signals.

Self-healing grid: Grid capable of detecting, isolating, and restoring service after faults.

Cybersecurity: Protection of digital systems, communications, and data from unauthorized or harmful actions.

Review Questions

Short Answer Questions

1. What are the four functional stages of an electric power system?
2. Why does increasing transmission voltage reduce resistive losses for a given transmitted power?
3. Define a distributed energy resource and give two examples.
4. Distinguish reliability from resilience.
5. What is the purpose of advanced metering infrastructure?

Long Answer Questions

1. Explain the operation of generation, transmission, distribution, and utilization as an interconnected system.
2. Discuss the principal limitations of the conventional grid and relate each limitation to a smart-grid capability.
3. Explain the roles of observability, automation, flexibility, and interoperability in smart-grid operation.
4. Describe the technical and governance responsibilities of major smart-grid stakeholders.
5. Compare the staged development pathway from foundational infrastructure to coordinated DER operation.

Analytical / Application Questions

1. A distribution feeder experiences increasing rooftop photovoltaic penetration and evening electric-vehicle charging. Analyze the likely effects on voltage, thermal loading, protection, and peak demand, and recommend a coordinated response.
2. Design an evaluation framework for a proposed distribution-automation project. Identify technical, economic, reliability, cybersecurity, privacy, and equity metrics.
3. A utility proposes collecting high-resolution customer data to improve forecasting. Assess the operational benefits, privacy risks, governance controls, and conditions under which the proposal should proceed.

Multiple Choice Questions with Answers

Question 1

Which function is primarily associated with transmission networks?

A. Converting electricity into lighting and motion

B. Bulk transfer of power at relatively high voltage

C. Measuring household appliance efficiency

D. Selling electrical appliances

Correct Answer: B. Bulk transfer of power at relatively high voltage

Question 2

For a balanced three-phase system, which expression gives real power using line voltage and line current?

A. P = V_L I_L

B. P = 3V_L I_L

C. P = √3 V_L I_L cosφ

D. P = I_L²/R

Correct Answer: C. P = √3 V_L I_L cosφ

Question 3

What is a major operational consequence of high rooftop photovoltaic penetration on a traditionally radial feeder?

A. Power can flow in reverse toward the substation

B. Transmission voltage becomes zero

C. All protection equipment becomes unnecessary

D. Customer demand becomes constant

Correct Answer: A. Power can flow in reverse toward the substation

Question 4

Which technology combines smart meters, communications, and meter-data systems?

A. SCADA only

B. Advanced metering infrastructure

C. A synchronous condenser

D. A mechanical governor

Correct Answer: B. Advanced metering infrastructure

Question 5

What does demand response involve?

A. Changing electricity consumption in response to signals or incentives

B. Increasing conductor resistance deliberately

C. Replacing all generators with batteries

D. Disconnecting every customer during peak demand

Correct Answer: A. Changing electricity consumption in response to signals or incentives

Question 6

Which characteristic means that different grid devices and systems can exchange and correctly use information?

A. Interoperability

B. Islanded operation

C. Dispatchability

D. Synchronization loss

Correct Answer: A. Interoperability

Question 7

A 10 MW battery has 40 MWh of stored energy. Ignoring losses and operating constraints, how long could it deliver 10 MW?

A. 0.25 hours

B. 2 hours

C. 4 hours

D. 40 hours

Correct Answer: C. 4 hours

Question 8

Which statement best distinguishes resilience from reliability?

A. Reliability concerns only voltage, while resilience concerns only frequency

B. Resilience emphasizes withstanding and recovering from disruptive events

C. Reliability applies only to generators

D. Resilience eliminates the need for maintenance

Correct Answer: B. Resilience emphasizes withstanding and recovering from disruptive events

Question 9

Which is an appropriate cybersecurity practice for a smart grid?

A. Using one shared password for all devices

B. Removing all system logs

C. Network segmentation and least-privilege access

D. Allowing unrestricted remote access

Correct Answer: C. Network segmentation and least-privilege access

Question 10

What is a microgrid?

A. A small transformer used only for voltage measurement

B. A bounded group of loads and resources that can operate with or without the main grid

C. A transmission line operating at extremely high frequency

D. A billing database with no electrical equipment

Correct Answer: B. A bounded group of loads and resources that can operate with or without the main grid