diff --git a/andes/models/area.py b/andes/models/area.py
index 4e2958e6f..04d5d5c8c 100644
--- a/andes/models/area.py
+++ b/andes/models/area.py
@@ -56,8 +56,8 @@ class ACEData(ModelData):
     def __init__(self):
         ModelData.__init__(self)
         self.bus = IdxParam(model='Bus', info="bus idx for freq. measurement", mandatory=True)
-        self.bias = NumParam(default=1.0, info='bias parameter', tex_name=r'\beta',
-                             unit='MW/0.1Hz', power=True)
+        self.bias = NumParam(default=-1.0, info='bias parameter', tex_name='B',
+                             unit='MW/0.1Hz', power=True, non_positive=True)
 
         self.busf = IdxParam(info='Optional BusFreq device idx', model='BusFreq',
                              default=None)
@@ -65,12 +65,20 @@ def __init__(self):
 
 class ACEc(ACEData, Model):
     """
-    Area Control Error model.
+    Area Control Error model, where bias (B) is an approximation of the actual frequency response (:math:`\\beta`).
+    Given NERC's definition, the bias should be a negative number.
 
     Continuous frequency sampling.
     System base frequency from ``system.config.freq`` is used.
 
     Note: area idx is automatically retrieved from `bus`.
+
+    Reference:
+
+    NERC, "Balancing and Frequency Control Reference Document, Chapter 1 Balancing Fundamentals: Bias
+    (B) vs. Frequency Response (Beta)", 2021. Available:
+
+    https://www.nerc.com/comm/RSTC_Reliability_Guidelines/Reference_Document_NERC_Balancing_and_Frequency_Control.pdf
     """
 
     def __init__(self, system, config):
@@ -103,10 +111,12 @@ def __init__(self, system, config):
                           info='Bus frequency',
                           unit='p.u. (Hz)'
                           )
+        # NOTE: here, we use -abs(bias) to ensure that bias is negative to ensure compatibility
+        # with previous versions
         self.ace = Algeb(info='area control error',
                          unit='p.u. (MW)',
                          tex_name='ace',
-                         e_str='10 * (bias * imva) * sys_f * (f - 1) - ace',
+                         e_str='- 10 * (-abs(bias) * imva) * sys_f * (f - 1) - ace',
                          )
 
 
@@ -122,6 +132,13 @@ class ACE(ACEc):
     can be specified in ``ACE.config.offset``.
 
     Note: area idx is automatically retrieved from `bus`.
+
+    Reference:
+
+    NERC, "Balancing and Frequency Control Reference Document, Chapter 1 Balancing Fundamentals: ACE Review",
+    2021. Available:
+
+    https://www.nerc.com/comm/RSTC_Reliability_Guidelines/Reference_Document_NERC_Balancing_and_Frequency_Control.pdf
     """
 
     def __init__(self, system, config):
@@ -140,4 +157,4 @@ def __init__(self, system, config):
                            info='Sampled freq.',
                            )
 
-        self.ace.e_str = '10 * (bias * imva) * sys_f * (fs_v - 1) - ace'
+        self.ace.e_str = '- 10 * (bias * imva) * sys_f * (fs_v - 1) - ace'
diff --git a/docs/source/release-notes.rst b/docs/source/release-notes.rst
index 3f4a1e22d..ac95a7798 100644
--- a/docs/source/release-notes.rst
+++ b/docs/source/release-notes.rst
@@ -46,6 +46,8 @@ Other changes:
 - Enhanced three-winding transformer parsing in PSS/E raw files by assigning the equivalent star bus ``area``,
   ``owner``, and ``zone`` using the high-voltage bus values.
 - Minor fix on model ``TGOV1NDB``
+- Clarification on models ``ACEc`` and ``ACE`` given NERC's definition, as NERC suggesst that both bias and beta
+  should be nevative
 
 v1.9.2 (2024-03-25)
 -------------------